Synthesis and raman characterisation of transition metal dichalcogenides

Abstract

In recent years there has been an increased interest in layered materials such as Transition Metal Dichalcogenides (TMDs). When isolated in their monolayer form, they are essentially a 2D material, with unique properties. The implementation of 2D materials into future electronic devices could be desirable for industry; as their potential applications include cheap, fast and flexible electronic devices. Furthermore, to fully exploit the unique electrical properties of these materials, such as high electron mobility, new synthesis, manipulation, integration and characterisation techniques must be developed. The first part of this thesis focuses on the reliable vapour phase synthesis of TMDs. Initial research focused on the thermally assisted chalcogenisation of pre-deposited layers. The successful growth of crystalline, uniform thin films of MoS2 indicated that this process was a significant step towards growth of TMD films in a manner compatible with techniques used in the semiconductor industry. Electrical transport in WS2 films was highly sensitive to the presence of NH3 gas, which was the first reported demonstration of a gas sensing response for WS2 thin films. A sensitivity of 1.4 ppm NH3 in nitrogen at room temperature was achieved, indicating device sensitivity comparable with commercially available metal-oxide-semiconductor and solid electrolyte sensors. However, films produced using these methods were typically polycrystalline, and therefore ?bottom-up? chemical vapour phase synthesis methods were required to create single crystals with a high degree of crystallinity in the lattice. Highly crystalline TMD monolayers were realised by using a close proximity precursor supply in a microreactor setup. Monolayer TMD single crystals were synthesised with high reproducibility. A wide range of spectroscopic, microscopic, and electrical characterisation techniques reveal the high quality of the TMD samples produced, which could be used in the future for the facile production of electronic device components. The second part of this thesis focuses on the spectroscopic characteristics of vapour phase grown TMDs. Raman characterisation of vapour phase grown PtSe2 thin polycrystalline films is presented. PtSe2 thin films were grown by conversion of Pt films to PtSe2 at low temperatures under Se atmosphere. Scanning transmission electron microscopy measurements of these films experimentally confirmed the crystal structure to be 1T, which allowed predictions to be made of the phonon dispersion curve and expected frequencies of vibrational modes. These modes were experimentally confirmed and found to depend on film thickness for a variety of laser excitation wavelengths. Chemical vapour deposition (CVD) grown TMDs were studied using low-frequency Raman spectroscopy. This allowed the measurement of the shear and layer-breathing modes in the multilayer crystals, which were visualised in maps. It was found that these low-energy Raman modes allowed the assessment of layer number, as well as stacking configuration (2H or 3R), of multilayer TMDs in a fast and reliable manner. Low-frequency Raman spectroscopy was further used to study suspended MoS2 layers, in order to determine material properties without substrate perturbations, allowing an estimation of strain in suspended bilayer CVD MoS2 to be made. These studies also allowed the measurement of resonance effects in the low frequency region of the WS2 Raman spectrum. In summary, reliable growth methods for highly-crystalline TMDs were realised, which were subsequently studied using Raman spectroscopy. This was found to be an ideal technique for the study of TMDs synthesised by different vapour phase methods, and revealed many insights into the behaviour of 2D materials.