Atomic structure and optical properties of color centers of silver in AgAu/glass irradiated by UV laser

A study of the structural states of silver in glass after Ag+↔Na+ ion-exchange and further irradiation with different numbers of UV laser pulses has been performed using Ag K-edge XANES combined with DFT simulations and the processing of Ag K-EXAFS. Theoretical Ag K-edge XANES, calculated for DFT-generated structural models, showed high sensitivity to the number of nearest oxygen neighbors of the absorbing Ag+ ion, to changes in the corresponding interatomic distances RAg-O (changes ≳ 0.05 Å) and bond angles (changes ≳ 10° at apical Ag in atomic chain O–Ag–O), and to the presence of Ag+ ions among the second neighbors of Ag. The effect of more distant O and Si neighbors can be considered negligible. The applied combination of complementary techniques for structural analysis of the color centers of silver in glass has revealed that, after ion-exchange, Ag+ ions are predominantly present in glass in the form of dimers, in which the two Ag+ ions are separated by an interatomic distance, RAg-Ag, of 3.4 Å. Other interatomic distances RAg-O in the dimers have also been obtained. It has been determined that, under laser irradiation with the number of pulses up to 1000, only a small proportion (up to 10%) of Ag+ ions in glass samples is reduced from the states of isolated ions and the revealed Ag dimers present after Ag+↔Na+ ion-exchange. The reduced Ag atoms form monometallic Ag NPs and are also deposited on the surface of Au NPs, forming bimetallic AgAu NPs with a solid solution structure. The processes of the formation, growth, and dissolution of monometallic Ag NPs and bimetallic AgAu NPs with increasing number of laser pulses are discussed. Analysis of the blue shifts of ca. 20 nm of localized surface plasmon resonances (LSPRs) of Ag NPs with increasing number of UV laser pulses up to 1000 revealed that these shifts occurred due to a decrease in the average size of optically visible Ag NPs caused by the growth of initially less visible small Ag NPs (≲5 nm) and partial dissolution of the largest particles.