Sensitive thermochromic behavior of InSeI, a highly anisotropic and tubular 1D van der Waals Crystal

Thermochromism, the change in color of a material with temperature, is the fundamental basis of optical thermometry. A longstanding challenge in realizing sensitive optical thermometers for widespread use is identifying materials with pronounced thermometric optical performance in the visible range. To this end, we demonstrate that single crystals of indium selenium iodide (InSeI), a 1D van der Waals (vdW) solid consisting of weakly-bound helical chains, exhibit considerable visible range thermochromism. We show a strong temperature-dependent optical band edge absorption ranging from 450 to 530 nm (2.8 to 2.3 eV) over a 380 K temperature range with an experimental (dEg/dT)max value extracted to be 1.26 × 10-3 eV K-1. This value appreciably lies above most dense conventional semiconductors in the visible range and is comparable to soft lattice solids. We further sought to understand the origin of this unusually sensitive thermochromic behavior and found that it arises from strong electron-phonon interactions and anharmonic phonons that significantly broaden band edges and lower the Eg with increasing temperature. Our identification of structural signatures resulting to sensitive thermochromism in exfoliable 1D vdW crystals opens avenues in discovering low-dimensional solids with strong temperature-dependent optical response across broad spectral windows, dimensionalities, and nanoscale size regimes.