Structure Of Silicene Research Articles

First-principles study on the adsorption of gas molecules on Fe, Ti-Doped silicene

In this study, we employ first-principles calculations to investigate the geometric and electronic properties of intrinsic silicene and silicene doped with metal elements Fe and Ti. We analyze the adsorption performance of five gas molecules (CO, SO2, H2S, CH4 and H2CO) on the surfaces of these two materials. For each gas molecule, we determine the optimal adsorption site and calculate parameters such as adsorption distance, adsorption energy, charge transfer, recovery time, and density of states to understand the adsorption mechanism. Our study reveals that the adsorption affinity of the selected gas molecules on intrinsic silicene is relatively weak. The distance between the gas molecules and the intrinsic silicene surface is relatively large, and no stable chemical bonds are formed between them. In contrast, Fe and Ti-doped silicene exhibit relatively higher stability, with the adsorption energies of the gas molecules on their surfaces increasing obviously. The Titanium doping exerts a considerable influence on increasing the band gap and the adsorption energy of silicene compared with the intrinsic one, thus the whole structure is able to reveal semiconductors’ properties. In Ti-doped silicene structures, the band gap opens, exhibiting semiconductor properties, and the adsorption energies increase relative to intrinsic silicene. These results indicate that the doping of Fe and Ti atoms enhances the adsorption performance of silicene materials, providing theoretical reference for the gas sensing performance of Fe and Ti-doped silicene materials and the semiconductor performance of Ti-doped silicene.

Vanadium embedded in monolayer silicene: Energetics and proximity-induced magnetism

Using the density-functional theory approach, including Hubbard U correction, we investigate the defect structures consisting of vanadium (V) atoms embedded in a monolayer silicene. Specifically, we consider V–V atom pairs in antiferromagnetic (AFM), ferromagnetic (FM), and non-magnetic states, which are embedded in substitutional and interstitial sites. We determine the ground-state structures, formation and binding energies, electronic structures, induced magnetization, as well as the spin-exchange coupling between the V–V pair. For the substitutional vanadium atom pair, the stability of the AFM and FM spin configurations depends on the sublattice sites in which the V atoms are sited. When the V pair is located on a similar sublattice site type, the AFM spin alignment is more energetically favored, whereas when the pair is located in a different sublattice site, the FM interactions are more stable. However, the relative stability of the AFM or FM configurations changes rapidly as the separation between the V pair increases. Regarding the interstitial-hole V–V pair configurations, the most stable structure is when the pair is at the nearest-neighbor hole sites and is in an FM alignment. Also, at larger separations, the AFM or FM hole configurations are approximately degenerate in energy. Furthermore, we elucidate on the Ruderman–Kittel–Kasuya–Yosida, direct-exchange, and the superexchange interaction mechanisms in the vanadium-embedded silicene. In addition, we estimate a Curie temperature (Tc) of up to ∼500 K for a silicene structure containing a V pair in the FM spin alignment. Such a high Tc, in addition to the stability of the material, suggests that vanadium-embedded silicene is a potential candidate material for spintronic device applications.

Robustness of linear bands in epitaxial planar silicene heterostructure against surface doping

In this study, we investigate the electronic structure of epitaxial planar silicene heterostructure grown on ultrathin gold films on the Si(111) substrates, covered by 0.1 monolayer of silver, antimony and lead. Angle-Resolved Photoemission Spectroscopy measurements reveal that the electronic structure of planar silicene, that shows bands with linear dispersion, remains largely unaffected by the presence of these dopants, with only a slight energy shift observed in the case of antimony. Density functional theory calculations corroborate these experimental observations, confirming the robustness of the epitaxial planar silicene heterostructure against surface doping. The results suggest that the intrinsic electronic properties of planar silicene are well-protected against surface contamination.

