First-principles study of graphenylene/MoX2 (X = S, Te, and Se) van der Waals heterostructures

Janus Type Monolayers of S-MoSiN2 Family and Van Der Waals Heterostructures with Graphene: DFT-Based Study

Novel representative 2D materials of the Janus type family X-M-ZN2 are studied. These materials are hybrids of a transition metal dichalcogenide and a material from the MoSi2N4 family, and they were constructed and optimized from the MoSi2N4 monolayer by the substitution of SiN2 group on one side by chalcogen atoms (sulfur, selenium, or tellurium), and possibly replacing molybdenum (Mo) to tungsten (W) and/or silicon (Si) to germanium (Ge). The stability of novel materials is evaluated by calculating phonon spectra and binding energies. Mechanical, electronic, and optical characteristics are calculated by methods based on the density functional theory. All considered 2D materials are semiconductors with a substantial bandgap (>1 eV). The mirror symmetry breaking is the cause of a significant built-in electric field and intrinsic dipole moment. The spin−orbit coupling (SOC) is estimated by calculations of SOC polarized bandstructures for four most stable X-M-ZN2 structures. The possible van der Waals heterostructures of considered Janus type monolayers with graphene are constructed and optimized. It is demonstrated that monolayers can serve as outer plates in conducting layers (with graphene) for shielding a constant external electric field.

Tunable band gaps and high carrier mobilities in stanene by small organic molecule adsorption under external electric fields

The effects of small organic molecule (SOM) adsorption with benzene (C6H6), hexafluorobenzene (C6F6), and p-difluorobenzene (C6H4F2) on the electronic properties of stanene under external electric fields are investigated through first-principles calculations. Different adsorption sites and molecular orientations are considered to determine the most stable configurations of small organic molecule (SOM) adsorption on the surface of stanene. The results show that the internal electric field caused by the adsorption of small organic molecules destroys the symmetry of the two sublattices of stanene in C6H6/stanene, C6F6/stanene and C6H4F2/stanene systems with the most stable configurations, opening the band gaps of stanene with 39.5, 18.9 and 14.5 meV, respectively. Under an external electric field, a wide range of linearly tunable and sizable direct band gaps (31.6-420.1 meV for the C6H6/stanene system, 14.8-587.2 meV for the C6F6/stanene system and 14.5-490.2 meV for the C6H4F2/stanene system) are merely determined by the strength of the composite electric field despite its direction. The mechanism of charge transfer between stanene and organic molecules under an external electric field can be revealed using an equivalent capacitor model to explain the tunable charge transfer. More importantly, the high carrier mobility of the stable SOM/stanene systems under an external electric field is largely retained due to the weak interactions at the interface. These results indicate that the electronic properties of stanene can be effectively modulated by the surface adsorption of organic molecules under an external electric field, providing effective and reversible routes to enhance the performance of stanene for novel electronic devices in the future.