Synthesis, properties and applications of 2D materials
Date of Issue2017-12-08
School of Materials Science and Engineering
Nanyang Technological University
Two dimensional materials (TMDs) have drawn more attention due to their excellent physical properties such as high mobility, valley spintronic and high on/off ratio. The band gap changing from indirect in bulk to direct in monolayer makes them as potential candidates for the next generation applications for field effect transistor (FET), flexible transparent electrodes, photonics sensors, conductive composites, gas separation membranes and nonlinear optics. So far, following the reported monolayer graphene, more than 40 different TMDs have been reported. However, many work only focus on synthesis and properties of TMDs in Mo-/W-based (MX2, M: Mo, W; X: S, Se) and MX2 based heterostructures. Other 2D TMDs are rare to be studied due to their difficulties synthesizing by chemical vapor method (CVD), such as MoTe2, WTe2, IV-, V-, Re-, and VIII-based TMDs, impeding researchers to study their properties such as superconductivity, spintronic, magnetism and applications. Furthermore, the heterojunctions based on the different structure of monolayer TMDs offer the possibility to fabricate the p-n junction, high-speed electronics, optoelectronic, LEDs and spin valleytronic devices. The most of studies focus on the heterojunctions with combination of WX2 and MoX2. The heterojunctions with different crystal structures are difficult to be realized, such as the heterostructures between MX2 and PtX2 (X: S, Se). In addition, compared with the TMDs, the IIIA-VIA layered materials such as GaS, GaSe, In2Se3 and InSe also show potential applications for next generation electronics and optoelectronics due to their suitable bandgaps. The properties such as the optical and optoelectronic have not been studied. Based on the above analysis of problems, in this thesis: First, the monolayer MoTe2 and WTe2 were synthesized by CVD method. The size of monolayer MoTe2 and WTe2 is up to 300 µm and 100 µm, respectively. Scanning transimission electron microscopy (STEM) was used to confirm their atomic structures. Devices fabricated using monolayer MoTe2 and few layer WTe2 show enhanced superconductivity in few layer MoTe2 and semimetal-insulator transfer between few layer in WTe2. Second, using the molten salt-assisted CVD method, varied types of TMDs were synthesized, including Ti (TiS2, TiSe2, and TiTe¬¬2), V (VS2, VSe2, and VTe2), Nb (NbS2, NbSe2, and NbTe2). Raman and photoluminescence (PL) spectroscopy were used to confirm as-synthesized TMDs. Atomic force microscopy (AFM) was used to check the thickness of as-synthesized crystals. STEM was used to confirm their atomic structure. Third, the heterojunction with different crystal structure and lattice constant between PtSe2 and MoSe2 was synthesized via one-step CVD method. The Raman and PL mapping reveal the vertically stacking between PtSe2 and MoSe2. X-ray photoelectron spectroscopy (XPS) and annular dark-field scanning transition electron microscopy (ADF-STEM) demonstrate the strong coupling epitaxial growth behavior of PtSe2 on the MoSe2. The device using PtSe2/MoSe2 heterostructure shows a typical p-n junction property. Forth, the monolayer In2Se3 for the first time was prepared by physical vapor deposition (PVD) method. Raman and PL spectra were identified in the monolayer In2Se3. STEM confirmed the hexagonal structure of In2Se3. The FETs fabricated using few layer In2Se3 perform excellent optoelectronic properties, indicating In2Se3 atomic layers as a promising candidate for the optoelectronic and photosensitive device applications.