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Register a new userInterband $pi$-like plasmon in silicene grown on silver
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Added by
Antonello Sindona Dr
Publication date
2017
fields
Physics
and research's language is
English
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Silicene, the two-dimensional allotrope of silicon, is predicted to exist in a low-buckled honeycomb lattice, characterized by semimetallic electronic bands with graphenelike energy-momentum dispersions around the Fermi level (represented by touching Dirac cones). Single layers of silicene are mostly synthesized by depositing silicon on top of silver, where, however, the different phases observed to date are so strongly hybridized with the substrate that not only the Dirac cones, but also the whole valence and conduction states of ideal silicene appear to be lost. Here, we provide evidence that at least part of this semimetallic behavior is preserved by the coexistence of more silicene phases, epitaxially grown on Ag(111). In particular, we combine electron energy loss spectroscopy and time-dependent density functional theory to characterize the low-energy plasmon of a multiphase-silicene/Ag(111) sample, prepared at controlled silicon coverage and growth temperature. We find that this mode survives the interaction with the substrate, being perfectly matched with the {pi}-like plasmon of ideal silicene. We therefore suggest that the weakened interaction of multiphase silicene with the substrate may provide a unique platform with the potential to develop different applications based on two-dimensional silicon systems.
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We attempt to simulate the heterogeneous nucleation of ice at model silver-iodide surfaces and find relatively facile ice nucleation and growth at the Ag+ termi nated basal face, but never see nucleation at the I- terminated basal face or the prism and normal faces. Water molecules strongly adsorb onto the Ag+ terminate d face to give a well-ordered hexagonal ice-like bilayer that then acts as a template for further ice growth.
We use the tight-binding model and the random-phase approximation to investigate the intrinsic plasmon in silicene. At finite temperatures, an undamped plasmon is generated from the interplay between the intraband and the interband-gap transitions. The extent of the plasmon existence range in terms of momentum and temperature, which is dependent on the size of single-particle-excitation gap, is further tuned by applying a perpendicular electric field. The plasmon becomes damped in the interband-excitation region. A low damped zone is created by the field-induced spin split. The field-dependent plasmon spectrum shows a strong tunability in plasmon intensity and spectral bandwidth. This could make silicene a very suitable candidate for plasmonic applications.
In this Letter, we present the first non-contact atomic force microscopy (nc-AFM) of a silicene on silver (Ag) surface, obtained by combining non-contact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). STM images over large areas of silicene grown on Ag(111) surface show both (sqrt13xsqrt13)R13.9{deg} and (4x4) superstructures. For the widely observed (4x4) structure, the nc-AFM topography shows an atomic-scale contrast inversion as the tip-surface distance is decreased. At the shortest tip-surface distance, the nc-AFM topography is very similar to the STM one. The observed structure in the nc-AFM topography is compatible with only one out of two silicon atoms being visible. This indicates unambiguously a strong buckling of the silicene honeycomb layer.
The plasmonic character of monolayer silicene is investigated by time-dependent density functional theory in the random phase approximation. The energy-loss function of the system is analyzed, with particular reference to its induced charge-density fluctuations, i.e., plasmon resonances and corresponding dispersions, occurring in the investigated energy-momentum region. At energies larger than 1.5eV, two intrinsic interband modes are detected and characterized. The first one is a hybridized pi-like plasmon, which is assisted by competing one-electron processes involving sp2 and sp3 states. The second one is a more conventional pi-sigma plasmon, which is more intense than the pi-like plasmon and more affected by one-electron processes involving the sigma bands, with respect to the analogous collective oscillation in monolayer graphene. At energies below 1eV, two extrinsic intraband modes are predicted to occur, which are generated by distinct types of Dirac electrons (associated with different Fermi velocities at the so-called Dirac points). The most intense of them is a two-dimensional plasmon, having an energy-momentum dispersion that resembles that of a two-dimensional electron gas. The other is an acoustic plasmon that occurs for specific momentum directions and competes with the two-dimensional plasmon at mid infrared energies. The strong anisotropic character of this mode cannot be explained in terms of the widely used Dirac-cone approximation. As in mono-, bi-, and few-layer graphene, the extrinsic oscillations of silicene are highly sensitive to the concentration of injected or ejected charge carriers. More importantly, the two-dimensional and acoustic plasmons appear to be a signature of the honeycomb lattice, independently of the chemistry of the group-IV elements and the details of the unit-cell geometry.
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 of 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.
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