  We report a scalable approach to synthesize a large-area (up to 4 mm) thin
black phosphorus (BP) film on a flexible substrate. We first deposited a red
phosphorus (RP) thin-film on a flexible polyester substrate, followed by its
conversion to BP in a high-pressure multi-anvil cell at room temperature. Raman
spectroscopy and transmission electron microscopy measurements confirmed the
formation of a nano-crystalline BP thin-film with a thickness of around 40 nm.
Optical characterization indicates a bandgap of around 0.28 eV in the converted
BP, similar to the bandgap measured in exfoliated thin-films. Thin-film BP
transistors exhibit a field-effect mobility of around 0.5 cm2/Vs, which can
probably be further enhanced by the optimization of the conversion process at
elevated temperatures. Our work opens the avenue for the future demonstration
of large-scale, high quality thin-film black phosphorus.
