1. science
2. physics
3. nanotechnology

2/13/2012
What is “nano”?
Nanoscience and
nanotechnology as seen
through quantum dots
Size scales
Categories
g
Pat Kambhampati
Department of Chemistry
McGill University
Nano in the news
A preliminary look at the buzz
'Nano' scientific publications
80000
70000
60000
Papers
•What happens when
the transistors are the
size of electrons?
50000
40000
'Nano' patents (US)
30000
20000
200
10000
1980
1990
Year
2000
Patents
1970
100
50
0
1970
(Eigler)
News articles
4000
1980
1990
2000
Year
3000
Articles
•How will computers
work when they are no
longer classical?
150
0
•What happens when
the wires are
molecules?
2000
1000
0
1970
1980
1990
2000
Year
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Nano in the news: controversy
• Toxicity (asbestos revisited?)
• ‘Grey goo’? (self-assembly run amok) – Eric Drexler
• Playing God? – Ray Kurzweil, Bill Joy
• A reality check:
courtesy of Rick Smalley (Rice) and George Whitesides (Harvard)
What is different about “nano”
Macroscopic device performing
macroscopic tasks:
fire  steam engines  computers
Microscopic objects serving macroscopic
tasks:
most of chemistry (“One word… plastics”)
A new paradigm:
microscopic functionality
NanoScience  NanoTechnology
•
•
•
•
•
Computers
Lasers
Solar energy
Medicine
Materials
The path from science to technology
• MRI – a good thing!
• But where did it come from?
– Quantum Mechanics (1920’s)
– I.I. Rabi and atomic physics (Nobel 1944)
• NMR: fundamental and applied science (1950 – present)
• MRI: 1977
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Nano and computers: Moore’s law
Hitting the brick wall
(Intel)
(Scholes)
Hitting the brick wall
Can we use nanoscience to make a
‘quantum leap’
• Analog circuits
Problem
All the incremental
progress
will stop in a couple of
decades!
• Digital circuits
• Quantum circuits???
(Intel)
Incremental  Transformative?
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Lasers: another good thing
Quantum computers?
•
•
•
•
•
•
• 101 - voltage or light (on/off/on)
• Lets turn on quantum mechanics..
• What does this 
Telecommunications
Surgery
Medical imaging
Machining
Defense
Fundamental Science
mean?
Energy:
Lasers & Nano: smaller, cheaper, better
Making the Internet on the
cheap – quantum wells
Making the
Internet really
cheap!
quantum dots
Statement of the problem
•
Increasing population + increasing energy needs/person
= global energy problem
•
Finding energy sources and ultimately finding clean energy sources
•
Current consumption at 13 TW-year
•
Projected need: 30 TW-year by 2050
•
600 TW of solar energy reaches earth at practical sites
(10,000 TW total)
(data from Nozik, Crabtree & Lewis)
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Solar cells: finding the Holy Grail
Nano and solar cells
• The problem: global energy needs
• Harvesting, collecting, and storing energy
• A solution: renewable energy (solar)
• Can we perform these operations cheaply and
efficiently?
• The reality:
• Solutions:
– efficient & expensive
– inefficient & cheap
– Light harvesting
– Quantum dots
– Conducting polymers
(NREL)
(Crabtree & Lewis)
A few other areas of NanoScience research
• Materials – learning from Nature
(Comsol)
Our activities in nanoscience
• How do quantum dots actually work?
• Dynamics: lasers & photovoltaics
• Artificial atoms: cryptography & basic physics
• Optical gain: lasers and light sources
• Medicine
–
“monitoring, control, construction, repair, defense, and
improvement of human biological systems, working from the
molecular level, using engineered nanodevices and
nanostructures.”
(R. Freitas)
A box for electrons
(Zunger Nano Lett 2003)
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2/13/2012
One way of looking at things
A brief history of a quantum dot:
with a fast camera
The “Galloping Horse”
Controversy
Palo Alto, CA 1872
Timescales
It’s routine to generate pulses < 1 nanosecond (10-9 s).
Researchers can generate pulses a few femtoseconds (10-15 s) long.
Leland Stanford Eadweard Muybridge
10 fs light
pulse
One
Age of
month pyramids
Computer Camera
clock cycle
flash
1 minute
Human existence
Age of universe
10-15 10-12 10-9 10-6 10-3 100 103 106 109 1012 1015 1018
1 femtosecond
Time (seconds)
Time Resolution:
1/60th of a second
Such a pulse is to one minute
as one minute is to the age of the universe.
