PD Research Report for the 2018 year
Name（Research group）Li Wang
（Tamai Research Group, Graduate School of Science and Technology）
Research Theme

半導体ナノ微粒子系の多励起子素過程の解明と光応答分子システムへの応用

Research PeriodApril, 1st, 2017 ~ March, 31st, 2018

Semiconductor nanostructures are expected to provide multiexciton generation for photo-switchablesystems. In semiconductor-metal hybrid nanosystems, metal components may influence exciton
dynamics by charge separation. Carrier dynamics in metal and semiconductor nanostructures provide
basic information to design high efficiency photo-switch systems.
1. Biexcitonic spectral shift and quasi-type II carrier distribution in type I CdSe/CdS core/shell QDs
Transient absorption measurementsof CdSe/CdS core/shell quantum dots(QDs) were performed with state selective
excitation of CdSe core or CdS shell todeclare assignment of spectral species andmonitor distribution of charge carriers.
Two different CdSe/CdS core/shell QDs
with one− and two−monolayer (ML) CdS
shells were prepared from 2.9 nm
diameter CdSe core QDs. CdSe/CdS
core/shell QDs represented a quasi-type IIcarrier

distribution

accordingtored-shifted spectra, increased quantum

Figure 1. Absorption spectrum (a), transient absorption spectrawith shell (black) and core (red) excitation at 1 ns delay timenormalized at 575 nm (b), transient absorption spectra with core
excitation at 1 ps (black) and 1 ns (red) delay times normalized at
575 nm (c), and the carrier relaxation diagram with shell
excitation (d) of CdSe/CdS core/shell QDs with the lowest
excitonic state at 575 nm.

yields, and elongated luminescence lifetimes. However, a type I band alignment of the core/shell QDs
was supported by electron localized in CdSe core with core excitation while delocalized in the whole
QDs with shell excitation. By state selective excitations, overlapped spectral species of 1S(e)-1S 3/2 (h)transition of CdS shell and 1S(e)-2S 1/2 (h) of CdSe core became distinguishable, and biexcitonic spectral
was found to be existed in all transitions to 1S(e) electronic state. Elongated trapping time constants ofboth electron and hole were observed with core excitation, which was induced by tunnel effect throughenergy barrier, CdS shell. Our results by transient absorption spectroscopy with state selective excitationof compositions are useful to clarify band alignment and carrier distribution of heteronanostructures,
which could help to objectively extract charge carriers in photovoltaic applications.

2. Coherent acoustic phonon vibration of Au nanocubes, nanoctahedrons, and nanocuboctahedrons.

Metal nanoparticles with the unique property of localized surface plasmon resonance (LSPR)
provide tunable optical property, surface enhanced electromagnetic field, high frequency mechanicalresonance, and nano-sized heat source. Their applications have been explored as light collectors for solarcells, as efficient catalysts in photocatalysis, supplying enhanced electric fields for non-destructive
spectroscopies, as resonators for detecting mechanical properties of surrounding environments, and astarget heaters in photothermal therapies. Coherent acoustic phonon vibration can be detected when its
displacement is sensitive to the plasmon mode.
Coherent
vibrational

acousticmodephononof

nanopolyhedrons

Au

includingnanocubes, nano-octahedrons, andTable 1. Parameters for coherent acoustic vibrational modes of
Au nanocubes, nanooctahedrons, and nanocuboctahedrons.φ and f
are phase and frequency of vibrational mode. fFEM is vibrationalfrequency calculated by COMSOL with bulk properties. Q isquality factor of the mode.

nanocubeoctahedrons were analyzed.
As shown in Table 1, for modes I and II,theirphases

are

less

thanAumodeφ(π)

f(GHz)f FEM(GHz)Q(πτf)

I-0.02 ± 0.01

29.9 ± 0.6

27.8

16.7

II0.02 ± 0.03

46.8 ± 0.3

42.7

16.5

I0.1 ± 0.1

30.4 ± 0.2

30.8

19.7II0.2 ± 0.1

53.7 ± 0.3

52.3

23.1

III-0.3 ± 0.1

68.1 ± 0.3

-0.9

π/10

difference but π/2 out-of-phase formode III of Au nano-octahedrons.

Cube

Morover, similar quality factors formodes I and II while that for mode III
is much smaller. Hence, modes I and II

Octa

were driven by lattice heating whilemode III was generated by impulsive
vibrational

Cub-I

0.05 ± 0.03

51.4

43.2

22.4

modes were simulated by finite element

octa

II0.09 ± 0.03

57.8

51.3

21.2

electronmethodpressure.
(FEM)

Thewithbulk

elastic

properties based on continuum elastic theory. The calculated frequencies f FEM are in good agreementwith the experimental results. Reverse spectral shift for expansion of the two modes has been reportedfor Ag nanocubes. The similar phases for mode I and mode II represent reverse displacement of modes Iand II, that means expansion of mode I is combined with shrinkage of mode IIConferences:1)

Li Wang, Kouhei Nonaka, Tomoki Okuhata, Naoto Tamai, “Biexcitonic spectral shift and quasi-type II carrier
distribution in type I CdSe/CdS core/shell QDs”, 第 11 回分子科学討論会 2017 仙台, 4B10

Publications:
1)

L. Wang, K. Nonaka, T. Okuhata, K. Tetsuro, and N. Tamai, “Quasi-Type II Carrier Distribution in CdSe/CdS
Core/Shell Quantum Dots with Type I Band Alignment” (submitted)

2)

L. Wang, S. Takeda, T. Okuhata, R. Sato, T. Teranishi, and N. Tamai, “Ultrafast Spectroscopy and Coherent Acoustic
Phonon of Au Nanopolyhedrons” (in draft)