Experimental and theoretical studies of electronic and mechanical properties of two-dimensional (2D) WSe₂

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with intrinsic band gaps are considered to be prospective alternatives for graphene in the applications of emerging nano-semiconductor devices. As a significant member of the TMDs family, WSe₂ with superior optical properties attracts increasing attention, especially in the optoelectronics. In this thesis, the electronic and mechanical properties of 2D WSe₂ have been studied experimentally and theoretically. Firstly, the fabrication of substrate-supported and suspended pre-patterned WSe₂ FETs with the low-cost optical lithography and vapour HF etching technology have been realised. The subsequent electrical measurement of the fabricated WSe₂ FETs indicates that the WSe₂/dielectric interface can affect the electrical performance of 2D WSe₂ negatively. To gain more insights on the impact of field-effect on 2D WSe₂, first-principle calculations have been conducted in this research to study the evolutions of the crystal structure, electronic band structure, conductive channel size, and electrical transport property of WSe2 under various levels of field-effect. Furthermore, a layer thinning and chemical doping method of 2D WSe₂ by vapour XeF₂ exposure featured with good air-stability, scalability, and controllability has been developed to enable the layer engineering of 2D WSe₂ and integration of 2D WSe₂ to logic circuits, solar cells, and light-emitting diodes (LED). The thinning and doping mechanism has been investigated with a combination of Raman spectroscopy, photoluminescence (PL) spectroscopy, and Xray photoelectron spectroscopy (XPS) characterization techniques. Afterwards, the inplane elastic properties (including the Young’s modulus, breaking strain, and etc.) of 2D WSe₂ have been measured with nanoindentation experiments implemented by atomic force microscopy (AFM). The results prove the suitability of 2D WSe₂ in the applications of flexible devices and nanoelectromechanical systems (NEMS) operating in the audio resonance frequency, such as acoustic sensors and loudspeakers. To provide a comprehensive understanding of the strain engineering of 2D WSe₂, the strain induced variations of the crystal structure, electronic band structure, and electrical transport property of 2D WSe₂ have been further studied with first-principle calculations, which paves the way for the performance tuning of 2D WSe₂ devices via strain and applications of 2D WSe₂ in strain sensors.