US researchers have made a voltage-controlled oscillator (VCO) using graphene and two other two-dimensional materials: tantalum sulphide and boron nitride.
This is the first useful device that exploits charge-density wave modulation of current through a 2D material, claims the University of California Riverside, which worked with the University of Georgia.
Graphene can conducts electrons rapidly, but has no usable bandgap from which to forge switching behaviour.
Instead of switching with graphene, a 2D crystal form material tantalum sulphide (TaS2) was used.
“There are many charge-density wave materials that have interesting electrical switching properties. However, most of them reveal these properties at very low temperature only. The particular type of TaS2 that we used can have abrupt changes in resistance above room temperature. That made a crucial difference,” said Professor Alexander Balandin of UC Riverside.
However, graphene still makes an entrance in the VCO, as in the design a gated graphene FET is connected in series with TaS2 switch and used as a variable load resistor for tuning.
Boron nitride (BN) is a two-dimensional insulator, and is coated over both TaS2 and graphene to keep oxygen away.
Oscillation was at MHz, but “the extremely fast physical processes that define the device functionality allow for the operation frequency to increase all the way to THz”, said UC Riverside.