(Trebino’s notes)
NanoTechnology and industry
Ultrafast Laser Spectroscopy:
i.e. building a better camera
• New materials
– Imaging, paints, coatings, structures
• Simple applications
Amplitude (arb. units)
– Drug delivery, lasers
• “Disruptive” applications
– Quantum computing
– Solar energy
0
1
2
3
4
Time (0.000000000001 second)
5
0.000000000000001 sec
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A brief introduction to
quantum dots
Outlook
• Popular buzz has upside and downside
• There probably will be a Nanotech bubble – brace
yourselves and invest wisely!
• After the bubble bursts, then the real fun will start!
• Consider the startup vs. the large company
•
•
•
•
•
What is a quantum dot
How to make a quantum dot
How to characterize it
How to think of it physically
Some key publications
• Get ready for some cool things in the coming decades!!!
What is a quantum dot
What is a quantum dot
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What is a quantum dot
What is a quantum dot
What is a quantum dot
What is a quantum dot
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How to make a quantum dot
How to make a quantum dot
How to characterize a quantum
dot
How to characterize a quantum
dot
1.
2.
3
3.
4.
Imaging – TEM, STM
Absorption, PL, PLE
TCSPC – lifetime
Dynamics – femtosecond spectroscopy
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How to characterize a quantum
dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
How to think of a quantum dot
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How to think of a quantum dot
How to think of a quantum dot
Some key results: synthesis
Some key results:
characterization
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Some key results: single dot
Some key results: electrical
Some key results: dynamics
Some key results: solar
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2/13/2012
Research in the Kambhampati
group
Excitons in Semiconductor Quantum Dots:
Design principles for QD lasers and photovoltaics
Patanjali Kambhampati
Department of Chemistry
McGill University
Montreal, Canada
Semiconductor quantum dots:
the early picture
•
Size dependent electronic structure
Size dependent properties
Size must be on the order of
the exciton Bohr radius
( 1 - 10 nm; 102 – 105 atoms)
•
Size tunable properties
•
See LE Brus (1982 – 1988)!
(NN Labs)
(Bawendi 2000)
(Zunger 2006)
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2/13/2012
The excitonics
of semiconductor quantum dots
•
Objective: understand the inner
workings of the quantum dot
•
Excitonics in QD revealed by StateResolved Femtosecond Pump/probe
spectroscopy
Exciton:
Bound electron-hole pai
Excitonics:
Dynamics connects structure and function
Physical Structure:
size, shape, composition, doping
Electronic Structure:
particle in a sphere, effective mass, atomistic
Dynamics:
•
Excitons establish design principles
for QD devices
–
–
–
–
photovoltaics
lasers and lighting
piezoelectrics
photon sources
relaxation, recombination, electron‐phonon coupling, multiexcitons
Function:
optical gain, multiple exciton generation, blinking
(Achermann)
Applications:
solar cells, lasers, photon sources, LEDs
Outline
1. Excitonics of Quantum Dots
2. Optical gain and lasing
3. Multiexcitons:
Excitons (X) in quantum dots
Coarse structure
Fine structure
1De
1Pe
Unraveling optical gain and multiple exciton generation
4. Getting back to the surface
1Se
(Efros, Bawendi)
1S3/2
1P3/2
2S3/2
3S1/2
(Zunger)
(Efros, Bawendi, Klimov)
F  (l  s )  L
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2/13/2012
Exciton vs. electron-hole representation
Illustration of hot exciton dynamics
Chronology of events
electron
/ hole
exciton
cooling
harge transport
harge trapping
(Bawendi)
multiple
exciton
generat
ion
Excitonic State-Resolved pump/probe spectroscopy
(focus on CdSe)
Optical gain in quantum dots
(Krauss Nature 2007)
•Threshold
•Temperature stability, Room temperature operation
•Wavelength range
•Single photon sources
•Integration, packaging, processability
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2/13/2012
Probing optical gain in QDs
The prevailing view of gain in QD
XX 
100
 OD0
(3) 
OD
NL
2. Nonlinear absorption:
X  XX
State-Resolved (toluene)
400 nm pump (hexane) - Klimov
400 nm pump (film?)
- Klimov
400 nm pump (tolune) - Klimov
80
Gain threshold ( <N> )
G
X
8
1. Linear
absorption: 0  X
(1)
6
4
2
0
1.5
3. Spontaneous emission: X  0
1.8
2.1
2.4
2.7
3.0
3.3
3.6
Radius (nm)
(Klimov/Bawendi Science 2000)
(Klimov JPCC 2004)
(Klimov Nature 2007)
1. Gain is inefficient
2. Gain is strongly size dependent
3. Gain dependents strongly upon matrix co
4. Phenomenology arises from structureless
4. Stimulated emission: XX  X
First observation of intrinsic gain performance
of CdSe QD
State resolved studies of optical gain
8
(PRL 2009, JCP 2009)
Exploring the control paramete
OD  i , N ,  ,   OD     0
?
