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US researchers claim competitor for graphene

Demonstration of ultrafast charge transfer in photo-excited 2D semiconductors

A new class of materials is emerging made up of heterostructures held together by weak Van der Waals forces.  In these materials, quantum coupling between stacked atomically thin 2D layers, including graphene, hexagonal-boron nitride and transition-metal dichalcogenides (MX2), give rise to fascinating new phenomena.

Now an international team of researchers led by a scientist with the US Department of Energy (DOE's) Lawrence Berkeley National Laboratory has reported the first experimental observation of ultrafast charge transfer in photo-excited MX2 materials, in this case MoS2/WS2. The recorded charge transfer time was under 50 femtoseconds, comparable to the fastest times recorded for organic photovoltaics. 

MX2 materials (comprising consist of a single layer of transition metal atoms, such as molybdenum or tungsten, sandwiched between two layers of chalcogen atoms, such as sulphur), are particularly exciting for novel optoelectronic and photovoltaic applications because 2D MX2 monolayers can have an optical bandgap in the near-infrared to visible range and exhibit extremely strong light-matter interactions. Theory predicts that many stacked MX2 heterostructures form type II semiconductor heterojunctions that facilitate efficient electron-hole separation for light detection and harvesting.

Pictured above is an illustration of the Berkeley Laboratory's MoS2/WS2 heterostructure. It shows  a MoS2 monolayer on top of a WS2 monolayer. Electrons and holes created by light are shown to separate into different layers. 

"We've demonstrated, for the first time, efficient charge transfer in MX2 heterostructures through combined photoluminescence mapping and transient absorption measurements," says Feng Wang, a condensed matter physicist with Berkeley Lab's Materials Sciences Division and the University of California (UC) Berkeley's Physics Department. "Our study suggests that MX2 heterostructures, with their remarkable electrical and optical properties and the rapid development of large-area synthesis, hold great promise for future photonic and optoelectronic applications."

Wang is the corresponding author of a paper in Nature Nanotechnology describing this research. The paper is titled 'Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures'. Co-authors are Xiaoping Hong, Jonghwan Kim, Su-Fei Shi, Yu Zhang, Chenhao Jin, Yinghui Sun, Sefaattin Tongay, Junqiao Wu and Yanfeng Zhang.

"Combining different MX2 layers together allows one to control their physical properties," says Wang, who is also an investigator with the Kavli Energy NanoSciences Institute (Kavli-ENSI). "For example, the combination of MoS2 and WS2 forms a type-II semiconductor that enables fast charge separation. The separation of photoexcited electrons and holes is essential for driving an electrical current in a photodetector or solar cell."

"MX2 semiconductors have extremely strong optical absorption properties and compared with organic photovoltaic materials, have a crystalline structure and better electrical transport properties," Wang says. "Factor in a femtosecond charge transfer rate and MX2 semiconductors provide an ideal way to spatially separate electrons and holes for electrical collection and utilisation."

Wang and his colleagues are studying the microscopic origins of charge transfer in MX2 heterostructures and the variation in charge transfer rates between different MX2 materials. "We're also interested in controlling the charge transfer process with external electrical fields as a means of utilising MX2 heterostructures in photovoltaic devices," Wang says.

This research was supported by an Early Career Research Award from the DOE Office of Science through UC Berkeley, and by funding agencies in China through the Peking University in Beijing.

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