  Many layered materials can be cleaved down to individual atomic planes,
similar to graphene, but only a small minority of them are stable under ambient
conditions. The rest reacts and decomposes in air, which has severely hindered
their investigation and possible uses. Here we introduce a remedial approach
based on cleavage, transfer, alignment and encapsulation of air-sensitive
crystals, all inside a controlled inert atmosphere. To illustrate the
technology, we choose two archetypal two-dimensional crystals unstable in air:
black phosphorus and niobium diselenide. Our field-effect devices made from
their monolayers are conductive and fully stable under ambient conditions, in
contrast to the counterparts processed in air. NbSe2 remains superconducting
down to the monolayer thickness. Starting with a trilayer, phosphorene devices
reach sufficiently high mobilities to exhibit Landau quantization. The approach
offers a venue to significantly expand the range of experimentally accessible
two-dimensional crystals and their heterostructures.
