Synthesis of Graphene and Two-Dimensional Transitional Metal Dichalcogenides and Their Optical Propertie

Since the first isolation of graphene from graphite in 2004, there has been a significant amount of research dedicated to two-dimensional (2D) materials. Graphene is arguably the most famous material which displays remarkable mechanical, thermal and optical properties. However, its lack of an electronic bandgap, limits its application in many areas. Transition metal dichalcogenides (TMDs) such as WSe2, WS2, and MoS2 are another kind of 2D material with semiconducting character. Its direct band-gap feature and favorable electronic and mechanical properties, complement graphene, leading to applications in high-end electronic and optoelectronic applications. This dissertation explores the optical properties of these 2D materials by Raman spectroscopy and their potential applications in molecular sensing have been explored.;Chapter 1 provides a brief introduction of various 2D materials, including graphene and TMDs. Crystal structure as well as electronic band structures for these materials have been introduced together with some potential applications of the 2D materials.;Chapter 2 gives an introduction on Raman spectroscopy. As one of the heavily used characteristic techniques to probe the optical properties of 2D materials, Raman scattering processes are studied from both macroscopic and microscopic approaches. In addition, normal Raman spectra of 2D materials are measured and by analyzing the Raman spectra, various information about the sample, such as crystallinity, thickness, doping and strain could be obtained indicating Raman can serve as a powerful tool to study properties of 2D materials.;In chapter 3, nitrogen-doped (NG) and silicon-doped (SiG) graphene are successfully synthesized through a chemical vapor deposition (CVD) setup. A variety of techniques, including transmission electron microscopy (TEM), scanning tunneling microscopy and spectroscopy (STM/STS), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy are used to characterize the sample and discover how nitrogen and silicon atoms are incorporated into the graphene lattice.;Chapter 4 demonstrates that when graphene monolayers are used as substrates to probe the Raman signal of various dye molecules, such as rhodamine B (RhB), crystal violet (CRV) and methylene blue (MB), intensities of the Raman signals from the dye molecules enhance a lot, giving the term as graphene-enhanced Raman scattering effect (GERS). In addition, NG exhibits an extraordinary sensing properties when compared to PG sheets. For example, the Raman vibrational modes of these particular dye molecules can be detected for concentrations as low as 10-11 mol/L and very close to single molecule detection limit. This is the lowest ever reported value when using graphene as a substrate so far. Electronic structure calculations and the simulation of the Raman spectra, performed by our collaborator Dr. Maria Cristina dos Santos from Brazil, suggests that a charge transfer excitation and the proper aligning of the HOMO-LUMO gap of the molecules with respect to the Fermi level of graphene are the key reasons for this enhancement..;Chapter 5 demonstrated that Raman spectroscopy could be a significant tool to study 2D materials by two simple examples. By applying polarized Raman techniques, a new Raman active peak, which is associated with first-order out-of-plane vibrational mode in TMDs is observed. Further experiments and calculations demonstrate that this peak only exist in few layer TMDs materials and does not show in monolayer or bulk samples. The unique property of this mode could potentially provide an easy method to distinguish monolayer TMDs sample just by Raman measurements. Another example is that a monodispersed, flower-like MoSe2 nanostructures have been synthesized by colloidal methods by our collaborator (Dr. Shaack's group). This 3D structure contains 2D-quasi-like MoSe2 nanosheets that protrude outwards from a dense central core. Laser power dependent Raman and temperature dependent Raman measurements demonstrate that the interlayer decoupling of these nanosheets could be easily tuned, providing insights on how to modulate the optical properties of TMD materials.;Finally, chapter 6 summarizes the studies discussed in this dissertation and provide some perspectives to the potential future works.