Nanofilm Allotrope and Phase Transformation of Ultrathin Bi Film on Si(111)-7×7

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}7\times 7$ surface. This pseudocubic $\{012\}$-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) phase, as the bulk contribution in the cohesion becomes dominant. Based on ab initio calculations, we propose that the new allotrope consists of black phosphorus-like puckered layers stabilized by saturating all the $p_z$ dangling bonds in the film.