Silicene:from monolayer to multilayer——A concise review

Silicene, a newly isolated silicon allotrope with a two-dimensional(2D) honeycomb lattice structure, is predicted to have electronic properties similar to those of graphene, including the existence of signature Dirac fermions. Furthermore,the strong spin–orbit interaction of Si atoms potentially makes silicene an experimentally accessible 2D topological insulator. Since 2012, silicene films have been experimentally synthesized on Ag(111) and other substrates, motivating a burst of research on silicene. We and collaborators have employed STM investigations and first principles calculations to intensively study the structure and electronic properties of silicene films on Ag(111), including monolayer, bilayer, and multilayer silicenes, as well as hydrogenation of silicene.     

Ar adsorptions on Al （111） and Ir （111） surfaces： a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites,i.e.,top,bridge,fcc-and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew,Burke and Ernzerhof functions.The geometric structures,the binding energies,the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces,the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated.Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure.The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML.For the Ar adsorption on Ir (111) surface at the same coverage,the most favourable site is the hcp-hollow site,with a corresponding binding energy of 0.493 eV.The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s,3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital.For Ar adsorption on Ir (111) surface,the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Fabrication of large-scale graphene/2D-germanium heterostructure by intercalation

We report a large-scale, high-quality heterostructure composed of vertically-stacked graphene and two-dimensional(2D) germanium.The heterostructure is constructed by the intercalation-assisted technique.We first synthesize large-scale,single-crystalline graphene on Ir(111) surface and then intercalate germanium at the interface of graphene and Ir(111).The intercalated germanium forms a well-defined 2D layer with a 2 × 2 superstructure with respect to Ir(111).Theoretical calculations demonstrate that the 2D germanium has a double-layer structure.Raman characterizations show that the 2D germanium effectively weakens the interaction between graphene and Ir substrate, making graphene more like the intrinsic one.Further experiments of low-energy electron diffraction, scanning tunneling microscopy, and x-ray photoelectron spectroscopy(XPS) confirm the formation of large-scale and high-quality graphene/2D-germanium vertical heterostructure.The integration of graphene with a traditional 2D semiconductor provides a platform to explore new physical phenomena in the future.     

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercala- tion, seven different metals have been successfully intercalated at the interface of graphene/Ru（O001） and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru（O001） and on Ir（l I 1）, have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.     

First-Principles Calculations of Atomic and Electronic Properties of T1 and In on Si（111）

The atomic and electronic structures of T1 and In on Si（111） surfaces are investigated using the firstprinciples total energy calculations. Total energy optimizations show that the energetically favored structure is 1/3 ML T1 adsorbed at the T4 sites on Si（111） surfaces. The adsorption energy difference of one T1 adatom between （√3 × √3） and （1 × 1） is less than that of each In adatom. The DOS indicates that TI 6p and Si 3p electrons play a very important role in the formation of the surface states. It is concluded that the bonding of TI adatoms on Si（111） surfaces is mainly polar covalent, which is weaker than that of In on Si（111）. So T1 atom is more easy to be migrated than In atom in the same external electric field and the structures of T1 on Si（111） is prone to switch between （√3 × √3） and （1 × 1）.     

Low-temperature growth of large-scale,single-crystalline graphene on Ir(111)

Iridium is a promising substrate for self-limiting growth of graphene. However, single-crystalline graphene can only be fabricated over 1120 K. The weak interaction between graphene and Ir makes it challenging to grow graphene with a single orientation at a relatively low temperature. Here, we report the growth of large-scale, single-crystalline graphene on Ir(111) substrate at a temperature as low as 800 K using an oxygen-etching assisted epitaxial growth method. We firstly grow polycrystalline graphene on Ir. The subsequent exposure of oxygen leads to etching of the misaligned domains.Additional growth cycle, in which the leftover aligned domain serves as a nucleation center, results in a large-scale and single-crystalline graphene layer on Ir(111). Low-energy electron diffraction, scanning tunneling microscopy, and Raman spectroscopy experiments confirm the successful growth of large-scale and single-crystalline graphene. In addition, the fabricated single-crystalline graphene is transferred onto a SiO_2/Si substrate. Transport measurements on the transferred graphene show a carrier mobility of about 3300 cm~2·V~(-1)·s~(-1). This work provides a way for the synthesis of large-scale,high-quality graphene on weak-coupled metal substrates.     

