Room-Temperature Solid-State Synthesis of CsCuI Thin Films and Formation Mechanism for Its Unique Local Structure.

Blue-emitting CsCuI has attracted attention owing to its near-unity PL quantum yield and applications in DUV photodetectors and scintillators. Its PL properties originate from the unique local structure around the luminescent center, the [CuI] polyhedron iodocuprate anion consisting of the edge-shared CuI triangle and the CuI tetrahedron dimer, which is isolated by Cs ions. We found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form CsCuI and/or CsCuI phases. High-quality thin films of these phases were obtained by the sequential deposition of CuI and CsI by thermal evaporation. We elucidated that the formation of interstitial Cu and the antisite of I at the Cs site in the CsI crystal through Cu and I diffusion results in the RT synthesis of CsCuI. The unique structure formation of the luminescent center was revealed using a model based on the low packing density of the CsCl-type crystal structure, similar sizes of Cs and I ions, and the high diffusivity of Cu. The self-aligned patterning of the luminous regions on thin films was demonstrated.