Allotropes of silicon

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Allotropes of silicon – a shorter, easy-to-understand version

Silicon comes in several structural forms, called allotropes. The main ones are amorphous silicon and crystalline silicon.

- Amorphous silicon: a brown powder with no regular crystal structure.

- Crystalline silicon: a shiny gray material. Single crystals can be grown by the Czochralski process. It can be doped with elements such as boron, gallium, germanium, phosphorus, or arsenic to change its electrical properties. Doped silicon is used in solar cells, diodes, and computer chips. Silicon crystals follow a diamond-like structure formed by two interpenetrating face-centered cubic lattices.

Notable 2D and 3D allotropes beyond ordinary crystalline silicon

- Silicene: a two-dimensional sheet of silicon in a hexagonal honeycomb pattern. It is similar to graphene but has a buckled structure and stronger interlayer coupling. Its oxidized form, 2D silica, is chemically different from graphene oxide. Silicene was first made in 2010.

- Penta-silicene: a two-dimensional sheet with a pentagonal structure, akin to penta-graphene. It was first synthesized in 2005.

- Si24: an orthorhombic silicon allotrope discovered in 2014. It is made using a high-pressure, high-temperature process starting from Na4Si24 in a tantalum capsule, reaching about 10 gigapascals, followed by eight days of vacuum degassing at 400 K. Si24 has a zeolite-like, porous structure built from 5-, 6-, and 8-membered silicon rings. It can conduct electricity better than diamond-structured silicon and can absorb and emit light. It can be doped as p-type or n-type, with boron and phosphorus as likely dopants. Potential uses include energy storage and filtration. A related hexagonal bulk form called 4H silicon exists as a more ordered version of Si24, with an indirect band gap near 1.2 eV.

- Silicyne: silicon analogues of carbon carbyne and graphyne. One-dimensional silicyne is a long chain of silicon atoms (like carbyne). Two-dimensional silicyne is a sheet analogous to graphyne.