Silicene Sheet Research Articles

Single-domain epitaxial silicene on diboride thin films

Epitaxial silicene, which forms spontaneously on ZrB2(0001) thin films grown on Si(111) wafers, has a periodic stripe domain structure. By adsorbing additional Si atoms on this surface, we find that the domain boundaries vanish, and a single-domain silicene sheet can be prepared without altering its buckled honeycomb structure. The amount of Si required to induce this change suggests that the domain boundaries are made of a local distortion of the silicene honeycomb lattice. The realization of a single domain sheet with structural and electronic properties close to those of the original striped state demonstrates the high structural flexibility of silicene.

Peculiar Magnetic Properties in Doped and Edge Functionalized Silicene Nanoribbons

Motivated by the exceptional properties of graphene, other two-dimensional (2D) nanomaterials beyond graphene such as silicene have emerged with growing interest. Silicene is attractive for its exceptional characteristics and compatibility with silicon based nanoelectronics. It is believed that silicene may challenge graphene due to a longer spin-diffusion time, a longer spin coherence length, and a much larger spin-orbit coupling gap. Half-metallic materials and spin-gapless semiconductors (SGS) are highly desirable for spintronics because they provide 100% spin-polarized currents around their Fermi level. Spin filters and spin field effect transistors (FETs) are one of the potential applications of these types of materials. Silicene nanoribbon (SiNR) as an approach to opening a band gap in a silicene sheet is a promising material for spintronics due to its unique magnetic properties. In this study, a first-principle method based on density functional theory (DFT) is used to investigate the stability, electronic, and magnetic properties of edge hydrogenated and fluorinated SiNRs. Edge functionalization of all possible types of SiNRs such as armchair, zigzag, Klein, reconstructed Klein, pentagon-heptagon, and the combination of the edges are studied. It is found that edge fluorinated structures are much more stable than that of edge hydrogenated. It is also discovered that tri-fluorinated Klein edge SiNR and di-hydrogenated armchair edge SiNR are the most stable structures when the edges are terminated with fluorine and hydrogen atoms, respectively. While, the symmetric edge functionalized zigzag SiNRs show antiferromagnetic semiconductor behaviour. Considering spin-polarization, the asymmetry in edge functionalization bring various and extraordinary magnetic states, such as SGS, ferromagnetic metals or semiconductors, and antiferromagnetic metals. In order to discover the effects of doping on the magnetic and electronic properties of edge functionalized SiNRs, N- or B-doped SiNRs are considered. It is revealed that the dopants energetically prefer to be substituted with Si atoms at the edge of SiNRs. Interestingly, the remarkable SGS and half-metal character, as well as ferromagnetic metallic behavior are observed in N- or B- doped armchair, zigzag, and Klein edge SiNRs which are highly advantageous for spintronics applications. These new findings encourage further experimental investigations in the development of SiNRs based nanoelectronics with spin tuning.

Study by EXAFS of the local structure around Si on silicene deposited on Ag(1 1 0) and Ag(1 1 1) surfaces

The local structure around the silicon atoms of silicene deposited onto Ag(1 1 0) and Ag(1 1 1) has been determined by extended x-ray absorption fine structure spectroscopy at the silicon K-edge. This study shows that silicon atoms are not in a flat honeycomb network locally buckled, but that this structure mimics the double Si(1 1 1)-plane of crystalline silicon with almost the same first and second interatomic distances (2.35 and 3.83 Å) on a regularly buckled geometry. Moreover the results evidence silver atoms at a well-defined distance from the silicon ones, a signature for an interaction between the silicene sheet and silver atoms released from the substrate.

Structural and electronic properties of zigzag InP nanoribbons with Stone–Wales type defects

By means of density-functional-theoretic calculations, we investigate the structural and electronic properties of a hexagonal InP sheet and of hydrogen-passivated zigzag InP nanoribbons (ZInPNRs) with Stone–Wales (SW)-type defects. Our results show that the influence of this kind of defect is not limited to the defected region but it leads to the formation of ripples that extend across the systems, in keeping with the results obtained recently for graphene and silicene sheets. The presence of SW defects in ZInPNRs causes an appreciable broadening of the band gap and transforms the indirect-bandgap perfect ZInPNR into a direct-bandgap semiconductor. An external transverse electric field, regardless of its direction, reduces the gap in both the perfect and defective ZInPNRs.

Topotactic synthesis of the overlooked multilayer silicene intercalation compound SrSi2.

