1. technology and computing
2. electronic components

CZECH TECHNICAL UNIVERSITY IN PRAGUE
DOCTORAL THESIS STATEMENT
Czech Technical University in Prague
Faculty of Nuclear and Physical Engineering
Department of Solid State Engineering
Mgr. Alexander Kovalenko
THE STUDY OF BIOACTIVE AND BIOCOMPATIBLE SURFACES AND
NOVEL NANOSTRUCTURAL COMPOSITES
Doctoral study program: Physical Engineering
Branch of study: Solid State Engineering
Doctoral thesis statement for obtaining the academic title of “Doctor”,
abbreviated to “Ph.D.”
Prague 2012
The doctoral thesis was produced in full-time manner
Ph.D. study at the department of Solid State Engineering of the Faculty of Faculty of Nuclear and
Physical Engineering of the CTU in Prague
Candidate:
Establishment
Address
Mgr. Alexander Kovalenko
Faculty of Nuclear and Physical Engineering
Břehová 7, 115 19 Prague 1
Supervisor:
Department
Prof. RNDr. Ivo Kraus, DrSc
Department of Solid State Engineering,
Faculty of Nuclear and Physical Engineering
Trojanova 13, 120 00 Prague 2
Address
Supervisor specialist:
Department
Address
Doc. Ing. Irena Kratochvílová, Ph.D.
Department of Solid State Engineering
Faculty of Nuclear and Physical Engineering
Trojanova 13, 120 00 Prague 2
Opponents:
Ing. Stanislav Záliš, CSc.
Doc. RNDr. Jiří Pavluch, CSc
Prof. RNDr. Tomáš Polívka, Ph.D.
The doctoral thesis statement was distributed on:
The defence of the doctoral thesis will be held on __________at ______ a.m./p.m. before the
Board for the defence of the Doctoral Thesis in the branch of study Solid State Engineering in the
meeting room No. ______ of the Faculty of Nuclear and Physical Engineering of the CTU in Prague.
Those interested may get acquainted with the doctoral thesis concerned at the Dean Office of the
Faculty of Nuclear and Physical Engineering of the CTU in Prague, at the Department for Science
and Research, Břehová 7, Prague 1.
prof. Ing. Stanislav Vratislav, CSc.
Chairman of the Board for the Defence of the Doctoral Thesis
in the branch of study Solid State Engineering
Faculty of Nuclear and Physical Engineering of the CTU in Prague
Trojanova 13, 120 00 Prague 2.
TABLE OF CONTENT
1
CURRENT SITUATION OF THE STUDIED PROBLEM
2
2
GOALS OF THE DOCTORAL THESIS
5
3
METHODOLOGY
7
4
SELECTED RESULTS
9
5
CONCLUSIONS AND RESEARCH CONTRIBUTION
20
REFERENCES
22
LIST OF CANDIDATE’S WORKS
25
SUMMARY
27
RÉSUMÉ
28
1
1 CURRENT SITUATION OF THE STUDIED PROBLEM
Recently nanodiamonds, like many other nanomaterials have attracted particular interest of
researchers. Nanocrystalline (ND) and ultrananocrystalline (UND) diamond nanoparticles [1] in
addition to the properties of bulk diamond have new qualities which make them a unique and
attractive material.
Another unique interesting physical feature of specifically modified nanodiamonds – is their
ability of an extremely intense fluorescence [2] [3-12]. Inherent photoluminescence is emitted from
structural defects and impurities. Of particular importance for the intrinsic photoluminescence
properties of diamond are defects containing complexes of substitutional impurity atoms and
vacancies. This property could be important for future development of more accurate methods of
bio-labeling used for diagnostic monitoring of complex biological processes.
In recent years a diversity of embodiments for single photon sources has been investigated.
Among them are schemes based on single molecules or atoms [2-5], single ions trapped in cavities
[6], color centers in diamonds [7], [8], quantum dots [9], [10] and parametric down conversion (PDC)
[11], [12]. These sources differ in the wavelength and purity of the emitted photons, their repetition
rate and whether they produce a photon on demand or heralded, i.e, announced by an event. Many
luminescent defects in diamond possess a single-photon behavior, thus can be successfully used for
quantum applications.
Due to their nanometer sizes, crystallites of nanodiamonds have a colossal specific surface
area. The surface of nanodiamond crystal has due to production and purification a number of
defects. ND, surface structure is different from the regular diamond, the surface carbon atoms have
uncompensated dangling bonds. This makes an ultra-high surface activity of nanodiamonds [1]. NDs
have unique properties and can be successfully used not only in technology and production, but also
as an effective drug delivery system in biomedicine.
1.1 NV centers
Aggregations of nitrogen admixtures, which are the dominant impurities in natural and
synthetic diamonds, create with vacancy thermally stable structures possessing high quantum
efficiency up to temperatures above 500 K. NV color centers are either neutral (NV0) or negatively
charged (NV-). Both of these centers are photostable and, irradiation, which is emitted from the
fluorescence nanodiamonds, can be easily identified at the individual level.
Nitrogen vacancy (NV) centers can be produces from synthetic bulk diamond of diamond
nanoparticles containing nitrogen impurities in the form of single substitutions (called C or P1 centers
in diamond literature) by irradiation with a high energy beam with further by annealing at
temperatures above 700°C for several hours. The post-irradiation annealing process allows carbon
atoms neighboring a vacancy to hop into a vacant place and leave an empty position in the diamond
lattice; by this process a vacancy can migrate through the diamond, forming complex lattice
compounds with other vacancies, interstitial atoms, (forming Frenkel pairs), or Schottky defects [13]
i.e. NV centers. The newly-formed compound defects are optically active; the precise optical
properties depend on the annealing time and type of initial defects and impurities present. [14] A
wide range of high-energy particle beams can be used for a primary irradiation, including electrons,
protons, neutrons, ions, and gamma photons. Irradiation produces lattice defects such as vacancies,
dispositions etc. Those defects are immobile at room temperature; however, at higher temperatures
defects can migrate in the diamond lattice. Single substitutional nitrogen creates strain in the
diamond lattice; [15] which are capable to capture moving vacancies, [16] creating the NV centers.
Due to the presence of relatively large lattice strain in diamonds, optical transitions are split and
shifted from individual centers by these strains from individual centers resulting in broad lines in the
ensembles of centers. [14]
Negatively charged NV color centers emit bright red light which can be easily excited by
visible light sources, such as argon or krypton lasers, frequency doubled Nd:YAG lasers, dye lasers, or
2
He-Ne lasers. Laser enlightenment, however, also leads conversion NV-into NV0 centers. [17]
Emission is relatively quick (relaxation time ∼10 ns). [18, 19] At room temperature, no sharp peaks
are observed owing to the thermal broadening. Nevertheless, cooling the NV- centers with liquid
nitrogen or by liquid helium dramatically narrows the lines down.
A significant feature of the luminescence from individual NV centers is its high temporal
stability. While bleaching occurs after the emission of 106 − 108 photons in organic dyes [20] and <
105 photons in autofluorescent proteins [21], no bleaching is observed for the NV centers at room
temperature. [2, 22, 23] Clearly, photoluminescent NDs are attractive alternative to molecular dyes
as biological markers and traceable molecular carriers, in the function of stable options of
fluorescent markers for special applications. As distinct from molecular dyes, photoluminescent NDs
do not photobleach. This results in a fluorescent signal of greater intensity and stability. A further
benefit over molecular dyes is that a wider diversity of biomolecules is able to be attached to
nanodiamond surface.
Photoluminescence emitted by nitrogen-vacancy defects can be altered by many parameters
- applying a magnetic field, electric field, microwave radiation and also by modifying a diamond
surface with a various molecular groups. In consequence of that fact the present work is based on a
research of the connection between the nanodiamond cluster surface termination and its
fluorescence. In the experimental part of the work experiments which were made in a purpose to
prove the possibility to use nanodiamond particles for in-vitro labeling are described. Also,
preparation of fluorescent ND particles by a high-energy proton beam implantation specific
luminescence nanodiamond particles with NV- & NV0 color centers and methods of surface
terminations are described.
