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ROYAL INSTITUTEOF TECHNOLOGY
Introduction to Quantum Dots andSingle Quantum Dot Spectroscopy
Jan Linnros
Materials Physics, ICT School, KTHKista
ADOPT Winter School 2012
200 nm
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Aim of lecture
Short review of physics of quantum dots
What can be learnt fom single quantum dot spectroscopy?
Our own work on silicon QDs – compared to direct bandgap QD’s
BenNiklas
Ilya Jan VMahtab(Fatemeh)
Acknowledgements:
Funding: Swedish Research Counsil (VR), VR project + Linné centre
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Outline
IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si
TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation
Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots
Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements
EpilogueSummaryApplications
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Ene
rgy
Atom Bulk semiconductor
Quantum dot – ‘artificial atom’
λλλ
kT
e-
h+
Quantum dot
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Metals - semiconductors
Density of states
Ene
rgy
Fermi level
Density of states
Ene
rgy
Fermi level
Conductionband
Valenceband
Metal Semiconductor
When size is reduced quantized states begin to form at band edges. This means that a semiconductor nanocrystal forms descrete states while a metal particle still has a very high density of states near Fermi level. Thus, metals turn to quantum dots only at very small sizes (< 1 nm)
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Quantum confinement
Size - d [nm]B
and
gap
[eV
]Quantum dot
Effectivebandgap
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Photon emission/absorption – quantum size effect
2.0
nm
2.4
nm
2.8
nm
3.2
nm
4.1
nm
5.0
nm
Diameter
Bulkbandgap
Emission Absorption
Murray, Norris, Bawendi, MIT
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’zoo’ of nanocrystals/QDs
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Nanocrystals
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• In(Ga)As – grown on GaAs• Lattice mismatch ~7.2 %• Compressive strained layers
- up to a few monolayers• Relaxation => island growth• Size: lateral: 10 – 20 nm; height: 5 – 8 nm
GaAs
InAs
Stranski-Krastanov growth: In(Ga)As QDs
ROYAL INSTITUTEOF TECHNOLOGYKouwenhoven et al., 1991
Quantum dot defined by top gates
Gates 1,2,3,4 are reverse-biased. This forms restrictions inan underlying 2-dimensional electron gas (GaAs layer embedded in AlGaAs). Thus, a quantum dot is formed below electrodes!
Quantum dot(in bured layer)
Top electrodes
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Porous silicon
Leigh Canham 1990, APL
Porous Si structureTEM, from Cullis et al.
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Outline
IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si
TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation
Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots
Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements
EpilogueSummaryApplications
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Schrödinger eq. for quantum dot
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Schrödinger eq. for quantum dot
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Energy states in a spherical well
Gaponenko ”Optical properties ofsemiconductor nanocrystals, Cambridge
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Exciton
Excitation
Luminescence
EC
EV
k
E
Coulombattraction
=>Bindingenergy!
Excitation of electron-hole pair: Exciton
-
+Exciton:
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Exciton – weak/strong confinement
-
+
-
+
-
+a aB
Weak confinement Strong confinement
a > aB
with exciton Bohr radius: aB ~ εħ2 / µe2
where µ is the electron-hole reduced mass:µ-1 = me*-1 + mh*-1
a < aB
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Exciton energy
,MR2n
RyEE 2
2ml
2
2
*
gnmlχ
+−=h
mlχ
B
2*
a2eRyε
= .**he mmM +=
The energy of an exciton in the weak confinement regime is:
where are roots of the spherical Bessel functions (n – number of the root, l – order of the function)Ry* is the exciton Rydberg energy given by:
, and
Thus, the exciton levels in the quantum dot are characterized by the quantum numbers n, m and l.
Weak confinement: R > aB
Strong confinement: R < aB
The confined electron and hole in such a case have no bound states correspondingto the hydrogen-like exciton. Then, position of energy levels become:
.R2
EE 2nl2
2
gnl χμ
+=h
1*1*1 −−− += he mmμwhere is the electron-hole reduced mass.
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Quantum confinement
r=0.7 nm
Exciton Bohr radius: red arrowsGaponenko ”Optical properties ofsemiconductor nanocrystals, Cambridge
Si
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Surface states - passivation
”dangling bonds”
”core shell quantum dot”
Quantum dot surface
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Recombination mechanisms
Augereeh(hhe)
hν
Radiative
ET
SRH:Defect/impurity
EC
EV
Shockley-Read-Hallrecombinationdominating in Si, Ge etc(ET= trap state energy)
dominates at highcarrier densities(e.g. in lasers)
common indirect-bandgapsemiconductors =>LEDs, lasers
In a perfect nanocrystal at low excitation
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”Phonon bottleneck” ???
