Carrier dynamics in 2D Optoelectronic Materials

The dimensionality of materials plays crucial role in their fundamental properties. For example quantum dots (CdTe, CdSe, CdS etc.) exhibit size dependent band-gap which results in novel optical properties important for applications. Graphene exhibits extraordinary properties not present in its bulk form, graphite [1]. The discovery of graphene has boosted the research of two-dimensional materials, and it has been found that graphene, with its vanishing band-gap, can only provide part of the required properties for applications such as optoelectronics, photovoltaics and sensing. The search to extend the properties of available 2D materials has brought various potential candidates in this category, a prominent one being transition metal dichalogenides (TMDs) [2]. The TMDs are represented by a general formula of MX2 where M is a transition metal of group 4-10 and X is a chalcogen (S, Se and Te), and therefore offers wide range of materials with different constituents and resulting properties. Particularly, Molybdenum disulphide (MoS2) has recently attracted much interest as a prominent example of monolayer materials with remarkable physical properties. Molybdenum covalently bonded to sulphur can form two-dimensional sheets that are linked to each other by weak van der Waals interactions, similar to graphene in graphite. The electronic band structure of MoS2 changes with thickness. Thinning the material down to a single monolayer transforms it into a direct bandgap semiconductor. The optical band gap of a monolayer MoS2 corresponds to the visible part of the spectrum with an energy of 1.9 eV. The group 6 transitions metals (Mo, W) with Sulphide (S) and Selenide (Se) form semiconductors. The optical and optoelectronics properties of these interesting layered materials (mono and bi-layer) suggest presence of strong excitonic transitions which dominate their optical/optoelectronics properties. The unique structural and optical properties of TMD monolayers and their material tunability makes them technologically relevant for various applications ranging from photodetectors, batteries and catalytic applications to solar cells.
The objective of this project is to investigate the spatiotemporal carrier dynamics and optoelectronic properties of small single crystal sheets of MX2 . This will be done by scanning tunnelling microscopy (STM), atomic force microscopy (AFM), correlative electric force microscopy (EFM) and photoluminescence spectroscopy, optical extinction microscopy, and Raman microscopy. This project is supervised by Prof Wolfgang Langbein.