倉田奨励金研究報告書 第 52 集｜2020 年度（第 52 回）助成
自然科学・工学研究部門：1 年助成■受領№1441

ロックイン探針増強非定常ラマン法の確立と２次元熱伝導マッピング
代表研究者

李秦宜

九州大学准教授

Lock-in tip-enhanced transient Raman method for2D heat conduction mapping
Principal Researcher

Qinyi Li,
Kyushu University, Associate Professor

ナノ材料の熱物性データは数 10％といった大きな誤差を含む場合が多く、欠陥と界面の影響についても調べる手法が限られている。本研究では、高い時空間分解能を有する熱物性計測法の開発を進めている。まず、走査型熱顕微鏡に基づいて 2 次元材料の熱物性分布を高精度にマッピングする手法を開発した。更に、高い時間分解能を有する非定常ラマン分光法を用いてグラフェンフォームの熱拡散率を測定し、ロックイン技術によってラマン分光法の計測精度も改善して進めている。
The measured thermophysical properties of nanomaterials often exhibit large uncertainties and there arevery limited methods to experimentally study the thermal transport at the defects and interfaces. In thisstudy, we develop methods to measure the thermophysical properties with high temporal and spatialresolutions. First, we developed an approach to map the distribution of thermophysical properties in 2D
materials with a high spatial resolution based on the scanning thermal microscope. Second, we measured
the thermal diffusivity of the graphene foam using the transient Raman method with a high temporal
resolution and utilized the lock-in technique to improve the accuracy of Raman thermometry.

1．研究内容1.1 Thermophysical property mapping along agraphene ribbon using scanning thermal microscopy
In this study, we set up the scanning thermal
microscopy (SThM) probe to the atomic force
microscope (AFM) and realized temperature
mapping with a spatial resolution as high as 50 nm.
Then we used SThM to measure the temperature
distribution along a defected graphene ribbon
under Joule heating, as illustrated in Fig. 1, anddetermined the interfacial thermal conductance
between graphene and the underneath SiO2 with

Figure 1. Schematic for the interfacial thermal

a high spatial resolution.

conductance mapping along a graphene ribbon

倉田奨励金研究報告書 第 52 集｜2020 年度（第 52 回）助成
自然科学・工学研究部門：1 年助成

The graphene ribbon sample was fabricated by ahigh as 3.52×105 S/m, indicating high quality ofseries of MEMS processes including wet transfer

the sample. We observed local hot spots at theof CVD-grown monolayer graphene onto thevicinity of defects and estimated the interfacial

SiO2/Si wafer, electron beam lithography for thethermalpatterning, physical vapor deposition of thegraphene and SiO2 to be 24.48MW/m2·K using

metallic electrodes, as well as wire bonding. We

the Joule heating power and the measured

confirmed the high quality of the graphene sample

graphene temperature.

conductance

betweenmonolayerwith Raman spectroscopy and measured thetemperature dependence of the G band and 2D

1.2 Thermal diffusivity measurement of graphene

band of the graphene sample. We measured thatfoam using the transient Raman method

both the Raman G band and 2D band shifts
decrease with increasing temperature at a slope of-0.0389 cm-1/K and -0.0394 cm-1/K, respectively.

Intheconventional

steady-state

Raman

optothermal method, continuous laser is used toheat the sample, while the local temperature of theWe optimized the scanning conditions for thesample is detected based on the temperature

cantilever probes of the AFM and SThM so that

dependence of the Raman spectrum of the sample

we can introduce defects into the graphene ribbon

in a non-contact manner. Then the thermal

in a controllable way. The spring constant of theconductivity of the sample can be derived from theprobe in our experiments was 9.9 N/m. When we

relationship between the absorbed laser power, i.e.

applied an operation potential of 0.1 V and a scanthespeed of 12 μm/s, there existed no observabletemperature. However, in this steady-state Raman

defects that are induced by the probe on themethod, the laser absorptance is difficult to begraphene sample. However, when we applied aaccurately determined, especially for micoscalehigh operation voltage of 1 V and a high scanand nanoscale samples, thus causing a large

speed of 40µm/s 〜 200µm/s, we can introduce

uncertainty.

