This illustration shows how edges are connected at the corners of a borophene flake. Image: Zhuhua Zhang/Rice University/Nanjing University of Aeronautics and Astronautics.
This illustration shows how edges are connected at the corners of a borophene flake. Image: Zhuhua Zhang/Rice University/Nanjing University of Aeronautics and Astronautics.

Scientists have found that silver can help the two-dimensional (2D) material borophene, an atom-thick allotrope of boron that so far can only be synthesized by molecular-beam epitaxy (MBE), to grow to unheard-of lengths.

By using a silver substrate and carefully manipulating the temperature and deposition rate, the scientists were able to grow elongated, hexagon-shaped flakes of borophene. This could facilitate the growth of ultrathin, narrow borophene ribbons.

The new work, reported in Science Advances by researchers at Rice and Northwestern universities, Argonne National Laboratory and Nanjing University of Aeronautics and Astronautics in China, will help streamline the manufacture of borophene, which shows potential for use in wearable and transparent electronics, plasmonic sensors and energy storage.

That potential has fueled efforts to make borophene easier to grow, led by Rice materials scientist Boris Yakobson, a theorist who predicted that borophene could be synthesized. He and collaborators Mark Hersam at Northwestern and Zhuhua Zhang, a Rice alumnus and now a professor at Nanjing, have demonstrated through theory and experimentation that not only are large-scale, high-quality samples of borophene possible, but they allow a qualitative understanding of their growth patterns.

Unlike the repeating atomic lattices found in graphene and hexagonal boron nitride, borophene incorporates a regular, woven-in array of ‘vacancies’, missing atoms that leave hexagonal holes among the triangular boron lattice. This not only affects the material's electronic properties but also influences how new atoms join the flake as it is being formed.

The Yakobson lab's calculations showed that the energies of atoms along the edges of a borophene flake, known as edge energies, are significantly lower than those in graphene and boron nitride, and that the conditions can be manipulated to tune the edges for optimum growth of ribbons.

Initial calculations showed borophene in equilibrium should form as a rectangle, but experiments proved otherwise. The confounding factor turned out to be in the flake's edges, which are forced by the vacancies to adopt variations of zigzag and armchair configurations. Boron atoms settle one-by-one into the ‘kinks’ that appear along the edges, but as armchairs are more energetically stable and present a higher barrier to the atoms, they prefer to join the zigzags. Rather than extending the flakes in all directions, the atoms are selective about where they settle and elongate the structure instead.

"On the atomic scale, edges don't act as though you cut the lattice with a pair of scissors," Yakobson explained. "The dangling bonds you create reconnect with their neighbors, and the edge atoms adapt slightly different, reconstructed configurations.

"So the origin of the shapes must not lie in equilibrium. They are caused by the kinetics of growth, how fast or slow the side edges advance. Opportunely, we had developed a theoretical framework for graphene, a nanoreactor model that works for other 2D materials, including boron."

Controlling the flow of atoms as well as temperature gives the researchers a simpler way to control borophene synthesis. "Silver provides a landing for boron atoms, which then diffuse along the surface to find the edges of a growing borophene flake," Zhang said. "Upon arrival, the boron atoms are lifted onto the edges by silver, but how difficult such a lift is depends on the edge's orientation. As a result, a pair of opposite zigzag edges grow very slowly while all other edges grow very fast, manifested as an elongation of the boron flake."

The researchers said the ability to grow needle-like ribbons of borophene means they could serve as atom-wide conductive wires for nanoelectronics devices. "Graphene-based electronics that have been conceived so far mostly rely on ribbon-like building blocks," Yakobson said. "Metallic boron ribbons with high conductivity will be a natural match as interconnects in circuitry."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.