Title: Single Quantum Dot Optical Spectroscopy
1Single Quantum Dot Optical Spectroscopy
- Presented by
- Rohini Vidya Shankar
- Amrita Urdhwareshe
2Motivation
- 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
3Observed quantum dot emission
- Optical spectra of 35 Ao CdSe nanocrystals no
discrete lines, even at low T - Ref 1
4Inhomogeneous broadening
- Ensemble averaging of optical properties
- Need to take single dot spectra
5Experimental techniques
- Samples of single quantum dots to look at
- Chemically prepared and spin coated on substrates
- Usually II-VI semiconductors. E.g. CdSe, PbS,
CdS, etc. - Particle size 10-100 Ao
- Core-shell quantum dots
- E.g. CdSe coated with ZnS or CdS, etc.
- Particle size 10-100 Ao
- Epitaxially deposited
- Usually III-V semiconductors. E.g. GaAs, InGaAs,
AlGaAs, etc. - Particle size 10-40 nm
6Experimental techniques (contd.)
- Optical techniques used
- Far-field epifluorescence microscopy/spectroscopy
- Near-field optical spectroscopy
7Far-field epifluorescence spectroscopy
- Light focused and collected using the same
objective - Both images and spectra obtained by switching
between a mirror and a diffraction grating - Need low areal densities one quantum dot per
µm2
8Far field images and spectrum
- A) Image of single CdSe 45 Ao 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)
9Near field optical spectroscopy
- Low temperature nano-probing system based on
shear-force distance regulation. - Near field excitation of the sample and
near-field collection of the luminescence - Useful for quantum dot areal densities of the
order of 100/µm2
10Near-field imaging
- Near-field luminescence image of a single
In0.4Ga0.6As/Al0.5Ga0.5As QD (T 5 K) (Ref 3) - Quantum dot emits light in a narrow band centered
at a wavelength of 733nm
11Observations
- Same 35 Ao CdSe spectra (Ref 1) dotted lines
show ensemble measurement. Solid lines single
quantum dot measurement - Narrow peakwidth at low T!
12Observations
- Ensemble vs single CdSe nanocrystal spectra (Ref
2) - Ensemble spectrum average of many single
nanocrystal spectra - Shift in energy peaks with average nanocrystal
size
13Fluorescence blinking
- On/off nature of fluorescence spectra (Ref 4)
- Typical on-off timescale .5 sec.
- Not observed for ensembles
14Blinking (contd.)
- On times dependent on excitation intensity
- Vary inversely as excitation intensity
- Off times Independent of excitation intensity
- Proposed explanation
- Photo ionization of nanocrystals
- Also possibly, thermally activated charge
trapping
15Spectral diffusion
- Different lineshapes for different nanocrystals
- Excitation intensity and integration time
dependent linewidths - Spectral diffusion result of locally changing
electric fields - Possibly correlated to fluorescence intermittency
Ref 2
16Spectra of capped nanocrystals
- Capping materials higher bandgap semiconductors
- Highly enhanced quantum yield of spectra (as high
as 50) - Red shift of the emission peak
- Decreases intermittency to a timescale several
seconds to few minutes
17Polarized photoluminescence studies
- Narrower linewidth enables precise measurements
of luminescence character - Information about the spin-related effects such
as Zeeman splittings. - Relaxation processes in single GaAs/InAs quantum
dots studied using polarized photoluminescence
(PL) spectroscopy in an external magnetic field
18Unpolarized and Polarized Spectra
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)
19Summary
- Need to observe single quantum dot spectra
- Techniques of sample preparation and spectrum
acquisition - Salient features of the spectra
- Narrow linewidths
- Size dependence of emission peaks
- Blinking/intermittency
- Spectral diffusion
- Polarization dependence
20Potential applications
- DNA and protein labeling
- Highly luminescent single quantum dots can
overcome the functional limitations encountered
with chemical and organic dyes - Easily tunable emission wavelength by changing
the particle size or composition - Optical coherence tomography using quantum dots
- Quantum-dot-based super-luminescent
light-emitting diodes - High-bandwidth high-power light sources
- Spectra of these devices can be largely tuned
21References
- 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, Advanced
Materials 11, No. 15, 1243-1256 (1999) - 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, Physical Review B 58 No. 16, R10
147-R10 149 (1998)
22 Thank You!