Graphene's Electrical Behavior Altered By Light Pulses

Graphene's Electrical Behavior Altered by Light Pulses

7 August 2014
Using short pulses of light, scientists have found a way to alter the photoconductive properties of graphene from semiconductor-like to metal-like.
A team of researchers from the Massachusetts Institute of Technology (MIT) discovered that the way the material reacts to light pulses is determined by the concentration of electrons it starts with. If it's low, a pulse will increase the graphene's electrical conductivity in a way that resembles semiconductors like silicon and germanium. If it's high, a light pulse will do the opposite and decrease the graphene's conductivity.
Publishing its findings in the journal Physical Review Letters, the team said that the reduction is a result of a unique quality that graphene possesses. Electrons within the material travel at a constant speed, similar to photons, and this is the reason their conductivity lowers when the light pulse increases their temperature.
In their research, the team managed to tune the number of electrons in the graphene, so it could control the response it was getting. To do so, researchers deposited graphene over a thin layer of metal - with an insulating layer between the two - and tuned its electron concentration by applying voltage.
Then the material was illuminated by two light pulses - a strong one to modify the material, and a second one with a lower frequency. This allowed researchers to measure the differences in electrical conduction between the two.
Because the scientists' approach was purely optical, it required no contact with the material and no addition of extra electrical contacts. Additionally, by using short light pulses, the team was able to look into the material's electrical response within trillionths of a second.