Photoemission of transition metal dichalcogenides and battery materials

Transition metal dichalcogenides (TMDs) are an emerging class of materials that are akin to graphene, but in many ways more useful as they provide a non-zero bandgap. They have interesting structural and electronic properties that make them very attractive for a range of applications including photovoltaics, battery anodes, transistors and sensors [1]. Similar to graphite, these are two-dimensional layered materials having the chemical composition MX2 (M=transition metal like Mo, W, Nb, Ta, Re etc and X=S, Se, Te), with a hexagonal microstructure similar to graphene. The hexagonally close-packed sheet of transition metal atoms is sandwiched between two sheets of chalcogen atoms forming a slab. The electronic properties range from metallic to semiconducting, and native defects such as chalcogenide or metal vacancies can be used to make the semiconducting TMDs n-type or p-type. Similarly, other electronic properties, such as band gap and workfunction can be tailored depending on the number of slabs [1].
The PhD project is multifaceted with a view to understanding how these materials work in energy conversion as well as storage devices. The idea initially is to investigate the bulk and surface properties of pristine thin films of TMDs using a variety of techniques in order to get both physical and electronic characterization. Primarily photoemission, both using laboratory based monochromated x-ray photoemission (XPS), and synchrotron-based high energy photoemission (HAXPES), will be used to probe the electronic structure of exfoliated TMDs. This method of using different energy PES allows the exploitation of the mean free path of electrons to probe different depth of the material, thus allowing the electronic structure of occupied states to be determined both of the bulk as well as near surface region of the TMDs. These measurements will be compared with density functional calculations performed in collaboration with colleagues from UCL. These measurements will be complimented by other standard optical techniques such as Raman and UV-Vis spectroscopy.
The inter-slab spacing in TMDs is larger than in graphite, providing large channels for ion intercalation and strain relief, leading to enhanced cyclic stability and capacity, making TMDs attractive for practical battery anode design. Due to such layered structure of TMDs they are increasingly considered as electrode materials for Li ion batteries. Li-ion batteries are used in our everyday electronics and have allowed for the success in electric vehicles [2]. Here, the idea is to first investigate the charge compensation mechanisms of Li-rich rock salt oxides using HAXPES and XPS. This will give insight into the different mechanisms that may occur at the surface and deeper into the bulk. These measurements will then be extended with the battery material supported on TMDs, in order to investigate the electronic properties at the interface of these materials.

[1] W. Choi et al, "Recent development of two-dimensional transition metal dichalcogenides and their applications", Materials Today, 20 (2017), 3.

[2] Y. Zhang et al, "Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2", Nature Nanotechnology, 9 (2014).

[3] J. B. Goodenough and K.-S. Park, "The li-ion rechargeable battery: A perspective," Journal of the American Chemical Society, 135, (2013), 4.