Doping Induced Band-gap widening in Transition-metal doped ZnO Nanocrystals
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.323
Doping Induced Band-gap widening in Transitionmetal doped ZnO Nanocrystals Azimatu Seidu, Martin Egblewogbe*, G. Gebreyesus, George Nkrumah-Buandoh Department of Physics, School of Physical and Mathematical Sciences, College of Basic and Applied Sciences, University of Ghana, Legon.
*Corresponding author. E-mail: [email protected]
Abstract
Pure and transition metal (TM)-doped ZnO nanocrystals were synthesized using a wetchemical process. Synthesis was carried out in distilled water at 85 oC followed by calcining the as-prepared powders at 280 oC and 600 oC. Co, Mn and Fe doping at 4 and 8 mol % was achieved by adding CoCl2.6H2O, MnCl2 .4H2O and FeCl2 .4H2O respectively during the synthesis. Crystal phase characterization was carried out by X-ray powder diffraction (XRD) which confirmed the formation of ZnO in the wurtzite polymorph. The band gap energy of the nanocrystals was measured by both photoluminescence spectroscopy (using the Near Band Edge Emission) and UV-Vis absorption spectroscopy, using a modified version of the Tauc law. Widening of the band gap energy from 3.23 eV to 3.33 eV with increased doping concentration was observed for all the dopants. Ab-initio simulations of doped and undoped ZnO crystals using density functional theory as implemented in the Quantum Espresso package confirmed the increase in the band gap energies with doping concentration.
INTRODUCTION ZnO is a II-VI wide-band gap material that has many useful and interesting properties and is easily prepared using a wide variety of methods. Many of these properties are enhanced at the nanoscale and can be tailored for specific applications. Furthermore, its physical and chemical stability as well as its low toxicity make it very attractive for device fabrication and deployment. ZnO nanomaterials hold significant promise for technological applications which range from piezoelectricity, luminescence in the optical and UV range, gas sensing, photovoltaics, and more (Wang [1], Özgur et al. [2]). Many of these applications depend on the electrical and optical properties of ZnO, and modifying these properties is key to optimising the material for applications. For example, the electrical and optical properties can be modified by doping to alter the
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band gap energy in a controlled and rational manner. Doping also opens up new material properties, for example, room temperature ferromagnetism in ZnO (Panigraphy et al. [3]). Therefore, it is important to determine and understand the nature of doping-induced changes in the material properties. Many experiments on transition-metal (TM) doping of ZnO have been reported. However, there are often conflicting reports of the effects of such doping and it is worth re-assessing the resu
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