Electric field modulated electronic, thermoelectric and transport properties of 2D tetragonal silicene and its nanoribbons

Using both first principles and analytical approaches, we investigate the role of a transverse electric field in tuning the electrical, thermoelectric, optical and transport properties of a buckled tetragonal silicene (TS) structure. The transverse electric field transforms the linear spectrum to parabolic at the Fermi level and opens a band gap. The gap is similar at the two Dirac points present in the irreducible Brillouin zone of the TS structure and increases in proportion to the applied field strength. However, a sufficiently strong electric field converts the system into a metallic one. A comparable band opening is also seen in the TS nanoribbons. Electric field-induced semiconducting nature improves its thermoelectric properties. Estimated Debye temperature reveals its superiority over graphene in terms of thermoelectric performance. The optical response of the structures is very asymmetric. Large values of imaginary and real components of the dielectric function are seen. The absorption frequency lies in the UV region. Plasma frequencies are identified and are red-shifted with the applied field. The current–voltage characteristics of the symmetric type nanoribbons show oscillation in current whereas the voltage-rectifying capability of anti-symmetric type nanoribbons under a transverse electric field is interesting.

Role of interfacial layer as PANI–silicene in Si-based photodiodes

Silicene is a 2D monoatomic sheet of silicon and can be used for various applications such as degradation, therapy, and biosafety. Polyaniline (PANI) is a conducting polymer employed for electronic devices. In this study, we synthesized PANI–silicene composites and operated as an external interfacial layer between Al and different type substrates of p-Si and n-Si to compare Schottky-type photodiodes of PANI–silicene/n-Si and PANI–silicene/p-Si. The silicene structures were investigated using X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Also, the light power intensity dependent of PANI–silicene/n-Si and PANI–silicene/p-Si photodiodes carried out in the range 0–100 mW/cm2 and I–t measurements utilized to determine the response time of the photodiodes. Basic parameters of devices such as ideality factors barrier, height, and series resistance were obtained by Norde and Cheung methods and thermionic emission (TE) theory from I–V graphs. While the PANI–silicene/n-Si exhibited high ideality factor values of 5.49, the PANI–silicene/p-Si photodiodes showed a low ideality factor of 1.48. The photodiode parameters such as detectivity and responsivity were calculated as 6.40 × 109 Jones and 38.9 mA/W for n-Si substrate and 78.2 mA/W and 8.81 × 109 Jones for p-Si substrate. The case of basic electrical properties for PANI–silicene composite interlayer-based photodiodes was analyzed in detail.

Theoretical Study of Electronic and Thermoelectric Properties of Ultra-Thin Silicene and Germanene

In this study, we used the Non-Equilibrium Green Function (NEGF) and Density Functional Theory (DFT) to analyze the nanoscale electronic properties of silicene and germanene structures, the band structure and Density of State (DOS) for the unit cell of silicene and germanene are found that the energy gap is (0 eV) in both. A comparison was made between silicene and germanene in terms of electrical and thermal properties, by calculating the transmission coefficient T(E) for silicene patterns (Si1, Si2, Si3, and Si4) and germanene patterns (Ge1, Ge2, Ge3, and Ge4). The results show that T(E) decreases with increasing etching depth, which also leads to a decrease in electronic conductivity. Moreover, the value of the Seebeck coefficient (S) increases with increasing drilling depth, and also the sign of S varies from a positive value for (Si1, Si4, Ge1, Ge2, Ge4) to a negative value for (Si2, Si3, Ge3) at Fermi level (Ef) equal to (0 eV). The highest value of the figure of merit is about (1.8) for the Ge4 structure.

Silicene growth mechanisms on Au(111) and Au(110) substrates

Despite the remarkable theoretical applications of silicene, its synthesis remains a complex task, with epitaxial growth being one of the main routes involving depositing evaporated Si atoms onto a suitable substrate. Additionally, the requirement for a substrate to maintain the silicene stability poses several difficulties in accurately determining the growth mechanisms and the resulting structures, leading to conflicting results in the literature. In this study, large-scale molecular dynamics simulations are performed to uncover the growth mechanisms and characteristics of epitaxially grown silicene sheets on Au(111) and Au(110) substrates, considering different temperatures and Si deposition rates. The growth process has been found to initiate with the nucleation of several independent islands homogeneously distributed on the substrate surface, which gradually merge to form a complete silicene sheet. The results consistently demonstrate the presence of a buckled silicene structure, although this characteristic is notably reduced when using an Au(111) substrate. Furthermore, the analysis also focuses on the quality and growth mode of the silicene sheets, considering the influence of temperature and deposition rate. The findings reveal a prevalence of the Frank–van der Merwe growth mode, along with diverse forms of defects throughout the sheets.

