Researchers have found it effective to combine different two-dimensional (2D) materials to create compound materials that have properties that no single 2D material would have on its own. For instance, hexagonal boron nitride with its enormous band gap—which gives it pseudo-insulator characteristics—has been combined with a pure conductor like graphene to create a semiconductor material for a host of applications, especially in flexible electronics.
Now researchers at the University of California, Riverside and the University of Georgia have taken this practice one step further and added a third 2D material to boron nitride and graphene: tantalum sulfide. The result is a compound material that the researchers used to make a voltage-controlled oscillator (VCO). These VCOs are ubiquitous, and are found in applications such as clocks, radios, and computers.
In research described in the journal Nature Nanotechnology, the researchers fabricated for the first time a practical device that exploits charge-density waves to modulate an electrical current through a 2D material. A charge-density wave is an ordered array of electrons in either a linear chain compound or layered crystal.
The VCO that the researchers made could be used as an ultralow power alternative to conventional devices that are now based on silicon. Because this VCO is flexible, it could be also used in wearables.
“It is difficult to compete with silicon, which has been used and improved for the past 50 years. However, we believe our device shows a unique integration of three very different 2D materials, which utilizes the intrinsic properties of each of these materials,” said Alexander Balandin, a professor at UC Riverside’s Bourns College of Engineering, who led the research team, in a press release. “The device can potentially become a low-power alternative to conventional silicon technologies in many different applications”
The researchers added the tantalum sulfide to the graphene and boron nitride to provide the on-off switching capabilities that graphene lacked on its own. In their research, the scientists demonstrated that voltage-induced changes in the atomic structure of the tantalum sulfide-enabled prototype allow the device to function as an electrical switch at room temperature.
“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,” Balandin explained. “The particular polytype of tantalum sulfide that we used can have abrupt changes in resistance above room temperature. That made a crucial difference.”
In the final device, the boron nitride is used to coat the tantalum sulfide to prevent oxidation. Meanwhile the graphene serves as an integrated tunable load resistor, which controls the voltage of the current and VCO frequency.
The first prototype devices developed by the researchers operated at the MHz frequency used in radios. Also, because of the extremely fast physical processes that characterize the device, it could operate at frequencies all the way up to THz.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.