New Findings on Multilayer Silicene on Si(111)√3×√3R30°-Ag Template

Epitaxial growth and structural properties of silicene and other 2D allotropes of Si

Since the breakthrough of graphene, considerable efforts have been made to search for two-dimensional (2D) materials composed of other group 14 elements, in particular silicon and germanium, due to their valence electronic configuration similar to that of carbon and their widespread use in the semiconductor industry. Silicene, the silicon counterpart of graphene, has been particularly studied, both theoretically and experimentally. Theoretical studies were the first to predict a low-buckled honeycomb structure for free-standing silicene possessing most of the outstanding electronic properties of graphene. From an experimental point of view, as no layered structure analogous to graphite exists for silicon, the synthesis of silicene requires the development of alternative methods to exfoliation. Epitaxial growth of silicon on various substrates has been widely exploited in attempts to form 2D Si honeycomb structures. In this article, we provide a comprehensive state-of-the-art review focusing on the different epitaxial systems reported in the literature, some of which having generated controversy and long debates. In the search for the synthesis of 2D Si honeycomb structures, other 2D allotropes of Si have been discovered and will also be presented in this review. Finally, with a view to applications, we discuss the reactivity and air-stability of silicene as well as the strategy devised to decouple epitaxial silicene from the underlying surface and its transfer to a target substrate.

Evidence of sp2-like Hybridization of Silicon Valence Orbitals in Thin and Thick Si Grown on α-Phase Si(111)√3 × √3R30°-Bi

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as a function of the electron beam incidence angle α and low-energy electron diffraction (LEED), as well as ex situ by grazing incidence X-ray diffraction (GIXRD). Scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) were also reported. The REELS spectra, taken at the Si K absorption edge (~1.840 KeV), reveal the presence of two distinct loss structures attributed to transitions 1s→π* and 1s→σ* according to their intensity dependence on α, attesting to the sp²-like hybridization of the silicon valence orbitals in both thin and thick Si films. The synthesis of a silicon allotrope on the α-phase of Si(111)√3 × √3R30°-Bi substrate was demonstrated by LEED patterns and GIXRD that discloses the presence of a Si stack of 3.099 (3) Å and a √3 × √3 unit cell of 6.474 Å, typically seen for multilayer silicene. STM and STS measurements corroborated the findings. These measurements provided a platform for the new √3 × √3R30° Si allotrope on a Si(111)√3 × √3 R30°-Bi template, paving the way for realizing topological insulator heterostructures from different two-dimensional materials, Bi and Si.

Temperature-Dependent Growth and Evolution of Silicene on Au Ultrathin Films-LEEM and LEED Studies

The formation and evolution of silicene on ultrathin Au films have been investigated with low energy electron microscopy and diffraction. Careful control of the annealing rate and temperature of Au films epitaxially grown on the Si(111) surface allows for the preparation of a large scale, of the order of cm², silicene sheets. Depending on the final temperature, three stages of silicene evolution can be distinguished: (i) the growth of the low buckled phase, (ii) the formation of a layered heterostructure of the low buckled and planar phases of silicene and (iii) the gradual destruction of the silicene. Each stage is characterized by its unique surface morphology and characteristic diffraction patterns. The present study gives an overview of structures formed on the surface of ultrathin Au films and morphology changes between room temperature and the temperature at which the formation of Au droplets on the Si(111) surface occurs.

Simplified Determination of RHEED Patterns and Its Explanation Shown with the Use of 3D Computer Graphics

The process of preparation of nanostructured thin films in high vacuum can be monitored with the help of reflection high energy diffraction (RHEED). However, RHEED patterns, both observed or recorded, need to be interpreted. The simplest approaches are based on carrying out the Ewald construction for a set of rods perpendicular to the crystal surface. This article describes how the utilization of computer graphics may be useful for realistic reproduction of experimental conditions, and then for carrying out the Ewald construction in a reciprocal 3D space. The computer software was prepared in the Java programing language. The software can be used to interpret real diffractions patterns for relatively flat surfaces, and thus it may be helpful in broad research practice.

Unoccupied electronic states of 2D Si on Ag-3-Si(111)

Optimizing substrate characterization to grow 2D Si layers on surfaces is a major issue toward the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the3×3-Ag/Si(111) surface (3-Ag). Exploiting the large upwards band bending of the3-Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, thek_∥-dispersion of the3-Ag free-electron-likeS₁surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on3-Ag maintained at ∼200 °C, the interface undergoes a metal-insulator transition with the complete disappearance of theS₁state. The latter is replaced by a higher-lying stateU₀with a minimum at 1.0 eV aboveE_F. The origin of this new state is discussed in terms of various Si 2D structures including silicene.

A re-evaluation of diffraction from Si(111) 7 × 7: decoding the encoded phase information in the 7 × 7 diffraction pattern

The diffraction features of Si(111) 7 × 7 are analyzed and related to various structural models of the Si(111) 7 × 7 surface as one part of a multivariate analysis of this system. The limitations in early sample preparation and measurements produce some uncertainty in previously proposed structures. More recent data is considered here. In addition, models used early on to evaluate the structure of 7 × 7 have been over simplified, idealized models. More complex models are considered within the projection rod method as used for surface crystallography. The origin of numerous diffraction features can be determined via their Fourier components for a wide range of 2-D layers, which provides new insight into the structure as well as the limitations of prior projection analyses. Structures which produce the key elements of the 7 × 7 diffraction are presented and various distortions are considered consistent with other experimental results. In general it is found that the presence of a strong set of 3/7th order beams and near extinction of neighbouring fractional order beams are features which are found experimentally and distinguish an important class of structures. This class has a particular type of 3-fold mirror symmetry, which is not apparent in the widely accepted dimer-adatom-stacking fault, DAS, model. Higher order diffraction features, of which many are weak, are also considered and provide important new structural information. Several new polymorphs of the 7 × 7 are identified which may also satisfy the diffraction derived features and possess some degree of pi-bonding so as to enable magnetic surface states not possible in a pure covalently bonded system such as DAS. The Patterson map of the 7 × 7 surface provide insight into the lost phase information encoded in diffraction and reveal why the DAS structure was experimentally favored. An unusual non-primitive 7 × 7 unit cell is also derived from the Patterson map that possesses unusual symmetry properties, a non-standard surface Brillouin zone with potentially unusual electronic properties.

Rich Magnetic Quantization Phenomena in AA Bilayer Silicene

The rich magneto-electronic properties of AA-bottom-top (bt) bilayer silicene are investigated using a generalized tight-binding model. The electronic structure exhibits two pairs of oscillatory energy bands for which the lowest conduction and highest valence states of the low-lying pair are shifted away from the K point. The quantized Landau levels (LLs) are classified into various separated groups by the localization behaviors of their spatial distributions. The LLs in the vicinity of the Fermi energy do not present simple wave function modes. This behavior is quite different from other two-dimensional systems. The geometry symmetry, intralayer and interlayer atomic interactions, and the effect of a perpendicular magnetic field are responsible for the peculiar LL energy spectra in AA-bt bilayer silicene. This work provides a better understanding of the diverse magnetic quantization phenomena in 2D condensed-matter materials.