Nanomaterials, such as carbon nanotubes (CNTs), silicon nanowire, and graphene, have been finding many applications in diverse fields relating to displays, sensors, data storage, nanoelectronics, and so on. The thermal characterization of nanomaterials is crucial for understanding energetics in electric and energy-conversion systems that utilize these nanomaterials, which is a precondition for their optimal design and application. However, despite their importance, reliable quantitative data on the thermal characteristics of these nanomaterials are hard to find, compared with their electric and chemical counterparts, due to the lack of proper tools for their thermal characterization
Although scanning thermal microscope (SThM) has been developed quite actively and sometimes applied for the thermal characterization of nanomaterials because of its exceptional spatial resolution, which is about 100 nm, it still remains as a qualitative measurement tool. Although we recently developed the double scan technique, which allows quantitative temperature profiling by eliminating the influence of heat transfer through the air, it does not always guarantee the quantitative measurement using SThM.
Herein, we exactly locate the subsurface microstructures by measuring the absolute phase lag of thermal wave with advanced scanning thermal wave microscopy (STWM). And by developing the quantitative temperature/temperature distribution measurement techniques using SThM, we measure the temperature of the electrically heated multiwall carbon nanotube (MWCNT) and the thermal contact resistance between CVD-grown graphene and SiO2.
At first, we differentiate the components of the absolute phase lag of thermal wave, isolate the component related to the position of the subsurface heater, and locate the subsurface heater accurately by using STWM, which can image the phase lag and amplitude of thermal wave with sub-micrometre resolution using SThM. Through the benchmark experiment, we profile the phase lags of thermal wave with the distortion due to heat conduction through the air with the driving frequency at 3.2 kHz and measure the phase lag due to the thermal isolation surrounding the subsurface heaters by measuring the 3f voltage, which is generated in the heater. And we experimentally calibrate the phase lags due to the thermal time constant of the SThM probe and thermal contact resistance between the probe tip and sample surface with exposed micro-heater. Finally we isolate the phase lag due to the distance traveled by the thermal wave in the medium and find out the exact location of two subsurface heaters whose depths are different.
Second, we developed the null-point method by which one can quantitatively measure the temperature of a sample without disturbances arising from the tip-sample thermal conductance, based on the principle of the double scan technique. We first checked the effectiveness and accuracy of the null-point method using 5 ?m- and 400 nm-wide aluminium lines. Then, we quantitatively measured the temperature of electrically heated multiwall carbon nanotubes (MWCNTs) using the null-point method.
At third, by expanding the principle of null-point method, we developed a theory and method of NP SThM, which can obtain quantitative temperature profiles even when tip-sample thermal conductance is disturbed due to abrupt changes in the surface topography or surface properties such as hydrophilicity. We first check the Seebeck coefficient of the SThM probe by using a 5 ?m wide aluminum line patterned in a four-probe configuration on pyrex glass. And then, we quantitatively profile the temperature distribution of electrically heated aluminum line with NP SThM.
Finally we directly measure the thermal contact resistance at the CVD-grown graphene/SiO2 interface by utilizing the NP SThM with relatively much higher accuracy than that of the previous studies(5.6 × 10-9 ~ 3.3 × 10-8 m2K/W). The abnormal thermal contact resistance measured in this study is caused by the extrinsic factors, such as ripples formed at the producing process. Therefore, one can characterize the intrinsic thermal contact nature by characterizing the thermal transport phenomena with NP SThM, which can directly and quantitatively profile the temperature distribution.