  Raman scattering and photoluminescence spectroscopy are used to investigate
the optical properties of single layer black phosphorus obtained by mechanical
exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy,
performed in situ on the same flake as the photoluminescence measurements,
demonstrates the single layer character of the investigated samples. The
emission spectra, dominated by excitonic effects, display the expected in plane
anisotropy. The emission energy depends on the type of substrate on which the
flake is placed due to the different dielectric screening. Finally, the blue
shift of the emission with increasing temperature is well described using a two
oscillator model for the temperature dependence of the band gap.