Silicene, a 2D honeycomb lattice of Si atoms similar to graphene, is expected to be a platform for nanoelectronics and home to novel quantum phenomena. Unlike graphene, free-standing silicene is notoriously difficult to stabilize, while strong hybridization of silicene with substrates destroys its desirable properties. On the other hand, Dirac cones of silicene are effectively realized in a bulk - stoichiometric ionic multilayer silicene intercalation compound CaSi2. Besides, a number of new 2D silicene-based materials are synthesized employing CaSi2 as a precursor. However, the rather complex atomic structure of CaSi2 and fresh opportunities of physical and chemical breakthroughs drive the search for alternative compounds with silicene networks. Here, a new polymorph of SrSi2 is synthesized, enjoying both the structure of intercalated multilayer silicene and the simplest possible stacking of silicene sheets. The MBE-quality synthesis accomplished on Si(001) and Si(111) surfaces leads to epitaxial films of SrSi2 with orientation controlled by the substrate, as revealed by XRD, RHEED and electron microscopy studies. The structural SrSi2/Si relation is mirrored in the transport properties of the films.

Impact of van der Waal’s interaction in the hybrid bilayer of silicene/SiC

DFT calculations find a noticeable interlayer van der Waal interaction in a silicene/SiC hybrid bilayer. The interaction leads to curvature in the planar SiC sheet and opens the band gap of the silicene sheet.

Characteristics of Li diffusion on silicene and zigzag nanoribbon**Project supported by the National Natural Science Foundation of China (Grant Nos. 11074212 and 11204123) and the Natural Science Foundation of Jiangsu province, China (Grant No. BK20130945).

We perform a density functional study on the adsorption and diffusion of Li atoms on silicene sheet and zigzag nanoribbons. Our results show that the diffusion energy barrier of Li adatoms on silicene sheet is 0.25 eV, which is much lower than on graphene and Si bulk. The diffusion barriers along the axis of zigzag silicene nanoribbon range from 0.1 to 0.25 eV due to an edge effect, while the diffusion energy barrier is about 0.5 eV for a Li adatom to enter into a silicene nanoribbon. Our calculations indicate that using silicene nanoribbons as anodes is favorable for a Li-ion battery.

Stable line defects in silicene

Line defects in two-dimensional (2D) materials greatly modulate various properties of their pristine form. Using ab initio molecular dynamics (AIMD) simulations, we investigate the structural reconstructions of different kinds of grain boundaries in the silicene sheets. It is evident that depending upon the presence of silicon adatoms and edge shape of grain boundaries (i.e., armchair or zigzag), stable extended line defects (ELDs) can be introduced in a controlled way. Further studies show the stability of these line-defects in silicene, grown on Ag(111) surface at room-temperature. Importantly, unlike most of the 2D sheet materials such as graphene and hexagonal boron nitride, 5-5-8 line defects modify the nonmagnetic semimetallic pristine silicene sheet to spin-polarized metal. As ferromagnetically ordered magnetic moments remain strongly localized at the line defect, a one-dimensional spin channel gets created in silicene. Interestingly, these spin channels are quite stable because, unlike the edge of nanoribbons, structural reconstruction or contamination cannot destroy the ordering of magnetic moments here. Zigzag silicene nanoribbons with a 5-5-8 line defect also exhibit various interesting electronic and magnetic properties depending upon their width as well as the nature of the magnetic coupling between edge and defect spin states. Upon incorporation of other ELDs, such as 4-4-4 and 4-8 defects, 2D sheets and nanoribbons of silicene show a nonmagnetic metallic or semiconducting ground state. Highlighting the controlled formation of ELDs and consequent emergence of technologically important properties in silicene, we propose new routes to realize silicene-based nanoelectronic and spintronic devices.

Controllable intrinsic DC spin/valley Hall conductivity in ferromagnetic silicene: Exploring a fully spin/valley polarized transport

We study intrinsic DC spin and valley Hall conductivity in doped ferromagnetic silicene in the presence of an electric filed applied perpendicular to silicene sheet. By calculating its energy spectrum and wavefunction and by making use of Kubo formalism, we obtain a general relation for the transverse Hall conductivity which can be used to obtain spin- and valley-Hall conductivity. Our results, in the zero limit of the exchange field, reduces to the previous results. Furthermore we discuss electrically tunable spin and valley polarized transport in ferromagnetic silicene and obtain the necessary conditions for observing a fully spin or valley polarized transport.

From Silicene to Half-Silicane by Hydrogenation.

