Growth Mechanism, Defects And Electronic Properties Tuning Of Silicene And Borophene

Silicene, a monolayer of silicon atoms assembling in a honeycomb lattice, has attracted more and more attention in recent years. Silicene shares most of the outstanding electronic properties of graphene, e.g., the "Dirac cone", high carrier mobility. Compared with graphene, silicene has a much stronger spin-orbit coupling and a better tunability of the band gap. Moreover the easy integration into the current Si based technology makes silicene a potential candidate for microelectronic devices. Borophene, a monolayer of boron atoms, is metallic. The presence of borophene enriches the variety of two-dimensional (2D) monolayer materialsVarious point defects in epitaxial silicene on Ag(111) surfaces have been systematically investigated using DFT calculations combined with experimental scanning tunneling microscopy (STM). The atomic structures of some point defects observed in experimental STM are identified. The concentrations of various point defects in different silicene superstructures were estimated through calculation of formation energy of these defects. At the growth temperature of silicene, the concentrations of single vacancy and double vacancy can be as high as 5.0 x 1013 cm-2. Moreover, two single vacancies can diffuse very fast in the epitaxial silicene and would coalesce into one double vacancy. The large concentration of point defects and easy diffusion and coalescence of single vacancy nicely explains the defective appearance of (?) ×(?) silicene in experiments. In contrast, the concentrations of point defects in epitaxial 4x4 silicene is very low. Thus, epitaxial 4x4 silicene is thought to be most suitable monolayer silicene phase for future device applications.The effect of inert substrates on the electronic properties of silicene, which is an important issue for the practical applications of silicene in nanoelectronic devices, was investigated by first-principle calculations. The characteristic Dirac cone is preserved for silicene on h-BN monolayer or hydrogenated Si-terminated SiC(0001) surface (Si-SiC), which provides a way to practically utilize the outstanding electronic properties of silicene based on the linear dispersing ? bands. On the other hand, the Dirac cone is destroyed when silicene is placed on a hydrogenated C-terminated SiC(0001) surface (C-SiC). This effect was explained by the work functions for silicene and the substrates. The present results provide some guidelines for selecting proper substrates for silicene in future microelectronic devices and electronic tuning of silicene. Moreover, for bilayer silicene intercalation of alkali metal (Li, Na, K and Rb) can effectively weaken interlayer interaction and further stabilize the system. In particular, potassium intercalation can recover the outstanding electronic properties of free-standing monolayer silicene by forming a double-degenerated Dirac cone with a small band gap of 0.27 eV below Fermi level.Using first-principle calculations, we predict the possible growth of borophene on Cu(111) surface. The monotonic decrease of formation energy of boron cluster BN with increasing cluster size and low diffusion barrier for a single B atom on Cu(111) surface ensure continuous growth of two-dimensional (2D) boron cluster. During growth process, hexagonal holes can easily arise at the edge of a planar triangular boron cluster and then diffuse entad. Hence, large-scale boron monolayer with mixed hexagonal-triangular geometry can be obtained via either depositing boron atoms directly on Cu(111) surface or soft landing of small planar BN clusters. Our theoretical prediction promoted the development of this field and successfully directed the experimental synthesization of borophene recently.