Electronic structure of single-crystalline graphene grown on Cu/Ni(111) alloy film

Graphene with a Dirac cone-like electronic structure has been extensively studied because of its novel transport properties and potential application for future electronic devices. For epitaxially grown graphene, the process conditions and the microstructures are strongly dependent on various substrate materials with different lattice constants and interface energies. Utilizing angle-resolved photoemission spectroscopy, here we report an investigation of the electronic structure of single-crystalline graphene grown on Cu/Ni(111) alloy film by chemical vapor deposition. With a relatively low growth temperature, graphene on Cu/Ni(111) exhibits a Dirac cone-like dispersion comparable to that of graphene grown on Cu(111). The linear dispersions forming Dirac cone are as wide as 2 e V, with the Fermi velocity of approximately 1.1×10~6 m/s. Dirac cone opens a gap of approximately 152 meV at the binding energy of approximately 304 meV. Our findings would promote the study of engineering of graphene on different substrate materials.     

Seeing Dirac electrons and heavy fermions in new boron nitride monolayers

Most three-dimensional(3D) and two-dimensional(2D) boron nitride(BN) structures are wide-band-gap insulators.Here, we propose two BN monolayers having Dirac points and flat bands, respectively. One monolayer is named as 5–7 BN that consists of five-and seven-membered rings. The other is a Kagome BN made of triangular boron rings and nitrogen dimers. The two structures show not only good dynamic and thermodynamic stabilities but also novel electronic properties.The 5–7 BN has Dirac points on the Fermi level, indicating that the structure is a typical Dirac material. The Kagome BN has double flat bands just below the Fermi level, and thus there are heavy fermions in the structure. The flat-band-induced ferromagnetism is also revealed. We analyze the origination of the band structures by partial density of states and projection of orbitals. In addition, a possible route to experimentally grow the two structures on some suitable substrates such as the PbO_2(111) surface and the Cd O(111) surface is also discussed, respectively. Our research not only extends understanding on the electronic properties of BN structures, but also may expand the applications of BN materials in 2D electronic devices.     

Thermal conductivity of carbon nanoring linked graphene sheets:A molecular dynamics investigation

Improving the thermal conduction across graphene sheets is of great importance for their applications in thermal management. In this paper, thermal transport across a hybrid structure formed by two graphene nanoribbons and carbon nanorings(CNRs) was investigated by molecular dynamics simulations. The effects of linker diameter, number, and height on thermal conductivity of the CNRs–graphene hybrid structures were studied respectively, and the CNRs were found effective in transmitting the phonon modes of GNRs. The hybrid structure with 2 linkers showed the highest thermal conductivity of 68.8 W·m~(-1)·K~(-1). Our work presents important insight into fundamental principles governing the thermal conduction across CNR junctions and provides useful guideline for designing CNR–graphene structure with superior thermal conductivity.     

Effect of surface morphology on the electron mobility of epitaxial graphene grown on 0° and 8° Si-terminated 4H-SiC substrates

Graphene with different surface morphologies were fabricated on 8° -off-axis and on-axis 4H-SiC(0001) substrates by high-temperature thermal decompositions. Graphene grown on Si-terminated 8° -off-axis 4H-SiC(0001) shows lower Hall mobility than the counterpart of on-axis SiC substrates. The terrace width is not responsible for the different electron mobility of graphene grown on different substrates, as the terrace width is much larger than the mean free path of the electrons. The electron mobility of graphene remains unchanged with an increasing terrace width on Siterminated on-axis SiC. Interface scattering and short-range scattering are the main factors affecting the mobility of epitaxial graphene. After the optimization of the growth process, the Hall mobility of the graphene reaches 1770 cm 2 /V·s at a carrier density of 9.8.×10 12 cm 2 . Wafer-size graphene was successfully achieved with an excellent double-layer thickness uniformity of 89.7% on a 3-inch SiC substrate.     

Characterizing silicon intercalated graphene grown epitaxially on Ir films by atomic force microscopy

An efficient method based on atomic force microscopy(AFM) has been developed to characterize silicon intercalated graphene grown on single crystalline Ir(111) thin films. By combining analyses of the phase image, force curves,and friction–force mapping, acquired by AFM, the locations and coverages of graphene and silicon oxide can be well distinguished. We can also demonstrate that silicon atoms have been successfully intercalated between graphene and the substrate. Our method gives an efficient and simple way to characterize graphene samples with interacted atoms and is very helpful for future applications of graphene-based devices in the modern microelectronic industry, where AFM is already widely used.     

