Jihoon Jeong, Ke Chen, Nilabh Roy, Michael A. Culling, Yaguo Wang
Nanostructured materials have come into the spotlight due to their potential application in energy device, sensor, and catalysis. In addition, nanostructured materials can be constructed to certain shape by 3-D printing technology and directly utilized for various applications. It is important to characterize thermal properties of nanostructured materials in order to design and fabricate the product in the desired purpose. Thermal transport properties in nanomaterials have been studied by various approaches including 3w method, time-domain thermoreflectance (TDTR) and transient thermal grating (TTG) technique. 3w method employs Joule heating of electrically conductive sample and measures the voltage change at frequency of 3w with heating of a current frequency w. This method experiences problems in measuring the interface resistance although it provides highly accurate measurement with bulk materials and low thermal conductivity dielectric film. TDTR utilizes pulse lasers (femto-, pico-, nano-second) to detect differential reflectivity change of materials due to temperature increase induced by laser heating. TDTR possesses experimental advantage of high spatial resolution and high sensitivity of thermal contact resistance, but is limited to detecting thermal properties along cross-plane direction. Lastly, transient thermal grating (TTG) technique generates interference using two different pump lasers to create transient thermal grating and is able to measure the thermal conductivity in plane direction while the efficiency of generated diffraction signal is relatively low. In this study, nanosecond transmission grating (NTG) technique is adopted to measure the thermal properties in metal nanofilms along both cross-plane and in-plane directions. In our experimental setup, a transmission grating is produced by an optical phase mask consisted of transparent slits on a thin metal film on transparent dielectrics. In order to produce the grating image on the sample, the pump and probe spots are overlapped in alignment and focused onto the same image plane of the desired sample via tube lens and objective lens. Two lasers including a nanosecond Nd:YAG laser (wavelength: 532nm) and a continuous He-Ne laser (wavelength: 632nm) are used as the pump and the probe, respectively. Two-dimensional thermal conduction model is applied to analyze heat diffusion in both cross-plane and in-plane direction and is compared with experimental results to extract thermal properties. This study covers measuring thermal conductivity of various thin metal films (e.g. Cu, Cr and etc.) on widely-used substrate (e.g. quartz, silicon and etc.). In particular, the research is focused on thin metal films fabricated with nano-size particles to see the effect of particle size and film thickness on thermal properties. Finally, nanosecond transmission grating (NTG) technique will be evaluated in comparison with other approaches in terms of merit and demerit.