Misaligned monolayers stack up for cheaper electronics

Stacking one atom thick materials can create semiconductor junctions that transfer charge more efficiently, regardless of whether the crystalline structure of the materials is mismatched.

The finding, by researchers at North Carolina State University (NC State) could lower the manufacturing cost for a variety of semiconductor devices such as solar cells, lasers and LEDs.

‘This work demonstrates that by stacking multiple two-dimensional materials in random ways we can create semiconductor junctions that are as functional as those with perfect alignment,’ said Dr Linyou Cao, senior author of a paper on the work and an assistant professor of materials science and engineering at NC State. ‘This could make the manufacture of semiconductor devices an order of magnitude less expensive.’

For most semiconductor electronic or photonic devices to work, they need to have a junction, which is where two semiconductor materials are bound together. In photonic devices like solar cells, lasers and LEDs, the junction is where photons are converted into electrons, or vice versa.

According to NC State, all semiconductor junctions rely on efficient charge transfer between materials, to ensure that current flows smoothly and that a minimum of energy is lost during the transfer. To do that in conventional semiconductor junctions, the crystalline structures of both materials need to match. However, that limits the materials that can be used, because you need to make sure the crystalline structures are compatible. And that limited number of material matches restricts the complexity and range of possible functions for semiconductor junctions.

‘But we found that the crystalline structure doesn’t matter if you use atomically thin, 2D materials,’ Cao said in a statement. ‘We used molybdenum sulphide and tungsten sulphide for this experiment, but this is a fundamental discovery that we think applies to any 2D semiconductor material. That means you can use any combination of two or more semiconductor materials, and you can stack them randomly but still get efficient charge transfer between the materials.’

Currently, creating semiconductor junctions means perfectly matching crystalline structures between materials, a process that requires expensive equipment, sophisticated processing methods and user expertise. This manufacturing cost is a major reason why semiconductor devices such as solar cells, lasers and LEDs remain very expensive. Stacking 2D materials, however, doesn’t require the crystalline structures to match.

‘It’s as simple as stacking pieces of paper on top of each other – it doesn’t even matter if the edges of the paper line up,’ Cao said.

The paper, Equally Efficient Interlayer Exciton Relaxation and Improved Absorption in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures, was published as a “just-accepted” manuscript in Nano Letters on December 3, 2014.