Influence of Cu induced crystallographic disorder on the optical and lattice vibrational properties of ZnO nanoparticles

Abstract

In this study, the vibrational and optical responses of 0–10% excess Cu incorporated ZnO nanoparticles (NPs) prepared by the low temperature (∼400 °C) wet chemical route were investigated experimentally and were found to be predominantly linked to the formation of both intrinsic and Cu induced crystallographic defects instead of substitutional Cu itself for the first time. For low temperature chemical synthesis, the effective band gap (E_g) of pristine ZnO NPs was found to be as low as 2.84 eV, which was followed by a further reduction by as high as ∼24.4% with gradual Cu inclusion. Excess Cu incorporation was found to drastically restrict the particle growth phenomenon by ∼61% and alter the shape morphology from anisotropic to irregular isotropic. Although all NPs showed a single phase structure with hexagonal P6₃mc symmetry, the X-ray peak profile analysis along with the characteristic E₂(High) Raman peak and electron–phonon coupling strength obtained from the 2nd order Raman mode suggested the presence of appreciable crystallographic disorders pertaining to point defects like Cu induced Zn and O interstitials and vacancies. Optical analysis revealed a gradual increase in Urbach tailing (E_u) from 0.35 to 0.96 eV directly associated with both intrinsic and Cu induced disorders arising from successive Cu incorporation at low processing temperature. A correlation between E_g and E_u predicted a direct-type band-to-band transition energy of ∼3.2 eV for the pristine ZnO crystal, suggesting both intrinsic and excess Cu induced extrinsic disorders to be the primary source of optical modulation of the NPs for the low temperature processing condition.