Quantum spin Hall effect and emergence of conducting edge states in silicene supported by MX (M=Ga, In; X=S, Se, Te) monolayer

The electronic structure of silicene supported by monolayer of different monochalcogenide MX (GaS, GaSe, GaTe and InSe) substrates has been investigated by first principle density functional theory. By calculating the formation energies and phonons, it has been seen that silicene supported by monolayer of MX remains stable. The systems retain their almost [Formula: see text] planner configurations with small buckling as that of the free standing silicene and also the Bader charge analysis shows that silicene hardly interacts with any of the MX substrates. The Dirac cone with a small gap ([Formula: see text] [Formula: see text]meV) has been observed in each of the cases. All the systems show quantum spin Hall effect and the quantum spin Hall conductivities have been estimated to be within the range [Formula: see text]S/cm, which are larger than that of the free standing silicene. Our calculations show that even if the systems have bulk band gaps but the edge states are conducting in nature.

P-block doped semi-metallic xenes as highly selective and efficient transition-metal free single atom catalysts for electrochemical CO reduction

Electronic structure of boron-doped silicene is optimized for CORR and selective towards methanol with an overpotential of less than 0.1 V.

Adsorption of Hydrogen Molecules in Nickel Decorated Silicene

First-principles simulations based on density functional theory (DFT) have been used to study the structural, electronic and magnetic properties of pristine and Ni decorated silicene sheets. Generalized Gradient Approximation (GGA) based exchange correlation functionals are used under software package Quantum ESPRESSO (QE), 6.5 versions. We have reconstructed the optimized unit cell of silicene, which has a face centered cubic (fcc) structure with two silicon atoms having lattice parameters a = b = 3.8 Å. The distance between two nearest silicene monolayers is found to be 20.5 Å which is large enough to neglect the interlayer interactions between 4×4 supercells of silicene monolayers. The atoms in the prepared supercell are fully relaxed under Bloyden-Fletcher-Goldfarb-Shanno (BFGS) scheme prior to the self-consistent, band structure and density of state (DoS) calculations. The pristine silicene is semi-metallic in nature possessing a Dirac-cone as in graphene. The h-site adsorption is found to be the most stable adsorption site of nickel in silicene with the binding energy of 4.69 eV. The addition of nickel atom completely distorts the hexagonal structure of silicene destroying the Dirac cone and the system becomes slightly insulating from its semi-metallic nature. We then construct a 4×4 nickel dimer silicene which further destroys the hexagonal silicene structure with further opening of the band gap. The charge transfer analysis in the Ni decorated systems shows the charge transfers of 0.163e and 0.294e in Ni adatom silicene and Ni dimer silicene respectively showing that the nickel atoms are adsorbed by weak van der Waals forces in both of the systems. We then proceed to hydrogen molecule adsorption in these prepared 4×4 silicene systems: pristine, Ni adatom and Ni dimer silicene systems. The adsorption energy of hydrogen in the Ni adatom silicene is found to be the largest making it the most effective system for hydrogen storage.

Machine-learned search for the stable structures of silicene on Ag(111)

Machine-learned search for the stable structures of silicene on Ag(111)

First-principles study of gas molecule adsorption on Ga-doped silicene

In this paper, based on first-principles calculations, the geometric structure and electronic properties of intrinsic silicene and metal element Ga doped silicene were studied, and three harmful gases CO, SO2 and NH3 gas molecules and H2O molecules were analyzed in two adsorption properties on the surface of two material. For each gas molecule, the optimal adsorption site was tried and determined, and parameters such as adsorption distance, adsorption energy, transfer charge, recovery time, and density of states were calculated to understand the adsorption mechanism. It was found that the adsorption capacity of the selected gas molecules on intrinsic silicene was weak except for NH3. While Ga doped silicene is a relatively stable structure, the adsorption energies of CO, SO2 and NH3 gas molecules on its surface increase in different degrees, the adsorption energies are −0.51 eV, −0.82 eV and −0.73 eV, but no adsorption to H2O. The results show that the doping of Ga atoms improves the adsorption performance of silicene materials, and is less affected by the humidity in the air in practical applications, which provides a theoretical reference for the gas-sensing properties of Ga doped silicene materials.

