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

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.