Researchers in the US have succeeded in growing single atomic layers of the naturally occurring mineral tungstenite for the first time. The sheets appear to have unusual photoluminescence properties that might be exploited in optics devices like lasers and light-emitting diodes.
2D materials 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 the most famous of 2D materials, graphene, but the fact that this material lacks a direct electronic band gap means that scientists are now starting to look at other 2D candidates too.
A team led by Mauricio Terrones and Vincent Crespi of Penn State university in the US grew monolayers of tungstenite (WS2) by depositing tiny crystals of tungsten oxide less than a nanometre tall and then passing these crystals though sulphur vapour at high temperatures of 850 °C. The result – monolayers of tungsten disulphide arranged in a honeycombed pattern of triangles comprising tungsten atoms bonded to sulphur atoms.
“We were astonished that we could grow such perfect, atomically thick triangle shapes using a chemical vapour deposition method,” Terrones told physicsworld.com. “Moreover, and again to our surprise, we observed that these triangles glow quite strongly at their edges rather than at their centres – a peripheral photoluminescence effect that we never expected and which has not been reported on before.”
Photoluminescence occurs when charge carriers (electrons and holes) recombine in a structure to emit light of a different wavelength from that used to initially excite the material. Normally, light emission is a delicate thing, explained Crespi, and structural defects – like edges – prevent light emission as they tend to give excited electrons and holes ways of recombining without emitting light. “We saw just the opposite effect,” he said, “in that the structural defects created close to the edges of a triangle seem to be the favoured place for emitting light.”
Direct band gap
2D systems are intrinsically different from their bulk 3D counterparts, and WS2 is no exception. While the bulk material is an indirect band gap semiconductor, the single-layer material boasts a direct band gap. Direct band gaps are important in semiconductors because they allow devices made from these materials to emit light efficiently – as in this case.
According to the Penn State team, the WS2 triangles might find applications in optoelectronics. “They might even come in handy as biomarkers or in drug delivery, but much more research still needs to be carried out before we can say this with any certainty,” added Terrones. “They could also be useful in a new generation of planar, 2D optoelectronic devices, such as light-emitting diodes – where we control the propagation of light in thin film layers of material – and even in laser technology.”
The researchers now plan to grow other 2D materials that have different optical and electronic properties. Some examples in the pipeline include MoSe2, NbS2 and WSe2, revealed Crespi. “We would also like to better understand and control the light emission from 2D materials in general, and try our hand at sculpting the triangles into multicomponent devices.”
The work is detailed in Nano Letters.