1. 光誘起キャリア緩和ダイナミクスおよびその偏光特性
13aWF-7
単層カーボンナノチューブにおける
光誘起キャリア緩和ダイナミクスおよびその偏光特性
Relaxation and polarization dynamics of photo-induced carriers in individually-suspended single-walled carbon nanotubes
Y. HashimotoA. B, A. SrivastavaB , J. KonoB, J. ShaverC, V.C. MooreC,
R.H. HaugeC, R.E. SmalleyC
Graduate School of Chiba Univ.A, ECE Dept., Rice Univ.B, Chemistry Dept., Rice Univ.C
Out line
Introduction & Purpose
Sample and Experimental setup
Results and discussion
Photo-induced carrier dynamics
Polarization memory
Summary
I would like to present my study of Long…
This is the brief our line of this presentation.
First, we will have an introduction and purpose.
Next, the sample and experimental setup is shown.
The experimental results will be discussed.
At last, I would like to summarize this study.
Supported by TATP and Welch Foundation

2. Single-Walled Carbon Nanotubes photo-induced carrier lifetimes
Hertel and Moos, Phys. Rev. Lett. 84, 5002 (2000)
Chen et al., Appl. Phys. Lett. 81, 975 (2002)
Han et al., Appl. Phys. Lett. 82, 1458 (2003)
Lauret et al., Phys. Rev. Lett. 90, (2003)
Korovyanko et al. Phys. Rev. Lett. 92, (2004)
< 1 ps Bundled SWNT
ps Isolated SWNT
G. N. Ostojic et al., Phys. Rev. Lett. 92, (2004)
Y.-Z. Ma et al., J. Chem. Phys. 120, 3368 (2004)
A. Hagen et al., Appl. Phys. A 78, 1137 (2004)
F. Wang et al., Phys. Rev. Lett. 92, (2004)
L. Huang et al., Phys. Rev. Lett. 93, (2004)
So far, there have been many reports about photo-induced carrier dynamics in bundle and isolated carbon nanotubes.
In the bundled SWNT, the carrier life time is extracted as less than 1 ps.
In the case of the isolated SWNT, the carrier life time get longer to 20 ps.
However, theoretical study predicts the radiative decay time of photo-induced carrier in SWNTs as 20 ns.
Discrepancy between the theoretical study and experimental result have not been clarified.
This study reports the long-lived carrier relaxation dynamics in single-walled carbon nanotubes which persist longer than 1ns.
~ ns Isolated SWNT
This work
~ 20 ns Theoretical C. D. Spataru et al., cond-mat/ v1 (2003)

3. Relaxation Dynamics of Photo-excited Carriers in SWNTs
Tube-tube interaction
Catalyst particles at the tube ends
Nonradiative recombination via surface defects
etc.
Radiative
Non-radiative
~ ps
~ ns
What kind of the Non-radiative relaxation is taking place ?
Transient absorption
t ~ 10 ps 1 – 30 mJ/cm2 (0.89eV) Phys. Rev. Lett. 92, (2004)
0.06 – 5.7 mJ/cm2 J. Chem. Phys. 120, 3368 (2004)
Time resolved fluence
t ~ 7 ps mJ/cm2 Estimate the radiatibe relaxation time as 110 ns
Phys. Rev. Lett. 92, (2004)
~640 e-h pairs in
1 mm SWNT
PRL 92, (2004)
~1 mJ/cm2
PRL 92, (2004)
Exciton-exciton interaction ?
First, we would like to discuss the carrier relaxation dynamics.
If the photo-induced carrier relax by two decay process; one is radiative and another is non-radiative decay process, the observed decay will be written as this.
If the non-radiative decay time is much faster than radiative decay time, then the observed decay will be dominated by the non-radiative decay process.
Thus we can attributed the previously reported fast decay time of 20 pico-second in isolated SWNT to the non-radiative decay process.
Then, what kind of the non-radiative decay process is taking place ?
Here, I would like to estimate the exciton density excited in carbon nanotubes in the previous study.
The typical excitation density was estimated as 1mJ/cm2.
This means 640 electron-hole pairs are excited in 1 micro-meter carbon nanotube.
Then, the mean distance between the excitons are estimated as 1.5 nm.
This length is shorter than the exciton diameter in SWNT predicted in the theoretical study.
Thus, we attributed the previously observed fast decay time as non-radiative decay via the exciton-exciton interaction.
Therefore in this study, we investigated the photo-induced carrier relaxation dynamics in low excitation limit.
In present study, the carrier density is extracted as 1 e-h pair per 1 micro-meter SWNT.
t ~ 10 ps 1 – 30 mJ/cm2 (0.89eV) G. N. Ostojic et al., Phys. Rev. Lett. 92, (2004)
t ~ 0.06 – 5.7 mJ/cm2 Y.-Z. Ma et al., J. Chem. Phys. 120, 3368 (2004)
A. Hagen et al., Appl. Phys. A 78, 1137 (2004)
t ~ 7 ps mJ/cm2 F. Wang et al., Phys. Rev. Lett. 92, (2004)
Purpose
Photo-induced carrier relaxation dynamics
in the low excitation limit
1 e-h pair per 1 mm SWNT

