High-mobility anisotropic transport in few-layer γ-B₂₈ films

Recent reports of successful synthesis of atomically thin boron films have raised great prospects of discovering novel electronic and transport properties in a new type of 2D materials. Here we show by first-principles calculations that monolayer and bilayer γ-B₂₈ films are intrinsically metallic while the thicker films possess intriguing electronic states that exhibit moderate to large bandgaps in all the interior layers but are nearly gapless at the surface. Remarkably, these surface electronic states are tunable by strain, allowing the outermost layer to transition between a semimetal and a narrow-gap semiconductor. Moreover, these surface states almost exclusively occupy a wide energy range around the Fermi level, thus dominating the electronic transport in γ-B₂₈ films. The dispersions of the surface electronic bands are direction sensitive, and with hole injection producing anisotropic and very high carrier mobility up to 104 cm² V^-1 s^-1. Surprisingly, surface passivation can open a sizable bandgap, which offers an additional avenue for effective band engineering and explains the experimental observation of a large bandgap in the synthesized film. These results make few-layer γ-B₂₈ films desirable candidate materials for catalysis and electronics applications.