Q. Shen1,2) and T. Toyodagcep.stanford.edu/pdfs/solar_workshop_10_04/SolarToyoda2004.pdf ·...
Transcript of Q. Shen1,2) and T. Toyodagcep.stanford.edu/pdfs/solar_workshop_10_04/SolarToyoda2004.pdf ·...
Photosensitization of nanostructured TiO2 electrodes with CdSequntum dots: effects of microstructure in substrates
Q. Shen1,2) and T. Toyoda1,2)
Department of Applied Physics and Chemistry1), and
Course of Coherent Optical Science2)
The University of Electro-Communications
Chofu, Tokyo 182-8585, Japan
(b) dye or semiconductor quantum dot sensitization toextend the photoresponse of TiO2 to the visible region.
Nanocrystalline TiO2 has received much attention in numerous fields of applications (e.g., photocatalysis, photoelectrochemical solar cell and gas sensor).
Dye-Sensitized Solar Cell (DSSC) with high energy conversion efficiency exceeding 10� has been developed by(a) preparation of highly porous, nanostructured TiO2
electrodes to increase the surface area;
BACKGROUNDSBACKGROUNDS
For higher efficiency, further research on the nanostructured TiO2 electrodes (morphology, optical absorption, charge injection, transport and recombination) is essential.
RESEARCH OBJECTIVESRESEARCH OBJECTIVES
TiO2 electrodes�Anatase-type TiO2 nanoparticles made from TiCl4 hydrolysis and oxidation processes.
�Composition of different size anatase-type TiO2 nanoparticles.
�Composition with rutile-type content on anatase-type TiO2nanoparticles.
�TiO2 nanotubes and nanowires.
�TiO2 photonic crystals.
SensitizerCdSe quantum dots.
Advantages of semiconductor Advantages of semiconductor quantum dots quantum dots
Quantum confinement allows for energygap tunable across the solar spectrum.
Large extinction coefficient resulting from quantum confinement.
Robust inorganic nature.
Large intrinsic dipole moment which maylead to rapid charge separation.
Impact ionization occurs with the possibility of high efficiency.
MOTIVATIONSMOTIVATIONSIn order to investigate morphological dependence,
two types of nanostructured TiO2 electrodes were prepared and CdSe quantum dots with various sizes were adsorbed on TiO2 as sensitizer.
TiO2 electrodes were characterized using SEM, XRD, photoacoustic (PA) and photoelectrochemical current (PEC) methods.
PA method is powerful for the investigation of optical absorption of optically opaque or scattering solid materials.
TiOTiO22 electrode preparationelectrode preparation
Nanocrystalline TiO2 powder (30 wt�)Pure waterAcetylacetone �10 wt%) Polyethylene glycol�PEG) (MW:500000)
(40 w�� vs TiO2)
A ( 15 nm)B ( 27 nm)
TiO2 paste
Stirring for one hour
Depositing TiO2 paste on FTO using squeegee method.
Heating in air at 450� for 30 min.Two types of TiO2
electrodes
Chemical solution deposition (CD) method of Chemical solution deposition (CD) method of CdSeCdSequantum dots on the TiOquantum dots on the TiO22 electrodeelectrode
S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338.
� 80mM CdSO4
� 120mM N(CH2COONa)3 (NTA)� 80 mM Na2SeSO3
mixed
(1:1:1)Chemical Deposition
(CD) Solution
Solution
TiO2
For some Time
Sample
CD Solution
1h 3h 5h 9h 23h 49h
SEM micrographs of TiO2 electrodesSEM micrographs of TiOSEM micrographs of TiO22 electrodeselectrodes
Size of TiO2 in the paste: 15 nm
500 nm
Electrode A
500 nm
Electrode B
Size of TiO2 in the paste: 27 nm
SEM micrographs of SEM micrographs of CdSeCdSe--quantum dots quantum dots adsorbed on TiOadsorbed on TiO22 electrodeselectrodes
�µm
CdSe 5h CdSe
�µm
CdSe 60h�µm
CdSe 13h CdSe
CdSe 28h
�µm
X-Ray diffraction patternsXX--Ray diffraction patternsRay diffraction patterns
25 30 35 40 45 50
C dSeC dSe
C dSe
FTOFTO
TiO 2
TiO 2TiO 2
C dSe / TiO 2/ FTO TiO 2/ FTO
(311)(220)
(111)
2θ (De gre e s )
Int
ensi
ty (
Arb
. U
nits
)
φβλ
cos9.0
=D
Scherrer equation:
D: CdSe quantum dot sizeβ: half-width of the
diffraction peakΦ: diffraction angleλ: 0.154 nm
40 41 42 43 44 45
CdSe/ TiO2/ FTO
(220)
2θ (Degrees)
Int
ensi
ty (
Arb
. U
nits
)
D�5-6 nm (CdSe120h)
Photoacoustic Spectroscopy ( PAS )
Light source : 300W xenon arc lampNormalization : carbon black sheet Wavelength range�250nm�800nmModulation frequency : 33Hz
Xe-Lamp0 5 0 0
Monochromator
Optical Lens
Microphone
Pre-Amplifier
Lock-in Amplifier
DATA
Computer
PA Cellsample
Chopper
1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.5
1.0
1.5A (TiO2: 15nm)
Wavelength (nm)
bulk 120h 23h 9h 5h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
EgE1
PA spectra of TiO2 electrodes A (TiO2:15nm) deposited with CdSe quantum dots for various times.