1.2 SiV centers
Intensive, narrow-band single photon emission from the silicon vacancy (SiV) complex defect
in diamond, which can be fabricated by Si ion irradiation has been reported [24-27]. Single SiV
centers are photostable and have a spectrum consisting of a sharp zero phonon line (FWHM is about
5 nm) at 738 nm exposing even at room temperature very weak vibronic sidebands. The short
luminescence lifetime of ∼2.7 ns (300K) enables a proficient generation of single photons. All this
features make the SiV centers promising candidate for building an efficient single photon source.
Silicon-vacancy centers emit strong narrow-line luminescence with emission energy in the far
red region – 1.681 eV (738 nm) [24]. This source of luminescence has been reported in films grown
by a variety of chemical vapor deposition (CVD) methods including hot-filament [25],
oxygenacetylene combustion [26] and microwave-plasma [27].
The most interesting and unique property of such centers, which makes them perfect
candidate for spintronics application is that, they have a very low vibronic structure (phonon wing)
both in luminescence and absorption, and no symmetry with a respect to the zero-phonon line [28].
Even at room temperatures luminescent peak is very sharp, showing FWHM of about 6nm with the
majority of the emission concentrated in the zero phonon line (ZPL). In comparison with widely
investigated NV centers, factor showing the ratio of the intensity of the one-phonon peak to the
intensity of the ZPL (Huang-Rhys factor), is about 15 times lower for the SiV centers (3.21 and 0.24
respectively) [29].
SiV centers show extremely short luminescence lifetime, which is often ascribed to the
existence of non-radiative transitions, which is confirmed by the observation that the intensity of the
ZPL of SiV centers drops with increasing temperature [30]. The emitting lifetime was measured to be
4 ns at 5 K and 2.7 ns at room temperature in the homoepitaxial CVD diamond film and about 1 ns
almost independent of temperature in a polycrystalline CVD diamond film [31] (as an example 2.156
eV center with 29 ns) [32].
SiV luminescent centers charge state mainly considered to be neutral, due to the fact, that
SiV centers show similar behavior in photochromism measurements as the neutral NV centers. The
3
fact that the intensity of the ZPL of SiV centers can be increased up to ten times by UV illumination
indicates the existence of a positively charged state. However, no narrow-line absorption, which
could be ascribed to that state, could be found in the range 0.5-5.5 eV [33]. The procedure for the
fabrication of SiV centers via ion implantation had been shown by C. Wang in [34]. Diamond surface
was implanted with 10 MeV Si2+ ions at room temperature. Post-implantation annealing was carried
out at 1000°C for 5 min in vacuum. Samples were washed in a solution of K2Cr2O7:H2SO4 at 180°C for
2 min to remove the graphite layer formed during the annealing process. The narrow emission
bandwidth of about 5 nm at room temperature, the high photostability and the short lifetime of
about 2.7 ns, make SiV centers interesting as a single photon source in practical quantum
cryptography.
1.3 Metal originated luminescent centers
Transition metals, especially nickel, are common impurities in synthetic high pressure high
temperature (HPHT) diamonds [35, 36, 37]. Throughout the HPHT process, metal atoms are
exclusively incorporated into the {1 1 1} growth sectors and their concentration can vary within the
same growth sector [38]. Nickel related centers can play the role of single photon emitters, due to its
sharp luminescent peak observed at room temperature.
Chromium-based luminescent centers is a new addition to the group of diamond-based
single-photon emitters. Such emitters were initially discovered by growing nanodiamond crystals on
a sapphire substrate [39]. Chromium is a regular impurity in sapphire, which can be integrated into a
diamond crystal during the CVD growth [40]. Chromium originated centers are for the most part
attractive owing to their narrow bandwidth emission observed even at room temperature. It has
been shown that Cr emitters can be successfully fabricated by ion implantation of chromium with
oxygen, sulfur or boron atoms [41-43]. Cr-containing luminescence centers have very short emission
lifetimes – from 1 to 4 ns, whereas NV centers have emission lifetimes an order of magnitude longer:
NV0 – 29 ns, NV- – 13 ns [44].
Also in this work it has been demonstrated the formation of new single-photon emitters
containing silicon and nickel atoms (Si+Ni centers), first produced by co-implantation of Ni and Si
atoms into the diamond. The formed centers are not previously known NE8 but show surprising and
potentially important properties including an estimated near unity quantum efficiency, a near
infrared emission 768 nm, and a 2 ns radiative lifetime. A further co-implantation of Ni and Si atoms
into pure bulk diamond indicates that the defect center belongs to a new class of nickel-silicon
composite centers [37]. The simple fabrication method and the extended optical properties make
this defect a very attractive candidate for integration with external microstructures. The coimplantation of Ni/Si is likely to spur substantial research into the physical structure and properties
of such composites.
4
2 GOALS OF THE DOCTORAL THESIS
In regard to NV centers one of the main object of the research is to provide a deep study of
the connection between the nanodiamond cluster surface terminations and its properties, such as
lattice geometry, molecular orbitals, charge distribution, and, especially, absorption and emitting
processes. To get qualitatively better understanding of the complicated and specific process of
nanodiamond particles luminescence, computer simulation based on the density functional theory,
was used; model processes and states influencing the luminescence of variously terminated ND
particles. Clusters containing from 35 to 900 atoms of carbon, with the impurities in a form of NV
color centers with two different charge states (NV- & NV0) terminated by carbonyl, carboxyl,
hydroxyl, fluorine and amine groups were modeled. Explored systems were studied by DFT based
calculations using Gaussian 09 program package. Main goals and objectives can be summarized as:
- Definition of the theoretical model and methods for the simulation of nanodiamond clusters;
measure of accuracy was the correlation of experimental and theoretical bandgap and excitation
energy;
- Optimization of the structure of nanodiamond particles to its minima, finding of the lattice
geometry; bond lengths and types;
- The study of the size dependence of nanodiamond particles (i.e. quantum dimensional effect); of
particular interest are influences on excitation energy for luminescent particles (i.e. adsorption
spectra) and bandgap of clusters with no defects in the lattice;
- The study of the influence of surface oxygen groups on the luminescent conditions; according to
the experimental research surface oxygen passivation was modeled in the form of carbonyl, hydroxyl
and bridge-type bonding;
- The study of the influence of other surface groups on the luminescent conditions; particularly
amine, fluorine and carboxyl groups;
- Definition of the density of states distribution, in particular electron density distribution in the
ground and excited states;
- The study of possible influence between NV centers and other crystallographic defects in terms
of positively charged single substituted nitrogen;
As respects silicon and nickel related centers, surface chemistry was not of particular
importance in this case, due to the fact that ND particles are not highly desired as single photon
sources and size is no the matter. However computational demand does not allow us to calculate
macrostructures, nevertheless we deal with point defects witch allows to reach high accuracy on the
small models. According to this clusters containing about 100 atoms of carbon with silicon and nickel
related defects were modeled. All structures were examined with a variety of charges and
multiplicities in order to find out energetically preferred system. Explored systems were studied by
DFT based calculations using Gaussian 09 program package. Main goals and objectives can be
summarized as:
- Definition of the theoretical model and methods for the simulation of nanodiamond clusters;
measure of accuracy was the correlation of experimental and theoretical bandgap and excitation
energy;
- Analysis of the bonding nature of Si and Ni diamond lattice defects in the semi-divacancy site
remained unclear, thus finding of the lattice geometry; bond lengths, types and orders was an
important task;
-
TD-DFT calculations in order to define excitation energies of the structures under study;
Cr and Ni+Si related luminescent centers are newly discovered and there are still many open
questions about their structure, bonding, charges and spin states. Using DFT based calculations using
Gaussian 09 program package we tried to approach to the experimental results. Clusters containing
5
chromium along with oxygen, sulfur and nitrogen in the variety of charges and multiplicities were
modeled. Goals and objectives can be defined as:
- Definition of the lattice geometry and components; the structure of Cr related centers remained
unclear so far;
- Definition of the theoretical model and methods for the simulation of nanodiamond clusters;
measure of accuracy was the correlation of experimental and theoretical bandgap and excitation
energy;
- Analysis of the bonding nature of Cr and Si+Ni related diamond lattice defects as far as structure
remained unclear, thus finding of the lattice geometry; bond lengths, types and orders as far as
charges and multiplicities was an important task;
-
TD-DFT calculations in order to define excitation energies of the structures under study;
6
3 METHODOLOGY
Numerous properties of molecules and crystals can be simulated using classical methods in
due form of molecular mechanics and dynamics. The classical force field is based on empirical results,
which are consequence of averaging over a large number of molecules. Because of this widespread
averaging, the results can be good for typical systems, but there are many important questions in
chemistry which can not be estimated at all by methods based on the empirical approach. If it is
required to know more than just structure or other properties that are derived only from the
potential energy surface, in particular properties that depend directly on the electron density
distribution, it is necessary to resort to a more fundamental and general approach: quantum
chemistry. The same holds for all non-standard cases for which molecular mechanics is simply not
applicable.
