Silicene

Replacement of carbon atoms from aromatic molecules and their two-dimensional extended analogues (graphene) have been predicted to have interesting structural diversity and tunable electronic properties. Recent progress in the experimental realization of such systems is discussed along with a conceptual understanding of the structural properties of planar organosilicon compounds and silicene. Psuedo Jahn-Teller (PJT) distortion is shown to contribute to the buckling distortions in silicene which make them excellent materials for band-gap tuning through hydrogenation. Chemical doping of silicene by cations is suggested to be a strategy to suppress buckling of silicene and regain its perfect planar two-dimensional silicon framework. TERS spectroscopy is proposed as a tool to probe the presence or absence of buckling distortions in silicene and cation doped silicene respectively.

A combination of density functional theory and a tight-binding model offers a robust means to describe the structure, vibrations, and electronic states of silicene. In this chapter we give an overview of the electronic structure and phonon dispersions of silicene and its fully hydrogenated derivative, silicane. We discuss the dynamical stability of the buckled silicene and silicane lattices and we present their phonon dispersions. We discuss the first-principles electronic band structure of ideal, free-standing silicene, paying particular attention to the small band gap opened by spin–orbit coupling, which renders the material a topological insulator. We look at the tight-binding description of silicene and examine the effects of an external electric field which, above a critical electric field, counters the spin–orbit gap and triggers a phase transition into a band-insulator state in which the band gap is linearly tunable by the electric field. We also present the tight-binding description of silicane which, parameterised by density functional theory, sheds light on the importance of long-range hopping in this material.

In this chapter, we review the recent progress on electronic and topological properties of monolayer topological insulators including silicene, germanene and stanene.We start with the description of the topological nature of the general Dirac system and then apply it to silicene by introducing the spin and valley degrees of freedom. Based on them, we classify all topological insulators in the general honeycomb system. We discuss topological electronics based on honeycomb systems. We introduce the topological Kirchhoff law, which is a conservation law of topological edge states. Field effect topological transistor is proposed based on the topological edge states. We show that the conductance is quantized even in the presence of random distributed impurities. Monolayer topological insulators will be a key for future topological electronics and spin-valleytronics.

Slightly buckled, graphene-like honeycomb crystals made by silicon, silicene, or by other group-IV elements such as germanene and stanene represent atomically thin films, i.e., two-dimensional (2D) systems. The theoretical description of their optical properties suffers from three difficulties, (i) a thickness much smaller than the wavelength of light, (ii) their common modeling by superlattice arrangements with sufficiently large layer distances, and (iii) the inclusion of many-body effects. Here, the solutions of all problems are discussed. (i) The optical response of an individual honeycomb crystal is described by a tensor of 2D optical conductivities or dielectric functions, which are related to the optical response of the corresponding superlattice. (ii) The influence of such a sheet crystal on the transmittance, reflectance and absorbance of a layer system is described. (iii) Excitonic and quasiparticle effects are demonstrated to widely cancel each other. Silicene sheets are investigated in detail. As a consequence of the linear bands and Dirac cones the low-frequency absorbance is defined by the Sommerfeld finestructure constant. Van Hove singularities represented by critical points in the interband structure are identified at higher photon energies. Clear chemical trends along the row C \(\rightarrow \) Si \(\rightarrow \) Ge \(\rightarrow \) Sn are derived. The influence of multiple layers is studied for the cases of bilayer silicene and graphene.

The key issue for the first proof-of-principle synthesis of the two-dimensional Si allotrope, silicene, realized in 2012, was to find an appropriate substrate. Here, we give a background for the initial choice of silver single crystals, determined by decades of studies of the reverse system: silver deposited onto silicon (111) templates. Next, clever serendipity lead to another platform: ZrB₂ thin films grown on Si(111). Clearly, non-metallic substrates would be preferred for many practical applications, they are currently searched for. A digest of this quest is presented in this chapter.

In this chapter we give an overview on the theoretical and experimental investigations of one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) Si nanoribbons (SiNRs) formed on the anisotropic Ag(110) substrate surface. We start by introducing briefly free-standing silicene, a silicon layer with Si atoms arranged in honeycomb lattice, with hexagonal Si-rings as structural units. These hexagonal Si units are subsequently discussed as possible candidates to explain the atomic arrangement of the experimentally synthesized Si nanoribbons on Ag(110). This interpretation is supported by properties such as the presence of the 1D projection of the π and π* bands, forming the so-called “Dirac cones” at the K points of the Brillouin zone, the sp²-like nature of the Si valence orbitals, and the strong resistance against oxidation. Besides these results, the atomic structure as well as the origin of the electronic properties of these Si nanoribbons are still controversially debated in the literature. We address this discussion in the last part of the chapter before summarizing it.

The expected properties of silicene and their theoretical background have already been discussed in Chaps. 1–3 and the different ways to synthesize this new 2D material in Chap. 5. It has already been mentioned that such a synthesis requires an adequate substrate material to accommodate the formation of a one-atom-thin silicon layer. Such a material is silver, in particular the Ag(111) surface plane. In this chapter the formation and properties of silicene formed epitaxially on the Ag(111)(\(1\times 1\)) surface are discussed. We will see that the properties of these silicene layers are modified with respect to the ones of free-standing silicene, due to the interaction with the substrate. For this reason we will refer to it as epitaxial silicene and look in detail at its two-dimensional (2D) character. A more detailed look at the formation of Si layers on Ag(111) shows that, depending on the specific preparation conditions, several 2D Si phase can be formed. Differences and similarities of these structures will be discussed. Furthermore, we will draw the intention on the chemical and temperature stability of these epitaxial silicene layers and unveil the limits for the silicene formation.

