Epitaxial Silicene Research Articles

Robustness of linear bands in epitaxial planar silicene heterostructure against surface doping

In this study, we investigate the electronic structure of epitaxial planar silicene heterostructure grown on ultrathin gold films on the Si(111) substrates, covered by 0.1 monolayer of silver, antimony and lead. Angle-Resolved Photoemission Spectroscopy measurements reveal that the electronic structure of planar silicene, that shows bands with linear dispersion, remains largely unaffected by the presence of these dopants, with only a slight energy shift observed in the case of antimony. Density functional theory calculations corroborate these experimental observations, confirming the robustness of the epitaxial planar silicene heterostructure against surface doping. The results suggest that the intrinsic electronic properties of planar silicene are well-protected against surface contamination.

Emergent Dirac Fermions in Epitaxial Planar Silicene Heterostructure.

Silicene, a single layer of Si atoms, shares many remarkable electronic properties with graphene. So far, silicene has been synthesized in its epitaxial form on a few surfaces of solids. Thus, the problem of silicene-substrate interaction appears, which usually depresses the original electronic behavior but may trigger properties superior to those of bare components. We report the direct observation of robust Dirac-dispersed bands in epitaxial silicene grown on Au(111) films deposited on Si(111). By performing in-depth angle-resolved photoemission spectroscopy measurements, we reveal three pairs of one-dimensional bands with linear dispersion running in three different directions of an otherwise two-dimensional system. By combining these results with first-principles calculations, we explore the nature of these bands and point to strong interaction between subsystems forming a complex Si-Au heterostructure. These findings emphasize the essential role of interfacial coupling and open a unique materials platform for exploring exotic quantum phenomena and applications in future-generation nanoelectronics.

Magnetism in Au-Supported Planar Silicene.

The adsorption and substitution of transition metal atoms (Fe and Co) on Au-supported planar silicene have been studied by means of first-principles density functional theory calculations. The structural, energetic and magnetic properties have been analyzed. Both dopants favor the same atomic configurations with rather strong binding energies and noticeable charge transfer. The adsorption of Fe and Co atoms do not alter the magnetic properties of Au-supported planar silicene, unless a full layer of adsorbate is completed. In the case of substituted system only Fe is able to produce magnetic ground state. The Fe-doped Au-supported planar silicene is a ferromagnetic structure with local antiferromagnetic ordering. The present study is the very first and promising attempt towards ferromagnetic epitaxial planar silicene and points to the importance of the substrate in structural and magnetic properties of silicene.

Adatom-induced dislocation annihilation in epitaxial silicene

The transformation of the stripe domain structure of spontaneously-formed epitaxial silicene on ZrB2 thin films into a single-domain driven by the adsorption of a fraction of a monolayer of silicon was used to investigate how dislocations react and eventually annihilate in a two-dimensional honeycomb structure. The in-situ real time scanning tunneling microscopy monitoring of the evolution of the domain structure after Si deposition revealed the mechanisms leading to the nucleation of a single-domain island into a domain structure through a stepwise reaction of partial dislocations. After its nucleation, the single-domain island extends by the propagation of edge dislocations at its frontiers. The identification of this particular nucleation-propagation formation of dislocation-free silicene sheet provides insights into how crystallographic defects can be healed in two-dimensional materials.

Band engineering in an epitaxial two-dimensional honeycomb Si6−xGex alloy

In this Letter, we demonstrate that it is possible to form a two-dimensional (2D) silicene-like Si$_5$Ge compound by replacing the Si atoms occupying on-top sites in the planar-like structure of epitaxial silicene on ZrB$_2$(0001) by deposited Ge atoms. For coverages below 1/6 ML, the Ge deposition gives rise to a Si$_{6-x}$Ge$_{x}$ alloy (with $x$ between 0 and 1) in which the on-top sites are randomly occupied by Si or Ge atoms. The progressive increase of the valence band maximum with $x$ observed experimentally originates from a selective charge transfer from Ge atoms to Si atoms. These achievements provide evidence for the possibility of engineering the bandgap in 2D SiGe alloys in a way that is similar for their bulk counterpart.

