1. science
2. physics
3. nanotechnology

Name: Abdulrahman Khalaf Ali
Thesis Title: (Preparation of Ag and Au Nanoparticles by Pulsed Laser
Ablation in Liquids)
Supervisor: Ass. Prof. Dr. Dayah N. Raouf
Date: 2010
No. of Pages: 114
Abstract:
This thesis has presented an alternative-novel, easy, fast and one-step
method for the preparation of pure and stable noble metal versatile nanoparticles
NPs in a high ablation rate and size-selected manner with a high concentration,
long period of stability, less aggregation, non toxic and contamination.
Noble metals silver and gold NPs were synthesized by pulsed (Q-switched,
1064 or 532nm doubled frequency-Nd: YAG) laser ablation of silver and gold
metal plates immersed in double distilled and deionised water DDDW or other
aqueous
solutions
such
as:
NaCl,
polyvinylpyrrolidone PVP, Ethanol
sodium
and
dodecyl
sulphate
Acetone, each with
SDS,
various
concentrations.
The optimum preparation parameters had been optimized for the best
formation efficiency of pulsed laser ablation in liquids (PLAL) process, which
are: laser shots is 15 and 10 pulses, laser energy is 600 and 500 mJ, liquid depth
is 8 and 7 mm, laser fluence is 47.4 and 105.8 J/cm2 for silver and gold,
respectively. Moreover, the optimum ablation laser wavelength was 1064 nm.
The formation efficiency of PLAL process was quantified in term of the
surface plasmon extinction SPE peaks and it was enhanced greater than 220
times, as well as the concentration of ablated Ag NPs which obtained via AAS;
however it was estimated to be about 8.66 μg/pulse, and it was increased about
21 times.
I
The SPE spectra shows a sharp and single peaks around 400 or 526 nm,
indicating the production of pure and spherical Ag and Au NPs with average
size of 14 nm, respectively. All the size measurements have been confirmed by
TEM.
There is a simultaneous possibility of on-line observation of the
nanoparticles formation via measuring the variation in nanoparticles absorption
at the peaks of SPE using 532nm, 1mW diode laser.
The optimum concentration for the formation of Ag nanoparticles in various
solutions were: NaCl (2.5 mM), SDS (25 mM) and PVP (5 mM) which optimize
the formation efficiency, reduces the size and size distribution, enhance
dispersity and prevents aggregation of the prepared nanoparticles.
Resizing and reshaping of Ag and Au nanoparticles have been performed
using pulsed Nd-YAG laser, λ=532nm; the average particle size dropped from
14 to 8nm.
Novel bimetallic of Au–Ag core-shell nanoalloys were synthesized. The
nanoalloys appear to be nearly spherical with average size of 19 nm. The
irradiation process has been done by Nd-YAG laser, with λ=532nm and E=900
mJ.
Keyword: Pulsed, Nanoparticles, Ag, Au, liquid, Nd: YAG
II
Preparation of Ag and Au
Nanoparticles by Pulsed Laser Ablation
in Liquids
By
Abdulrahman Khalaf Ali
A Thesis
Submitted to the Department of Applied Sciences at the University of
Technology
as a Partial Fulfilment
ulfilment of the Requirement for the Degree
egree of
Doctor of Philosophy in Laser Physics
Supervised by
Assistant
istant Prof. Dr.Dayah N. Raouf
Baghdad (2010)
I
‫ﻣ‪‬ﻦ‪ ‬ﻋ‪‬ﺒ‪‬ﺎﺩ‪‬ﻩ‪‬‬
‫ﺇﻧ‪‬ﻤ‪‬ﺎ ﻳ‪‬ﺨ‪‬ﺸ‪‬ﻰ‬
‫ﺍﻟﹾﻌ‪‬ﻠﹶﻤ‪‬ﺎﺀُ‬
‫ﺻﺪق اﷲ اﻟﻌﻈﯿﻢ‬
‫)ﻓﺎﻃﺮ‪(٢٨ -‬‬
‫‪II‬‬
Supervisor Certification
I certify that this thesis entitled: (Preparation of Ag and Au
Nanoparticles by Pulsed Laser Ablation in Liquids) was prepared under my
supervision at the University of Technology, department of applied sciences, in
partial fulfilment of the requirements for the degree of Doctor of Philosophy in
Laser Physics.
Signature:
Name: Dr. Dayah N. Raouf
Title: Assistant Professor
Date:
/ / 2010
III
Linguistic Certification
I certify that this thesis entitled: (Preparation of Ag and Au Nanoparticles
by Pulsed Laser Ablation in Liquids) was prepared under my linguistic
supervision. It was amended to meet the style of the English language.
Signature:
Name: Dr. Mohamed Saleh Ahmed
Title: Assistant Professor
Date:
/
/ 2010
IV
Examination Committee Certificate
We certify that we have read this thesis entitled (Preparation of Ag and Au
Nanoparticles by Pulsed Laser Ablation in Liquids) and as an examining
committee examined the student '' Abdulrahman Khalaf Ali'' in its contents
and that in our opinion; it meets the requirements of awarding the degree of
Doctor of Philosophy of Science in Laser Physics.
Signature:
Name: Dr. Bassam G. Rasheed
Title: Assistant Professor
(Member)
Date: /
/ 2010
Signature:
Name: Dr. Khawla Salah Khashan
Title: Assistant Professor
(Member)
Date: / / 2010
Signature:
Name: Dr. Adawiya J. Haider
Title: Professor
(Member)
Date: / / 2010
Signature:
Name: Dr. Nadir Fadhil Habbubi
Title: Professor
(Member)
Date: / / 2010
Signature:
Name: Dr. Dayah N. Raouf
Title: Assistant Professor
(Supervisor)
Date: / / 2010
Signature:
Name: Dr. Mazin M. Elias
Title: Professor
(Chairman)
Date: / / 2010
Approved by the School of Applied Sciences, University of Technology.
Signature:
Name: Kassim S. Kassim
Title: Head of School of Applied Sciences
Date: / / 2010
V
Acknowledgment
After thanks my “glorious ALLAH”, I am pleased to acknowledge the
assistance and support for many people whom had support me in completion of
this work.
First I would like to thank my supervisor Dr. Dayah N. Raouf for skilful
scientific guidance, who always supported my research effort, during the long
period time of this project.
I would like to present my spatial thanks and great gratitude to Dr. Bassam
G. Rashid, the head of laser branch, for his unlimited support for the
experimental requirements.
Also, I would like to express my deep appreciation and respect to all the
department of applied science staff, especially of laser branch.
I am indebted to the electronic microscope centre-collage of medicine/ AlNahrain University for their helpful in TEM measurements. I am also grateful to
the chemical Lab.-collage of science/ Baghdad University for unlimited
assistant in AAS measurements.
Abdulrahman
15-1-2010
VI
Dedication
I would like to dedicate my Doctoral Thesis to my family: parent, wife,
brothers and sisters.
VII
Abstract
This thesis has presented an alternative-novel, easy, fast and one-step method
for the preparation of pure and stable noble metal versatile nanoparticles NPs in a
high ablation rate and size-selected manner with a high concentration, long period
of stability, less aggregation, non toxic and contamination.
Noble metals silver and gold NPs were synthesized by pulsed (Q-switched,
1064 or 532nm doubled frequency-Nd: YAG) laser ablation of silver and gold
metal plates immersed in double distilled and deionised water DDDW or other
aqueous solutions such as: NaCl, sodium dodecyl sulphate SDS,
polyvinylpyrrolidone PVP, Ethanol and Acetone, each with various concentrations.
The optimum preparation parameters had been optimized for the best
formation efficiency of pulsed laser ablation in liquids (PLAL) process, which are:
laser shots is 15 and 10 pulses, laser energy is 600 and 500 mJ, liquid depth is 8
and 7 mm, laser fluence is 47.4 and 105.8 J/cm2 for silver and gold, respectively.
Moreover, the optimum ablation laser wavelength was 1064 nm.
The formation efficiency of PLAL process was quantified in term of the
surface plasmon extinction SPE peaks and it was enhanced greater than 220 times,
as well as the concentration of ablated Ag NPs which obtained via AAS; however
it was estimated to be about 8.66 μg/pulse, and it was increased about 21 times.
The SPE spectra shows a sharp and single peaks around 400 or 526 nm,
indicating the production of pure and spherical Ag and Au NPs with average size
of 14 nm, respectively. All the size measurements have been confirmed by TEM.
There is a simultaneous possibility of on-line observation of the nanoparticles
formation via measuring the variation in nanoparticles absorption at the peaks of
SPE using 532nm, 1mW diode laser.
The optimum concentration for the formation of Ag nanoparticles in various
solutions were: NaCl (2.5 mM), SDS (25 mM) and PVP (5 mM) which optimize
the formation efficiency, reduces the size and size distribution, enhance dispersity
and prevents aggregation of the prepared nanoparticles.
Resizing and reshaping of Ag and Au nanoparticles have been performed using
pulsed Nd-YAG laser, λ=532nm; the average particle size dropped from 14 to 8nm.
Novel bimetallic of Au–Ag core-shell nanoalloys were synthesized. The
nanoalloys appear to be nearly spherical with average size of 19 nm. The
irradiation process has been done by Nd-YAG laser, with λ=532nm and E=900 mJ.
VIII
List of Abbreviations
AAS
Atomic Absorption Spectroscopy
AD
After Date
AgNPs
Silver Nanoparticles
Au-Ag
Bimetallic of Gold–Silver Nanoalloys
AuNPs
Gold Nanoparticles
BC
Before Christ
DDDW
Double Distilled and Deionised Water
e.g.
for example (exempli gratia)
et al.
and others (et alia)
i.e.
That is (id est)
LISR
Laser Induced Size Reduction
LSPR
Localized Surface Plasmon Resonance
NP (NPs)
Nanoparticle (Nanoparticles)
PLA
Pulsed Laser Ablation
PLAL
Pulsed Laser Ablation in Liquids
PVP
Polyvinylpyrrolidone
SDS
Sodium Dodecyl Sulphate
SERS
Surface Enhanced Raman Scattering
SPE
Surface Plasmon Extinction
SPR
Surface Plasmon Resonance
TEM
Transmission Electron Microscope
List of Symbols
cm-1
α
Absorption Coefficient
C
Concentration
ε
Complex dielectric constant
J-1C2m-1
ε0
Vacuum permittivity (dielectric constant)
J-1C2m-1
k
Extinction Coefficient
cm-1
M
Molary: (number of moles per litre)
mol/l
Pa
Pascal (=10-5 bar =10-5 atm.=760×10-5 mmHg=760×10-5 torr)
N/m2
S
Specific Surface Area
m2/g
g/l or Molary (M)
IX
Contents
CHAPTER ONE: INTRODUCTION
Page
1-1 Nanotechnology
1-2 Noble Metal Nanoparticles
1-3 Advantage of Pulsed Laser Ablation in Liquid Media
1-4 Literatures Review
1-5 Scientific Problems
1-6 Aims of the Work
1-7 Thesis Outline
1
3
4
6
11
12
12
CHAPTER TWO: THEORETICAL CONSIDERATIOS
2-1 Introduction
2-2 Laser ablation and Particle Generation
2-2-1 Laser-Induced Heating and Melting
2-2-2 Explosive Boiling
2-2-3 Evaporization
2-2-4 Plasma Formation
2-2-5 Solid Exfoliation
2-2-6 Hydrodynamic Sputtering
2-2-7 particle Ejection (Spallation)
2-2-8 Nucleation and Condensation
2-2-9 Coagulation and Agglomerates (Groth)
2-3 Nanoparticles-Liquid Reaction
2-4 Nanoparticles Suspension
2-5 Synthesis of Nanoparticles
2-5-1 Dispersion Methods
2-5-2 Reduction Methods
2-6 Colloids
2-7 Pulsed laser Ablation in Liquid Medium
2-8 Nanoparticles Formation Mechanism
2-9 Interaction of Light with Noble Metal Nanoparticles
2-9-1 Surface Plasmon Resonance in Metal Nanoparticles (SPR)
2-9-2 Mie Theory
2-10 Modification of Metal Nanoparticles
2-10-1 Size Reduction: Secondary Laser Irradiation
2-10-2 Mechanism of Size Reduction Process
2-10-3 Laser-Induced Growth Tuneable Nanoparticles
2-10-4 Target Modification
2-11 Surface Area of Nanoparticles
2-12 Properties of Nanoparticles
2-12-1 Optical Properties
2-12-2 Thermal Properties
2-12-3 Catalytic Properties
2-13 Application of Nanoparticles
2-13-1 Antibacterial of Silver Nanoparticles Agent
2-13-2 Surface Enhanced Raman Scattering
X
13
13
14
14
15
15
16
16
17
18
18
19
20
21
21
21
22
22
23
25
25
28
29
29
30
31
32
33
34
34
35
35
36
36
37
2-13-3 Diabetic Delayed Wound Healing
2-13-4 Cooling Challenge
39
39
CHAPTER THREE: EXPERIMENTAL WORKS
3-1 Laser Ablation System
3-2 Laser Source and Measurements Device
40
41
3-2-1 Nd-YAG Laser
3-2-2 Semiconductor Laser
3-2-3 Transmission Electron Microscope
3-2-4 Spectrophotometer
3-2-5 Atomic Absorption Spectroscopy
3-2-6 Optical Microscope
3-2-7 Evaporation System
3-3 Materials
3-3-1 Target Materials
3-3-2 Distilled and Deionised Water
3-3-3 Chemical Solution Preparation
3-4 General Experimental Process for Nanoparticles Formation
41
41
41
42
42
42
42
43
43
43
43
44
CHAPTER FOUR: RESULTS AND DISCUSSION
4-1 Introduction
4-2 Effect of Laser Shots
4-2-1 Silver Nanoparticles Concentration
4-2-2 Ablation Monitoring
4-3 Effect of Laser Energy
4-4 Effect of Laser Fluence
4-5 Effect of Liquid Depth
4-6 Effect of Laser Wavelength
4-7 Effect of Chemical Solutions
4-7-1 Effect of NaCl Solution
4-7-2 Effect of SDS Solution
4-7-3 Effect of PVP Solution
4-7-4 Effect of Organic Solutions
4-8 Effect of Temperature
4-9 Effect of Aging Time
4-10 Nanoparticles Modification and Size Controlling
4-11 Nanoalloys: Core-Shell Nanostructure Synthesis
4-12 Color Changing: Indicator
4-13 Thin Films Deposition
4-13-1 Bulk Thin Films
4-13-2 Nanoparticles Thin Films
4-14 Target Effects
45
45
50
54
55
61
64
67
70
70
73
77
82
85
87
91
95
97
98
98
99
103
CHAPTER FIVE: CONCLUSIONS AND FUTURE WORK
5-1 Conclusions
5-2 Suggestion for Future Works
References
106
106
107
XI
‫ﺟﻤﮭﻮرﯾﺔ اﻟﻌﺮاق‬
‫وزارة اﻟﺘﻌﻠﯿﻢ اﻟﻌﻠﻲ واﻟﺒﺤﺚ اﻟﻌﻠﻤﻲ‬
‫اﻟﺠﺎﻣﻌﺔ اﻟﺘﻜﻨﻮﻟﻮﺟﯿﺔ‬
‫ﻗﺴﻢ اﻟﻌﻠﻮم اﻟﺘﻄﺒﯿﻘﯿﺔ‬
‫ﺗﺤﻀﻴﺮ ﺟﺴﻴﻤﺎﺕ ﺍﻟﻔﻀﺔ ﻭﺍﻟﺬﻫﺐ ﺍﻟﻨﺎﻧﻮﻳ‪‬ﺔ‬
‫ﺑﻄﺮﻳﻘﺔ ﺍﻟﹶﺘﺒ‪‬ﺨﺮ ﺍﻷﻧﻔﺠﺎﺭﻱ ﻓﻲ ﺍﻟﺴﻮﺍﺋﻞ‬
‫ﺑﺄﺳﺘﺨﺪﺍﻡ ﻟﻴﺰﺭ ﻧﺒﻀﻲ‬
‫ﻣﻦ ﻗﺒﻞ‬
‫ﻋﺒﺪ ﺍﻟﺮﺣﻤﻦ ﺧﻠﻒ ﻋﻠﻲ‬
‫رﺳﺎﻟﺔ‬
‫ﻣﻘﺪﻣﺔ اﻟﻰ ﻗﺴﻢ اﻟﻌﻠﻮم اﻟﺘﻄﺒﯿﻘﯿﺔ ﻓﻲ اﻟﺠﺎﻣﻌﺔ اﻟﺘﻜﻨﻮﻟﻮﺟﯿﺔ وھﻲ ﺟﺰء ﻣﻦ‬
‫ﻣﺘﻄﻠﺒﺎت ﻧﯿﻞ درﺟﺔ اﻟﺪﻛﺘﻮراه ﻓﻲ اﻟﻠﯿﺰر‬
‫إﺷﺮاف‬
‫ﺃ‪.‬ﻡ‪.‬ﺩ‪.‬ﺿﻴﺎﺀ ﻧﻮﺭﻱ ﺭﺅﻭﻑ‬
‫‪٢٠١٠‬م‬
‫‪١٤٣١‬ھ‬
‫‪XII‬‬
‫ﺍﻟﺨﻼﺻﺔ‬
‫ﺗﻘﺪم ھﺬه اﻷروﺣﺔ ﻃﺮﯾﻘﺔ ﺟﺪﯾﺪة‪ -‬ﺳﮭﻠﺔ وﺳﺮﯾﻌﺔ ﻟﺘﺤﻀﯿﺮ ﺟﺴﯿﻤﺎت ﻧﺎﻧﻮﯾﺔ ﺑﺨﻄﻮة واﺣﺪة وذات ﻛﻔﺎءة‬
‫ﺗﺤﻮﯾﻞ ﻋﺎﻟﯿﺔ وﺑﺤﺠﻢ وﺧﺼﺎﺋﺺ ﻣﺴﯿﻄﺮ ﻋﻠﯿﮭﺎ وﺑﺘﺮاﻛﯿﺰ ﻋﺎﻟﯿﺔ ﺑﺪون ﺗﻜﺘﻞ‪،‬ﻏﯿﺮ ﺳﺎﻣﺔ وﺧﺎﻟﯿﺔ ﻣﻦ اﻟﺘﻠﻮث‪.‬‬
‫ﺣﻀﺮت اﻟﺠﺴﯿﻤﺎت اﻟﻨﺎﻧﻮﯾﺔ ﻣﻦ اﻟﻤﻌﺎدن اﻟﻨﺒﯿﻠﺔ ﻣﺜﻞ اﻟﺬھﺐ واﻟﻔﻀﺔ ﺑﺄﺳﺘﺨﺪام ﻟﯿﺰر اﻟﯿﺎك اﻟﻨﺒﻀﻲ ذو اﻟﻄﻮل‬
‫اﻟﻤﻮﺟﻲ ‪ 1064 nm‬و ‪ 532 nm‬ﻣﻀﺎﻋﻒ اﻟﺘﺮدد‪ ،‬ﺑﻄﺮﯾﻘﺔ اﻟﺘﺒﺨﺮ اﻻﻧﻔﺠﺎري ﻟﻘﻄﻌﺔ ﻣﻌﺪﻧﯿﺔ ﻓﺎﺋﻘﺔ اﻟﻨﻘﺎوة‬
‫ﻣﻦ اﻟﺬھﺐ واﻟﻔﻀﺔ ﻣﻐﻤﻮرة ﻓﻲ ﻣﺎء ﻻأﯾﻮﻧﻲ وﺛﻨﺎﺋﻲ اﻟﺘﻘﻄﯿﺮ ﯾﻌﺮف ڊ ‪ DDDW‬أو ﻣﺤﺎﻟﯿﻞ ﺳﺎﺋﻠﺔ أﺧﺮى‬
‫ﻣﺜﻞ ‪ (polyvinylpyrrolidone PVP, sodium dodecyl sulfate SDS, NaCl Ethanol,‬و‬
‫‪ (Acetone‬وﺑﺘﺮاﻛﯿﺰ ﻣﺨﺘﻠﻔﺔ‪.‬‬
‫ﺗﻢ ﺣﺴﺎب أﻓﻀﻞ ﻣﻌﻠﻤﺎت اﻟﺘﺤﻀﯿﺮ واﻟﺘﻲ ﺣﺴﻨﺖ ﻛﻔﺎءة ﺗﻮﻟﯿﺪ اﻟﺠﺴﯿﻤﺎت اﻟﻨﺎﻧﻮﯾﺔ ﺑﻄﺮﯾﻘﺔ اﻟﺘﺒﺨﺮ اﻻﻧﻔﺠﺎري‬
‫ﻓﻲ اﻟﻤﺤﺎﻟﯿﻞ ﺑﻮاﺳﻄﺔ ﻟﯿﺰر ﻧﺒﻀﻲ واﻟﻤﻌﺮﻓﺔ ڊ ‪ ، PLAL‬وھﺬه اﻟﻤﻌﻠﻤﺎت ھﻲ ﻛﻞ ﻣﻦ‪ :‬ﻋﺪد ﻧﺒﻀﺎت اﻟﻠﻠﯿﺰر‬
‫ﻛﺎﻧﺖ ‪١٥‬و‪ ١٠‬ﻧﺒﻀﺔ‪ ،‬وﻃﺎﻗﺔ اﻟﻠﯿﺰر ﻛﺎﻧﺖ ‪ 600 mJ‬و‪ ،500 mJ‬وﻋﻤﻖ اﻟﺴﺎﺋﻞ ﻓﻮق اﻟﻘﻄﻌﺔ اﻟﻤﻌﺪﻧﯿﺔ ﻛﺎن‬
‫‪ ٨mm‬و ‪ ،7 mm‬وﻛﺜﺎﻓﺔ ﻃﺎﻗﺔ اﻟﻠﯿﺰر ﻛﺎﻧﺖ ‪ 47.4 J/cm2‬و ‪ 105.8 J/cm2‬ﻋﻠﻰ اﻟﺘﻮاﻟﻲ ﺑﺎﻟﻨﺴﺒﺔ‬
‫ﻟﺠﺴﯿﻤﺎت اﻟﻔﻀﺔ واﻟﺬھﺐ اﻟﻨﺎﻧﻮﯾﺔ‪ .‬ﻛﺬﻟﻚ ﻛﺎن أﻓﻀﻞ ﻃﻮل ﻣﻮﺟﻲ ﻷﺟﺮاء اﻟﺘﺒﺨﺮ اﻻﻧﻔﺠﺎري ﻋﻨﺪ ‪1064‬‬
‫‪.nm‬‬
‫ﻛﻔﺎءة ﻋﻤﻠﯿﺔ اﻟﺘﻮﻟﯿﺪ ﻠ ‪ PLAL‬ﻗﺪرت ﺑﺪﻻﻟﺔ أﻋﻀﻢ ﺷﺪة ﻟﻞ ‪،(surface plasmon extinction) SPE‬‬
‫ﺣﯿﺚ أزدادت اﻛﺜﺮﻣﻦ ‪ ٢٢٠‬ﻣﺮة وﻛﺬﻟﻚ ﺑﺪﻻﻟﺔ ﺗﺮاﻛﯿﺰ ﺟﺴﯿﻤﺎت اﻟﻔﻀﺔ اﻟﻨﺎﻧﻮﯾﺔ اﻟﻤﻘﺎﺳﺔ ﺑﻮاﺳﻄﺔ ﻣﻄﯿﺎف‬
‫اﻻﻣﺘﺼﺎص اﻟﺬري ‪ AAS‬واﻟﺘﻲ ﻗﺪرت ڊ ‪ ،8.66 μg/pulse‬ﺣﯿﺚ ازدادت ﺑﻤﻘﺪار ‪ ٢١‬ﻣﺮة‪.‬‬
‫أﻃﯿﺎف اﻠ ‪ SPE‬ﺗﻈﮭﺮ ﻗﻤﻢ أﻣﺘﺼﺎص ﺣﺎدة وﻣﻨﻔﺮدة ﺣﻮل اﻟﻘﯿﻤﺔ ‪ 400nm‬أو ‪ ،526 nm‬واﻟﺘﻲ ﺗﺪل ﻋﻠﻰ‬
‫ﺗﻮﻟﯿﺪ ﺟﺴﯿﻤﺎت ﻧﺎﻧﻮﯾﺔ ﻧﻘﯿﺔ وﻛﺮوﯾﺔ اﻟﺸﻜﻞ ﻣﻦ اﻟﻔﻀﺔ واﻟﺬھﺐ ﻋﻠﻰ اﻟﺘﻮاﻟﻲ وﺑﻘﻄﺮ ﻣﻌﺪﻟﮫ ‪ .13 nm‬ﻛﻞ‬
‫ﻗﯿﺎﺳﺎت أﻗﻄﺎر اﻟﺠﺴﯿﻤﺎت اﻟﻨﺎﻧﻮﯾﺔ اﺛﺒﺘﺖ ﺑﻮاﺳﻄﺔ اﻠ ‪. TEM‬‬
‫ھﻨﺎﻟﻚ أﻣﻜﺎﻧﯿﺔ ﻟﻤﺮاﻗﺒﺔ ﻋﻤﻠﯿﺔ ﺗﻮﻟﺪ اﻟﺠﺴﯿﻤﺎت اﻟﻨﺎﻧﻮﯾﺔ ﻣﺒﺎﺷﺮة ﺑﻮاﺳﻄﺔ ﻗﯿﺎس اﻟﺘﻐﯿﺮ اﻟﺤﺎﺻﻞ ﻓﻲ ﻗﯿﻤﺔ اﻠ‬
‫‪ SPE‬ﺑﺄﺳﺘﺨﺪام ﻟﯿﺰر اﻟﺪاﯾﻮد ذو اﻟﻄﻮل اﻟﻤﻮﺟﻲ ‪ 532nm‬وﺑﻄﺎﻗﺔ ‪.1 mW‬‬
‫أﻓﻀﻞ ﺗﺮﻛﯿﺰ ﻟﺘﺤﻀﯿﺮ ﺟﺴﯿﻤﺎت اﻟﻔﻀﺔ ﻓﻲ اﻟﻤﺤﺎﻟﯿﻞ اﻟﻤﺨﺘﻠﻔﺔ ﻣﺜﻞ ‪ NaCl‬ﻛﺎن ﻋﻨﺪ ‪ ،2.5 mM‬و ‪SDS‬‬
‫ﻋﻨﺪ ‪ ،25 mM‬و ‪ PVP‬ﻋﻨﺪ ‪ ،5 mM‬ﺣﯿﺚ ﺣﺴﻦ ﺣﯿﺚ ﺣﺴﻦ ھﺬا اﻟﺘﺮﻛﯿﺰ ﻛﻞ ﻣﻦ ﻛﻔﺎءة اﻟﺘﻮﻟﯿﺪ واﻟﺤﺠﻢ‬
‫واﻟﺘﻔﺮق وﻣﻨﻊ اﻟﺘﻜﺘﻞ ﻟﻠﺠﺴﯿﻤﺎت اﻟﻤﺤﻀﺮة‪.‬‬
‫ﺑﺎﻻﻣﻜﺎن ﺗﺤﺴﯿﻦ ﺣﺠﻢ وﺷﻜﻞ ﺟﺴﯿﻤﺎت اﻟﻔﻀﺔ واﻟﺬھﺐ واﻟﻤﺤﻀﺮة ﺑﻄﺮﯾﻘﺔ اﻠ ‪ ، PLAL‬ﺣﯿﺚ أﻧﺠﺰت ھﺬه‬
‫اﻟﻌﻤﻠﯿﺔ ﺑﺄﺳﺘﺨﺪام ﻟﯿﺰر اﻟﯿﺎك اﻟﻨﺒﻀﻲ ذو اﻟﻄﻮل اﻟﻤﻮﺟﻲ ‪ ،532 nm‬ﻣﻌﺪل ﻗﻄﺮ اﻟﺠﺴﯿﻤﺔ اﻟﻨﺎﻧﻮﯾﺔ اﻧﺨﻔﺾ‬
‫ﻣﻦ ‪ 14 nm‬اﻟﻰ ‪.8 nm‬‬
‫ﺗﻢ ﺗﺤﻀﯿﺮ ودراﺳﺔ ﺗﺮﻛﯿﺐ ﺳﺒﯿﻜﺔ ﻧﺎﻧﻮﯾﺔ ﺛﻨﺎﺋﯿﺔ اﻟﻤﻌﺪن ﻣﻦ اﻟﺬھﺐ ﻓﻲ اﻟﻠﺐ ﻣﺤﺎط ﺑﻄﺒﻘﺔ ﻣﻦ اﻟﻔﻀﺔ‪.‬‬
‫اﻟﺴﺒﯿﻜﺔ اﻟﻨﺎﻧﻮﯾﺔ ﺗﺒﺪو ﺑﺸﻜﻞ ﻛﺮوي وﺑﻘﻄﺮ ‪ .19 nm‬ﻋﻤﻠﯿﺔ اﻟﺘﺸﻌﯿﻊ ﺗﻤﺖ ﺑﻠﯿﺰر اﻟﯿﺎك اﻟﻨﺒﻀﻲ ﺑﻄﻮل ﻣﻮﺟﻲ‬
‫‪ ٥٣٢ nm‬وﻃﺎﻗﺔ ‪. 900 mJ‬‬
‫‪XIII‬‬
Chapter One: Introduction
1-1 Nanotechnology
When Neil Armstrong stepped onto the moon, he called it small step for
man and giant leap for mankind. Nano may represent another giant leap for
mankind, but with step so small that it makes Neil Armstrong look the size of
solar system! However, nanoscience and nanotechnology are steering mankind
into new realms of efficient and miniature tools and gadgetry [1].
