Wonder materials: 2D phosphorene nanoribbons and 2D borophene get a closer look

Graphene can come from graphite. But borophene? There's no such thing as borite.

Unlike its carbon cousin, two-dimensional borophene can't be reduced from a larger natural form. Bulk boron is usually only found in combination with other elements, and is certainly not layered, so borophene has to be made from the atoms up. Even then, the borophene you get may not be what you need.

For that reason, researchers at Rice and Northwestern universities have developed a method to view 2D borophene crystals, which can have many lattice configurations - called polymorphs - that in turn determine their characteristics.

Knowing how to achieve specific polymorphs could help manufacturers incorporate borophene with desirable electronic, thermal, optical and other physical properties into products.

Boris Yakobson, a materials physicist at Rice's Brown School of Engineering, and materials scientist Mark Hersam of Northwestern led a team that not only discovered how to see the nanoscale structures of borophene lattices but also built theoretical models that helped characterize the crystalline forms.

Their results are published in Nature Communications.

Borophene remains hard to make in even small quantities. If and when it can be scaled up, manufacturers will likely want to fine-tune it for applications. What the Rice and Northwestern teams learned will help in that regard.

Graphene takes a single form - an array of hexagons, like chicken wire - but perfect borophene is a grid of triangles. However, borophene is a polymorph, a material that can have more than one crystal structure. Vacancies that leave patterns of "hollow hexagons" in a borophene lattice determine its physical and electrical properties.

Yakobson said there could theoretically be more than 1,000 forms of borophene, each with unique characteristics.

"It has many possible patterns and networks of atoms being connected in the lattice," he said.

The project started at Hersam's Northwestern lab, where researchers modified the blunt tip of an atomic force microscope with a sharp tip of carbon and oxygen atoms. That gave them the ability to scan a flake of borophene to sense electrons that correspond to covalent bonds between boron atoms. They used a similarly modified scanning tunneling microscope to find hollow hexagons where a boron atom had gone missing.

Scanning flakes grown on silver substrates under various temperatures via molecular-beam epitaxy showed them a range of crystal structures, as the changing growth conditions altered the lattice.

"Modern microscopy is very sophisticated, but the result is, unfortunately, that the image you get is generally difficult to interpret," Yakobson said. "That is, it's hard to say an image corresponds to a particular atomic lattice. It's far from obvious, but that's where theory and simulations come in."

Yakobson's team used first-principle simulations to determine why borophene took on particular structures based on calculating the interacting energies of both boron and substrate atoms. Their models matched many of the borophene images produced at Northwestern.

"We learned from the simulations that the degree of charge transfer from the metal substrate into borophene is important," he said. "How much of this is happening, from nothing to a lot, can make a difference."

The researchers confirmed through their analysis that borophene is also not an epitaxial film. In other words, the atomic arrangement of the substrate doesn't dictate the arrangement or rotational angle of borophene.

The team produced a phase diagram that lays out how borophene is likely to form under certain temperatures and on a variety of substrates, and noted their microscopy advances will be valuable for finding the atomic structures of emerging 2D materials.

Looking to the future, Hersam said, "The development of methods to characterize and control the atomic structure of borophene is an important step toward realizing the many proposed applications of this material, which range from flexible electronics to emerging topics in quantum information sciences."

Research paper

University College London
Wonder material: Individual 2D phosphorene nanoribbons made for the first time
Tiny, individual, flexible ribbons of crystalline phosphorus have been made by UCL researchers in a world first, and they could revolutionise electronics and fast-charging battery technology.

Since the isolation of 2-dimensional phosphorene, which is the phosphorus equivalent of graphene, in 2014, more than 100 theoretical studies have predicted that new and exciting properties could emerge by producing narrow 'ribbons' of this material. These properties could be extremely valuable to a range of industries.

In a study published in Nature, researchers from UCL, the University of Bristol, Virginia Commonwealth and University and Ecole Polytechnique Federale de Lausanne, describe how they formed quantities of high-quality ribbons of phosphorene from crystals of black phosphorous and lithium ions.

"It's the first time that individual phosphorene nanoribbons have been made. Exciting properties have been predicted and applications where phosphorene nanoribbons could play a transformative role are very wide-reaching," said study author, Dr Chris Howard (UCL Physics and Astronomy).

The ribbons form with a typical height of one atomic layer, widths of 4-50 nm and are up to 75 um long. This aspect ratio is comparable to that of the cables spanning the Golden Gate Bridge's two towers.

"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," explained first author, Mitch Watts (UCL Physics and Astronomy).

While nanoribbons have been made from several materials such as graphene, the phosphorene nanoribbons produced here have a greater range of widths, heights, lengths and aspect ratios. Moreover, they can be produced at scale in a liquid that could then be used to apply them in volume at low cost for applications.

The team say that the predicted application areas include batteries, solar cells, thermoelectric devices for converting waste heat to electricity, photocatalysis, nanoelectronics and in quantum computing. What's more, the emergence of exotic effects including novel magnetism, spin density waves and topological states have also been predicted.

The nanoribbons are formed by mixing black phosphorus with lithium ions dissolved in liquid ammonia at -50 degrees C. After twenty-four hours, the ammonia is removed and replaced with an organic solvent which makes a solution of nanoribbons of mixed sizes.

"We were trying to make sheets of phosphorene so were very surprised to discover we'd made ribbons. For nanoribbons to have well defined properties, their widths must be uniform along their entire length, and we found this was exactly the case for our ribbons," said Dr Howard.

"At the same time as discovering the ribbons, our own tools for characterising their morphologies were rapidly evolving. The high-speed atomic force microscope that we built at the University of Bristol has the unique capabilities to map the nanoscale features of the ribbons over their macroscopic lengths," explained co-author Dr Loren Picco (VCU Physics).

"We could also assess the range of lengths, widths and thicknesses produced in great detail by imaging many hundreds of ribbons over large areas."

While continuing to study the fundamental properties of the nanoribbons, the team intends to also explore their use in energy storage, electronic transport and thermoelectric devices through new global collaborations and by working with expert teams across UCL.

Research paper

Related Links
Rice University
Space Technology News - Applications and Research

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