Silicene
Silicene is a two-dimensional
History
Although theorists had speculated about the existence and possible properties of free-standing silicene,[2][3][4] researchers first observed silicon structures that were suggestive of silicene in 2010.[5][6] Using a scanning tunneling microscope they studied self-assembled silicene nanoribbons and silicene sheets deposited onto a silver crystal, Ag(110) and Ag(111), with atomic resolution. The images revealed hexagons in a honeycomb structure similar to that of graphene, which, however, were shown to originate from the silver surface mimicking the hexagons.[7] Density functional theory (DFT) calculations showed that silicon atoms tend to form such honeycomb structures on silver, and adopt a slight curvature that makes the graphene-like configuration more likely. However, such a model has been invalidated for Si/Ag(110): the Ag surface displays a missing-row reconstruction upon Si adsorption [8] and the honeycomb structures observed are tip artifacts.[9]
This was followed in 2013 by the discovery of dumbbell reconstruction in silicene[10] that explains the formation mechanisms of layered silicene[11] and silicene on Ag.[12]
In 2015, a silicene field-effect transistor was tested.[13] that opens up opportunities for two-dimensional silicon for fundamental science studies and electronic applications.[14][15][16]
In 2022, it was found that silicene/Ag(111) growth on top of a Si/Ag(111) surface alloy, functions as a foundation and scaffold for the two-dimensional layer.[17] This, however, raises questions of whether silicene can be truly regarded as two-dimensional material at all, due to its strong chemical bonds to the surface alloy.
Similarities and differences with graphene
Silicene and graphene have similar electronic structures. Both have a Dirac cone and
Unlike carbon atoms in graphene, silicon atoms tend to adopt sp3 hybridization over sp2 in silicene, which makes it highly chemically active on the surface and allows its electronic states to be easily tuned by chemical functionalization.[20]
Compared with graphene, silicene has several prominent advantages: (1) a much stronger spin–orbit coupling, which may lead to a realization of quantum spin Hall effect in the experimentally accessible temperature, (2) a better tunability of the band gap, which is necessary for an effective field effect transistor (FET) operating at room temperature, (3) an easier valley polarization and more suitability for valleytronics study.[21]
Unlike graphene, it has been shown that, at least silicene supported by Ag(111) grows on a surface alloy.[17] Hence, decoupling silicene is much less trivial, if possible at all, than decoupling graphene.
Surface alloying
Silicene on Ag(111) grows on top of a Si/Ag(111) surface alloy, which has been shown by a combination of different measurement techniques.[17] The surface alloy precedes the growth of silicene, acting both as foundation and as scaffold for the two-dimensional layer. Upon further increase of silicon coverage, the alloy is covered by silicene, yet pervasivley exists for all coverages. This implies that the properties of the layer are strongly influenced by its alloy.
Band gap
Early studies of silicene showed that different
Inducing
Power dissipation within traditional metal oxide semiconductor field effect transistors (
Properties
2D silicene is not fully planar, apparently featuring chair-like puckering distortions in the rings. This leads to ordered surface ripples. Hydrogenation of silicenes to
The buckling of the hexagonal structure of silicene is caused by pseudo Jahn–Teller distortion (PJT). This is caused by strong vibronic coupling of unoccupied molecular orbitals (UMO) and occupied molecular orbitals (OMO). These orbitals are close enough in energy to cause the distortion to high symmetry configurations of silicene. The buckled structure can be flattened by suppressing the PJT distortion by increasing the energy gap between the UMO and OMO. This can be done by adding a lithium ion.[19]
In addition to its potential compatibility with existing semiconductor techniques, silicene has the advantage that its edges do not exhibit oxygen reactivity.[23]
In 2012, several groups independently reported ordered phases on the Ag(111) surface.
