LAUSR

Silver phenylselenolate (AgSePh), known as mithrene, is a 2D metal‐organic chalcogenolates (MOC) semiconductor with a wide direct bandgap, narrow blue emission, and in‐plane anisotropy. However, its application in next‐generation optoelectronics is limited by crystal size and orientation, as well as challenges in large‐area growth. Here, a controlled tarnishing step is introduced on the silver surface prior to the solid‐vapor‐phase chemical transformation into AgSePh thin films. Mithrene thin films are prepared through thermally assisted conversion (TAC) at 100 °C, incorporating a water (H2O) vapor pulse treatment and propylamine (PrNH2) as an organic ligand. The AgSePh thin films are characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and grazing incidence wide‐angle X‐ray scattering (GIWAXS). The pre‐tarnishing process, combined with organic ligand, results in large crystals exceeding 1 µm and improves homogeneous in‐plane orientation, while also enabling the selective, wafer‐scale synthesis of mithrene on 100 mm wafers. Furthermore, the films are integrated on planar graphene field‐effect phototransistors (GFETs) and demonstrated photoresponsivity beyond 100 A W−1 at 450 nm, highlighting mithrene's potential for blue light‐detection applications.