In quantum chemistry, the system is described by a wavefunction which can be found by
solving the Schrödinger equation. This equation relates the stationary states of the system and their
energies to the Hamiltonian operator, which can be viewed as the way to obtain the energy
associated with a wavefunction describing the positions of the nuclei and electrons in the system. In
practice the Schrodinger equation cannot be solved exactly and approximations have to be made
using the ab-initio approach using no empirical information, except for the fundamental constants of
nature such as the mass of the electron, Planck's constant etc. that are required to arrive at
numerical predictions. In spite of the necessary approximations, ab-initio theory has the conceptual
advantage of generality, and the practical advantage that its successes and failures are more or less
predictable.
The major drawback of ab-initio quantum chemistry calculations is a big demand on
computation technique power. Consequently, such approximations have been applied for a long time
with a good correlation which allows introducing the empirical parameters into the theoretical
model. This has led to a number of semi-empirical quantum chemical methods, which can be applied
to larger systems, and give reasonable electronic wavefunctions so that electronic properties can be
predicted. Compared with ab-initio calculations their correlation is less and their applications are
restricted by the condition for parameters, just like in molecular mechanics.
In general, ab-initio quantum chemistry calculations is preferred for relatively small systems,
which are need to be explored at a very high level, when electronic properties are required and for
structures, for which valid molecular mechanics parameters are not obtainable. Electronic structures
of all systems examined were calculated by density functional theory (DFT) methods using the
Gaussian 09 [42] program package. DFT is a formally exact method for investigation of many-body
system electronic structure. The DFT methods attain considerably greater accuracy than HartreeFock theory at only a modest augment in the calculation time. This effect is obtained by specific
including of electron correlation.
However, for the simulation of relatively complicated systems, sophisticated hybrid
exchange-correlation functionals are needed [43-46]. In our case Becke’s three-parameter hybrid
functional (B3LYP) [47] correlation functional was used. This functional include a mixture of exact
exchange from Hartree-Fock theory with the Lee, Yang, and Parr exchange - correlation functional.
Open shell systems were treated by the unrestricted Kohn – Sham (UKS) procedure. The geometry
optimizations and spectral calculations were carried out without any symmetry restriction.
For geometry optimization of smaller nanodiamond clusters (up to 200 C atoms) 6-31G(d)
Pople split-valence polarized double-ζ basis sets for H, C, N and O atoms were used. Relatively large
nanodiamond particles (No. of C atoms up to 700) were optimized by the DFTB method. Electronic
excitations were calculated by time dependent DFT (TD-DFT) at optimized geometries. TD-DFT is an
extension of DFT determined to investigate excited states and non-equilibrium properties of manybody systems in the presence of time-dependent potentials. This method enables the analysis of the
character and localization of individual excited states [23-25]. A. S. Zyubin et al. [48] used TD-DFT for
the analysis of optical properties of approximately 100 atoms containing ND particles with neutral
and negatively charged vacancy-related ND point defects (N2V0, N2V-, and N3V0). DFT methods have
7
been successfully used for calculations of the optical gap, absorption spectrum and luminescence of
small Si nanocrystals, with hydrogen and oxygen at the surface [49, 50]. For clusters containing more
than 82 carbon atoms (more than 1 nm size), the changes of the parameters studied here with
cluster size were negligible and the lowest lying excitation energies were close to the experimental
data.
Gaussian is a connected system of programs for performing ab-initio and semi-empirical
molecular orbital (MO) quantum chemical calculations. It has been designed with the needs of the
user in mind. Thus, all the standard input is free-format and mnemonic. Gaussian 09 provides an
analysis of molecular potential energy surfaces to determine molecular structures and spectral
properties of stable and unstable species. It provides a wide number of ab-initio models and some
semi-empirical models.
Program package provides calculations of one- and two-electron integrals over s, p, d and f
contracted Gaussian functions. The basis set can be either Cartesian Gaussians or pure angular
momentum functions, and a variety of basis sets are stored in the program and can be requested by
name. Integrals can be stored externally or recomputed as needed via the direct self-consistent field
procedures. Variety of methods can be used, self-consistent field calculations for restricted closed
shell, unrestricted open-shell, and open shell restricted Hartree-Fock wavefunctions as
multiconfigurational wavefunctions that fall within the generalized valence bond-perfect pairing
formalism. Automated geometry optimization to either minima, or saddle points, numerical
differentiation to produce force constants, polarizabilities and dipole derivatives and reaction path
following is also one of the Gaussian capabilities.
8
4 SELECTED RESULTS
4.1 NV centers
It has been reported [2], that for ND particles containing NV- centers close to the hydrogen
terminated surface (resulting in a positive electrostatic potential of the surface layer), the
luminescence probability is significantly reduced or/and the NV- centers in hydrogen terminated ND
particles are converted into NV0 centers. In the case of oxygen containing a ND termination, the layer
with an excess of electrons is created (resulting in negative electrostatic potential of the surface
layer). Previously, it has been reported according to the experimental results [2] that ND particles
NV0 centers luminescence is not practically changed by surface states (terminations). Thus in the
present work deep study of the surrounding/particle surface impact on nanodiamond NV- centers
luminescence is presented.
There are many parameters affecting NV- centers luminescence process in ND particles and
some of them have been discussed in previously published works [2, 48-50]. Here it is shown how
specific orbitals in NV- containing NDs occupations affect luminescence. It can be said that all lowest
lying excitation transitions are between β (spin down) orbitals.
With the change of particles’ size or NV center position in particle degenerated triplet state
splits - which was shown experimentally [51], and simulated theoretically [52]. Pinto et al. [52]
described models of the diamond vacuum slab, resulting 0.2 eV split of two empty spin-down orbitals
when NV center was closer than 1 nm to the surface. In the present work excitation energies,
absorption lines split and absorption spectra broadening were calculated. Simulated absorption
spectra (black) and calculated transitions (red vertical lines) of the variously terminated and sized ND
particles are shown on Figure 1. Lowest two lines correspond to triplet-triplet transitions between
frontier β orbitals. For 1.2 nm hydrogen terminated ND the difference (split) between two lowest
electronic transitions equals to 0.18 eV. For OH terminated particles the lowest lying electron
transition shifts from ~730 to ∼850 nm, which corresponds to larger energy levels’ split - 0.44 eV.
Similar situation is observed for fluorine terminations: 0.42 eV is the difference between lowest
transitions (Figure 1). Surfaces of hydrogenated particles were 100 % covered by hydrogen, in case of
oxygenation/fluorination 10% of hydrogen was substituted by carbonyl groups/fluorine. Also, several
carbonyl/fluorine rations were studied (2%, 4%, 6%, 8%, and 10%); in all cases the tendency of
βHOMO to βLUMO transition energy withdraws remained the same.
