  Thin layers of black phosphorus have recently raised interest for their
two-dimensional (2D) semiconducting properties, such as tunable direct bandgap
and high carrier mobilities. This lamellar crystal of P atoms stacked together
by weak van der Waals forces can be exfoliated down to the stratophosphane
monolayer (also called phosphorene) using procedures similar to those used for
graphene. Properties of this 2D material are however challenging to probe due
to a fast and ubiquitous degradation upon exposure to ambient conditions.
Herein, we investigate the crystal degradation using in-situ Raman and
transmission electron spectroscopies and highlight a process involving a
photo-induced oxidation reaction with adsorbed oxygen in water. The
experimental conditions to prepare and preserve mono-, bi- and multilayers of
stratophosphane in their pristine states were determined. Study on these 2D
layers provides new insights on the effect of confinement on the chemical
reactivity and the vibrational modes of black phosphorus.
