Transcript Intraband transitions in semiconductor nanocrystals P

Intraband transitions in
semiconductor nanocrystals
(references) 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)
ITOH Lab.
Hiroaki SAWADA
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.
Contents
• Introduction
Quantum dot, Quantum-size effect
• Motivation
• Experiment
(1)Size-dependence of transient absorption
(2)Time evolution of transient absorption
• Summary
Quantum dot
bulk
DOS
well
DOS
wire
DOS
dot
DOS
• 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.
Quantum well
bulk
Quantum wire
Quantum dot
E
E
E
E
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
2
2
nh
E 
2
8m a
.
n: principal quantum number
h: Planck constant
m: effective mass
a: dot radius
size
energy
Quantum confinement effect
Consider the effect on an exciton in a spherical dot.
(exciton:an electron-hole pair combined by Coulomb force)
Weak confinement
aB≪a
Strong confinement
aB≫a
aB：Bohr radius
a:dot radius
excited state
lowest state
excited state
lowest state
lowest state
excited state
electron
hole
2a
2a
Motions of electron and hole are
Center-of-mass motion is confined. confined individually.
Applications of quantum dot
Quantum dots show interesting optical properties
and are expected to be used for many optical devices.
For example
1. 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.
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.
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
lowest state
pump beam
probe beam
lowest state
valence band
By using two beams, we can
observe the intraband transition
that cannot be observed with the
single beam.
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
TOPO 360℃
CdSe-TOPO
Experiment Ⅰ
Three CdSe samples
CaF2
window
CdSe
The diameters are
・31.5Å
・38Å
・43Å
size distributions of samples
31.5Å:7%
38Å:7%
43Å:12%
chloroform
1mm
Optical densities at 532 nm are
adjusted between 0.5 and 1.5 .
Absorption spectra
Absorption spectra of the three colloids in chloroform
545nm
566nm
516nm
size
energy
The dot sizes can be
estimated from the peak
energy.
Experimental setup
IR probe beam
About 1μm～
VIS pump beam
532nm
sample
delay
Time evolution of
transient absorption
detctor
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
δE
E0:peak energy
δE:spectral variance
Absorption energy increases
with decreasing the dot radius.
E0
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.
:±δE
The vertical error bars
The horizontal error bars :±r0(δE/2E0)
a one-electron transition with
the hole being rather localized,
possibly as a surface trap
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.
Experiment Ⅱ
New CdSe samples
Sapphire
window
Capping molecules
・TOPO
・thiocresol
・pyridine
CdSe nanocrystal
3.5nm and 4.3nm
Liquid
Optical densities at 532 nm are
adjusted between 1 and 2 .
400μm
IR absorbance change Ⅰ
For different surface treatment
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
TOPO
shallow Se hole traps
pyridine
charge-separated complex
Fast decay increases.
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
fast decay
electron-electron Auger
interband relaxation
slow decay
one electron per dot
Saturation of the electron
density of 1S state
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.