The structure of radiative tunnel recombination sites in emulsion microcrystals of AgBr(I)

To identify the structure of emissive tunnel recombination sites in the emulsion microcrystals of silver bromide AgBr(I) with iodine contaminations and to determine the role of an emulsion medium in their formation, the temperature dependence of the luminescence spectra in the range from 77 to 120 K, the kinetics of the growth of the maximum luminescence intensity value at $\lambda$ $\approx$ 560 nm, and the luminescence flash spectrum stimulated by the infrared light are investigated. Two types of the AgBr$_{1-x}$(I$_{x}$) ($x$ = 0.03) microcrystals–namely, obtained in an aqueous solution and on a gelatin substrate–are used in the studies. It is established that the emissive tunnel recombination sites with a luminescence maximum at $\lambda$ $\approx$ 560 nm in AgBr$_{1-x}$(I$_{x}$) ($x$ = 0.03) are the $\{$(I$_{a}^{-}$I$_{a}^{-}$)Ag$_{i}^{+}\}$ donor–acceptor complexes with the I$_a^-$ iodine ions located in neighbor anionic sites of the AgBr(I) crystal lattice, next to which the Ag$_i^+$ interstitial silver ion is positioned. With an increase in the temperature, the $\{$(I$_{a}^{-}$I$_{a}^{-}$)Ag$_{i}^{+}\}$ sites undergo structural transformation into the $\{$(I$_{a}^{-}$I$_{a}^{-}$)Ag$_{i}^{+}\}$ sites, where $n$ = 2, 3, $\dots$. Moreover, the $\{$(I$_{a}^{-}$I$_{a}^{-}$)Ag$_{in}^{+}\}$ sites ($n$ = 2) after the capture of an electron and hole also provide the tunnel recombination with a luminescence maximum at $\lambda$ $\approx$ 720 nm. The influence of an emulsion medium consists in that gelatin interacts with the surface electron-localization sites, i.e., the interstitial silver ions Ag$_{in}^{+}$, $n$ = 1, 2, and forms the complexes $\{$Ag$_{in}^{0}$G$^{+}\}$ ($n$ = 1, 2) with them. The latter are deeper electron traps with a small capture cross section as compared to the Ag$_{in}^{+}$ sites ($n$ = 1, 2) and that manifest themselves in that the kinetics of the luminescence growth in AgBr(I) to a stationary level at $\lambda$ $\approx$ 560 nm is characterized by the presence of “flash firing.” At the same time, the luminescence flash stimulated by IR light, for which the Ag$_{in}^{+}$ ($n$ = 1, 2) electron-localization sites are responsible, is absent. It is supposed that the electrons localized on the $\{$Ag$_{in}^{0}$G$^{+}\}$ complexes ($n$ = 2) retain the capability for emissive tunnel recombination with holes localized on paired iodine sites with a luminescence maximum at $\lambda$ $\approx$ 750 nm.