As it has been explained βHOMO - βLUMO+1 transition energy shows value (∼1.9 eV) which
is in a good correspondence with the experimental results, in case of smaller particles (1.2 nm), or
oxygenated/fluorinated surface βHOMO to βLUMO transition energy is lower than measured
adsorption/emission energy of NV-. This is consistent with the fact that LUMO orbital of cluster
(C696H300N1) is located on the surface (i.e. electron can be pulled back from its initial position);
however HOMO and LUMO+1 are on the NV center’s immediate vicinity (see Figure 2). Thus, electron
which dropped down from the LUMO+1 to LUMO is dislocated from its initial position and with high
probability relaxes to the ground state possessing no standard luminescence. Moreover electron can
be trapped by vicinity defects or surface terminations. For fluorinated and oxidized NDs the βLUMO
and βLUMO+1 energy difference and k-space position difference is larger (comparing to Hterminated systems) and the probability of the electron relaxation to βLUMO decreases, thus
βLUMO+1 to βHOMO relaxation, which with high probability possess zero phonon line transition is
more preferable. In the case of close arrangement of βLUMO and βLUMO+1 (H termination) electron
can transit to the ground state non-radiatively. Therefore, fluoridating or oxygenating of the ND
particles preserve conditions for standard zero phonon line emission.
9
a)
b)
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Figure 1: Simulated absorption spectra and calculated transitions (vertical lines) of the 100% hydrogenated 1.7
nm (a), 100% hydrogenated 1.2 nm (b), 10% fluorinated 1.7 nm (c) and 10% fluorinated 1.2 nm (d) ND particles.
a)
b)
c)
Figure 2: βHOMO (a), βLUMO (b) and βLUMO+1 (c) orbitals of cluster (C696H300N1). In the first and third cases
(a) and (c), electron density located in the immediate vicinity to the NV center, in the second case (b) electron
density is shifted to the surface.
Due to the fact that for small particles degenerated 3E state splits into two levels there are
two different transitions - βHOMO to βLUMO and βHOMO to βLUMO+1. In case of hydrogenated ND
particles excited electron relaxation to βLUMO causes it’s delocalization from the NV center. Electron
can be trapped by ND’s surface or vicinity defects. For O/F terminated ND the energy difference
between βLUMO and βLUMO+1 is higher (compared to H-terminated NDs) and βLUMO+1 to βHOMO
relaxation with no shelving, is more preferable.
10
4.2 Silicon related centers
Silicon in diamond has been identified with a Silicon-Vacancy complex in the semi-divacancy
site. Theoretical approach shows the distance between Si and the nearest carbon is about 2.05 Å
which is more than regular carbon-silicon bond. The bond orders between Si and C atoms were
found in the range of 0.3-0.4. Thus, silicon atom is located in the cavity between two carbon
vacancies. Calculated excitation energy using TD-DFT method for neutrally charged Si atom educes
that the most probable irradiative electronic transition has energy equal to 1.8875 eV (656 nm,
experimental 738 nm) and oscillator strength f=0.0244 (see Table 1).
Table 1: TD-DFT calculated lowest energies excitations parameters for neutrally charged Si containing defects:
excitation energy and oscillator strengths.
Defect type
Orbitals involved
Excitation energy, eV
Oscillator strength
SiV0 singlet
HOMO -> LUMO
0.2628
0.0001
HOMO-2 -> LUMO
1.8875
0.0244
HOMO-1 -> LUMO
2.0112
0.0812
βHOMO -> βLUMO+1
1.8709
0.0370
βHOMO-1->βLUMO+1
1.8820
0.0232
βHOMO -> βLUMO+2
2.0800
0.0818
SiV0 triplet
a)
b)
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5000
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6000
ε [arb. un.]
0,08
0,08
7000
600
800
1000
1200
Wavelength [nm]
Wavelength [nm]
Figure 3: Simulated absorption spectra and calculated transitions (vertical lines) of the neutrally charged Si
related luminescent centers in the singlet (a) and triplet (b) spin states.
As it can be in Figure 3, neutrally charged Si related centers have similar absorption spectra in
the singlet and triplet spin states, however singlet state is 0.97 eV lower. Thus, singlet state is more
energetically favorable, but triplet close enough to the singlet ground state to be excited by laser
forming triplet state, consequently two spin states can emit luminescence at the similar wavelengths,
which can cause overlapping of emission.
Negatively (-1) charged Si-related center shows strong absorption lines in the range of 2.21
and 3.58 (see Table 2) possessing splitting of the orbitals due to the low symmetry of studied cluster,
however similar splitting into four lines was reported in [53, 54] Simulated absorption spectrum of
negatively charged Si related center depicted in Figure 4.
11
Table 2: TD-DFT calculated lowest energies excitations parameters for negatively charged Si
containing defects: excitation energy and oscillator strengths.
Defect type
Orbitals involved
Excitation energy, eV
Oscillator strength
SiV- singlet
βHOMO-1 -> βLUMO
2.2196
0.0101
βHOMO-2 -> βLUMO
2.3841
0.0507
βHOMO-3 -> βLUMO
3.2988
0.0692
βHOMO-4 -> βLUMO
3.5821
0.0145
0,07
6000
0,06
ε [arb. un.]
0,05
4000
0,04
3000
0,03
2000
0,02
1000
0,01
0
Oscillator strength
5000
0,00
400
600
800
1000
1200
wavelength [nm]
Figure 4: Simulated absorption spectrum and calculated transitions (vertical lines) of the negatively charged Si
related luminescent centers in the doublet state.
4.3 Chromium and nickel related centers
Geometry optimizations were performed for neutral and doubly charged Ni centers both in
singlet and triplet states in C101H72 ND clusters. Calculations on model clusters indicate that the single
neutrally charged nickel should be placed in divacancy microcavity of the diamond lattice. In this
case, the triplet state lies 0.22 eV lower than the singlet state. For the doubly positively charged Ni-V
center, the singlet state has the lowest energy while the triplet state energy is about 0.80 eV higher.
In the case of neutral Ni, the calculated C-Ni bond lengths are in the range of 2.05–2.13 Å (bond
orders 0.30–0.42). The octahedral nickel atom is weakly bonded to the surrounding carbons. For the
doubly positively charged Ni-V center, the Ni-C bond orders (0.36) and bond lengths (from 2.07 to
2.11 Å) were close to these parameters of neutral nickel containing centers. In both cases, the Ni-C
bonds are weak. The oscillator strengths represent the probability of a transition from the ground
state to the excited state. The excitation energy calculated for a system with a neutral Ni center (3A
state) is 2.34 eV, which is close to the experimental 2.51 eV ZPL [47]. For a Ni2+ luminescence center
(1A state), was calculated an excitation energy value of 1.73 eV, which is also close to the
experimental value of 1.883 eV (see Figure 5) [45].
12
a)
b)
4000
0,035
2500
0,020
2000
1500
0,015
1000
0,010
500
0,005
0
200
0,025
3000
ε [arb. un.]
ε [arb. un.]
0,025
600
800
1000
0,015
2000
0,010
1000
0,005
0
200
0,000
400
0,020
1200
Oscillator strength
3000
Oscillator strength
0,030
0,030
4000
3500
0,000
400
600
800
1000
1200
Wavelength [nm]
Wavelength [nm]
Figure 5: Simulated absorption spectra and calculated transitions (vertical lines) of 1.25 nm sized ND particles
containing NiV centers (a) in a neutral triplet state, (b) in a double positively charged singlet state.
