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arXiv2026

When Cubic Is Not Isotropic: Phonon-Exciton Decoupling in CuInSnS$_4$ Single Crystals

Atomic-scale disorder can create hidden optical anisotropy even in crystals that are structurally cubic on average. Here, we show that CuInSnS$_4$ single crystals host locally symmetry-broken environments arising from intrinsic In/Sn cation disorder, which affect vibrational and excitonic properties in markedly different ways. Combining polarization- and temperature-dependent Raman spectroscopy, infrared near-field microscopy, steady-state and time-resolved photoluminescence, and first-principles calculations, we find that phonons remain largely symmetry-averaged and locally homogeneous on the nanoscale. In contrast, photoluminescence reveals a lower-energy band-tail emission with pronounced polarization anisotropy following a well-defined angular symmetry, highlighting the strong sensitivity of excitonic states to local symmetry breaking. This phonon-exciton decoupling reveals that intrinsic disorder can localize excitons while preserving vibrational coherence and dielectric homogeneity, thereby opening new opportunities for polarization-sensitive light sources, anisotropic photodetectors, and exciton-based optical functionalities even in nominally cubic multinary semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Raman and IR Signatures of Mo3S4 and Mo3S13 Molybdenum Sulphide Molecular Catalysts for Solar Hydrogen Evolution

Molybdenum sulfide clusters, [Mo3S4]4+ and [Mo3S13]2-, have emerged as key molecular models for understanding active sites in Mo-S-based catalysts and as promising candidates for energy conversion applications. Despite their importance, comprehensive vibrational characterization of these clusters remains limited. Here, we present a detailed Raman and infrared spectroscopic analysis of both clusters, supported by density functional theory (DFT) calculations. High-quality crystalline samples were synthesized and characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to confirm morphology and stoichiometry. Raman spectra, acquired using 488 nm and 532 nm laser excitation, were deconvoluted using Lorentzian fitting. Vibrational mode assignments were made through direct comparison with DFT predictions. For [Mo3S4]4+, major Raman bands appear near 200 cm^-1, 350 cm^-1, and 450 cm^-1, corresponding to Mo-S-Mo bending, Mo-S stretching, and terminal sulfur vibrations. [Mo3S13]2- shows two distinct spectral regions: 100-400 cm^-1 for Mo-S and S-S bending and stretching, and 450-550 cm^-1 for terminal disulfide (S-S) stretching. Complementary IR spectra calculations reveal additional vibrational features, yielding a more complete fingerprint for each cluster. Finally, we demonstrate that Raman spectroscopy offers greater sensitivity than X-ray diffraction (XRD) in detecting these clusters on supporting materials. This work provides a detailed vibrational reference for [Mo3S4]4+ and [Mo3S13]2-, establishing Raman and IR spectroscopy as powerful tools for characterizing Mo-S molecular clusters in both fundamental and applied contexts.

cond-mat.mtrl-sci↗