“Our result is an important step towards making high-speed communications that can be integrated into flexible and transparent platforms,” team leader Zhaohui Zhong told nanotechweb.org. “What is more, we show that we are able to achieve quaternary digital modulation using no more than a couple of graphene transistors at a time, something that also represents a milestone for all-graphene-based high-speed data communication circuits.”

Graphene consists of a single, flat sheet of carbon arranged in a honeycombed lattice and has numerous unique electronic and mechanical properties. For example, its extremely high carrier mobility allows it to function as both an interconnect and high mobility channel material in ultrafast transistors. Graphene also has attractive optical properties over a range of wavelengths in the electromagnetic spectrum from the visible to mid-infrared and is highly transparent to light. The fact that it is mechanically flexible while being incredibly strong is another important advantage.

Zhong and colleagues made their modulator from circuits containing only graphene components, including transistor channels, the interconnects between the transistors, the load resistance and source/drain/gate electrodes. To encode digital information using the devices, they relied on the unique “ambipolar” gate response of graphene transistors. This means that both electrons and holes can carry current in the material, and that the type of carrier utilized can be switched by simply applying a gate bias. This is rather different to conventional semiconductors, where the type of carrier is pre-determined by the doping in the device.

From binary to quaternary

Most modulation techniques map information by varying either the amplitude, frequency or phase of the carrier signal – or all three of these parameters. When just one of these parameters is varied, this represents binary digital information (of 0 and 1), but it is possible to extend the technique into quaternary modulation by combining two or more binary modulation schemes. Quaternary digital information means four symbols represented by two bits of digital information: 00, 01, 10 and 11. The new graphene digital modulator made by the Michigan team can encode such information.

“Quaternary modulation schemes (such as Quadrature Phase-Shift Keying or QPSK) are the fundamental building blocks for more efficient coding schemes widely used in today’s telecommunication standards such as Code Division Multiple Access (CDMA) and Long Term Evolution (LTE),” said Zhong. “Our demonstration of QPSK using transparent all-graphene circuits thus shows great promise for graphene-based flexible wireless communication.”

The new graphene modulator circuits are also better than traditional such circuits that are based on rigid crystalline materials like silicon and which cannot be used in flexible electronics. And by exploiting the ambipolarity of graphene transistors, the modulators themselves use no more than two graphene devices at a time, which makes them much less complex than their conventional counterparts.

Spurred on by its preliminary results, the team says that it would now like to improve the graphene circuits so that they operate at GHz frequencies.

The current work is published in Nature Communications.