Cr-based color centers in ND particles were as it has been mentioned previously engineered
by Cr implantation followed by the implantation of oxygen, sulfur or nitrogen. Subsequent annealing
causes the formation of luminescent centers [37, 39–41]. Such centers can be leading candidates for
future quantum devices thanks to their room temperature operation, photostability, narrow
bandwidth and short lifetime. To model such center, Cr atom was placed in the semi-divacancy site,
due to the fact, that standard Cr-C bond length is longer than a C-C bond, and thus substitutional
chromium atom leads large lattice stresses in the diamond, during the annealing (such treatment was
also applied in the process of creating Cr luminescent defects [41]) chromium atom placed in the
lattice site can push away carbon atom forming octahedral coordination semi-divacancy complex.
Moreover, it also has been proved by calculations that Cr-V complex is energetically more favorable
than substitutional single chromium atom placed in the diamond lattice. The experiments performed
have shown that the presence of nitrogen is a crucial condition for creating chromium-based
luminescent centers. Thus, nitrogen atom was placed in the immediate vicinity to Cr atom. Further
irradiation with either sulfur, oxygen or boron ions allows the replacement of nitrogen or carbon
atoms – Cr’s nearest neighbors – which causes an augmentation of the single photon emitters.
a)
b)
Figure 6: The optimized geometries of Cr-based centers, a chromium atom placed in the cavity of two carbon
atoms and bonded to five adjacent carbons and one nitrogen atom- (a), or nitrogen substituted by sulfur - (b).
The DFT calculations were done for C98H72Cr1N1 clusters containing triply charged Cr centers
for 1A, 3A and 5A states (Figures 6 (a), (b)). Such structures allow the placement of atoms with
minimal lattice stresses corresponding to energetically advantageous positions. Thus, the octahedral
triply charged chromium atom in a semi-divacancy site (the Cr-C bond was found to be longer than
the C-C bond (1.93–2.02 Å)) and the adjacent carbon was substituted by nitrogen or sulfur was
modeled. Figure 6 (a) shows a model of a ND cluster containing an imbedded Cr atom in a semidivacancy site with a bonded nitrogen substitution.
Geometry optimizations indicate 3A state as the most stable. For the triply charged Cr-related
center between C and Cr, two shorter bonds (1.93 Å, Wiberg index 0.64) and four longer bonds (in
the range from 2 to 2.51 Å and Wiberg index 0.29–0.12, respectively) were found. The Cr-N bond
13
value on the Wiberg index is 0.28. Thus, in a chromium-related center, the Cr3+ is strongly bonded to
the two adjacent carbons while the other bonds are weak. The calculated triplet–triplet electronic
transition (from αHOMO to αLUMO, Cr3+) found at 747 nm (see Figure 8 (b)) corresponds well to the
measured excitation energy of a Cr-containing center in a diamond. As in this luminescence center
the chromium atom is strongly bonded to the two adjacent carbons and weakly to the other carbons
and nitrogen and the N-C bond is weak; the nitrogen atom can be again easily substituted by
implanted ions. Ulterior irradiation with sulfur atoms can cause the substitution of either nitrogen or
carbon in the center’s immediate vicinity, and new configurations expand the excitation/emission
frequency scale. If nitrogen is substituted by sulfur, it then shows the sulfur is bonded to the adjacent
Cr3+ more strongly than the initial nitrogen (Wiberg index 0.54, bond length 2.16 Å). The chromium
atom is in turn strongly bonded to the two carbon atoms (Wiberg indexes ∼0.73, bond lengths ∼1.98
Å) and relatively weakly to other carbons (Wiberg indexes ∼0.28, bond lengths 2.08–2.31 Å). It can
thus be suggested, that the sulfur-containing center is more stable. Bonding lengths for Cr-based
centers containing one atom of chromium bonded either with nitrogen of sulfur are depicted in
figures 6 (a) and 6 (b) respectively.
a)
b)
Figure 7: Bond lengths of optimized Cr-based centers, a chromium atom placed in the cavity of two carbon
atoms and bonded to five adjacent carbons and one nitrogen atom- (a), or nitrogen substituted by sulfur - (b).
b)
1000
0,010
800
0,008
0,006
600
0,004
400
0,002
200
Oscillator strength
ε [arb. un.]
a)
0,000
0
400
600
800
1000
1200
Wavelength [nm]
Figure 8: (a) Lattice geometry of Cr related center in a semi-divacancy site with a bonded nitrogen substitution
and simulated absorption spectrum of Cr-N center. (b) Simulated absorption spectra and calculated transitions
(vertical lines) of the Cr-N center.
14
a)
b)
0,012
1800
1600
0,010
ε [arb. un.]
0,008
1200
1000
0,006
800
0,004
600
400
0,002
Oscillator strength
1400
200
0
0,000
400
600
800
1000
1200
Wavelength [nm]
Figure 9: (a) Lattice geometry of the Cr-N-B center. (b) Simulated absorption spectra and calculated transitions
(vertical lines) of the Cr-N-B center.
Further implantation with boron can cause substitution of carbon in the CR-N center.
Geometry optimizations indicate positively doubly charged 3A state as the most stable in case of CrN-B center (Figure 9 (a)). For the triply charged Cr-related center between C and Cr, two strong bonds
with lengths 1.93 Å and 1.94 Å, and Wiberg indexes 0.67 and 0.70 respectively and three longer Cr-C
bonds in the range from 1.99 to 2.49 Å and Wiberg index 0.33–0.16, respectively. The Cr-N bond
value on the Wiberg index is 0.33 and bond order of the Cr-B bond is equal to 0.12. In the Figure 9 (b)
it can be seen, that Cr-N-B centers has strong absorption with the wavelength about 745 nm, which is
in a good correspondence with the experimental ZPL emission. Total calculated Wiberg index for
chromium is 3.63.
a)
b)
1800
0,018
1600
0,016
0,014
ε [arb. un.]
1200
0,012
1000
0,010
800
0,008
600
0,006
400
0,004
200
0,002
0
Oscillator strength
1400
0,000
400
600
800
1000
1200
Wavelength [nm]
Figure 10: (a) Lattice geometry of the Cr-N-O center. (b) Simulated absorption spectra and calculated
transitions (vertical lines) of the Cr-N-O center.
The triply charged Cr-N-O center has two shorter bonds 1.91 and 1.98 Å, Wiberg indexes are
0.72 and 0.65 respectively, three longer bonds (in the range from 2.11 to 2.36 Å and Wiberg index
0.52–0.20, respectively). The Cr-N bond value on the Wiberg index is 0.29, Cr-O bond is very weak
with a length 2.40 Å and Wiberg index 0.07 Thus, in a Cr-N-O center, the Cr3+ is strongly bonded to
the two adjacent carbons while the other bonds are weak. Cr atom is placed in the double vacancy
cavity, its total Wiberg index equals to 3.65. Simulated absorption spectrum of the Cr-N-O center
shows intensive absorption at ∼760 nm, which shows possibility of existence such type of
luminescence centers (Figure 10).
15
a)
b)
0,035
4000
ε [arb. un.]
0,025
3000
0,020
2000
0,015
0,010
1000
Oscillator strength
0,030
0,005
0
0,000
400
600
800
1000
1200
Wavelength [nm]
Figure 11: (a) Lattice geometry of the Cr-N-S center. (b) Simulated absorption spectra and calculated transitions
(vertical lines) of the Cr-N-S center.
For the triply charged Cr-N-S (Figure 11 (a)) center there are three strong bindings between C
and Cr, with a length in the range from 1.91 to 2.07 Å, and bond orders 0.68–0.65 respectively and
one longer Cr-C bond with the length 2.37 Å and Wiberg index 0.20. The Cr-N bond value on the
Wiberg index is 0.28. Thus, in a chromium-related center, the Cr3+ is strongly bonded to the two
adjacent carbons while the other bonds are weak. Calculated absorption spectrum for Cr-N-S center
showed the strongest and the sharpest absorption line (figure 11 (b)) among all Cr-related centers
considered. In the case of sulfur containing center total Wiberg index of chromium atom has the
highest value – 3.99.
