Conducting Boron Sheets Formed by the Reconstruction of the $\alpha$-Boron (111) Surface

Systematic ab initio structure prediction was applied for the first time to predict low energy surface reconstructions by employing the minima hopping method on the $\alpha$-boron (111) surface. Novel reconstruction geometries were identified and carefully characterized in terms of structural and electronic properties. Our calculations predict the formation of a planar, monolayer sheet at the surface, which is responsible for conductive surface states. Furthermore, the isolated boron sheet is shown to be the ground state 2D structure in vacuum at a hole density of $\eta =1/5$ and is therefore a potential candidate as a precursor for boron nanostructures.

Figure 1

View from the $⟨11\overline{2}⟩$ (a) and the $⟨1\overline{1}0⟩$ (b) direction of $(111)\mathrm{\text{−}}{\mathrm{I}}_{R,(h)}$. (c) Top view toward the surface from the $⟨\overline{1}\overline{1}\overline{1}⟩$ direction. The colors red (light gray), green (medium gray), and blue (dark gray) denote the first, second, and third atomic layer, respectively. An isosurface of the ELF is shown for the top two layers at the bottom, and the STM image is overlayed at the top.

Figure 2

The second atomic layer of the nine lowest energy surface structures $(111)\mathrm{\text{−}}{\mathrm{I}}_{R,(\alpha )}$, $\alpha =a,b,\dots$, are shown.

Figure 3

The electronic surface band structures and the density of states of the (111)-I surface are shown. Orange lines (shaded) indicate bulk bands, whereas the red (solid) lines represent the surface states of the reconstructed surface. The green (dashed) lines represent the surface bands of the unreconstructed surface of $\alpha \mathrm{\text{−}}\mathrm{B}$. The energy zero is set at the Fermi energy of the bulk.

Figure 4

Structural transformation of the topmost atomic layer of the reconstructed surface during a local geometry optimization. (a) Initial structure, (b) intermediate during relaxation, and (c) final structure, relaxed into the global minimum of 2D sheets at $\eta =1/5$ hole density.