Lead iodide (PbI2) is a layered material with unique optical and electrical properties. Despite being extensively studied in its bulk form and being used as a precursor for perovskite materials, the recently developed PbI2 nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors, heterostructure photodetectors and nanolasers. These novel applications of PbI2 nanosheets, however, require careful characterisation of their crystalline structure. Here we experimentally and numerically demonstrate the nonlinear microscopy of PbI2 nanosheets. It is a non-invasive optical technique to precisely determine the thickness, crystalline orientation and strain.
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