New in the exclusive family of single-layered two-dimensional materials that includes graphene molybdenum disulfide and silicene: phosphorene! as reported by Han Liu et al. here (not peer-reviewed so absorb with caution). Not exactly monolayer, the researchers call their invention 'few-layer phosphorene'. The raw material is black phosphorus, first synthesised in 1914 with a well-known two-dimensional structure very much like graphite. On average three phosporene layers were peeled from this material (scotch-tape!, again!) into a regular silicon layer.
A field-effect transistor based on a fleck of this new material is reported to have an on/off ratio of ten thousand which is the ratio of current passing through from source to drain when switching between two arbitrary voltages at the gate.
Update 28 January: Borophene just around the corner!