Synthesis and properties of 2d layered semiconductors and heterostructures

Thesis

More than a decade after the successful isolation of the first monolayer two-dimensional (2D) layered materials in 2004, a major focus in contemporary material science research is to expand the 2D family with novel layered materials or vertical layered heterostructures (VLHs) presenting extraordinary properties. Especially, 2D layered semiconductors with various bandgaps are considered as promising candidates for versatile applications in nanoelectronics, optoelectronics, sensors, etc. This DPhil project aims to contribute to the field by developing fundamental knowledge in the synthesis of 2D layered semiconductors and their VLHs via vapour phase deposition, and also in subsequent characterisation of their material properties for optoelectronic applications.

Simple direct sulfurization process was first developed for 2D semiconductor MoS2 which employs glass as a precursor-soluble substrate. This approach enables low cost and large-area uniformity as well as self-limiting crystal thickness and small nucleation density. Understanding of the diffusion-controlled dissolution and interface-limited precipitation processes facilitated morphological tuning of the MoS2 single crystals. Then, thickness- and stoichiometric-controlled growth of 2D GaS, a wide bandgap semiconductor, was achieved by chemical vapour deposition (CVD). A detailed investigation was performed into the stoichiometry- and thickness-dependent stability, optical and optoelectronic properties of GaS.

Both layer-by-layer growth and stacking assembly are commonly used for fabrication of 2D VLHs. Based on the Frenkel-Kontorova model, a general framework was proposed for understanding the origin of small twist angles in high-mismatch epitaxially-grown VLHs. Experimentally, the framework was tested on CVD-grown epitaxial GaS/MoS2 and GaS/graphene by scanning transmission electron microscopy. Finally, the non-stoichiometric GaS0.87 was stacked with WS2 to investigate optoelectronic behaviour of the bilayer VLH. The band alignment and charge transfer behaviour of the isotype type-II heterojunction were probed by various spectroscopic characterisation techniques. In addition, both large linear dynamic range and large photoresponsivity across the visible light spectrum were observed for GaS0.87/WS2 photodetectors.

Authors: Lu, Y

• DOI: 10.5287/bodleian:ZO90dAma9
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: epitaxy; chemical vapour deposition; photodetectors; defects; gallium sulfide; two-dimensional (2D) materials; transition metal dichalcogenides
• Subjects: monolayer; Detectors; Heterostructures; Synthesis; Semiconductors; Two-dimensional materials; Crystals