Metastable phases of 2D boron sheets on Ag(1 1 1).

Two reproducible new phases of 2D boron sheets have been found on Ag(1 1 1). One of them shares the identical atomic structure of the previously reported S1 phase (β 12 sheet) but has a different rotational relationship with the substrate, and thus exhibits very different features in scanning tunneling microscopy (STM) images. The other new phase has a hexagonal symmetry and is proposed to be the long-expected α-sheet. Both of these two boron sheets are confirmed to be metallic by scanning tunneling spectroscopy.