1. ITOH Lab. Hiroaki SAWADA
Intraband transitions in semiconductor nanocrystals P. Guyot-Sionnest and M. A. Hines Appl. Phys. Lett. 72, 686 (1998) and P. Guyot-Sionnest et al/ Phys. Rev. B 60, R2181 (1999)
(references)
ITOH Lab Hiroaki SAWADA

2. Abstract
Intraband transition of one-electron confined in CdSe quantum dots has been observed by infrared pump-probe spectroscopy.
These report ・
The transition energy depends on dot-size. ・
The time profile of transient absorption is influenced by surface modifications of the quantum dots.

3. Contents Introduction Quantum dot, Quantum-size effect Motivation
Experiment
(1)Size-dependence of transient absorption
(2)Time evolution of transient absorption
Summary

4. Quantum dot
A quantum dot is a nanometer-sized semiconductor. It consists of 103~106 atoms.
Quantum effects appear due to three dimensionally confined electrons.
The energy levels are discrete.
bulk
DOS E
DOS
well E
Quantum well
bulk
DOS
wire E
Quantum wire
Quantum dot
DOS
dot E

5. Quantum size effect
Confined electrons have higher energy than those in bulk crystal, and it depends on dot size. The energy shift is derived by
n: principal quantum number
h: Planck constant
m: effective mass
a: dot radius .
size
energy

6. Quantum confinement effect
Consider the effect on an exciton in a spherical dot.
(exciton:an electron-hole pair combined by Coulomb force)
Weak confinement
Strong confinement
aB≪a
aB：Bohr radius
a:dot radius
aB≫a
excited state
lowest state
excited state
lowest state
lowest state
electron
excited state 2a
hole 2a
Motions of electron and hole are confined individually.
Center-of-mass motion is confined.

7. Applications of quantum dot
Quantum dots show interesting optical properties
and are expected to be used for many optical devices.
For example
Quantum dot laser
Electron-hole pair confinement leads to the efficient recombination.
Superior lasing efficiency over existing devices
2. Optical switch
The network communication carrier shifts from electric to optical.
Large optical nonlinearity of quantum dot realizes optical switch.

8. Motivation Intraband transition
･･･
electronic transition in conduction band (1S-1P transition etc)
Intraband transition energy (1S-1P etc) in quantum dot exists in infrared region, and the energy depends on the dot size.
Infrared laser etc.
In these reports
Authors clarify the details of intraband dynamics of electrons in quantum dots.

9. Transient infrared absorption
To study intraband transition in quantum dots
Transient infrared absorption by pump-probe spectroscopy is useful.
Pump-probe spectroscopy
conduction band
excited state
By using two beams, we can observe the intraband transition that cannot be observed with the single beam.
lowest state
pump beam
probe beam
lowest state
valence band

10. Colloidal CdSe quantum dot
CdSe colloids are the best characterized semiconductor quantum dots in the strong confinement regime.
colloid method
(ref) C. B. Murray et al/ J.Am.chem.Soc. 115,8706(1993)
CdSe quantum dot is produced by colloid method.
Me2Cd＋TOPSe
in TOP
TOP:Tri-n-octylphosphine
Me:methyl
<1sec
CdSe-TOPO
TOPO 360℃

11. Experiment Ⅰ Three CdSe samples CdSe The diameters are ・31.5Å ・38Å
・43Å
CaF2 window
size distributions of samples
31.5Å:7%
38Å:7%
43Å:12%
chloroform
Optical densities at 532 nm are adjusted　between 0.5 and 1.5 .
1mm

12. Absorption spectra size energy
Absorption spectra of the three colloids in chloroform
545nm
516nm
566nm
size
energy
The dot sizes can be estimated from the peak energy.

13. Time evolution of transient absorption
Experimental setup
IR probe beam
About 1μm～
VIS pump beam
532nm
sample
detctor
delay
Time evolution of transient absorption

14. Infrared absorption spectra
Visible-induced infrared absorption of the colloids.
:43Å
:38Å
:31.5Å
The dotted lines are Gaussian fits.
exp-(E-E0)2/2δE2
E0:peak energy
δE:spectral variance δE
Absorption energy increases with decreasing the dot radius. E0

15. Experimental transition energy
Experimental transition energy(solid square)
:the 1Se–1Pe transition neglecting the Coulomb interaction.
:considering the Coulomb interaction to a 1S hole state.
:considering the Coulomb interaction for Se -Pez and Se -Pexy transitions with the hole at the pole.
a one-electron transition with the hole being rather localized, possibly as a surface trap
The vertical error bars
:±δE
The horizontal error bars
:±r0(δE/2E0)

16. Time evolution of IR absorption
Time evolution of the visible-induced IR absorption.
:43Å
:38Å
:31.5Å
All samples exhibit qualitatively similar behavior.
Fast(20~60ps) and slow decays are observed.

17. Experiment Ⅱ New CdSe samples Capping molecules ・TOPO ・thiocresol
・pyridine
Sapphire window
CdSe nanocrystal
3.5nm and 4.3nm
Liquid
Optical densities at 532 nm are adjusted　between 1 and 2 .
400μm

18. IR absorbance change Ⅰ For different surface treatment at 1mJ cm-2
IR absorbance change α as function of delay of the pump visible beam
：thiocresol-capped　in Paeptamethylnonani
：TOPO-capped　in Paeptamethylnonani
：Pyridine-capped　in pyridine solvent
at 1mJ cm-2
thiocresol
deep S hole traps
Fast decay increases.
TOPO
shallow Se hole traps
pyridine
charge-separated complex

19. electron-electron Auger interband relaxation
IR absorbance change Ⅱ
Excitation density dependence
IR absorbance change α for a chloroform　solution of thiocresol-capped CdSe
:at 6 mJ cm-2
:at 3 mJ cm-2
:at 1 mJ cm-2
Saturation of the electron density of 1S state
fast decay
slow decay
electron-electron Auger interband relaxation
one electron per dot

20. Summary
How transient infrared absorption measurement can be used to study intraband transition in quantum dots is demonstrated.
The size dependence of intraband transitions have been measured in strongly confined CdSe quantum dots.
The surface modifications of the quantum dots effect on the coupling of the electron with hole.