Gain
n threshold ( <N> )
100
State-Resolved (toluene)
400 nm pump (hexane) - Klimov
400 nm pump (film?)
- Klimov
400 nm pump (tolune) - Klimov
80
6
4
2
Result
Excitonic state
controls
optical gain
0
1.5
1.8
2.1
2.4
2.7
Radius (nm)
3.0
3.3
3.6
Result
The ‘worst’
materials can yield
the best
performance ever
measured.
If driven correctly!
21
2/13/2012
CdSe/ZnS/CdSe:
Controlling gain bandwidth
Tailoring optical gain via material design
(PRL 2009, JCP 2009)
CdSe/ZnS/CdSe
CdSe
Result
Optical gain bandwidth
can be tailored
An effect not seen in
bulk or molecules!
core
shell
A new platform for
ultrafast optical
switching?
Battaglia et al., J. Am. Chem. Soc., 2005.
Dias et al., J. Phys. Chem C, 2007.
CdSe/ZnS/CdSe:
Multiexcitons in QD:
Massive gain bandwidth in a coupled heterostructure
dynamics connects structure & function
Ne expts
New
e pts
THz optica
switching
Nonlinear
Spectra
SE Spectra
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MX & Multiple Exciton Generation
MX & Multiple Exciton Generation
Observing multiexcitons using
pump / probe spectroscopy
First observation of structure of biexciton
XX 1  1 1
XX 2  2 1
X  (1)  OD0
XX  (3)  ODNL
XX
X
0
(w/ A Zunger)
XX 3  3 1
XX 4  4 1
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Higher multiexcitons
Biexciton Stokes shift
controlling gain bandwidth
controlling gain threshold
•Gain bandwidth arises from
MX
•Preparation of specific MX
enables shaping of gain
bandwidth
•Pulse sequences enables
ultrafast optical switching
on Stokes shift (X) controls spontaneous emission
iton Stokes shift (XX) controls stimulated emission
Structural dynamics of multiexcitons:
hot exciton relaxation
Hot exciton relaxation dynamics:
Illustrating the processes
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2/13/2012
Radiationless transitions on the nanoscale:
A unified picture of state-resolved exciton
dynamics
Structural dynamics of multiexcitons
Controlling gain dynamics
Rev. B 2006, Phys. Rev. Lett. 2007, Phys. Rev. B 20
J. Phys. Chem. C 2008, Phys. Rev. B 2008
The prevailing view of QD surface
1. Broad
emission
at low
energy is
due to
surface
states
(Brus JPC 1986)
Thermally relating the surface to the core
1) The surface strongly
brightens at low temp.
2) Surface spectrum
does not narrow at low
temp.
2. Breadth
arises from
wide
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2/13/2012
An electron transfer approach
to describing the nature of the surface
Hot exciton surface trapping
1) Classical bath
polarizability gives
rise to thermal
equilibrium
q
between surface
and core
2) Strong phonon
coupling gives rise
to large energy
shift and large
Hot exciton surface trapping in time resolved
& CW spectroscopy
Summary and outlook
•
Overturning prevailing view of gain in NC QD
– Gain is extremely efficient and size universal in QD
– First observation of gain bandwidth control
•
First observation of electronic structure of multiexcitons in QD using
pump/probe spectroscopy
– Gain determined by XX Stokes shift
– Gain dynamics controlled by structural dynamics of XX
– Hot exciton surface trapping and gain / MEG / blinking
•
Development of complete amplitude / phase / polarization shaping of
femtosecond pulses for Coherent Multidimensional Spectroscopy of
multiexcitons in nanostructures
•
Development of lasers and LEDs using QDs and nanowires
NOPA
 ~ 100 nm
shaper 1
shaper 2
sample
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2/13/2012
Publications
Acknowledgements
Kevin
Anderson
Ryan
Cooney
2.
3.
The Group (in order of appearance)
Sam
Sewall
1.
Eva
Dias
Dr. D.M.
Sagar
4.
5.
6.
PHD 2009
MSC 2006
Pooja Tyagi
PHD 2009
PDF 2008
7.
Jon Saari Jonathan Mooney Michael Krause Brenna Walsh
8.
9.
10.
11.
Funding:
12.
