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Synthesis, characterization and spectroscopic properties of ­Cu2+:ZnO, ­Ce3+:ZnO, and ­Cu2+, ­Ce3+:ZnO Winfred Mueni Mulwa1  Received: 11 January 2020 / Accepted: 17 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Pristine ZnO, C ­ u2+:ZnO, ­Ce3+:ZnO and C ­ u2+, ­Ce3+:ZnO nanopowders with different doping concentrations (0, 0.31, 0.62, 0.93 and 1.24% of dopant) were synthesized by sol–gel technique with low sintering temperature of 600 °C. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), UV–Vis optical absorption and photoluminescence (PL) spectroscopy analysis. XRD patterns revealed that all the compounds are hexagonal wurtzite crystalline structure and that all the dopant atoms substituted Zn atoms in the ZnO lattice and there was no formation of extra Phases. SEM photographs displayed morphology of the prepared nanopowders. The UV–Vis absorption spectrum presented an absorption peak at 355 nm which was ascribed to ZnO nanoparticles. The photoluminescence spectrum displayed emission peaks at 486 nm and 527 nm. The 486 nm peak conformed to bandgap excitonic emission and the 527 nm peak was attributed to the existence of independently ionized oxygen vacancies. Sol–gel technique has capability for application in manufacturing units, because its process is simple and the reagents used are economical. Particle sizes in the range 10–51 nm were realized from the TEM analysis. Keywords  ZnO · Nanostructures · Sol–gel · Defects · Photoluminescence

1 Introduction In the recent past, nanostructured semiconducting oxides have drawn a lot of attention. From the group II–IV semiconductors, many scientists have developed interest to a great extent in ZnO nanostructures. This is because ZnO is a low cost material, optically transparent in the observable zone of the spectrum and n-type semiconductor properties which is as a result of ionization of extra Zinc atoms at substitutional sites and the oxygen vacancies [1]. It forms crystals of hexagonal wurtzite structure with lattice parameters (c = 5.205 Å a = 3.249 Å) [2]. This material is greatly appreciated in the field of optoelectronic applications in the range of blue, green and UV (short wavelength) [3], due to its broad and direct bandgap of 3.37 eV [4, 5]. ZnO has properties very similar to those of GaN [6–8]; therefore, it is used in information storage and sensors. ZnO is found to have high exciton binding energy of 60 meV at room * Winfred Mueni Mulwa [email protected] 1



Department of Physics, Egerton University, P.O Box 536‑20115, Egerton, Kenya

temperature [9]. This type of exciton binding energy helps in excitonic transition at low temperatures (room temperature), resulting to increased radiative recombination effectiveness for impulsive emission not forgetting a comparatively low threshold voltage recommended for laser emission. Due to its high electrical conductivity, and optical transmittance in