Dichalcogenides are layered semiconducting films that might be used to make circuits for low-power electronics, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces. These so-called van der Waals materials have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). They go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. These monolayers efficiently absorb and emit light and so could also be ideal for making a variety of optoelectronics devices such as light-emitting diodes and solar cells.

“With covalent bonds binding these monolayers, and with no dangling bonds, these materials are chemically perfect,” explains co-team leader Eli Yablonovitch. “What is more, since these layers are just one molecule thick, they might be geometrically perfect on the atomic scale too. Such perfection would greatly benefit miniaturized electronics devices.”

The researchers, co-led by Ali Javey of the University of California at Berkeley studied WSe2/MoS2 heterobilayers (made by stacking individual monolayers of the two transition-metal dichalcogenides on top of each other) using photoemission electron microscopy (PEEM). The data obtained showed that the WSe2 layer has a net negative charge while the MoS2 layer has a net positive charge.

Atomic-scale perfection

“By photoluminescence and absorption spectroscopies, we were also able to observe a large shift of around 100 meV between the photoluminescence peak and the lowest absorption peak of the heterobilayer,” team member Hui Fang told nanotechweb.org. “Such observations imply a type II band alignment in the heterojunction – a hypothesis that is also backed up by the PEEM data. The result proves that we have real control over the monolayers, since band alignment is the first question to ask, and we have answered it through experiment.”

For the first time, we have materials that might be perfect on the atomic scale, she added. “Such perfection would make energy levels in these structures sharper and would be very advantageous for when it comes to miniaturizing devices and making energy efficient electronics and optoelectronics.”

The researchers say that they will now be trying to create bottom-up heterostructures similar to the ones that they studied by varying chemical compositions and interlayer spacing. “We are still very much focused on characterizing the basic physical and electronic properties of these heterojunctions,” said Fang, “but the next step in our experiments will be to ‘squeeze out’ all of the out-of-place atoms and other imperfections from these structures.”

The team revealed that it will shortly be looking into the electroluminescence efficiency of van der Waals heterostructures too and testing them in nanoscale light-emitting and lasing devices.

The present work is detailed in PNAS doi: 10.1073/pnas.1405435111.