Clusters of metals are known from ancient times. It is widely known that the
brilliant colors of noble metals like gold or silver have fascinated the human
being from the very beginning of mankind. They were employed to dye glass
and fabrics and as a therapeutic aid in treatment of arthritis. While the most
ancient use colloidal Au is believed to have been BC in Egypt by alchemists [2],
the brilliant colors of nanosized colloidal particles of Ag and Au were used in
ancient Romans glass artefact dated to 4th century AD, that appears red in
transmitted light and green in reflected light was found to be due to Au and Ag
nanocrystals impregnated in glass. These metals were already used to decorate
glass, exhibited in the British Museum shows. This technique to color glass was
extensively used in the 10th century AD for the many cathedrals in Europe [3].
The history of metal nanoparticles begins with Faraday’s study of gold
colloids, as early as 1857 [4]. He established that several dyes were indeed
made of metal particles. After a thorough study of gold sols, Faraday concluded
“The introduction into a ray of separate particles [...]The gold is reduced in
exceedingly fine particles which becoming diffuse, produce a beautiful fluid[. .
.] the various preparations of gold, whether ruby, green, violet or
blue[...]consist of that substance in a metallic divided state known phenomena
appeared to indicate that a mere variation in the size of its particles gave rise to
a variety of resultant colours”. The British physicist Michael Faraday
recognized that this variety of colors was due to the interaction of light with
1
small metal particles. [3]. In 1908, Mie explained the origin of the bright colors
to the colloid by solving Maxwell's electromagnetic equation for the interaction
th
of light with spherical particles (d<<λ) [2,5].On December 29 1959, Nobel
Prize winner, physicist Feynman said in his famous speech “There is Plenty of
Room at the Bottom” [6].
The term “Nanotechnology” has been in use as early as 1974. It was defined
by Taniguchi [7]. Additionally, the definition of nanoscience and
nanotechnology as it is given by the US National Nanotechnology Initiative
NNI, in 2000[6]: nanoscience or nanotechnology are “Development at the
atomic levels in the length scale of approximately 1-100 nanometer range, to
provide a fundamental understanding of phenomena and materials at the
nanoscale and to create and use structures, devices and systems that have novel
properties and functions”. The term “Nano” refers to 10-9 meter is so small that
things smaller than it can only be molecules, clusters of atoms or particles in the
quantum world [8]. The term “Nanoparticle” is referring to a particle where all
the three dimensions are nanometer in scale; contain small enough a number of
constituent atoms or molecules that they differ from the properties inherent in
their bulk counterparts, exist in diverse shapes such as spherical, triangular,
cubical, pentagonal, rod-shaped, shells, ellipsoidal and so forth[8]. The term
“Colloid” is more elusive, the particle size can range from nanometers to
several hundreds of micrometers. The term “Cluster” is usually used for small
nanoparticles that have well-defined composition and surface structure as finite
aggregates of atoms or molecules which are bound by forces of metallic,
covalent, ionic, hydrogen bonded or van der Waals [9].
Nanomaterials display unique, superior and indispensable properties and
have attracted much attention for their distinct characteristics that are
unavailable in conventional macroscopic materials. Their uniqueness arises
specifically from higher surface to volume ratio and increased percentage of
atoms at the grain boundaries. They represent an important class of materials in
2
the development of novel devices that can be used in various physical,
biological, biomedical and pharmaceutical applications [10]. Therefore are an
objects of active research in various applications such as: photo-thermal
therapy[11],
surface-enhanced
Raman
spectroscopy[12],
biochemical
sensors[13], nanophotonics devices[14], biology[15], carrier systems for drug
delivery[16] , biosensing in vivo or in vitro diagnostic[17], solar cells[18],
optoelectronic
device[19],
diabetic
healing[20],
cooling
system[21],
antibacterial against[10], cancer treatment[2], catalysis[3], sensor[22], imaging,
sensing, biology and medicine[23], inkjet-printer[24]... etc.
1-2 Noble Metal Nanoparticles
Noble metal nanoparticles such as Ag and Au NPs have been a source of
great interest due to their novel electrical, optical, physical, chemical and
magnetic properties [25,26]. They were very attractive for biophysical,
biochemical, and biotechnological applications due to their unusual physical
properties, especially due to their sharp plasmon absorption peak at the visible
region. Another important advantage Ag and Au nanoparticles prepared by
PLAL process were stable for a period of months. Additionally, Gold and silver
nanoparticles are chemically stable and typically exhibit surface enhanced
Raman scattering SERS in the visible wavelength range, where they may cause
a tremendous increase in various optical cross-sections. The resonance
frequencies strongly depend on particle shape and size as well as on the optical
properties of the material within the near-field of the particle [14]. Silver, for
example, has been for thousands of years, used as a disinfectant; from the other
side nobody can neglect its value as a catalyst [27]. On the other hand, Gold
nanoparticles have gained considerable attention in recent years for potential
applications in nanomedicine due to their interesting size dependent chemical,
electronic and optical properties. Also, gold nanoparticles show promise in
3
enhancing the effectiveness of various targeted cancer treatments such as
radiotherapy and photothermal therapy [8]. However, the field of nanotechnology
has received much attention, specially gold and silver nanoparticles with the
number of publications of growing exponentially (as shown in Fig. 1-1)[28].
6000
.
5000
Articles
4000
3000
2000
1000
0
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
years
Fig. 1-1: Articles published on gold and silver NPs from 1990 to 2005[28].
1-3 Advantage of Pulsed Laser Ablation in Liquids
Nanoparticles have been prepared by a wide variety of techniques such as
pulsed laser deposition[29], flame metal combustion[30], chemical reduction
[31], photo-reduction[32], electrochemical reduction[33], solvothermal[34],
electrolysis[27],
green
method[35],
Microwave-induced[36],
sono-
electrochemical[37], aerosol flow reactor[38], photochemical reduction[40],
chemical fluid deposition[41], spray pyrolysis[39,42] , and spark discharge[43].
Among them, the pulsed laser ablation in liquids PLAL has become an
increasingly popular top-down approach [44] for producing nanoparticles. It's a
relatively new method that was first introduced by Fojtik et al. in 1993[45] as is
a promising technique for the controlled fabrication of nanomaterials via rapid
4
reactive quenching of ablated species at the interface between the plasma and
liquid with high-quality nanoparticles free from chemical reagents. Therefore,
PLAL process has received much attention as a novel NPs production technique.
In general, there are an ability to prepare various kinds of nanoparticles
such as metals[46,47], noble metals[48], semiconductors[49,50], nanoalloys[51], oxides[52], magnetic[53], biaxial heterostructures[54] and core–shell
nanostructure[55]. Moreover, the interesting feature of this technique, which
distinguishes it from laser ablation in gas or vacuum, is the influence of the
surrounding solvent. The solvent can provide (i) physical effects such as
confinement and cooling, in addition to (ii) chemical reactions effects such as
oxidation or reduction and control on the size and the aggregation state of
nanoparticles NPs by changing the surface charge of the nuclei. In addition, the
surfactant molecules can prevent particle size increase by their adsorption on
the nanoparticles as (iii) coating reagents effect [56]. Crystallized NPs can
easily be obtained in one-step procedures without subsequent heat-treatments,
because of the high energetic state of ablated species. Certain pure NPs
colloidal solutions can be formed without the formation of by-products.
Resizing and reshaping of colloidal NPs synthesized by other technique are also
possible through melting and fragmentation technique by laser irradiation. The
production system is easy, simple and cheap, does not require costly vacuum
chambers [3,57]. The ejected nanoparticles completely collected in solutions
forming thus a colloidal solution make them very easier to handle as suspended
or powder (by centrifuged) [58]. Moeover, the PLAL method is free of any
reducing agents, which are potential impurities no pollution and contamination
[59]. However the strong confinement of the expanding plasma produced by
laser ablation of a target in solution, which can realize extremely high
temperature and high pressure [56]. Another interesting feature is the weak
dependence of the size of generated NPs on the duration of laser pulse. For laser
ablation in vacuum, the generated NPs are almost immediately deposited in the
5
chamber walls or on a substrate and do not absorb forthcoming laser pulses. In
liquids, NP remains in liquid and can therefore appear in the laser beam path
upon convective motion of the liquid [60]. In addition, the physical approach
feature was originally used to produce colloidal metal nanoparticles with
controlled size and shape. There is ability to surface modification as partial
oxidation, charge distribution, coating ...etc, using a surfactant solution in a
simple one step process. Moreover, there is an ability to disperse the aggregated
nanoparticles, as: (i) changing the pH value of suspension; (ii) using surface
activator and dispersants [13] (iii) using ultrasonic vibration to deagglomeration
[61,62].
1-4 Literatures Review
This review aims to summarize recent research has attracted much attention
during last decade on the preparation of noble metal NPs using PLAL method.
In 2000, Mafune et al. [63] produced silver nanoparticles by laser ablation
of a silver plate in water or SDS. The laser wavelength and laser shots are 532
nm and 50000 pulses, respectively. He found that the size distribution of the
nanoparticles shifts to a smaller size with increase in the concentration of SDS.
In 2001, Tsuji et al. [64] studied the influence of the laser wavelength,
focusing conditions on the ablation efficiency of silver and copper targets in
water to prepare nanoparticles. It was finding that the ablation efficiency at
shorter wavelengths was higher at low fluence, while the ablation efficiency at
longer wavelengths was higher at high fluence.
In 2002, Dolgaev et al.[65] reported the formation of Ag and Au
nanoparticles under pulsed laser ablation metal target in liquid environment
(H2O, C2H5OH, C2H4Cl2) by using Cu vapour laser (λ=510nm). This method
allow high rate of nanoparticles formation. Tsuji et al.[66] studied the
influences of laser wavelength of 1064, 532, or 355 nm laser in order to prepare
6
silver nanoparticles in water. It was found that the ablation efficiency decrease
with decrease the laser wavelength.
In 2003, Kabashin et al.[67] reported two different mechanisms of material
ablation in the liquid environment to produce gold nanoparticles. The first,
associated with thermal-free femtosecond ablation, manifests itself at relatively
low laser fluences F,400 J/cm2 and leads to small ~3–10 nm. The second one,
attributed to the plasma-induced heating and ablation of the target, takes place.
Tsuji et al.[68] studied the preparation of Ag NP by laser ablation in water with
femtosecond laser pulses at 800 nm. The formation efficiency for femtosecond
pulses was significantly lower than nanoseconds pulse.
In 2004, Sylvestre et al.[69] reported the femtosecond laser ablation of a
gold target in aqueous solutions to produce Au nanoparticles with controlled
surface chemistry. A detailed chemical analysis showed that the nanoparticles
formed were partially oxidized by the oxygen present in solution.
In 2005, Pyatenko et al.[70] prepared silver nanoparticles with 8 nm, by
irradiating a silver colloid, prepared via the citric reduction method, using 532
nm Nd:YAG laser, with laser fluence more than about 0.2 J/cm2. Tarasenko et
al.[71] studied effects of laser irradiation of silver colloids prepared by laser
ablation technique in acetone at different wavelengths (532, 266, 400 and 800
nm). The experimental conditions favoured a dimension reduction of the initial
particles and a formation of spherical size-controlled nanoparticles.
In 2006, Kawasaki et al.[72] studied the laser-induced fragmentation of
thin Au and Ag flakes in acetone by 1064-nm nanosecond laser (with the
fluence typically about 2 J/cm2) potentially offers a highly productive pathway
to stable metal nanoparticles, at a remarkable high production rate of 1.1
mg/min in liquid. Zhao et al.[73] synthesized of Ag NP by laser ablation in
water with excitation of 532 and 248 nm. It is proved that all of them are
effective SERS-active substrates. Zhu et al.[74] investigated the pulsed laser
7
ablation of Ag bulk in distilled water to synthesize Ag colloid nanoparticles.
Amendola et al.[75] Prepared of free and functionalized gold nanoparticles
laser ablation of a gold metal plate immersed in dimethyl sulfoxide, acetonitrile,
and tetrahydrofuran. It is possible to have significant control of the
concentration, aggregation, and size of the particles by varying few parameters.
Kazakevich et al. [60] presented of new nanoparticles formation under laser
ablation of Ag, Au, and Cu-containing solid targets in liquid environments
(H2O, C2H5OH and C2H4Cl2). The Formation of alloyed Au–Ag and Ag–Cu
nanoparticles is reported under laser exposure of a mixture of individual
nanoparticles. Yamada et al.[76] presented of new Gold nanoparticles with an
average diameter of 8 nm were irradiated with a tightly focused pulse laser at
355 nm in an aqueous solution of SDS. Tarasenko et al.[77] reported the
formation of gold nanoparticles during laser ablation of gold target in water by
pulsed Nd:YAG laser, operating at the second (532 nm, 10 ns, 10 Hz), or the
fourth harmonic (266 nm) wavelengths. The properties of the nanoparticles
were found to be both the mean size of the nanoparticles and their stability to
the additional 532 and 266 nm laser irradiation. Mafune et al.[78] produced
silver nanoparticles by laser ablation of a metal silver plate in SDS. The
concentration dependence of the abundance implies that the surfactant coverage
and the charge state on the nanoparticle surface are closely related to the
stability of the nanoparticles in the solutions.
In 2007, Zheng et al.[79] presented a new method for the tunable
production of monodisperse silver nanoparticles by different laser wavelengths
to irradiate an initial solution of seed crystals, the size and shape of the products
can be controlled. The absorption maximum shifts to longer wavelengths and
broadens, indicating an increase in particle size and size dispersion. They found
that the final size and shape are depending on laser wavelength and power.
Muto et al.[80] prepared of partially oxidized gold nanoparticles by laser
ablation of a gold metal plate in water. The nanoparticles are negatively charged
8
because the surface atoms are partially oxidized to Au-O−. It was found that the
surfactant cations attach to the particle surface, neutralizing the particle charge.
Phuoc et al.[61] fabricated the multi-pulse Nd-YAG lasers operating at 1064
nm laser ablation of silver in deionized water, arranged in a cross-beam
configuration. It’s found that the cross-beam ablation can increase the ablation
rate and promote reduction of the particle sizes and particle size distribution.
Kim et al.[81] synthesized metal and oxide nanoparticles(Ag, Al and Cu) by
pulsed laser ablation of the compacted metal microparticles using a Q-switched
Nd:YAG laser in water. It was shown that the process is effective for preparing
nanoparticle suspensions having relatively uniform size distributions. Truong et
al.[82] studied the formation of dense arrays of nanospikes occurs under laser
ablation of bulk targets (Ag, Au, Ta, Ti) immersed in liquids. The effect is
observed with sufficiently short laser pulses, either a 350 ps or a 90 ps Nd:YAG
lasers. The nanostructured Ag surface shows enhanced Raman scattering.
Giusti et al.[83] reported the preparation of Au nanoparticles by picosecond
laser ablation in water, with the fundamental and second harmonic of a
picoseconds Nd:YAG laser. The ablation process at 532 nm reached early
saturation because of both linear and nonlinear absorption mechanisms,
accompanied by fragmentation of existing nanoparticles. Compagnini et al.[51]
synthesized Au/Ag colloidal nano-alloys by laser ablation of single metal
targets in water and a re-irradiation of mixed colloidal suspensions.
In 2008, Werner et al.[84] studied the formation of silver nanoparticles by
nanosecond pulsed-laser irradiation (1064 and 532 nm, at 1 J/cm2) of silver
flakes in alcohols such as methanol and ethanol, the NPs are extremely unstable
and easily settled down to form precipitates. Siskova et al.[85] synthesized Ag
nanoparticles by laser ablation of a Ag target immersed in water and in aqueous
electrolyte solutions (HCl, NaCl, NaOH) as stabilization of the resulting Ag
nanoparticles. Tsuji et al.[86] prepared silver nanoparticles by laser ablation of
a silver plate in PVP aqueous solutions. Secondary laser irradiation onto the
9
prepared colloidal solutions was also carried out. It was found that the
formation efficiency was increased by addition of PVP as well as the stability of
nanoparticles with fine particles no more than 4 nm. Smejkal et al.[87] showed
that the laser fragmentation of Ag nanoparticles proceeds during first 20 pulses
and then reaches saturation. Fluences above 303 mJ/cm2 caused the formation
of less stable, aggregating nanoparticles, while fluences below 90 mJ/cm2 do not
provide sufficient energy for efficient fragmentation. Muto et al.[88] prepared
of gold nanoparticles by laser ablation of a gold metal plate in an SDS aqueous
solution with an average diameter of about 11 nm. Jimenez et al.[89]
introduced novel technique, which consists of the laser ablation of a solid target
immersed in a water solution of a metal salt. Silicon was chosen as the most
adequate target to synthesize silver and gold nanoparticles from a water solution
of either AgNO3 or HAuCl4. Compagnini et al.[55] prepared a novel Au-core–
Ag-shell nanoparticles by irradiated of mixed Au–Ag nanoparticles, were
obtained by pulsed laser ablation of metallic targets in liquid medium.
In 2009, Yang et al.[90] fabricated the Ag nanostructured films, based on
electrophoretic deposition EPD in the Ag colloidal solution produced by laser
ablation in water, under a constant current deposition mode. It has been found
that the obtained films are of tunable and controllable morphologies and
structures depending on EPD parameters. Pyatenko et al.[91] presented
mechanisms for silver and gold particle size reduction by laser (1064nm)
irradiation. The results presented here will be useful to all specialists using
lasers in particle size controlling, resizing, and reshaping. Petersen et al.[17]
studied the generation of gold NPs using a femtosecond laser system delivering
120 fs laser pulses at a wavelength of 800 nm at a repetition rate of 5 kHz.
