When silicene, the two-dimensional version of silicon, was first introduced back in 2010, some called it a "wonder material." Silicene offered something akin to what graphene had been promising for half-a-decade but this time with an intrinsic (rather than engineered) band gap and without all the headaches of retooling an industry that had shaped itself around silicon for the last 50 years.
But research into silicene has been relatively quiet compared to graphene and its other 2D cousins. Now research out of the MESA+ Research Institute of the University of Twente in the Netherlands has discovered something about silicene that may help explain why silicene success remains elusive: As the researchers put it, the material has "suicidal tendencies."
The research, which was published in the journal Applied Physics Letters (“The Instability of Silicene on Ag”), has thrown into question the practical uses of silicene. “We find that silicene layers are intrinsically unstable against the formation of an 'sp3-like' hybridized, bulk-like silicon structure,” says the abstract of the research paper.
The Dutch researchers used electron microscopy to image in real time the formation of silicene on a film. They evaporated silicon atoms on a surface of silver so that a nearly-closed surface of silicene was formed.
The researchers didn't notice anything out of the ordinary up to this point, but then they observed that as soon as silicon atoms started to be deposited on the silicene layer “a silicon crystal" (silicon in a diamond crystal structure instead of in a honeycomb structure) appeared. Soon all of the material became crystalized, with only silicon remaining in the structure.
In the video below you can watch this all happen step by step. At the beginning, you see the silicene forming on the silver surface (this is the gray you see at the start). Then you see it turn gradually black—this is the formation of silicene islands on the surface of the silver. When this black takes over the surface, the silicene collapses into silicon crystals.
The reason that silicene always reverts back to silicon as soon as more layers are added onto it is that the regular crystal structure of silicon is more favorable than the honeycomb structure of the silicene. Silicene seems to just "kill" itself and simple silicon takes its place.
The researchers believe that attempts to create multiple layers of silicene will always intrinsically fail, but whether this means the end of silicene applications in electronics is hard to say at this point.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.