Dichalcogenides are easily processed 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. They 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). The materials 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 be ideal for making a variety of optoelectronics device applications such as light-emitting diodes and solar cells.

All dichalcogenides studied to date undergo this indirect to direct bandgap transition – thanks to strong coupling between the neighbouring layers in the materials – but the newly discovered member of this family, ReS2, does not, according to a team led by Junqiao Wu of the University of California at Berkeley and the Lawrence Berkeley National Lab. Both bulk and monolayer ReS2 are direct bandgap semiconductors and have the same gap value. “Such contrasting behaviour comes as a result of crystal symmetry differences and a lack of interlayer coupling,” explains team member Sefaattin Tongay. “Essentially, the ReS2 layers do not talk to each other.”

New 3D crystal in which to study 2D physics

In fact, the ReS2 layers behave as though they are pure monolayers, without actually being isolated monolayers,” adds team member Hasan Sahin at the University of Antwerp. “This is rather exciting since we now have a new 3D crystal in which we can study 2D physics. This means that, for such studies, we no longer need to prepare large-area quality monolayers – something that is technically very difficult to do.”

The researchers synthesised ReS2 at high temperatures using a technique called assisted vapour transport. They then characterised the monolayers formed, both as multilayers and in the bulk, using Raman, photoluminescence and photoreflectance spectroscopy, as well as optical transmission experiments.

Completely different optical spectrum

The analyses revealed that ReS2 has a completely different optical spectrum to that of the other members of the dichalcogenide family. For example, its photoluminescence intensity increases with the number of layers and the peak photoluminescence remains constant, which implies that the bandgap of the material does not vary with interlayer coupling. On the contrary, other transition metal dichalcogenides, such as MoS2, emit less light as the numbers of layers increase and their bandgap value changes drastically.

These results were backed up by detailed oriented density functional theory, vibration spectrum analyses, and electronic calculations, says Tongay, and reveal that the decoupling between the layers comes thanks to the so-called Peierls distortion of the ReS2 structure.

The Peierls distortion

“An Re atom has one extra electron compared to a Mo atom,” he explains. “This extra electron does not know quite where to go and it is eventually shared between two Re atoms. As a result, Re atoms move closer to each other forming quasi-1D chains within each layer. The Peierls distortion describes the interactions between neighbouring Re atoms, and the formation of the chains.”

Neighbouring ReS2 monolayers appear to be only very weakly coupled – in contrast to MoS2 in which they are coupled much more strongly, says Wu. “Such weak coupling in ReS2 might be useful in low-friction applications, such as ‘van der Waals heterostructures', as well in novel optoelectronics devices and solar cells.”

The team says that it is now busy working on tuning the physical properties of ReS2 in the bulk and monolayer limits by engineering defects in the material and by doping. “We are also alloying ReS2 with other members of the dichalcogenide family and looking at the hybrid properties of these ‘composites’”, he revealed.

The present work is detailed in Nature Communications doi:10.1038/ncomms4252.

Further reading

MoSe2 single layers for solar cells (Nov 2012)
Making 2D semiconductors emit more light (May 2013)
Defects help enhance photoluminescence in 2D semiconductors (Sep 2013)
Microwaves serve up nanostuctured chalcogenides and chalcogens (Mar 2009)