In 2010, Manjon et al.[92] studied the influences of temperature variation
on the hydrodynamic diameter of the resulting colloidal nanoparticles when a
gold target is ablated by an IR femtosecond laser in water at different stabilized
liquid temperatures in the range of 283-353 K. The maximum hydrodynamic
10
diameter was observed at 330 K, the temperature at which the compressibility
of water reaches its minimum. Karimzadeh et al.[48] synthesized silver
nanoparticles by nanosecond pulsed laser ablation of silver plate in distilled
water. The results showed narrow size distribution of the nanoparticles with
radius centered at about 9 nm with a standard deviation of 3 nm.
1-5 Scientific Problems
As reported in previous literatures, we found that the preparation of metal
nanoparticles via PLAL method suffers from low production yields. However,
have not been sufficiently explored until very recently, though Kawasaki [72]
and Kim [81] recently shown that a suspension of fine metal powders as the
target material for a 1064-nm laser could largely improve the production yield.
To effectively prepare metal nanoparticles of high ablation rate quantities, it is
necessary to understand how the laser parameters affect the final nanoparticle
product mass yield. We suggest that the rate of NP generation depends on
different experimental parameters such as the metal reflectivity at the laser
wavelength, the liquid depth, energy...etc, not only on the laser fluence as
reported. These last parameters have been studied only in marginal until now.
Another disadvantage is that the size distribution of the NPs prepared by
this technique tends to be broadened due to agglomeration of nanoclusters and
to the possible ejection of the relatively large target fragments during the laser
ablation process. The formation mechanism and methods to control the
properties have not been sufficiently explored until very recently. To achieve
that, the particle-size reduction accomplished by re-irradiation, it was one of the
most important mechanisms. To date, to the best of our knowledge, only one
report has described laser ablation of metal in a PVP solution recently by Tsuji
et al.[86]. However, because that study specifically examined the optical
properties of Ag nanoparticles, other essential information about the influences
11
of PVP on the properties of produced nanoparticles such as formation efficiency
were not reported.
In general, the PLAL process is not controlled; we refer to on-line
monitoring of nanoparticles formation as a topic of great present interest study.
We regard as a pioneer study in our thesis.
1-6 Aims of the Work
•To prepare pure silver and gold nanoparticles in easy, fast and one step method
via PLAL process.
•On-line monitoring to controlling on the formation process of nanoparticles.
•To optimise the PLAL process and improve the formation rate of nanoparticles
by studying the effects of experimental parameters.
•To controlling and optimise the formation rate, NPs size, size distribution,
dispersity and aggregation of Ag NPs by studying the effect of surfactants
solutions, as capping agent
•Modification and size controlling of the noble metal nanoparticles by reirradiation to narrow the size and size distribution of metal nanoparticles.
•Synthesis novel composite of Au-Ag core-shell nanoalloys in two steps.
•Study the stability of nanoparticles upon ages.
1-7 Thesis Outline
This thesis is divided into five chapters. The First Chapter describes the
metal nanoparticles and their literature review. Chapter Two explains in detail a
theoretical background approach to characterize the properties of nanoparticles
and their interaction with light. Chapter Three gives the experimental
procedures and methodologies adopted in synthesis. Chapter Four describes the
experimental results of the studies and its discussions. Chapter Five gives the
summary the conclusions for the current work and future pathways.
12
Chapter Two: Theoretical Considerations
2-1 Introduction
Production of nanoparticles under laser ablation of solids either in gas or in
vacuum has been extensively explored during two last decades. A new
methodology based on laser ablation in liquids has received much attention as a
novel nanoparticle-production technique. Laser ablation represents dramatic
laser-material-interaction phenomenon. The amount of mass removed was
depending on the laser parameters such as: pulse duration, energy, wavelength,
target properties and the surrounding environment [93,94]
2-2 Laser ablation and Particle Formation
When a laser pulse reaches a sample surface, some of the energy is reflected
by the surface. It is noted that the reflectivity depends on the material and laser
wavelength [95]. The energy absorbed by the sample is transferred from optical
photons to electrons and then to the lattice, which then diffuses the energy into
the material [93]. Extremely high energy pulses may cause photochemical
reactions which remove atoms and molecules from the surface. The heated
surface can reach temperatures close to the critical temperature and cause rapid
vaporization process. The vaporization resulting in plasma that consists of
ionized vaporized atoms and electrons. It is possible that the plasma cloud
absorbs some of the incident laser energy and thereby only allows a fraction of
the laser energy to reach the surface (plasma shielding). The plasma expands
and is heated by photon absorption. Later the vapour cools and aerosol particles
begin to form. The rest of the energy diffuses into the material via heat transfer.
Depending on the applied laser energy, the surface may be melted into a liquid
with a moving solid-liquid interface. Liquid may be removed from the molten
pool as droplets that result in a higher ablation rate. However, a series of events
during laser ablation has been take place, can be discussed as follows [93,95].
13
2-2-1 Laser-Induced Heating and Melting
The absorption of laser light by metal nanoparticles gives rise to a
succession of energy transformation processes. These involve the successive
excitation and relaxation of the metal electrons, its interaction with the lattice,
i.e. electron-phonon relaxation and the phonon-phonon thermalization.
Afterwards, several thermal processes like melting or evaporation can be
activated. As discussed above, in the case of nanosecond-pulsed laser light, the
heat diffusion from the metal particle to the support takes place on a time scale
much shorter than the pulse width. This enables a simple thermodynamic
treatment of the laser induced temperature rise [3,96].
The laser energy was not vaporized the material, heats it and raises their
temperature, propagates via heat conduction inside the material. The
temperature distribution is governed by the heat conduction equation [93]:
࣋࡯࢖
ࣔࢀ
࢚ࣔ
ൌ સ ȉ(ࡷ સࢀ) + (૚ െ ࡾ)ࡵ૙ࢻࢋିࢻࢠ
(2-1)
where ߩ, Cp, K, T: represent density, specific heat, thermal conductivity and
temperature, respectively. The second term on the right hand side of equation 21 represents the source term which is the laser energy absorbed by the material
at a depth z from the surface, where R is the surface reflectivity, and I0 is the
laser irradiance and α is the absorption coefficient (the imaginary part of the
complex refractive index, assumed to be constant) [93,97]
2-2-2 Explosive Boiling
When a very high-energy fluence laser pulse is applied on the target surface, a
dramatic change of crater shape and volume is reported. The ablation rate
measured from the crater depth changes abruptly when the energy power density is
larger than a critical value. This mechanism has been analyzed based on classical
thermodynamics. When the surface region is heated beyond the limit of its
14
thermodynamic stability during short-pulse laser irradiation, the surface is
presumed to undergo a rapid transition from a superheated liquid to a mixture of
vapour and liquid droplets. Experimental results of the existence of well-defined
threshold fluence for the onset of the droplet ejection, as well as a steep increase of
the ablation rate at the threshold, have been reported and interpreted as evidence of
the transition from normal vaporization to phase explosion [93,98].
2-2-3 Evaporation
The mass evaporated from the target forms a vapour plume and moves away
from the surface with a high temperature and pressure. The surface temperature
primarily determines the vaporization rate. The evaporation rate from a liquid
surface is given by [93]:
૚
࢑ ࢀ ૛
ࢎ
࢐ࢋ࢜ୀ࢔࢒ ቀ ࡮ ࢒ቁ ‫ ܘܠ܍‬ቀ− ࢒࢜ ቁെ
૛࣊࢓ ࢇ
࢑࢈ࢀ࢒
૚
࢑ ࢀ ૛
ࣂ࢙࢔࢜ ቀ ࡮ ࢜ ቁ
૛࣊࢓ ࢇ
(2-2)
Where n is the number of atoms per unit volume, the subscripts l and v represent
liquid and vapour, respectively, and are the latent heat of vaporization hlv and Tv
temperature of the vapour, ma is the atomic mass and kB is the Boltzmann constant.
The first term in equation (2-2) represents the evaporization rate from a liquid
surface temperature and the second term represents the condensation rate of
molecules back to the liquid surface. The sticking coefficient θs is the probability
that a vapour atom returning to the liquid surface is adsorbed [93,99].
2-2-4 Plasma Formation
The laser-irradiated region on the target surface is heated to high
temperatures. The large portion of the absorbed laser energy is used to
evaporate the atoms of the target material thereby reducing the energy transport
into the interior of the target material. The front of the vapour formed plasma
contacts the environment and forms a shockwave, which travels faster than the
speed of sound. The vapour plasma is in a strong nonequilibrium state with an
15
ions density reaching 1016−1018 ions/cm3. The characteristic time for the cloud
formation is about 10-7 sec. The temperature inside the plasma can reach several
tens of thousands of K when all of the atoms or molecules are ionized
[100,101].
2-2-5 Solid Exfoliation
Exfoliation is the removal of fractured material in the solid state caused by
photomechanical effects due to strong laser-induced thermal expansion and
stress. This kind of removal is particularly important for brittle, refractory
materials such as single crystal materials including silicon, graphite or glass
type materials. The magnitude of the laser-induced stresses becomes significant
under conditions of stress confinement, when the laser pulse duration is shorter
than the time needed for mechanical equilibration of the absorbing volume. The
particles are usually cleaved along crystallographic planes. The particles are of
large size and evidently irregular shape. The sample surface after an exfoliation
process can also be easily identified by the shape of the crater [93].
2-2-6 Hydrodynamic Sputtering
The term hydrodynamic sputtering is refer to a range of processes in which
large droplets are ejected as a result of a transient melting and motion of a liquid
caused by steep thermal gradients and relaxation of the laser induced pressure.
However, even though it is easy to identify those particles that are ejected in the
liquid state, there are several different mechanisms that could cause the liquid to
leave the surface. Among them, hydrodynamic sputtering which is caused by
cyclic heating and cooling of the surface which is frequently observed in laser
ablation. When the laser fluence is sufficient to melt the surface, cyclic heating and
cooling of the surface leads to the growth of perturbations and produces finger-like
or ridge-like surface structures. Liquid droplets could be ejected from a melted
layer as the acceleration of a liquid asperity (protrusion which is related to thermal
expansion) exceeds the force holding the liquid to the surface [102].
16
2-2-7 Particle Ejection (Spallation)
The spherical shape of the particles demonstrates its origin from the liquid
state. The large particles are formed from the liquid-solid interface. In the
regime of stress confinement the laser-induced stresses can exceed the dynamic
tensile strength of the target material, causing disruption of a liquid-solid
interface. When the high-energy laser pulse, caused fast heating and vaporized
of target material, results in the formation of thermomechanically stressed state.
Its unloading may cause frontal cavitation of subsurface layer. The compression
wave propagating deep into material hits the rear-side of the target with the
formation of rarefaction wave. The last may produce cracks and rear-side
spallation[103]. However, the vaporized material generated a vapour plume that
propagated outward in the ambient. The pressure and temperature of the plume
are very large especially in the initial stage of the vapour plume formation. The
high pressure may push the liquid melt out from the centre of the laser-heated
region and after the melt is cooled form a “volcano-shaped” crater (Fig.2-1).
When the pressure is sufficiently large the resulting momentum surpasses the
surface tension, which is holding the liquid to the surface, and droplets are
ejected [93, 95,103, 104].
Fig. 2-1: Material removal via vaporization (left), pressure induced melt displacement
(middle) and explosive melt ejection (right) [104].
17
2-2-8 Nucleation and Condensation
As the vapour plume propagates outward and starts to cool, the cooler front of
the plume cloud allows aerosol particles to form. The characteristic times for
particle formation are typically on the order of nanoseconds (10-9 sec), which is
much shorter than the duration time of the cloud. The much shorter time scale for
the particle formation ensures that the vapour will be transformed into particles.
Condensation of the vaporized atoms is the mechanism for the generation of nanosized particles. The ambient pressure, gas properties and temperature are critical
factors in condensation processes. Most numerical studies that have been made of
this process calculate the shock location and the gas properties. Condensation
processes were predicted when the plasma temperature decreased to a sufficiently
low value related to the vapour number density. The molecules collide with each
other and form larger aerosols; however the probability of forming particles as
large as microns is very low since extremely high collision frequencies and atom
number densities would be required [93,105].
2-2-9 Coagulation and Agglomeration (Growth)
Besides the main particle generation mechanisms there are several minor
mechanisms, which will not generate particles but may alter the particle size
distribution or the total particle number concentrations. These processes as
coagulation can occur from several nanoseconds to several milliseconds after the
laser pulse. Two most important processes were coagulation and agglomeration,
which would usually increase the final particle size. Particles after forming from
laser ablation collide with each other in the gas ambient, and if the momentum is
large enough or the participant particles are in liquid phase, particles coalesce to
form new large particles. This process is called coagulation and usually happens in
the later time stage. Agglomerates are formed as early soon condensation starts,
when the nanosizes aerosols are formed from vapour, they are strongly charged
with electrons existing in the plasma. The charged particles then attach to each
18
other by the electronic bond and the agglomerates are identifiable by their chain or
web shape. Figure 2-2 shows several images of particles ejecting from silver target
that were ablated in liquid medium [86,93].
0 μs
Fig.2-2: 10-ns resolved shadowgraph images of laser ablation phenomena observed for
the 18 mM PVP solution. Some remarkable phenomena were selected: (a) optical
emissions, (b) shockwave, (c) cavitations bubble and (d) secondary shockwave generated
at the bubble collapse [86].
2-3 Nanoparticles-Liquid Reaction
Strong chemical reactions and physical processes will take place among the
laser ablated metal species in the aqueous solution, such as the reaction between
water molecules with the ablated species (as charge-transfer interaction and
electrostatic forces), since the ablated active species are electronically excited
and hence highly reactive, which induces the formation of nanoparticles in
solution. The structure, morphology, size, and hence properties of nanoparticles
will differ for different media including solvent and surfactant [106]. So, the
properties of nanoparticles can be controlled by solution composition; On the
other hand, the effect of such aqueous oxidation should be controllable through
surface modification by surfactant coverage and manipulation of laser
parameters. Hereby, it is possible to obtain the metal oxide metal composite
NPs by rapid reactive quenching with surfactant aqueous solution [52,107]
19
2-4 Nanoparticles Suspension
Nanosized particles suspensions provide many advantages: (1) Nano-sized
particles can have a surface force that balances the gravity force keeping the
particles from being sunk or floated, thus nano-sized particles dispersions can form
a stable systems with very little settling in static conditions; the mechanism of
stability of nanoparticles was shown in Fig.(2-3) [85]. (2) The thermal, optical,
mechanical, electrical, rheological, and magnetic properties of nanoparticles
depend significantly on size and shape and they are superior to those of the base
material. Thus, a variety of different nanofluids with significant properties can be
designed to be environmentally friendly for a wide range of applications. Recent
experiments have shown some promising nanofluids with amazing properties such
as fluids with advanced heat transfer, drag reduction, binders for sand
consolidation, gels, products for wettability alteration, and anti corrosive coatings
For heat transfer applications, the presence of these nanoparticles has been shown
to increase the static thermal conductivity of the base fluid by as much as 160%
with the addition of carbon nanotubes.
Fig.2-3: Electrostatic repulsion between the outer parts of the electric bilayer
enveloping each of the two Ag nanoparticles adapted for the case of Ag [85].
20
2-5 Synthesis of Nanoparticles
Synthesis techniques to generate metal nanoparticles depend on isolation of
small amounts of a material. There are two general strategies mechanism to
obtain materials on the nanoscale; I-The top down method (dispersion method)
is where material is removed from the bulk material, leaving only the desired
nanostructures. II-The bottom up method (reduction method) is one where the
atoms produced from reduction of ions, are assembled to generate
nanostructures [28,108].
2-5-1 Dispersion Methods (Top down method)
The Top down method typically starting from bulk, involves laser ablation
[47], arc discharge [43], etc... Nucleation takes place starting from the plume
and continues till a solid substrate comes in its way. Control of particle size is
achieved by tuning the fluence, wavelength irradiation time ...etc. The above
crude method may be modified by altering the design of the cluster. Top down
techniques suffer from the need to remove large amounts of material [28, 108].
2-5-2 Reduction Methods (bottom up method)
The bottom up method starting from atoms, include chemical [31],
electrochemical [33], sono-chemical[37], thermal and photochemical reduction
[32,40,109],...etc, have been used to generate nanoparticles. Bottom up
synthesis techniques usually employ an agent to stop growth of the particle at
the nanoscale. Capping materials, such as a surfactant or polymer are used to
prevent aggregation and precipitation of the metal nanoparticles out of solution.
Choice of the reduction technique, time, and capping material determine the
size and shape of the nanoparticles generated. Spheres,
rods,
cubes, disks,
wires, tubes, branched, triangular prisms and tetrahedral nanoparticles have
been generated in gold, silver and platinum with various reduction techniques
and capping materials [28,108,109].
21
2-6 Colloids
Nanosized particles of metals are ordinary insoluble in organic or organic
solvent, but if they can be prepared in colloidal form, they can function more
readily as catalysts. A colloid is a suspension of particles in range from 1 nm to
1 µm in size. Many colloidal particles can, however, be detected by the way the
scatter light, such as dust particles in air. This particles are in state of constant
random movement (Brownian motion) arising from collisions with solvent
molecules, which themselves are in motion. Particles are kept in suspension by
repulsive electrostatic forces between them. The addition of salt to a colloid can
weaken these forces and cause the suspended particles to gather into aggregates,
and eventually they collect as sediment at the bottom of the solvent. This
process of the settling out of colloid is called flocculation. Some of colloidal
systems to be discussed are colloidal dispersions of insoluble materials
(nanoparticles) in organic liquids, and these are called organsols. Analogous
colloidal dispersions in water are called hydrosols [67,110].
2-7 Pulsed Laser Ablation in Liquids
Pulsed laser ablation in liquids PLAL is currently explored as a
prospective top-down (dispersion method) strategy of metals nanoparticles
preparation [70]. It’s simple no chemistry is involved and basically free from
limitations because it can generate nanoparticles without counter-ions or
surface-active substances [61]. When a high-power pulsed laser beam irradiates
on a metal target in a transparent liquid, a local plasma, with super high
temperature (about 6000 K) and high pressure (about 1 GPa)[60,93], will
instantly be produced on the solid-liquid interface and quench quickly after one
pulse due to adiabatic expansion of the plasma and its interaction with
surrounding media. The whole process is finished in about some microsecond.
The thickness of the molten layer on the target is about 300 nm [60]. The
22
formation of nanostructures can be mainly attributed to the combination of
ultrafast quenching of hot plasma produced via evaporated of molten thin layer
and its interaction with surrounding media [111]. Moreover the nanoparticles
ejected with velocity about 200 m/s[112].
2-8 Nanoparticles Formation Mechanisms
The complexity of the mechanisms of metal nanoparticle formation during
PLAL includes various reactions in high-temperature and high-pressure plasma
which are nonequilibrium process [60]. Upon laser ablation, various materials
such as metal atoms, ions, clusters, fracture and droplets [113] are emitted from
the metal plate. Nanoparticles are formed via nucleation, phase transition, and
crystal growth of these emitted substances [86]. On the basis of that mentioned
above, the formation of nanoparticles could be described in three mechanisms
and every mechanism started with three steps :(I) After the interaction between
pulsed laser and the metal target, the electron–phonon coupling leads to a
transfer of the electronic excess energy into lattice heat. The high-temperature
and high-pressure of plasma (without solvent) is produced in the solid-liquid
interface quickly after the interaction between pulsed laser and the metal target.
(II) The subsequent ultrasonic and adiabatic expansion of the high temperature
and high-pressure metal plasma leads to cooling of the metal plume region and
hence to formation of metal clusters. (III) With the extinguishment of the
plasma, the formed metal clusters encounter the solvent and surfactant
molecules in the solution, which induces some chemical reactions and capping
effects. The final structure and morphology of the particles are dependent on the
surfactant concentration in solution or on the competition between aqueous
oxidation of metals particles and surfactant protection [107].
•However, The first mechanism, associated with aggregation of the ablated
atoms and clusters into small embryonic nanoparticles and their growth by
23
assembling the clusters and attachment of free atoms. The density of ablated
species (atoms) plays an important role in the nanoparticles growth, can be
changed by adjusting the laser fluence. By controlling the density of the ablated
species it is possible to control the final size of the formed nanoparticles [104].
•The second mechanism attributed to the plasma-induced, with super high
temperature and high pressure, plasma expanding result in ablation of
particulates as fragment from the target or crater walls. Moreover, exfoliation as
removal of fractured material in the solid state caused by photomechanical
effects due to strong laser-induced thermal expansion and stress [93]. It should
be noted that the second mechanism gives rise to much larger particle sizes and
broader size distributions [46,104].
•The third mechanism started when the solid at the focal point initially
melts and is vaporized above ablation threshold (explosive boiling). The thin
liquid layer adjacent to the solid surface is heated to the same temperature of the
solid. The liquid is vaporized and in the vapour phase. Expanding vapours of
the liquid splash this reservoir resulting in the removal of the molten layer. This
molten drops and fractures split into nano-sized droplets, which are super
cooled by the surrounding liquid [111]. Note that formation of NP via
evaporation of the metal is unlikely, since the pressure of metal vapour at
temperature close to melting is too low compared to vapour pressure of
surrounding liquid. Surface tension stabilizes the molten drop of the metal,
while the pressure of surrounding vapour of the liquid tends to split this drops
[60,104,112]. It was source of the bimodal distributions was attributed to
nanoparticles formed from ejected species from the initial ultrafast, nonthermal
laser target interaction and thermal vaporization due to plasma heating of the
target the bimodal distributions found here are attributed to thermally induced
vaporization and explosive boiling. We identify the larger-sized mode of the
distribution as arising from explosive boiling that ejects molten nanoparticles
directly and the small size from thermal vaporization [112].
24
2-9 Interaction of Light with Noble Metal Nanoparticles
The intensity of light which propagates through a medium containing small
particles is reduced by scattering and absorption. The extinction of the light
beam is given by [3]:
I(z) = I0 exp(−n0σextz),
(2-3)
where I(z) is the intensity of the incoming beam after a distance z, n0 the
number of particles per unit volume and σext the extinction cross section of a
single particle. It holds [3]:
σext = σabs +σsca,
(2-4)
where σabs and σsca: is the absorption and scattering cross sections of a single
particle, respectively. The optical properties of such particles, as a consequence
of their reduced dimensions, are dominated by a coherent collective oscillation
of their conduction band electrons. As a result, the absorption cross section,
which scales with their volume, can reach values several orders of magnitude
larger compared to common organic dye molecules. Such collective oscillation
is known as surface plasmon resonance [3].
2-9-1 Surface Plasmon Resonance in Metal Nanostructures (SPR)
The term plasmon is used to refer to plasma oscillations in metals, i.e.
collective oscillation of conductive electron driven by light. The term
resonance refers to a plasma oscillation excited by electromagnetic waves and
the term surface is used because a surface polarization is the origin of the
plasma oscillation [11]. For this purpose, the Surface Plasmon Resonance SPR
in metal nanoparticles is an oscillation plasmon absorbs or scatters light
resonantly of certain wavelength, also is known as surface plasmon extinction
SPE, localized surface plasmon resonance LSPR, polariton resonance or Mie
resonance.