“State-to-state exciton dynamics in semiconductor quantum dots”
Phys. Rev. B., 74, 235328 (2006).
“Light harvesting and carrier transport in core/barrier/shell semiconductor nanocrystals”
J. Phys. Chem. C, 111, 708 (2007).
“Breaking the phonon bottleneck for holes in semiconductor quantum dots”
Phys. Rev. Lett., 98, 177403 (2007).
“Unified picture of electron and hole relaxation pathways in semiconductor quantum dots”
Phys. Rev. B., 78, 245311 (2007).
“State-resolved exciton-phonon couplings in CdSe semiconductor quantum dots”
J. Phys. Chem. C, 112, 9124 (2008).
“Size dependent, state-resolved studies of exciton-phonon couplings in strongly confined semiconductor
quantum dots”
Phys.
y Rev. B., 77, 235321 ((2008).
)
“State-resolved processes in semiconductor quantum dots: biexciton interactions and surface trapping
dynamics”
J. Chem. Phys, 129, 084701 (2008).
“Single dot spectroscopy of two-color quantum dot/quantum shell nanostructures”
J. Phys. Chem. C, 112, 14229 (2008).
“Gain tailoring in semiconductor quantum dots via state-resolved optical pumping”
Phys. Rev. Lett, 102, 127404 (2009).
“Experimental tests of effective mass vs. atomistic pictures of quantum dot electronic structure”
Appl. Phys. Lett., 94, 243116 (2009).
“Direct observation of the structure of band-edge biexcitons in quantum dots”
Phys. Rev. B, 80, 081310(R) (2009)
“State-Resolved Manipulations of Optical Gain in Semiconductor Quantum Dots: Size Universality, Gain
Tailoring, and Surface Effects”
J. Chem. Phys., In Press (2009)
CdSe/ZnS/CdSe:
CdSe/ZnS/CdSe:
Optical gain in a coupled heterostructure
Optical gain in a coupled heterostructure
CdSe/ZnS/CdSe
Core
Pump
Shell
Pump
27
2/13/2012
State-resolved exciton dynamics
Excitons in quantum dots
•
Quantum dots have a
resolvable eigenstate
spectrum like atoms
or molecules.
•
Consequently there
should be statespecific dynamics
which may be
measurable.
•
Phys Rev B 2006, Acc. Chem. Res. 2011
•
These state-specific
processes include
relaxation dynamics,
many-body
interactions, optical
gain, etc.
•
Or… what really
Or
happens inside a
quantum dot?
All work on CdSe QD
to explore principles
Real time observation of excited state (hot exciton)
surface trapping
Hot exciton relaxation dynamics:
Electron and hole transition rates
28
2/13/2012
Precision measurements of relaxation:
Hot exciton relaxation dynamics
radiationless transitions on the nanoscale (PRB 2006, PRL 2007, PRB 2007)
•How to climb down the ladder?
•Is there a phonon bottleneck?
E ~ 10LO
Auger
E ~ 4LO
(Klimov 2006)
(Efros & Nozik, Nano Lett 2006)
Excitonics:
Hot exciton relaxation dynamics:
Fundamental dynamics connects structure & function
Exciton-phonon coupling
•What is the strength of excitonphonon coupling?
•Unanimous disagreement about
nearly every aspect of excitonphonon coupling!
•Key result:
First simultaneous observation of
coherent optical and acoustic
phonons
•Intrinsic couplings for each state
(femto)
•Extrinsic coupling for trapped state
(CW – Raman, PL)
29
2/13/2012
Hot exciton relaxation dynamics:
Hot exciton relaxation dynamics:
Exciton-phonon coupling
Weak coupling to phonons
How to observe biexcitons
Fundamental dynamics connects structure & function
Excitonics:
(PRB 2006, JCP 2008, PRB 2009)
XX
2
OD0
OD
X  (1)  OD0
ODNL
X
1
XX  ((3))  ODNL
0
0
-1
Energy
30
2/13/2012
Future work
•
Exploring excitonics with novel probes:
•
Relating excitonics to design principles for
devices:
–
–
Excitonics:
Fundamental dynamics connects structure & function
2D Spectroscopy, coherent control, development
photovoltaics, piezoelectrics, optoelectronics (laser,
LED, THz),
NOPA
shaper
p 1
 ~ 100 nm
shaper 2
sample
Excitonics:
Excitonics:
Fundamental dynamics connects structure & function
Fundamental dynamics connects structure & function
31
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Excitonics:
Excitonics:
Fundamental dynamics connects structure & function
Fundamental dynamics connects structure & function
Excitonics:
Fundamental dynamics connects structure & function
32