The structure and potential applications of borophene Jacob Tutt Lower Sixth
Before discussing further characteristics, it is important to understand the terms used to describe the planar directions in which 2D molecules are described – the two directions which are perpendicular to each other are described as the armchair and the zigzag directions. In contrast to most 2D honeycomb materials, borophene has a highly anisotropic crystal structure in space, implying a structure in which the armchair and zigzag directions have varying arrangements of atoms and characteristics. Due to the strongly anisotropic structure, the molecule can be manipulated for ideal application by correctly orientating the molecule so there is the correct combination of zigzag and armchair characteristics.
The synthesis of graphene, at the University of Manchester in 2004, initiated huge excitement and financial investment in the nanomaterial industry. The planar, honeycomb lattice of carbon promised a remarkable and wide range of potential applications across the scientific spectrum. Graphene is a semi-metallic material with previously unparalleled mechanical, electrical, chemical, and optical properties. The material is a zero-gap semiconductor with extremely high carrier mobility and is therefore a potentially efficient transistor. Furthermore, by only absorbing 2.3% of visible light, it offered the first major steps towards a transparent conductor. Material scientists imagined a new era of “graphenebased computer processing and a lucrative graphene chip industry”.
plications, its regular honeycomb structure could not be greatly manipulated, especially to the degree that was required on the microscopic scale of nanomaterial research. The material borophene has gained recent attention and it is suggested to rival graphene in many applications. Borophene is a 2D allotropic material, with a triangular base structure. All three phases (allotropes) of borophene have recently been synthesised on single-crystal silver substrates, as the regular arrangement of silver atoms within the crystal force boron atoms into a regular pattern of hexagons with bonding to a maximum of 6 boron atoms each. However, under certain conditions it is common for boron atoms to form only 4 or 5 B-B bonds, creating vacancies in the structures. This is one of the two properties that make borophene so promising. Its ability to form regular vacancy patterns within a structure, which create subtle changes to characteristics, allows it to be strategically tuned for the desired properties.
Borophene is a 2D allotropic material, with a triangular base structure.
This visionary graphene-based world is yet to materialise on the scale intended by material scientists 16 years ago. However, it accelerated the research into the fabrication of other 2D materials. One of the major limitations of graphene was, despite its incredibly wide range of conceivable ap-
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It is important, before looking at the uses of each structure, to show the 2D arrangement of atoms within the 3 major allotropes/ phases (Striped, B12, and X3). The ‘base structure’, known as the stripped phase, has the densest arrangements of atoms, in which bonds travelling in the armchair direction are 1.6 Angstrom (Å) and those travelling in the zigzag direction are 2.8 Å. It is shown to have a ‘planar’ structure with anisotropic corrugation, with ‘out of plane’ buckling along the zigzag direction, and an un-corrugated structure in the armchair direction. This buckling along the zigzag direction is caused by the arrangements of atomic orbitals in this direction, as they have a majority of SP3 hybridisation rather than less stable SP2 hybridisation. This can be proven by the accumulation of electrons on the top of the upper plane and the bottom of the lower plane. Although we can see the dissociation of electrons from the bond along the zigzag direction, the electrons along the armchair direction are localised near strongly covalent bonds. The two other phases of borophene (B-12 and X-3) are planar without vertical undulations. These structures can also be referred to by their boron vacancy concentration, which is defined as the ‘ratio between the number of hexagon boron vacancies and the number of atoms in the original triangular sheet’, with B-12 borophene having a vacancy concentration of 1/6. Finally, it is important to appreciate the 3 major allotropes discussed are only those which have been physically synthesised, and many more allotropes have been theoretically predicted.