25
For electromagnetic wave at a certain frequency (ν) incident on a spherical
nanoparticles much smaller than the wavelength of light (λ˃˃R), which induce
a resonant, coherent oscillation of metal free electrons across the nanoparticles
(Fig.2-4-a). Since the diameter of the particle is on the order of a few
nanometers, and the penetration depth of electromagnetism waves in metals is
of about 30 nm, the incident light is able to propagate through the particle. The
propagated electric field inside the particle drives the conduction band electrons
collectively with respect to the fixed positive lattice ions. As a result, a net
charge difference appears on the surface at one side of the particle. Its attraction
with the lattice ions on the opposite side leads to a restoring force. The
resonance frequency is mainly determined by the strength of the restoring force.
This force depends on the separation of the surface charges, i.e. the particle
dimensions, and the polarizability of the medium between and around the
charges. In other words the frequency, intensity and bandwidth of the SPR
absorption and scattering depend on the incident wave, metal composition,
nanoparticles
size
and
shape,
dielectric
properties
of
surrounding
medium/substrate [114], spaced particles inter-particle interaction and particleto-particle interactions [23]. A photon confined to the small size of the
nanostructure, constituting an intense electric field around the particle. The
surface plasmon oscillation decays by radiating its energy resulting in light
scattering or decays non-radiatively as a result of conversion of absorbed light
to heat. The alternating surface charges form an oscillating dipole, which
radiates electromagnetic waves. This oscillation is known as SPR (SPE), the
resonance that lays at visible frequency for noble metals as Au and Ag, given
those intense colours and interesting optical properties, as reflected are due to
their unique interaction with light. Some of the photons will be released with the
same frequency in all directions and this process is known as scattering. At the
same time, some of the photons will be converted into phonons or vibrations of
the lattice and this process is referred to as absorption. In general, the SPR peak
26
of metal nanostructure should include both scattering and absorption
components. If the frequency of the incident light is in resonance with this
surface plasmon oscillation of metal electrons, results in strong enhancement of
absorption and scattering of electromagnetic radiation [3].
This simple model for particle plasmons is reminiscent of an optical antenna
such that all the conduction electrons move in-phase producing only dipole-type
oscillations manifested by a single, narrow peak in the SPR spectrum (Fig. 24b). As the size increases, the field across the particle becomes nonuniform, and
this phase retardation broadens the dipole resonance and excites higher
multipole resonances, such as the quadrupole, octupole, etc. (Fig 2-4-c) leading
to several peaks in the spectra [3,114].
Fig. 2-4: Interaction of a small metal nanoparticle with light (λ˃˃R) (a), particle
dipolar radiation(b) and quadrupole radiation of larger particles (c) [3].
27
2-9-2 Mie Theory
The general solution of the interaction problem of a single homogeneous
sphere, of the radius R, and of arbitrary material with an incident
electromagnetic field was first given by Mie in 1908[5]. Mie presented a
solution to Maxwell’s equations that describes the extinction spectra of
spherical particles of arbitrary size embedded in a homogeneous medium. One
of the reasons why Mie’s theory has remained important for so long is that it is
the only simple, exact solution to Maxwell’s equations that is relevant to
particles. It is also worth mentioning that in his calculation, he introduces the
dielectric function ε(ω,R) at the angular frequency ω to treat the material
problem, which can incorporate all the size effects. The spherical symmetry
suggests the use of a multipole extension of the fields, giving Mie’s calculations
a series of multipole oscillations (dipole, quadrupole, etc.) for the absorption
and the scattering cross section of the particles as a function of the particle
radius. The extinction spectrum is then composed of the sum of absorption and
scattering modes, each of which has a contribution that depends on the particle
size. Higher-order modes become more dominant with increasing particle size.
Physically, this can be explained by the fact that for larger particles, the light
cannot polarize the nanoparticles homogeneously and retardation effects lead to
the excitation of higher-order modes. Mie’s theory and experimental spectra
agree well until for bulk metals, the normal incidence absorption no longer
shows a plasmon resonance. Although his theory describes accurately the
optical extinction spectra of metal nanoparticles, it does not explain the physical
process, i.e. the collective oscillation of the conduction band electrons. The term
plasmon for the Mie resonances was proposed first by Schopper in
1931[2,5,115].
28
2-10 Modification of Metal Nanoparticles
One of the most important challenges in the preparation of metal
nanoparticles is the control of their size, shape and morphology. Laser-induced
modification of the size and shape of nanoparticles are powerful tooling to
enhancement the properties. Since the plasmon frequency of each single particle
is determined by its dimension and shape, the optical absorption profiles of the
whole distributions are inhomogeneously broadened. Therefore, irradiation of
colloids with laser (pulsed or CW) of definite photon energy yields resonant
plasmon excitation in particles with specific size and shape. By changing the
excitation wavelength it is possible to selectively excite particles within a range
of sizes and/or shapes. The observed changes in the absorption spectra caused
by laser irradiation appear to correspond to changes in the size of the particles.
Experimental parameters such as laser fluence, wavelength and irradiation time
were found to influence the efficiency of the modification process [71,79,116].
2-10-1 Size Reduction- Secondary Laser Irradiation
Laser-induced modification of the size and shape of nanoparticles carried
out when the surface Plasmon of nanoparticles in solution is excited under
irradiation of a 532-nm laser, the photon energy is readily converted to the
internal modes of the nanoparticles as heat [76]. During a single laser pulse (5
ns), one nanoparticle is considered to absorb consecutively more than one
thousand photons, and its temperature rises significantly so that the nanoparticle
starts to fragment. After the single laser pulse, the heat diffuses into the solution
and the temperature of the nanoparticles returns to room temperature before the
next one arrives. The heating and cooling of the nanoparticles occur in every
laser pulse. We employed irradiate the growth solution containing silver
nanoparticles. However that size was influenced by the frequency and power of
the incident light. The optical properties of the prepared nanoparticles were
29
linearly dependent on the excitation wavelength [117]. So, the laser irradiation
onto metal colloids induces both fragmentation and fusion of the colloidal
particles [79,86].
2-10-2 Mechanism of Size Reduction Process
The efficiency of coupling of radiation to NP depends on the proximately of
laser wavelength to plasmon frequency of charge carriers. The energy from
electrons to the lattice is transferred within 3–5 ps, and the temperature T of the
nanoparticles can be estimated on the basis of conventional heat diffusion
equation. For small NP one obtains [60]:
ࢀൌ
૛ࡵ૙࢑
ࣅࡷ ࢒
࣊࢘૛
(2-5)
Here r, I0, λ, Kl are: radius of nanoparticles, peak power of the laser, laser
wavelength, and thermal conductivity of surrounding liquid, respectively. In this
approximation, the temperature of the nanoparticle in the laser beam is
proportional to its geometric cross-section πr2. The extinction coefficient kex
under large detuning from the plasmon resonance is close to that of the bulk
metal. However, in the vicinity of plasmon resonance k = k(λ) shows resonant
behaviour, as well as the temperature T of the particle. Note that T is
proportional to the peak power I0 of the laser beam, which is due to its small
size. Fragmentation of NP occurs under their melting. As one can see from
Eq.(2-5) the temperature T depends on the particle radius and can be lower than
the melting point of its bulk material. At given value of laser peak power further
fragmentation of nanoparticles stops as soon as they reach some critical size.
Also, the temperature of a nanoparticle in the laser beam depends on the
detuning of its plasmon resonance from the laser wavelength. This factor
determines the efficiency of interaction of laser beam with nanoparticles
generated by laser ablation of solid targets in liquids [60,118,].
30
2-10-3 Laser-Induced Growth Tuneable Nanoparticles
The correlations between the SPE properties of silver or gold nanoparticles,
their size, and their morphology have become an important subject. Preparation of
tuneable nanoparticles were employed various laser lines as the exciting source to
irradiate the growth solution containing nanoparticles. As the growth proceeds, the
SPE peaks shifts to longer wavelength and broadens. Size, shape and optical
properties of the prepared nanoparticles are influenced by the frequency and power
of the incident light. Fig. 2-5 shows samples of the prepared silver colloids by
using different laser lines as the incident light and their corresponding UV-VIS
spectra. The vials marked from a to e, as shown in Fig. 2-5-A, correspond to the
samples prepared with the excitation wavelengths of 514.5, 501, 488, 476.5, and
457.9 nm. The vial marked f is the original growth solution. These samples
together display a series of colors, from mauve to faint yellow, due to differences
in the size and shape of the prepared nanoparticles in the final colloid solution. The
corresponding UV-VIS spectra, shown in Fig.2-5-B, display a series of absorption
bands in the range from 480 to 560 nm, which implies that the optical properties of
the prepared nanoparticles can be finely adjusted by changing the excitation
wavelength surface of nanoseeds that impelled the growth of silver nanoparticles
and formation of specific shape and size [79].
Fig. 2-5: (A) Optical pictures of the final products prepared by the irradiation of laser
beam with different excitation wavelengths. (B) Corresponding UV-VIS absorption
spectra of the final products, from a to e, with the excitation at 514.5, 501, 488, 476.5,
and 457.9 nm, respectively. Spectrum f corresponds to the growth (parent) solution [79].
31
2-10-4 Target Modification
For instance, exposure of a solid by a stationary laser beam produces a crater
(Fig. 2-6-a) [58,119]. If a scanning laser beam is used, then a new type of periodic
structure arises (Fig.2-6-b). The formation of periodic structures under laser
ablation of solid targets (such as Cu, brass) in liquids can be explained as follows
[58,82]. During the laser pulse the target material melts and expelled from the pit
by the recoil pressure of vapours of surrounding liquid. If the scanning velocity of
the laser beam is small enough, then the laser radiation is captured by the pit in a
sense that the side walls of the pit reflect the laser beam into the pit. The formation
of the adjacent pit is therefore inhibited until the laser spot crosses the first pit, and
the cycle repeats. The recoil pressure of vapours induces the melt motion along the
pit surface. The molten material is partially ejected into surrounding liquid as
nanoparticles. Then the melt solidifies, and one can see it in the form of small
protrusions on the tips of structures. The gaps between the adjacent structures are
of special interest. Their width rapidly decreases with depth down to few µm and is
therefore much smaller than the laser spot size. The formation of these channels
attributed to the instability of a flat front of a melt under high recoil pressure
[58,120]. The channels become ‘hot spots’ due to reflection of laser light by side
walls of the cones into in the target where the intensity of the laser beam may
exceed by far the initial value of intensity on a flat target surface. [58].
Fig. 2-6: Typical craters on the gold target in water after 5000 laser pulses at F=60 (a)
and F=1000 J/cm2 (b) [67]. And SEM view of periodic structures formed under
scanning laser ablation in ethanol of bronze (c), and brass (d). A Cu vapour laser,
fluence of 50 J/cm2 (c). An Nd: YAG laser, fluence of 16 J/cm2 (d)[58].
32
2-11 Surface Area of Nanoparticles
A number of properties of materials composed of nanometres-sized
particles depend strongly on the surface area. For example, the chemical activity
of conventional heterogeneous catalyst is proportional to the overall specific
surface area per unit volume, so the high areas of nanoparticles provide them
with the possibility of functioning as efficient catalysts. Fig. (2-7) shows an
interest depended on the surface area on the nanoparticles size. The specific
surface area (S) of sphere nanoparticles with diameter r is given by:[121].
ࡿൌ
ሺࢇ࢘ࢋࢇሻ
࣋ሺ࢜࢕࢒࢛࢓ ࢋሻ
=
࡭
࣋ࢂ
=
૟ൈ૚૙૜
࣋࢘
Using the units square meters per gram (m2/g) (2-6)
Where: ߩ and r are the density (g/cm3) and nanoparticles size, respectively.
It is of interest to examine how the specific surface area depends on the shape of
nanoparticles. Consider cube of side d with the same volume as a sphere of
radius r, that 4/3ߨr3=d3, we obtain for this case Scub=1.24 Ssph., so a cube has
more specific surface than a sphere in same volume.
In summary, the efficient way to increase the surface area of material is to
decrease its particle size or shape. Another way to increase the surface area is to
fill material with void or empty spaces, as porous material [121].
Surface Area per Weight(m2/g)
300
250
200
150
100
50
0
0
10
20
30
40
50
60
size of nanopaticle (nm)
70
80
90
100
Fig. 2-7: The surfaces area of GaAs nanoparticles as a function of their size [121].
33
2-12 Properties of Nanoparticles
2-12-1 Optical Properties
In small nanoclusters the effect of reduced dimensionality on electronic
structure has the most profound effect on the energies of the highest occupied
molecular orbital, essentially the valence band, and the lowest unoccupied
molecular orbital, essentially the conduction band. Optical emission and absorption
depend on transitions between these states; semiconductors and metals, in
particular, show large changes in optical properties, such as colour, as a function of
particle size. Colloidal solutions of gold nanoparticles have a deep red colour
which becomes progressively more yellow as the particle size increases. Fig.2-8
shows the images for colloidal gold and silver nanoparticles of varying shape and
sizes. Other properties which may be affected by reduced dimensionality include
photocatalysis,
photoconductivity,
photoemission
and
electroluminescence
[63,122,123,124].
Fig. 2-8: Synthetic tunability of noble metal nanoparticles. Transmission electron
micrographs of (a) Au nanospheres, (b) Au nanorods, and (c) Ag nanoprisms(Left).
Photographs of colloidal dispersions of (d) Au-Ag alloy nanoparticles with increasing
Au concentration, (e) Au nanorods of increasing aspect ratio, and (f) Ag nanoprisms
with increasing lateral size(Right) [2].
34
2-12-2 Thermal Properties
The large increase in surface energy and the change in interatomic spacing
as a function of nanoparticle size have a marked effect on material properties.
For instance, the melting point of gold particles, which is really a bulk
thermodynamic characteristic, has been observed to decrease rapidly for particle
sizes less than 10 nm, as shown in Fig. 2-9. There is evidence that for metallic
nanocrystals embedded in a continuous matrix the opposite behaviour is true;
i.e., smaller particles have higher melting points [123].
1400
Meltig Temperature(K)
1300
Bulk→
1200
1100
1000
900
800
0
5
10
15
20
25
30
35
size of nanopaticle (nm)
Fig. 2-9: variation in melting point of gold NPs as a function of particle size [123].
2-12-3 Catalytic Properties
Catalysis involve the modification of the rate of a chemical reaction, usually
speeding up or acceleration of the reaction rate , by the addition of a substance,
called a catalyst, that is not consumed during the reaction. Ordinary the catalyst
participates in the reaction by combining with one or more of the reactants, and
at the end the process it is regenerated without change. The catalyst is being
constantly recycled as the reaction progresses. When two or more chemical
reactions are proceeding in sequins or in parallel, a catalyst can play the role of
selectively accelerating one reaction relative to other. [121,125].
35
2-13 Applications of Nanoparticles
Gold and silver nanoparticles exhibit strong optical extinction at visible and
near-infrared wavelengths which can be tuned by adjusting the size. With recent
advances in their high-yield synthesis, stabilization, functionalization and
bioconjugation, gold nanoparticles are an increasingly applied nanomaterial.
Bulk gold is well known for being inert; however, the nanoparticulate sizes of
gold display astronomically high chemical reactivity [3,109].
2-13-1 Antibacterial of Silver Nanoparticles Agent
Synthesis of nanosized drug particles with tailored physical and chemical
properties is of great interest in the development of new pharmaceutical
products. Investigations have shown encouraging results about the activity of
different drugs and antimicrobial formulation in the form of nanoparticles.
However silver is a nontoxic, safe inorganic antibacterial agent used for
centuries and is capable of killing about 650 types of diseases causing
microorganisms. Silver has been ability to exert a bactericidal effect at minute.
It has a significant potential for a wide range of biological applications such as
antifungal agent, antibacterial agents for antibiotic resistant bacteria, preventing
infections, healing wounds and anti-inflammatory. Silver ions (Ag+) and its
compounds are highly toxic to microorganisms exhibiting strong biocidal
effects on many species of bacteria but have a low toxicity towards animal cells.
Therefore, silver ions, being antibacterial component, are employed in
formulation of dental resin composites, bone cement, ion exchange fibers and
coatings for medical devices. Bactericidal behaviour of nanoparticles is
attributed to the presence of electronic effects that are brought about as a result
of changes in local electronic structures of the surfaces due to smaller sizes.
These effects are considered to be contributing towards enhancement of
reactivity of silver nanoparticles surfaces. Ionic silver strongly interacts with
36
vital enzymes and inactivates them. It has been suggested that DNA loses its
replication ability once the bacterium are treated with silver ions. Two
dimensional electrophoresis and proteins identification analysis of antibacterial
action of silver nanoparticles have disclosed accumulation of envelope proteins
precursors. Silver nanoparticles destabilize plasma membrane potential and
depletion of levels of intracellular adenosine triphosphate by targeting bacterial
membrane resulting in bacterial cell death. Antibacterial activity of these silver
nanoparticles as a function of particles concentration against gram-negative
bacterium Escherichia coli (E: coli), that silver nanoparticles after interaction
with E: coli have adhered to and penetrated into the bacterial cells. Antibacterial
properties of silver nanoparticles are attributed to their total surface area, as a
larger surface to volume ratio of nanoparticles provides more efficient means
for enhanced antibacterial activity [10].
2-13-2 Surface Enhanced Raman Scattering
Noble metallic nanostructures exhibit a phenomenon known as surfaceenhanced Raman scattering SERS. The SERS technique is a powerful analytical
tool in the fields of surface science, electrochemistry, biology, analytical
chemistry, biochemistry, catalysis, and materials research. The excellent
sensitivity and selectivity of SERS allow for the determination of chemical
information from single monolayer on planar surfaces and extend the
possibilities of surface vibrational spectroscopy to solve a wide array of
problems [53]. In which, the intensity of Raman spectroscopy are dramatically
enhanced through adsorbing the molecules onto metal surfaces. The aggregate
of noble metal particles is prerequisite for stronger SERS enhancement. This is
due to the existence of so-called “hot spots” having intense local
electromagnetic fields in which highly efficient Raman scattering can be
obtained [126].The sensitivity of SERS obtained from noble metal nanoparticles
37
strongly depends on the size and shape [127]. The basic theory of Raman Effect
was developed before its discovery in 1928. Briefly, in this effect, incident light
is inelastically scattered by molecules and shifted in frequency by the energy of
the characteristic molecular vibrations. Raman scattering provides information
about vibrational levels of molecules or, in other words, its structural
fingerprint. However, the applications of Raman scattering are strongly limited
by the weak intensity of the Raman-scattered light. When light is scattered from
an atom or a molecule, most photons are elastically scattered (Rayleigh
scattering), and only a small fraction is scattered at frequencies different from
that of the incident photons [8]. When molecules deposited on rough noble
metal surfaces showed greatly enhanced Raman scattering. Rough surfaces are
decorated with nanoparticle shapes with surface plasmon oscillations. Also
Surface Enhanced Raman Scattering occurs when the molecule is either
absorbed or is in close proximity of metallic nanostructures (Fig. 2-10). The
enhancement in the Raman signal in SERS is thought to occur due to two
mechanisms:
chemical
(electronic)
enhancement
and
electromagnetic
enhancement [8,128].
+
Biomolecules: Raman
Signal is weak
Metallic
Biomolecule Adsorbed on the NPs:
Nanoparticles NPs
Enhancement Raman Signal
Fig. 2-10: Schematic of SERS process [8].
38
2-13-3 Diabetic Delayed Wound Healing
Diabetes mellitus is most common disease of the altered glucose
homeostasis. One of the common degenerative diseases affecting people in the
world today is diabetes mellitus. People with diabetes mellitus have five times
the risk of having heart disease as people without diabetes. Diabetics have
impaired wound healing and impaired formation of coronary collaterals. The
abnormal apoptosis or angiogenesis may cause many of the clinical
manifestations of diabetes. Silver has been known to have effective bactericidal
properties for centuries. Nowadays, silver-based topical dressings have been
widely used as a treatment for infections in burns, open wounds, and chronic
ulcers. Silver nanoparticles are novel nanosized and highly crystalline
antibacterial agent which carries Ag+ ions by ion-exchanging [20].
2-13-4 Cooling Challenge
Ultrahigh- performance cooling is one of the most vital needs of many
industrial technologies. However, inherently low thermal conductivity is a primary
limitation in developing energy-efficient heat transfer fluids that are required for
ultrahigh-performance cooling. Nanofluids are engineered by suspending
nanoparticles in traditional heat transfer fluids such as water, oil...etc. A very small
amount of guest nanoparticles, when dispersed uniformly and suspended stably in
host fluids, can provide dramatic improvements in the thermal properties of host
fluids. Cooling is indispensable for maintaining the desired performance and
reliability of a wide variety of products, such as computers, power electronics, car
engines, and high-powered lasers or x-rays. With the unprecedented increase in
heat loads (in some cases exceeding 25 kW) and heat fluxes (in some cases
exceeding 2000 W/cm2) caused by more power and/or smaller feature sizes for
these products, cooling is one of the top technical challenges facing high-tech
industries such as microelectronics, transportation, and defence. Nanoparticles
used in nanofluids have been made of various materials, such as (Ag, and Au) [21].
39
Chapter Three- Experimental Works
3-1 Laser Ablation System
Fig. 3-1 shows the experimental setup for laser ablation of solid metal target
immersed in water or aqueous solution, which includes two lasers: Nd-YAG
laser 1064nm and/or 532 nm (frequency doubled) wavelength was used for laser
ablation process. A diode laser, 532 nm wavelengths, was used for on-line
monitoring the formation and growth advance of nanoparticles. The
measurement system consists of a detector type RS BPW 21 was connected to
an electric circuit that convert the electric signal into transmittance/absorbance
values; the calibration measurement was done as in reference [133]. The NdYAG laser beam was focused by using a lens onto a metallic target. The
ablation process was typically done for 1 minute at room temperature. The
target is fixed by a holder at the bottom of a quartz container.
Nd-YAG laser
1064 or 532 nm
Laser beam
Absorbance meter
Lens
Quartz container
Electronic circuit
Diode laser(532nm)
Detector
Liquid (NPs solution)
Target
Plasma
Target holder
Magnetic stirrer
Melting layer
Fig. 3-1: Experimental setup for nanoparticles synthesis, by laser ablation technique.
40
3-2 Laser Source and Measurements Device
3-2-1 Nd-YAG Laser
Q-switched Nd/YAG laser system type HUAFEI providing pulses of
1064nm and 532 nm(frequency doubled) wavelength with maximum energy per
pulse of 1000 mJ, pulse width of 10 ns, repetition rate of 10 Hz and effective
beam diameter of 5 mm, was used for laser ablation. The laser is applied with a
lens with 110 mm focal length is used to achieve high laser fluence.
3-2-2 Semiconductor Laser
Diode laser type IIIB laser product- 21CFR, Taiwan, was used. Its
wavelength is 532nm; maximum output power is 10 mW. The beam diameter
and divergence angle were experimentally measured about 2 mm and 3 mRad,
respectively.
3-2-3 Transmission Electron Microscope
Samples of nanoparticles were identified by the transmission electron
microscope TEM type CM10 pw6020, Philips-Germany (electronic microscope
centre-collage of medicine/ Al-Nahrien University). The test samples were
prepared by placing a drop of suspension of interest on a copper mesh coated
with an amorphous carbon film. The drop was dried with an infrared lamp
(Philips, 100 W) until all the solvent had evaporated. This process was repeated
three to four times. The TEM carbon grids were loaded into the sample. The
images were obtained at an accelerating voltage of 60 kV, with maximum
magnification of 25000x-450000x. The diameter of produced was calculated
from the following equation (taken from CM10 TEM sheet):
‫ܦ‬ൌ
ௗ
ெ
×
ଷ
ସ
(nm)
(3-1)
Where; D, d and M are: nanoparticle diameter, real diameter on image, and
magnification of TEM respectively.
