Growth Of Silicene Research Articles

Growth mechanism of silicene onAg(111)determined by scanning tunneling microscopy measurements andab initiocalculations

Scanning tunneling microscopy has been used to real-time study the growth of silicene on $\mathrm{Ag}(111)$ in the 200--533-K temperature range. We show that the growth mode depends strongly on the deposition temperature $T$. At $T=300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and above, the formation of silicene results from the exchange between surface Ag atoms and Si atoms, which are inserted in the substrate top layer. Density functional theory calculations confirm that Si insertion is thermodynamically favored, and we propose an energetic model for explaining the observed growth processes as a function of $T$. For $T\ensuremath{\ge}400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, ordered structures are observed as soon as silicene domains are large enough. With increasing coverage, disordered and dotted phases progressively transform into stable $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.{9}^{\ensuremath{\circ}}$ and $(4\ifmmode\times\else\texttimes\fi{}4)$ structures.

The rise of silicene

Silicene is the silicon counterpart of graphene, that is, it consists of a single layer of Si atoms arranged in a hexagonal network. This new two-dimensional material, first predicted by theory, has been recently grown on different metallic surfaces.1, 2, 3 An obvious advantage of silicene (over graphene) for nanoelectronic applications is its better compatibility and expected integration with the existing Si nanotechnology platform. A new breakthrough on this material has been recently reported by Tao et al.,4 who have successfully fabricated the first silicene-based field effect transistors (FETs) operating at room temperature. Their success relies on the development of a layer transfer process, called ‘silicene-encapsulated delamination with native electrodes’ (SEDNE). This innovative process includes the following key steps: (1) epitaxial growth of silicene on Ag(111) thin films grown on mica substrates; (2) Al2O3 in situ encapsulation of the silicene layer, followed by its delamination transfer on a p++Si/SiO2 substrate; and (3) subsequent Ag source/drain contact formation by e-beam lithography. A resulting silicene-based FET, with the p++Si substrate used as a back-gate contact, is shown in Figure 1a.

Monitoring Si growth on Ag(111) with scanning tunneling microscopy reveals that silicene structure involves silver atoms

Using scanning tunneling microscopy (STM), the elaboration of the so-called silicene layer on Ag(111) is monitored in real time during Si evaporation at different temperatures. It is shown that the growth of silicene is accompanied by the release of about 65% of the surface Ag atoms from the Si covered areas. We observe that Si islands develop on the Ag terraces and Si strips at the Ag step edges, progressively forming ordered (4×4), (13×13) R13.9°, and dotted phases. Meanwhile, displaced Ag atoms group to develop additional bare Ag terraces growing round the Si islands from the pristine Ag step edges. This indicates a strong interaction between Si and Ag atoms, with an important modification of the Ag substrate beneath the surface layer. This observation is in contradiction with the picture of a silicene layer weakly interacting with the unreconstructed Ag substrate, and strongly indicates that the structure of silicene on Ag(111) corresponds either to a Si-Ag surface alloy or to a Si plane covered with Ag atoms.

Epitaxial growth of silicene on ultra-thin Ag(111) films

Epitaxial growth of silicene on atomically flat ultra-thin Ag(111) films was investigated using scanning tunneling microscopy. The Ag films were prepared by low-temperature deposition of Ag on Si substrates, followed by soft annealing at room temperature. Patchy 4 × 4, √13 × √13, √7 × √7, and 2√3 × 2√3 silicene domains were nucleated on regions that were one monolayer lower than the pristine Ag surface after the initial morphological change had been completed. On the first-layer silicene containing the above domains, two types of √3 × √3 silicene domains (√3 × √3-α and β) were formed in the second layer. A bias voltage-independent Moiré pattern was observed for the √3 × √3-α silicene, and a bias voltage-dependent standing wave pattern for the √3 × √3-β silicene. The √3 × √3-β silicene had a dispersion relation that could be fitted using a linear function, although the resulting Fermi velocity was twice as large as the expected value.

Silicite: The layered allotrope of silicon

Based on first-principles calculation we predict two new thermodynamically stable layered-phases of silicon, named as silicites, which exhibit strong directionality in the electronic and structural properties. As compared to silicon crystal, they have wider indirect band gaps but also increased absorption in the visible range making them more interesting for photovoltaic applications. These stable phases consist of intriguing stacking of dumbbell patterned silicene layers having trigonal structure with $\sqrt{3} \times \sqrt{3}$ periodicity of silicene and have cohesive energies smaller but comparable to that of the cubic diamond silicon. Our findings also provide atomic scale mechanisms for the growth of multilayer silicene as well as silicites.

