Raman-based study of micro/nanoscale structure and thermal transport

Nanoparticles, two-dimensional (2D) materials, and carbon fibers are widely used in nano electronic devices, aerospace structures due to their extraordinary properties. Among these properties, the surface morphology and the thermal transport are two important properties. The Raman scanning technique is used to characterize the surface morphology. But, the various asymmetries of Raman scattering due to structure variation in space are not considered. The optothermal method based on Raman spectroscopy is widely used to study the thermal transport of these materials. However, a significant drawback of this method is that both temperature and power dependent Raman study should be done to extract the thermal conductivity of the sample. The laser absorption is also subject to very large errors induced by unknown sample-to-sample optical property variation. In addition, for measuring the thermal conductivity of carbon fibers, it is very important to verify whether the thermal conductivities in axial and radial directions are isotropic. In this work, a Raman scanning technique is developed to explore the asymmetry of Raman scattering signal caused by structure variation in space. A nanosecond energy transport state-resolved Raman (ns ET-Raman) technique is developed to measure the thermal conductivity of suspended 2D atomic-layer molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2). And a novel method by combining the frequency domain energy transport state-resolved Raman (FET-Raman) technique and the transient electrothermal (TET) technique is developed to measure the anisotropic thermal conductivities of lignin-based microscale carbon fibers. The Raman scanning technique combines the confocal Raman system with a three-dimensional (3D) scanning stage. Silica microparticles, glass micro fibers, and MoSe2 nanosheet are used in the experiments. Three asymmetry types of Raman scattering signal due to physical structure variation in space are discovered, which indicates this technique could be used to study the surface morphology of the sample. In the ns ET-Raman technique, two energy transport states in time domain are constructed to eliminate the need of temperature calibration and laser absorption measurement. The ratio of temperature rise under the two energy transport states is determined by the in-plane thermal conductivity of 2D atomic-layer materials. As a result, the in-plane thermal conductivity could be determined accurately without doing Raman temperature calibration and knowing the laser absorption. Four suspended MoS2 (45 – 115 nm thick) and four suspended MoSe2 (45 – 140 nm thick) samples are characterized using ns ET-Raman. With the increased sample thickness, the measured thermal conductivity increases from 40.0 ± 2.2 to 74.3 ± 3.2 W·m-1·K-1 for MoS2, and from 11.1 ± 0.4 to 20.3 ± 0.9 W·m-1·K-1 for MoSe¬2. This is attributed to the decreased significance of surface phonon scattering in thicker samples. To measure the thermal conductivities in the two directions of carbon fibers, the TET technique is used to measure the axial thermal conductivity of carbon fibers, and the FET-Raman technique is then used to determine the radial thermal conductivity with the measured axial thermal conductivity. Four lignin-based microscale carbon fibers are characterized. The temperature effect on the thermal conductivities in the two directions is also explored. Detailed Raman study of the axial and radial structures uncovers very strong structure anisotropy and explains the observed anisotropic thermal conductivities. The future work about the energy coupling between optical and acoustic phonons under photon excitation is also discussed at the end of this work