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Band gap energy as function of distance: (a) for AA1 geometry , (b) AA2 geometry and (c) Flat geometry.
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Using ab intio numerical calculations based on the all-electron full-potential local-orbital minimum-basis scheme FPLO9.00-34, we discuss the interdistance e�ect on the energy gap of two parallel layers of the silicone systems. The like- bilayer systems we dealt with here are relying on a dynamic monolayer of silicene located at distance d along th...
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... present calculations show that the band energy gap can be opened up due to the influence of the variation of the distance d for the flat and buckled geometries for the inter layer distance staring from the Van der Waals bond length. This result, which has been obtained from the distance corresponding to a maximal gap, is shown in figure 5. In particular, we see that the band gap varies exponentially with the distance d. ...
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We have numerically calculated electron exchange and correlation energies and dynamical polarization functions for recently discovered silicene, germanene and other buckled honeycomb lattices at various temperatures. We have compared the dependence of these energies on the chemical potential, field-induced gap and temperature and we have concluded...
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... This indicates that part of the apparent height difference arises from differences in the electronic structure of the two layers. Importantly, this distance is less than that of the ZrB 2 step height of 0.35 nm [39], and less than predicted values for multi-layer silicene [28,40,41]. It however suggests the formation of a layered silicon structure thicker than silicene. ...
... We note that the apparent interlayer spacing of 0.15 nm is different from the 0.24-0.42 nm spacing expected for an isolated bilayer of silicene [28,40,41], though the sensitivity of silicene to the supporting substrate leaves open the possibility for a novel surface reconstruction of a silicene multilayer structure. It is also noted that although surface segregation of Zr or B to the surface cannot be ruled out, it is considered to be unlikely due to the high thermal stability or ZrB 2 [47] and the lack of a band gap as would be expected for B-terminated silicon [48,49]. ...
Using low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM), we observe a new two-dimensional (2D) silicon crystal that is formed by depositing additional Si atoms onto spontaneously-formed epitaxial silicene on a ZrB2 thin film. From scanning tunnelling spectroscopy (STS) studies, we find that this atomically-thin layered silicon has distinctly different electronic properties. Angle resolved photoelectron spectroscopy (ARPES) reveals that, in sharp contrast to epitaxial silicene, the layered silicon exhibits significantly enhanced density of states at the Fermi level resulting from newly formed metallic bands. The 2D growth of this material could allow for direct contacting to the silicene surface and demonstrates the dramatic changes in electronic structure that can occur by the addition of even a single monolayer amount of material in 2D systems.
... This suggests that part of the apparent height difference arises from differences in the electronic structure of the two layers. Importantly, this distance is less than that of the ZrB 2 (0001) step height at 0.35 nm [142], and less than predicted values for multilayer silicene [88,90,143]. The small interlayer spacing is an indication that the new layered silicon nanostructure is not formed from weakly Van der Waals coupled layers. ...
As the pursuit for more powerful electronic devices progresses, individual components have had to be produced at ever smaller dimensions. Today, conventional technologies are at the edge of feasibility as they approach a fundamental limit at the atomic scale. Much research is aimed at overcoming the barrier to atomic scale devices, and indeed some of the explosion of interest into two-dimensional materials over the past decade has its roots in this goal. Graphene, the first atomically thin two-dimensional material, has since been followed by a growing number of intriguing materials with a wide variety of interesting properties, many of which may prove useful in technological devices. One of these two-dimensional materials is silicene, the silicon analogue to graphene that shares many of its properties. Moreover, owing to it being made of silicon, it may be more easily integrated into existing industrial processes. In this thesis, silicene grown upon conductive zirconium diboride is investigated by scanning tunnelling microscopy and spectroscopy. It is found that the structural and electronic properties of epitaxial silicene can be fine-tuned by depositing small amounts of silicon on its surface. However, with continued silicon deposition a significant change is observed: the additional silicon leads to the formation of a layered metallic silicon nanostructure that could be utilised as an atomically precise metallic contact to silicene. Beyond this, the magnetic interactions of individual cobalt atoms on the silicene surface are investigated and it is found that the combination of the semiconducting silicene surface with the metallic zirconium surface yields an unusual spatially distributed Kondo effect. When cobalt atoms are in close proximity to one another on the silicene surface, they exhibit an incredibly strong indirect exchange (RKKY) interaction even at significant separations above 1 nm. The results in this thesis highlight the rich array of phenomena that can manifest in two-dimensional materials and point towards potential future developments for atomic scale electronic and spintronic devices.
We investigated, using DFT calculations, the electronic, optical, polaron properties and estimate the upper light yield of Tl2ZrCl6 material. The DFT calculations were performed with GGA+mBJ and GGA+mBJ+SOC approximations to obtain precise electronic, optical and polaron properties. The calculated results showed that Tl2ZrCl6 has localized valence and narrow discrete conduction bands with many sharp peaks and low dispersion which reflects the heavier effective mass of electron at CBM, 4.63 me, in comparison with that of hole at VBM, 1.05 me. Indirect band gap energy of 4.30 eV was observed and which is close to the experimental value, 4.22 eV. Additionally, in the emission range, this material exhibited a high transmittance rate of around 90% which is an advantage of the scintillation properties. Following DFT calculations, polaron properties and also the temperature-dependence of polaron mobility and relaxation time were calculated by solving the Feynman polaron model, variationally, with the free-energies minimization. The obtained results showed that Tl2ZrCl6 has a large electron (hole)-phonon coupling, αe=8.55(αh=4.1), leading to large polaron mass, small relaxation time, high scattering rate and low electron (hole) mobility of 0.13 cm2V-1s-1 (4.17 cm2V-1s-1) at room-temperature. Such low mobility has a positive role only in the initial ionization track by increasing the recombination yield. Moreover, using the polaron and simple phenomenological models, the upper light yield was estimated to be 87,757 ph/MeV and 75,187 ph/MeV. Such findings could have important implications for understanding the scintillation behavior of this material to improve its performance as a potential scintillator for γ- and X-rays detection.