Interaction of Silicene and Germanene with Non-Metallic Substrates

Silicene, the silicon counterpart of graphene, has recently attracted a lot of attention as an alternative 2D materials to graphene for nanoelectronic devices. So far, the likely existence of silicene on Ag(111) surfaces has recently been reported, by combining experimental and first-principles simulations (1,2). The growth of silicene has also been reported on other metallic substrates, like (0001) ZrB2 (3) and (111)Ir (4).However, the characterization of the electronic and electrical properties of silicene on metallic substrates is very challenging, since these properties can be largely dominated by the substrates. In addition, potential applications of silicene in nanoelectronic devices will also require its growth on non-metallic substrates.We report here on the theoretical study of the interaction of silicene and germanene (2D Ge counterpart of graphene) with non-metallic substrates, using first-principles simulations. We first studied the weak (van der Waals) interaction between silicene or germanene with metal chalcogenide substrates (5,6). The buckling of the silicene/germanene layer is found to be correlated to the in-plane lattice mismatch between the free-standing silicene or germanene layer and the metal dichalcogenide layer. Highly buckled silicene (e.g. on MoS2) is predicted to be metallic, while low buckled silicene (on e.g. GaSe) is predicted to have Dirac cones at the K-points.We next investigated the interaction between silicene or germanene with (0001) ZnS or (0001) ZnSe surfaces (7,8). In this case, part of the Si (Ge) atoms form covalent bonds with the substrate, leading to the opening of a gap in their energy band structure. Very interestingly, the value of this energy gap can be controlled by an out-of-plane electric field, leading to field-driven semiconductor to metal transitions at these interfaces – which could be very promising for the potential use of silicene and germanene in field effect transistors.Part of this work has been financially supported by the European Project “2D Nanolattices” within the Future and Emerging Technologies (FET) program of the European Commission, under FET-grant number 270749, as well as the KU Leuven Research Funds, project GOA/13/011.

Growth and structural properties of silicene at multilayer coverage

At monolayer coverage, silicene on Ag(1 1 1) may present different structural phases depending on the growth conditions. At multilayer coverage, only one structural phase has been reported: the phase. However, no link between the structural arrangement of the monolayer and that of the multilayer has been addressed. In this paper, reporting experimental work based on low-energy electron diffraction and scanning tunneling microscopy, we focus on the structural aspects of a multilayer film of silicene. We demonstrate that it exhibits one structural arrangement, namely the form, but with different domain orientations resulting from the structural properties of the initial wetting monolayer.

Silicon-induced faceting at the Ag(110) surface

The Si/Ag(110) interface is attracting increasing interest since the recent claim that the one-dimensional (1D) nanostructures formed at this surface would be silicene nanoribbons. Lately, the synthesis of multilayer silicene nanoribbons was reported as well. In this paper, we report on the scanning tunneling microscopy, low-energy electron diffraction, and density functional theory study of the 1D nanostructures forming on Ag(110) upon Si deposition at temperatures above 460 K. Two different kinds of nanostructures are here described: nanodikes and nanotrenches, respectively protruding from and engraved on the Ag(110) substrate. The first kind of structure was recently reported to be constituted by multistacks of silicene nanoribbons. We show here that nanodikes and nanotrenches are the result of silver faceting stabilized by silicon atoms. Indeed, such nanostructures share the very same facet crystallographic features, namely inclination and reconstruction: the top and base parts are Ag(110) planes, while their sides are Ag(221) and Ag(22$\overline{1}$) facets stabilized by Si atoms. Finally, density functional theory calculations suggest that Si atoms may substitute Ag atoms in such facets, casting doubts on the usual assumption that Ag is a suitable substrate for silicene growth because it is inert against silicon.

Two-dimensional silicene nucleation on a Ag(111) surface: structural evolution and the role of surface diffusion

The structural evolution of planar Si clusters and the nucleation mechanism of silicene in the initial stages of silicene epitaxial growth on a Ag(111) surface are studied by using ab initio calculations and two-dimensional nucleation theory. The ground-state SiN clusters (1 ≤ N ≤ 25) on the Ag(111) surface are found to undergo a significant structural transition from non-hexagonal plane structures to fully-hexagonal ones at N = 22, which is a crucial step for growing a high-quality silicene nanosheet. Furthermore, important parameters for controlling silicene growth, including the diffusion barriers of Si clusters, nucleation barrier, nucleus size, and nucleation rate are explored. Compared to graphene nucleation on transition-metal (TM) surfaces, the low diffusion barrier of Si atoms and the low nucleation barrier are responsible for the rapid nucleation of silicene on a Ag(111) surface. Our calculations demonstrate that silicene should be synthesized at a relatively low growth temperature (~500 K) in order to reduce the defect density. The results can be successfully applied to explain the broad experimental observations where the growth temperature of silicene is below 550 K.