Time-resolved X-ray photoelectron diffraction using an angle-resolved time-of-flight electron analyzer

X-ray photoelectron diffraction (XPD) provides atomic resolution, element sensitive local structure information about the surfaces and interfaces of materials. In the work reported in this paper, a two-dimensional angle resolved time-of-flight (2DARTOF) system is used to perform time-resolved XPD experiments on epitaxial silicene. The two Si 2p peak components from the different atomic sites in the silicene layer allow extraction of the individual XPD patterns. Time-resolved measurements captured small angle shifts in the forward focusing peak, indicating laser-induced changes in the silicene structure. At 10 ns, the XPD patterns appear to relax back to the equilibrium state. This work demonstrates that 2DARTOF systems are well suitable for time-resolved XPD measurements.

Disassembling Silicene from Native Substrate and Transferring onto an Arbitrary Target Substrate

AbstractHere, two novel approaches for disassembling epitaxial silicene from the native substrate and transferring onto arbitrary target substrates are presented. From the processing perspective, the two methodologies open up a new route for handling silicene, and in general any epitaxial Xene, in view of establishing reliable process flows for the development of a Xene‐based nanotechnology. Integration of silicene in a back‐gated controlled device architecture is demonstrated and the built up of unique multi‐stack heterostructures between silicene, but potentially every Xene, and other technological relevant materials, like transparent conductive oxides and transition metal dichalcogenides is shown.

Planar Silicene: A New Silicon Allotrope Epitaxially Grown by Segregation

Abstract2D sheets of graphene‐like silicon, namely planar silicene, are synthesized. This new silicon allotrope is prepared on Au(111) thin films grown on a Si(111) substrate in the process of surface segregation. Owing to its almost perfectly flat geometry it shares the atomic structure with graphene rather than with low‐buckled silicene. Scanning tunneling microscopy measurements clearly display an atomically resolved planar silicene honeycomb lattice. Ab initio density functional theory calculations fully support the experimental findings and predict a pure sp2 atomic configuration of Si atoms. The present work is the first experimental evidence of epitaxial planar silicene.

Formation of hBN monolayers through nitridation of epitaxial silicene on diboride thin films

The formation process of hexagonal boron nitride (hBN) monolayer sheets on single-crystalline ZrB2(0001) thin films grown on Si(111) wafers has been investigated by electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy. A two-step reaction was identified, resulting first in the formation of a silicon nitride layer by room temperature exposure of the silicene-terminated ZrB2 thin film surface to nitrogen radicals and then in the formation of an hBN monolayer replacing the silicon nitride layer through annealing at 900 °C. A large-scale moiré pattern and a clear dispersion of the π-electronic band provide evidence for the formation of an epitaxial hBN monolayer sheet directly on the diboride surface. The unique ability of the ZrB2(0001) surface, upon which both silicene and hBN monolayers can be formed, opens a way toward the integration of these two very different two-dimensional materials.

Substrate-induced half-metallic property in epitaxial silicene

For most practical applications in electronic devices, two-dimensional materials should be transferred onto semiconducting or insulating substrates, since they are usually generated on metallic substrates. However, the transfer often leads to wrinkles, damages, contaminations and so on which would destroy the intrinsic properties of samples. Thus, directly generating two-dimensional materials on nonmetallic substrates has been a desirable goal for a long time. Here, via a swarm structure search method and density functional theory, we employed an insulating N-terminated cubic boron nitride(111) surface as a substrate for the generation of silicene. The result tells that the silicene exhibits a ferromagnetic half-metal with a half-metallic band gap of . This feature is driven by the strong interaction between silicon and surface nitrogen atoms. The magnetic moments are mainly located on surface nitrogen sites without bonding silicon atoms and the value is . In the spin-up channel, it behaves as a direct band gap semiconductor with a gap of , while it exhibits metallic characteristic in the spin-down channel. Besides, both the magnetic and electronic properties are not sensitive to the external compressive strain. This work maybe opens the way for the utility of silicene in the spintronic field.

