title: The chemical vapour deposition of MoTe2 thin films and their applications
Creators: name: Fraser, James P.
Publisher: name: University of Glasgow
Year: 2021

description: The isolation of graphene, a single layer of graphite, in 2004 highlighted the important role dimensionality plays in determining the properties of materials. This discovery led to attention being focused on other types of two-dimensional materials, in particular the family of compounds known as the transition metal dichalcogenides (TMDCs). The chemical and structural diversity of the TMDCs means they exhibit many exotic properties that make them interesting for both fundamental science and suitable for many applications. Of the TMDCs, perhaps one of the most interesting is molybdenum ditelluride. MoTe2 can exist in two polymorphs, one of which is semiconducting and the other metallic and their properties vary depending on their thickness. This polymorphism also presents a challenge towards the synthesis of phase pure MoTe2 films by chemical vapour deposition (CVD), a method which shows promise for being able to produce these materials at scale and with high throughput. Both polymorphs tend to exist at the typical deposition temperatures used and as such are in competition with each other. This work sought to develop a CVD set up that can retain control over the phase and thickness of the MoTe2 product films. Following the determination of conditions that allowed for the deposition of 2H- and 1T’-MoTe2, the films were characterised extensively with a variety of techniques to ensure that they were indeed phase pure. Subsequently, films of both phases with a variety of thicknesses were produced, highlighting the ability of the CVD method to accurately control the thickness and phase of MoTe2. The method also allowed for the simultaneous deposition of 1T’- and 2H-MoTe2 on the same substrate by taking advantage of the type of precursor layer used. The CVD grown films of MoTe2 were then trialled in two different applications. Firstly, as electrocatalytic films for the production of hydrogen from water, where the impact of the phase and film thickness on the catalytic activity was studied. Secondly, 1T’-MoTe2 was applied as a surface enhanced Raman spectroscopy substrate. A preliminary study was conducted using a model analyte, rhodamine 6G, with the intention of confirming the activity of the 1T’-phase and performing brief thickness dependent measurements. Subsequently a thorough study was conducted using a clinically relevant biomolecule, β-sitosterol, as an analyte. This highlighted the potential applications of the TMDCs as SERS substrates with the achieved limit of detection of 10-9 M being one of the lowest reported so far for sterol-like lipophilic molecules. Furthermore, the mechanism of Raman enhancement was determined to be related to the formation of a surface complex between the analyte and 1T’-film, which was then characterised extensively. Finally, the Mo3Sb7-xTex (where x = 0, 1, 1.6 and 2) group of compounds were synthesised and investigated. These materials offer an interesting opportunity to see how varying the tellurium amount impacts their performance as hydrogen evolution catalysts. It was found that increasing the amount of tellurium led to increased catalytic performance as well as improving the stability of the materials. Efforts were made to utilise the previously developed CVD system to try and deposit thin films of Mo3Sb7 and Mo3Sb5.4Te1.6 as there are no literature reports of this being achieved. This was somewhat successful but overall hampered by the difficulty in characterising the product films.