Is silicene the next graphene?
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Introduction The study of nanomaterials took off some 40 years ago with the design of so-called quasi-two-dimensional (2D) (solids thin in one direction, typically around tens of nanometers or tens of atomic layers in thickness), quasi-one-dimensional (nanowires), and quasi-zero-dimensional solids (quantum dots). In 2004, the study of exact 2D solids became a renewed focus, as graphene was isolated from graphite,1 and its many fascinating properties were demonstrated. Graphite can be viewed as a stack of graphene sheets held together by the weak van der Waals bonding. Graphene has a hexagonal arrangement of carbon atoms in two dimensions, with a very strong covalent bonding between them. One consequence is the extremely high mechanical strength of the graphene sheet. Another key consequence is that the electrons behave as if they are massless or relativistic electrons (so-called Dirac electrons), whereas electrons in standard semiconductors such as silicon behave as traditional electrons with mass; this has been shown to lead to electron mobilities a hundred times larger for graphene2 with the potential for much faster electronics than with conventional silicon (see the December 2012 special issue of MRS Bulletin on graphene). Furthermore, graphene is neither a semiconductor nor a metal, rather it is exactly in between and can be called a semimetal; technically, it is said to not have an electronic energy gap. The ease of making graphene by mechanical
exfoliation (simply by peeling off graphene sheets from graphite using Scotch tape) and the demonstration of various properties of graphene led to Geim and Novoselov receiving the Nobel Prize in Physics in 2010. A simple question after the discovery of graphene would be why not silicene (the silicon analog of graphene) or germanene (germanium analog) (i.e., the formation of 2D sheets from other atoms from group IV of the periodic table). Elements are grouped in the periodic table because of periodic trends in their physical and chemical properties, and thus carbon, silicon, germanium, tin, and lead are expected to have certain similar properties. A simple reason why the silicon analog of graphene was not immediately considered is related to the types of chemical bonding that are common for carbon and silicon. Carbon is known to form various types of covalent bonding (known as hybridization) involving two, three, and four electrons forming such materials as ethylene, graphite, and diamond, whereas silicon has been known to favor sharing four electrons equally, leading to bulk silicon. The key to making 2D silicon, however, is to take a broader view of two-dimensionality by not requiring all the atoms to form a flat sheet; for chemists, this means one is not strictly looking for the elusive sp2 bonding for silicon. This broader understanding was already achieved at least as early as 1994 in a paper by Takeda and Shiraishi3 in which they asked what kind of 2D structures of silicon and germanium
L.C. Lew Yan Voon, School of Science and Mathematics, The Citadel; llewyanv@c
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