Anisotropic photodetection in monoclinic SnP2S6 single crystals

Anisotropic crystals possess unique direction-dependent physical properties, making them potentially promising for optoelectronics, nonlinear optics, and photodetection applications. Metal thiophosphates (MTPs), with their layered structures and tunable electronic properties, are of particular interest. However, the link between crystal symmetry, stacking order, and anisotropic optical behavior remains inadequately understood in MTPs. Herein, we synthesized high-quality, large-sized monoclinic phase of SnP2S6 single crystals via the chemical vapor transport method. Structural analysis revealed that the monoclinic phase is featured by a unique bilayer stacking configuration with a 180° in-plane rotation of adjacent metal sites. This stacking arrangement disrupts the threefold rotational symmetry of the monolayer, resulting in in-plane structural anisotropy as revealed by polarized Raman spectroscopy and consequently optical birefringence. To showcase its application potential, photodetectors based on monoclinic SnP2S6 were fabricated, exhibiting exceptional polarization-sensitive performance under 405 nm excitation. This work highlights the importance of interlayer stacking patterns on the structural and optical properties of van der Waals MTPs, delivering a unique strategy for property tailoring.