Optimized Synthesis and Characterization of Janus RhSeCl with Uniform Anionic Valences, Nonlinear Optical and Optoelectronic Properties

Abstract The symmetry‐breaking nature of Janus materials enables the design of multifunctional compounds with distinct properties that are inaccessible to traditional materials. However, the limited availability of intrinsically stable Janus materials hinders a complete understanding of their full potential. Here, the first millimeter‐sized Janus material, RhSeCl, is successfully synthesized through the precisely controlled chemical vapor transport (CVT) method. Single‐crystal X‐ray diffraction and high‐resolution transmission electron microscopy analyses reveal the Janus character of RhSeCl, emphasizing its strong correlation with steric hindrance. X‐ray absorption near‐edge structure (XANES) analyses demonstrate the highly unusual oxidation states of Rh and Se, underlining their critical role in determining the formation of the inherent Janus structure. Interestingly, a clear second‐harmonic generation (SHG) is observed in RhSeCl, weakening with the temperature. DFT calculations attribut the strong SHG response to the band nesting effect, with an intensity modulated by the temperature‐dependent interlayer coupling. Notably, its damage threshold surpassed that of Janus MoSSe. Furthermore, devices based on RhSeCl exhibit promising optoelectronic performance at the visible wavelength range of 405–650 nm, showing a great opportunity for solar applications. These findings deepen the understanding of inherent Janus structures, paving the way for the development of new Janus compounds with versatile functionalities in advanced materials.