Two-dimensional molybdenum ditelluride (MoTe2): synthesis, characterization, and application
Ajayan, Pulickel M
Doctor of Philosophy
Recent research efforts in two-dimensional (2D) materials have shown an increasing focus on molybdenum ditelluride (MoTe2). Unlike other TMDs, MoTe2 is distinguished by the existence of two stable phases, hexagonal 2H phase and monoclinic 1T’ phase, both of which can be synthesized directly. 2H MoTe2 is a semiconductor with a band gap of ~ 1 eV, while 1T’ MoTe2 is metallic which can be transformed to a type-II Weyl semimetal at low temperature. The semiconductor-metal junction between 2H and 1T’ MoTe2 shows the potential to resolve the issue of the existence of high Schottky barrier in traditional devices. MoTe2 does not only possesses a myriad of physical properties to further unravel but also shows great potential towards various industrial applications such as analog circuits and spintronics. Here in this thesis, chapter 1 gave a brief introduction to 2D materials. Several common synthesis methods, characterization methods, and applications were discussed. Chapter 2 focused on the phase-controlled synthesis of large-area MoTe2 films and 2H/1T’ MoTe2 heterostructures by chemical vapor deposition. A series of techniques have been used to systematically characterize the synthesized MoTe2 films. In chapter 3, several types of MoTe2-based devices were fabricated and measured. We demonstrated an electrical device across the one-step-synthesized 2H/1T’ MoTe2 in-plane heterostructure, where 1T’ phase serves as the contact electrodes for the 2H phase channel. An improved current density was observed compared with deposited metal electrodes on top. In chapter 4, we studied the Raman enhancement on MoTe2 films. MoTe2-based 2D heterostructures exhibit the potential as novel platforms for surface-enhanced Raman scattering (SERS) application. Our recent efforts in the spatial phase-targeted synthesis of 2H and 1T’ MoTe2 were presented in chapter 5. This strategy was suitable not only for large-scale patterns but also for small features. In chapter 6, we used the ultrafast electron diffraction (UED) to investigate the nonradiative process in 2H MoTe2 at SLAC National Accelerator Laboratory. This thesis doesn’t only study the fundamental properties of MoTe2, but also paves the way towards the large-scale application of MoTe2 in electronic and optoelectronic devices.