  The ability to uniquely identify an object or device is important for
authentication. Imperfections, locked into structures during fabrication, can
be used to provide a fingerprint that is challenging to reproduce. In this
paper, we propose a simple optical technique to read unique information from
nanometer-scale defects in 2D materials. Flaws created during crystal growth or
fabrication lead to spatial variations in the bandgap of 2D materials that can
be characterized through photoluminescence measurements. We show a simple setup
involving an angle-adjustable transmission filter, simple optics and a CCD
camera can capture spatially-dependent photoluminescence to produce complex
maps of unique information from 2D monolayers. Atomic force microscopy is used
to verify the origin of the optical signature measured, demonstrating that it
results from nanometer-scale imperfections. This solution to optical
identification with 2D materials could be employed as a robust security measure
to prevent counterfeiting.
