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Manchester team proposing graphene-based ballistic rectifier for waste heat recovery

Researchers at the University of Manchester (UK) have developed a graphene-based nano-rectifier (“ballistic rectifier”) that can convert waste heat to electricity. The nano-rectifier was built by a team led by Professor Aimin Song and Dr. Ernie Hill, in collaboration with a team at Shandong University (China).

The device exploits graphene’s phenomenally high electron mobility—a property which determines how fast an electron can travel in a material and how fast electronic devices can operate. The resulting device is the most sensitive room-temperature rectifier ever made, the researchers said. Conventional devices with similar conversion efficiencies require cryogenically low temperatures.

Professor Song and other colleagues first outlined their concept of a ballistic rectifier for the study of electron transport through semiconductor devices in a 1998 paper:

Here we introduce a novel device geometry which is particularly suitable to study the effects of reduced symmetry on the nonlinear ballistic transport properties. By inserting an asymmetric scatterer into the center of a ballistic cross junction, we observe unusual nonlinear current-voltage characteristics which we show to be dominated by the symmetry properties of the scatterer. The size of the artificial scatterer is much larger than the Fermi wavelength λF of the conducting electrons and comparable to their elastic mean free path le (le » λF). We demonstrate a successful guidance of ballistic electrons to a predetermined spatial direction independent of input current direction. As a result, the device works as a ballistic rectifier with a mechanism entirely different from that of an ordinary diode.

—Song et al. (1998)

Rectifying diodes let electrical current flow in only one direction; mainly used for power supply operation, they convert AC to DC current. Conventional rectifying diodes are based on either p–n doped junction or a Schottky barrier. The built-in electric field or a threshold voltage in these devices requires an applied bias high enough to overcome the internal field and generate a significant current flow.

The ballistic rectifier has no intrinsic threshold or turn-on voltage because there is no p–n junction or Schottky barrier along the direction of electrical current in the device.

In 2012, a team from the US, Spain and Sweden (Matthews et al.) demonstrated a thermally driven ballistic rectifier:

Here we present measurements of the response to a thermal bias of a four-terminal, quasiballistic junction with a central scattering site. We find a novel transverse thermovoltage measured across isothermal contacts. Using a multiterminal scattering model extended to the weakly nonlinear voltage regime, we show that the device’s response to a thermal bias can be predicted from its nonlinear response to an electric bias. Our approach forms a foundation for the discovery and understanding of advanced, nonlocal, thermoelectric phenomena that in the future may lead to novel thermoelectric device concepts.

—Matthews et al. (2015)

The device in that experiment consisted of an InP/Ga0.23In0.77As 2DEG wafer.

In 2015, Dr. Song and his colleagues reported the development a graphene-based ballistic-rectifier.

Singh2015

Singh2015
Top: Schematic illustration of the different geometries of ballistic rectifiers (a) with and (b) without a triangular antidot, an artificial scatterer, at the center of device active region. The dark areas represent the etched regions. Arrows indicate the typical directions of flow of the charge carriers from S or D to L terminal. (c) The typical scanning electron microscope image (c) and atomic-force micrograph (d) of the devices with and without a triangular antidot, respectively. Singh et al. (2015)

Bottom: Illustration of graphene-based ballistic rectifier. Manchester University. Click to enlarge.

The Manchester team is now proposing that the graphene-based ballistic rectifier can convert waste from engine exhaust into a useable electrical current to power additional automotive features such as air conditioning and power steering, or be stored in the car battery.

Graphene has exceptional properties; it possesses the longest carrier mean free path of any electronic material at room temperature. Despite this, even the most promising applications to commercialize graphene in the electronics industry do not take advantage of this property. Instead they often try to tackle the the problem that graphene has no band gap.

The working principle of the ballistic rectifier means that it does not require any band gap. Meanwhile, it has a single-layered planar device structure which is perfect to take the advantage of the high electron-mobility to achieve an extremely high operating speed.

Unlike conventional rectifiers or diodes, the ballistic rectifier does not have any threshold voltage either, making it perfect for energy harvest as well as microwave and infrared detection.Greg Auton, who performed most of the experiment

Graphene was the world’s first two-dimensional material, isolated in 2004 at The University of Manchester; since then a whole family of other 2D materials have been discovered.

The Manchester-based group is now looking to scale up the research by using large wafer-sized graphene and perform high-frequency experiments. The resulting technology can also be applied to harvesting wasted heat energy in power plants.

Resources

  • Arun K. Singh, Gregory Auton, Ernie Hill, Aimin Song (2015) “Graphene based ballistic rectifiers,” Carbon, Volume 84, Pages 124-129 doi: 10.1016/j.carbon.2014.11.064

  • J. Matthews, D. Sánchez, M. Larsson, and H. Linke (2012) “Thermally driven ballistic rectifier” Physical Review B doi: 10.1103/PhysRevB.85.205309

  • Song, A. M. and Lorke, A. and Kriele, A. and Kotthaus, J. P. and Wegscheider, W. and Bichler, M. (1998) “Nonlinear Electron Transport in an Asymmetric Microjunction: A Ballistic Rectifier” Phys. Rev. Lett. doi: 10.1103/PhysRevLett.80.3831

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