Researchers at the Massachusetts Institute of Technology and Harvard University have found that single layer molybdenum disulfide semiconductors have reduced electrical conductivity under light excitation. The use of this new photoconductive mechanism is expected to develop the next generation of exciton devices. The findings were published in the recent Physical Review Letters.
As we all know, silicon semiconductors used in computer chips and solar cells have enhanced electrical conductivity under light irradiation. Researchers at the Massachusetts Institute of Technology and Harvard University have discovered that under intense laser pulses, the conductivity of only three atom thick monolayers of molybdenum disulfide is reduced to one third of the initial conductivity. The research team used optical laser pulses to create this effect and used delayed terahertz pulses to detect the material's conductive response.
The researchers said that this is a new mechanism for semiconductor photoconductors that has never been observed before. Although some semiconductor systems have been reported to have negative photoconductivity, they are mostly due to external factors such as defects, and the current discovery is the intrinsic property of crystals.
When light stimulates a semiconductor, its conductivity increases, because after light absorption, loose electrons and hole pairs can form, making it easier to pass current through the material. This phenomenon is the basis for designing and optimizing photovoltaic devices such as solar cells and digital cameras.
Layered atomic crystals have been a hot research topic in recent years. These materials have a prominent feature, namely the strong binding of charge carriers in their two-dimensional planes. The electrostatic interaction between its charge carriers is stronger than that of three-dimensional solids. Strong electrostatic interactions have a very interesting effect: When light excites electron-hole pairs, instead of creating free motion in a three-dimensional solid, they are still bound together. This bound state is also known as exciton. .
In fact, the interaction of a single layer of molybdenum disulfide is so strong that excitons can trap additional free electrons, forming a bound state of two electrons and one cavity. These complex particles resemble negatively charged hydrogen ions and consist of two electrons and one proton. In a single layer of molybdenum disulfide, the exciton is the same as the electron and has a negative charge, but its mass is about three times larger than that of an electron. Therefore, under light excitation, instead of increasing free electrons, the same charge density is heavier. Excited. Under the action of an electric field, the reaction is slow, resulting in a decrease in the conductivity of the material.
Excitons are known unstable particles, which usually occur at very low temperatures and have very short durations. Therefore, it is very challenging to test their influence on the material conductivity. The monolayer of molybdenum disulfide has a very strong exciton and can be found at room temperature. Although exciton life is less than one-billionth of a second, ultra-fast terahertz technology can be detected before they decay.
This discovery helps to achieve room temperature exciton devices, otherwise exciton devices will need to work at extremely low temperatures. In addition, because this effect can be switched using light pulses, these devices are easy to control and can be connected wirelessly.
So far, the research team only studied molybdenum disulfide, a new two-dimensional semiconductor. They speculated that other two-dimensional materials may have stronger effects and may also exhibit the same photoconductivity. The researchers said that this result is another proof of the strong Coulomb interaction of molybdenum disulfide, which is consistent with the results of the previous monolayer transition metal dichalcogenide exciton study. (Reporter He Wei)
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