Observation of Size Confinement Effects of Excitons in AgX Nanocrystals by Cryo-Energy-Filtering TEM/EELS

Abstract

ABSTRACTThe non-uniform size-dependent contrast of AgBr0.95I0.05 nanocrystals (NCs) ranging from 22 to 80 nm in equivalent diameter (dc) observed by cryo-energy-filtering TEM is referred to predominant excitations at the surfaces and near the edges. When the fields due to surface losses reach throughout the structure, they couple and the probability for their generation becomes periodic in the NC size. Since electronic sum rules must be satisfied, the surface excitations reduce the strength of the bulk excitations. Coupling of surface and volume losses may cause oscillations of the image intensities with the NC size. The appearance of such oscillations demonstrates a size confinement of excitations of valence electrons due to contributions to the energy-level structure from carrier confinement and surface states. The imaginary part of relative dielectric permittivity derived from electron energy-loss spectra shows an enhanced intensity of the band at 4 eV for NCs with dc = 50±4 nm as compared to those of 109±7 nm in size, while the bands at 7 eV and at 10 eV appear to be suppressed. An increase of the intensity of exciton-assisted direct interband transition at 4 eV (Γ8-, Γ6- → Γ6) correlates with the size-dependent enhancement of free exciton luminescence from AgBr NCs, when their size is less than 100 nm.