4. Single-Walled Carbon Nanotube Samples
Absorption spectrum
Raman spectrum
SDS miscelled SWNT
SWNT
The sample is SDS micelled SWNT.
The absorption spectrum is shown in this graph and shows sharp peaks.
This indicates that the SWNTs are well isolated.
Absorption shows sharp peaks
Excited SWNTs are
(12,5), (12,1), (11,3)
(10,5), (9,8), (9,7)
SWNT is well isolated
SDS micelle
Science VOL (2002)

5. Laser wavelength: 1.550 eV (E2H2)
Experimental Setup
Laser wavelength: eV (E2H2)
Pulse picker
80 MHz  800kHz
Ti:S laser
80MHz
Delay stage (2 ns)
Aperture
SWNT
Si detector
l / 2
Lock in
The excitation source is a mode-locked Ti:s laser with a repetition rate of 80MHz.
To avoid the sample heating, an EO modulator is used.
The EO modulator changed repetition rate from 80MHz to 800kHz.
The standard pump and probe method is employed.
To investigate the polarization dependence, the polarization of the pump pulse is controlled by a half-wave plate.
The laser wavelength is fixed at 800 nm where the absorption spectrum shows a sharp peak.
The probe intensity is 1 tenth of the pump intensity.
The Excitation fluence is estimated as 100 nJ/cm2.
Excitation fluence: 100 nJ/cm2
Pump : Probe = 10 : 1

6. Checking the Experimental Setup
GaAs
Polarization of the
pump and probe pulse
We checked the experimental setup by using the GaAs sample.
If the pump beam spot is moved by changing the polarization, then the pump-probe signal should change.
The signal was not changed, thus the experimental setup is OK.
No difference

7. Photo-Induced Carrier Dynamics in SWNT in Low Excitation Limit
Room temperature
Repetition rate: 8 MHz
Polarization of the
pump and probe:
Previous reports
in high excitation
t < 120 ps
With this experimental setup, the transient absorption is observed.
The polarization of the pump and probe pulses is set to parallel polarization.
Surprisingly, the transient absorption shows a very slow decay which persist even after 1 ns.
This decay time is much faster than the previously observed fast decay of 20 ps.
We should note here that this signal does not depend on the repetition rate so this signal is not caused by thermal heating.
Next, we discuss the decay process.
Pump-probe signal exists even at 1 nano-second !!!

8. Decay Dynamics 1: t < 1 ps 2: t ~ 1 ns
The transient absorption shows two decay process of very different decay time of less than 1 ps and as long as 1 ns.

9. E2H2  E1H1 intraband transition
Decay Dynamics
E2H2  E1H1 intraband transition E E1
DOS E2 H2 H1
< 1 ps
~ ns
E1H1 carrier recombination
The fast decay of less than 1 ps is attributed to the inter-subband transition from E22 subband to E11 subband.
On the other hand, the long decay of 1 ns is attributed to the E11 carrier recombination.

10. Polarization memory exists even at 1 ns !!!
The red and blue line shows the transient absorption observed with parallel and perpendicular polarized configuration.
The polarization memory defined by this equation is calculated and shown as a function of time delay.
Surprisingly, the polarization memory persists even at 1 ns.
In bundled carbon nanotubes, the polarization memory decays in 10 ps.
This is the special feature of the isolated single-walled carbon nanotube.
Polarization memory exists even at 1 ns !!!
In bundled SWNT, the polarization decay time ~ 10 ps
O. J. Korovyanko et al., Phys. Rev. Lett (2004)

11. Pump pulse polarization
Polarization Memory
Pump
Absorption is reduced
No change
Pump
n  I pump cos2q q
Pump pulse polarization
Then I would like to discuss the meaning of the polarization memory 0.45.
First, I consider just one carbon nanotube case.
I should note here again that, in the isolated SWNTs, the absorption is taking place only for light polarized parallel to the tube axis.
In other words, no absorption is taking place for light polarized perpendicular to the tube axis.
Here I assumed the following case.
When the pump pulse is absorbed by the carbon nanotube, the absorption of the probe pulse polarized parallel to the tube axis is reduced.
On the other hand, for the probe pulse polarized perpendicular to the tube axis does not affected.
In the experimental case, the carbon nanotubes are oriented in random ways.
Then, not only the carbon nanotubes which orient to the pump polarization, but also the carbon nanotubes which orient to other ways are also excited.
Only the carbon nanotube, which orient to the perpendicular way to the pump polarization can not be excited.
Then, the distribution of the excited carbon nanotube is proportional to cosine theta square.
In addition, the distribution of the probed CNT by probe pulse is also proportional to the cosine theta square for parallel polarization case and to the sine theta square for perpendicular polarization case.
In this case, the observed transient absorption with parallel and perpendicular configurations are written as these equations.
The calculated P from these equations is 0.5.
This number is almost same value of observed 0.45.
As I showed here, I am interested in the polarization dependence of the carbon nanotubes.
Then, to investigate the polarization dependence of individually suspended SWNT is difficult because the carbon nanotube direct to random ways.
To investigate the polarization dependence of the carbon nanotube, it will be much better to use an aligned carbon nanotube.

12. Summary
The transient absorption of the isolated SWNTs in low excitation regime shows very fast (< 1 ps) E2H2  E1H1 intraband transition and very long (~ 1ns) E1H1 carrier recombination
The polarization memory in isolated SWNTs shows no decay even at 1 ns.
Future work
Two color pump-probe in the isolated SWNT
in the low excitation limit