While light enhances the conductivity of most semiconductors, it has the opposite effect on 2-D molybdenum disulfide. A team from MIT and Harvard University discovered the phenomenon, in which intense laser pulses reduced by about two-thirds the conductivity of a three-atom-thick MoS2 crystal structure. The photoconductivity response was measured using time-delayed terahertz spectroscopy. “By measuring the transmission of the terahertz radiation through the material, we can extract its electrical conductivity,” said MIT professor Dr. Nuh Gedik. “This approach is more convenient than conventional methods that attach electrical contacts to the samples and measure the current.” The crystal structure of MoS2 is shown in blue and yellow. When it is hit with a burst of laser light, electrons and holes are freed and form trions, shown in orange and green. Courtesy of Jose-Luis Olivares/MIT. Optical illumination causes more electrostatic interactions between charge carriers in 2-D materials than in 3-D materials. In this case, the result was the creation of bound states made of up two electrons and one hole — called trions. Each trion has the same net charge as an electron but roughly three times the mass. “Their much heavier mass dulls their response to the electric field, and lowers the material’s conductivity,” said MIT postdoctoral researcher Joshua Lui. Trions are known to be unstable and usually appear only at very low temperatures. In MoS2, the trionic effect was strong enough to be observed at room temperature using terahertz spectroscopy, Lui said. The findings could lead to development of “room-temperature excitonic devices” that could be controlled wirelessly, the researchers said. The research was funded by the U.S. Department of Energy and the National Science Foundation. The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.113.166801). For more information, visit www.mit.edu.