Hongsheng Liu

Structures, mobilities, electronic and magnetic properties of point defects in silicene

In the fabrication and processing of silicene monolayers, structural defects are almost inevitable. Using ab initio calculations, we systemically investigated the structures, formation energies, migration behaviors and electronic/magnetic properties of typical point defects in silicene, including the Stone-Wales (SW) defect, single and double vacancies (SVs and DVs), and adatoms. We found that SW can be effectively recovered by thermal annealing. SVs have much higher mobility than DVs and two SVs are very likely to coalesce into one DV to lower the energy. Existence of SW and DVs may induce small gaps in silicene, while the SV defect may transform semimetallic silicene into metallic. Adatoms are unexpectedly stable and can affect the electronic properties of silicene dramatically. Especially, Si adatoms as self-dopants in silicene sheets can induce long-range spin polarization as well as a remarkable band gap, thus achieving an all-silicon magnetic semiconductor. The present theoretical results provide valuable insights into identification of these defects in experiments and understanding their effects on the physical properties of silicene.

From Boron Cluster to Two-Dimensional Boron Sheet on Cu(111) Surface: Growth Mechanism and Hole Formation

As attractive analogue of graphene, boron monolayers have been theoretically predicted. However, due to electron deficiency of boron atom, synthesizing boron monolayer is very challenging in experiments. Using first-principles calculations, we explore stability and growth mechanism of various boron sheets on Cu(111) substrate. The monotonic decrease of formation energy of boron cluster BN with increasing cluster size and low diffusion barrier for a single B atom on Cu(111) surface ensure continuous growth of two-dimensional (2D) boron cluster. During growth process, hexagonal holes can easily arise at the edge of a 2D triangular boron cluster and then diffuse entad. Hence, large-scale boron monolayer with mixed hexagonal-triangular geometry can be obtained via either depositing boron atoms directly on Cu(111) surface or soft landing of small planar BN clusters. Our theoretical predictions would stimulate further experiments of synthesizing boron sheets on metal substrates and thus enrich the variety of 2D monolayer materials.

Tuning the Band Gap in Silicene by Oxidation

Silicene monolayers grown on Ag(111) surfaces demonstrate a band gap that is tunable by oxygen adatoms from semimetallic to semiconducting type. With the use of low-temperature scanning tunneling microscopy, we find that the adsorption configurations and amounts of oxygen adatoms on the silicene surface are critical for band gap engineering, which is dominated by different buckled structures in √13 × √13, 4 × 4, and 2√3 × 2√3 silicene layers. The Si-O-Si bonds are the most energy-favored species formed on √13 × √13, 4 × 4, and 2√3 × 2√3 structures under oxidation, which is verified by in situ Raman spectroscopy as well as first-principles calculations. The silicene monolayers retain their structures when fully covered by oxygen adatoms. Our work demonstrates the feasibility of tuning the band gap of silicene with oxygen adatoms, which, in turn, expands the base of available two-dimensional electronic materials for devices with properties that is hardly achieved with graphene oxide.

Atomistic insight into the oxidation of monolayer transition metal dichalcogenides: from structures to electronic properties

Monolayer transition metal dichalcogenides (TMDs) stand out in two-dimensional (2D) materials due to their potential applications in future microelectronic and optoelectronic devices. In experiments, field effect transistors (FET) based on the MoS2 monolayer are sensitive to environmental gases, especially O2. Thus, the oxidation of monolayer TMDs is a critical concern. By first-principles calculations, we reveal that a perfect single-layer sheet of TMDs stays intact when exposed in O2 due to the weak physical adsorption of O2. However, O2 can be chemically adsorbed onto the monolayer of TMDs (including MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2) with single vacancies of chalcogen, which are the most common defects in realistic TMD materials. The adsorption configurations and dissociation behavior of the O2 molecule at vacancy sites, as well as the possible diffusion behavior of oxygen adatoms on the TMD monolayer surface were explored. Oxidation significantly influenced the electronic properties of a defective MoS2 monolayer, while other defective TMD monolayers (especially MoTe2 and WTe2) suffered less from oxidation. Our theoretical results provide valuable atomistic insight into the oxidation of TMD monolayers and are useful for the future design of TMD-based 2D devices.

