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Competition for graphene: Researchers demonstrate ultrafast charge transfer in new family of 2-D semiconductors

Date:
August 26, 2014
Source:
DOE/Lawrence Berkeley National Laboratory
Summary:
The first experimental observation of ultrafast charge transfer in photo-excited MX2 materials, the graphene-like two-dimensional semiconductors, has been conducted. Charge transfer time clocked in at under 50 femtoseconds, comparable to the fastest times recorded for organic photovoltaics.

Illustration of a MoS2/WS2 heterostructure with a MoS2 monolayer lying on top of a WS2 monolayer. Electrons and holes created by light are shown to separate into different layers.
Credit: Image courtesy of Feng Wang group

A new argument has just been added to the growing case for graphene being bumped off its pedestal as the next big thing in the high-tech world by the two-dimensional semiconductors known as MX2 materials. An international collaboration of researchers led by a scientist with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has reported the first experimental observation of ultrafast charge transfer in photo-excited MX2 materials. The recorded charge transfer time clocked in at under 50 femtoseconds, comparable to the fastest times recorded for organic photovoltaics.

"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. "Having quantitatively determined charge transfer time to be less than 50 femtoseconds, 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."

Feng Wang is a condensed matter physicist with Berkeley Lab's Materials Sciences Division and UC Berkeley's Physics Department. (Photo by Roy Kaltschmidt)

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.

MX2 monolayers consist of a single layer of transition metal atoms, such as molybdenum (Mo) or tungsten (W), sandwiched between two layers of chalcogen atoms, such as sulfur (S). The resulting heterostructure is bound by the relatively weak intermolecular attraction known as the van der Waals force. These 2D semiconductors feature the same hexagonal "honeycombed" structure as graphene and superfast electrical conductance, but, unlike graphene, they have natural energy band-gaps. This facilitates their application in transistors and other electronic devices because, unlike graphene, their electrical conductance can be switched off.

"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."

In demonstrating the ultrafast charge separation capabilities of atomically thin samples of MoS2/WS2 heterostructures, Wang and his collaborators have opened up potentially rich new avenues, not only for photonics and optoelectronics, but also for photovoltaics.

Photoluminescence mapping of a MoS2/WS2 heterostructure with the color scale representing photoluminescence intensity shows strong quenching of the MoS2 photoluminescence. (Image courtesy of Feng Wang group)

"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 utilization."

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 utilizing MX2 heterostructures in photovoltaic devices," Wang says.


Story Source:

The above story is based on materials provided by DOE/Lawrence Berkeley National Laboratory. The original article was written by Lynn Yarris. Note: Materials may be edited for content and length.


Journal Reference:

  1. Xiaoping Hong, Jonghwan Kim, Su-Fei Shi, Yu Zhang, Chenhao Jin, Yinghui Sun, Sefaattin Tongay, Junqiao Wu, Yanfeng Zhang, Feng Wang. Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures. Nature Nanotechnology, 2014; DOI: 10.1038/nnano.2014.167

Cite This Page:

DOE/Lawrence Berkeley National Laboratory. "Competition for graphene: Researchers demonstrate ultrafast charge transfer in new family of 2-D semiconductors." ScienceDaily. ScienceDaily, 26 August 2014. <www.sciencedaily.com/releases/2014/08/140826142537.htm>.
DOE/Lawrence Berkeley National Laboratory. (2014, August 26). Competition for graphene: Researchers demonstrate ultrafast charge transfer in new family of 2-D semiconductors. ScienceDaily. Retrieved October 23, 2014 from www.sciencedaily.com/releases/2014/08/140826142537.htm
DOE/Lawrence Berkeley National Laboratory. "Competition for graphene: Researchers demonstrate ultrafast charge transfer in new family of 2-D semiconductors." ScienceDaily. www.sciencedaily.com/releases/2014/08/140826142537.htm (accessed October 23, 2014).

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