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Bond-forming: Making good use of negative photoconductivity

09 Oct 2014  | David Chandler

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"These complex particles are called trions," Lui says. "They are analogous to negatively charged hydrogen ions, which consist of two electrons and one proton."

In single-layer MoS2, trions have the same net charge as an electron, but a mass roughly three times that of an electron. "Their much heavier mass dulls their response to the electric field, and lowers the material's conductivity," Lui says.

Instead of increasing the population of free charges, the illumination actually converts the original free electrons into heavier trions with the same charge density. This is the reason for the reduction of conductivity of single-layer MoS2 under illumination.

"This is a new mechanism of photoconduction in semiconductors," Gedik says, "which has not been observed before."

"Although negative photoconductivity has been reported in some semiconducting systems, it always arises from extrinsic factors, such as defects," Lui adds. "In this case, it is an intrinsic property of the crystal."

Trions are known to be unstable particles that usually appear at very low temperature and last for an extremely short period of time—so it has been very challenging to detect their influence on materials' conductivity. "In single-layer MoS2, the trionic effect is so strong that we can see it even at room temperature," Lui says. "Although the trions live for less than a billionth of a second, our ultrafast terahertz technique can detect them before they decay."

Gedik says that the work "might help us to realise room-temperature excitonic devices," which would otherwise require extremely low temperatures. In addition, because the effect can be switched on and off using light pulses, such devices could be easy to control without wired connections.

So far, the team has only studied the effect in MoS2, which belongs to a family of new two-dimensional semiconductors. "There are other types of two-dimensional materials with [similarly] strong trionic effect," Lui says. "They are likely to exhibit the same photoconduction phenomenon."

Xiaodong Xu, a professor of physics at the University of Washington who was not involved in this research, says it is "a piece of interesting and solid work." He adds, "This result is another evidence of strong Coulomb interaction in MoS2, consistent with previous studies of robust trions in monolayer transition metal dichalcogenides."

- David Chandler

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