Structures And Properties Of Junctions In Two-Dimensional Materials

Conventional bulk material junctions are the key components in fabricating devices such as field-effect transistors, photovoltaic cells and light-emiiting diodes. Extensive effort has been made to improve their performance and fabricate new promising junctions. Over the past two decades, the 2-dimensional (2D) materials have attracted great interests because of their promising electronic and mechanical properties. They also provide new opportunities to fabricate novel 2D junctions to build 2D nano devices. These 2D junctions can be fabricated by stacking two 2D monolayers together to form vertical junctions, or by connecting two materials within one monolayer to form lateral junctions. Recent studies show that these 2D junctions have promising novel properties and good performance in various devices. The goal of this dissertation is to deepen understanding of the structure of these 2D junctions and look for new ways to tune their properties for various applications. In particular, we investigate the structures of vertical and lateral 2D junctions by calculating the generalized stacking fault energy in bilayer 2D materials and the heterophase interface energy in lateral transition metal dichalcogenide (TMD) junctions. Moreover, we investigate the effects of alloying, twisting and lattice misfit on the band structure of TMD vertical junctions. We prove that the heterojunction of Type I band alignment with preferred direct bandgap can be achieved within a large composition region and a wide range of twist angles. The work of this dissertation provides a deeper understanding of the structures of 2D junctions, and provides new opportunities and methods to fabricate novel 2D nano-devices.