Yaqiang Ma

Tuning the Schottky contacts at the graphene/WS2 interface by electric field

Using the first-principle calculations, we study the electronic structures of graphene/WS2 van der Waals (vdW) heterostructures by applying an external electric field (Eext) perpendicular to the heterobilayers. It is demonstrated that the intrinsic electronic properties of graphene and WS2 are quite well preserved due to the weak vdW contact. We find that n-type Schottky contacts with a significantly small Schottky barrier are formed at the graphene/WS2 interface and p-type (hole) doping in graphene occurs during the formation of graphene/WS2 heterostructures. Moreover, the Eext is effective to tune the Schottky contacts, which can transform the n-type into p-type and ohmic contact. Meanwhile, p-type (hole) doping in graphene is enhanced under negative Eext and a large positive Eext is required to achieve n-type (electron) doping in graphene. The Eext can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/WS2 interface. The present study would open a new avenue for application of ultrathin graphene/WS2 heterostructures in future nano- and optoelectronics.

Band structure engineering in a MoS2/PbI2 van der Waals heterostructure via an external electric field

Band structure engineering in a MoS2/PbI2 van der Waals (vdW) heterostructure under an external electric field (Efield) is investigated using density functional theory (DFT). It is demonstrated that the MoS2/PbI2 vdW heterostructure has a type-II heterojunction with a direct bandgap, and thus the lowest energy electron-hole pairs are spatially separated. Meanwhile, the band structure could be effectively modulated under an Efield and the bandgap shows linear variations with the Efield, indicating a giant Stark effect. This gets further support from the band edges of MoS2 and PbI2 in the heterostructure. Moreover, the MoS2/PbI2 vdW heterostructure experiences transitions from type-II to type-I and then to type-II under various Efields. Our calculated results pave the way for experimental research and provide a new perspective for the application of the vdW heterostructure in electronic and optoelectronic devices.

Monoclinic Ga2O3 (100) surface as a robust photocatalyst for water-splitting

The β-Ga2O3 (100) surface, with or without defects, as a robust photocatalyst for water decomposition was studied on the basis of density functional theory (DFT). The surface defects considered, herein, were oxygen vacancies and doping with higher chalcogens, such as S, Se and Te. Narrowed bandgaps of the defective surfaces, leading to a high utilization of solar energy with respect to pure Ga2O3, were observed. By optimizing the geometrical structures of the initial molecular adsorption states (IS), the transition states (TS) and the final dissociative adsorption states (FS), the reaction activation energy and the adsorption energy of each species in the reaction pathway were obtained. Water acts as a Lewis base and provides electrons to the surfaces. The presence of water on the surfaces more likely preferred the molecular modes. The reaction results demonstrate that the surface is robust for water decomposition, where the defects, both vacancies and doping with high chalcogens, have no evident influence on the reaction parameters. The reaction pathway can be improved by vacancies or Se doping. These findings for water decomposition on Ga2O3 (100) surfaces can be used in synthesis of photocatalysts and for understanding the interactions across the reaction pathway.

Effect of an external electric field on the electronic properties of SnS2/PbI2 van der Waals heterostructures

The future development of optoelectronic devices will require an advanced control technology in electronic properties, for example by an external electric field (Efield). Here we demonstrate an approach that the heterostructure based on van der Waals (vdW) heterobilayer built by monolayer SnS2 and PbI2 has a well-controlled electronic properties with Efield. A type-II staggered-gap band alignment is achieved from the SnS2/PbI2 vdW heterostructure with which SnS2 dominated the lowest energy holes as well as the lowest energy electrons are separated in PbI2. The charge redistribution with an Efield is mainly on the surface of SnS2 layer and PbI2 and the numbers of polarized electrons on the monolayers display a linear evaluation with external Efield. The band structure under different Efield experiences not only a transition from semiconductor to metal but also conversions between type-I straddling-band alignment and type-II staggered-gap, which results in different spatial distribution of the lowest energy electrons and holes. Moreover, when the Efield is between −0.06 V A−1 and −0.34 V A−1, the material manifests a varied direct bandgap which is more favor to optoelectronics and solar cell. Consequently, this vdW heterobilayer with well-controlled manner shows expectation for huge potential in optics and electronics.

Strain effects on the magnetism of transition metal-doped MoTe2 monolayer

Abstract Using first-principles calculations, we investigated the electronic and magnetic performance of MoTe2 monolayer doped by transition metal (TM) Ti, V, Cr, Mn, Fe, Co and Ni atoms as well as strain effects. The dopants of Ti, V, Mn, Fe, Co and Ni atoms can induce magnetic moments in MoTe2 monolayer, and the magnetic moments mainly originate from the localizing unpaired TM-3d electrons. Mn- and Fe-doped MoTe2 nanostructures show the half-metallic character with 100% spin polarization near the Fermi level. The elastic strain applied on TM-doped MoTe2 monolayer systems leads to the redistribution of the electrons in TM-3d states, which results in the magnetic state transition in doped systems. The magnetic moments of Ti-, Co- and Ni-substituted MoTe2 sheets monotonously increase with the increase in strain, while the magnetic moment of V-substituted MoTe2 sheet has an oscillatory variation with the increase in strain. The Hubbard potential U has no significant effect on our main conclusions. The research results offer an important theoretical support for further application of strain-driven spin devices on MoTe2 nanostructures.