Presented by
Rohini Vidya Shankar
Amrita Urdhwareshe

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Discrete atom-like states in 0 D quantum dots
Discrete exciton levels just below the bandgap
Quantum confinement effect for excitons
Ultra narrow transitions and spectra expected

Optical spectra of 35 A o CdSe nanocrystals: no discrete lines, even at low T
Ref [1]

 Ensemble averaging of optical properties
 Need to take single dot spectra

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Samples of single quantum dots to look at
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Chemically prepared and spin coated on substrates
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Usually II-VI semiconductors. E.g. CdSe, PbS, CdS, etc.
Particle size ~ 10-100 A o
Core-shell quantum dots
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E.g. CdSe coated with ZnS or CdS, etc.
Particle size ~10-100 A o
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Epitaxially deposited
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Usually III-V semiconductors. E.g. GaAs, InGaAs, AlGaAs, etc.
Particle size ~ 10-40 nm

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Optical techniques used
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Far-field epifluorescence microscopy/spectroscopy
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Near-field optical spectroscopy

Far-field epifluorescence spectroscopy
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Light focused and collected using the same objective
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Both images and spectra obtained by switching between a mirror and a diffraction grating
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Need low areal densities ~ one quantum dot per µm 2

A) Image of single CdSe 45 A o nanocrystals at 10 K (Ref [2])
B) Image of the same region as in (A) with narrowed entrance slit
C) Spectrally dispersed image of the entrance slit in (B)

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Low temperature nanoprobing system based on shear-force distance regulation.
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Near field excitation of the sample and near-field collection of the luminescence
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Useful for quantum dot areal densities of the order of
100/µm 2

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Near-field luminescence image of a single In
0.4
QD (T = 5 K) (Ref [3])
Ga
0.6
As/Al
0.5
Ga
0.5
As
Quantum dot emits light in a narrow band centered at a wavelength of
733nm

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Same 35 A o CdSe spectra (Ref [1]): dotted lines show ensemble measurement. Solid lines: single quantum dot measurement
Narrow peakwidth at low T!

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Ensemble vs single CdSe nanocrystal spectra (Ref [2])
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Ensemble spectrum: average of many single nanocrystal spectra
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Shift in energy peaks with average nanocrystal size

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On/off nature of fluorescence spectra (Ref [4])
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Typical on-off timescale ~.5 sec.
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Not observed for ensembles

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On times: dependent on excitation intensity
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Vary inversely as excitation intensity
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Off times: Independent of excitation intensity
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Proposed explanation
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Photo ionization of nanocrystals
Also possibly, thermally activated charge trapping

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Different lineshapes for different nanocrystals
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Excitation intensity and integration time dependent linewidths
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Spectral diffusion: result of locally changing electric fields
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Possibly correlated to fluorescence intermittency
Ref [2]

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Capping materials: higher bandgap semiconductors
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Highly enhanced quantum yield of spectra (as high as 50%)
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Red shift of the emission peak
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Decreases intermittency to a timescale ~several seconds to few minutes

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Narrower linewidth enables precise measurements of luminescence character
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Information about the spin-related effects such as Zeeman splittings.
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Relaxation processes in single GaAs/InAs quantum dots studied using polarized photoluminescence (PL) spectroscopy in an external magnetic field

Typical unpolarized photoluminescence spectra from a single GaAs quantum dot ~20nm at various magnetic fields (Ref [5])
Luminescence spectra for all polarization geometries at 8 T (Ref [5])

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Need to observe single quantum dot spectra
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Techniques of sample preparation and spectrum acquisition
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Salient features of the spectra
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Narrow linewidths
Size dependence of emission peaks
Blinking/intermittency
Spectral diffusion
Polarization dependence

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DNA and protein labeling
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Highly luminescent single quantum dots can overcome the functional limitations encountered with chemical and organic dyes
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Easily tunable emission wavelength by changing the particle size or composition
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Optical coherence tomography using quantum dots
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Quantum-dot-based super-luminescent light-emitting diodes
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High-bandwidth high-power light sources
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Spectra of these devices can be largely tuned

[1] U. Banin, M. Bruchez, A. P. Alivisatos, T. Ha, S. Weiss and D. S.
Chemla, Journal of Chemical Physics 110 No. 2, 1195 – 1201 (1999)
[2] Stephen A. Empedocles, Robert Neuhauser, Kentaro Shimizu and
Moungi G. Bawendi,
(1999)
Advanced Materials 11, No. 15, 1243-1256
[3] A. Chavez-Pirson, J. Temmyo, H. Kamada, H. Gotoh, and H. Ando,
Applied Physics Letters 72, No. 6, 3494-3496 (1998)
[4] M. Nirmal, B. O. Dabbousi, M. G. Bawendi, J. J. Macklin, J. K.
Trautman, T. D. Harris and L. E. Brus, Nature 383, 802-804 (1996)
[5] Y. Toda, S. Shinomori, K. Suzuki and Y. Arakawa,
No. 16, R10 147-R10 149 (1998)
Physical Review B 58