Graphane is graphene fully hydrogenated from both sides, forming a 1×1 structure, where all C atoms are in sp(3) configuration. In silicene, the Si atoms are in a mixed sp(2)/sp(3) configuration; it is therefore natural to imagine a silicane structure analogous to graphane. However, a monatomic silicene sheet grown on substrates generally reconstructs into different phases, and only partially hydrogenated silicene with reconstructions had been reported before. In this work, we produce half-silicane, where one Si sublattice is fully H-saturated and the other sublattice is intact, forming a perfect 1×1 structure. By hydrogenating various silicene phases on a Ag(111) substrate, we found that only the (2√3×2√3)R30° phase can produce half-silicane. Interestingly, this phase was previously considered to be a highly defective or incomplete silicene structure. Our results indicate that the structure of the (2√3×2√3)R30° phase involves a complete silicene-1×1 lattice instead of defective fragments, and the formation mechanism of half-silicane was discussed with the help of first-principles calculations.

Spin- and valley-dependent transport properties for metal-silicene-metal junctions

Detailed knowledge relating to the interactions between silicene and normal metal is crucial to understanding silicene growth on metal surfaces and metal/silicene interfaces in nanoelectronic devices. In this work, we study the valley- and spin-dependent transport properties of a metal/silicene/metal junction (MSM) with end and side metal-silicene contacts, respectively, where the central silicene sheet is simultaneously in proximity to a ferromagnet and a perpendicular electric field. By connecting the wave amplitudes obeying the lattice Schrodinger equation for the interfaces within the tight-binding model, the tunable conductance of both end-contacted (EC) and side contacted (SC) MSM junctions have been calculated. The current through MSM junctions is spin and valley polarized due to the coupling between valley and spin degrees of freedom, and the conductance and polarization show oscillating behavior as a function of the length of the silicene sheet. In particular, we find that the full spin and valley polarized conductance can be achieved by introducing proper electric and exchange fields. Further, the conductance is heavily dependent on the hopping integrals of simple metal, silicene and metal/silicene interfaces for EC junctions, and as long as the hopping integrals satisfy certain condition (with suitable incident energy) there is no difference in the transport between EC and SC junctions. The findings here may be meaningful in understanding the nature of metal/silicene interfaces.

Valley Hall effect in silicene and hydrogenated silicene ruled by grain boundaries: Anab initioinvestigation

We have performed an ab initio theoretical study of the energetic stability and the electronic properties of pristine and hydrogen-adsorbed grain boundaries (GBs) in silicene. We find that GBs in silicene present lower formation energy when compared with their counterparts in graphene. Removing the inversion symmetry, by applying an external electric field perpendicular to the silicene sheet, we verify the formation of valley-indexed metallic states lying along the GBs, characterizing the quantum valley Hall effect (QVHE). Here, we find the maintenance of the QVHE upon the presence of disordered and asymmetric geometries along the GBs. Those metallic states are suppressed upon the adsorption of H adatoms along the GBs. The H adatoms promote an unbalance on the electronic occupation of the unsaturated $\ensuremath{\pi}$ electrons beside the hydrogenated GB rows, giving rise to (i) a net magnetic moment on the Si atoms along the edge sites of the hydrogenated GBs and (ii) an electronic band structure characterized by spin-polarized valley states protected against backscattering processes.

High hydrogen storage capacity in calcium‐decorated silicene nanostructures

We report our first‐principle's calculations on the possibility of Ca‐decorated silicene sheet and zigzag silicene nanoribbons (ZSiNRs) as hydrogen storage medium. We predict that Ca atoms prefer to disperse on the silicene or at the edges of ZSiNRs without clustering, due to the strong binding between Ca and Si atoms. By adsorbing Ca atoms on the both sides of silicene, the hydrogen storage capacity can reach to 6.17 wt% (gravimetric density) with an average adsorption energy of 0.265 eV H2−1, which are quite optimial for reversible hydrogen adsorption and desorption at ambient conditions. The hydrogen storage capacity can be further improved to 8.43 wt% with the average adsorption energy in the range of 0.182–0.269 eV H2−1 in the Ca‐decorated ZSiNRs. The adsorption of H2 on Ca‐decorated Si nanostructures is mainly dominated by polarization and the orbital hybridizations. These findings indicate that the Ca‐decorated silicene and ZSiNRs have potential application in hydrogen storage.

Ordered and reversible hydrogenation of silicene.

The hydrogenation of monatomic silicene sheet on Ag(111) was studied by scanning tunneling microscopy and density functional theory calculations. It was observed that hydrogenation of silicene at room temperature results in a perfectly ordered γ-(3×3) superstructure. A theoretical model, which involves seven H atoms and rearranged buckling of Si atoms, was proposed and agrees with experiments very well. Moreover, by annealing to a moderate temperature, about 450K, a dehydrogenation process occurs and the clean silicene surface can be fully recovered. Such uniformly ordered and reversible hydrogenation may be useful for tuning the properties of silicene as well as for controllable hydrogen storage.