41
3-2-4 Spectrophotometer
Absorbance spectra (SPE spectra) of NPs solution were measured by UVVIS double beam spectrophotometers, CECIL C. 7200 (France) and
SHIMADZU. All spectra were measured at room-temperature in a quartz cell
with 1 cm optical path. Additionally, spectrophotometer was used to estimate of
metals nanoparticles [129].
3-2-5 Atomic Absorption Spectroscopy
Atomic absorption spectroscopy AAS measurement was carried out for the
prepared samples using AAS spectrometer model GBS 933, Australia. Standard
solution with concentration 1000 µg/l of silver was prepared by dissolving 1.57
g of AgNO3 powder in 10 ml of HNO3 (40%), then the solution completed to 1
litre by adding distilled water. To obtain the calibration curve of silver, five
standard samples were prepared ranged from (1.5, to 5.5 µg/ml). The
corresponding absorbance values of the above samples were measured by AAS.
3-2-6 Optical Microscope
The optical microscope, type KRUSS-OPTICAL IV (Germany) and
KRUSS-OPTRONIC, with amplification of (1000x, 1600x), were used to
observe morphology of metals target and thin films.
3-2-7 Evaporation System
The bulk silver and gold have been deposited on glass slides using thermal
evaporation technique. The evaporation system is Edwards (UK). The
deposition rate was 1 nm/sec. The vacuum chamber evacuated down to 10-5 torr
using rotary pump and diffusion pump simultaneously. The thickness of the thin
films has been evaluated about 20 nm, using the interference method.
42
3-3 Materials
3-3-1 Target Materials
Metals plates silver and gold ounces are purchased from Al-Rafedian bank,
with high purity listed of (99.999) for Ag and Au foil. The plates were polished,
washed in ethanol and DDDW and cut off to pieces with dimensions to suite the
experimental arrangement. The surface of the noble metals plate (ounce) was
polished with 600-grade emery paper and applying to ultrasonically rinse in
organic solvents before being prior to each experiment.
3-3-2 Distilled and Deionised Water
Double distilled and deionised water DDDW is necessary for the
preparation of all samples and solution in this work. Even such water is entirely
pure; however, it is contaminated by salts ions, dissolved gases and dissolved
materials. Deionised water prepared in Mansur factory-Baghdad by process of
ion exchange [130]. The dissolved gases were removed by boiling the water at
100 0C for 10 minutes. Filter papers used to filter out and remove particulate
matter. Distilled and Redistilled (duple distilled) water was prepared in our
laboratory in glasses containers to avoid the contamination. After that
preparation, the pH and resistance of water were measured to be near 7 and
5×106 ohms/cm respectively.
3-3-3 Chemical Solution Preparation
Aqueous solution containing sodium dodecyl sulphate SDS, (M.W 289
C12H25SO4Na) (Kanto Chemical Co., Inc., 96%), as a surfactant (ionic
detergent) is determined by particle growth by the laser ablation and its
termination by SDS coating. Evidently, SDS plays an important role in
determining stability and size of the nanoparticles. It is negatively charged, has
43
lathering properties [78,131]. SDS solution was prepared by adding 0.3 g of
pure SDS powder to 10 ml DDDW and shake carefully. Then different
concentrations samples (5-100 mM) are prepared by dilution procedure.
Polyvinylpyrrolidone PVP (C6H9NO)x aqueous solution is a typical polymer
(M.W.5000) that is used extensively as a stabilizing agent of metal colloids. In
addition, since because PVP will also interact with ablated matter (atoms,
clusters, and droplets) produced by laser ablation and prevent their aggregation,
it is expected that PVP will affect on the particle size [85]. PVP solutions are
prepared by adding 0.5 g of pure PVP powder to 10 ml of DDDW and shake
carefully. Different concentrations samples (2-8 mM) are prepared.
3-4 General Experimental Process for Nanoparticles Formation
Silver and gold NPs were synthesized by pulsed laser ablation of a piece of
silver and gold metal plates (ounces: 99.999%) placed on the bottom of quartz
vessel containing 1ml of DDDW, or different solution such as NaCl, SDS, PVP,
Ethanol and acetone. Those solutions have analytical grade were prepared in
DDDW. The Nd-YAG laser was utilized as an. The spot size of the laser beam on
the surface of the metal plate was varied in the range of 0.4-2.37 mm in diameter
by changing the distance between the focusing lens and the metal plate. The laser
fluence was varied in the range from 477.7 to 13.6 J/cm2. The pulse energy was
varied in the range (100-900 mJ). The pulse duration and the repetition rate of the
laser pulse were 10 ns and 10 Hz respectively. The liquid thickness was changed in
the range from 2-14 mm. The liquid thickness adjusted by using different
dimensions of cells. The number of laser shots applied for the metal target ranged
from 5 to 90 pulses. A TEM was employed to take the electron micrographs of the
solutions studied. SPE spectra of the nanoparticles solution were measured by UVVIS double beam spectrophotometer.
44
Chapter Four: Results and Discussion
4-1 Introduction
Laser ablation of bulk target immersed in liquid environment [48] which is
simple method, recently has attracted much attention. The characteristics of the
metal nanoparticles formed and the ablation efficiency strongly depend upon many
parameters such as the wavelength[64,71] of the laser impinging the metallic
target[60], the duration of the laser pulses[68], the laser fluence[67], the ablation
time duration[85] and the effective liquid medium[65], with or without the
presence of surfactants[86]. Moreover, nanoparticles can be modified in shape and
size due to their further interaction with the laser light passing through it [1].
However, the SPR is a collective excitation of the electrons in the conduction band
near the nanoparticle surface. Electrons are limited to specific vibration modes by
particle size and shape [109]. Therefore, metallic NPs have characteristic SPE
spectra in the UV–VIS region and the SPE position is relates to particle size [117].
4-2 Effects of Laser Shots
Fig. 4-1 (A and B) shows the SPE spectra of silver and gold nanoparticles
solutions, respectively, synthesized by pulsed laser ablation of a piece of silver and
gold plate placed on the bottom of quartz vessel containing 1ml of ultra pure
DDDW. The liquid depth was selected 8 and 7 mm above the target for silver and
gold, respectively. The piece of metal was irradiated by focused energy of 600
mJ/pulse and 1064 nm Nd: YAG laser. The beam spot diameter at the metal
surface was 1.27 and 0.85 mm for silver and gold, respectively. The number of
pulses applied for the metal target ranged from 5 to 90 pulses. When the laser pulse
struck the metal surface immersed in liquid; it created a spark plume with a strong
shockwave that propagated in all directions. The spark emitted light and cracking
noise, which were followed by a visible cloud of metal particles oozing out of the
metal surface and dispersed slowly in all directions floating in liquid, easily
noticed by naked eye. The colour of solution was changed and the intensity was
45
increased when advancing in the laser shots, showing the formation of colloidal
metals nanoparticles. The SPE peaks in visible region are the characteristic metals
NPs formation [3] while confinement in nanoscale was proved by blue shift in
plasmon absorption peak relative to the bulk[114] (Fig. 4-42, sec. 4-15-1). When
an increase in laser shots results in an increase in the SPE intensity, while the peak
position remaining practically constant. The height and the width of the SPE peaks
were found to be dependent upon the laser shots. This spectral change indicates
that the abundance of the NPs is enhanced more under irradiation of the laser.
Fig 4-1-A shows the SPE spectrum of the silver nanoparticles solution,
displays a quasisymmetric absorption band centred at 400 nm, which indicates that
the nanoparticles in the growth solution are quasispherical approximately 8 nm in
size [79]. The silver nanoparticles, was faint yellow in color.
Fig 4-1-B shows the plasmon peak position of AuNPs was around 525–535 nm
indicating the formation of particles with dimensions of 5–30 nm in the solution
for laser energy used in the experiment. The formation of the gold nanoparticles in
the solution was also verified by the TEM results, which are discussed below. The
presence of the single surface plasmon peak implied that the formed nanoparticles
were nearly spherical; in the case of ellipsoidal particles the absorption spectrum
would have two plasmon peaks [77]. The height and the width of the SPR peaks
were found to be dependent upon the laser shots. The gold nanoparticles, was faint
pink in color, due to plasmon absorption [3]. The losses in the ablation of Au
compared with Ag is attributed to the large reflectivity from the metal surface[112]
The inset in Fig. 4-1-A shows the SPE band of Ag nanoparticles has been
performed at 10 Hz pulses, band a, and, alternatively, in stepwise ablation (4 sec.
break) band b, for 15 pulses. The SPE band of Ag NPs in spectrum (b) is more
intense, narrower and more symmetrical than in spectrum (a), and its maximum is
located at shorter wavelengths. In particular, the SPE peaks have been increased
from 0.69 to 0.83. That attributed to the mobility of generated particles which
are much lower in solution than in gas [117], they can stay in the light-path of
subsequent laser pulses, and then attenuate the laser energy.
46
3.5
15 Pulses
Ag Nanoparticles
3
(a)
SPE
Laser Shots (Pulses)
5 Pulses
10 P
15 P
30 P
45 P
60 P
75 P
90 P
2.5
2
SPE
1
A
1.5
(b)
0
350
400
λ(nm)
450
500
1
0.5
0
325
350
375
400
425
450
475
500
525
Wavelength (nm)
1
B
Au Nanoparticles
Laser shots
0.8
10 Pulses
15 Pulses
30 Pulses
0.6
SPE
40 Pulses
60 Pulses
0.4
90 Pulses
0.2
0
390
415
440
465
490
515
540
565
590
615
640
Wavelength (nm)
Figure 4-1: The SPE spectra of the plasmon band of Ag (A), and Au NPs (B), obtained
by laser ablation of metal plates immersed in DDDW. The laser shots are changed in the
range 5 to 90 pulses at laser energy of 600 mJ and λ=1064 nm. The inset shows the
difference in SPE values between 10 Hz (a) and stepwise ablation (b), for 15 pulses.
47
Also it was attributed to a more efficient redistribution of nanoparticles due to
long period time and efficient electric bilayer [85] build-up around the
nanoparticles. It was noticed the stepwise laser ablation in pure water actually
provides better results than of 10 Hz pulses ablation.
Figure 4-2(A and B) shows the SPE peaks of silver and gold nanoparticles at
400 nm and 526 nm, respectively, as a function of the laser shot. As shown, the
peaks were found to increase exponentially and saturate at more number of laser
shots, attributed to the effect of accumulated nanoparticles to attenuate the laser
intensity. The ejected NP remains in the liquid that surrounds the target resulting in
formation of so called colloidal solution and prolonged interaction with laser
radiation proceeds via its absorption by free electrons is possible. Thus, the
particles have a considerable extinction coefficient at wavelength of laser light,
they can absorb energy of laser light and the intensity of the incident laser light
will be reduced. However, when the number of laser shots increases, the
concentration of the atoms ejected in solution increases, whereas the ejection rate
decreases. The degree of the reduction must depend on the concentrations of the
nanoparticles. We expect that the ablation efficiency reduced and effected by three
opposite parameters: a change of the polarizability of the solution [59], increase
scattering due to present high concentration of nanoparticles and surface defect.
3.5
1.2
A
3
Ag Nanoparticles
2.5
Au Nanoparticles
0.8
2
SPE Peaks
SPE Peaks
B
1
1.5
1
0.6
0.4
0.2
0.5
0
0
0
15
30
45
60
75
0
90
Laser Shots
15
30
45
60
75
90
Laser Shots
Fig. 4-2: SPE peaks as a function of number of laser shots for silver (A) and gold
nanoparticles (B), respectively
48
Fig. 4-3 shows the PLAL efficiency, in terms of the SPE peaks, as a function
of laser shot. It was found that ablation efficiency of samples is very weak, and
found increases with the number of laser shot until 15 pulses, then turns to
decrease. The linearly increase of the efficiency is interpreted as that initially the
target surface of gold and silver was smooth and very shining, so that it reflects
some of incident photon and reduces the ablation efficiency . But after application
of the first few laser shots 2 to 3 pulses, the surface roughness and area increase.
The decrease in ablation efficiency above 15 laser pulses was attributed to
absorbance effect by advance laser shots [134]. Here the concentration of ejected
NPs in solution increases, whereas the ejection rate decreases. The degree of the
reduction must depend on the concentrations of the nanoparticles. It observed that,
the laser shots of 15 pulses is the optimum, therefore it was selected to be applied
as an effective parameter in following experimental study. Under our optimum
parameters for Ag nanoparticles (liquid depth=8 mm, spot size=1.27, laser
energy=600 mJ and laser wavelength =1064 nm), the formation efficiency was
enhanced and estimated to be 30 times greater compared with results was reported
by Siskova [85], 50 times as reported by Smejkal[132], 220 times as reported by
Tsuji[64] and 700 times as reported by Tsuji[68]. This change in formation rate
attributed to different parameters, one of them was our optimizations.
0.12
0.4
Ag Nanoparticles
Ablation Efficiency (a.u)
Ablation Efficiency (a.u)
A
0.3
0.2
0.1
0.1
B
Au Nanoparticles
0.08
0.06
0.04
0.02
0
0
0 10 20 30 40 50 60 70 80 90 100
Laser Shots
0 10 20 30 40 50 60 70 80 90 100
Laser Shots
Fig. 4-3: PLAL efficiency as a function of laser shots for silver (A) and gold
nanoparticles (B), obtained by laser ablation of metal plates immersed in DDDW.
49
4-2-1 Silver Nanoparticles Concentration
To obtain the concentration of silver nanoparticles in liquid, atomic absorption
spectrometer AAS was employed. Fig. 4-4-A shows the calibration curve, referring
to atomic absorbance values obtained from AAS, as a function of silver standard
concentration samples. The silver concentration exhibited an almost linear increase
with atomic absorption.
Fig. 4-4-B shows the amounts of ablated silver nanoparticles as a function of
laser shots, corresponding to the samples shown in Fig. 4-1-A. The amount of
ablated silver nanoparticles in agreement with SPE peaks as in Fig. 4-2-A. Under
our optimum conditions, the rate of nanoparticles formation for silver and gold was
enhanced. For examples, the formation rate of Ag nanoparticles is estimated to be
5200 μg/min in one step, compared with 240 μg/min, has been reported by Smejkal
et al.[132], and 1100 μg/min as reported by Kawasaki et al.[72] in two steps (at 10
Hz of laser ablation).
600
A
0.25
Calibration Curve
0.2
0.15
y = 0.05x
0.1
300
200
100
0
0
1
2
3
4
5
6
0
Ag standard concentration (μg/ml)
Ag Nanoparticles
400
0.05
0
B
500
Ablated Ag (µg/ml)
Atomic Absorbance
0.3
15
30
45
60
75
90
105
laser Shots
Fig. 4-4: Calibration curve of atomic absorbance as a function of Ag standard
concentration (A) and ablated concentration of Ag nanoparticles as a function of laser
shots (B), obtained by laser ablation of silver plate immersed in DDDW.
From the observations of the electric potential of Ag colloid, the mechanism of
desperation and aggregation depending on the pH value of solution [135]. The
50
Ag nanoparticles having negative surface charge can demonstrate a highly
dispersed state without aggregation because of the electrostatic repulsion
between the Ag NPs. However the Zeta potential inversely proportional with
pH[80,135]. So that, because of their negative charges among the nanoparticles,
the repulsive forces are likely to exceed the van der Waals attractive forces
leading to coalescence[59], and hence, the nanoparticles are present in a
solution without being coalesced even under centrifuge application
Fig.4-5-A
shows
good
agreement
and
correlation
between
the
concentrations of ablated Ag nanoparticles was determined by AAS and SPE
peaks obtained by spectrophotometer. These results have two important features.
First it is suggested that we obtain coherent result for quantify the PLAL efficiency
in terms of SPE peak, as well as of the amount of ablated silver nanoparticles. The
second feature, one can estimate the amount of silver nanoparticles produced from
the spectrophotometer measurements without need to AAS. Fig.4-5-B shows the
ablation efficiency in term of the concentration of ablated Ag nanoparticles.
These results are in agreement with efficiency obtained in term of the SPE
peaks; it has been proven by Fig. 4-5-A.
0.6
B
500
Ag Nanoparticles
400
300
200
y = 179.2x
100
0
0
0.5
1
1.5
2
B
0.5
Ablation Efficiency (a.u)
Ablated Ag concentrations (µg/ml)
600
2.5
Ag Nanoparticles
0.4
0.3
0.2
0.1
0
3
0
Maximum Absorbance (SPE)
15
30
45
60
75
90
laser Shots
Fig. 4-5: Amount of ablated Ag nanoparticles as a function of SPE peaks (A) and
ablation efficiency in terms of the amount of ablated Ag nanoparticles (B), as a function
of laser shots.
51
Figure 4-6(A and B) shows the TEM images and the corresponding size
distributions of silver nanoparticles produced by laser ablation of a silver plate
immersed in 1 ml of DDDW, at 15 pulses (A) and 90 pulses (B), respectively.
The Nd-YAG laser of 1064 nm and energy of 600 mJ was used. The
nanoparticles thus produced were calculated to have an average diameter of 1٣
and 15 nm at 15 and 90 pulses, respectively. The result revealed that the average
diameter of nanoparticles increase with an increase in laser shots.
40
35
A
15 pulses
Ag Nanoparticles
30
Frecuency (%)
25
20
15
10
5
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
28
24
B
Ag Nanoparticles
20
Frequency (%)
90 pulses
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
Fig. 4-6: TEM images and size distribution of silver nanoparticles produced by 1064-nm
laser ablation (E=600 mJ/pulse) of silver plate immersed in 1ml of DDDW. The laser
shots set of 15 pulses (A) and 90 pulses (B), respectively.
52
Figure 4-7(A and B) shows a typical TEM images and the corresponding
size distributions of gold nanoparticles produced by laser ablation of gold plate
immersed in 1 ml of DDDW, at different laser shots of 15 (A) and 90 pulses
(B), respectively. The nanoparticles thus produced were calculated to have the
average diameters of 1٤ and 16 nm at 15 and 90 pulses, respectively. The result
shows that the average diameter increases with an increase of the laser number
of shots. However, laser irradiation of the nanoparticles can stimulate further
change of their morphology or can change the rate of their aggregation [77].
25
A
Frecuency (%)
20
15 Pulses
Au Nanoparticles
15
10
5
0
200 nm
5
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
25
B
90 Pulses
20
Au Nanoparticles
Frecuincy (%)
15
10
5
0
200 nm
5
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
Fig.4-7: TEM images and size distribution of Au nanoparticles produced by laser
ablation of Au plate immersed in DDDW. The laser shots set of 15 pulses (A) and 90
pulses (B).
53
4-2-2 Ablation Monitoring
The SPE spectra of Au nanoparticles solution has an intense peaks centering
at 525-535 nm (Fig 4-1-B). So that when the gold nanoparticles solution is
excited under irradiation of 532-nm diode laser (1 mW, CW), the wavelength is
in the vicinity of the SPE. The photon energy is interact [3] with nanoparticles
solution can be result of absorb by nanoparticles. Therefore, when PLAL is take
place, the Au nanoparticles formation was on-line monitored by simultaneous
measurements SPE at the peaks via diode laser (Fig.4-8-A) for sample have of
15 pulses . It was found that the absorption values increased at more pulses,
indicating an increase in particle abundance. Fig.4-8-B shows a fairly good linear
correlation between SPE peaks obtained by spectrophotometer and laser; therefore
we suggest a relationship and agreement between them. Remarkably, all the
calibration and correlation between laser and spectrophotometer measurements
were done in reference [133]. These results are very important for testing and
providing good reference to comparison with other results for evaluation of PLAL
efficiency. It was believed that the result of the SPE peaks obtained by laser is a
good tool to investigate the ablation efficiency. The on-line monitoring of Au
nanoparticles formation represents a topic of great present interest in our study.
1
A
0.25
Au Nanoparticles
SPE(Obtained by Laser)
SPE (Obtained by laser)
0.3
0.2
0.15
0.1
0.05
B
0.8
Au Nanoparticles
0.6
0.4
y = 0.9806x
0.2
0
0
0
2
4
6
8 10
Laser Shots
12
14
0
16
0.5
1
SPE (Obtained by Spectrophtpmeter)
Fig.4-8: The SPE peaks obtained by laser as a function of laser shots (A), and the
correlation of between the SPE peaks obtained by laser and spectrophotometer(B).
54
4-3 Effects of Laser Energy
Another parameter having an important effect on the formation of metal
nanoparticles is the laser energy. Fig. 4-9(A and B), shows SPE spectra of Ag
and Au samples, respectively, prepared at different laser ablation energies. The
Nd-YAG laser of 1064 nm was utilized as an ablation source. The pulse energy
at the target surface was varied in the range (100-900 mJ) and the beam was
focused to have a diameter near the outer edges of the target of 1.27 and 0.85
mm for Ag and Au, respectively. The metal plate was fixed in a glass vessel
filled with 1 ml DDDW thus the smokelike colloids above the metal plate was
observed. The plate was located at 8 and 7 mm from the liquid surface for Ag
and Au, respectively. Laser ablation listed for 15 pulses and the solution
gradually turned to coloured with the increase of the number of laser pulses. We
measure a sensible increase in the SPR intensity, accompanied by a slight
change in bandwidth and maximum wavelength, when increasing in the laser
energy. This enhancement in intensity can be explained by the increase in the
concentration of metals nanoparticles formed in solution during the ablation
process. The height and the width of the SPR peaks were found also to be
dependent upon the laser energy. Fig. 4-9(A) shows the SPE peaks that occurred
at around 400 nm is the characteristic SPE signature of Ag nanoparticles [74],
the SPR intensity increase broadens and slightly shifts to the red. This effect
could be an indication of the formation of Ag nanoparticles with larger average
size [93]. Fig 4-9-B shows broad band with the SPR peak around 526 nm with
the peak position remaining practically constant, that indicates the production
of gold nanoparticles [77]. We observed a visible coloration of the solution after
several pulses of the experiment. The solution was coloured with faint pink. In
the absorption spectra of the solutions, the surface plasmon related peak could
be clearly distinguished. This peak was around 520–530 nm, which was
consistent with the presence of small 3–30 nm particles in the solution [136].
55
A
1
Ag Nanoparticles
Laser Energy (mJ)
100 mJ
0.8
300 mJ
500 mJ
SPE
0.6
600 mJ
700 mJ
0.4
900 mJ
0.2
0
350
375
400
425
450
475
500
Wavelength(nm)
0.35
B
Au Nanoparticles
0.3
Laser energy (mJ)
100 mJ
0.25
300 mJ
500 mJ
SPE
0.2
600 mJ
700 mJ
0.15
900 mJ
0.1
0.05
0
390
415
440
465
490
515
540
565
590
615
640
Wavelength(nm)
Fig. 4-9: SPE spectra of the plasmon band of silver nanoparticles (A), and gold NPs (B),
obtained by laser ablation of metal plates immersed in DDDW with laser energy of (100
to 900 mJ, laser shots of 15 pulses and wave length is 1064 nm of Nd-YAG.
56
Fig. 4-10(A and B) shows the SPE peaks of ablated metals nanoparticles
were increased as a function of laser energy. These absorption processes
increase as number of particles increase in the solution. The curve shows linear
increase as a function of energy, until the energy reaches 600 and 500 mJ for
silver and gold, respectively, then the curve seems to saturate at high energies.