As regards to Cr and Ni containing centers, their excitations were of 3d origin and the
number of states lying between ground and excited state is relatively large. This great variety of
excited states can interact via spin-orbit coupling and lead to shorter lifetimes and deactivation.
Significant advance of the present research is the discovery of possible structure of the chromium
related centers with a good correlation with the experiment performed by Arahonovich, Muller et al.
[40-43]. It has been found that single chromium atom placed in the semi-divacancy site is bonded to
the nitrogen atom in its initial state. Ulterior irradiation with oxygen/sulfur/boron can cause
substitution of adjacent carbon with implanted atoms increasing number of emitters.
Geometry optimizations indicate 3A state as the most stable. For the triply charged Cr-related
center between C and Cr, two shorter bonds (Wiberg index 0.64) and four longer bonds (Wiberg
index 0.29–0.12) were found. The Cr-N bond value on the Wiberg index is 0.28. Thus, in a chromiumrelated center, the Cr3+ is strongly bonded to the two adjacent carbons while the other bonds are
weak. The calculated triplet–triplet electronic transition found in the range of 750 nm corresponds
well to the measured excitation energy of a Cr-containing center in a diamond. Centers with one
carbon atoms substituted by oxygen, boron and sulfur showed similar spectra, which means
overlapping of luminescence originated from different defects. I case of oxygen and sulfur
substitution defects were found to be triply charged in the triplet state, defect containing boron was
found to be doubly charged (triplet). The specificity of bonds strongly influences the excitation
process.
4.4 Ni+Si luminescent centers
Very new and interesting color centers in diamond are containing complexes with Ni
[39, 40, 41]. A very short lifetime of only 2 ns, a total high count rate (from two APDs) of 280 kHz and
operation at room temperature makes some of these systems perfect solid state candidate to be
used in integrated quantum optics [55, 56] i.e. various optical light guide cavities which couple
individual photons with minimum loss. In preceding papers it has been reported various emission
lines characteristic of nickel impurities in diamond [57]; however, an emission around 770 nm has
not been observed or assigned to any particular nickel related centre up to now. Silicon, which is the
most common diamond impurity besides N, is favorably incorporated within the diamond lattice
16
forming common Si-V centers. Therefore, in [37] it has been assumed that the new centre is related
to both nickel and silicon complexes. To check these assumptions, a reference experiment by
implanting both silicon and nickel into pure synthetic, type IIa, diamond was performed. Nickel ions
were implanted with an energy of 37.5 keV while silicon ions were implanted using 25 keV. A similar
PL line centered at 766 nm was observed (figure 12) whereas when only nickel was implanted into
the same crystal did not result in the formation of this specific emission line. Doublet at around
883/885 nm associated with an interstitial nickel defect in diamond [41, 57] appeared in both
performed nickel implantations. These observations provide solid evidence that the new emission
line at around 770 nm is due to a complex containing both nickel and silicon. Those two atoms create
a large distortion in the diamond crystal. Hence, it is possible that upon annealing a vacancy is
combined with those two atoms and the actual structure consists of Si and Ni impurities associated
to a vacancy.
Figure 12: PL spectra from Ni only (black) and a co implantation of Ni/Si into type IIa e6 CVD diamond. The
measurement is taken at 77K under 514 nm excitation. The red curve is recorded from the non implanted
region. [37]
a)
b)
0,010
1800
1600
0,008
ε [arb. un.]
1200
0,006
1000
800
0,004
600
400
0,002
Oscillator Strength
1400
200
0
0,000
400
600
800
1000
1200
Wavelength [nm]
Figure 13: (a) Lattice geometry of the Si+Ni center. (b) Simulated absorption spectra and calculated transitions
(vertical lines) of the Si+Ni center.
In order to find optimal state of the defect geometry optimizations were performed simulations for
single and triplet stated for charges 0, and +2; doublet and quadruplet states were used with charges
+1 and +3. Calculations on model clusters indicate that the nickel and silicon should be placed in
close vicinity to each other in the divacancy sites of the diamond lattice (Fig. 13). For the mentioned
structure intensive electron transition at ∼1000 nm (oscillator strength ∼0.01) was found in the case
of +1 charged system in a quadruplet spin state. In this case quadruplet state lays 0.22 eV lower than
the doublet state. In this case Ni-C bond lengths are in the range of 1.89-2.06 Å, Wiberg indexes were
17
found in the range of 0.40-0.33 respectively. Si-C bonds were slightly longer than Ni-C bonds and
were found to be in the range of 1.98-2.10 Å, bond orders defined by Wiberg indexes were in the
range of 0.44-0.34 respectively. Simulated bond between Si and Ni atoms was equal to 2.34 Å, bond
order 0.20. Total Wiberg indexes for Ni and Si were 3.37 and 3.01 respectively.
a)
b)
0,012
2500
ε [arb. un.]
0,008
1500
0,006
1000
0,004
500
0,002
0
Oscillator strength
0,010
2000
0,000
400
600
800
1000
1200
Wavelength [nm]
Figure 14: (a) Lattice geometry of the Si+Ni center with a nitrogen bonded to nickel. (b) Simulated absorption
spectra and calculated transitions (vertical lines) of the Si+Ni center.
However, addition of nitrogen bonded to nickel atom causes the appearance of strong
electron transition near 700 nm with oscillator strength ∼0.012, which is closer correspondence with
the experimental results showing luminescence at 766 nm. In this case, the bond between Si and Ni
atoms remains almost the same (2.33 Å, Wiberg index 0.20), bonds Ni-C are also in the range of 1.892.06 Å, with the bond orders 0.53-0.35, Si-C bonds were found to be slightly longer than in the case
of system without nitrogen: bong lengths 2.00-2.11 Å, bond orders 0.44-0.34 respectively. The
lengths of the Ni-N bond were found to be equal to 2.01 Å with the bond order 0.27. Total Wiberg
indexes by atoms were: Si – 3.0, Ni – 3.4, N – 3.2.
a)
b)
5000
0,05
4000
ε [arb. un.]
3000
0,03
2000
0,02
1000
0,01
0
Oscillator Strength
0,04
0,00
400
600
800
1000
1200
Wavelength [nm]
Figure 13. (a) Lattice geometry of the Si+Ni center with a nitrogen bonded to nickel and silicon. (b) Simulated
absorption spectra and calculated transitions (vertical lines) of the Si+Ni center.
Further analysis of the studied system showed that addition of nitrogen bonded to both
silicon and nickel atoms causes intensification of the 700 nm line and totally diminishes 1000 nm line.
Oscillator strength in this case is ∼0.05, which corresponds to the most intensive transition studied in
the present work. In case of +1 charged, triplet state, system with the nitrogen bonded to Si and Ni
atoms simultaneously showed, the expansion of Si-Ni bond length (from 2.34 Å to 2.41 Å, Wiberg
index – 0.23), Si-C bonds were found to be in the range of 2.00-2.14 Å, Wiberg indexes: 0.31-0.52. NiC bonds had lengths in the range of 1.98-2.04 Å, which is also longer than in previously studied
systems, bond orders were in the range of 0.38-0.26 respectively. Ni-N and Si-N bond were found to
18
be 1.91 (Wiberg index 0.27) and 1.86 Å (Wiberg index 0.23) respectively. Total Wiberg indexes by
atoms were: Si – 3.2, Ni – 3.7, N – 2.9. Thus, it can be concluded, that this type of luminescence
defect is weakly influenced by diamond phonons, due to lower Si-C and Ni-C bond orders, which
causes sharp luminescence peak and weak phonon wing.
19
5 CONCLUSIONS AND RESEARCH CONTRIBUTION
ND particles with luminescent centers are very promising for a wide range of applications. In
the present doctoral thesis, (using TD-DFT) parameters which mainly affect optical centers’ behavior,
were studied. Main purpose of the present work was the relevance of surface chemistry (hydrogen
vs. oxygen termination) and the type of luminescence defects in diamond lattice (transition metals
(Ni, Cr) vs. nonmetals (Si, N)).