First-principles calculations study of Na adsorbed on silicene

Geometry, electronic structure, and diffusion properties of Na atom on the pristine and single vacancy-defected silicene sheet have been systematically investigated by using density functional theory with generalized gradient approximation. The most stable positions of Na atom on the perfect and the defected silicene sheet are determined, and four diffusion paths have been explored. The minimum diffusion barrier of Na atom is 0.25eV on pristine silicene, and 1.44eV on single vacancy-defected silicene sheet. The vacancy exhibits a more significant influence than the adsorption of the Na atom on the electronic structure of silicene.

Electronic and magnetic properties of zigzag silicene nanoribbons with Stone–Wales defects

The structural, electronic, and magnetic properties of zigzag silicene nanoribbons (ZSiNRs) with Stone–Wales (SW) defects were investigated using first-principles calculations. We found that two types of SW defects (named SW-Ι and SW-ΙΙ) exist in ZSiNRs. The SW defect was found to be the most stable at the edge of the ZSiNR, independently of the defect orientation, even more stable than it is in an infinite silicene sheet. In addition, the ZSiNRs can transition from semiconductor to metal or half-metal by modifying the SW defect location and concentration. For the same defect concentration, the band structures influenced by the SW-Ι defect are more distinct than those influenced by the SW-ΙΙ when the SW defect is at the edge. The present study suggests the possibility of tuning the electronic properties of ZSiNRs using the SW defects and might motivate their potential application in nanoelectronics and spintronics.

The rare two-dimensional materials with Dirac cones

Abstract Inspired by the great development of graphene, more and more research has been conducted to seek new two-dimensional (2D) materials with Dirac cones. Although 2D Dirac materials possess many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphene, silicene, germanene, graphynes, several boron and carbon sheets, transition-metal oxides (VO2)n/(TiO2)m and (CrO2)n/(TiO2)m, organic and organometallic crystals, so-MoS2, and artificial lattices (electron gases and ultracold atoms). Their structural and electronic properties are summarized. We also investigate how Dirac points emerge, move, and merge in these systems. The von Neumann–Wigner theorem is used to explain the scarcity of Dirac cones in 2D systems, which leads to rigorous requirements on the symmetry, parameters, Fermi level, and band overlap of materials to achieve Dirac cones. Connections between existence of Dirac cones and the structural features are also discussed.

Defects in silicene: vacancy clusters, extended line defects, and Di-adatoms.

Defects are almost inevitable during the fabrication process, and their existence strongly affects thermodynamic and (opto)electronic properties of two-dimensional materials. Very recent experiments have provided clear evidence for the presence of larger multi-vacancies in silicene, but their structure, stability, and formation mechanism remain largely unexplored. Here, we present a detailed theoretical study of silicene monolayer containing three types of defects: vacancy clusters, extended line defects (ELDs), and di-adatoms. First-principles calculations, along with ab initio molecular dynamics simulations, revealed the coalescence tendency of small defects and formation of highly stable vacancy clusters. The 5|8|5 ELD – the most favorable extended defect in both graphene and silicene sheets – is found to be easier to form in the latter case due to the mixed sp2/sp3 hybridization of silicon. In addition, hybrid functional calculations that contain part of the Hatree-Fock exchange energy demonstrated that the introduction of single and double silicon adatoms significantly enhances the stability of the system, and provides an effective approach on tuning the magnetic moment and band gap of silicene.

Covalent nitrophenyl diazonium functionalized silicene for spintronics: a first-principles study.

We predict some novel electronic and magnetic properties of a functionalized silicene sheet by nitrophenyl diazonium (NPD) using first-principles calculations in the framework of density functional theory with dispersion corrections. Our calculations at the HSE06 level show that for the three coverage ratios of NPD considered in this work (i.e., NPD : Si = 1 : 8, 1 : 18 and 1 : 32), spin-polarized electronic structures can be always realized with NPD adsorption although the bandgap decreases upon reducing the NPD coverage ratio. The quasi-localized pz electrons of Si are identified to be responsible for the ferrimagnetism in these two-dimensional systems. Remarkably, the system with the NPD : Si = 1 : 8 ratio is predicted to be a bipolar magnetic semiconductor. As such, half-metallicity can be realized by applying a gate voltage with reversible spin polarization, making NPD-1/8 a potential candidate for future spintronic applications. This work offers a new tailor-made functionalization approach to realize magnetic semiconducting silicene.

First-principles study of half-fluorinated silicene sheets

Strain dependence of the energy difference between FM and AFM order of chair-F@Si2 sheet. Inserted image shows the spin density distribution of chair-F@Si2 sheet with no strain.