Quasi-Topological Insulator and Trigonal Warping in Gated Bilayer Silicene

Bilayer silicene has richer physical properties than bilayer graphene due to its buckled structure together with its trigonal symmetric structure. The buckled structure arises from a large ionic radius of silicon, and the trigonal symmetry from a particular way of hopping between two silicenes. It is a topologically trivial insulator since it carries a trivial $\mathbb{Z}_{2}$ topological charge. Nevertheless, its physical properties are more akin to those of a topological insulator than those of a band insulator. Indeed, a bilayer silicene nanoribbon has edge modes which are almost gapless and helical. We may call it a quasi-topological insulator. An important observation is that the band structure is controllable by applying the electric field to a bilayer silicene sheet. We investigate the energy spectrum of bilayer silicene under electric field. Just as monolayer silicene undergoes a phase transition from a topological insulator to a band insulator at a certain electric field, bilayer silicene makes a transition from a quasi-topological insulator to a band insulator beyond a certain critical field. Bilayer silicene is a metal while monolayer silicene is a semimetal at the critical field. Furthermore we find that there are several critical electric fields where the gap closes due to the trigonal warping effect in bilayer silicene.Comment: 8 pages, 11 figures, to be published in J. Phys. Soc. Jp

Adsorption and absorption of Boron, Nitrogen, Aluminium and Phosphorus on Silicene: stability, electronic and phonon properties

Ab initio calculations within the density-functional theory formalism are performed to investigate the chemical functionalization of a graphene-like monolayer of silicon - silicene - with B, N, Al or P atoms. The structural, electronic, magnetic and vibrational properties are reported. The most preferable adsorption sites are found to be valley, bridge, valley and hill site for B, N, Al and P adatoms, respectively. All the relaxed systems with adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B, N, Al, and P atoms accompanied by an appreciable electron transfer from silicene to the B, N and P adatom/substituent. The Al atoms exhibit opposite charge transfer, with n-type doping of silicene and weaker bonding. The adatoms/substituents induce characteristic branches in the phonon spectrum of silicene, which can be probed by Raman measurements. Using molecular dynamics we found that the systems under study are stable up to at least T = 500 K. Our results demonstrate that silicene has a very reactive and functionalizable surface.Comment: 9 pages, 5 figure

Atomistic Representation of Anomalies in the Failure Behaviour of Nanocrystalline Silicene

Silicene, a 2D analogue of graphene, has spurred a tremendous research interest in the scientific community for its unique properties essential for next generation electronic devices. In this work, for the first time, we present a molecular dynamics (MD) investigation to determine the fracture strength and toughness of nanocrystalline silicene (nc silicene) sheet of varied grain size and pre existing crack length at room temperature. Our results suggest that the transition from an inverse pseudo Hall Petch to a pseudo Hall Petch behavior in nc silicene occurs at a critical grain size of 17.32 nm. This phenomenon is also prevalent in nanocrystalline graphene. However, nc silicene with pre existing cracks exhibits anomalous crack propagation and fracture toughness behaviour. We have observed two distinct types of failure mechanisms (crack sensitive and insensitive failure) and devised the mechanophysical conditions under which they occur. Fracture toughness calculated from both Griffiths theory and MD simulations indicate that the former overpredicts the fracture toughness of nc silicene. The most striking outcome, however, is that despite the presence of a pre existing crack, the crack sensitivity of nc silicene is found to be dependent on the grain size and their orientations. This study is the first direct comparison of atomistic simulations to the continuum theories to predict the anomalous behaviour in deformation and failure mechanisms of nc silicene

Distortion and electric field control of band structure of silicene

Density functional theory with local density approximation for exchange and correlation functional is used to tune the electronic band structure of silicene monolayer. The cohesive energy of free standing monolayer is increasing (decreasing) with external electric field (distortion). Electrons in silicene behave like Dirac fermions, when the bond angle between the Si atoms is larger than $\sim 102^{0}$. Large distortions destroy the electronic structure of silicene and silicene is no longer a semi-metallic material, and the distorted silicene acts like an $n$-doped system. Electric field opens a band gap around $K$ point in the Brillouin zone, which increases with electric field. The bond angle between the Si atoms is a key player to determine the presence or absence of Dirac cones in silicene.Comment: Europhysics Lett. Accepted (2014