Polymorphs of the 2D Layered Semiconducting Gallium Monosulfide (GaS) and Antimony Trisulfide (Sb₂S₃)

Layered van der Waals (vdW) materials are garnering increasing attention due to their unique optical, electronic, and mechanical properties, paving the way for novel applications across various fields such as optoelectronics, photonics, and catalysis. Among these materials, layered post-transition metal chalcogenides (PTMCs) like gallium monosulfide (GaS) and antimony trisulfide (Sb₂S₃) are emerging as low-loss phase change materials (PCMs). GaS and Sb₂S₃ with optical band gap varying from 1.7 to 2.5 eV to 3.2 eV depending on its dimensionality and phase, fill the gap between semiconducting TMDs and two-dimensional insulators, enabling new optoelectronic devices. For GaS, the stable 2H phase is well-documented. We report on discovery of the rhombohedral 3R phase and on the successful growth of 3R GaS thin films using chemical vapor deposition (CVD) and present a comprehensive study of their structural, phononic, and electronic properties. Experimental techniques such as photoluminescence, Raman spectroscopy, and X-ray diffraction, combined with density functional theory (DFT) calculations. Unlike the indirect band gap of the 2H phase, the R-3m GaS exhibits a direct band gap of 2.55 eV, accompanied by near-blue light emission at room temperature.<br/>Similarly, we report on the discovery of Type-I and Type-II spherulitic crystalline phases of Sb₂S₃, whose appearance is related to the crystallization temperature, with different stability and photonic properties. Guidelines to use one or the other phase in photonic devices will de provided.