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John O | February 2019

Physicists create the first electron liquid at room temperature, paving way for more study


By Josh Perry, Editor
[email protected]

 

Scientists at the University of California, Riverside (UCR) created the first electron liquid at room temperature by bombarding a sandwich of ultrathin semiconductor molybdenum ditelluride layered between graphene with superfast, powerful lasers.

 


Electrons (blue) and holes (red) condense into liquid droplets akin to liquid water in devices composed of ultrathin materials. (UCR)

 

According to a report from UCR, rather than seeing the typical properties of a gas, researchers instead detected evidence of condensation into a liquid equivalent. Rather than consisting of molecules, such as a typical liquid like water, the electron liquid consisted of electrons and holes within the semiconductor.

 

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

 

This breakthrough could lead to practical devices for generating and detecting terahertz wavelengths for communicating in space or detecting cancer or it could be used to further study of the basic physics of matter at the nanoscale. For instance, it could lead to the engineering of quantum metamaterials.

 

“In further studies of the electron-hole ‘nanopuddles,’ the scientists will explore their liquid properties such as surface tension,” the article added. “In their experiments, the researchers used two key technologies. To construct the ultrathin sandwiches of molybdenum ditelluride and carbon graphene, they used a technique called ‘elastic stamping.’ In this method, a sticky polymer film is used to pick up and stack atom-thick layers of graphene and semiconductor.”

 

The second technology was multi-parameter dynamic photoresponse microscopy, which uses ultrafast laser pulse to map the current that is generated.

 

The research was recently published in Nature Photonics. The abstract stated:

 

“In semiconductors, photo-excited charge carriers exist as a gas of electrons and holes, bound electron–hole pairs (excitons), biexcitons and trions. At sufficiently high densities, the non-equilibrium system of electrons (e) and holes (h+) may merge into an electronic liquid droplet.

 

“Here, we report on the electron–hole liquid in ultrathin MoTe2 photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene–MoTe2–graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power at T = 297 K.

 

“The sudden onset to these patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional electron–hole liquid droplet. The electron–hole liquid, which features a macroscopic population of correlated electrons and holes, may offer a path to room-temperature optoelectronic devices that harness collective electronic phenomena.”

 

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