Size Confinement Effects on Electronic and Optical Properties of Silver Halide Nanocrystals as Probed by Cryo-EFTEM and EELS

Cryo-energy-filtering transmission electron microscopy and electron energy-loss spectroscopy have been applied to study size confinement effects on electronic, dielectric, and optical properties of Ag(Br, I) nanocrystals. The dielectric permittivity, optical joint density of states, refractive index, extinction, and absorption for nanocrystals derived via Kramers–Kronig relations have been compared with experimental data for Ag(Br, I) tabular microcystals and ab initio linear muffin-tin orbital method in its atomic spheres approximation calculations for AgBr. Contrast tuning with the selected energy window between 0 and 100 eV enabled visualizing valence electron excitations in silver halides because of plasmons superimposed with interband transitions and Mott–Wannier excitons. The nonuniform contrast of nanocrystals revealed by cryo-energy-filtering transmission electron microscopy was referred to a size-confined coupling of surface and volume losses that could lead to oscillations of the image intensity with the nanocrystal size. A size-dependent enhancement of interband transitions at 4 eV correlated with the enhancement of exciton luminescence from nanocrystals because of contributions to the energy-level structure from carrier confinement and surface states.