Synthesis and thermal stability of TMD thin films: A comprehensive XPS and Raman study

Transition metal dichalcogenides (TMDs) have been a core constituent of 2D material research throughout the last decade. Over this time, research focus has progressively shifted from synthesis and fundamental investigations, to exploring their properties for applied research such as electrochemical applications and integration in electrical devices. Due to the rapid pace of development, priority is often given to application-oriented aspects while careful characterisation and analysis of the TMD materials themselves is occasionally neglected. This can be particularly evident for characterisations involving X-ray photoelectron spectroscopy (XPS), where measurement, peak-fitting, and analysis can be complex and nuanced endeavours requiring specific expertise. To improve the availability and accessibility of reference information, here we present a detailed peak-fitted XPS analysis of ten transition metal chalcogenides. The materials were synthesised as large-area thin-films on SiO2 using direct chalcogenisation of pre-deposited metal films. Alongside XPS, the Raman spectra with several excitation wavelengths for each material are also provided. These complementary characterisation methods can provide a more complete understanding of the composition and quality of the material. As material stability is a crucial factor when considering applications, the in-air thermal stability of the TMDs was investigated after several annealing points up to 400 °C. This delivers a trend of evolving XPS and Raman spectra for each material which improves interpretation of their spectra while also indicating their ambient thermal limits. This provides an accessible library and set of guidelines to characterise, compare, and discuss TMD XPS and Raman spectra.