Bulk – conduction band Quantum dot – descrete states
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Outline
IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si
TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation
Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots
Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements
EpilogueSummaryApplications
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Need for single quantum dot studies
Energy
Ensemble ofnanocrystals
Inhomogenous broadening!
Energy
Single nanocrystal
Sharp linewidth
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Techniques to probe single QD
Adapted after M. Sugisaki in’Semiconductor quantum dots’, Springer 2002
quantumdots
substrate
microscope SNOM metal mask mesa etching tip-enhanced
fiber
lens
(dot spacing > λ)
AFM metal tip
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Empedocles, Norris, BawendiMIT, PRL 77, 3873, -96
Width lessthan kT !
Ensemble average
Single dot
Spectroscopy of single CdSe nanocrystals
Energy (eV)
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To see individual quantum dots....
disperse QDs > ~1 um
use high numerical aperture microscope lens
in cryostat – window corrected lens
filter out excitation light and background fluorescence
cooled CCD camera (< -90 C)
mechanically stable system
You need to:
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The unexpected.....
On/Off blinking
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Single CdSe nanocrystals
Shimizu et al.MIT, PRB 2001
Blinking
on
off
On/Off blinking observed in:
• II-VI nanocrystals• single molecules• in III-V Q-dots - rarely
Time
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The unexpected.......
spectral diffusion
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Empedocles, Norris, BawendiMIT, PRL 77, 3873, -96E=0
Spectraldiffusion Conclusions:
• Randomly oriented local fieldschanging over time
• Increasing by high excitation
• Charges at the dot surface?
+ -
E
E in kV/cm
Starkshift
CdSe nanocrystals: Spectral diffusion
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high excitations
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Bayer et al.Nature -00
In0.6Ga0.4As self-assembled dots on GaAs
Single QD
Multi-excitonic emission
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Bayer et al.Nature -00
In0.6Ga0.4As self-assembled dots on GaAs
Single QD
Multi-excitonic emission
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STM of single quantum dots
Martin Janson, NANOSPECTRA2004
Wave-Function Mapping of Strain-Induced InAs QDsSTS imaging - |ψ|2(x, y, V)
0.8 0.9 1.0 1.1 1.2 1.3 1.4
01234567
dI/d
V [a
rb. u
nits
]
sample voltage [V]
QD wetting layer
STS 1.14 V
(100)
STS 0.89 V
(000)
Martin Janson, NANOSPECTRA2004
Wave-Function Mapping of Strain-Induced InAs QDsSTS imaging - |ψ|2(x, y, V)
14 nm
STM H=9.4 nm
14 nm1.0 1.2 1.4 1.6 1.8 2.0
0.0
0.2
0.4
0.6
0.8
Vstab=2.4 VIstab=70 pAdI
/dV
[arb
. uni
ts]
sample voltage [V]
ROYAL INSTITUTEOF TECHNOLOGYU. Banin et al. Nature 1999
Size evolutionSTM I-V curve
InAs QD: atomic-like electronic states
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Outline
IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si
TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation
Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots
Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements
EpilogueSummaryApplications
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fabrication – our approachto see individual Si QD’s
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Single Si quantum dots by lithography
Fabrication1. e-beam lithography2. plasma etching3. oxidation (self limiting)4. PL?5. repeat 3 & 4.....
35 Kpillars0D dots?
walls - 1D lines?planes - 2D quantum wells?
SOI wafer
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Pillar array – PL image
White light reflection image(arrays of 30x30 pillars)
PL imagePumping: 325 nm cw (HeCd)
Each single light source can be traced down to a certain pillar
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200 nm
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Some structures after oxidation
100 nm
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Make one or two dots?
Schematic of a wall with varying thickness (repeated continuously along line):
Oxi
datio
n
Double dots
Single dots
Bruhn et al, PSS (2010)
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New setup for combined AFM/PL
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Single-dot spectra – room temperature
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35 K => 3 meV
Width less than kT!
Descrete lines!– Quantum dot!
