The exotic physical phenomena in quantum matters, such as topological properties, superconductivity, and quantum spin Hall (QSH) effect, have attracted much attention in the academic community due to their interesting basic principles and potential novel applications. Moreover, researchers are searching for quantum materials that can exhibit these interesting physical phenomena. In this thesis, we study structural and electronic properties of polymorphic structures in two-dimensional transition metal dichalcogenides, which exhibit valley polarization and QSH effect. We also predict novel phase of carbon and silicon using the evolutionary genetic algorithm and analyze their exotic electronic properties.
First, through first-principles calculations, we study the electronic properties of monolayer and bilayer $MoS_2$ on $SiO_2$ substrate. We find that the interface interaction between $MoS_2$ and $SiO_2$ strongly depends on the surface polarity and thus significantly affects the electronic structure of MoS2. Especially, when dangling bonds exist at the interface, bilayer $MoS_2$ exhibits the strong spin-valley coupling at the valence band edge, similar to that found in monolayer $MoS_2$. Then, we systematically investigate the strain-induced modification of both the bulk band and topological edge states in QSH insulators based on 1T’-$MoX_2$ nanoribbons with X = (S, Se, Te). Although the location of the Dirac point depends on the chalcogen species, we show the possibility of tuning the Dirac point in the band gap by applying compressive or tensile strain. Considering the size of band gap and the amount of strain, we suggest that $MoSe_2$ nanoribbons would be the best candidates for QSH devices. Our results will represent a viable strategy for engineering TMDCs-based devices.
Next, we report a novel semimetallic carbon allotrope in monoclinic C2/m space group, discovered using first-principles evolutionary structure search. The new carbon phase, termed $m-C_8$, consists of five-membered rings with $sp^3$ bonding interconnected by $sp^2$-bonded carbon networks. Enthalpy calculations reveal that $m-C_8$ is more favorable over recently reported topological semimetallic carbon allotropes, and the dynamical stability of $m-C_8$ is verified by phonon spectra and molecular dynamics simulations. Simulated x-ray diffraction spectra propose that $m-C_8$ would be one of the unidentified carbon phases observed in detonation shoot. The analysis of electronic properties indicates that $m-C_8$ belongs to a class of topological nodal line semimetals, exhibiting the nodal line protected by both inversion and time-reversal symmetries in the absence of spin-orbit coupling and the surface band connecting the projected nodal points.
Finally, we report the prediction of pure metallic Si allotropes with open channels at ambient pressure, unlike a cubic diamond structure in covalent bonding networks. The metallic phase termed $P6/m-Si_6$ can be obtained by removing Na after pressure release from a novel Na-Si clathrate called $P6/m-NaSi_6$, which is discovered through first-principles study at high pressure. We confirm that both $P6/m-NaSi_6$ and $P6/m-Si_6$ are stable and superconducting with the critical temperatures of about 13 and 12 K at ambient pressure, respectively. The discovery of new Na-Si and Si clathrate structures presents an opportunity to explore new Si allotropes useful for Si-based devices.