Electronic structure and phase transition engineering in NbS_2:Crucial role of van der Waals interactions

Based on first-principles simulations, we revisit the crystal structures, electronic structures, and structural stability of the layered transition metal dichalcogenides(TMDCs) Nb S_2, and shed more light on the crucial roles of the van der Waals(vd W) interactions. Theoretically calculated results imply that the vd W corrections are important to reproduce the layered crystal structure, which is significant to correctly describe the electronic structure of NbS_2. More interestingly,under hydrostatic pressure or tensile strain in ab plane, an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered. The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects. The interlayer Nb–S distances collapse and the interlayer vd W interactions disappear, concomitant with new covalent bond emerging and increasing coordination number.Present work highlights the significance of the vd W interactions, and provides new insights on the unconventional structural transitions in NbS_2, which will attract wide audience working in the hectic field of TMDCs.     

Spin and valley half metal induced by staggered potential and magnetization in siliceneSpin and valley half metal induced by staggered potential and magnetization in siliceneSpin and valley half metal induced by staggered potential and magnetization in silicene

We investigate the electron transport in silicene with both staggered electric potential and magnetization; the latter comes from the magnetic proximity effect by depositing silicene on a magnetic insulator. It is shown that the silicene could be a spin and valley half metal under appropriate parameters when the spin-orbit interaction is considered; further, the filtered spin and valley could be controlled by modulating the staggered potential or magnetization. It is also found that in the spin-valve structure of silicene, not only can the antiparallel magnetization configuration significantly reduce the valve-structure conductance, but the reversing staggered electric potential can cause a high-performance magnetoresistance due to the spin and valley blocking effects. Our findings show that the silicene might be an ideal basis for the spin and valley filter analyzer devices.     

Epitaxial fabrication of two-dimensional TiTe_2 monolayer onAu(111) substrate with Te as buffer layer

Two-dimensional(2 D) materials provide a platform to exploit the novel physical properties of functional nanodevices.Here, we report on the formation of a new 2 D layered material, a well-ordered monolayer TiTe_2, on a Au(111) surface by molecular beam epitaxy(MBE). Low-energy electron diffraction(LEED) measurements of the samples indicate that the TiTe_2 film forms(√3 ×√7) superlattice with respect to the Au(111) substrate, which has three different orientations. Scanning tunneling microscopy(STM) measurements clearly show three ordered domains consistent with the LEED patterns.Density functional theory(DFT) calculations further confirm the formation of 2 H-TiTe_2 monolayer on the Au(111) surface with Te as buffer layer. The fabrication of this 2 D layered heterostructure expands 2 D material database, which may bring new physical properties for future applications.     

Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe_2 Films

Understanding the interplay between superconductivity and charge-density wave(CDW) in NbSe_2 is vital for both fundamental physics and future device applications. Here, combining scanning tunneling microscopy, angleresolved photoemission spectroscopy and Raman spectroscopy, we study the CDW phase in the monolayer NbSe_2 films grown on various substrates of bilayer graphene(BLG), SrTiO_3(111), and Al_2O_3(0001). It is found that the two stable CDW states of monolayer NbSe_2 can coexist on NbSe_2/BLG surface at liquid-nitrogen temperature.For the NbSe_2/SrTiO_3(111) sample, the unidirectional CDW regions own the kinks at ±41 meV and a wider gap at 4.2 K. It is revealed that the charge transfer from the substrates to the grown films will influence the configurations of the Fermi surface, and induce a 130 meV lift-up of the Fermi level with a shrink of the Fermi pockets in NbSe_2/SrTiO_3(111) compared with the NbSe_2/BLG. Combining the temperature-dependent Raman experiments,we suggest that the electron-phonon coupling in monolayer NbSe_2 dominates its CDW phase transition.     

Surface structures of equilibrium restricted curvature model on two fractal substrates

With the aim to probe the effects of the microscopic details of fractal substrates on the scaling of discrete growth models, the surface structures of the equilibrium restricted curvature(ERC) model on Sierpinski arrowhead and crab substrates are analyzed by means of Monte Carlo simulations. These two fractal substrates have the same fractal dimension df, but possess different dynamic exponents of random walk zrw. The results show that the surface structure of the ERC model on fractal substrates are related to not only the fractal dimension df, but also to the microscopic structures of the substrates expressed by the dynamic exponent of random walk zrw. The ERC model growing on the two substrates follows the well-known Family–Vicsek scaling law and satisfies the scaling relations 2α + df≈ z ≈ 2zrw. In addition, the values of the scaling exponents are in good agreement with the analytical prediction of the fractional Mullins–Herring equation.     