Optimization of the tunneling magnetoresistance and spin-valley polarization in complex magnetic silicene structures

Aperiodic order is ubiquitous in nature and quite relevant in science and technology. There are extensive works in aperiodic structures studying fundamental characteristics in physical properties, such as fractality, self-similarity, and fragmentation. However, there are fewer reports in which aperiodicity signifies an improvement in physical quantities with practical applications. Here, we show that the aperiodicity of fractal or self-similar type optimizes the tunneling magnetoresistance and spin-valley polarization of magnetic silicene structures, raising the prospects of spin-valleytronics. We reach this conclusion by studying the spin-valley-dependent transport properties of complex (Cantor-like) magnetic silicene structures within the lines of the transfer matrix method and the Landauer–Büttiker formalism. We find that the self-similar arrangement of magnetic barriers in conjunction with structural asymmetry reduces the conductance oscillations typical of periodic magnetic silicene superlattices and more importantly makes the K′-spin-down conductance component dominant, resulting in nearly perfect positive and negative spin-valley polarization states accessible by simply reversing the magnetization direction. The tunneling magnetoresistance is not as prominent as in periodic magnetic silicene superlattices; however, it is better than in single magnetic junctions. Furthermore, the optimization of the spin-valley-dependent transport properties caused by the complex structure is superior than the corresponding one reported in typical aperiodic structures, such as Fibonacci and Thue–Morse magnetic silicene superlattices.

First principle modeling of a silicene-aluminum composite anode for lithium ion batteries

The creation of new environmentally friendly and portable energy sources requires the development of technology used to produce lithium-ion batteries, especially their anodes. The aim of this study was to show the possibilities of the state-of-the-art ab initio approach for modeling battery anode materials. A quantum-mechanical study of the limiting filling of a silicene/aluminum anode with lithium has been performed. In this case, the changes in the structure, energy, and electronic properties of silicene that occur upon filling the anode have been studied. Lithium atoms are deposited both inside and on top of the channel formed by two parallel silicene sheets. The ratio of lithium to silicon varies in the range of 0.1–1.5. The gap in the silicene channel increases as the channel is filled with lithium. The filling of the silicene channel with lithium is associated with an increase in the Si–Si bond length in both silicene sheets without destruction of the channel. The infiltration of lithium into the space between the aluminum substrate and silicene sheet has been determined. Our calculation of the band structure has shown that, regardless of filling with lithium, the silicene-aluminum system exhibits metallic conductivity. The storage capacity of the combined anode (729 mAh g−1) significantly exceeds the capacity of graphite anodes. It was found that the top sheet of silicene acquires a negative electric charge upon significant lithium filling of the anode, exceeding the absolute value of the negative charge of the bottom sheet in contact with the aluminum substrate. Therefore, our model study has elucidated that the new silicene-aluminum anode is a promising material for the creation of a new generation of lithium-ion batteries.

Atomic arrangement of Si adatom on the Silicene/Ag(111) surface

Silicene is a two-dimensional material with a honeycomb structure of silicon atoms that exhibits exotic electronic properties. Exploring Si adatom on monolayer silicene opens up new possibilities for studying tunable all-silicon-based devices at an atomic scale. Here we use non-contact atomic force microscopy and scanning tunneling microscopy to visualize and control the Si adatom on monolayer silicene at atomic resolution. We demonstrate that Si adatom forms a two-dimensional island on monolayer silicene. Si adatom can be manipulated on monolayer silicene, thereby delivering a tunable low-dimensional Si structure at an atomic scale. Local reconstruction of monolayer silicene with Si adatom has been confirmed by density functional theory calculations. Our results suggest that controlling the buckling structure of monolayer silicene by positioning Si adatom will locally tune the band structures and functionalities of monolayer silicene which is important for applications.

Molecular dynamics study of the finite-size effect in 2D nanoribbon silicene

ABSTRACT We present the molecular dynamics (MD) simulation of the finite-size effect in 2D silicene nanoribbons (SiNRs). Five silicene nanoribbon structures were modelled with the same length and buckling, but different width sizes. All models were melted from the perfect honeycomb silicene structure and then cooled from the disordered liquid state. Structural properties such as ring sizes, radial distribution function, coordination numbers, and interatomic distances have been carefully analysed. All models’ critical melting and cooling temperature were determined via total energy per atom. We found that 2D SiNRs transition temperature is lower than the pristine silicene. Particularly, the SiNRs present a noticeable finite-size effect on nanoribbons, resulting in a domain-type structure of 4, 5, and 6-fold rings.

