Study on Optical and Electrical Property Changes of Molybdenum Diselenide by Reversible Hydrogenation

Abstract

Hydrogenation is one of the chemical functionalization methods, which has been investigated as an approach for modulate electronic structure in nanomaterials, since it was theoretically suggested that electronic structure can be modified by degree of hydrogenation of graphene like two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs). In the case of graphene, its electronic structure with zero band-gap has been expected to be opened by surface hydrogenation. Compared to wide researches for hydrogenation reactions on graphene surface, the other 2D materials, ones on TMD was not sufficiently studied. In this thesis, I have studied the surface hydrogenation reaction on TMDs. Similar to the concept of graphene functionalization, chemical functionalization is known for leading to modification of their electronic and optical properties. Theoretical calculations predicted that the electronic structure of single-layer TMDs also can be tuned by hydrogenation. However, its experimental demonstration has not been realized so far. In addition, TMDs can be used as outstanding catalyst for hydrogen evolution reaction (HER). Therefore, the systematic investigation of hydrogenation on TMDs cannot only unveil the modified electronic structures in TMDs, but also can provide the critical information to understand the interaction between hydrogen atom (or molecule) and TMDs, which is fundamentally important for improving HER efficiency. Here we show modification of electronic properties in MoSe2, one of TMD materials, which is synthesized by chemical vapor deposition (CVD) process. The photoluminescence (PL) intensity and peak position indicates a direct band gap of 1.54 eV for the single-layer MoSe2. After the hydrogenation by H2 plasma treatment, semiconducting properties of single-layer MoSe2 turn into insulator. In a step-by-step PL results, hydrogenation reaction started from edge to center. Also, we confirmed the hydrogen atoms only react with selenium atoms (Se) in X-ray photoelectron spectroscopy (XPS) analysis. This study demonstrates the great potential of controlling electronic property of single-layer MoSe2 and fundamentally understanding about hydrogenation as a surface functionalization study.