The isolation of graphene sheets from its parent crystal graphites has given the kick to experimental research on its prototypical 2D elemental cousin, silicene [1]. Unlike graphene, silicene lacks a layered parent material from which it could be derived by exfoliation. Hence, the efforts of making the silicene dream a reality were focused on epitaxial growth of silicene on substrates. The first synthesis of epitaxial silicene on silver (111) [27, 46] and zirconium diboride templates [16] and next on an iridium (111) surface [31], has boosted research on other elemental group IV graphene-like materials, namely, germanene and stanene [30, 48]. The boom is motivated by several new possibilities envisaged for future electronics, typically because of the anticipated very high mobilities for silicene and germanene [49], as well as potential optical applications [30]. It is also fuelled by their predicted robust 2D topological insulator characters [14, 28] and potential high temperature superconductor character [5, 50]. One of the most promising candidates as a substrate is Ag because from the studies of the reverse system, where Ag atoms were deposited on silicon substrate, it was known that Ag and silicon make sharp interfaces without making silicide compounds [24]. Indeed, studies on synthesiz and characterization of silicene is mainly focused on using Ag(111) as substrates and hence we think it is important to understand this particular system. In this Chapter, we present a theoretical perspective on the studies investigating the atomic and electronic structure of silicene on Ag substrates.

Following the successful preparation of silicene, the focusing issue is whether silicene indeed exhibits the exotic electronic properties predicted theoretically. To study the electronic structure of silicene, angle resolved photoelectron emission spectroscopy and scanning tunneling microscopy/spectroscopy are the two major experimental techniques. The development of cryogenic STM allows the investigation of physical phenomena that occur only at low temperatures or are best seen at low temperature, such as superconductivity, a quantum Hall effect, metal-insulator transition and charge density waves. However, it is equally important that low temperature environment provides a stable and well-regulated platform and greatly improves the performance of STM. In this chapter, we give an overview on the recent progress of silicene on Ag(111) at low temperatures by utilizing cryogenic STM.

Silicene has so far been successfully grown on metallic substrates, like Ag(111), ZrB₂(0001) and Ir(111) surfaces. However, the characterization of its electronic structure is hampered by the metallic substrate. In addition, potential applications of silicene in nanoelectronic devices will require its growth/integration with semiconducting or insulating substrates. In this chapter, we review recent theoretical works about the interaction of silicene with several non-metallic templates, distinguishing between the weak van der Waals like interaction of silicene with e.g. AlN or layered metal (di)chalcogenides, and the stronger covalent bonding between silicene and e.g. ZnS surfaces. Recent experimental results on the possible growth of silicene on MoS₂ are also highlighted and compared to the theoretical predictions.

For two-dimensional (2D) materials, an attractive feature is that all the atoms of the materials are exposed on the surface. Thus tuning the structure and properties by surface treatments becomes straightforward. Similar as graphene, the nearly zero-gap character of silicene hinders its applications in electronic and optoelectronic devices. In the case of graphene, functionalization through hydrogenation, halogenation, oxidation, have been widely explored in order to modify the electronic structure of graphene. However, the stable aromatic π-bond network of graphene makes it very inert and difficult to bond with foreign atoms. For example, hydrogen atoms on graphene usually form clusters instead of an ordered structure. In contrast, silicene possesses hybrid sp²-sp³ bonding, which is more readily to be modified or functionalized. Since the early stage of silicene research, theoretical investigations on the hydrogenation, halogenation, and oxidation of silicene have been widely reported in literature. Recently, increasing experimental successes have been achieved on functionalization of silicene. It is now imperative to review the progresses in the fast-growing field. In this chapter, we will discuss hydrogenation, halogenation oxidization individually. In each section, we first describe those theoretical predictions and then illustrate recent experimental successes. Finally, we will give some overview and outlook of this field.

Besides theoretical studies, experimental investigations on silicene began with the synthesis of silicene on ceramic or metallic catalyst substrates such as ZrB₂, Ir and Ag. Among various reported methods, the epitaxial growth of silicene sheet atop Ag(111) has received increasing attention and a derivative approach of using evaporated Ag(111) film as catalyst on a cleavable substrate will be specifically discussed in this Chapter for the ease of following device studies. Despite these research progresses in silicene synthesis, there is a lack of experimental investigation on silicene devices. One of the most key challenges is the material preservation during device fabrication and measurement process. This chapter will summarize recent understanding and progress in air-stability of silicene and viable device fabrication choices, to enable the debut of the first silicene field-effect transistor. A survey will be conducted on experimental probing of electrical properties of silicene via scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and experimental transport measurement on field-effect transistors. These results not only provide experimental feedback to existing theoretical studies, but also encourages further interest in novel device concepts and prospects of silicene and other emerging 2D materials like germanene, stanene and phosphorene.

Soon after the discovery of graphene, the first two-dimensional material, many other two-dimensional materials have been developed. Due to their \(s^{2}p^{2}\) type of electronic structure the elements of the ‘carbon’ column of the periodic system i.e. silicon, germanium and tin have received a lot of attention as potential two-dimensional materials. The silicon, germanium and tin analogues of graphene are coined silicene, germanene and tinene or stanene, respectively, and share many properties with graphene. There are, however, also a few distinct differences with graphene. Here we will give a brief update on the current status of germanene. We briefly review the various routes to synthesize germanene and elaborate on its structural and electronic properties as well as its potential for application in future electronic devices.