Tunable valley filter efficiency by spin–orbit coupling in silicene nanoconstrictions

Valley filter is a promising device for producing valley polarized current in graphene-like two-dimensional honeycomb lattice materials. The relatively large spin–orbit coupling in silicene contributes to remarkable quantum spin Hall effect, which leads to distinctive valley-dependent transport properties compared with intrinsic graphene. In this paper,quantized conductance and valley polarization in silicene nanoconstrictions are theoretically investigated in quantum spinHall insulator phase. Nearly perfect valley filter effect is found by aligning the gate voltage in the central constriction region. However, the valley polarization plateaus are shifted with the increase of spin–orbit coupling strength, accompanied by smooth variation of polarization reversal. Our findings provide new strategies to control the valley polarization in valleytronic devices.     

Thermo-controllable self-assembled structures of single-layer 4,4'-diamino-p-terphenyl molecules on Au(110)

Here we report the thermo-controllable self-assembled structures of single-layer 4, 4'-diamino-p-terphenyl(DAT)molecules on Au(110), which are investigated by scanning tunneling microscopy(STM) combined with density functional theory(DFT) based calculations. With the deposition of monolayer DAT molecules on Au(110) and subsequent annealing at 100℃, all DAT molecules adsorb on a(1×5) reconstructed surface with a ladder-like structure. After annealing the sample at about 200℃, STM images show three distinct domains, including DAT molecules on a(1×3) reconstructed surface, dehydrogenated molecules with two hydrogen atoms detached from one amino group(–2H-DAT) on a(1×5)reconstructed surface and dehydrogenated molecules with four hydrogen atoms detached from two amino groups(–4HDAT) on a(1×3) reconstructed surface through N–Au bonds. Furthermore, after annealing the sample to 350℃, STM image shows only one self-assembled structure with –4H-DAT molecules on a(1×3) reconstructed surface. Relative STM simulations of different self-assembled structures show excellent agreements with the experimental STM images at different annealing temperatures. Further DFT calculations on the dehydrogenation process of DAT molecule prove that the dehydrogenation barrier on a(1×5) reconstructed surface is lower than that on(1×3) one, which demonstrate the experimental results that the formation temperature of a(1×3) reconstructed surface is higher than that of a(1×5) one.     

Surface Structure and Reconstructions of HgTe(111) Surfaces

Hg Te(111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy(STS).In addition to th√e prim√itive(1 × 1)√ hexagonal lattice,six reconstructed surface structures are observed:(2 × 2),2 × 1,4 × 1,3 ×(1/2)3,2(1/2)2 × 2 and (1/2)11 × 2.The(2 × 2) reconstructed lattice maintains the primitive hexagonal symmetry,whi√le the lattices of the other five reconstructions are rectangular.Moreover,the topographic features of the3 ×(1/2)3 reconstruction are bias dependent,indicating that they have both topographic and electronic origins.The STSs obtained at different reconstructed surfaces show a universal dip feature with size ～100 mV,which may be attributed to the surface distortion.Our results reveal the atomic structure and complex reconstructions of the cleaved Hg Te(111) surfaces,which paves the way to understand the rich properties of Hg Te crystal.     

Spectroscopic and scanning probe analysis on large-area epitaxial graphene grown under pressure of 4 mbar on 4H-SiC （0001） substrates

We produced epitaxial graphene under a moderate pressure of 4 mbar(about 400 Pa) at temperature 1600℃. Raman spectroscopy and optical microscopy were used to confirm that epitaxial graphene has taken shape continually with slight thickness variations and regularly with a centimeter order of magnitude on 4H-SiC(0001) substrates. Then using X-ray photoelectron spectroscopy and Auger electron spectroscopy, we analyzed the chemical compositions and estimated the layer number of epitaxial graphene. Finally, an atomic force microscope and a scanning force microscope were used to characterize the morphological structure. Our results showed that under 4-mbar pressure, epitaxial graphene could be produced on a SiC substrate with a large area, uniform thickness but a limited morphological property. We hope our work will be of benefit to understanding the formation process of epitaxial graphene on SiC substrate in detail.