For ND particles with NV- centers close to the hydrogen terminated surface region (resulting
in a positive electrostatic potential of the surface layer), excited triplet electrons are localized on C
atoms. In this configuration, the luminescence probability is significantly reduced and the NV- centers
in hydrogen terminated ND particles are converted into NV0 centers. In the case of oxygen containing
a ND termination, the layer with an excess of electrons is created (resulting in negative electrostatic
potential of the surface layer). Electrons excited from NV- centers are partly localized at highelectronegative oxygen atoms preserving higher luminescence probability. Excited states in
nanodiamond NV0 centers are not modified by surface states – the ND particle NV0 center
luminescence is practically not affected by the particle’s termination. Based on the mentioned
phenomenon, it can be expected the significance of nanodiamond particles’ terminations on their
functionality in spintronic applications.
Due to high surface dipole moment in the case of oxygen or fluorine groups on the surface
NDs singlet state a1A is shifted to higher energy level than excited triplet state b3A and exclude
transition between excited triplet and basic singlet state - strong presence of standard NVluminescence. Direct excitations from triplet ground state to singlet excited states are in this case
forbidden. Furthermore, excited electrons which are localized in surface layer have lower probability
to be easily drawn up by atoms which originally acted as electron donors. For oxidized ND particles
containing NV- center the excitation energy shifts to lower values, which means that the probability
of luminescence is higher than in the case of hydrogenated ND particle.
It has been identified that for NV centers containing NDs the excitation energy (close to
experimental results) is stabilized for clusters having more than 82 C atoms. However, for defect-free
NDs the lowest lying excitation energy of particles containing up to 900 C atoms was gradually
changing and did not reach a stable value. This means that for NV center containing NDs the local
defect plays dominant role in luminescence process and standard optical parameters can be
obtained on nanoparticles of sizes from nm upwards. In defect-free diamond the extension of the
particle must be enough for the definitive establishing collective solid state properties (namely
forbidden gap). Moreover, it has to be noticed, that the excitations of non-metal atoms (nitrogen)
containing centers are of 2p origin. In the case of NV centers, the number of states for deactivation
between the ground state and the excited state is low.
In the present work we only nibbled at a subject of Silicon related centers. The undertook
research showed, that in the negative state silicon related centers showed similar absorption spectra
in the singlet and triplet spin stated, moreover triplet state is just 0.9 eV higher, than singlet, thus it
can be exited by laser, resulting conversion to a triplet state and emission overlap. In both neutral
and negative charge states the bond order of the single silicon atom placed in the semi-divacancy site
is weaker and longer than standard bond, which proves suggestion, that silicon atom are weakly
influences by phonons of diamond lattice, which results short lifetime.
Metal atom containing center excitations were of 3d origin and the number of states lying
between basic and excited state is relatively large. This great variety of excited states can interact via
spin-orbit coupling and lead to shorter lifetimes and deactivation. In regard to nickel centers, it was
found that NE1 center has two states which can be proper luminescence sources: theoretical model
neutrally charged nickel center in the triplet state showed strong transition at the wavelength 530
nm, while doubly charged (+2) nickel center in the singlet state exposed 717 nm, which are green and
red light respectively. Octahedral nickel atom was found to be weakly bonded to the surrounding
carbons. For the Ni-C bonds Wiberg indexes were defined ∼0.36.
20
Significant advance of the present research is the discovery of possible structure of the
recently discovered chromium related and Si+Ni centers with a good correlation with the experiment
performed by the Australian group. It has been found that single chromium atom placed in the semidivacancy site is bonded to the nitrogen atom in its initial state. Ulterior irradiation with
oxygen/sulfur/boron can cause substitution of either nitrogen of adjacent carbon with
oxygen/sulfur/boron atoms increasing number of emitters. Geometry optimizations indicate 3A state
as the most stable. For the triply charged Cr-related center between C and Cr, two shorter bonds
(Wiberg index 0.64) and four longer bonds (Wiberg index 0.29–0.12) were found. The Cr-N bond
value on the Wiberg index is 0.28. Thus, in a chromium-related center, the Cr3+ is strongly bonded to
the two adjacent carbons while the other bonds are weak. The calculated triplet–triplet electronic
transition found at 747 nm corresponds well to the measured excitation energy of a Cr-containing
center in a diamond.
In regard to the complex silicon-nickel centers, first very important approach has been done.
It was found that, single charged Si and Ni in the quadruplet state, placed in the immediate vicinity to
each other (semi-divacancy site) possess two strong absorption line in the range of 1000 and 600
(nm), forming coordination complex with Si and Ni placed in the cavity (Wiberg indexes in a range of
0.33-0.44), however addition of nitrogen bonded to nickel atom causes the appearance of strong
electron transition near 700 nm with oscillator strength ∼0.012, which is closer correspondence with
the experimental results showing luminescence at 766 nm. In case of nitrogen atom bonded to both
Si and Ni, absorption intensity of 700 nm line increases (oscillator strength ∼0.50), and 1000 nm line
diminishes, which allows suggestion of existence such luminescence lattice defects. Moreover this
kind of defects has impurity atoms weakly bonded to carbon atoms in the diamond lattice, which
causes feeble interaction with diamond phonons, thus sharpens luminescence peak. Further analysis
of the studied system showed that addition of nitrogen bonded to both silicon and nickel atoms (in
case of +1 charged, triplet state) causes intensification of the 700 nm line and totally diminishes 1000
nm line. Oscillator strength in this case is ∼0.05, which corresponds to the one of the most intensive
transition studied in the present work. Thus it can be said that Si+Ni centers consist of Si and Ni
atoms placed in the cavity of diamond lattice vacancies with a bonded nitrogen atoms. Moreover
close values of transition energies can cause overlap of the emission.
Luminescence is a very complex phenomenon. Generally, it holds true that after a charge
transfer to the excited state, its surrounding is more or less deformed so that the excited state is
affected by its vicinity, the state and luminescence properties depend on the local structure
(bonding, states) particularities. Specifically, it is important to know the bond, structural and charge
parameters of each center in order to estimate the impact of the specific external
effects/technological processes (ion implantation) on the whole system of state/luminescence
properties. Our structures, charge and bonding descriptions brought new results and comparisons in
all of the investigated cases (all the calculated excitation energies were comparable to the
experimental results).
Finally, it should be stressed that all of the systems examined were calculated by density
functional theory (DFT) methods. From the agreement of the theoretically and experimentally
obtained parameters, it can be concluded that ND color centers have a strongly local character.
Working in this way with point effects (color-center luminescence), we were able to describe (using
DFT methods) details of the systems on a truly fundamental level yielding very useful information
about the systems.
21
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24
LIST OF CANDIDATE’S WORKS
[1]
[2]
[3]
[4]
[5]
[6]
I. Kratochvílová, A. Taylor, A. Kovalenko, F. Fendrych, V. Řezáčová, V. Petrák, S. Záliš,J.
Šebera, M. Nesládek Fluorescent Nanodiamonds: Effect of Surface Termination Mater. Res.
Soc. Symp. Proc., Vol. 1203, 1203-J03-05, pages 5
I. Kratochvílová, A. Taylor, F. Fendrych, A. Kovalenko and M. Nesládek Surface potential of
functionalised nanodiamond layers Mater. Res. Soc. Symp. Proc, Vol. 1203, 1203-J17-01,
pages 5
I. Kratochvílová, A. Kovalenko, A. Taylor, F. Fendrych, V. Řezáčová, J. Vlček, S. Záliš, J. Šebera,
P. Cígler, M. Ledvina, M. Nesládek Fluorescence of Variously Terminated Nanodiamond
Particles: Quantum Chemical Calculations phys. status solidi a 207, No.9 (2010) 2045 - 2048.
I. Kratochvílová, A. Kovalenko, F. Fendrych, V. Petráková, S. Záliš, M. Nesládek Tuning of
Nanodiamond Particles’ Optical Properties by Structural Defects and Surface Modifications:
DFT Modelling J. Mater. Chem. 21 (2011) 18248 - 18255.
