‘Electron Liquid’ Created at Room Temperature

A team of physicists at the University of California, Riverside, has produced the first ‘electron-hole liquid’ at room temperature.

In conventional electronic devices, electricity requires the movement of electrons (blue spheres) and their positive counterparts, called holes (red spheres), which behave much like the gas molecules in our atmosphere. Although they move rapidly and collide infrequently in the gas phase, electrons and holes can condense into liquid droplets akin to liquid water in devices composed of ultrathin materials. Image credit: QMO Lab, University of California, Riverside.

In their experiments, University of California, Riverside’s Dr. Nathaniel Gabor and co-authors constructed an ultrathin sandwich of the semiconductor molybdenum ditelluride between layers of graphene. The layered structure was just slightly thicker than the width of a single DNA molecule.

The researchers then bombarded the material with superfast laser pulses, measured in quadrillionths of a second.

“Normally, with such semiconductors as silicon, laser excitation creates electrons and their positively charged holes that diffuse and drift around in the material, which is how you define a gas,” Dr. Gabor said.

However, the team detected evidence of condensation into the equivalent of a liquid. Such a liquid would have properties resembling common liquids such as water, except that it would consist, not of molecules, but of electrons and holes within the semiconductor.

“We were turning up the amount of energy being dumped into the system, and we saw nothing, nothing, nothing — then suddenly we saw the formation of what we called an ‘anomalous photocurrent ring’ in the material. We realized it was a liquid because it grew like a droplet, rather than behaving like a gas,” Dr. Gabor said.

“What really surprised us, though, was that it happened at room temperature. Previously, researchers who had created such electron-hole liquids had only been able to do so at temperatures colder than even in deep space.”

“The electronic properties of such droplets would enable development of optoelectronic devices that operate with unprecedented efficiency in the terahertz region of the spectrum.”

Terahertz wavelengths are longer than infrared waves but shorter than microwaves, and there has existed a ‘terahertz gap’ in the technology for utilizing such waves.

Terahertz waves could be used to detect skin cancers and dental cavities because of their limited penetration and ability to resolve density differences.

Similarly, the waves could be used to detect defects in products such as drug tablets and to discover weapons concealed beneath clothing.

“Terahertz transmitters and receivers could also be used for faster communication systems in outer space,”

“And, the electron-hole liquid could be the basis for quantum computers, which offer the potential to be far smaller than silicon-based circuitry now in use.”

“More generally, the technology used in his laboratory could be the basis for engineering ‘quantum metamaterials,’ with atom-scale dimensions that enable precise manipulation of electrons to cause them to behave in new ways.”

The research is published in the journal Nature Photonics.

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Trevor B. Arp et al. Electron-hole liquid in a van der Waals heterostructure photocell at room temperature. Nature Photonics, published online February 4, 2019; doi: 10.1038/s41566-019-0349-y