Synthesis of 2D Crystals and Structures for Electronic Devices

Developing new type of 2D materials remains highly desirable given their versatile properties and applications in electronic nanodevices. Conventional 2D materials such as graphene and transition metal dichalcogenides, whose bulks are van der Waals layered materials, have been realized through top-down mechanical exfoliation and self-limited in plane growth using chemical vapor deposition (CVD), thanks to the weak binding among layers in their bulks. Consequently, unprecedented properties have been observed in the monolayers compared to the bulk. However, the majority materials in nature, are atomically bonded in three-dimensional mode. Synthesizing atomically thin 2D crystals of these materials using the conventional synthetic strategies remains a great challenge. In addition, although progress has been made in successful synthesis of a library of van der Waals 2D materials such as graphene and transition metal dichalcogenides, there are still a lot of members in the family underexplored. Driven by the need of high crystallinity and scalable 2D materials for electronic devices, as well as a low thermal budget growth condition enabling the 3D monolithic integration, our group focuses on developing effective synthesis strategies for various 2D crystals with diverse electronic properties. In this talk, I will first introduce the atomic substitution method where we chemically convert layered metal chalcogenides such as MoS2, WS2 and GaS, to ultrathin metal nitrides such as MoN_x, WN_x, and GaN. The thickness of the obtained nitrides can be tuned by using different number of layers of metal chalcogenides, providing a platform for the study of their fundamental properties at quantum region and for the application in nanoelectronics devices. Moreover, the conversion process is investigated using transmission electron microscope, where edge and surface conversion are observed, leading to the formation of lateral and vertical heterostructures. Particularly, a mask-assisted method is used to obtain the MoS₂-MoNx-MoS₂ lateral junction, where metallic MoNx serves as the electrode for semiconducting MoS₂ field-effect transistors (FET). Second, I will introduce our recent work on realizing ultrathin 3d metal hydroxides with long-range ordering. The UV-vis. spectra bandgap and electrical measurements show the semiconductor behavior of the materials.