Nannan Han

Atomistic insight into the oxidation of monolayer transition metal dichalcogenides: from structures to electronic properties

Monolayer transition metal dichalcogenides (TMDs) stand out in two-dimensional (2D) materials due to their potential applications in future microelectronic and optoelectronic devices. In experiments, field effect transistors (FET) based on the MoS2 monolayer are sensitive to environmental gases, especially O2. Thus, the oxidation of monolayer TMDs is a critical concern. By first-principles calculations, we reveal that a perfect single-layer sheet of TMDs stays intact when exposed in O2 due to the weak physical adsorption of O2. However, O2 can be chemically adsorbed onto the monolayer of TMDs (including MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2) with single vacancies of chalcogen, which are the most common defects in realistic TMD materials. The adsorption configurations and dissociation behavior of the O2 molecule at vacancy sites, as well as the possible diffusion behavior of oxygen adatoms on the TMD monolayer surface were explored. Oxidation significantly influenced the electronic properties of a defective MoS2 monolayer, while other defective TMD monolayers (especially MoTe2 and WTe2) suffered less from oxidation. Our theoretical results provide valuable atomistic insight into the oxidation of TMD monolayers and are useful for the future design of TMD-based 2D devices.

Band gap opening in bilayer silicene by alkali metal intercalation

Recently, bilayer and multilayer silicene have attracted increased attention following the boom of silicene, which holds great promise for future applications in microelectronic devices. Herein we systematically investigate all stacking configurations of bilayer silicene and the corresponding electronic properties. Strong coupling is found between two silicene layers, which destroys the Dirac cones in the band structures of pristine silicene and makes bilayer silicene sheets metallic. However, intercalation of alkali metal (especially potassium) can effectively decouple the interaction between two silicene layers. In the K-intercalated bilayer silicene (KSi4), the Dirac cones are recovered with a small band gap of 0.27 eV located about 0.55 eV below the Fermi level. Furthermore, intercalation of K(+) cations in bilayer silicene (K(+)Si4) results in a semiconductor with a moderate band gap of 0.43 eV, making it ideal for microelectronic applications.

Lateral heterostructures of monolayer group-IV monochalcogenides: band alignment and electronic properties

Lateral semiconductor/semiconductor heterostructures made up of two-dimensional (2D) monolayer or few-layer materials provide new opportunities for 2D devices. Herein, we propose four lateral heterostructures constructed by phosphorene-like monolayer group-IV monochalcogenides, including GeS/GeSe, SnS/GeSe, SnSe/GeS and GeS/SnS. Using first-principles calculations, we investigated the energetics and electronic properties of these lateral heterostructures. The band structures and formation energies from supercell calculations demonstrate that these heterostructures retain semiconducting behavior and can be easily synthesized in the laboratory. The band offsets of monolayer, bilayer and trilayer heterojunctions at the Anderson limit are calculated from the valence/conduction band edges with respect to the vacuum energy level for each individual component. Among them, some heterostructures belong to type II band alignment and are promising for a high-efficiency solar cell.

Schottky barrier at graphene/metal oxide interfaces: insight from first-principles calculations

Anode materials play an important role in determining the performance of lithium ion batteries. In experiment, graphene (GR)/metal oxide (MO) composites possess excellent electrochemical properties and are promising anode materials. Here we perform density functional theory calculations to explore the interfacial interaction between GR and MO. Our result reveals generally weak physical interactions between GR and several MOs (including Cu2O, NiO). The Schottky barrier height (SBH) in these metal/semiconductor heterostructures are computed using the macroscopically averaged electrostatic potential method, and the role of interfacial dipole is discussed. The calculated SBHs below 1 eV suggest low contact resistance; thus these GR/MO composites are favorable anode materials for better lithium ion batteries.

2D lateral heterostructures of group-III monochalcogenide: Potential photovoltaic applications

Solar photovoltaics provides a practical and sustainable solution to the increasing global energy demand. Using first-principles calculations, we investigate the energetics and electronic properties of two-dimensional lateral heterostructures by group-III monochalcogenides and explore their potential applications in photovoltaics. The band structures and formation energies from supercell calculations demonstrate that these heterostructures retain semiconducting behavior and might be synthesized in laboratory using the chemical vapor deposition technique. According to the computed band offsets, most of the heterojunctions belong to type II band alignment, which can prevent the recombination of electron-hole pairs. Besides, the electronic properties of these lateral heterostructures can be effectively tailored by the number of layers, leading to a high theoretical power conversion efficiency over 20%.