Effect of pre-annealing on the structural evolution and optical response of Ag films exposed to iodine vapours

Ag thin films of 5 nm thickness were deposited on glass substrates by thermal evaporation. The films were divided into two sets, out of which, one set was not annealed and the other set was subjected to pre-annealing at $$300^\circ \hbox {C}$$ for 2 h in air. The un-annealed and pre-annealed films were exposed to iodine vapours at room temperature for the durations from 5 min to 10 h. The un-annealed films were crystallized into the $${\upbeta }$$ phase of AgI after exposure for 5 h. In contrast, for the pre-annealed films, crystallization into the $${\upbeta }$$ phase occurred within the first 5 min. Both sets of films, however, exhibit a strong preferential c-axis orientation in the $${\upbeta }$$ -AgI phase. Optical absorption studies reveal that the un-annealed films exhibit a localized surface plasmon resonance (LSPR) with a peak at 545 nm and a long wavelength shoulder at 620 nm, which shifts to 516 nm after iodization for a few minutes. This peak position does not change with further iodization. The LSPR for the pre-annealed films has a single peak at 538 nm. After iodization for a few minutes, this peak shifts to 525 nm. Iodization for 3 h results in a further blue-shift of this resonance to 475 nm. The photoluminescence spectrum reveals two peaks, one at 368 nm and the other at 712 nm. The first one is assigned to the excitons of AgI, whereas the long wavelength peak is attributed to the presence of disorder in the films. The reasons for the difference in behaviour of the un-annealed and pre-annealed films are discussed.