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Bond-forming: Making good use of negative photoconductivity

09 Oct 2014  | David Chandler

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A team of researchers claims to have discovered that light can make a semiconductor less conductive—the exact opposite of the widely-known phenomenon that has been the basis for designing and optimising optoelectronic devices such as solar cells, digital cameras and other light detectors.

The team made the discovery in an exotic two-dimensional semiconductor—a single layer of molybdenum disulfide (MoS2), just three atoms thick. They found that when illuminated by intense laser pulses, single-layer MoS2 is reduced to approximately one-third of its initial conductivity.

In detecting the conductive response of the material, optical laser pulses generated the effect and time-delayed terahertz pulses. The finding is reported in a paper in Physical Review Letters by MIT postdoc Joshua Lui; Nuh Gedik, the Lawrence C. and Sarah W. Biedenharn Career Development Associate Professor of Physics; and six others at MIT, Harvard University, and in Taiwan.

"By measuring the transmission of the terahertz radiation through the material, we can extract its electrical conductivity," Gedik says. "This approach is more convenient than conventional methods that attach electrical contacts to the samples and measure the current."

When a semiconductor is illuminated by light, its conductivity tends to increase. This is because light absorption generates pairs of loose electrons and holes—places in a material with "missing" electrons—that facilitate the flow of electrical current through the material.

The MIT team, however, observed the opposite behaviour in a two-dimensional semiconductor. "Atomically thin layered crystals have been the subject of intense research in recent years," Lui says. "One remarkable property of these materials is the strong confinement of charge carriers in a two-dimensional plane. ... As a consequence, the electrostatic interactions between the charge carriers are much stronger than those in three-dimensional solids."


Crystal structure of MoS2, with molybdenum atoms shown in blue and sulphur atoms in yellow. When hit with a burst of laser light, freed electrons and holes combine to form combinations called trions, consisting of two electrons and one hole, and represented here by orange and green balls. Source: Jose-Luis Olivares, MIT

Forming bonds

The strong electrostatic interactions give rise to an interesting effect: When light generates an electron-hole pair in the material, instead of flying off freely as they would in a three-dimensional solid, they remain bound together. Such a bound state is called an exciton.

In fact, the interactions in single-layer MoS2 are so strong that excitons can capture extra free electrons in the material and form bound states with two electrons and one hole.

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