Penta-silicene (also written as pentasilicene) is a silicon-based two-dimensional (2D) structure, a cousin of silicene, composed entirely of silicon pentagons, in analogy with penta-graphene (a hypothetical variant of graphene). As of 2017, such a structure has only been obtained synthetically as one-dimensional nanoribbons (1D-NRs) grown on a silver (110) substrate.
Discovery and Structure
These nanoribbons adopt a highly ordered chiral arrangement in single- and/or double-strands (SNRs and DNRs, respectively). They were first discovered in 2005 upon depositing silicon onto an Ag(110) surface held at room temperature or at about 200 °C, and were observed using scanning tunneling microscopy (STM). However, their unique atomic structure was unveiled only in 2016 through thorough density functional theory (DFT) calculations and simulations of STM images.
The structure consists of alternating silicon pentagons residing along a missing row formed at the silver surface during the growth process. In penta-silicene nanoribbons, each Si pentagonal moiety displays an envelope conformation whereby four atoms are coplanar and a fifth flap atom protrudes out of the surface. The pentagons do not deviate much from regular pentagons. Double-strand nanoribbons consist of two single-strand nanoribbons with the same handedness running in parallel along two missing rows separated by two Ag lattice constants (aAg = 4.1 Å).
Significance
The uniqueness of penta-silicene nanoribbons resides in the fact that pentagonal silicon motifs are hardly found in nature. Despite large efforts devoted to designing Si-based structures analogous to those of carbon, the existence of Si pentagonal rings had only been reported in clathrate bulk phases or in Si surface reconstructions (e.g., the cleaved Si(111)2×1 surface).
Properties and Potential Applications
- A monolayer of penta-silicene is dynamically unstable, but few-layer and multilayer configurations (such as bilayers) exhibit remarkable stability.
- Multilayers of penta-silicene form stable materials with semiconducting or metallic properties, depending on the layer thickness.
- Penta-silicene has been studied for its ultra-low lattice thermal conductivity.
- Theoretical studies have explored Li-decorated penta-silicene as a high-capacity hydrogen storage material.
- The possibilities offered by this one-dimensional pentagonal structure include enlarged spin–orbit effects and Si-based nanowires.
Current Status
The discovery of 1D-penta-silicene nanoribbons increases the chances of future isolation of this new low-dimensional Si allotrope, provided these epitaxial nanoribbons can be detached from the silver surface. The theoretical results have been corroborated by detailed surface X-ray diffraction studies.
