The Interfacial Effect And Topological Phase In Two-Dimensional Materials

The graphene/h-BN heterojunction has been extensively studied in theory and experiment. Graphene can form the Moire superstructures with the h-BN substrate.30 Different rotation angles can form the different supercells. The interactions between graphene and h-BN are the vdW interactions. The heterojunction can keep the Dirac cones of graphene and its high Fermi velocity. From the view of graphene/h-BN heterojunction, we propose the silicene (germanene)/h-BN vdW heterojunction.Using first-principles calculations, we predict h-BN with flat surface is an ideal substrate for silicene (germanene). Similar to the graphene/h-BN heterojunction, the attachment of silicene (germanene) on h-BN substrate may result in a random rotational orientation between the silicene (germanene) and h-BN lattices. By the four integers, (m n/p q), we can construct the Moire superstructures with different sizes and the relative rotation angle θ between the two lattices is given.The interactions between silicene (germanene) and h-BN are the vdW interactions, which can be confirmed by calculating the adsorption energies in different interlayer distances with PBE and PBE+vdW calculations. The PBE+vdW calculations indicate that silicene (germanene) can stably attach on h-BN substrate without any energy barriers, whereas PBE functional without vdW correction fails to give stable adsorption state.The adsorption energy is independent of rotation and translation modulations, which mean that all the superstructures may be achieved in experiments, similar to that of graphene/h-BN heterojunctions. The rotation angle and stacking pattern between silicene (germanene) and h-BN are random.For the band structure, all heterojunctions are the direct semiconductor with a small band gap opened in the Dirac cones. For silicene/h-BN heterojunctions, the gap is about 30meV and it is 50meV for germanene/h-BN heterojunctions. The band gap is also independent of rotation and translation modulations. By changing the pressure, the band gap can be effectively tuned. High Fermi velocity of silicene (germanene) is preserved in the heterojunctions. These features are helpful in achieving applications of silicene in nanoscale electronic devices.In experiment, silicene is successfully synthesized on bulk MoS2 surface, showing the metallicity. The buckling height of the silicene is about 2A, much larger than that of low-buckled (LB) silicene (0.45 A), so it can be regard as the high-buckled (HB) silicene. Then we researched the multifarious stacking patterns for HB and LB silicene on MoS2 monolayer substrate.We used a lattice match model to study the HB silicene on MoS2 substrate (HSMS heterojunction). According to the relative position of the two lattices, we chose six stacking patterns. After structure optimization, we found the energetically most favorable stacking pattern and its interlayer distance and buckling height are in good agreement with the experimental data. For LB silicene on MoS2 substrate (LSMS heterojunction), we used the lattice mismatch model, forming the Moire superstructures. The method of constructing Moire superstructures of LSMS heterojunction is similar to that of silicene (germanene)/h-BN.The interactions between HB and LB silicene and MoS2 substrate are the vdW interactions. This can be confirmed by calculating the adsorption energies in different interlayer distances with PBE and PBE+vdW calculations. The PBE+vdW calculations indicate that HB and LB silicene can stably attach on MoS2 substrate showing an obvious adsorption energy minimum, whereas PBE functional without vdW correction fails to give stable adsorption state, due to the failure of PBE in describing weak interactions. The vdW interactions between LB and HB silicene and MoS2 substrate can also be confirmed by the electron localization function (ELF), which shows that no covalent bonds are formed between silicene and MoS2 substrate.All the LSMS heterojunctions have the similar formation energy, showing it is independent of rotation angle and stacking pattern. The formation energy of LSMS heterojunction is much less than that of HSMS heterojunction, indicating that LSMS heterojunctions can be achieved in experiment. We conclude that the lower temperature is beneficial to the synthesis of LB silicene on MoS2 substrate. For free-standing HB silicene, its band structure shows metallicity. The metallicity band structure is also kept in the HSMS heterojunctions. For LSMS heterojunctions, they all are the direct semiconductor with a small band gap opened in the Dirac cones. The band gaps are dependent of rotation angle and stacking pattern, but their Fermi velocity all are 85.8%-89.7% of the value of free-standing LB silicene. The band gaps can be tunable by applying an external electric field, which is promising for applications in nanoscaled devices.Experimentally, QSH effect has been observed in HgTe/CdTe36 and InAs/GaSb37 quantum wells, but the operating temperature is quite low due to their small bulk gap arising from weak SOC. For most 2D topological insulators, they have not been synthesized in experiment, so searching for a new 2D topological insulator with an observable bulk gap and high stability is significant for future practical applications. We also proposed a QSH insulator, chloridized gallium bismuthide (GaBiCl2) monolayer.The pristine GaBi monolayer has the atomic structure resembles those of silicene and germanene.38 However, it is noteworthy that both the Ga and Bi atoms are under-coordinated compared with the stable crystal structures of III-V compounds. The phonon spectrum calculations clearly show that the GaBi monolayer has imaginary frequency modes. After chlorination, the Cl atoms are chemically bonded to the GaBi monolayer. The dynamical stability of the GaBiCl2 monolayer is confirmed by the phonon spectrum without imaginary frequency modes.Compared to GaBi monolayer and CI2, the formation energy of GaBiCl2 is-1.45eV/Cl atom. The negative formation confirms the superiority of GaBiCh over GaBi in stability and plausibility. A suitable reaction is also given:nGa2Cl6+2nBiCl3+8nNaâ†'[GaBiCl2]2n+8nNaCl. The energy release is 0.79eV/atom by DFT calculations.For the band structure without SOC, the s-pxy band inversion at F point in the GaBiCh has occurred. Considering the SOC, the direct band gap at the F point (0.95eV) and indirect band gap (0.65eV) in the GaBiCl2 are both much larger than the values of pristine GaBi which are about 0.19 eV (direct) and 0.16 eV (indirect). The s-pxv band inversion at F point is also kept, indicating the topological nontrivial property of the band gaps. To confirm the nontrivial property, we calculated the edge states of the GaBiCb. Obviously, the gapless edge states are in the bulk gap. The Fermi velocity of the edge states is about 5.0×105m/s, which is are very useful for electronics and spintronics owning to their robustness against scattering.The stability of the bulk gap of GaBiCl2 under external strain is quite crucial for device applications where strain is inevitable. In the strain range of ±10%, the topological nontrivial phase is kept, indicating the topological nontrivial robustness of GaBiCl2.