Borophene – a new, exciting 2D material

Borophene – a new, exciting 2D material

 

Not so long ago, graphene was proclaimed a new wonder-material thanks to its outstanding mechanical, chemical, and electronic properties. While mass-production of graphene-based devices is yet to come true, the extensive research within one-atom-thick materials has helped to discover a few other “flatlands”: silicene, germanene and borophene (one-atom-thick sheets of silicon, germanium and boron). Borophene is particularly interesting because it is believed to be even stronger than graphene while maintaining many of its outstanding properties: excellent heat and electricity conductivity.

However, heat and electricity conductivity are not only the strengths making borophene so important to study. Unlike carbon atoms in graphene, which must be bound to four neighbours, boron atoms in 2D sheets of borophene may be bound to three, four, five or six neighbours, which creates a periodic motif of vacancies or hexagonal (by a number of neighbours) holes in the flat sheet of boron. It is the number of these hexagonal holes and the motif of their positions in the lattice that mostly define the properties of a specific borophene.

The first experimentally observed borophene was grown on silver substrate a few years ago. Since then, several other metals have been shown to be suitable for hosting 2D boron layers, amongst them, gold, aluminium and copper. The question, however, remained whether these borophenes would be stable and not crumble substantially if detached from their supporting metallic substrates.

In a research project recently initiated and led by a MAX IV team, a novel borophene has been discovered that is possible to grow on iridium substrates in a broad range of experimental conditions. According to theoretical calculations, this material may be suitable for catalytic or electrochemical applications. Moreover, by inserting atoms of gold between this borophene and its substrates, the scientists have demonstrated the stability of this novel structure itself. More results to come!

The experimental work has been conducted at the Scanning Probe Microscopy lab at MAX IV, an auxiliary lab opened for general users to help them characterize their samples using STM (now) and AFM (planned).

a) Theoretically predicted STM image (greyscale) of borophene/Ir based on the refined atomic positions versus experimental data (orange colour map). Ball-and-stick model of χ6-borophene (green) is overlaid in the centre; b) Unit cell of χ6-borophene (borophene on iridium) coloured according to theoretically predicted Bader charge of B atoms; c) The stability of the freestanding χ6-borophene is tested by lifting it off Ir substrate via gold intercalation. The inset in the top-right corner is the height profile along the red line.

Artificial two-dimensional (2D) materials, which host electronic or spatial structure and properties not typical for their bulk allotropes, can be grown epitaxially on atomically flat surfaces; the design and investigation of these materials are thus at the forefront of current research. In this paper, the research team report the formation of borophene, a planar boron allotrope, on the surface of Ir(111) by exposing it to the flux of elemental boron and consequent annealing.

By means of scanning tunnelling microscopy and density functional theory calculations, the research team reveal the complex structure of this borophene, different from all planar boron allotropes reported earlier. This structure forms as a single phase on iridium substrate in a wide range of experimental conditions and may then be decoupled from the substrate via intercalation. These findings allow for the production of large, defect-free borophene sheets and advance theoretical understanding of polymorphism in borophene.

Read the article here
N. A. Vinogradov, A. Lyalin, T. Taketsugu, A. S. Vinogradov, and A. B. Preobrajenski
ACS Nano 2019, 13, 12, 14511-14518. DOI: 10.1021/acsnano.9b08296


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