In 2012, research groups in Japan and Europe reported the production of a single atomic layer of silicon, dubbed silicene. Disagreement in some of the experimental results led to an interesting debate about whether silicene had truly been formed, and if so, what its structure and physical properties are.
Silicene is the thinnest possible form of silicon, and could have promising applications in micro-electronics if it can be manufactured commercially. Silicene is the silicon equivalent of graphene, the wonder material that has gripped the world of nanoscience since 2004.
Properties of Silicene
Two-dimensional silicene is not completely planar - like graphene it consists of a hexagonal pattern of atoms, but in silicene the hexagonal rings include puckering distortions due to a "chair" configuration. Hence the surface seems to have ordered ripples.
Silicene can be converted to silicane by addition of hydrogen in an exothermic chemical reaction. This process could lead to applications in hydrogen storage.
As silicene is composed of silicon, it is compatible with present semiconductor manufacturing techniques. It's edges do not show oxygen reactivity, which suggests it will be stable enough to use in electronics. Some experiments show that silicene has an electronic structure which is analogous to that of graphene, which allow electrons to move through the material as though they had zero mass. Not all the evidence agrees with this, however, and the exact electronic properties have yet to be determined.
Silicene Upon Graphene
This video shows a simulation of a single silicene sheet overlayed on a sheet of graphene, to demonstrate the similarity between the materials. Both have a clear hexagonal lattice structure - the atoms are more widely spaced in silicene because silicon atoms are larger than the carbon atoms in graphene. Run time: 0:51 min.
Comparison to Graphene
Graphene is often referred to as a wonder material. It has superb electron mobility, and is so strong that it is possible to hold a single atomic sheet. It's density is around a million times that of copper at room temperature.
If silicene can be produced reliably, it could be even more exciting than graphene, since it will be more directly compatible with present silicon-based electronics, allowing its benefits to be more easily exploited. According to the European research team, silicene behaved in a similar way to graphene in terms of it's electronic properties - they can move like particles without mass, traveling through the lattice at the speed of light on a continuous range of energy levels.
Graphene does not have any bandgap - it is a semi-metal, which means it has very good conductivity. However, a non-zero bandgap is what makes a material a semiconductor, and what allows semiconductors to be used in electronic components like diodes and transistors. Silicene does have a bandgap, hence making it suitable to be used in novel transistors and other.
Recent work used simple vapor deposition techniques to grow a silicon layer one atom thick on a silver crystal surface. Observing the chemical structure, the dimensions and electronics of the produced material, the researchers concluded that this must be silicene. The structural parameters such as lengths and bond angles matched density functional theory (DFT) calculations perfectly.
The interlayer silicene coupling is very strong when compared to graphite, according the European research. However, results reported by the Japanese team do not show an identical behaviour. A physicist from the Italian National Research Council in Rome, Paola De Padova, who was involved in the original European study says that the absence of the Dirac cone leaves room for doubt that silicene is very similar to graphene. A senior author of the Japanese research team felt that it is better not to define silicene too rigidly since the work is still it its early stages.
According to an associate professor of inorganic chemistry at the University of Nottingham, UK, silicene may be more convenient to be integrated into electronic devices when compared to graphene.
An Aix-Marseille group tried creating silicene in 2010 but they could not find considerable evidence as to its presence. According to Michel Houssa of the Catholic University of Leuven (KUL) in Belgium, this study proved that it is possible to grow silicene on silver. Angle-resolved photo-emission spectroscopy was used and it was observed that silver sheets synthesized on silver surface show electrical characteristics similar to graphene. A Fermi-velocity of νF = 1.3 × 106 ms-1 was determined for these silicene sheets that is comparable to graphene. Silicene has also been reported to grow on a ZrB2 substrate. He added that it will truly be a challenge to grow silicene on insulating substrates so that more is understood about its electrical properties and study how they can be used to build electronic devices.
Applications of Silicene
Density functional theory calculations that were done in 2011 showed that silicene clusters will be suitable for FET applications.
Since silicene has a band gap unlike graphene, it will be possible to develop nanoscale semiconductors with a wide range of applications including light emitting diodes and microprocessor chips.
Conclusion
The study of silicene is still in its infancy. There needs to be a lot more clarity and agreement between the different research groups working on the material before silicene can be truly considered as a potential contender to take over silicon as the dominant commercial semiconductor material. If its properties are proved, however, it may well prove to be another wonder material - whether it will overtake graphene in popularity remains to be seen.
Sources and Further Reading