Spin-dependent Schottky barriers and vacancy-induced spin-selective ohmic contacts in magnetic vdW heterostructures.

2D ferromagnets, such as CrX3 (X = Cl, Br and I), have been attracting extensive attention since they provide novel platforms to fundamental physics and device applications. Integrating CrX3 with other electrodes and substrates is an essential step to their device realization. Therefore, it is important to understand the interfacial properties between CrX3 and other 2D materials. As an illustrative example, we have investigated the heterostructures between CrX3 and graphene (CrX3/Gr) by first-principles calculations. We found a unique Schottky contact type with strongly spin-dependent barriers in CrX3/Gr. This can be understood by synergistic effects between the exchange splitting of the semiconductor band of CrX3 and interlayer charge transfer. The spin-asymmetry of Schottky barriers may result in different tunneling rates of spin-up and down electrons, and then lead to spin-polarized current, namely the spin-filter (SF) effect. Moreover, by introducing X vacancies into CrX3/Gr, an ohmic contact forms in the spin-up direction. It may enhance the transport of spin-up electrons, and improve the SF effect. Our systematic study reveals the unique interfacial properties of CrX3/Gr, and provides a theoretical view of the understanding and designing of spintronic devices based on magnetic vdW heterostructures.

The enhanced ferromagnetism of single-layer CrX3 (X = Br and I) via van der Waals engineering.

The recent experimental discovery of intrinsic ferromagnetism in single-layer CrI3 opens a new avenue to low-dimensional spintronics. However, the low Curie temperature, TC ∼ 45 K, is still a large obstacle to its realistic device application. In this work, we demonstrate that the TC and magnetic moment of CrX3 (X = Br, I) can be enhanced simultaneously by coupling them to buckled two-dimensional Mene (M = Si, Ge) to form magnetic van der Waals (vdW) heterostructures. Our first-principles calculations reveal that n-doping of CrX3, induced by a significant spin-dependent interlayer charge-transfer from Mene, is responsible for the drastic enhancement of TC and magnetic moment. Furthermore, the diversified electronic properties including half-metallicity and semi-conductivity with a configuration-dependent energy gap are also predicted in this novel vdW heterostructure, implying broad potential applications in spintronics. Our study suggests that vdW engineering may be an efficient way to tune the magnetic properties of 2D magnets, and Mene/CrX3 magnetic vdW heterostructures are wonderful candidates in spintronics and nanoelectronics devices.

Achieving a direct band gap and high power conversion efficiency in an SbI3/BiI3 type-II vdW heterostructure via interlayer compression and electric field application.

Type-II van der Waals (vdW) heterostructures are considered as a class of competitive candidates of high-efficiency photovoltaic materials, due to their spontaneous electron-hole separation. However, most of the vdW heterostructures possess an indirect gap and a large band offset, which would lead to low photon-to-electron conversion efficiency. Taking an SbI3/BiI3 vdW heterostructure as an illustrative example, we propose interlayer compression and vertical electric field application as two effective strategies to modulate the electronic and photovoltaic properties of type-II vdW heterostructures. Our results reveal that a lattice-matched SbI3/BiI3 vdW heterostructure has an indirect band gap of 1.34 eV with the conduction band minimum (CBM) at the Γ point and the valence band maximum (VBM) between the Γ and M points. The power conversion efficiency (PCE) of an SbI3/BiI3-based excitonic solar cell (XSC) is predicted to be about 14.42%. When compressing the heterostructure along the vdW gap direction, the highest valence band state at the Γ point is lifted significantly and the VBM gradually approaches the Γ point, implying an indirect-direct gap transition. This interesting evolution can be attributed to the increasing k-dependent electronic hybridization of the pz orbitals of interlayer adjacent I atoms with a reduced interlayer distance. Moreover, the interlayer compression also enhances the PCE of the system monotonically. When applying a vertical electric field, the band alignment of the heterostructure undergoes a transition from type-II to type-I and then returns to type-II between 0.1 and 0.6 V Å-1. Meanwhile, the PCE of the SbI3/BiI3 XSC could be enhanced up to 21.63%. This work provides guidance for improving the electronic and photovoltaic properties of type-II vdW heterostructures.

Electronic Structure and Magnetic Anisotropy of Single-Layer Rare-Earth Oxybromide.

The discovery of intrinsic magnetism in two-dimensional (2D) limit has triggered increasing investigations in layered magnetic materials. However, most of the available candidates involves 3d transition metals, while the layered rare-earth magnetic materials are largely unexplored at present. Here, we proposed a series of 2D rare-earth magnetic semiconductors REOBr (RE = Tb, Dy, Ho, Er and Tm) with large magnetic moments and magnetic anisotropy energies using the PBE + U method. Our calculations indicate a half-metallic meta-stable state and a low-energy semi-conducting ground state in these 4f single-layers, which can be characterized by the location of the two-fold degenerate x(x 2 - 3y 2) orbital. The dynamical stability of single-layer REOBr is further confirmed using phonon dispersions. The predicted energy gaps ranging from 2.47 to 4.26 eV decrease with the atomic number of the rare-earth element. Meanwhile, very large spin moments and orbital moments up to 6.018 and 2.872 µB are found, which seem to be insensitive to the magnetic state. Furthermore, the magnetic anisotropy energies are evaluated and understood by a fourth-order non-uniaxial anisotropy mode. Diverse anisotropy energy landscapes including easy cone, easy plane, and easy axis are found, and an extremely high magnetic anisotropy energy of about 8 meV per RE atom is found in the single-layer DyOBr. Our investigations provide a unique insight into layered rare-earth magnetic materials and suggest the single-layer REOBr as competing candidates for low-dimensional data storage applications.