Van der Waals integration of silicene and hexagonal boron nitride

Silicene, the silicon analogue of graphene, consists of an atomically buckled honeycomb lattice of silicon atoms. Theory predicts exceptional electronic properties, including Dirac fermions and a topological spin Hall insulator phase. An important obstacle impeding exploration of such properties in electronic devices is the chemical sensitivity of silicene, hampering its incorporation in layer stacks. Here we show experimentally that epitaxial silicene and hexagonal boron nitride (h-BN) can be stacked without perturbing the electronic properties of silicene. Intercalated silicene underneath epitaxial h-BN on ZrB2(0 0 0 1) substrate films is obtained by depositing Si atoms at room temperature. Using (angle resolved) photoelectron spectroscopy (ARPES, PES) and scanning tunneling microscopy (STM) we find that the intercalated silicene exhibits the same electronic properties as epitaxial silicene on ZrB2, while it resists oxidation in air up to several hours. This is an essential step towards the development of layer stacks that allow for fabrication of devices.

Two‐Dimensional Materials: Nanomechanical Properties of Epitaxial Silicene Revealed by Noncontact Atomic Force Microscopy (Adv. Mater. Interfaces 2/2019)

Two‐Dimensional Materials: Nanomechanical Properties of Epitaxial Silicene Revealed by Noncontact Atomic Force Microscopy (Adv. Mater. Interfaces 2/2019)

Engineering Epitaxial Silicene on Functional Substrates for Nanotechnology

Two-dimensional materials are today a solid reality in condensed matter physics due to the disruptive discoveries about graphene. The class of the X-enes, namely, graphene-like single element artificial crystals, is quickly emerging driven by the high-momentum generated by silicene. Silicene, in addition to the graphene properties, shows up incidentally at the end of Moore's law debate in the electronic era. Indeed, silicene occurs as the crafted shrunk version of silicon long yearned by device manufacturers to improve the performances of their chips. Despite the periodic table kinship with graphene, silicene and the X-enes must deal with the twofold problem of their metastable nature, i.e., the stabilization on a substrate and out of vacuum environment. Synthesis on different substrates and deep characterization through electronic and optical techniques of silicene in the early days have been now following by the tentative steps towards reliable integration of silicene into devices. Here, we review three paradigmatic cases of silicene grown by molecular beam epitaxy showing three different possible applications, aiming at extending the exploitation of silicene out of the nanoelectronics field and thus keeping silicon a key player in nanotechnology, just in a thinner fashion.

Embedding epitaxial (blue) phosphorene in between device-compatible functional layers.

The newly predicted allotropic phase of phosphorus termed blue phosphorus has been recently synthesized in its two-dimensional (2D) single layer fashion via epitaxial growth on a Au(111) substrate. The large scale epitaxy and the semiconductive character with a reported bandgap of ∼1.1 eV suggest that epitaxial phosphorene might be a suitable candidate to overcome the lack of a sizeable bandgap in semimetal X-enes. In close similarity to other X-enes, like silicene, the epitaxial phosphorene shows technological issues towards possible integration into devices, such as the metallic supporting template at the bottom and oxidation under ambient conditions on the top interface of its 2D lattice. Here, we report on a detailed structural and chemical analysis of epitaxial phosphorene and a newly developed methodology to allow for easy transfer of the chemically protected epitaxial phosphorene in between amorphous Al2O3 and thin Au(111) films grown on mica. The large scale epitaxy achieved on a portable Au(111)/mica template and the low reactivity with molecular oxygen of phosphorene pave the way for easy encapsulation of epitaxial phosphorene fostering its exploitability in devices through a versatile transfer methodology, as in the case of epitaxial silicene.

Nanomechanical Properties of Epitaxial Silicene Revealed by Noncontact Atomic Force Microscopy

AbstractSilicene, the graphene‐like allotrope of silicon, has a great potential for the development of novel silicon‐based nanotechnologies. Its unique buckling makes the atomic structure of this silicon 2D material particularly flexible and tunable. The nanomechanical properties of silicene are investigated by probing epitaxial silicene on ZrB2 by atomic‐resolution noncontact atomic force microscopy. Δf–distance curve and scan direction dependence of topographic images point out, respectively, the out‐of‐plane and in‐plane flexibility of the Si honeycomb structure. A contrast visible only in dissipation images reveals the stress distribution across the periodic domain structure texturing the silicene sheet.