)2( 8
:ionapproximat mass Effective
2
2
1 aDa
hEEE g ==−=∆µ
0 20 40 60 80 100 1200
2
4
6
8
10
Deposition Time (Hours)
Dia
met
er (
nm)
Dependence of CdSe quantum dot size on deposition time.
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.00.0
0.5
1.0
1.5 B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PA I
nten
sity
(A
rb.
Uni
ts)
Photon Energy (eV)
900 750 600 450 300
PA spectra of TiO2 electrodes B (TiO2:27nm) deposited with CdSe quantum dots for various times.
Photoelectrochemical Current (PEC)Spectroscopy
Light source : 300W xenon arc lampNormalization : carbon black sheetMaterial of the PEC cell � quartzWavelength range�250nm�800nmElectrolyte : 1M KCl + 0.1M Na2S
AmmeterLock-in Amplifier����
Computer
Monochromator
Xe-Lamp1 2 3 4
PEC Cell
Pt
Sample
Chopper
Optical Lens
Electrolyte
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
100
200
300
400A (TiO2: 15nm) 120h
49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
PEC spectra of TiO2 electrodes A (TiO2:15nm)deposited with CdSe quantum dots for various times.
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
200
400
600
800B (TiO2: 27nm)
Wavelength (nm)
120h 49h 23h 9h 5h 3h 0h
PEC
Int
ensi
ty (
Arb
. U
nits
)
Photon Energy (eV)
750 600 450 300
PEC spectra of TiO2 electrodes B (TiO2:27nm)deposited with CdSe quantum dots for various times.
IPCE: incident photon-to-current conversion efficiency for monochromatic radiation
)cmW(photoflux (nm) wavelength)cmA(density nt photocurre nm)(eV 1240
numbersphoton incident numberselectron injectedIPCE(%)
2-
2-
⋅×⋅×⋅
=
=
µµ
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28A (TiO2: 15nm)
Wavelength (nm)
CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
IPCE spectra of TiO2 electrodes A (TiO2:15nm)deposited with CdSe quantum dots for various times.
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
1.5 2.0 2.5 3.0 3.5
0
4
8
12
16
20
24
28
Wavelength (nm)
B (TiO2: 27nm) CdSe120h CdSe49h CdSe23h CdSe9h CdSe5h CdSe3h CdSe0h
IPC
E (
%)
Photon Energy (eV)
800 600 400
IPCE spectra of TiO2 electrodes B (TiO2:27nm)deposited with CdSe quantum dots for various times.
SummaryTwo types of nanostructured TiO2 electrodes A (15nm) and B (27nm) deposited with CdSe quantum dots by chemical deposition, have been characterized using PA, PEC spectra and IPCE measurements.
Photosensitization by CdSe quantum dots was demonstrated and red shift of the spectra with increasing CdSe sizes can be clearly observed.
PEC and IPCE spectra in the visible region are quite different for the two types of TiO2 electrodes, even for the same deposition time of CdSe quantum dots.
The electron diffusion coefficient of the TiO2 electrode A was found to be two times larger than that of electrode B using tran-sient photocurrent responses.
PEC and IPCE in the visible region in the CdSe-sensitized TiO2 nanostructured electrodes largely depend on both the microstructure and electron transport property in the TiO2 electrodes as well as the size and quantity of CdSe nanoparticles .
Future studies:TiO2/CdSe interfacial property;Energy conversion efficiency; Ultrafast relaxation dynamics etc.
UltrafastUltrafast Carrier Dynamics of Carrier Dynamics of CdSeCdSe Quantum Quantum Dots Adsorbed on Dots Adsorbed on NanostructuredNanostructured TiOTiO2 2 FilmsFilms
The ultrafast carrier dynamics were investigated by using lens-free heterodyne detection transient grating (LF-HD-TG) technique.
One kind of optical pump and probe technique:
Measurements of the decay of excited carrier densities after pulsed injection.
