nanomaterials Review Two-Dimensional Silicon Carbide: Emerging Direct Band Gap Semiconductor Sakineh Chabi * and Kushal Kadel Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131, USA; [email protected] * Correspondence: [email protected] Received: 8 October 2020; Accepted: 6 November 2020; Published: 9 November 2020 Abstract: As a direct wide bandgap semiconducting material, two-dimensional, 2D, silicon carbide has the potential to bring revolutionary advances into optoelectronic and electronic devices. It can overcome current limitations with silicon, bulk SiC, and gapless graphene. In addition to SiC, which is the most stable form of monolayer silicon carbide, other compositions, i.e., SixCy, are also predicted to be energetically favorable. Depending on the stoichiometry and bonding, monolayer SixCy may behave as a semiconductor, semimetal or topological insulator. With different Si/C ratios, the emerging 2D silicon carbide materials could attain novel electronic, optical, magnetic, mechanical, and chemical properties that go beyond those of graphene, silicene, and already discovered 2D semiconducting materials. This paper summarizes key findings in 2D SiC and provides insight into how changing the arrangement of silicon and carbon atoms in SiC will unlock incredible electronic, magnetic, and optical properties. It also highlights the significance of these properties for electronics, optoelectronics, magnetic, and energy devices. Finally, it will discuss potential synthesis approaches that can be used to grow 2D silicon carbide. Keywords: silicon carbide; two-dimensional materials; semiconductor; optoelectronics 1. Introduction The discovery of monolayer silicon carbide will accelerate various technological innovations in the post-Moore era. As a wide bandgap semiconducting material with high thermal capability, SiC is a leading material for high-power electronics and high- temperature applications. However, due to quantum confinement and surface effects, 2D SiC offers tremendous unprecedented properties, which are absent in bulk SiC materials [1–5]. Unlike graphene, which is a pure one atom carbon material, 2D silicon carbide is a heteroatomic material that may exist in a variety of compositions and hence structures i.e., SixCy e.g., SiC, SiC3, SiC7, among others. Further, unlike graphene which can be exfoliated from bulk graphite via mechanical exfoliation, the synthesis of single-layer SiC is one of the most challenging and tricky syntheses among 2D materials, demanding deep understanding of the atomic structure of the bulk SiC and its crystal structures. The main challenge is that bulk SiC is not a layered van der Waals material. It is a covalently bonded material with sp3 bonding between carbon and silicon along the c axis. As such, the formation of a monolayer silicon carbide requires phase transformation from sp3 to sp2. These structural challenges lead to the following fundamental questions: How will hexagonal 2D SiC be isolated from the tetrahedrally coordinated bulk SiC if the top-down approach would be adopted? Additionally, how easy would be the phase transformation from sp3 to sp2? Or at what thickness does the transformation take place? Furthermore, how stable is 2D SiC? Does it have ideal planar structure or slightly buckled form? And more importantly is 2D SiC stable in air? Or is it highly reactive? Nanomaterials 2020, 10, 2226; doi:10.3390/nano10112226 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 19 Herein, we address these outstanding questions by reviewing the latest efforts and progress in the field of 2D silicon carbide, focusing on the structure, properties, and potential applications of these emerging 2D materials. This paper is organized as follows. The first section will provide a fundamental understating of the structure of 2D silicon carbide. Then, the key properties of 2D SiC will be discussed. Finally, we will outline future opportunities and challenges. Nanomaterials 2020, 10, 2226 2 of 20 2. The Structure of 2D Silicon Carbide Structurally,Herein, we address 2D SiC these is predicted outstanding to have questions a graphene-like by reviewing honeycomb the latest eff stortsructure and progressconsisting in theof alternatingfield of 2D Si silicon and C carbide, atoms. focusingIn the monolayer on the structure, SiC, the properties, carbon and and silicon potential atoms applications will bond through of these emerging hybrid 2D orbitals materials. to form This the paper SiC is sheet, organized Figure as follows.1. Various The research first section groups will providehave investigated a fundamental the stabilityunderstating of planar of the 2D structure SiC, and of all 2D these silicon studies carbide. have Then, confirmed the key that properties 2D SiC ofis 2Denergetically SiC will be stable discussed. and hasFinally, a 100% we planar will outline structure future with opportunities inherent dynamic and challenges. stability [6–8]. They found that, while there may be a competition between hybridization preferred by C in its planar form and preferred by Si,2. Thethe ground Structure state of of 2D 2D Silicon SiC is Carbidecompletely flat, as planar 2D SiC has the lowest energy [9–11]. TheStructurally, predicted 2D planarity SiC is predicted feature is to very have aimportant, graphene-like and honeycombit contributes structure significantly consisting to the of developmentalternating Si of and several C atoms. unprecedented In the monolayer properties. SiC, the In carbon fact, except and silicon graphene atoms and will hexagonal bond through boronsp2 nitride,hybrid orbitalsh-BN, most to form of the the explored SiC sheet, 2D Figure materials,1. Various do not research have stable groups planar have investigatedstructures. 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