A. Kovalenko, Parameters Affecting the Fluorescence of Nanodiamond Particles: Quantum
Chemical Calculations. In “Sborník příspěvků 1. Studentské vědecké konference fyziky
pevných látek, Tetřeví Boudy 2011” 2011, p. 100-107. ISBN 978-80-01-04869-6.
A. Kovalenko, V. Petráková, P. Ashcheulov, S. Záliš, M. Nesládek, I. Kraus, I. Kratochvílová:
Parameters affecting the Luminescence of Nanodiamond Particles: Quantum chemical
calculations, physica status solidi, accepted.
List of candidate’s works not referring to the doctoral thesis
[1]
A. A. Leksikov, A. A. Daviduk, A. A. Kovalenko, “Microstrip Sensors for Local Measuring
Surface Resistance on Microwaves” Microwave and Telecommunication Technology, 2006.
CriMiCO '06. 16th International Crimean Conference, pp 743 – 745, ISBN: 966-7968-92-8
[2]
A.A. Leksikov, A.V. Daviduk, A.A. Kovalenko, “Miscrostrip Sensors for the local measurements
of microwave surface impedance”, Izvestiya Vuzov (University Tidings), Russia, Tomsk 2006,
vol. 9, pp. 85-88.
25
SUMMARY
Present Ph.D. research is focused on the study of the luminescent nanodiamond particles
which is the elaborateness of the initial title “bioactive and biocompatible surfaces and novel
nanostructural composites”. Particularly quantum chemical modeling of luminescent diamond
defects using Gaussian 09 program package were performed.
Nanodiamonds (ND), due to their specific properties is a very attractive material for the large
scale of applications. Implicitly unique property is their ultra-high surface activity which is a result of
a colossal ratio of atoms on the surface to the number of atoms in the bulk. The surface of the crystal
always has defects due to methods of production and purification, its structure differs from the
diamond, and the surface carbon atoms have dangling bonds, such fact leads the unique sorption
capacity, and can be successfully used not only in technology and production, but also in
biomedicine. The particular advantages of the use of nanodiamonds in medicine are also their
extraordinary stability, their solubility in water, but, of course, the main priority of their use to the
other drug delivery systems is their safety penetrating into the cells without evoking a toxic cell
response.
Another interesting physical feature of specifically modified nanodiamonds – is their ability of
an extremely intense fluorescence. Inherent photoluminescence is emanated from structural defects
and impurities. Well known, so called nitrogen-vacancy (NV) luminescent centers can be easily
detected at the individual level in their neutral (NV0) or negative charge (NV-). Such defects emit an
extremely bright fluorescence, on wavelengths in visible spectrum of 575 and 635 nm respectively,
which is very suitable for in-vivo molecular imaging and bio-labeling. It has been discovered that,
luminescence of the nanodiamond particles containing NV centers strongly depends on surface
chemistry due to high surface-to-bulk ratio. This phenomenon was the inspiration for the first part of
the present PhD thesis. Clusters containing up to 900 atoms of carbon, with the impurities in a form
of NV color centers with two different charge states (NV- & NV0) terminated by OH, H, NH2, carbonyl,
carboxyl and hydroxyl groups terminations were modeled. Explored systems were studied by DFT
based calculations using Gaussian 09 program package.
The application of nanodiamonds as single-phonon sources is a promising field of work in
quantum mechanics and application of quantum information. Silicon and nickel related defects in
diamond lattice have sharp luminescence in the far-red and infrared region, clearly visible even at
room temperature and very weak phonon absorption. In addition very short life time of the emission
enables a proficient generation of single photons.
Recently discovered metal related (Cr and Si+Ni) luminescent defects in diamond are of
particular interest. As well as silicon centers they emit in the far-red region, but they are much more
stable, easier to produce and considered to be the brightest single-photon emitters (Cr) known for
today. However, structure of these defects remains unclear so far. So the of the main object of the
second part of the research was to provide a deep study of the metal containing luminescent centers
in diamond and their properties, such as lattice geometry, molecular orbitals, charge distribution,
and, especially, to analyze absorption and emitting processes.
To unveil properties unknown so far and to get qualitatively better understanding of the
complicated and specific process of nanodiamond related luminescence computer simulation based
on the density functional theory was used; model processes and states influencing the luminescence
of variously terminated ND particles.
26
RÉSUMÉ
V předložené práci se v rámci původního obecného tématu
"Studium nových
nanostrukturních kompozitů" autor soustředil na studium luminiscenčních vlastností specificky
upravených nanodiamantových částic.
K pochopení specifických a komplikovaných luminiscenčních procesů v nanodiamantových
částicích byly vytvořeny ab-initio (program Gaussian/Funkcionál hustoty stavů) modely aplikačně
perspektivních luminiscenčních defektů v nanodiamanech (centra dusík-vakance (NV), centra
obsahující Si, Ni, Cr v kombinaci s dalšími prvky/poruchami).
Unikátní vlastnost nanodiamantů je jejich velmi vysoká povrchová aktivita, resp. významný
vliv povrchu na luminiscenční odezvu zejména záporně nabitých NV center. Díky této skutečnosti
mohou být nanodiamanty úspěšně používány nejen v oblasti technologií a výrob, ale také v medicíně
pro zobrazování buněčných/tkáňových procesů a transport léčiv. Absorpční vlastnost nanodiamantů,
díky hydrofilní a hydrofobní interakci, je tak vysoká, že je umožněno efektivní zachycení bílkovin, jako
např. cytochromu, myoglobinu a albuminu.
Tzv. dusík-vakance (NV) luminiscenční centra lze snadno detektovat na individuální úrovni v
neutrálním (NV0) nebo záporném (NV-) stavu. Tyto defekty mají intenzivní fluorescenci na vlnových
délkách 575 a 635 nm, což je velmi vhodné pro in-vivo molekulární zobrazování. V rámci dizertační
práce byly modelovány klastry obsahující až 900 atomů uhlíku, s defekty ve formě NV barevných
center ve dvou různých nábojových stavech (NV-& NV0) obsahujících na povrchu karboxylové,
karbonylové, hydroxylové, aminové a fluorové skupiny. V první části dizertační práce byla podrobně
popsána interpretace dosud nejasné souvislosti mezi specifickým stavem povrchu a změnou
luminiscenční odezvy systému.
Další unikátní fyzikální vlastnost speciálně upravených nanodiamantů je jejich mimořádně
intenzivní fluorescenční odezva. Použití nanodiamantů jako zdrojů jednotlivých fotonů je velmi
perspektivní pro řadu moderních technických a technologických směrů. Křemík obsahující defekty
diamantové struktury mají ostrou luminiscenční odezvu v červené/infračervené oblasti. Tato
luminiscence je velmi dobře detekovatelná i při pokojové teplotě a obsahuje jen slabou fononovou
absorpci. Kromě toho velmi krátká doba života emisí umožňuje rychlou generaci jednotlivých fotonů.
Nedávno objevená/popsaná luminiscence iontovou implantací připravených, kovy
obsahujících diamantových defektů (Cr a Si+Ni), je předmětem velkého zájmu. Stejně jako křemíková
i kovová centra emitují na okraji červené části spektra, luminiscenční odeva je ale mnohem
stabilnější, systémy lze připravit relativně dobře definovaným postupem, navíc jsou považovány za
jedny z nejjasnějších zářičů (Cr) v dané oblasti. Ve věci pochopení/popisu funkce těchto center je
ovšem celá škála nejasností - struktura, vazby, stavy excitovaných elektronů. Proto je v druhé části
doktorandské práce prezentováno velmi podrobné studium diamantových defektů obsahujících
zmíněné kovy - geometrie mřížky, molekulové orbitaly, náboje, multiplicity a hlavně analýzy
luminiscenčních procesů.
27
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