Two-dimensional silicon: silicene

INTRODUCTION
Silicon based approaches have attracted enormous attention since the first steps of nanotechnology have become a reality [1]. The fruitful results obtained for graphene [2,3] gave new momentum to the research in two dimensional (2D) materials [4,5]. Being directly compatible with existing semiconductor technologies, silicon in a 2D configuration, namely ‘silicene’, has received a notable interest in the last few years. Silicene consists of a honeycomb lattice of atoms akin to those in graphene. The main difference, however, resides in its structural arrangement as silicene is not atomically flat like graphene, but it shows a buckled configuration with a low, although sizeable, wrinkling of the surface [6-8]. Silicene may possess unique properties which provide rich opportunities for further engineering [9]: for instance, an external electric field [10,11] or interface interactions [12,13] can modify its band structure, as shown in a recent proof-of-concept silicene field effect transistor [14].

Since silicon crystallizes in a diamond cubic crystal structure and does not naturally form any 2D structures as carbon does in the form of graphite [15], considerable research efforts have been reported for the synthesis of silicene on silver [16-22] and on other substrates [23,24]. In spite of the large number of experimental results, the epitaxial growth of silicene on silver has been recently questioned. It was observed that Si atoms strongly interact with Ag substrates and actually penetrate into the first layer of the silver surface, expelling Ag atoms, and inducing reconstructions, faceting, or growing reconstructed islands inside the first layer rather than remaining on top of the substrate [25-27]. This drawback is mostly due to the significant p-d hybridization occurring between the electronic states of the silicon layer and those of the metal underneath [28]. Furthermore, the structural nature of silicene multilayers grown on Ag has also been severely questioned. It was revealed that the √3 x √3 R30° surface reconstruction may actually be traced back to a layer of silver atoms rising to the surface and not to pristine Si [29]. These observations imply that the growth dynamics of an ultra-thin silicon layer on any metal substrate is extremely complicated and does not necessarily result in genuine silicene [30].

In view of a viable solution to the above mentioned issues related to metallic substrates, it has been shown by first-principles calculations that a silicene sheet remains stable on different inert and non-metallic substrates [31-33]. Here, following the same idea, we use a highly oriented pyrolytic graphite (HOPG) substrate that, due to its sp2 configuration, approximates well the electronic and structural properties of a fully honeycombed structure and provides chemical inertness. Hence, we show that silicon atoms spontaneously arrange themselves in several locations of the HOPG substrate to form a honeycomb structure with a lattice parameter of about 0.4 nm, which is expected to be typical for genuine free- standing silicene, with negligible interaction with the supporting substrate. Similar to the C atoms in graphene, each of those Si atoms only has three nearest neighbor atoms necessary for the anticipated outstanding properties of true silicene.

We have performed a combined experimental and theoretical study providing strong evidence that 2D alloy-free silicene can be grown on top of HOPG. Atomic force microscopy shows that a quasi- continuous layer of silicene is deposited on the substrate together with small Si 3D clusters. Scanning tunneling microscopy (STM) measurements show that our silicene patches are atomically resolved showing in detail the unit cell and the small buckling of the structure. Our density functional theory (DFT) calculations nicely agree with the structural results obtained experimentally. Moreover, scanning tunneling spectroscopy (STS) shows the metallic character of the silicene areas. In addition, ab initio molecular dynamics (AIMD) simulations are performed in order to study the thermal stability of the bi-dimensional structure at two different temperatures (RT and 350°C) and to investigate the growth mechanism of silicene on a graphite substrate.

ACS Nano:  https://doi.org/10.1021/acsnano.6b06198

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