  Phosphorene is a new family member of two-dimensional materials. We observed
strong and highly layer-dependent photoluminescence in few-layer phosphorene (2
to 5 layers). The results confirmed the theoretical prediction that few-layer
phosphorene has a direct and layer-sensitive band gap. We also demonstrated
that few-layer phosphorene is more sensitive to temperature modulation than
graphene and MoS2 in Raman scattering. The anisotropic Raman response in
few-layer phosphorene has enabled us to use an optical method to quickly
determine the crystalline orientation without tunneling electron microscope
(TEM) or scanning tunneling microscope (STM). Our results provide much needed
experimental information about the band structures and exciton nature in
few-layer phosphorene.
