Electric Field, Strain, Magnetic Proximity Effect in Two-Dimensional Heterostructures: A Theoretical Study
Due to the self-passivated and dangling bond free surfaces of two-dimensional (2D), a variety of vertical heterostructures are designed with a wide range of bandgap and material properties. We use first-principles simulations to investigate the electric field, strain and magnetic proximity effect in 2D heterostructures.
Both monolayer WSe2/monolayer MoSe2 and bilayer WSe2/monolayer MoSe2 form intrinsic type II heterojunction. As the electric field is ramped from negative to positive, the band structure of both structures shows a transition from indirect to direct bandgap. The bilayer WSe2/monolayer MoSe2 even shows a transition from type I heterojunction to type II heterojunction under negative electric field.
HfSe2/SnS2 is indirect bandgap heterostructure and shows a coherent superposition of the conduction band wavefunctions of the individual layers at conduction band minimum (CBM). The CBM without electric field is weighted towards SnS2 layer, a vertical electric field of 0.2 V/Å, pointing from HfSe2 to SnS2 layer, reverses the weights of the conduction band wavefunction. Placing graphene on HfSe2/SnS2 results in significant charge transfer from graphene to the heterostructure, and the trilayer system forms a negative Schottky barrier contact. The contact resistance of graphene on HfSe2/SnS2 calculated from a tunneling Hamiltonian indicates an excellent low-resistance contact.
PtSe2/SnS2 forms a Mexican hat in the valence bands around Γ. The in-plane biaxial strain can significantly tune the band structures of PtSe2/SnS2. Under tensile strain the height of Mexican hat is more than six times of the value without strain; while under compressive strain, a semiconducting to metallic transition is observed. Graphene in contact with SnS2 layer of PtSe2/SnS2 heterostructure forms negative Schottky barrier.
Recent experiments demonstrating proximity induced ferromagnetism in graphene motivate this study of commensurate EuO/graphene/EuO heterostructures. Using insights from lattice symmetries of EuO/graphene/EuO heterostructures, we developed a model Hamiltonian that includes proximity induced exchange splitting, spin-orbit coupling, and inter-valley interactions with parameters fitted to ab initio calculations. The intervalley interaction opens a trivial gap preventing the system from crossing into a non-trivial state. The model Hamiltonian is analyzed to determine the conditions under which the heterostructures can exhibit topologically non-trivial bands.