We demonstrate a nonlinear microscopy-based non-invasive technique for characterisation of PbI2 nanosheets. By using the polarisation, wavelength and thickness dependences of the harmonic emissions we can precisely determine their thickness, strain and crystalline orientation. 160.4236 160.4330 Lead iodide (PbI2) is a layered material with unique optical and electrical properties, including direct bandgap in the bulk and a layered crystalline structure, consisting of close-packed Pb atoms sandwiched between two layers of Iodine atoms . Compared to the widely-studied TMDCs, 2D PbI2 is a halide semiconductors with a larger visible bandgap (Eg ∼ 2.4 eV) which endows its distinct optical 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  and nanolasers . However, such novel applications of PbI2 nanosheets require careful characterization of their crystalline structure, thickness, strain and nonlinearity. Here we deploy a nonlinear optical microscopy as a non-invasive technique to investigate PbI2 nanosheets and fully determine their crystalline properties. To achieve such complete characterisation we analyse the polarisation, thickness and strain dependence of the second harmonic generation (SHG) and third harmonic generation (THG) from solution-grown PbI2 nanosheets. Our experimental results are also compared with numerical simulations of the conversion efficiency of the nonlinear harmonic generation with different thickness and wavelength, revealing the influence of the layered crystalline structure. Our measurements allow, for the first time, to precisely determine their thickness, crystalline orientation and strain with a non-invasive optical technique.
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