Last Month's Most Accessed Feature: 2D UV: Making nanoscale UV detectors with 2D materials

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  • Published: Oct 3, 2017
  • Categories: Detectors
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2D UV: Making nanoscale UV detectors with 2D materialsInspired by graphene

The discovery of graphene, a one-atom-thick layer of carbon with a chicken-wire structure, in 2004 not only led to a Nobel Prize for its discoverers, but also to the uncovering of a whole host of other atom-thick materials, collectively known as two-dimensional (2D) materials. Many of these materials are made up of several layers of atoms for example, transition metal dichalcogenides (TMDs) comprise a layer of a transition metal such as tungsten sandwiched between two layers of a chalcogenide such as sulfur but they are essentially all surface, hence the term 2D.

Between them, 2D materials possess a whole host of interesting properties. Graphene is still their most famous member, with its impressive strength and electrical conductivity, but 2D materials also include semiconductors and insulators. As such, scientists are experimenting with joining different 2D materials together into stacks and combining them with thin films to produce novel nanoscale devices such as transistors and solar cells. Now, they have shown that a similar approach can also be used to produce nanoscale ultraviolet (UV) detectors.

Microcrystalline diamond

To produce their UV detector, Liwei Lin and his colleagues at the University of California, Berkeley in the US combined graphene with a thin film of microcrystalline diamond (MCD) to create a heterojunction that can absorb UV light and then generate a detectable current between two electrodes. This was no easy task, however, because in order to absorb UV light effectively the heterojunction needs to be nice and smooth. Current MCD fabrication techniques, which involve growing the MCD film on a silicon substrate, tend to produce films with fairly rough surfaces, preventing the graphene from forming a smooth junction when deposited on top.

Lin and his team have now found a fairly straightforward solution to this problem, by simply peeling the MCD film from the silicon substrate and turning it over, because the underside of the film is much smoother than the top side. This also allows them to grow a film of tungsten and gold on the top of the MCD film, before turning it over and depositing graphene onto the underside of the film, forming a smooth heterojunction.

The graphene and the tungsten-gold layer then act as the electrodes, with UV light passing through the transparent graphene to be absorbed by the heterojunction. In tests, this novel device proved to be a sensitive and selective UV detector that didn’t suffer from a great deal of background noise.

Metal phosphorus trichalcogenides

Taking a slightly different approach, a team of Chinese scientists led by Jun He at the National Center for Nanoscience and Technology in Beijing has shown that a special form of TMD can act as a sensitive UV detector all on its own. This special form of TMD is known as metal phosphorus trichalcogenides (MPTs), which comprise a layer of metal atoms sandwiched between two joint layers of phosphorus and chalcogenide atoms. Like most TMDs, MPTs are semiconductors, but they have a much wider bandgap than TMDs, potentially making them suitable as UV detectors.

To test this potential, He and his colleagues created a stack made up of sheets of an MPT comprising atoms of nickel, phosphorus and sulphur (NiPS3), with these sheets produced by a novel chemical vapor deposition technique. Rather than sandwich the stack between two thin-film electrodes, He and his colleagues attached two chromium and gold nanoelectrodes to either side of the stack, leaving its upper surface free to receive incoming UV light. They found that this stack could detect UV light with a sensitivity that exceeded some existing UV detectors and with minimal background noise.

Chemical and biological sensing is one obvious application for these nanoscale UV detectors, especially as they could easily be incorporated onto chip-based electrophoresis and chromatography systems.

Small (Article in Press): "A solar-blind UV detector based on graphene-microcrystalline diamond heterojunctions"

Advanced Functional Materials (Article in Press): "High-performance ultraviolet photodetector based on a few-layered 2D NiPS3 nanosheet"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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