Junqiao Wu and colleagues at the University of California at Berkeley, the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing, and the Massachusetts Institute of Technology studied anion vacancies in monolayers of TMDCs. These materials, which have the chemical formula MX2 (where M = Mo, W and X= S, S) are promising for a variety of electronics and optoelectronics device applications such as light-emitting diodes (LEDs) and solar cells because they become direct bandgap semiconductors when scaled down to monolayers.

Knocking out atoms

The researchers studied the effects of defects in monolayer TMDCs by creating chalcogen (sulphur and selenium) vacancies. They did this by knocking out atoms of the monolayers using high-energy alpha-particle irradiation and then measuring the light emission spectrum of the semiconductors and comparing this to the same spectrum from a pristine sample.

The team saw that an additional light emission line appears below the band gap and that the materials became more luminescent as the density of vacancies increased. The new peak originates from the recombination of bound excitons (electron-hole pairs strongly bound to the defects) explains team member Sefaattin Tongay. The effect was absent in vacuum, something that suggests that gas molecules (such as nitrogen and oxygen) in the atmosphere are needed to interact with the defect sites and produce the light. The scientists further confirmed their result by density functional calculations, which revealed that the anion vacancies indeed create new energy levels around 0.2 eV below the bandgap of the materials when the gas molecules are absorbed at the defect sites.

Electron depeletion

Light emission is enhanced by draining free electrons from the materials by enhanced gas-material interactions at these defect sites, adds Tongay. This charge transfer depletes n-type TMDCs (MoS2 and MoSe2) and stabilizes the excitons (which otherwise would not last long enough) so that they are able to recombine and produce much more light.

"Our work shows that defects are not such a bad thing in this case," team member Joonki Suh told nanotechweb.org. "Our results not only shed more light on how defects affects the photoluminesence of the 2D materials we studied, but also point to a new way of tailoring their physical properties by defect engineering."

Wu says that his team is now looking at quantifying the effects of defects in chalcogenides and exploring the phenomenon in other 2D semiconductors.

The present work is detailed in Scientific Reports doi:10.1038/srep02657.

Further reading

MoSe2 single layers for solar cells (Nov 2012)
Making 2D semiconductors emit more light (May 2013)
Graphene solar cells break new efficiency record (Jun 2012)
Microwaves serve up nanostuctured chalcogenides and chalcogens (Mar 2009)