Theoretical aspects of graphene-like group IV semiconductors

Silicene and germanene are the silicon and germanium counterparts of graphene, respectively. Recent experimental works have reported the growth of silicene on (111)Ag surfaces with different atomic configurations, depending on the growth temperature and surface coverage. We first theoretically study the structural and electronic properties of silicene on (111)Ag surfaces, focusing on the (4×4)silicene/Ag structure. Due to symmetry breaking in the silicene layer (nonequivalent number of top and bottom Si atoms), the corrugated silicene layer, with the Ag substrate removed, is predicted to be semiconducting, with a computed energy bandgap of about 0.3eV. However, the hybridization between the Si 3p orbitals and the Ag 5s orbital in the silicene/(111)Ag slab model leads to an overall metallic system, with a distribution of local electronic density of states, which is related to the slightly disordered structure of the silicene layer on the (111)Ag surface. We next study the interaction of silicene and germanene with different hexagonal non-metallic substrates, namely ZnS and ZnSe. On reconstructed (0001)ZnS or ZnSe surfaces, which should be more energetically stable for very thin layers, silicene and germanene are found to be semiconducting. Remarkably, the nature and magnitude of their energy bandgap can be controlled by an out-of-plane electric field, an important finding for the potential use of these materials in nanoelectronic devices.

The quasiparticle band dispersion in epitaxial multilayer silicene

The growth of multilayer silicene is an exciting challenge for the future of silicon nano-electronics. Here, we use angle-resolved photoemission spectroscopy to map the entire Brillouin zone (BZ) of (√3 × √3)R30° reconstructed epitaxial multilayer silicene islands, growing on top of the first (3 × 3) reconstructed silicene wetting layer, on Ag(111) substrates. We found Λ- and V-shape linear dispersions, which we relate to the π and π* bands of massless quasiparticles in multilayer silicene, at the BZ centre and at all the centres of the (√3 × √3)R30° Brillouin zones in the extended scheme, due to folding of the Dirac cones at the and points of the (1 × 1) silicene BZ. The Fermi velocity of ∼0.3 × 106 m s−1 obtained is highly promising for potential silicene-based devices.

Initial geometries, interaction mechanism and high stability of silicene on Ag(111) surface

Using ab initio methods, we have investigated the structures and stabilities of SiN clusters (N ≤ 24) on Ag(111) surface as the initial stage of silicene growth. Unlike the dome-shaped graphene clusters, Si clusters prefer nearly flat structures with low buckling, more stable than directly deposition of the 3D freestanding Si clusters on Ag surface. The p-d hybridization between Ag and Si is revealed as well as sp2 characteristics in SiN@Ag(111). Three types of silicene superstructures on Ag(111) surface have been considered and the simulated STM images are compared with experimental observations. Molecular dynamic simulations show high thermal stability of silicene on Ag(111) surfaces, contrast to that on Rh(111). The present theoretical results constitute a comprehensive picture about the interaction mechanism of silicene on Ag(111) surface and explain the superiority of Ag substrate for silicene growth, which would be helpful for improving the experimentally epitaxial growth of silicene.

Local Electronic Properties of Corrugated Silicene Phases

IO N Exploring new physical-chemical properties of atomically thin graphitic materials is retaining a booming interest and it is expected to have a tremendous impact on the development of future nanoelectronic devices. So far, the enormous consideration for graphene [ 1 , 2 ] has overshadowed other two dimensional (2D) counterparts which may overcome the intrinsic limitations of graphene as active material. Indeed, despite the recent advances either in graphene bandgap opening [ 3–5 ] or integration in ad hoc device structures, [ 6 , 7 ] the zero-gap character of free standing graphene poses a severe drawback for an electric fi eld modulation suitable to graphene based logic devices. A strong effort is currently made to provide theoretical and experimental evidence of stable graphene-like nanolattices of the other group-IV semiconductors, the so called silicene and germanene, [ 8–11 ] which might take benefi t from being naturally compatible with the Si technology and from an intrinsically higher spin orbit coupling. Unlike graphene, the spontaneous formation of silicene is not expected to be energetically favorable because the sp 3 hybridization of Si atoms is more stable than the sp 2 one. However, a graphene-like form of silicene has been theoretically predicted [ 8 , 9 , 12 ] and, in just a few years, experimental investigations have moved up this fascinating hypothesis to concrete evidence. Recent experiments [ 13 , 14 ] report on the epitaxial growth of silicene on Ag(111) substrates with a non-trivial surface arrangement and relevant electronic features such as a linear electronic dispersion at the K points and an estimated Fermi velocity v F = 1.3 × 10 6 ms − 1 . [ 13 ]

Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111)

In the search for evidence of silicene, a two-dimensional honeycomb lattice of silicon, it is important to obtain a complete picture for the evolution of Si structures on Ag(111), which is believed to be the most suitable substrate for growth of silicene so far. In this work we report the finding and evolution of several monolayer superstructures of silicon on Ag(111), depending on the coverage and temperature. Combined with first-principles calculations, the detailed structures of these phases have been illuminated. These structures were found to share common building blocks of silicon rings, and they evolve from a fragment of silicene to a complete monolayer silicene and multilayer silicene. Our results elucidate how silicene forms on Ag(111) surface and provides methods to synthesize high-quality and large-scale silicene.