  Phosphorene, a single atomic layer of black phosphorus, has recently emerged
as a new twodimensional (2D) material that holds promise for electronic and
photonic technology. Here we experimentally demonstrate that the electronic
structure of few-layer phosphorene varies significantly with the number of
layers, in good agreement with theoretical predictions. The interband optical
transitions cover a wide, technologically important spectrum range from visible
to mid-infrared. In addition, we observe strong photoluminescence in few-layer
phosphorene at energies that match well with the absorption edge, indicating
they are direct bandgap semiconductors. The strongly layer-dependent electronic
structure of phosphorene, in combination with its high electrical mobility,
gives it distinct advantages over other twodimensional materials in electronic
and opto-electronic applications.
