Complex Raman Tensors of CrPS4 and Polymorphism of Two-Dimensional Pentacene Crystals Studied with Polarized Raman Spectroscopy

DoctorTwo-dimensional materials have attracted tremendous attention in a wide range of applications for future electronic devices due to their interesting properties such as atomic-level thinness high charge mobility, thermal conductivity, transparency, and flexibility. In my doctoral studies, I conducted research on two-dimensional inorganic and organic semiconductor materials using Raman spectroscopy. In Chapter Ⅰ, I present the specificity and necessity of two-dimensional inorganic and organic materials. I explain how Raman spectroscopy is used to study two-dimensional inorganic materials such as graphene and MoS2. Here, I mentioned some of the results of the strain study conducted in the master's course. In addition, It described the research background and necessity of two-dimensional organic matter. Furthermore, I showed the possibility that ultra-low frequency Raman spectroscopy is utilized to study interfaces of two-dimensional organic-inorganic heterostructures. In Chapter Ⅱ, Raman spectroscopy is described as a major experimental. In this thesis, the study was conducted by polarized Raman spectroscopy and ultra-frequency Raman spectroscopy. I explain each theory and the schematic diagram of Raman spectroscopy that extends from normal Raman spectroscopy. Raman spectroscopy is a representative analysis method of two-dimensional materials because it has the advantages of measuring samples quickly and nondestructively. Polarization Raman spectroscopy can distinguish and assign Raman modes, providing information on crystalline orientation and interfacial structures. In addition, ultra-low frequency Raman spectroscopy is a powerful tool for observing interactions between materials, revealing the thickness, stacking order, and polymorphism. In a schematic diagram, I showed that a specific component implements each Raman spectroscopy. In Chapter Ⅲ, I conducted the study of CrPS4 that was the assignment of Raman mode and determining of complex Raman tensor by setting the polarized Raman spectroscopy. CrPS4 is a ternary layered semiconductor with an optical bandgap of 1.4 eV and exhibits antiferromagnetism below 36 K. Despite its potential in optoelectronic and magnetic applications, the symmetry of its lattice vibrations has not been systematically studied. I performed a polarized Raman spectroscopy of bulk CrPS4 using three different excitation wavelengths of 457, 514, and 633 nm. High-quality crystals grown by the chemical vapor transport method were mechanically exfoliated or polished to expose three orthogonal crystallographic facets. Polarized Raman spectra were obtained in parallel and cross configurations by rotating samples about the surface normal to each of the facets. Among 33 Raman active modes (16 A and 17 B) at the Brillouin zone center, 19 (8) peaks observed in the parallel (cross) configuration were assigned as A (B) modes. Complex-valued Raman tensors were determined for 7 major A modes using the angle-dependent Raman spectra of the three facets. In Chapter Ⅵ, by fabricating the high-quality two-dimensional organic semiconductor thin films, I conducted the study of interfacial interaction by ultra-low frequency Raman spectroscopy. Understanding the interfacial interaction is essential for the 2D organic semiconductor to be applied in high-performance electronic devices with maximum efficiency. The ultra-low frequency Raman spectrometer was set to observe the interlayer vibration of the pentacene crystal. I installed holographic notch filters at specific angles to attenuate the strong Rayleigh lines by about OD9. As a result, the observable Raman spectral boundaries were extended from about 100 cm-1 to 10 cm-1. In addition, a high-quality pentacene thin film was grown uniformly on graphene using a thermal evaporator. Then, pentacene was encapsulated with graphene to form a sandwich structure. The encapsulation has shown the effects of the prevention of molecular desorption and enhancement of optical stability and ultra-low frequency Raman signal. As a result, the pentacene thin-film polymorphisms were not distinguished at the high frequency range but differed significantly at ultra-low frequency Raman mode. I effectively distinguished polymorphs of organic thin films by ultra-low frequency Raman spectroscopy