micrometer scale defects on the graphene sample.
Furthermore, we fulfilled line scan with the AFM
probe and successfully introduced line defects in
the graphene sample.
intwomodes

flux,

and

theRaman-measured

In this study, we measured the thermal
diffusivity of CVD grown graphene foam using thetransient Raman method. The graphene foam wassuspended between two Si wafers that served as

We heated the graphene sample with direct
currentheat

andmeasured

heat sinks. We used pulsed laser to heat thethesample and measured the transient temperaturetemperature distribution along the grapheneincrease from the temperature dependence of theribbon. In the first mode, we drove current in aRaman shift of the graphene foam. We observed athin metallic (Au) film near the graphene sample

strong G band and a weak D band in the Ramanand mapped the temperature distribution on bothspectrum. which indicates high quality of thegraphene and SiO2 areas. When the heatingsample. As shown in Fig. 2, by changing the laser

current in the thin Au film was 70 mA, thepulse duration, we measured the change of thetemperature on the graphene area was 1.37 K

G-band Raman shift that reflects the temperature

lower than that on the SiO2 area, which indicatedevolution. From this temperature change as a

the cooling effect of graphene. In the second mode,

function of the pulse duration, we can extract thewe drove current in the graphene ribbon andthermal diffusivity without the need of knowing

investigated its temperature distribution under

the absorptance of the heating laser. Specifically,

Joule self-heating. We measured the electricalfor this graphene foam sample, the laser spot size

conductivity of the monolayer graphene to be as

is much larger than the sample size and the倉田奨励金研究報告書 第 52 集｜2020 年度（第 52 回）助成
自然科学・工学研究部門：1 年助成

heating laser can be regarded as a point heateliminated and we can improve the temperaturesource. We performed finite-element simulations

resolution to about 0.5 K. Using this method, wefor the transient temperature evolution of themeasured the thermal conductivity distribution ingraphene foam under point heat source anda carbon fiber network with a high resolution.

establishedtherelationshipbetween

thenormalized temperature rise and the thermaldiffusivity. Then, by comparing the simulation
results with our experimental data, we candetermine the thermal diffusivity of the sample.
We can further determine the thermal conductivity

2．発表（研究成果の発表）[1] 李 秦宜，高橋 厚史. ラマン分光法によるマイクロファイバー同士の接触熱抵抗計測. 第58回日本伝熱シンポジウム. 2021年5月25日-27日, オンライwith the specific heat and density. In this way, weン.

can eliminate the uncertainty of the laser[2] 呉 雨沢, 李 秦宜, 森下 和彦, 高橋 厚史. 走査absorptance and measure the thermal property型熱顕微鏡を用いたグラフェンの熱物性計測. 第42with a high accuracy. We successfully measured

回日本熱物性シンポジウム. 2021年10月25日-27日,

the effective thermal conductivity of this 3Dオンライン.

porous graphene foam to be 5.4W/mK, which isabout 1/500 of that of monolayer graphene.

[3] 池田 航洋, 横田 誠二朗, 李 秦宜, 高橋 厚史,河野 正道. レーザーフラッシュラマン分光法によるカーボンファイバーの熱拡散率計測. 第42 回日
本熱物性シンポジウム. 2021年10月25日-27日, オン
ライン.
[4] 趙 子楽, 李 秦宜, 高橋 厚史. グラフェンの熱電特性の計測及び制御. 第 12 回マイクロ・ナノ工学シンポジウム. 2021年11月9日-11月11日, オンライン.

Figure 2. Thermal diffusivity measurement ofgraphene foam
1.3 Development of the lock-in Raman method
The Raman based temperature measurement
suffers from the relatively low temperature
resolution, typically about 5 K. In this study, we
propose a lock-in technique to improve thetemperature measurement accuracy. We use
sinewave modulated current to heat the sample
and pick up the change in the Raman shift that isat the same frequency of the heating power. In thisway, the noises at other frequencies can be