Novel strategies for chemical vapor deposition growth and engineering of two-dimensional materials

• Main Author: Lin, Jinjun
• Other Authors: Teo Hang Tong Edwin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/144187
• Institution: Nanyang Technological University

Two-dimensional (2D) materials are atomically thin materials that possess many superior and outstanding properties as compared to its bulk counterpart. Depending on its elemental composition, 2D materials encompass a full spectrum of electronic properties ranging from semi-metallic to insulating and hold great promise for next-generation nanoelectronics. In order to achieve practical utilization, large-scale fabrication of 2D materials and engineering of their properties are fundamental. In this thesis, chemical vapor deposition (CVD) growth of various types of 2D materials including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) were investigated and new methods were developed to enhance or modify their properties. Firstly, CVD growth and characterization of hexagonal shaped single-crystal graphene domains on Cu foil was demonstrated. Following this growth strategy, doping of these single crystals was performed by using a nitrogen (N)-containing single-source precursor known as hexamethylenetetramine (HMTA). Importantly, it was discovered for the first time that segregation of dopant concentration exists even in monolayer N-doped graphene (NG) single crystals. This study provides a critical insight into the growth mechanism of CVD-grown NG and enables new opportunities to tailor the properties of graphene toward applications in high-performance 2D electronics. 2D h-BN, an electrical insulator, is a perfect complement to graphene and the best-known substrate material for all 2D materials to date owing to its atomic smoothness and lack of dangling bonds. CVD has been recognized as one of the most pragmatic approach to produce large-area and high-quality h-BN films. However, the strain-induced wrinkles in CVD-grown h-BN films, which cause high surface roughness, still remained a major drawback which severely degrade device performance. Here, by employing a post-synthesis annealing process, wrinkles on the h-BN film were effectively eliminated. The unwrinkled h-BN film showed significant surface smoothness enhancement and resulted in a much cleaner surface, which has high potential use for scalable fabrication of high-performance 2D heterostructure devices. Layered TMDs is another important class of 2D materials with exceptional electronic and optoelectronic properties. Particularly, vertically aligned TMDs are highly promising for optoelectronics and electrochemical devices due to the much higher density of exposed active edge compared to their laterally oriented counterparts. In this work, a versatile and scalable CVD growth of vertically aligned MoTe2 on reusable Mo foil is demonstrated for the first time. Importantly, the as–grown MoTe2 can be directly dispersed in solvent to produce high–quality MoTe2 nanosheets. Furthermore, the versatility of this growth strategy was demonstrated by synthesizing other vertically aligned TMDs such as TaTe2 and MoSe2. Hence, this work paves the path towards achieving unique TMDs structures to enable high–performance optoelectronic and electrochemical devices.