Monolayer structures made up of purely one kind of atoms are fascinating. Many kinds of honeycomb systems including carbon, silicon, germanium, tin, phosphorus and arsenic have been shown to be stable. However, so far the structures are restricted to
group IV and V elements. In this letter, we systematically investigate the stability of monolayer structures made up of aluminium, in four different geometric configurations (planar, buckled, puckered and triangular), by employing density functional theory based electronic structure calculation. Our results on cohesive energy and phonon dispersion predict that only planar honeycomb structure made up of aluminium is stable. We call it aluminene according to the standard naming convention. It is a metal. Results of electronic band structure suggest that it may be regarded as a highly hole doped graphene. We also present the tight-binding model and the Dirac theory to discuss the electronic properties of aluminene.
In this work, we report our results on the geometric and electronic properties of hybrid graphite-like structure made up of silicene and boron nitride (BN) layers. We predict from our calculations that this hybrid bulk system, with alternate layers o
f honeycomb silicene and BN, possesses physical properties similar to those of bulk graphite. We observe that there exists a weak van der Waals interaction between the layers of this hybrid system in contrast to the strong inter-layer covalent bonds present in multi-layers of silicene. Furthermore, our results for the electronic band structure and the density of states show that it is a semi-metal and the dispersion around the Fermi level (E_F) is parabolic in nature and thus the charge carriers in this system behave as textit{Nearly-Free Particle-Like}. These results indicate that the electronic properties of the hybrid bulk system resemble closely those of bulk graphite. Around E_F the electronic band structures have contributions only from silicene layers and the BN layer act only as a buffer layer in this hybrid system since it does not contribute to the electronic properties near E_F. In case of bi-layers of silicene with a single BN layer kept in between, we observe a linear dispersion around E_F similar to that of graphene. However, the characteristic linear dispersion become parabola-like when the system is subjected to a compression along the transverse direction. Our present calculations show that the hybrid system based on silicon and BN can be a possible candidate for two dimensional layered system akin to graphite and multi-layers of graphene.
We carry out a computational study on the geometric and electronic properties of multi-layers of silicene in different stacking configurations using a state-of-art abinitio density functional theory based calculations. In this work we investigate the
evolution of these properties with increasing number of layers (n) ranging from 1 to 10. Though, mono-layer of silicene possesses properties similar to those of graphene, our results show that the geometric and electronic properties of multi-layers of silicene are strikingly different from those of multi-layers of graphene. We observe that there exist strong inter-layer covalent bondings between the layers in multi-layers of silicene as opposed to weak van der Waals bonding which exists between the graphene layers. The inter-layer bonding strongly influences the geometric and electronic structures of these multi-layers. Like bi-layers of graphene, silicene with two different stacking configurations AA and AB exhibits linear and parabolic dispersions around the Fermi level, respectively. However, unlike graphene, for bi-layers of silicene, these dispersion curves are shifted in band diagram; this is due to the strong inter-layer bonding present in the latter. For n > 3, we study the geometric and electronic properties of multi-layers with four different stacking configurations namely, AAAA, AABB, ABAB and ABC. Our results on cohesive energy show that all the multi-layers considered are energetically stable. Furthermore, we find that the three stacking configurations (AAAA, AABB and ABC) containing tetrahedral coordination have much higher cohesive energy than that of Bernal (ABAB) stacking configuration. This is in contrast to the case of multi-layers of graphene where ABAB is reported to be the lowest energy configuration.
We study optical properties of two dimensional silicene using density functional theory based calculations. Our results on optical response property calculations show that they strongly depend on direction of polarization of light, hence the optical
absorption spectra are different for light polarized parallel and perpendicular to plane of silicence. The optical absorption spectra of silicene possess two major peaks: (i) a sharp peak at 1.74 eV due to transition from pi to pi* states and (ii) a broad peak in range of 4-10 eV due to excitation of sigma states to conduction bands. We also investigate the effect of external influences such as (a) transverse static electric field and (b) doping of hydrogen atoms (hydrogenation) on optical properties of silicene. Firstly, with electric field, it is observed that band gap can be opened up in silicene at Fermi level by breaking the inversion symmetry. We see appreciable changes in optical absorption due to band gap opening. Secondly, hydrogenation in silicene strongly modifies the hybridization and our geometry analysis indicates that the hybridization in silicene goes from mixture of sp^2 + sp^3 to purely sp^3. Therefore, there is no pi electron present in the system. Consequently, the electronic structure and optical absorption spectra of silicene get modified and it undergoes a transition from semi-metal to semiconductor due to hydrogenation.
We predict the existence of a new ferromagnetic shape memory alloy Ga_2MnNi using density functional theory. The martensitic start temperature (T_M) is found to be approximately proportional to the stabilization energy of the martensitic phase (delta
E_tot) for different shape memory alloys. Experimental studies performed to verify the theoretical results show that Ga_2MnNi is ferromagnetic at room temperature and the T_M and T_C are 780K and 330K, respectively. Both from theory and experiment, the martensitic transition is found to be volume conserving that is indicative of shape memory behavior.
