Hierarchical Lead-Free Cs–Cu–I Multilayers Fabricated by Vacuum Molten Salt Deposition for Hydrochromic and Spectrally Multiplexed Optical Encryption

Abstract

Optical security labels are the front line of product authentication, yet mainstream photoluminescent tags emit a static, single-color signal that is easily replicated. Lead-free ternary copper halides of the Cs–Cu–I system combine nontoxicity and air stability with bright luminescence arising from self-trapped excitons. In solution processing, the lack of solvent orthogonality makes it difficult to stack CsCu2I3 and Cs3Cu2I5 layers within a single device. Here, we adapt the molten salt evaporation concept, previously developed for lead-halide perovskite photovoltaics, to the Cs–Cu–I system through a vacuum molten salt deposition strategy. Premixed CsI and CuI form a homogeneous molten source that mitigates the disparate volatilities of the two precursors and enables compositionally coupled evaporation. Phase-selective X-ray diffraction, X-ray photoelectron spectroscopy, and optical characterization confirm that the 1:2 and 3:2 sources yield the targeted one-dimensional (1D) CsCu2I3 and 0D Cs3Cu2I5 films, respectively. Because the process is solvent-free, the two phases can be deposited sequentially between transparent PDMS spacers to form an encapsulated PDMS/CsCu2I3/PDMS/Cs3Cu2I5/PDMS core, and adding a spatially patterned, unencapsulated Cs3Cu2I5 dynamic layer on top yields an integrated label that combines three independent security primitives. First, macroscopic transparency provides physical stealth, revealing nothing to the naked eye. Second, spectral gating with 325 nm excitation and a 510 nm long-pass filter unmasks the yellow CsCu2I3 pattern beneath the intense blue Cs3Cu2I5 emission. Third, moisture triggers a reversible 0D-to-1D phase conversion of the dynamic layer that executes a localized bit-flip in the optical code. A standalone unencapsulated Cs3Cu2I5 film and the integrated label remain stable for 200 days in ambient air, the dynamic layer withstands 20 hydration–dehydration cycles, and the encapsulated core withstands 20 days of water immersion. By combining solvent-free, phase-selective deposition with multistimulus optical encoding, this work provides a lead-free multilayer platform for optical encryption.