Understanding the behaviour of nanoscale systems is of great importance to tailor their properties. To this aim, we investigate the Young's modulus (YM) of defect-free and defective armchair bilayer silicene nanoribbons (SNRs), at room temperature, as a function of length and distance between layers. In this study, we perform molecular dynamics simulations using the environment-dependent interatomic potential to describe the interaction of the Si atoms. We show that the Young's modulus of pristine and defective bilayer SNRs increases with the ribbon length exhibiting size dependence. In general, YM of defective bilayer SNRs is smaller than the value obtained for the defect-free case, as a result of the number of missing bonds. In all cases, as the interlayer distance increases YM decreases and the buckling increases. It is shown that the YM exhibits a quadratic interlayer distance dependence. Finally, when only one layer has a mono-vacancy defect, the atomic stress distribution of the pristine layer is affected by the presence of the vacancy. This effect can be considered as a “ghost vacancy” since the deterioration of the pristine layer is similar to that shown by the defective one. These results show that YM of pristine and defective bilayer SNRs could be tailored for a given length and interlayer distance. It is also found that the fracture stress and the fracture strain of defective bilayers are both smaller than those obtained for the defect-free ones.
Silicene, a silicon analogue of graphene, has attracted increasing attention during the past few years. As early as in 1994, the possibility of stage corrugation in the Si analogs of graphite had already been theoretically explored. But there were very few studies on silicene until 2009, when silicene with a low buckled structure was confirmed to be dynamically stable by ab initio calculations. In spite of the low buckled geometry, silicene shares most of the outstanding electronic properties of planar graphene (e.g., the “Dirac cone”, high Fermi velocity and carrier mobility). Compared with graphene, silicene has several prominent advantages: (1) a much stronger spin–orbit coupling, which may lead to a realization of quantum spin Hall effect in the experimentally accessible temperature, (2) a better tunability of the band gap, which is necessary for an effective field effect transistor (FET) operating at room temperature, (3) an easier valley polarization and more suitability for valleytronics study. From 2012, monolayer silicene sheets of different superstructures were successfully synthesized on various substrates, including Ag(1 1 1), Ir(1 1 1), ZrB2(0 0 0 1), ZrC(1 1 1) and MoS2 surfaces. Multilayer silicene sheets have also been grown on Ag(1 1 1) surface. The experimental successes have stimulated many efforts to explore the intrinsic properties as well as potential device applications of silicene, including quantum spin Hall effect, quantum anomalous Hall effect, quantum valley Hall effect, superconductivity, band engineering, magnetism, thermoelectric effect, gas sensor, tunneling FET, spin filter, and spin FET, etc. Recently, a silicene FET has been fabricated, which shows the expected ambipolar Dirac charge transport and paves the way towards silicene-based nanoelectronics. This comprehensive review covers all the important theoretical and experimental advances on silicene to date, from the basic theory of intrinsic properties, experimental synthesis and characterization, modulation of physical properties by modifications, and finally to device explorations.
Using first-principles density functional theory, we have investigated the mechanical and electronic properties of monolayer and bilayer arsenenes under in-plain biaxial strains. It is interesting to find that under large enough tensile strains, the monolayer arsenene transfers from buckled honeycomb structure to planar honeycomb phase while the interlayer distance D of bilayer arsenene maintains at about 1.371 Å. Both monolayer and bilayer arsenenes possess indirect band gaps and their electronic properties can be tuned via in-plane biaxial strains. The variations of the band gap energy are diverse with respect to the compressive and tensile biaxial strains. Under compressive strains, the band gap of both monolayer and bilayer arsenenes initially increases, and then rapidly decreases. In addition, the monolayer arsenene exhibits an indirect-to-direct band gap transition when the compressive strains reach to -10%. However, under tensile strains, the band gap of monolayer arsenene monotonously decreases with the strains, while the band gap of bilayer arsenene reduces quickly to zero under small tensile strains. Our present results will be conductive to design strain-based arsenene materials for applications in nanoelectronic and optoelectronic devices.
Temporal behavior of entanglement between electrons in silicene and single mode radiations is reported. We show that the corresponding total Hamiltonian and time evolution operators are block diagonal. Initial states are divided into two categories for which buckling and the intrinsic spin-orbit effects are either of opposite or the same signs. Negativity shows that π-electrons and photons periodically become entangled for both categories. The entanglement spontaneously shows abrupt variations when buckling and the spin-orbit effects are equal but opposite in sign, leading to quantum phase transitions. As photonic excitations increase, the entanglement exhibits plateaus of constant durations for such initial states.