This is attributed to the ejection rate that is reduced because: (1) The laser light
is absorbed by nanoparticles present over the metal plate [59] and the ablation
efficiency is reduced, (2) the surface properties of the metal plate are changed
by the high laser pulses, and the ablation efficiency reduced drastically, (3)
change of the polarizability of the solution in the presence of the nanoparticles
[59], that causes the ablation efficiency to decrease. On the other hand, silver
nanoparticles shows faster saturated compared with gold nanoparticles. It was
attributed to abundance of silver nanoparticles is greater than that in gold at
same parameters.
0.35
1.2
A
1
Ag Nanoparticles
Au Nanoparticles
0.25
0.8
0.2
0.6
SPE Peaks
SPE Peaks
B
0.3
0.4
0.15
0.1
0.2
0.05
0
0
0
200
400
600
800
1000
0
Laser Energy (mJ)
250
500
750
1000
Laser Energy (mJ)
Fig. 4-10: Intensity of the SPE peaks as a function of laser energy, for silver (A) and
gold (B) NPs, respectively, obtained by laser ablation of metal plates immersed in
DDDW, laser shots is 15 pulses at λ=1064 nm.
57
Figure 4-11(A and B) shows electron micrographs and corresponding size
distributions of silver nanoparticles, produced by laser ablation of silver plate
immersed in pure water. The laser wavelength is 1064 nm and energies of 300
(A) and 900 mJ (B), respectively. The nanoparticles thus produced were
calculated to have the average diameters of 13 and 17 nm at 300 and 900 mJ,
respectively. It is observed that the average diameter and size distribution was
increased with the increase of the laser energy.
12
A
10
Ag Nanoparticles
Frquency(%)
8
200 nm
300 mJ
6
4
2
0
5
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
9
B
8
7
Ag Nanoparticles
6
Frequency (%)
900 mJ
5
4
3
2
1
200 nm
0
5
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
Fig. 4-11: TEM images and size distributions of the silver nanoparticles, produced by
laser ablation of silver plate immersed in DDDW. The laser energies are 300 mJ (A) and
900 mJ (B) , respectively(λ=1064 nm, laser shots of 15 pulses).
58
Figure 4-12(A and B) shows TEM pictures and size distributions of gold
nanoparticles, produced by laser ablation of metal plates immersed in DDDW;
the laser wavelength is 1064 nm. The nanoparticles thus produced were
calculated to have the average diameters of 14 and 17 nm at the laser energies
of 300 and 900 mJ, respectively. The average particles sizes increase and the
size distribution broadens with an increase of applied laser energy. The origin of
the surface morphology of the irregularly shaped particles in case of high
energy can be explained by absorption by defects and thermally induced
pressure pulses which cause cracking [47].
28
A
24
300 mJ
Frequency (%)
20
Au Nanoparticles
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
20
B
900 mJ
16
Au Nanoparticles
Frequency (%)
12
8
4
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
200 nm
Particle Diameter (nm)
Fig. 4-12: TEM images and size distributions of gold nanoparticles, produced by laser
ablation of metal plats immersed in DDDW, (λ=1064 nm and laser shots of 15 pulses).
The laser energies are 300 (A) and 900 mJ (B), respectively.
59
The TEM data shows a drastic particle size reduction under the laser fluence
decrease. Indeed, the mean particle size dropped from 18 to 13 nm as energy
decreased from 900 to 300 mJ, as shown in Fig (5-12). Our result is in an
agreement with Phuoc et al. [61] and Said et al [137]. However, increasing the
fluence means delivering more energy that implies ablating larger amount of
material. The strong effect of the laser intensity on the particle size and size
distribution observed by our present study suggests that these parameters
depend strongly on the plasma conditions in particular temperature, pressure,
and species density [112]. However in ablation mechanism, the laser pulse
produces melting and evaporating mass. The evaporated mass is an optimum
case to produce soft nanoparticles by aggregation of evaporated atoms. On the
other hand, in case of melting mass, small metal droplets are ejected [46] and
nanoparticles formed as fragment of these droplets with large diameter and
broad particles distribution, so that it is undesirable for application. At high
energy ablation caused melting mass of the target surface with less evaporation,
therefore it is not preferred in ablation mechanism. Therefore increase in size
distribution at high energy can be attributing to that high energy laser caused
target melting and expanding vapours splashes the liquid this solid melt and
fractures into large nano-sized drops [112], which are super cooled by the
surrounding liquid. At high energy, the inter- absorption of laser light is also
taken place during the ablation, which retards the efficiency of ablation and
minimize rate of nanoparticles production. These absorption processes increase
as number of particles in the solution rise at small size. The melting and
fragmentation mechanism of larger particles and formation of smaller ones
could also be taken place due to the absorption of laser light [118,138]. This
fragmentation mechanism explained the variation in size distribution. Therefore
the population of particles smaller than 5 nm increased markedly in solution
when laser energy at 900 mJ, compared to 300 mJ. However the density of the
ablated species can be changed by adjusting the laser energy.
60
4-4 Effects of Laser Fluence
Fig. 4-13(A and B) shows the absorption spectra of colloidal solutions
prepared by ablating silver and gold target respectively in ultra pure water
ambient. The laser fluence was varied in the range from 477.7 to 13.6 J/cm2.
The effective spot diameter of the laser beam on the surface of the metal plate
was varied in the range of (0.4-2.37 mm). The laser energy was adjusted at 600
mJ, and the ablation was made by 15 pulses of laser. The metal plate was placed
on the bottom of a glass vessel filled with 1 ml of pure water. The liquid depth
was selected 8 and 7 mm for silver and gold, respectively, above the target. The
metal plate was irradiated with a focused output of the fundamental (1064 nm)
of Nd-YAG laser. Upon irradiating the target by the laser beam, the solution
was gradually turned into contamination in nanoparticles. The spectra consist of
strong absorption varied drastically with our conditions, due to plasmon band
around visible region.
Figure 4-14(A and B), shows the SPE peaks of silver and gold nanoparticles
at 400 and 526 nm respectively, as a function of the laser fluence. the SPE peaks
was found to increase linearly with the laser fluence, until (optimum) 47.4 and
105.8 J/cm2 for silver and gold, respectively, and then turns to decrease
exponentially. A drastic change in the SPE peaks was occurred, when the spot
diameter is slightly changed. An interesting phenomenon was found in our
study that the lowest efficiency was obtained at the focal length of used lens
(i.e. at the minimum spot size, 0.4 mm). Moreover, our results shows high
formation rate compared with similar studies were prepared of Ag NPs at spot
size of 2 mm over the Ag target, as reported by Zhao et al. [73] and 0.2 mm as
reported by Tsuji et al.[64]. Both literatures shows low formation rate. These
can be attributed to the effects of spot size on the metal surface. It is obvious
that, by decreasing in spot diameter less than an optimum values decrease the
formation rate. That attributed to enhancement of blast wave.
61
1
(A) Ag Nanoparticles
0.9
Laser Fluence(J/cm2)
447.7
0.8
68.4
0.7
47.4
SPE
0.6
34
0.5
20.5
0.4
13.6
0.3
0.2
0.1
0
350
375
400
425
450
475
500
Wavelength(nm)
0.3
(B) Au Nanoparticles
Laser Fluence (J/cm2)
477.7
0.25
156
105.8
0.2
SPE
47.4
13.6
0.15
0.1
0.05
0
390
415
440
465
490
515
540
565
590
615
640
Wavelength(nm)
Figure 4-13: SPE spectra of the SPE band of silver (A), and gold nanoparticles (B),
obtained by laser ablation of metal plates immersed in DDDW. The laser fluence was
varied in the range from 477.7 to 13.6 J/cm2 with spot diameter ranged of (0.4-2.371
mm), respectively. The laser energy is 600 mJ and laser shots are 15 pulses.
62
Because the laser power is much higher than the focusing threshold, a blast
wave is induced as a result of the high pressure exerted by the hightemperature filament like plasma [101].On the other hand, increase the spot
size upon the target surface the radiation intensity is decreased and thus
enlarges the radiation spot. This weakened the plasma intensity and thus
minimized the impact of plasma-related ablation effects [69], which were
found to broaden the size distribution of the ablated nanoparticles and reduce
the ablation efficiently. However, the optimum adjusted laser fluence was
very impotent parameter to increase ablation efficiency both in silver and
gold.
0.3
A
1
Ag Nanoparticles
Au Nanoparticles
0.2
0.6
SPE Peaks
SPE Peaks
0.8
B
0.25
0.4
0.2
0.15
0.1
0.05
0
0
0
100
200
Laser
300
400
500
0
Fluence(J/cm2)
100
200
Laser
300
400
500
Fluence(J/cm2)
Fig. 4-14: Intensity of the SPE peaks as a function of laser fluence, of silver (A)
and gold nanoparticles (B), respectively, obtained by laser ablation of metal
plates immersed in DDDW with an laser energy of 600 mJ, laser shots are 15
pulses and λ=1064 nm.
63
4-5 Effects of Liquid Depth
Liquid depth above the metal target is very important parameter that
enhanced the ablation efficiency of nanoparticles that confirm by absorbance
measurement for the ablated suspended solution.
Fig. 4-15 (A and B) shows the UV-VIS absorption spectra that indicated
the characteristic SPE feature of silver and gold nanoparticles, respectively.
This was carried out by PLAL of a metal plate in DDDW. The liquid depth
was changed in the range from 2 to 14 mm. A focused Nd-YAG laser
operated at 1 Hz with a wavelength of 1064 nm was vertically irradiated onto
a metal plate placed in the aqueous solution. The beam diameter was selected
at 1.27 and 0.85 mm for silver and gold plate, respectively. The ablating
energy of 600 mJ was employed to ablate a target for 15 pulses. The products
formed in the ambient liquid were transparent just after ablation, and then
changed to contaminated ones after more application of NPs. We noticed a
drastic change in the SPE peak found by few change in liquid depth. Fig. 415-A shows UV–VIS absorption spectra of Ag NPs. All the spectra exhibit a
characteristic peak around 400 nm, indicating the formation of Ag
nanocolloids [139]. Fig. 4-15-B shows absorption spectra of gold
nanoparticles, the surface plasmon related peak could be clearly
distinguished. This peak was around 520–530 nm, which was consistent with
the presence of small 3–30 nm Au nanoparticles in the solution [67]. We
observed a visible coloration of the solution after several pulses during the
experiment. The color of solutions is faint pink.
64
1
(A) Ag Nanoparticles
Liquid Depth (mm)
2 mm
0.8
4 mm
6 mm
0.6
8 mm
SPE
10 mm
14 mm
0.4
0.2
0
350
375
400
425
450
475
500
Wavelength(nm)
0.3
(B) Au Nanoparticles
Liquid Depth (mm)
2 mm
0.25
4 mm
7 mm
0.2
SPE
8 mm
10 mm
0.15
14 mm
0.1
0.05
0
390
415
440
465
490
515
540
565
590
615
640
Wavelength(nm)
Fig. 5-15: SPE spectra of the plasmon band of Ag (A), and Au nanoparticles (B),
obtained by laser ablation of metal plates immersed in DDDW with laser energy of 600
mJ, laser shots of 15 pulses and λ=1064 nm. The liquid depth varied in the range of (214 mm).
65
Figure 4-16 (A and B), shows the SPE peaks for silver and gold
nanoparticles around 526 and 400 nm respectively, as a function of the liquid
depth. We notice drastic changes in SPE when a little changes in liquid depth.
The peak of SPR tends to increase linearly with the increase of depth, to reach 8
and 7 mm for silver and gold, respectively, and then turns to decrease
exponentially, attributed to high absorption of IR wavelength by water.
However, these values which were considered as optimum depths only in our
study, as energy and wavelength ...etc.
Since the laser burst were greater than 8mm below the liquid surface, the
expanding plume was always confined within the liquid, no water splashing was
observed due to strong pressure and shockwave propagation. The reduction in
formation rate observed when the liquid depth less than 4 mm attributed to water
droplet as well as plasma splashing out of the liquid ambit at struck pulse. By
compared with our result, Tsuji et al.[64] and Zhu et al.[74] reported low
formation rate. These attributed to different parameters, one of them the depth of
water layer was selected 15 mm and 5 mm by Tsuji and Zhu, respectively;
compared with 8 mm was reported in our study.
1
0.4
A
Ag Nanoparticles
0.8
Au Nanoparticles
0.3
0.25
SPE Peaks
0.6
SPE Peaks
B
0.35
0.4
0.2
0.15
0.1
0.2
0.05
0
0
0
1
0
2
Liquid Depth (cm)
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6
Lquid Depth (cm)
Fig 4-16: Intensity of the SPE peaks as a function of liquid depth, of silver (A) and gold
nanoparticles (B), respectively, obtained by laser ablation of metal plates immersed in
DDDW with an laser energy of 600 mJ and laser shots 15 pulses at λ=1064 nm.
66
4-6 Effects of Laser Wavelength
Fig. 4-17(A and B), shows the SPE spectra for Ag and Au samples,
respectively, that were the nanoparticles synthesized by PLAL technique. The
Nd-YAG laser of wavelength 1064nm and 532 nm(frequency doubled) with
energies of 600 mJ per pulse in either wavelength. The beam was focused with
near the target surface to of 1.27 mm for Ag, and 0.85 mm for Au. The metal
plate was fixed in a glass vessel filled with 1 ml DDDW. After the laser ablation
process is advanced, so that smokelike colloids above the metal plate were
observed. The plate is located at 8 mm from the liquid surface for silver, and 7
mm for gold. Laser ablation listed for 15 pulses and the solution gradually
turned to coloured with the increase of the number of pulses. An interesting
phenomenon is observed that is the color of water is changed faster for the laser
wavelength of 1064 nm than 532 nm at the same laser energy.
Fig. 4-17-A shows UV–VIS absorption spectra of Ag and Au nanoparticles,
respectively. The spectra exhibit a characteristic peak around 400nm, indicating
formation of Ag and Au nanocolloids, respectively [139]. It was found that
augment is broadening in spectrum and decreased in the maximal value of
absorption peak, when 532nm takes place (band -b). Fig 4-17-B shows broad
band with the SPR peak around 526 nm with the peak position remaining
practically constant, that indicates production of gold nanoparticles. However it
was found that the SPE peaks of samples produced at 532 nm laser wavelengths
is lower than that produced at 1064 nm as shown in Fig. 4-17. Therefore, it
suggests that the particle densities of samples prepared at 1064 nm laser
wavelength are an optimum. It may be concluded that laser wavelength of 532
nm is less efficient in fabricating nanoparticles in water. It is obvious that the
efficiency of nanoparticle production increases when the wavelength decreases.
By comparison of the SPE peaks of noble metals nanoparticles produced by
PLAL at laser wavelength of 1064 and/or 532nm, indicates that the efficiency
67
of PLAL obtained by 1064 nm is largely exceeded that of 532 nm pulses. In
particular, the maximum efficiencies of Ag ablated by 1064 nm pulses are
greater than that obtained by using the same parameters with the 532 nm pulses
for Ag and Au nanoparticles, respectively. There are some factors that can be
attributing to the changes in PLAL efficiency between the two wavelengths.
First, it may be contributing to light absorption properties of the target
materials. The Ag and Au target have the capability to absorb the longwavelength (specifically1064 nm) laser energy as efficiently as at 532 nm. At
laser wavelength 532 nm, the value of the abortion coefficient k of bulk noble
metals (silver or gold) is less than that at wavelength 1064 nm [70]. Another
factor is that at green wavelength, the absorbance by metal hydrosol is
substantially higher than that at the 1064 nm. Although the visible wavelength
exhibits a lower ablation threshold than that of the infrared one, the ablation
process at 532 nm reaches early saturation because of both linear and nonlinear
absorption mechanisms, accompanied by fragmentation of existing nanoparticles [83].
1
0.4
(A)
0.8
(a)-1064 nm
SPE
0.4
Au Nanoparticles
(c)-1064 nm
0.3
(b)-0532 nm
0.6
SPE
(B)
Ag Nanoparticles
(d) -0532 nm
0.2
0.1
0.2
0
0
350
400
450
500
390
430
470
510
550
590
Wavelength(nm)
Wavelength(nm)
Fig. 4-17: SPE spectra of the plasmon band of silver (A), and gold nanoparticles (B),
obtained by laser ablation of metal plates immersed in DDDW exposed by 15 laser
pulses, with laser energy 600 mJ, at laser wavelength of 1064 nm (band a and c), and
532 nm (bands b and d), respectively.
68
Fig. 4-18(A and B) shows TEM images and the corresponding size
distributions of silver and gold nanoparticles, respectively. The laser wave
length is 532 nm. The nanoparticles thus produced were calculated to have the
average diameters of 16 and 18 for silver and gold, respectively. The result
shows that the average diameter and size distribution increase with the decrease
of wavelength. The average sizes increases and the distribution broadens with
decrease in the laser wavelength.
25
A
Ag Nanoparticles
20
Frequency (%)
15
10
5
0
150 nm
5
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
20
B
18
Au Nanoparticles
16
14
Frequency (%)
12
10
8
6
4
2
0
150 nm
5
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
Fig. 4-18: TEM images and size distributions of the silver (A) and gold nanoparticles
(B), respectively, produced by laser ablation of metal plates immersed in DDDW, (laser
energy of 600 mJ/pulse and laser wavelength is 532 nm).
69
4-7 Effects of Chemical Solutions
4-7-1 Effects of NaCl Solution
Fig. 4-19 shows the SPE spectra of colloidal solutions obtained by laser
ablation of a silver plate in neat water and NaCl solutions at various
concentrations. The absorption intensity of the plasmon band at 400 nm
corresponding to the SPE, were increased by addition of NaCl. On the other hand,
the spectral shapes of the plasmon bands were almost identical among those
colloidal solutions. For that reason, the increase in the absorption intensity of the
plasmon bands implies that the formation efficiency of nanoparticles was increased
by addition NaCl. The SPE spectrum of Ag nanoparticles produced at 15 mM
NaCl was characterized by a much weaker plasmon-related peak, which is shifted
to 404 nm, this suggesting both a relatively large nanoparticle sizes and size
dispersions, this is confirmed by TEM analysis
The inset in Fig. 4-19 shows the SPE peaks of Ag nanoparticles are rapidly
increased and reaching a maximum value of 1.23, when NaCl concentration
changed from 0 to 2.5 mM, and then gradually decreased for higher concentrations.
We believe that the abundance of silver nanoparticles is attributed to the presence
of Cl− ions, which in particular increase of the efficiency of the formation of Ag
NPs [140]. Also it was noticed that the formation efficiency decreases with the
increase of the concentration of NaCl from 2.5 to 15 mM. According to our result
the optimum NaCl concentration to produce maximum amount of Ag NPs is 2.5
mM. We believe that convenient amount of NaCl in water will enhance the
efficiency of nanoparticles formation, reduce the size, and prevent the aggregation.
The increase in the absorption intensity of the plasmon bands implies that the
formation efficiency of nanoparticles was increased; this fact was confirmed by
Tsuji et al. [86]. The presence of Cl− has a positive effect on the progress and the
outcome of laser ablation, since they prevent the formation of large Ag
nanoparticles and their aggregations, as witnessed by sharp, of narrow and nearly
symmetric SPE bands with maxima in the 390-400 nm regions. This effect was
70
attributed to an efficient adsorption of Cl− ions by the Ag nanoparticle surfaces and
an efficient build-up of the electric bilayer around particles [85]. However the Cl−
increases the absolute value of this negative charge. The highly negatively charged
nanoparticles produced in basic solution will tend to repel each other, thus limiting
particle coalescence. Metal NPs surface reacted efficiently with Cl- and
to
augment its net surface charge. This limited the coalescence of the particles, due to
electrostatic repulsion, and led to a significant reduction of their size. Taking
advantage of the repulsion effect, efficient size control is achieved. Metal surface
was also suitable for surface modification through both covalent and electrostatic
interactions during particle formation [69]. This was confirmed by TEM analysis
indicating the absence of any reduction effect compared to that of pure deionised
water.
1.4
1.5
NaCl Concentration (mM)
DDDW
1.2
SPE peaks
0.5 mM
2.5 mM
1
5 mM
10 mM
0.8
NaCl Solution
1.3
Ag Nanoparticles
1.1
0.9
0.7
0.5
15 mM
SPE
0.3
0.1
0.6
-1
1
3
5
7
9
11
13
15
NaCl Concentration (mM)
0.4
0.2
0
350
375
400
425
450
475
500
525
Wavelength(nm)
Fig. 4-19: SPE spectra of silver colloids prepared by laser ablation of a silver plate
immersed in DDDW or in NaCl solutions at various concentrations, ranged as (0.5 - 15
mM), for laser energy of 600 mJ, λ=1064 nm and laser shots is 15 pulses. The inset
illustrates the change in the SPE peaks as a function of NaCl concentration.
71
Fig. 4-20 shows TEM and the corresponding size distributions of Ag NPs
produced by laser ablation of a Ag plate immersed in NaCl solutions, the laser
energy of 600 mJ/pulse. The nanoparticles that produced were calculated, have an
average particle diameters of 11 and 13 nm in the 2.5 and 15 mM solutions
respectively. The result shows that the average diameter and size distribution were
decreased with the increase of NaCl. Ag nanoparticles are prepared in 2.5 mM
faceted disperse without aggregation and do not have irregular shapes. However, at
higher concentrations of NaCl (>10 mM), we observed quite different colloidal
solution properties. In these cases, agglomeration of nanoparticles was occurred.
16
A
14
2.5 mM
Frequency (%)
12
Ag Nanoparticles
10
8
6
4
2
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
28
B
24
Frequency (%)
20
15 mM
Ag Nanoparticles
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
Fig. 4-20: TEM images and size distributions of the Ag NPs produced by laser ablation
(E=600 mJ/pulse, laser shots of 15 pulses) in 2.5 mM (A) and 15 mM (B) of NaCl
solution, respectively.
72
4-7-2 Effects of SDS Solution
Metal nanoparticles have a tendency to agglomerate, and therefore, it is
necessary to protect those using surfactants or polymers [63,78,131,141]. The
surfactant which surrounds each nanoparticle prevents direct contact of the
other nanoparticles.
Fig. 4-21 shows a typical optical SPE spectrum of silver nanoparticles
produced in different concentrations of sodium dodecyl sulphate SDS and pure
water respectively. So that the laser ablation in an aqueous solution containing
SDS: C12H25SO4Na, (M.W 289), as a surfactant is determined by particle
growth in a plume by the laser ablation and its termination by SDS coating. The
formation mechanism of silver nanoparticles in the solution was examined by
changing the concentration of SDS. Laser ablation was carried out with a
Nd:YAG laser 1064 nm, maximum energy 600mJ/pulse, during all of the
experiments. The silver target was placed on the bottom of a 1ml quartz vessel
filled with 1 ml of aqueous solution. The depth of the deionised water layer
above the target was 8 mm. The focal plane was adjusted to 1.27 mm beyond
the target surface to decrease the radiation intensity on its surface and enlarge
the radiation spot. This weakened the plasma intensity and thus minimized them
impact of plasma-related ablation effects that were found to broaden the size
distribution of the ablated nanoparticles [69]. All aqueous solutions used as
ablation environments were prepared from double distilled deionised water. The
extinction band of capped silver colloid suspended in SDS solution has SPE
peaks at 402 nm. The spectrum exhibits a broad band extending toward the long
wavelength range while the extinction band of uncapped silver colloid
(suspension in water) has a maximum at 400 nm. This is the characteristic of
silver colloid [142]. It is observed that a shift in the SPE peaks between capped
and uncapped Ag nanoparticles is almost 2 nm. It may be concluded that the
SPE depending on a number of parameters involving not only the particles size
and shape but also the environmental dielectric function, the surface coating,
73
and the particle- particle interaction [110]. The red shift of the SPE peaks could
be explained as formation of the chemical band between SDS ions and silver
atom [142].The shapes and the intensities of the plasmon bands in the
absorption spectra depended on the regime of laser operation. The inset shows
the peaks of SPE of the Ag nanoparticles as a function of the SDS concentration
corresponding to be the samples shown in same figure. The peak of SPE tends
to increased linearly by addition of SDS, until 25 mM, and then turns to
decrease gradually. According to our result the optimum SDS concentration to
produce maximum amount of Ag nanoparticles is 25 mM. Note that the
optimum concentration depends on the experimental parameters. Evidently,
SDS plays an important role in determining the stability and size of the
nanoparticles, because the termination of the nanoparticle growth is controlled
by the diffusion and the attachment rates of SDS on the NPs.
