One of the critical functions for electronic devices is a process known as rectification in which an electrical current can be forced to move in one direction and then the opposite one. Rectification makes possible electronics devices such as transistors, diodes and memory circuits.
Now researchers at Purdue University's School of Mechanical Engineering and Birck Nanotechnology Center have determined that a material based on graphene is capable of producing this rectification effect not on electric current but on heat flow.
While still just a simulation, the research shows that graphene can confine phonons to create a kind of “thermal rectifier” in which heat flow is directed like electrical diodes direct electrical current. Phonons are vibrations in a crystal lattice that carry heat, but outside of this work they are usually viewed as a nuisance to be eradicated because they produce noise in electronic circuits and cause other problems.
"In most systems, heat flow is equal in both directions, so there are no thermal devices like electrical diodes. However, if we are able to control heat flow like we control electricity using diodes then we can enable a lot of new and exciting thermal devices including thermal switches, thermal transistors, logic gates and memory," said Xiulin Ruan, an associate professor at Purdue, in a press release. "People are just starting to understand how it works, and it is quite far from being used in applications."
In this simulation, the details of which were published in the journal Nano Letters (“Phonon Lateral Confinement Enables Thermal Rectification in Asymmetric Single-Material Nanostructures”), the researchers have shown that tiny triangular structures formed by the graphene laterally confine the phonons to produce the effect of thermal rectification. But the researchers believe that a number of nanomaterials, such as quantum dots and nanowires, could also potentially fill the bill.
"We demonstrate that other asymmetric materials, such as asymmetric nanowires, thin ﬁlms, and quantum dots of a single material can also be high-performance thermal rectiﬁers, as long as you have lateral confinement," Ruan said. "This really broadens the potential of this rectification to a much wider spectrum of applications."
The key feature that the material has to have to confine the phonons laterally is smallness. To achieve the lateral confinement, the cross structure of the material has to be smaller than the path of the phonon, which means that the cross structures need to be smaller than a hundred nanometers and sometimes down to a few nanometers.
While Ruan and his colleagues concede that this simulation is a long way from actual applications, a speculative but exciting one would be to create thermal transistors. These transistors would not require silicon and would function around the movement of phonons rather than electrons. The benefit of this type of transistor is that they could make use of all the waste heat generated in the electronics instead of trying to find ways of getting rid of it.
Illustration: Purdue University