2D materials, like graphene, have dramatically different electronic and mechanical properties from their 3D counterparts and so may find use in a host of novel device applications. Until now, however, most research in this field has focused on graphene, but the fact that this material lacks a bandgap means that scientists are also now starting to turn their attention elsewhere.

2D materials based on molybdenum, such as molybdenite (MoS2) can be prepared using the same "sticky tape" method used to obtain graphene. Although atomically thin MoS2 has been studied before (the material was found to have a bandgap of 1.9 eV), similar compounds containing selenium have never been experimentally synthesized. Sefaattin Tongay and Junqiao Wu of the University of California, Berkeley and colleagues at the Massachusetts Institute of Technology and Lawrence Berkeley National Laboratory have now put things right and have found that single layers of this material have very interesting optical and electronic properties.

Direct bandgap

For example, they have discovered that 2D MoSe2 has a direct bandgap at 1.5 eV. This is a non-negligible advantage because it is easier to make optoelectronics and photonics devices that exploit electron-hole pair excitation with direct rather than indirect gap semiconductors, which indeed silicon is. The material is also easy to isolate from bulk MoSe2, which for its part is also an indirect bandgap semiconductor.

What is more, when more single layers are stacked on top of each other, few-layer MoSe2 has almost the same direct and indirect bandgap values at 1.3–1.5 eV and these can be tuned using temperature and the number of MoSe2 layers as desired, explains Tongay. So far single-layer MoSe2 also appears to be stable, something that is needed for reliable device function. Single layers of MoSe2 photoluminesce strongly too, making the material interesting for light emitting diode (LEDs) applications.

Well matched to sunlight's spectrum

"We have shown that that 2D MoSe2 has an ideal bandgap value for solar-energy harvesting (because its bandgap is well matched to the solar spectrum) and possibly for many other optoelectronics applications too," said Tongay. "The results we have obtained are exciting for us because, according to the 'Shockley-Queisser limit' for the theoretical maximum efficiency of a solar cell, semiconductors with a bandgap between 1.0 and 1.6 eV have the greatest potential to yield an efficient cell – and we are in this range for a 2D material," he told nanotechweb.org.

"2D materials should be better than their 3D counterparts for solar-cell applications because they are exposed to 100% of sunlight," said Tongay. "Making a 3D foam or network out of these 2D MoSe2 layers would be a very good and efficient way to making excellent solar cells and we are currently making good progress in this area."

The work is detailed in Nano Letters.