Interaction effects in a two-dimensional AlSi[formula omitted]P nanosheet: A first-principles study on the electronic, mechanical, thermal, and optical properties

Physical quantities such as the electronic, the mechanical, the thermal, and the optical properties of Al and P codoped silicene forming AlSi6P nanosheets are investigated by first-principle calculations within density functional theory. A particular attention of this study is paid to the interaction effect between the Al and the P atoms in the buckled silicene structure. We infer that the interaction type is attractive and it leads to an asymmetry in the density of states opening up a band gap. In contrast to BN-codoped silicene, the buckling in the Al–P codoped silicene is not much influenced by the dopants as the bond lengths of Si–P and Al–P are very similar to the Si–Si bond lengths. On the other hand, the longer bond length of Si–Al decreases the stiffness and thus induces fractures at smaller values of applied strain in AlSi6P. The elastic and the nonelastic regions of the stress–strain curve of AlSi6P depend on the placement of the Al and the P atoms. The finite band gap caused by the Al–P dopant leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of peaks in the dielectric response, and the optical conductivity. Finally, properties of the real and the imaginary parts of the dielectric function, the excitation spectra, the refractive index, and the optical response of AlSi6P are reported.

Intrinsic exchange bias state in silicene and germanene materials EuX2.

2D magnets have recently emerged as a host for unconventional phases and related phenomena. The prominence of 2D magnetism stems from its high amenability to external stimuli and structural variations. The low dimensionality facilitates competition between magnetic orders which may give rise to exchange bias, in particular in magnetic heterostructures. Here, we propose a strategy for the search of exchange bias state in 2D individual compounds. We track the evolution of magnetic orders driven by the number of monolayers in a system exhibiting antiferromagnetism in the multilayer and ferromagnetism in the monolayer limit. The material, EuSi2, has the structure of multilayer silicene intercalated by Eu. A strong intrinsic exchange bias effect accompanies the dimensional crossover. Comparison with silicene-based GdSi2 and germanene-based EuGe2 suggests the competition between magnetic orders to be a common property of this class of materials that may be useful in spintronic applications.

Crystal phase engineering of silicene by Sn-modified Ag(111).

The synthesis of silicene by direct growth on silver is characterized by the formation of multiple phases and domains, posing severe constraints on the spatial charge conduction towards a technological transfer of silicene to electronic transport devices. Here we engineer the silicene/silver interface by two schemes, namely, either through decoration by Sn atoms, forming an Ag2Sn surface alloy, or by buffering the interface with a stanene layer. Whereas in both cases Raman spectra confirm the typical features as expected from silicene, by electron diffraction we observe that a very well-ordered single-phase 4 × 4 monolayer silicene is stabilized by the decorated surface, while the buffered interface exhibits a sharp phase at all silicon coverages. Both interfaces also stabilize the ordered growth of a phase in the multilayer range, featuring a single rotational domain. Theoretical ab initio models are used to investigate low-buckled silicene phases (4 × 4 and a competing one) and various structures, supporting the experimental findings. This study provides new and promising technology routes to manipulate the silicene structure by controlled phase selection and single-crystal silicene growth on a wafer-scale.

Revealing the hydrogenated structure of silicene- (√13×√13)R13.9∘ by tip-induced dehydrogenation

The application of voltage pulse in a scanning tunneling microscope has been demonstrated to induce dehydrogenation on hydrogenated silicene/Ag(111), recovering intact silicene phases. This manipulation provides us a method to build a relationship between various silicene phases and their hydrogenated structures. Combining with density-functional theory calculations, hydrogenation of silicene-$(\ensuremath{\surd}13\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13)R13.{9}^{\ensuremath{\circ}}$ is found to produce the same half silicane as hydrogenated silicene-$(2\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}2\ensuremath{\surd}3)R{30}^{\ensuremath{\circ}}$. These two silicene phases are suggested to be the same monolayer silicene on Ag(111), albeit with different Si buckling configurations and lattice strains. Our study not only reveals the hydrogenated structure of silicene-$(\ensuremath{\surd}13\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}13)R13.{9}^{\ensuremath{\circ}}$, but also offers significant potential for lithography and modification of hydrogenated silicene at atomic scale for future design of silicene-based electronic devices.