Oxidation of Epitaxial Silicene on Ag(111)

The high chemical reactivity of epitaxial silicene on Ag(111) still remains a debated subject in the literature. In particular results on the oxidation of epitaxial silicene and its related lifetime under ambient conditions are controversially discussed. Here, a detailed investigation of the oxygen exposure to epitaxial silicene layers investigated by means of X‐ray photoemission and in situ Raman spectroscopy is reported. The results should clearly cease the discussion on the stability of epitaxial silicene against oxygen as it becomes completely oxidized after an exposure to only 100 L of oxygen. Such a small dose sets strict limits for ex situ studies of epitaxial silicene. Besides the formation of silicon oxide also the silver substrate surface oxidizes, suggesting that the silicene layer can hardly protect it, probably owing to the high number of domain boundaries within the silicene layer.

Characterization of epitaxial silicene with Raman spectroscopy

Silicene, the silicon equivalent of graphene, is most commonly grown on Ag(111) substrates where it undergoes reconstruction due to the strong interaction between the Si and Ag atoms. We demonstrate through first-principles density functional theory for eight reconstructions that the Raman spectrum is unique for each configuration. We argue that the reconstructions can, in fact, be identified by their Raman spectra and suggest key features within the spectra as points of reference to be used for identification.

Optical properties of silicene, Si/Ag(111), and Si/Ag(110)

We present a state-of-the-art study of the optical properties of free-standing silicene and of single-layer Si 1D and 2D nanostructures supported on Ag(110) and Ag(111) substrates. Ab initio simulations of reflectance anisotropy spectroscopy (RAS) and surface differential reflectivity spectroscopy (SDRS) applied to the clean Ag surface and Si/Ag interfaces are compared with new measurements. For Si/Ag(110) we confirm a pentagonal nanoribbon geometry, strongly bonded to the substrate, and rule out competing zigzag chain and silicenelike models. For Si/Ag(111) we reproduce the main experimental features and isolate the optical signal of the epitaxial silicene overlayer. The absorption spectrum of a silicene sheet computed including excitonic and local field effects is found to be quite similar to that calculated within an independent particle approximation, and shows strong modifications when adsorbed on a Ag substrate. Important details of the computational approach are examined and the origins of the RAS and SDRS signals are explained in terms of the interface and substrate response functions. Our study does not find any evidence for Si adlayers that retain the properties of freestanding silicene.

The mechanism for the stabilization and surfactant properties of epitaxial silicene

Using real-time in situ scanning tunneling microscopy and density functional theory simulations, we have studied the growth of Si films on Ag(111) beyond the silicene monolayer, evidencing the existence of metastable phases and an original growth mechanism. Above monolayer Si coverage, an initial structure forms, which is identified as an Ag-free Si bilayer with additional Si adatoms. With further deposition, this structure is replaced by a distinct bilayer structure covered by Si trimers and Ag atoms. The formation of these bilayers follows counterintuitive dynamics: they are partially inserted within the Ag substrate and form by expelling, from the underlying substrate, the atoms that reinsert below the adjacent silicene layer. The growth is therefore characterized by an unexpected "surfactant competition" between Ag and silicene: while silicene is a metastable surfactant for the Ag(111) surface, Ag plays the role of a surfactant for thicker diamond-like Si islands. In spite of being thermodynamically unfavoured, the silicene monolayer is, thus, a remarkably stable structure because of the high kinetic barrier for the growth of thicker layers.

Hydrogen-induced sp2−sp3 rehybridization in epitaxial silicene

We report on the hydrogenation of $(3\ifmmode\times\else\texttimes\fi{}3)/(4\ifmmode\times\else\texttimes\fi{}4)$ silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of $(3\ifmmode\times\else\texttimes\fi{}3)/(4\ifmmode\times\else\texttimes\fi{}4)$ silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed ${sp}^{2}\text{\ensuremath{-}}{sp}^{3}$ to a dominating ${sp}^{3}$ state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries.