Two-dimensional silicon: silicene
Silicon based approaches have attracted enormous attention since the first steps of nanotechnology have become a reality . 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 : 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 .
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 , 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 . 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 . 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 .
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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The potentially crucial role of quasi-particle interferences for the growth of silicene on graphite
A comprehensive picture of the initial stages of silicene growth on graphite is drawn. Evidence is shown that quasiparticle interferences play a crucial role in the formation of the observed silicene configurations. We propose, on one hand, that the charge modulations caused by those quantum interferences serve as templates and guide the incoming Si atoms to self-assemble to the unique (√3×√3)R30° honeycomb atomic arrangement. On the other hand, their limited extension limits the growth to about 150 Si atoms under our present deposition conditions on graphite. The here proposed electrostatic interaction finally explains the unexpected stability of the observed silicene islands over time and with temperature. Despite the robust guiding nature of those quantum interferences during the early growth phase, we demonstrate that the window of experimental conditions for silicene growth is quite narrow, making it an extremely challenging experimental task. Finally, it is shown that the experimentally observed three-dimensional silicon clusters might very well be the simple result of the end of the silicene growth resulting from the limited extent of the quasi-particle interferences.
Charge modulations caused by quantum interferences (QI) on graphene guide Si atoms to self-assemble to the unique (3×3)R30° honeycomb atomic arrangement. The limited extend of those QI leads to the experimentally observed transition from 2D to 3D growth at the outskirts of the silicene islands.(TOP) Ab initio simulation of the STM image resulting from the predicted island structure. (Bottom) Experimental STM image of two silicon clusters that formed at the outskirts of a silicene island.
Nano Research 13 (9), 2378-2383 (2020); https://link.springer.com/article/10.1007/s12274-020-2858-x
High graphene permeability for room temperature silicon deposition: The role of defects
Graphene (Gr) is known to be an excellent barrier preventing atoms and molecules to diffuse through it. This is due to the carbon atom arrangement in a two-dimensional (2D) honeycomb structure with a very small lattice parameter forming an electron cloud that prevents atoms and molecules crossing. Nonetheless at high annealing temperatures, intercalation of atoms through graphene occurs, opening the path for formation of vertical heterojunctions constituted of two-dimensional layers. In this paper, we report on the ability of silicon atoms to penetrate the graphene network, fully epitaxially grown on a Ni(111) surface, even at room temperature. Our scanning tunneling microscopy (STM) experiments show that the presence of defects like vacancies and dislocations in the graphene lattice favor the Si atoms intercalation, forming two-dimensional, flat and disordered islands below the Gr layer. Ab-initio molecular dynamics calculations confirm that Gr defects are necessary for Si intercalation at room temperature and show that: i) a hypothetical intercalated silicene layer cannot be stable for more than 8 ps and ii) the corresponding Si atoms completely lose their in-plane order, resulting in a random planar distribution, and form strong covalent bonds with Ni atoms.
Side views of the initial optimized Ni(111)/silicene/graphene structure at zero Kelvin (left side) and the same structure at room temperature (right side).
Carbon 158, 631-641 (2020); https://www.sciencedirect.com/science/article/abs/pii/S0008622319311601
Raman investigation of air-stable silicene nanosheets on an inert graphite surface
The fascinating properties of two dimensional (2D) crystals have gained increasing interest for many applications. The synthesis of a 2D silicon structure, namely silicene, is attracting great interest for possible development of next generation electronic devices. The main difficulty in working with silicene remains its strong tendency to oxidation when exposed to air as a consequence of its relatively highly buckled structure. In this work, we univocally identify the Raman mode of air-stable low-buckled silicene nanosheets synthesized on highly oriented pyrolytic graphite (HOPG) located at 542.5 cm-1. The main focus of this work is Raman spectroscopy and mapping analyses in combination with ab initio calculations. Scanning tunneling microscopy images reveal the presence of a patchwork of Si three-dimensional (3D) clusters and contiguous Si areas presenting a honeycomb atomic arrangement, rotated by 30° with respect to the HOPG substrate underneath, with a lattice parameter of 0.41 ± 0.02 nm and a buckling of the Si atoms of 0.05 nm. Raman analysis supports the co-existence of 3D silicon clusters and 2D silicene. The Raman shift of low-buckled silicene on an inert substrate has not been reported so far and it is completely different from the one calculated for free-standing silicene and the ones measured for silicene grown on Ag(111) surfaces. Our experimental results are perfectly reproduced by our ab initio calculations of deposited silicene nanosheets. This leads us to conclude that the precise value of the observed Raman shift crucially depends on the strain between the silicene and the HOPG substrate.
Nano Research 11, 5879-5889 (2018); https://link.springer.com/article/10.1007/s12274-018-2097-6