Besides silver, silicene has been reported to grow on ZrB
2,[36] and iridium.[37] Theoretical studies predicted that silicene is stable on the Al(111) surface as a honeycomb-structured monolayer (with a binding energy similar to that observed on the 4x4 Ag(111) surface) as well as a new form dubbed "polygonal silicene", its structure consisting of 3-, 4-, 5- and 6-sided polygons.[38]
The p-d hybridisation mechanism between Ag and Si is important to stabilise the nearly flat silicon clusters and the effectiveness of Ag substrate for silicene growth explained by DFT calculations and molecular dynamics simulations.[33][39] The unique hybridized electronic structures of epitaxial 4 × 4 silicene on Ag(111) determines highly chemical reactivity of silicene surface, which are revealed by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. The hybridization between Si and Ag results in a metallic surface state, which can gradually decay due to oxygen adsorption. X-ray photoemission spectroscopy confirms the decoupling of Si-Ag bonds after oxygen treatment as well as the relative oxygen resistance of Ag(111) surface, in contrast to 4 × 4 silicene [with respect to Ag(111)].[33]
Functionalized silicene
Beyond the pure silicene structure, research into functionalized silicene has yielded successful growth of organomodified silicene – oxygen-free silicene sheets functionalized with
Silicene transistors
The U.S. Army Research Laboratory has been supporting research on silicene since 2014. The stated goals for research efforts were to analyze atomic scale materials, such as silicene, for properties and functionalities beyond existing materials, like graphene.[41] In 2015, Deji Akinwande, led researchers at the University of Texas, Austin in conjunction with Alessandro Molle's group at CNR, Italy, and collaboration with U.S. Army Research Laboratory and developed a method to stabilize silicene in air and reported a functional silicene field effect transistor device. An operational transistor's material must have bandgaps, and functions more effectively if it possesses a high mobility of electrons. A bandgap is an area between the valence and conduction bands in a material where no electrons exist. Although graphene has a high mobility of electrons, the process of forming a bandgap in the material reduces many of its other electric potentials.[42]
Therefore, there have been investigations into using graphene analogues, such as silicene, as field effect transistors. Despite silicene's natural state also having a zero-band gap, Akinwande and Molle and coworkers in collaboration with U.S. Army Research Laboratory have developed a silicene transistor. They designed a process termed “silicene encapsulated delamination with native electrodes” (SEDNE) to overcome silicene's instability in the air. The stability that resulted has been claimed to be due to Si-Ag's p-d hybridization. They grew a layer of silicene on top of a layer of Ag via epitaxy and covered the two with alumina (Al2O3). The silicene, Ag, and Al2O3 were stored in a vacuum at room temperature and observed over a tracked period of two months. The sample underwent Raman spectroscopy to be inspected for signs of degradation, but none were found. This complex stack was then laid on top of a SiO2 substrate with the Ag facing up. Ag was removed in a thin strip down the middle to reveal a silicene channel. The silicene channel on the substrate had a life of two minutes when exposed to air until it lost its signature Raman spectra. A bandgap of approximately 210 meV was reported.[43][42] The substrate's effects on silicene, in developing the bandgap, have been explained by the scattering of grain boundaries and limited transport of acoustic phonons,[43] as well as by symmetry breaking and hybridization effect between silicene and the substrate.[44] Acoustic phonons describe the synchronous movement of two or more types of atoms from their equilibrium position in a lattice structure.
Silicene nanosheets
2D silicene nanosheets are used in high-voltage symmetric supercapacitors as attractive electrode materials.[45]
See also
References
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- ^ Iyengar, Rishi (February 5, 2015). "Researchers Have Made Computer-Chip Transistors Just One Atom Thick". TIME.com.
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External links
- Yamada-Takamura, Y.; Friedlein, R. (2014). "Progress in the materials science of silicene". Science and Technology of Advanced Materials. 15 (6): 064404. PMID 27877727.
- Anthony, Sebastian (April 30, 2012). "Silicene discovered: Single-layer silicon that could beat graphene to market".