LFLF--HDHD--TGTG methodmethod
By approaching a sample to the grating, 1: Grating excitation2: Heterodyne detection
Excitation of sample
Heterodyne detection
Reference
Signal
Detector
Probe
Detection of signal
→ Easy TG method without lenses (No daily setup)
Sample
Pump
Transmission grating
K. Katayama et al., APL 82, 2775 (2003).
Lens-free heterodyne detection transient grating technique ( LF-HD-TG )
Grating
Titanium/sapphire laserWavelength: 800 nm; Pulse width: 150 fs; Intensity: 5 µJ/pulse
Lock-in Amp.
Probe beam
Sample
Optical Delay Line
SHG
Chopper Advantages1) Easy and compact.2) High sensitivity. → Measurements under
low pump intensity.3) Suitable for rough surfaces
or optically scattering samples.
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0
Delay Time (ps)
LF
-HD
-TG
Sig
nal (
arb.
uni
ts)
LF-HD-TG response of nanostructured TiO2electrode without CdSe quantum dots.
0 10 20 30 40 50 60 70
0.0
0.2
0.4
0.6
0.8
1.0 5h 20h 44h 125h
LF-H
D-T
G S
igna
l (ar
b. u
nits
)
Delay Time (ps)
LF-HD-TG responses of nanostructured TiO2 electrodes B (TiO2: 27 nm) with CdSe quantum dots for different deposition times.
0 20 40 60 80 100
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Pump intensitySi
gnal
inte
nsit
y (a
rb.
unit
s)
Time (ps)
Pump intensity dependence of the LF-HD-TG response of nanostructured TiO2 electrode B (TiO2: 27 nm) deposited with CdSe quantum dots for 49h. The pump intensity is ranging from 2.5 to 16.5 µJ/pulse.
0 10 20 30 40 50 60 700.0
0.5
1.0
1.5
2.0
2.5
3.0
Sign
al in
tens
ity
(arb
. un
its)
Relative pump intensity (arb. units)
The pump intensity dependence of the maximum signal intensity for nanostructured TiO2 electrode B (TiO2: 27 nm) deposited with CdSe quantum dots for 49 h.
Analyses of LF-HD-TG response
21 /2
/10
ττ tt eAeAyy −− ++=
0 10 20 30 40 50 60 700.0
0.5
1.0
y0 0.11043 �}0.00715A1 1.00703 �}0.03463τ1 2.55526 �}0.12042A2 0.39276 �}0.00834τ2 27.48046 �}1.86301
cdse20h fit t ing
Tra
nsie
nt G
rati
ng S
igna
l
Delay Time (ps)
0 10 20 30 40 50 60 700.0
0.5
1.0 cdse20h fit t ing
Tra
nsie
nt G
rati
ng S
igna
l
Delay Time (ps)
0 10 20 30 40 50 60 700.0
0.5
1.0
Tra
nsie
nt G
rati
ng S
igna
l
Delay Time (ps)
Results of the Fitting Parameters
CdSedeposition time (h)
CdSesize (nm)
τ1(ps)
τ2(ps) A1 A2 y0
5 4.8 1.8 16 0.74 0.34 0.15
9 5.1 2.1 19 0.68 0.37 0.16
44 6.5 2.2 29 0.63 0.28 0.18
20 5.8 2.1 20 0.65 0.31 0.20
125 6.7 1.9 30 0.60 0.31 0.27
tr: trapped et: electron transferEg: energy gap
: excitation energynonradiativeradiative
υh
Eg υh
e-
h+
trtr
et
excitation
Energy
(1) Fast decay process (τ1 ): decrease of hole carrier numbers by trapping at the CdSe QD surface states;
(2) Slow decay process (τ2 ): photoexcited electron relaxation process, i.e. recombination and/or transfer to the TiO2 electrode;
(3) Base line (y0): thermal diffusion signal owing to thermal grating.
Summary
Three decay processes can be shown with decay time of 2 ps (τ1), 16-30 ps (τ2) and larger than a few hundreds ps, respectively.
Nanostructured TiO2 electrodes adsorbed with CdSe quantum dots by chemical deposition, have been characterized using LF-HD-TG measurements for the first time.
It is found that τ1 is independent of quantum dot size, but τ2 depends on quantum dot size.
AcknowledgementsD. Arae (now at Anritsu)M. Hayashi (now at CANON)I. Tsuboya (now at SONY)Y. KumagaiK. Katayama (University of Tokyo, now at MIT)T. Sawada(University of Tokyo, now at Tokyo University of Agriculture and Technology)
Financial supports:�A Grant-in Aid for Scientific Research (No. 14750645 and No. 15510098) and one on on Priority Area 417 (No. 15033224) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.�The 21st Century COE program on “Coherent Optical Science”.