First-principles Study Of Ultra-thin Pb/MoTe2 Heterojunction Materials

Transition metal dichalcogenides have a graphene-like layered structure. When it changes from bulk material to monolayer, the band structure changes from an indirect band gap to a direct band gap and the band gap becomes larger. The band gap lies at visible light and near-infrared frequency range, so the monolayer transition metal dichalcogenides have a good optical properties. In addition, two valleys at (?) and (?)' are not equivalent due to the the break of the inversion symmetry when the thickness of the transition metal dichalcogenides decreases to monolayer. It results in the optical transitions having a valley dependent selection rule. So, the transition metal dichalcogenides have been aroused a great attention due to their certain potential application in electronics, photonics, valley electronics, spin electronics and other aspects. In addition, it is also possible to construct heterostructures by selectively stacking two different two-dimensional materials, that may lead to new physical properties and phenomena. It is important technically to study the interface between ultrathin metal films and semiconductor substrates since metal-semiconductor contacts are widely present in electronic devices. In this paper,we consider the ultrathin heterostructures constructed by metal Pb and semiconductor MoTe2. On the one hand, the lattice mismatch between MoTe2 (lattice constant 3.52?)with Pb ((111) plane is 3.50A) is very small. On the other hand, Pb and MoTe2 have large spin-orbit coupling effect. Large spin splitting is very important in spin electronics. It can be seen that the ultra-thin Pb(111)/ MoTe2 heterostructures is a good candidate material, and its research is helpful to understand the new effect of quantum confinement, the broken of inversion symmetry, spin-orbit coupling effect in the hybrid system. In this paper, we study the electronic structure and spin splitting of two-dimensional MoTe2 and ultrathin Pb/MoTe2 heterostrctures by using the first principle. The spin texture in momentum space and the hybrid effect at the interface on the band structure are analyzed and discussed.The main research work has two aspects:1.We studied the evolution of band structure with the number of layers of two-dimensional MoTe2 and the spin splitting of monolayer MoTe2 in the case of spin-orbit coupling. By using the first-principles calculation based on density functional theory, we find that the MoTe2 changes from direct band gap to indirect band gap and the band gap becomes smaller when MoTe2 changes from monolayer layer to the bulk. Through the relaxation of the geometrical structure, we find that the van der Waals coupling between the two layers of MoTe2 is slightly stronger than that in the bulk MoTe2. After taking into account the spin-orbit coupling, the most remarkable feature of the band is disappear of the spin degenerate except at ? and M high-symmetry points. The size of splitting is 224meV at the (?) point, and the spin-polarization orientation is opposite at (?) and (?)' points due to time reversal symmetry. MoTe2 have large spin splitting than MoS2 and MoSe2 due to its strong spin-orbit coupling effect. This character of monolayer MoTe2 makes it have some potential applicaltions in spintronics, optics and so on.2.The band structure and the spin splitting of Pb/ MoTe2 heterostructures are calculated. By first-principles calculations based on the density functional theory, we find that there are different types of Rashba spin splitting at different high symmetry point on the band structure of Pb/1L-MoTe2. Because of the charge transfer at the interface, the out-plane potential gradient leads to in-plane Rashba spin splitting near the ? and M points. At the (?) ((?)') point, there is only out-of-plane spin-polarization due to the C3 rotational symmetry of the honeycomb layered structure and the spin-polarization orientation is opposite at (?) and (?)' points due to time reversal symmetry. The spin splitting of the bottom of the conduction at (?) reaches about 350 meV. To help understand the influence of hybridize effect at the Pb/1 L-MoTe2 interface on the band structures, we have also performed a set of calculations by artificially increasing the interfacial distance. With increasing the interfacial distance, the hybridization becomes weaken and the antibonding band shifts downward and even locates below the Fermi level, resulting in large changes of the dispersions of upper valence bands near the ? point. Furthermore,with increasing the interfacial distance, surface potential gradient decreases due to the decrease of charge transfer. So the spin splitting of the bands from Pb becomes smaller. Finally,we also calculate and discuss the interaction between Pb and multilayer MoTe2, and we find that the band structure and spin splitting of Pb/nL-MoTe2(n (?) 2) are similar to that in Pb/1L-MoTe2 heterostructure. This is because the multilayer MoTe2 connected by the van der Waals coupling has little effect on the interfacial interaction, so that the number of layers of MoTe2 has little effect on the band structure and the corresponding Rashba spin splitting of the heterostructures. Furthermore, our calculations show that the spin-polarized bands closely approach the Fermi level in Pb/MoTe2 heterostructure,showing that this heterostructure may be a good candidate in valleytronics or spitronics.