SDS
DDDW
5 mM
10 mM
25 mM
50 mM
100 mM
1.2
1
0.8
SPE
1.5
SDS Concentration (mM)
SPE Peaks
1.4
1.25
1
0.75
0.5
0.6
0.25
-2
8
18
28
38
48
SDS Concentration (mM)
0.4
0.2
0
350
375
400
425
450
475
500
525
Wavelength (nm)
Fig. 4-21: SPE spectra of silver colloids prepared by laser ablation of a silver plate
immersed in DDDW and SDS solutions at various concentrations (5, 10, 25, 50 and 100
mM) .The pulsed laser parameters are (E=600 mJ, λ=1064 nm and 15 laser pulses).The
inset shows intensity of the SPE peak as a function of SDS concentrations (5-50 mM).
74
Fig. 4-22(A and B) shows TEM images and corresponding size distributions
of silver nanoparticles produced by laser ablation (the wavelength of 1064 nm,
600 mJ/pulse) of a silver plate immersed in aqueous solutions of SDS having
concentrations of 25 mM (A) and 50 mM (B),respectively. The nanoparticles
thus produced were calculated to have an average diameter of 9 and 7 nm for
25, and 50 mM solutions, respectively. These results show that the average
diameter reduction occurred when the SDS concentration is increased. The
products are composed of the particles with nearly spherical shape. It is
expected that the size distribution and the stability of the nanoparticles depend
critically on the properties of the surfactant employed.
48
44
A
40
25 mM
36
Frequency (%)
32
28
Ag Nanoparticles
24
20
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
60
B
54
50 mM
Frequency (%)
48
42
Ag Nanoparticles
36
30
24
18
12
6
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
Fig. 4-22: TEM images and size distributions of the Ag NPs produced by laser ablation of
silver plate immersed in 25 mM (A) and 50 mM (B) of SDS aqueous solution, respectively.
75
For the samples prepared in SDS solution, the particles are covered with
surfactant (especially for high SDS concentration). It can be seen that with the
increasing of SDS concentration, the size distribution width becomes narrow.
TEM data, revealed a drastic particle size reduction as SDS concentration
increased. Indeed, the mean particle size dropped from 14 to 7 nm as SDS
concentration increased to 50 mM (Fig. 4-22).
Highly negative charged nanoparticles can repel each other more
effectively, thus allowing the SDS molecules to cover them before contact
occurs. These kinetics considerations are expected to limit the coalescence of
the forming clusters, leading to smaller particles. SDS which interact with the
nanoparticles during the condensation and, thus, prevent them from further
coalescence and agglomeration, have been used to effectively stabilize and
reduce the size of Ag nanoparticles covered with SDS could be problematical
for intended biosensing applications, because this surfactant might interfere
with the subsequent biomolecule immobilization step[44].
At SDS concentrations of 10 mM, silver nanoparticles are born to be coated
by a sufficient number of SDS molecules and are negatively charged. The
repulsive force exerted among the negatively charged nanoparticles exceeds the
van der Waals attractive force among them, so that they are dispersed stably in
the solution and resistive against the aggregation. In a much higher
concentration region (greater than 50 mM), the adsorption reduces the surface
charge, increasing the van der Waals attractive interaction between the metal
surfaces, and resulting in the attractive interaction. The attraction among metal
nanoparticles acts as driving factors for the fusing process. The metal
nanoparticles in higher concentration have been reported to show a tendency to
fuse [126]. This phenomenon is explained by reduction of the negative charge
on the nanoparticles because of neutralization on the nanoparticles by an excess
amount of SDS .The repulsive forces among the different nanoparticles are
weakened [59,141], so that, the Ag nanoparticles are tendency to coagulated.
76
4-7-3 Effects of PVP Solution
Fig. 4-23 shows the SPE spectra of silver nanoparticles produced by laser
ablation of a silver plate immersed in neat water or in PVP aqueous solutions
at various concentrations of 2, 4, 5, 8, 10 mM. Already, PVP is a typical
polymer that is used extensively as a stabilizing agent of metal colloids [46]
(as capping agent). Therefore, it is expected that colloids obtained in PVP
solution will be more stable than those obtained in neat water. In addition,
because PVP will also interact with ablated matter (atoms, clusters and
droplets) produced by laser ablation and prevent their aggregation, it is
expected that PVP will also affect on the particle size. Laser ablation was
carried out with a Nd:YAG, (wavelength 1064 nm). The laser energy is fixed
at 600 mJ/pulse and using 15 laser pulses during all experiments. The depth
of the deionised water layer above the target was 8 mm, typically the focal
plane was adjusted to 1.27 mm beyond the target surface to decrease the
radiation intensity on its surface and enlarge the radiation spot. This
weakened the plasma intensity and thus minimized their impact of plasmarelated ablation effects that were found to broaden the size distribution of the
ablated nanoparticles. The SPE peaks are sensitive by PVP concentration.
The plasmon absorption peak at 400 nm is the characteristic plasmon
absorption peak of silver nanoparticles [74]. The position of the plasmon
absorption peak depends on the particle size and shape and the adsorption of
surfactant to the particle surface. It was noticed that the plasmon absorption
peak shifts toward longer wavelengths (red shift) as we increased PVP
concentration, usually is associated with an increase in particle size [143].
77
1.2
Ag Nanoparticles
PVP Concentration (mM)
DDDW
1
2 mM
4 mM
0.8
5 mM
8 mM
0.6
SPE
10 mM
0.4
0.2
0
350
375
400
425
450
475
500
Wavelength(nm)
Fig. 4-23: SPE spectra of silver nanoparticles prepared by laser ablation of a silver plate
immersed in DDDW and PVP solutions at various concentrations. The laser energy is
600 mJ, laser wavelength is1064 nm and laser shots of 15 pulses.
Fig. 4-24: Shows the SPE peaks (A) and peak position (B), respectively, as
a function of PVP concentration corresponding to the samples shown in Fig. 423. The SPE peaks of silver nanoparticles increased linearly by the addition of
PVP, until 5 mM and then gradually decreased at higher concentrations. The
formation efficiency, as a function of SPE peaks, is increased by 12% by the
increase of the concentration of PVP from 0 to 5 mM. We believe that the
concentration of silver nanoparticles are increased, which is attributes to high
viscosity in particular increase the plasma confinement and enhancing the
shockwave, that increase the efficiency of the formation of Ag nanoparticles. At
high concentration of PVP greater than 5 mM, the formation efficiency of the
formation of Ag nanoparticles decreased, that attributed to the production of
78
nanoparticle with large size. According to our result the optimum PVP
concentration to produce maximum amount of Ag NPs is 5 mM. We note that
the optimum concentration that depends on the experimental parameters as
pulse energy, laser fluence wavelength... etc. The results described above
indicate that the addition of PVP enhanced the formation efficiency of silver
nanoparticle.
1.2
420
A
Ag Nanoparticles
1
Ag Nanoparticles
410
SPE Locaction
0.8
SPE Peaks
B
415
0.6
0.4
405
400
395
390
0.2
-1
1
3
5
7
9
11
PVP Concentration (mM)
-1
1
3
5
7
9
PVP Concentration (mM)
Fig. 4-24: Intensity of the SPE peaks (A) and SPE location (B), respectively, as a
function of PVP concentrations of silver NPs obtained by laser ablation of Ag plate
immersed in DDDW, the laser energy of 600 mJ, laser shots is 15 pulses at laser
wavelength of 1064 nm.
Fig. 4-25 shows a typical TEM images and the particle size distributions of
silver nanoparticles produced by laser ablation (λ=1064 nm and laser energy of
600 mJ/pulse) of a silver plate immersed in PVP aqueous solution. The Ag
nanoparticles have an average diameter of 12 and 16 nm were produced in 5 and
10 mM PVP aqueous solution, respectively. The silver nanoparticles prepared in
PVP solutions were more dispersed on the TEM grids than those prepared in
neat water and the particle size was clearly decreased by addition of PVP
compared with pure water (Fig. 4-6-A). It was found that, the size distribution
increased by addition of PVP. The products are composed of the particles with
79
11
nearly spherical shape. For the samples prepared in PVP solution, the particles
are covered with surfactant (especially for high PVP concentration). It can be
seen that with the PVP concentration increasing, the size distribution increased.
According to our result the optimum size was obtained when PVP concentration
was at 5 mM. Note that the optimum size was depending on the experimental
parameters. However, the size that decreases by addition of PVP is attributable
to the interaction between PVP molecules and materials emitted by laser
ablation.
32
28
A
24
5 mM
Ag Nanoparticles
Frequency (%)
20
16
12
8
4
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
200 nm
Particle Diameter (nm)
24
20
Frequency (%)
16
B
10 mM
Ag Nanoparticles
12
8
4
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 80
200 nm
Particle Diameter (nm)
Fig. 4-25: TEM images and size distributions of the silver nanoparticles produced by
laser ablation of silver plate immersed in PVP aqueous solution have the concentration
of 5 mM (A) and 10 mM (B) respectively. The laser parameters are (E=600 mJ, λ=1064
nm and laser shots is 15 pulses).
80
Throughout the laser ablation process, various materials such as silver
atoms, clusters, and droplets are emitted from the silver plate. Nanoparticles are
formed via nucleation phase transition, and crystal growth of these emitted
substances. As for the PVP concentration dependence of the particle size, it can
be attributed to the capping effect of PVP on the particles. The commercially
available PVP molecules are terminated in the hydroxyl group due to the
involvement of water and hydrogen, so that PVP in solution adsorbs on the
nanoparticles and prevent their aggregation and growth. Such surface capping
will prevent not only growth of the particles but also the coalescence among
particles due to the repulsive interaction [111], leading to stable colloidal
solution and small sized particles. Obviously, the higher PVP concentration will
lead to more PVP molecules in solution attaching on the surface of the
nanoparticles, inducing more stable colloidal solution and smaller particle size.
On the other hand, the TEM images shows significant two type of nanoparticles,
the first one larger than 50 nm and smaller than 10 nm were very similar in all
PVP solutions samples. These phenomena attributed to thermal vaporization
and explosive boiling mechanisms [112]. It was shown that the size distribution
of particles formed after thermal vaporization is relatively small and constant in
size and. We suggest this is due to the strong confinement of the ablation plume
by the liquid atmosphere resulting in a nearly constant vapour density at the
onset of nanoparticle growth. The explosive mode resulted in larger
nanoparticles with a wide distribution with a significantly larger yield. Size
control is an important measure of the quality of the nanomaterials synthesis.
For synthesis, laser ablation of silver in PVP is a trade-off of i- larger
nanoparticle yields at the expense of a much larger size distribution and less
control in the final product or ii- lower yields of smaller, narrower size
distribution. Additionally, the vapour plume also expands faster at higher PVP
concentration. These reasons contribute to more irregular shape [112].
81
4-7-4 Effects of Organic Solutions
Fig. 4-26 shows the compression of optical SPE spectra of solutions (H2O,
Ethanol and acetone) of Ag nanoparticles, respectively. The characteristic peak
of the Ag colloid prepared in different organic solution centred around 400 nm.
There colloids were prepared by pulsed laser ablation of a piece of silver plate
immersed in 1ml of ultra DDDW or organic solution. The liquid depth and spot
size above the target was selected 8 mm and 1.27 mm, respectively. The laser
energy of 600 mJ/pulse, λ=1064nm of Nd: YAG laser operating at 1 Hz.
0.9
Ag nanoparticles
0.8
(i) DDDW
0.7
(ii) Ethanol
SPE
0.6
(iii) Acetone
0.5
0.4
0.3
0.2
0.1
0
350
375
400
425
450
475
500
Wavelength (nm)
Fig. 4-26: SPE spectra of silver colloids prepared by laser ablation of a silver plate
immersed in DDW (i), ethanol (ii) and acetone (iii).The laser parameter are (laser
energy of 600 mJ, λ=1064 nm and laser shots of 15 laser pulses)
Fig.4-27 shows typical electron micrographs and corresponding size
distribution of silver nanoparticles produced by laser ablation (λ=1064 nm, laser
energy 600 mJ/pulse and 15 laser pulses) of a silver plate immersed in ethanol
(A), acetone (B). The nanoparticles thus produced were calculated to have the
average diameters of 15 and 12 for ethanol and acetone, respectively.
82
28
(A)
24
Ethanol
Frequency (%)
20
Ag Nanoparticles
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
28
(B)
24
Frequency (%)
20
Acetone
Ag Nanoparticles
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
Fig. 4-27: TEM images and size distributions of the silver nanoparticles produced by
laser ablation (λ=1064-nm, laser energy is 600 mJ/pulse and laser shots of 15 pulses) of
silver plate immersed in ethanol (A), and acetone (B).
a- Ethanol Effects
During laser ablation of silver in ethanol small bubbles can be observed in
the solution, while in water no bubbles were observed. It is found that the SPE
of samples produced in ethanol is lower than that produced in water. The lower
of SPE indicates lower abundance of particles in solution. Thus, it implies that
the ablation efficiency in ethanol is lower. After coupling of pulse energy to the
surface of target, plasma species with very high temperature are generated. A
83
thin layer of solution at the interface of ablated area is vaporized to high
temperature that far beyond its boiling point and to a high pressure of orders of
tens atmospheres. The thermal conductivity of ethanol is 0.17 W/mK at 25 oC,
which is much smaller than that of water (0.60 W/mK at 25 oC) [53]. Ethanol
solution can be easily vaporized to such an enhanced condition. At high
pressure and high temperature, ethanol can decompose to form permanent
gases. The formed permanent gases in ethanol solution aggregate to bubbles that
can be seen during laser ablation. The gases bubbles in the path of laser beam in
combination with ablated plasma and formed nanoparticles in earlier pulses
weaken the laser light that couples to the target. The nanoparticle generation
reduces into a lower nanoparticle concentration. It suggests that no oxidation
occurs under the protection of ethanol solution. Therefore, ethanol is proposed
to be an optimal substitute of water for fabricating pure metals colloid.
b- Acetone Effects
Acetone is preserving good dispersity of the nanoparticles. This superior
function of acetone most probably stems from the interaction between the
acetone carbonyl group and the metal nanoparticle surface. Strong interaction of
the carbonyl group with nanoparticle surface is occurred. This interaction most
probably involves a net charge transfer between the metal surface and the
carbonyl oxygen atom. When the acetone molecules are adsorbed in this manner
around the metal nanoparticle, they develop a protective surface dipole layer
with the methyl groups in the outermost plane. This is probably how the
interactions between nanoparticles become overall repulsive in the acetone
medium [72]. Therefore acetone is good stabilizing power, serves as a superior
liquid medium that keeps fine metal nanoparticles free from precipitation and
oxidation.
84
4-8 Effects of Temperature
Fig. 4-28 band a-d shows the optical SPE spectra of four kinds of Ag
nanoparticles in the colloids, respectively. The characteristic peak of the Ag
colloid measured in different temperature treatment as denoted by inset caption
for same colloids. The growth solution was synthesis by pulsed laser ablation of
a piece of metals plates immersed in 1ml of ultra DDDW. The piece of metal
was irradiated with the focused 700 mJ/pulse, 1064nm Nd: YAG laser.
Fig.4-28 band a shows the SPE spectra of fresh (growth) solution, have a
peak at 400 nm, and then it increase in intensity and shifted to 405 nm for
storage of 15 days at room temperature (band b). This phenomenon attributed to
a laser-ablated plume containing of an atoms [113] and small clusters during the
ablation process. This phenomena was discussed as pioneer study by
Tarasenko et al.[77] only in short period time along 60 minutes for gold
nanoparticles. The ablated atoms and clusters tend to aggregate into small
embryonic nanoparticles. These nanoparticles continue to grow by assembling
the clusters and attachment of free atoms to the nanoparticles until all atoms in
the vicinity of embryonic nanoparticles are consumed. Therefore the density of
ablated species as atoms in the gas phase plays an important role in the
nanoparticle growth, leading to a formation of new particles [51,77,136]. On the
other hand band (c) shows reduce and red shift of the plasmon frequency of
virgin nanoparticles solution, when heated for 5 minutes at 900C. This is
characteristic for increase of the size. It is attributed to increase the kinetic
energy of nanoparticles, lead to increase collision between nanoparticles that
caused fusion. By compared with a, the band (d) is narrowed and the relative
intensity of the plasma peak is enhanced and blue shift observed. The difference
in the shape of the plasmon band suggested the change in particle size under
frizzing treatment. An interesting phenomenon is observed that after frizzing,
the ice of nanoparticles is slightly expand that caused a broken of contact points
between nanoparticle that make it in small size.
85
1.2
Ag NPs
1
Fresh NPs
0.8
Keeping(15 Days)
SPE
Heating(90 C. Dgree)
0.6
Freezing (3 Months)
0.4
0.2
0
350
375
400
425
450
475
500
Wavelength(nm)
Fig 4-28: SPE spectra of silver NPs prepared by laser ablation of Ag plate immersed in
DDDW. The laser energy was selected of 700 mJ/pulse and wavelength is 1064 nm.
Fig. 4-29: shows TEM images and corresponding size distributions of silver
nanoparticles after freezing it a long period of 3 months. The nanoparticles were
calculated have the average diameters of 11 nm. The result shows that the
average size and size distribution decreases at freezing and more dispersed.
36
32
Three Months of Freezing
Frequency (%)
28
24
Ag Nanoparticles
20
16
12
8
4
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
200 nm
Particle Diameter (nm)
Fig. 4-29: TEM image and size distributions of the silver nanoparticles after frizzing it
along period of 3 months, produced by laser ablation of Ag plate immersed in DDDW.
86
4-9 Effects of Aging Time
To examine a stability of the prepared nanoparticles, the changes in the
absorption spectrum with time were studied.
Fig. 4-30 shows the change in the SPE of AgNPs, the plasmon bands at 400
nm became noticeable on the time scale of several months. The growth solution
of nanoparticles was prepared by PLAL process in optimum condition at laser
energy of 700 mJ. We noticed that the width of the 400-nm peak is broadened
and the height is getting lowered for a longer time periods. This spectral change
indicates that the diameters of the nanoparticles are increased more under by
aging effect; which is interpreted to the metal nanoparticles are aggregated and
fused by and hence the number of nanoparticles which are available decreased
for as long time storage. Note that the nanoparticles in the solution fuse
sufficiently fast that the spatial distribution of the nanoparticles is regarded as
homogeneous.
1.2
Ag Nanoparticles
Storage Time (Month)
1
Fresh
2 Months
0.8
4 Months
6 Months
0.6
SPE
8 Months
0.4
0.2
0
350
375
400
425
450
475
500
Wavelength (nm)
Fig. 4-30: SPE spectra of the plasmon band of silver NPs, obtained by laser ablation in
DDDW with various aging time ranges from, 2 to 8 months using laser energy ,700 mJ,
λ=1064 nm and 15 laser pulses.
87
Another significant change, red shifts is by observing in the absorption maximum
that is shifted to longer wavelengths and broadening bandwidths until, on
completion, a final size which is relatively broaden, indicating an increase in
particle size and size dispersion remarkably [140]. We observed changes in color
of the solution after several months of the experiment. The color of initial solutions
was yellow for silver, whereas the aging solution looked red-yellow tint. This
spectral change implies that the diameters of the nanoparticles increased. The
decreased absorption intensity is caused by the sedimentation of the colloidal
particles. The agglomeration of silver nanoparticles at first four months occurred
fast than after six months.
Fig. 4-31(A and B) shows peak intensity and the peak position (inset), for
silver and gold, respectively, as a function of storage time (months); it was reduced
and shifted to the longer wavelengths. These spectral changes attributed to that the
metal nanoparticles tend to aggregate slowly with time in the solution [77,140].
The nanoparticles grow by collecting the small nanofragments. As a result the
optical spectrum changes with time, the peak position of silver slightly shifted to
Peak position(nm)
A
SPE Peaks
1
0.95
0.28
414
406
0.245
402
398
394
-1
0.9
1 3 5 7
Time (Month)
9
0.85
0
1
2
3
4
5
6
7
8
9
Time (Month)
560
B
550
540
530
520
0.21
-1
0.175
AgNPs
-1
B
A
410
SPE Peaks
1.05
Peak Position(nm)
the longer wavelengths and then stops after approximately 5 months.
1
3
5
7
Time (Month)
AuNPs
0.14
-1
1
3
5
Time (Month)
7
Fig. 4-31: Peaks intensity and peak position (inset) of the SPE of silver (A) and gold
nanoparticles (B) as a function of storage time (months) obtained by laser ablation in
DDDW with laser energy of 700 mJ, laser shots is 15 pulses at λ=1064 nm.
88
9
Fig. 4-32 shows TEM images and corresponding size distributions of Ag
nanoparticles, produced by laser ablation of Ag metal plate immersed in 1 ml
DDDW. The silver nanoparticles were storage at room temperature for one year
before carrying out TEM measurement, without addition of any surfactants .The
nanoparticles thus produced were calculated to have the average diameter of 20
nm. The result shows that the average diameter and size distribution increased
with aging time. The nanoparticles where strongly aggregated due to a small
electrostatic repulsive force between them, since the absolute value of the
potential was small. When the aggregation and precipitation of Ag nanoparticles
occurs, the color of colloidal solution changed. The SPR intensity diminishes,
broadens and shifts to the red. On the other hand, bigger particles can precipitate
more easily than much smaller ones due to the larger weight overcoming the
interaction forces among particles and the buoyancy force of water. An
interesting phenomena was noticed that the stability of the Ag nanoparticles in
water decreases very slowly with the time compared with gold, a half-life time
period is estimated about >720 days). That attributed formation of a partially
oxide layer [84] on the surface of Ag nanoparticles that hampers further
aggregation of Ag NPs.
25
Ag
Ag Nanoparticles
Frecuency (%)
20
15
10
5
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
Fig. 4-32: Selected TEM image and size distributions of the AgNPs after one year from
preparation, produced by 1064-nm laser ablation (E=700 mJ/pulse and 15 pulses).
89
Fig. 4-33: summarize the stability of the colloidal nanoparticles solution
prepared (in previous sections at the optimum parameters) by laser ablation of
silver plates immersed in DDDW (a), NaCl (b), SDS (c), acetone (d), PVP (e)
and ethanol (f). Its shows the change in the absorption intensity of the plasmon
bands at 400 nm as a function of the months after the preparation to observe the
stability of the colloids. The sedimentation of silver nanoparticles in the PVP,
acetone and ethanol solutions occurred very fast than in neat water.
Consequently, these data show that colloidal particles obtained in Ethanol
solutions are less stable than those obtained in neat water. Consequently, these
data show that colloidal particles obtained in NaCl solutions are less stable than
those obtained in neat water.
