  Ultrathin black phosphorus, or phosphorene, is the second known elementary
two-dimensional material that can be exfoliated from a bulk van der Waals
crystal. Unlike graphene it is a semiconductor with a sizeable band gap and its
excellent electronic properties make it attractive for applications in
transistor, logic, and optoelectronic devices. However, it is also the first
widely investigated two dimensional electronic material to undergo degradation
upon exposure to ambient air. Therefore a passivation method is required to
study the intrinsic material properties, understand how oxidation affects the
physical transport properties and to enable future application of phosphorene.
Here we demonstrate that atomically thin graphene and hexagonal boron nitride
crystals can be used for passivation of ultrathin black phosphorus. We report
that few-layer pristine black phosphorus channels passivated in an inert gas
environment, without any prior exposure to air, exhibit greatly improved n-type
charge transport resulting in symmetric electron and hole trans-conductance
characteristics. We attribute these results to the formation of oxygen acceptor
states in air-exposed samples which drastically perturb the band structure in
comparison to the pristine passivated black phosphorus.
