The team, led by Junqiao Wu of the University of California, Berkeley, studied layered transition metal dichalcogenides (TMDCs) that have the chemical formula MX2, where M = Mo, W and X= S, Se. These materials are promising for a variety of electronics and optoelectronics device applications such as light-emitting diodes and solar cells thanks to the fact that they go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. The electrons in TMDCs also interact exceptionally strongly with light, which means that even though these materials are just a few atoms thick, a large portion of absorbed photons can be used to produce electric current.

However, there is one problem in that the monolayers do not emit light very efficiently – their photoluminescence quantum yield is low.

Wu and colleagues have now found, however, that they can increase the light-emitting efficiency of the 2D materials by 100-fold simply by exposing the samples to O2 and/or H2O vapour after thermal vacuum annealing. The effect is completely reversible at room temperature by controlling the gas pressure.

Charge transfer

According to the researchers, the O2 and H2O molecules interact weakly with the TMDC monolayers, with binding energies ranging from 70–140 meV, and withdraw a significant number of electrons from the 2D materials. This charge transfer has the effect of depleting n-type TMDCs (MoS2 and MoSe2) and stabilises electron-hole pairs (or excitons), which would otherwise not live long enough, so that they are able to recombine and produce much more light.

"Our result shows that – just like graphene – monolayer semiconductors are also very sensitive to their environment, " explained team member Sefaattin Tongay of the University of California at Berkeley. "Interaction with surrounding gas molecules changes the materials’ physical properties by redefining excitonic interactions in the system."

Molecular gating

Physi-sorption of the gas molecules on the monolayer semiconductors results in significant charge transfer that changes the free electron density in the materials, and we call this effect "molecular gating", he added. "Molecular gating is better than conventional electrical gating because we are no longer limited by the breakdown field of a dielectric."

Tongay says that the new work should help scientists better understand how sensitive monolayer semiconductors really are to the molecules in their environment, and how they interact with different gases. "From a technology point of view, a 100-fold enhancement in photoluminescence is exciting since PL is one of the most important parameters in most optoelectronic and optical devices," he told nanotechweb.org.

The team is now busy looking at how imperfections (such as atomic defects) in such 2D semiconductors affect their photoluminescence. "We are also trying to create new TMDC materials with unusual physical properties," revealed Tongay.

The present work is detailed in Nano Letters.