Graphene is a very curious material, which is still being explored. Usually it is described as a two dimensional material, just one atom thick, which is extremely strong and conductive.
However, despite numerous researches devoted to this material, scientists still discover something new about it. In reality, graphene is not necessarily two dimensional – it may form wrinkles, which may have different application when using graphene in electronic devices. Furthermore, this material can also interact with the substrate upon which it is laid.
Now scientists at the RIKEN institute, Japan, have discovered that wrinkles in graphene can restrict the motion of electrons to one dimension, forming a junction-like structure that changes from zero-gap conductor to semiconductor back to zero-gap conductor. Furthermore, they discovered a new way to manipulate graphene not by chemical means (adding other elements). In a process they called “graphene engineering” they were using the tip of a scanning tunnelling microscope to manipulate the formation of wrinkles, which opens a new way to form such graphene semiconductors.
The discovery was made when scientists were using chemical vapour deposition to create graphene films. This method currently is considered to be the most reliable one. During the experiments researchers were working in order to form graphene on a nickel substrate. The difficulty here is the temperature and cooling speed – success of the experiment heavily relies on these two variables. Then just by accident scientists discovered wrinkles on their graphene.
Hyunseob Lim, the first author of the paper, described the accidental discovery: “we were attempting to grow graphene on a single crystalline nickel substrate, but in many cases we ended up creating a compound of nickel and carbon, Ni2C, rather than graphene. In order to resolve the problem, we tried quickly cooling the sample after the dosing with acetylene, and during that process we accidentally found small nanowrinkles, just five nanometres wide, in the sample.”
Researchers managed to image these minute nanowrinkles using the scanning tunnelling microscopy. This allowed them to find that there were band gap openings within them. This indicated that graphene with wrinkles could potentially act as semiconductor. In normal conditions electrons and electron holes flow freely through a conductor without a band gap.
The difference with semiconductor is that there are band gaps between the permitted electron states, which means that the electrons can only pass through these gaps under certain conditions. Therefore, wrinkled graphnee under certain conditions could be used as a semiconductor.
At first research team was analysing two possibilities for the emergence of this band gap. The first one stated that the mechanical strain could cause a magnetic phenomenon. This possibility soon was ruled out by scientists because the second possibility was closer to truth. Scientists concluded that the phenomenon was caused by the confinement of electrons in a single dimension due to “quantum confinement”.
Scientists note that up to this day most of efforts to manipulate the electronic properties of graphene were focused around chemical means. However, there is a significant disadvantage to that – chemical manipulation of graphene may lead to degraded electronic properties due to chemical defects.
But now scientists have discovered a way to dodge such downside by using mechanical manipulation. Their discovery means that the electronic properties of the graphene can be manipulated by simply changing the shape of the carbon structure. Scientists say that it is exciting to think that this discovery ultimately could be used in new generation of electronics. We can only wonder what new uses and properties graphene may still have undiscovered.