Sketch and Peel Focused Ion Beam Patterning of Black Phosphorus for Mid-Infrared Photonics

Black phosphorus (BP) is an important mid-infrared semiconductor, having a direct bandgap from monolayer (≈1.7 eV) to bulk (≈0.31 eV) thicknesses. The ability to nanopattern BP could enable new optoelectronic devices. However, existing nanopatterning techniques are either limited to thin flakes or produce BP with poor optical properties. Here, focused neon ion beam lithography is used to produce functional BP nanostructures. We demonstrate a “sketch and peel” method. The “sketch” step removes BP in a narrow line that outlines the desired structure. In the “peel” step, a hardened polymer droplet is used as a handle to pull off the surrounding flake. The “sketch and peel” method minimizes the damage to BP. We show that thermal treatment allows the small amount of damage that still occurs to be mitigated, as confirmed by Raman spectroscopy and mid-infrared photoluminescence. For the first time, this work demonstrates a method to nanopattern BP with very high resolution and to repair damage by producing high quality BP nanostructures. It thus could be an important enabling technology for future BP nanophotonic devices.