FIRST PRINCIPLES STUDY OF 2D POLAR HETEROSTRUCTURES

Abstract

Recently, two-dimensional (2D) heterostructures have attracted extensive attention in nanomaterials science. They have been successfully fabricated and applied to nanotechnology in many fields, such as nanoelectronics, solar cells, sensors, energy stores, quantum information, etc. The most common heterostructures are 2D-lateral heterostructure (LH) and 2D-vertical heterostructure (VH) where each of them exhibits unique features depending on the direction of assembly, i.e., along in-plane or out-of-plane direction. Beyond the van der Waals-VH which possess of van der Waals (vdW) interaction, there are other types of heterostructures made of 2D polar materials that possess different types of chemical bonding nature, e.g., chemical bonds with less (e.g., SiC monolayer) or more (e.g., GeC and SiGe monolayers) charge transfer between atoms, forming covalent bonds with a certain ionicity. The goal of this work focused on shedding light on the physical aspects of 2D LH and VH, constructed by such polar materials (e.g., 𝑆𝑖𝐶, GeC, and SiGe monolayers). This work is a theoretical study by employing Density Functional Theory to unravel the unique physical properties of such heterostructures. Because an artificial strain will be induced by the lattice mismatch in building heterostructures, the effect of strain on the electronic properties of 𝑆𝑖𝐶, 𝐺𝑒𝐶, and 𝑆𝑖𝐺𝑒 monolayers was first investigated. It was found that these monolayers can tolerate strain up to 8%, and such strain can induce modifications on the physical properties. Interestingly, it was found that 𝑆𝑖𝐶 and 𝐺𝑒𝐶 monolayers undergo a direct-indirect band gap transition; while, 𝑆𝑖𝐺𝑒 monolayer undergoes a metal-semimetal transition, which made them attractive candidates for building heterostructures. Second, a systematic study on the aspect of 2D polar-LH of 𝑆𝑖𝐶/𝐺𝑒𝐶, and 𝑆𝑖𝐺𝑒/𝐺𝑒𝐶 has been conducted. It was found that the synergistic effect of the lattice mismatch induced strain, the chemical bonding nature at the interface, and quantum confinement can lead to several interesting phenomena. For instance, their electronic properties can be modulated by tuning the domain size, the chemical bonding nature, and the designing of interface. Accordingly, a lateral spontaneous p-n junction triggered by the in-plane charge transfer was detected which implies the promising applications such as visible light photocatalyst. Third, the roles of the stacking species arrangement and the interlayer interactions (including vdW and electrostatic forces) on stabilizing the structure and modulating electronic properties of 2D polar-VH of 𝑆𝑖𝐺𝑒/𝐺𝑒𝐶 were deeply studied. It was found that, in addition to the redistribution of the in-plane net-charge transfer, a net charge redistribution also occurs between layers and leads to a polarization in the interfacial region that induces a built-in electric field and helps to reduce the recombination of photogenerated electron-hole pairs.