Quasi-freestanding epitaxial silicene on Ag(111) by oxygen intercalation

Quasi-freestanding silicene with massless Dirac fermion characteristics has been successfully obtained by oxygen intercalation. Silicene is a monolayer allotrope of silicon atoms arranged in a honeycomb structure with massless Dirac fermion characteristics similar to graphene. It merits development of silicon-based multifunctional nanoelectronic and spintronic devices operated at room temperature because of strong spin-orbit coupling. Nevertheless, until now, silicene could only be epitaxially grown on conductive substrates. The strong silicene-substrate interaction may depress its superior electronic properties. We report a quasi-freestanding silicene layer that has been successfully obtained through oxidization of bilayer silicene on the Ag(111) surface. The oxygen atoms intercalate into the underlayer of silicene, resulting in isolation of the top layer of silicene from the substrate. In consequence, the top layer of silicene exhibits the signature of a 1 × 1 honeycomb lattice and hosts massless Dirac fermions because of much less interaction with the substrate. Furthermore, the oxidized silicon buffer layer is expected to serve as an ideal dielectric layer for electric gating in electronic devices. These findings are relevant for the future design and application of silicene-based nanoelectronic and spintronic devices.

Band gap opening in bilayer silicene by alkali metal intercalation

Recently, bilayer and multilayer silicene have attracted increased attention following the boom of silicene, which holds great promise for future applications in microelectronic devices. Herein we systematically investigate all stacking configurations of bilayer silicene and the corresponding electronic properties. Strong coupling is found between two silicene layers, which destroys the Dirac cones in the band structures of pristine silicene and makes bilayer silicene sheets metallic. However, intercalation of alkali metal (especially potassium) can effectively decouple the interaction between two silicene layers. In the K-intercalated bilayer silicene (KSi4), the Dirac cones are recovered with a small band gap of 0.27 eV located about 0.55 eV below the Fermi level. Furthermore, intercalation of K(+) cations in bilayer silicene (K(+)Si4) results in a semiconductor with a moderate band gap of 0.43 eV, making it ideal for microelectronic applications.

Silicene on substrates: interaction mechanism and growth behavior

Silicene, a monolayer of silicon atom assembling in a honeycomb lattice, has attracted more and more attention due to its outstanding electronic properties. The recently successful synthesis of silicene on several metal surfaces takes a big step towards the utilization of silicene in the future microelectronic devices. On the roadmap for the applications of silicene, two critical issues have to be addressed: (1) how to improve the quality of silicene; (2) how to preserve the extraordinary electronic properties of silicene. These two problems can be solved by deeply understanding the substrate effect on silicene. In this review, we systematically discuss the substrate effect on the atomic structure and electronic properties of a silicene sheet as well as the growth behavior of silicene on Ag surface, which are important for both fabrication and application of silicene.

Point defects in epitaxial silicene on Ag(111) surfaces

Silicene, a counterpart of graphene, has achieved rapid development due to its exotic electronic properties and excellent compatibility with the mature silicon-based semiconductor technology. Its low room-temperature mobility of about 100 cm2V-1s-1, however, inhibits device applications such as in field-effect transistors. Generally, defects and grain boundaries would act as scattering centers and thus reduce the carrier mobility. In this paper, the morphologies of various point defects in epitaxial silicene on Ag(111) surfaces have been systematically investigated using first-principles calculations combined with experimental scanning tunneling microscope (STM) observations. The STM signatures for various defects in epitaxial silicene on Ag(111) surface are identified. In particular, the formation energies of point defects in Ag(111)-supported silicene sheets show an interesting dependence on the superstructures, which, in turn, may have implications for controlling the defect density during the synthesis of silicene. Through estimating the concentrations of various point defects in different silicene superstructures, the mystery of the defective appearance of v13*v13 silicene in experiments is revealed, and 4*4 silicene sheet is thought to be the most suitable structure for future device applications.