Elettra | MCX Crystalline modification in AgI thin films loaded with Ag nanoparticles by laser beam irradiation Silver-silver iodide (Ag-AgI) films are photosensitive materials in the visible light region. The photosensitivity of Ag-AgI films can be explained by the electron transfer at silver and AgI interface, which is schematically shown in Fig. 1. Ag-AgI films also present antibacterial and photocatalytic activities. The size variation of silver nanoparticles on the AgI thin films results in the colour changes in the sample after laser irradiation (Fig. 1 b). This colour variation depends on the wavelength of the laser beam. Laser irradiation is employed not only for the colouration of Ag-AgI films but also for promoting the crystallinity of Ag and AgI in these films. Ag–AgI films combine the plasmonic behaviour of silver nanoparticles with the semiconductor properties of AgI, allowing for enhanced light absorption and charge transfer under visible light. In this work, two types of AgI films (approximately 20 nm and 60 nm thick) were fabricated via silver layer iodization and then coated with a 10 nm silver layer. These samples were irradiated using continuous-wave lasers of red (632.8 nm), green (532 nm), and blue (450 nm) light. Structural and optical changes before and after irradiation were analyzed using synchrotron X-Ray Diffraction (XRD), UV-Vis spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). The optical absorption spectra revealed Localized Surface Plasmon Resonance (LSPR) peaks around 550 nm due to the presence of AgNPs. Upon laser irradiation, colour changes were observed in the films, such as green, red, orange, or yellow, depending on the laser wavelength and film thickness (Figs. 1 b, c). These changes were linked to modifications in the LSPR spectrum caused by silver nanoparticles growth and aggregation, as confirmed by SEM imaging and UV-Vis analysis. The white spots in the SEM images are the silver nanoparticles. The appearance of larger silver nanoparticles on the surface after laser irradiation is evident (Fig. 1 d). Structurally, the average crystallite sizes of Ag and AgI grains have been calculated from the XRD patterns of Ag(10 nm)–AgI(20 nm) and Ag(10 nm)- AgI(60 nm) films before and after laser irradiation, represented respectively in Figs. 1 (e, f), by using the DebyeScherrer equation. The results showed an increase in the crystallinity of both Ag and AgI phases after laser exposure. In thinner AgI films, silver crystallites grew more significantly under green and blue lasers, while in thicker films, AgI crystal growth was more pronounced. These effects were explained by photoinduced charge transfer across the Schottky junction at the Ag–AgI interface, which promotes both nanoparticle growth and crystalline ordering. XPS results further confirmed an increase in the metallic silver (Ag⁰) component after laser exposure, especially under green and blue light. Moreover, the iodine species on the surface exhibited signs of partial oxidation in thinner films exposed to high-energy light, forming iodate and other oxygenated compounds. The mechanisms involve hot electron generation from LSPR, silver ion reduction, nanoparticle aggregation, and crystal reorganization. Notably, the laser wavelength relative to the bandgap energy of the AgI film determines whether photocarrier generation or direct electron transfer dominates the process. The Ag-AgI films also demonstrated promising antibacterial activity against Staphylococcus aureus, with inhibition zones dependent on irradiation conditions. Samples irradiated with green and blue lasers showed enhanced antibacterial effects, likely due to increased silver nanoparticles presence and optimized charge dynamics. Furthermore, the films displayed photocatalytic activity, successfully degrading methylene blue dye under UV light, achieving around 95% efficiency in 2.5 hours. These findings show that Ag–AgI films can be optically and structurally tuned using low-power lasers, with potential applications in colour printing, biosensing, antibacterial coatings, and photocatalytic water purification. The combination of plasmonic and semiconductor features, together with controllable laser induced modifications, positions these hybrid films as multifunctional materials for sustainable and light responsive technologies. Elettra Highlights 2024–2025 128
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