Collaboration produces first-ever 2D phosphorene nanoribbons

"We've characterised the ribbons in great detail finding they are extremely flat, crystalline and unusually flexible. Most are only a single-layer of atoms thick but where the ribbon is formed of more than one layer of phosphorene, we have found seamless steps between 1-2-3-4 layers where the ribbon splits. This has not been seen before and each layer should have distinct electronic properties.”Chris Howard

Researchers have shown how to produce individual 2D phosphorene nanoribbons for the first time, a breakthrough that could lead to a step-change in integrated high-speed electronic circuitsand fast-charging battery technology. A team from University College London, the University of Bristol, Virginia Commonwealth University and École Polytechnique Fédérale de Lausanne producedribbons with remarkably uniform widths along their entire lengths, and which are extremely flexible.

As reported in Nature [Watts et al. Nature (2019) DOI: 10.1038/s41586-019-1074-x], the high-quality nanoribbons are formed by combining crystals of black phosphorus and lithium ions dissolved in liquid ammonia at –50⁰C. After 24 hours, the ammonia is removed and replaced with an organic solvent that produces a solution of nanoribbons of different sizes. It is thought that the very fast initial diffusion of lithium ions along the corrugated channels in the crystals produces ‘stripes” of the ions along these channels, while the associated high local electron doping causes bond breaking along the stripes and ribbon formation.

There have been high hopes for such narrow ribbons of phosphorene since they were isolated in 2014, and because the ribbons produced here have a greater range of widths, heights, lengths and aspect ratios than other materials, and are also tunable,this opens up potential applicationsmany areas being explored for 2D materials,from batteries and transistors, to solar cells, optoelectronics,thermoelectric devices, photocatalysis, nanoelectronics and in quantum computing. They could alsohelp the emergence of effects such as novel magnetism, spin density waves and topological statessince a number of other exotic properties have also been predicted, and as they are produced in liquids their use in volume could be relatively inexpensive.

The team assessed the dimensions of the ribbons produced in close detail through imaging hundreds of them over large areas. As study author Chris Howard points out, “By using advanced imaging methods, we've characterised the ribbons in great detail finding they are extremely flat, crystalline and unusually flexible. Most are only a single-layer of atoms thick but where the ribbon is formed of more than one layer of phosphorene, we have found seamless steps between 1-2-3-4 layers where the ribbon splits. This has not been seen before and each layer should have distinct electronic properties.”

First author Mitch Watts also said “Our process produces high-quality ribbons at a scale that could now enable measuring these properties, and testing PNRs in various applications. We hope that with our discovery, phosphorene nanoribbons becomes a field of its own, similar to graphene nanoribbons, which are studied by hundreds of groups worldwide.” The researchers now want to further establish the optical and electronic properties of the ribbons, investigate how they can be tuned for specific applications, and to continue developing scanning probe techniques to study the ribbons on surfaces.