1.3
Ag Nanoparticles
1.1
a=DDDW
0.9
SPE Peaks
b=NaCl(2.5mM)
0.7
c=SDS(25mM)
d=Acetone
0.5
e=PVP(5mM)
0.3
f=Ethanol
0.1
-1
0
1
2
3
4
5
6
7
8
9
10
11
Time ( Months)
Fig. 4-33: Peak intensity of the SPE of silver nanoparticles as a function of storage time
(months) obtained by laser ablation of silver plate immersed in DDDW (a), NaCl (b),
SDS (c),acetone(d) PVP (e)and ethanol (f) with laser energy of 600 mJ, laser shots is 15
pulses and at λ=1064 nm.
90
4-10 Nanoparticles Modification and Size Controlling
To obtain the laser induced size reduction LISR onto the produced metals
nanoparticles was carried out by irradiation of unfocused pulsed 532 nm
(second harmonic). After the laser ablation, the metal plate was removed from
the solution and prepared colloids were put in tube with diameter of 5 mm under
the laser beam irradiated 20 pulses unfocused double frequency 532 nm beams
of the same Nd:YAG laser. The maximal laser irradiation energy was 900 mJ.
Fig 4-34 shows the SPE spectrum of the fresh solution of silver
nanoparticles, displays a quasi-symmetric absorption band centered at 400 nm
(band a), which indicates that the nanoparticle in the virgin solution are
quasispherical nanoparticles. By irradiating an initial solution of silver colloid,
prepared via PLAL method, using the second harmonic of a Nd-YAG laser 532
nm, at laser energy of 900mJ. The size and shape of the products can be
controlled; we prepared a colloid consisting of small spherical silver
nanoparticles with average size of 8 nm.
By monitoring the absorption
spectrum during growth, it is found that initially the solution irradiated displays
symmetric absorption band centered at 395 nm (band b), which indicates that
the nanoparticle in the embryonic solution are spherical nanoparticles and of
darken yellow in color. The absorption maximum shifts to shorter wavelengths
and narrower bandwidths until, on completion, a final size and relatively narrow
is obtained, indicating a decrease in particle size and size dispersion remarkably
[136]. Since the plasmon frequency of each single particle is determined by its
dimension and shape, the optical absorption profiles of the whole distributions
are inhomogeneously broadened. Therefore, irradiation of colloids with laser
pulses of definite photon energy yields resonant plasmon excitation in particles
with specific size and shape. By changing the excitation wavelength it is
possible to selectively excite particles within a range of sizes and/or shapes
[71].The ability to fabricate nanoparticles of varying shape and size, is a
91
hopefully improvement. So by irradiation of nanoparticles with a laser beam,
which wavelength is in the vicinity of the surface plasmon excitation the laserinduced size reduction can be caused in result of the interaction between pulsed
laser light and particles, the heating effect was suggested to be the cause in the
case of the gold nanoparticles. We can prepare silver nanoparticles with wellcontrolled size, shape, and tunable SPR properties. This suggests that there must
be a relationship between the excitation wavelength and the corresponding SPR
absorption band of the prepared nanoparticles [77]. The final size and shape is
found to depend on irradiation pulses. Our result in agreement with Smejkal et
al.[87], study the effects of laser fluence and laser shots on fragmentation
process. The full width at half-maximum FWHM of band (b) was decreased and
the profile narrowed and became monosymmetric.
1
Ag Nanoparticles
(a) Virgin Nanoparticles
0.8
(b) Embryonic Nanoparticles
SPE
0.6
0.4
0.2
0
350
375
400
425
450
475
500
Wavelength(nm)
Fig. 4-34: SPE of Ag nanoparticles produced by laser ablation of 1064 nm (600 mJ) in
water (band a) and after irradiation (band b) by pulsed 532 nm laser, having laser
energy of 900 mJ for laser shots of 20 pulses.
92
Figure 4-35 shows the SPE spectra of gold NPs produced in water before
(band a) and after irradiation (band b) by a pulsed 1064 and 532 nm laser,
having energies of 600 and 900 mJ, respectively. The spectra of virgin solution
exhibit the characteristic peak of the surface plasmon band at 526 nm (band a).
The embryonic nanoparticles exhibit the characteristic peak of the surface
plasmon band at 520 nm (band b). The width of the 520-nm peak is broadened
and the height is lowered more greatly by introducing more laser shots. This
spectral change indicates that the diameters of the nanoparticles are reduced
more [77] under irradiation of the laser with a more laser shots. Another
significant change is observed is the blue shifts. According to the Mie’s theory,
the peak shift to be observed in an absorption spectrum when the mean diameter
of the particles changes. It was observed that the Au NPs with smaller diameter
exhibit SPE at the shorter wavelengths. Therefore, we observed spectral
changes imply that the initial nanoparticles were most likely fragmented by the
laser irradiation.
0.3
(a) Virgin Nanoparticles
Au Nanoparticles
(b) Embryonic Nanoparticles
0.25
SPE
0.2
0.15
0.1
0.05
390
415
440
465
490
515
540
565
590
615
640
Wavelength(nm)
Fig. 4-35: SPE spectra of gold nanoparticles produced by laser ablation of 1064 nm (600
mJ) in water (band a) and after irradiation (band b) by pulsed 532 nm laser, the laser
energy is 900 mJ and laser shots of 20 pulses.
93
Figure 4-36 shows the TEM image and corresponding size distribution of
nanoparticles products prepared by the irradiation of unfocused 532 nm laser for
20 pulses with excitation energy of 900 mJ. The TEM image shows that the
nanoparticles in the embryonic colloid solution are spherical in shape, with the
average diameter of 9 and 8 nm for silver and gold respectively. The parent
silver and gold nanoparticles were shifted to a small diameter and sizedistribution is narrowed by the size reduction because the smallest possible.
40
A
36
32
Ag Nanoparticles
28
Frequency (%)
24
20
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Nanoparticles Diameter (nm)
48
B
44
40
36
Au Nanoparticles
32
Frequency (%)
28
24
20
16
12
8
4
0
5
200 nm
10 15 20 25 30 35 40 45 50 55 60
Nanoparticle Diameter (nm)
Fig. 4-36: TEM images and size distributions of the embryonic silver (A) and gold (B)
nanoparticles, produced by laser irradiation virgin nanoparticles with laser pulsed at
λ=532 nm, E=900 mJ and laser shots of 20 pulses. (The virgin NPs prepared by 1064-nm
laser ablation (E=600 mJ/pulse, λ=1064 nm) of metals plats immersed in DDDW.
94
We noticed that the population of particles smaller than 5 nm increased markedly
in solution after the secondary laser irradiation; large particles should be pulverized
to small particles, which revealed a drastic particle size reduction under the laser
fluence decrease. Our results are in agreement with Tsuji et al. [86]. Moreover,
metal NPs are much smaller than the wavelength of the exciting light so that all
conduction elections of particles are excited in phase. The collective electron
motion (plasmon) inside the metal clusters arouses strong interaction of metal
clusters with the exciting light at their SPE frequency [144].
We expect that the reduction size efficiency must be reached to saturation limit. It
was attributed to smaller particles show time constants of heat dissipation that are
proportional to the square of their radius [75]. So that smaller nanoparticles show,
therefore, fast heat dissipation and they are less expected to reach a temperature
sufficient for reduction. Therefore, smaller nanoparticles can be pulverized even by
weak laser energy [117,136]. Similar study was reported by Pyatenko et al.[91].
4-11 Nanoalloys: Core-Shell Nanostructure Synthesis
Pure Ag and Au nanoparticles were synthesised by pulsed laser ablation of
metallic targets in liquid media, subsequently, mixing in same volume ratio and
re-irradiating by unfocused and pulsed Nd-YAG laser. The laser energy, laser
wavelength and shots are 900 mJ, 532 nm and 25 pulses, respectively. Fig (437) reports the extinction spectra of two sets of Ag (band a) and Au NPs (band
b) samples. The SPE spectra (band c) of irradiated Ag-Au mixed nanoparticles
solution show double peaks, indicating the presence of discrete-pure silver and
gold particles [51], respectively. On the other hand, the SPE reduced and the
tow peaks at 400 and 525 nm were shifted to the red and blue, respectively,
indicating of producing of bimetallic Au–Ag core-shell nanoalloys [55] instead
of separate Ag and Au nanoparticles. This phenomenon was confirmed by
TEM. However, the Ag NPs have been observed to melt at significantly lower
temperatures than the melting temperature of bulk Ag [145].
95
0.9
Au-Ag Core-Shell Nanostructure
0.8
0.7
(a) Ag Nanoparticles
0.6
(b) Au Nanoparticles
SPE
0.5
(c) As Irradiated(532 nm)
0.4
0.3
0.2
0.1
0
340
380
420
460
500
540
580
Wavelength (nm)
Fig. (4-37): SPE spectra of pure Ag (a), pure Au (b) and as mixed and irradiated (λ=532
nm, laser shots of 25 pulses and E=900 mJ) of Au-Ag core-shell nanostructure (c).
Fig. 4-38 shows the TEM images and corresponding size distribution of
colloid obtained after laser irradiation. The clusters appear to be nearly spherical
with average sizes of 19 nm. Similar study was reported in [51,55,146].
24
200 nm
Size Distribution
20
Au-Ag Core-Shell:
Nanostructure
Au-core
Frequency (%)
16
Ag-shell
12
8
4
0
30 nm
5
10 15 20 25 30 35 40 45 50 55 60
Particle Diameter (nm)
Fig. 4-38: TEM image and corresponding size distribution of Au-Ag core-shell
nanostructure solution, prepared via laser irradiation of mixed Ag-Au nanoparticles
solution. The lighter particles are Ag ones while darker ones are Au. The irradiated
laser parameters are:(λ=532 nm, E=900 mJ and laser shots is 25 pulses).
96
4-12 Color Changing:
nging: Indicator
During the laser ablation of noble metal target, it was observed that the color
of solution changed from light yellow and pink for Ag and Au, respectively, and
then finally deep. The deeper color indicates higher concentration of noble metal
nanoparticles. Fig.(4-39) shows the color changing as a function of nanoparticles
concentration by changing some parameter as mentioned in each vial
vial. The
interesting optical is attributes of metal nanoparticles, as scattered
red in their bright
intense colors [3].. In the presence of the oscillating electromagnetic field of the
light, the free electrons of the metal nanoparticle undergo a co
collective coherent
oscillation with respect to the positive metallic lattice. This process is resonant at a
particular frequency of the light and is termed the SPR oscillation (SPE) [114].
This electronic oscillation can be simply visualized as a photon confined to the
small size of the nanostructure, constituting an intense electric fie
field around the
particle. The SPR oscillation decays by radiating its energy resulting in light
scattering [23]. Whose
hose color and color intensity can be determined both their
particle size and concentration, respectively. Therefore, the color of the colloidal
solution is a good indicator of the particle size and solution concentration [69]. The
color of metal nanoparticle is resulted from the coherent oscillation of the
conduction band electrons for metallic nanoparticles can be induc
induced by the
interacting electromagnetic field, which is named as SPE [46].
Fig.4-39: Photo-pictures
pictures of the final products of NPs solutions prepared by PLAL.
97
4-13 Thin Films Deposition
4-13-1 Bulk Thin Films
To estimate the absorbance behaviour of bulk noble metals target as a
function of incident wavelength, silver and gold thin films have been prepared
by deposited on glass substrates using thermal evaporation technique. The
thickness of the thin films was 20 nm, has been evaluated using interference
method. We noted that the surface is smooth because the thickness of the Ag
and gold films was only 20 nm. The optical spectra of the films were
investigated in the wavelength range of 350–1200 nm by a spectrophotometer.
Fig. 4-40 shows the absorption spectra of the Ag and Au films deposited for
20 sec. The Ag thin films deposited, which exhibit a linear increase in
absorbance intensity with the increasing of the wavelength and have flat
absorption peak around 600 nm, suggesting that the Ag films are homogeneous
according to Moore et al. [147] .The Au thin film shows decrease in absorbance
intensity, when the wavelength is increased until 500 nm, then will be increased
and it is subsequently saturated.
0.8
0.7
Thin Film
0.6
Silver thin film
Absorbance
0.5
Gold thin film
0.4
0.3
0.2
0.1
0
300
400
500
600
700
800
900
1000
1100
1200
Wavelength (nm)
Fig.4-40: Absorption spectra of silver and gold bulk thin films as a function of wavelength.
98
4-13-2 Nanoparticles Thin Films
Two-dimensional nanostructures have been studied and categorized as “thin
films”. Because of their confinement to the nanoscale, they have been developed
for use in fields as diverse as electronic devices and photovoltaic applications, due
to their large surface area [148]. Therefore transparent thin films were prepared by
precipitated nanoparticles solutions on glass substrate by drying at room
temperature in an auto-dry method. The nanoparticles solutions were synthesized
by pulsed laser ablation of metal plates immersed in DDDW. This method is
dependent by Kazakevich et al [58]. On the other hand, deposition of noble metal
nanoparticle via PLD in liquid has been reported by Cui et al.[12].
Fig. 4-41 shows the optical microscope images of silver (a-g) and gold (h-o)
NPs thin films, respectively. These thin films are prepared by precipitate of liquid
nanoparticles samples (produced by PLAL in previous section). However the SPE
peaks of the silver nanoparticles solutions in panels a-g are 0.4-2.2, increased by
0.3, and the SPE peaks of the gold nanoparticles solutions in panels h-o are 0.170.87, increased by 0.1. The thickness of the nanoparticles thin films was 20 nm in
panels a and h for silver and gold respectively, has been evaluated using
interference method. Correspondingly, the films structure changes from
discontinuous film, consisting of the isolated aggregates of the building blocks, to
the dense film. Further experiments have revealed that formation of the irregularly
shaped building blocks is attributed to the randomly growth of the nanoparticles.
We noted that the surface is not quite smooth because the films prepared in an
auto-dry precipitation. We suggest that the relatively large roughness of the thin
films is a result from two aspects. Firstly, the films were deposited at room
temperature. Secondly, the films were grown at long period time about 5 days,
which may give rise to some particles during the depositing process, affecting on
surface roughness. The metal film shows a shiny and metallic appearance may be
come from the agglomerating into bulk metal [32]. The coloration is due to
plasmon oscillations of free electrons in nanoprotrusions structure [82,149].
99
50 μm
Fig.4-41: Optical microscope images of silver a-g and gold h-o nanoparticles thin films.
All the images were done in the same magnification.
100
On the other hand, metal films which contain large networks of elongated
nano- voids, granular films with a small quantity of voids, and finally dense
void free films which however have properties which differ from the bulk due
to their nano-grain.
The corresponding UV-VIS absorption spectra of silver (panel a) and gold
(panel h) in Fig. 4-41, are shown in Fig. 4-42. All the spectra exhibit bands due
to the SPE, the position and intensity depending on several factors such as the
diameter of the nanocrystals, the nature of the light and the refractive index of
the surrounding medium.
Fig. 4-42(A and B) shows the SPE of silver and gold nanoparticles,
respectively, in case of solution and thin film. Band (i) in Fig 4-42-A shows the
SPE spectrum of the silver nanoparticles solution, displays a quasisymmetric
absorption band centred at 400 nm, which indicates that the nanoparticles in the
growth solution are quasispherical nanoparticles approximately 8 nm in
size[79]. Band (ii) shows the UV-VIS absorption spectrum that the thin film
silver nanoparticles, displays asymmetric absorption band centred at 430 nm
which indicates that the nanoparticle in the thin film are hemispherical
nanoparticles. The absorption maximum shifts to longer wavelengths and
broadening bandwidths, a final size and relatively expand, indicating an
increase in particle size and size dispersion [86], is in good agreement with the
red shift and widening of the SPR peak with the increase in the thickness of
nanostructured Ag films prepared by PLD, reported by Alonso et al.[150]. Band
(iii) shows the SPE spectrum of the gold nanoparticles solution, was around
525–535 nm, indicating the formation of particles with dimensions of 5–30
nm[151]. The presence of the single surface plasmon peak implied that the
formed nanoparticles were nearly spherical. In the case of ellipsoidal particles
the absorption spectrum would have two plasmon's peaks [77]. Band (iv) shows
the SPE spectrum of the thin film gold nanoparticles. The spectra exhibit the
101
characteristic peak at 545 nm compared with Thomas et al. was reported a broad
band centred at 575 nm [108]. The width of the 545-nm peak is broadened and
the height is lowered more greatly. This spectral change indicates that the
diameters of the nanoparticles are increased more under precipitation [136]. The
increase in size is interpreted as those gold nanoparticles are fusion by
precipitation and hence the number of nanoparticles which are available to the
fusion decreases. Note that the nanoparticles in thin film fused sufficiently fast
that the spatial distribution of the nanoparticles is regarded as inhomogeneous.
Another significant change is observed red shifts. According to the Mie’s
theory, the peak shift to be observed in an absorption spectrum when the mean
diameter of the particles changes [136]. Gold nanoparticles with larger diameter
exhibit maximum absorption at the longer wavelengths. However, the thin film
shows a strong decrease of the UV-VIS absorbance appeared in the absorption
spectrum, which corresponds to the decrease concentration of the Au atoms in
thin film. On the other hand broadening of the red tail of the plasmon resonance
band can be attributed to the fusion of the particle aggregates. The SPR peak
shifts to longer wavelengths with increasing thickness which is consistent with
our previous work on Donnelly et al. [29].
0.22
0.5
(A)
Silver Nanoparticles
0.4
(B)
Gold Nanoparticles
(iii) Solution
(i) Solution
0.18
(ii) Thin Film
(iv)Thin Film
SPE
SPE
0.3
0.2
0.14
0.1
0.1
0
0.06
350
375
400
425
450
475
500
390
Wavelength(nm)
440
490
540
590
Wavelength(nm)
Fig. 4-42: SPE spectra of silver (A) and gold nanoparticles (B), respectively, in case of
solutions (i, iii) and thin films (ii, iv).
102
4-14 Target Effects
Pulsed Laser ablation (desorption) of solid targets in liquids has attracted
much attention not only for nanoparticles formation but also due to possibility
to fabricate debris -free microstructures on various solids. Fig. 4-43 shows the
surface morphology of Ag target, panels a-i and Au, panels j-o, respectively,
was
studied
using
optical
microscope.
All
experimental
parameters
corresponding to panels a-m are mentioned in table (4-l). The structures are
closely packed periodic micro-grooves with deep ablation, depending on some
parameters such as laser wavelength, fluence and environment liquid. The
surface of the target undergoes eye–visible changes. Namely, the exposed areas
of the Ag and Au substrate take on a yellow coloration, suggesting (1) its
relevance to the plasmon resonance of metal NPs and (2) the formation of a
nanostructure directly on the metal substrate as a result of laser exposure in
water.
Table 4-1: Shows the ablation stateous of figure a-o as a function of target type,
environment solution, laser shots, energy and wavelength, respectively.
Figure
Target
medium
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
Silver
Silver
Silver
Silver
Silver
Silver
Silver
Silver
Silver
Gold
Gold
Gold
Gold
Gold
Gold
H2 O
H2 O
NaCl
H2 O
H2 O
H2 O
Ethanol
PVP
SDS
H2 O
H2 O
H2 O
H2 O
H2 O
Ethanol
Laser
shots
15
15
15
15
5
90
15
15
15
15
15
15
15
90
15
Energy
(mJ)
600
600
600
900
600
600
600
600
600
600
600
900
300
600
600
103
Wavelength (nm)
1064
532
1064
1064
1064
1064
1064
1064
1064
1064
532
1064
1064
1064
1064
Ablation Stateous
Homogeneous
Shallow, homogeneous
Homogeneous
Inhomogeneous, melt drops
Partially, inhomogeneous
Deep, inhomogeneous
Partially, inhomogeneous
Deep, inhomogeneous
Deep, inhomogeneous
Homogeneous,
Shallow, Homogeneous
Deep, Homogeneous
Partially, inhomogeneous
Deep, inhomogeneous
Partially, inhomogeneous
The formation of the structures is attributed to the instability of the flat front of
the melt under recoil pressure of vapours of surrounding liquid.
One can see that the periodic ripples on the Ag and Au target are
predominantly (Fig. 4-43). Formation of these ripples is typical of the
interaction of a laser radiation of sub-nanosecond duration with the surface
electromagnetic wave induced in the material. However, the formation of
microstructures on a rough metal surface subjected to laser ablation in water is
due to the modification of the topology of the molten area upon decreasing the
laser fluence. At high laser energy (900 mJ, Fig 5-43-d), the molten area is
continuous, and this melt is effectively dispersed as nanoparticles by the recoil
pressure of the surrounding liquid. At lower laser energy (300 mJ, Fig. 5-43-n),
the melting occurs in the areas with weak thermal bounding to the substrate, e.g.
summits of the initial micro- relief, edges of scratches, etc. Therefore, at low
laser fluence the molten area is a discontinuous set of small molten areas. The
recoil pressure of the vapour surrounding the target pushes these molten areas
from the target and generates a microstructure. In other words, formation of
microstructure is a result of hydrodynamic instability at the interface ‘‘liquid
vapour–melt’’ characterized by very small period of order of hundreds of
nanometers. The described mechanism is independent on the target material,
and similar nanostructures may be grown on silver or gold target that absorb at
laser wavelength.
Typically the period of ripples is close to laser wavelength, which is 1064 or
532 nm in our work. However, ripples on Ag and Au have the period about of
1000 nm (Fig.4-43-a, j) and 500 nm (Fig.4-43-b, k) corresponding to ablation
wavelength. This discrepancy should be assigned to the elevated refractive
index of surrounding water compared to air or vacuum. Yellowish coloration of
laser-treated areas is also observed in the case of Ag or Au target, though its
origin is not clear.
104
50 μm
Fig. 4-43: Surface morphology of Ag, panels a-i and Au target, panels j-o, respectively, after
ablation by Nd-YAG laser at the parameters was mentioned in table 4-1. All the images were
done in the same magnification.
105
Chapter Five: Conclusions and Future Works
5-1 Conclusions
1-Certain pure NP colloidal solutions can be formed. In addition, surfactants can
be added to liquids in order to control the size and the aggregation state.
2-The formation rate, mean particle size and stability could be controlled by proper
selection of the laser parameters.
3-On-line monitoring is regarded as a good tool to controlling on PLAL process.
4- There is an agreement in the PLAL efficiency was quantified in term of the SPE
peaks as well as of the concentration of Ag NPs.
5-Metals nanoparticles can be produced at laser wavelength of 1064-nm laser
energy more efficiently than at 532 nm.
6-The silver nanoparticles are shifts to a smaller size due to increasing in SDS
concentration; slow particle growth is due to SDS coating.
7-The Cl ions were formed of a negatively charged electric bilayer which enhances
dispersity and prevents the aggregation.
8-The PVP solution enhanced the formation efficiency of Ag NPs and prevents
aggregates (as capping agent).
9-LISR (modification) is caused by heating, melting and evaporation of the initial
particles, which reduced the size and size distribution.
10-We have successfully prepared of Au–Ag core-shell NPs in two steps.
11-The SPE properties as peak position and peak intensity is reliable indicator for
identify the size and concentration.
12-Fabrication of metal nanostructured films using auto-dry deposition provides a
new method to tuning the morphology and thickness.
5-2 Suggestion for Future Works
1. Using the produced nanoparticles in practices application such as: solar cell,
sensor, cancer treatment and antibacterial reagent.
2. Study the effect of NaOH solution on the size and size distribution of metal NPs.
106
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114