Sychugov et al. PRL (2005)
Low temperature spectra
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Narrow spectra - phonons
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k-conservation relaxed
Si
Phononline (TOL)
Zerophononline (ZPL)
Δx • Δp ≥ h
Silicon nanocrystals– still indirect bandgap
CdSenanocrystals
EmpedoclesPRL 1996
Sinanocrystals
SychugovPRL 2005
TO
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Time resolved imaging
Delay: 0 µs 5 µs 10 µs 15 µs
100 µs
20 µs
Delay
5 µs
Laser modulation
Image intensifier
on
off
open
shut
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Lifetimes
τ ~30 µs
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Summary – single dot studies on Si
100 excitons/nanocrystal??
1 exciton/nanocrystal
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On/Off blinking
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Blinking
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Blinking - stability
Data for one Si nanocrystal
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Model for blinking
-
+
-
+
QD trapstates
Light state
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Model for blinking
-
+
-
+
QD trapstates
-+
-
+
Dark state
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Model for blinking
-
+
-
+
QD trapstates
Light state
P(t) = A * t –α
with α = 1.5
For Gaussiantrap distribution:
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Blinking at high excitation
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Supression of blinking using graded shell
Wang et al.Nature 2009
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Outline
IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si
TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation
Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots
Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements
EpilogueSummaryApplications
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Comparison - quantum dots
ZnSe nanocrystals InAs quantum dots Si quantum dots
type of bandgap
lifetime
on/off blinking
quantum efficiency
tuning
direct
ns - range
YES
10 – 60 %
visible
direct
ns - range
NO (rarely)
?
IR
indirect
10 µs - range
YES
10 – 80 % ?
visible - IR
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Summary – single dot spectroscopy
excellent for studying the physics of quantum dotshas revealed new phenomenaindividual variation
But…..
rich scenario of individual properties may exist:size, shape, stress, passivation, local environment
=> large number of dots must be analyzed forproper conclusions
Silicon quantum dots:
Similar physics as direct bandgap QD – less intenseHigh quantum efficiencies?Nontoxic
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• Labelling of (bio-) molecules(fluoresence tagging)
• Single-photon source:Quantum cryptography
• Luminescent devicesLED, laser
• ”Dots in well” infrared detectors
• Phosphor for new lighting
Time
Ideal singlephoton source
Low thresholdcurrent
Applications of quantum dots
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Rhodamine Green Dextran
CdSe quantum dot
Dubertret et al.Nature -02
Bleaching – QD’s vs dyes
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nano silicon image gallery
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nanosilicon group
seniors
PhD students
M.Sc. students
Jan Linnros(prof)
Ilya Sychugov(ass prof)
Torsten Schmidt(postdoc)
Benjamin Bruhn ’Mahtab’ Sangghaleh
Roodabeh Afrasiabi Yashar Hormozan Karolis Gulbinas
Viktor Tullgren Fatjon Qejvanaj
Miao Zhang(starting March)
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Thank you for your attention!
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Bibliography
Books:’Optical properties of semiconductor nanocrystals’
S.V. Gaponenko (Cambridge Univ. Press 1998)’Semiconductor quantum dots: Physics, spectroscopy and applications’
Y.Masumoto, T. Takagahara (Eds) (Springer 2002)’Quantum dots’ L.Jacak, P. Hawrylak, A. Wojs (Springer 1998)‘The physics of low-dimensional semiconductors’
John Davies (Cambridge 1998)
Articles:‘Semiconductor clusters, nanocrystals and quantum dots’ A.P. Alivisatos, Science 271, 933 (1996)‘The use of nanocrystals in biological detection’ A.P. Alivisatos, Nature biotech.22, 47 (2004)‘Biologists join the dots’ News feature, Nature 413, 450 (2001)‘Artificial atoms’ M.A. Kastner, Physics Today January 1993, p 24‘Electrons in artificial atoms’ R.C. Ashoori, Nature 379, 413 (1996)‘Identification of atomic-like states in……’ U. Banin et al., Nature 400, 542 (1999)‘The structural and luminescence properties of porous silicon’
Cullis, Canham, Calcott, J. Appl. Phys. 82, 909 (1997)‘Overview of fundamentals and appl. of electrons, excitons and photons in confined structures’
C. Weisbuch, H. Benisty, R. Houdre, J. of Luminescence 85, 271 (2000)‘Excitons in nanoscale systems’ G.D. Scholes, G. Rumbles, Nature Materials 5, 683 (2006).
Home page:Evident technologies (selling QD’s): http://www.evidenttech.com/