EFFECTS OF SUBSTRATE ON 2D MATERIALS, GRAPHENE, MOS2, WS2, AND BLACK PHOSPHORUS, INVESTIGATED BY HIGH TEMPERATURE AND SPATIALLY RESOLVED RAMAN SCATTERING AND PHOTOLUMINESCENCE
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Abstract
The exploration of a group of new 2D materials, such as graphene and transition metal dichalcogenides, has become the hottest research of interest in recent years. With the dependable techniques of producing 2D materials, particularly mechanical exfoliation and chemical vapor deposition, we are able to study all kinds of their unique properties in mechanical, electrical and optical fields. In this dissertation, we examine the vibrational and thermal properties of four 2D materials – graphene, MoS2, WS2 and black phosphorus – as well as their interaction with the supporting substrates, by using temperature-dependent Raman spectroscopy. Regarding the increasing interests of studying on the fabrication and applications of 2D materials, the role of 2D-material/substrate interaction has seldom been taken into consideration which would significantly affects the quality of the grown films and the performance of the devices. To the best of our knowledge, we are the first to systematically investigate on this issue. At first, we performed temperature-dependent Raman spectroscopy on two graphene samples prepared by CVD and ME up to 400 °C, as well as graphite as a reference. The temperature dependence of both graphene samples shows very non-linear behavior for G and 2D bands, but with the CVD-grown graphene more nonlinear. Comparing to the Raman spectra collected before the measurenents, the spectra after the measurements exhibit not only a shift of peak position but also a huge broadening of linewidth, especially for CVD-grown graphene. This study implies that the polymeric residues from either scotch tape or PMMA during transfer process are converted to amorphous carbon after annealed at high temperature, which may significantly change the optical and electrical properties of graphene.With the same temperature-dependent Raman technique as graphene, we examine on monolayer MoS2 and WS2, and thin-film black phosphorus and demonstrate that the film morphology and the substrate play very important roles in modifying the properties of the materials. For the films transferred onto SiO2/Si substrates, the E2g1 mode is only weakly affected by the substrate, whereas the A1g mode is strongly perturbed, showing a highly nonlinear temperature dependence in Raman peak shift and linewidth. In contrast, for the films epitaxially grown on sapphire substrate, E2g1 is tuned more significantly by the substrate by showing a much smaller temperature coefficient than the bulk, while A1g is less. A two-round temperature dependent Raman measurements on a transferred MoS2 on SiO2 sample confirm these findings. These experiments suggest that the film-substrate coupling depends sensitively on the preparation method, and in particular on the film morphology for the transferred film. Additionally, temperature-dependent PL spectroscopy of monolayer WS2 shows a 0.2 eV activation energy for CVD-grown films on SiO2/Si substrate.Besides temperature dependent Raman spectroscopy, we also perform PL and Raman mappings on monolayer WS2 triangles prepared by both chemical vapor deposition and transfer, and find both Raman and PL are very sensitive to strain and doping effects. The non-uniform strain distribution over one single triangle is determined both qualitatively and quantitatively through the shift of E2g1 mode and PL peak energy. In transferred WS2 monolayer, comparing to suspended WS2 film a very strong PL quench in WS2 film supported by SiO2/Si substrate is observed, which is attributed to charge transfer between the film and the substrate. Finally, the thermal conductivity of thin-film black phosphorus is determined by its temperature and laser power dependent Raman spectroscopy. An average thermal conductivity of a suspended black phosphorus film has been determined to be 15.8 K/mW.