Effect Co -, and Sc- Doping on the Functional Properties of Nanocrystalline Powders and Thin Films of ZnO
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1256-N11-23
Effect Co -, and Sc- Doping on the Functional Properties of Nanocrystalline Powders and Thin Films of ZnO Marco Galvez1, Oscar Perales-Perez2 and S. P. Singh2 1 Department of Physics, University of Puerto Rico at Mayaguez, Mayaguez 00980, PR 2 Department of Engineering Science and Materials, University of Puerto Rico at Mayaguez, Mayaguez, PR, 00680-9044
ABSTRACT A modified sol-gel approach to synthesize well-crystallized pure and doped ZnO nanocrystalline powders and thin films has been developed. The attachment of ZnO films onto quartz substrates was optimized by selecting suitable organic agents to control the viscosity of precursor solutions. Thermo-gravimetric analyses on pure and doped precursor solids suggested the need for annealing temperatures above 400 ºC to assure the effective crystallization of the oxide phase. The average crystallite size in powders and thin films varied from 25.9nm to 33.7nm when pure ZnO films were annealed for 1 hour in the 450 oC - 600 oC range. The average crystallite size ranged between 30 nm and 33 nm in the presence of cobalt (5 at%) and decreased from 33.7 nm to 21.1 nm when scandium ions was used in the 0.0 at% - 10 at% range under similar annealing conditions. UV-vis measurements showed a sharp exciton peak centered at 370 nm whereas photoluminescence analyses detected the characteristic ZnO emission band in the UV region. No photoluminescence band in the visible region, usually attributed to defect states in ZnO, was observed in our measurements. Magnetization measurements revealed a weak ferromagnetism in Co-doped ZnO whereas a clear diamagnetism was evident in the Sc-doped sample.
I. INTRODUCTION The research and development of multifunctional nanomaterials and devices are receiving increased attention from both the industry and academia. ZnO is one of the most attractive alternatives among various semiconductor oxides that are expected to become suitable platforms for multifunctional applications. ZnO is an II-VI group compound semiconductor, having a hexagonal wurtzite structure with lattice parameters ‘a’=3.249 Ǻ and ‘c’=5.207 Ǻ. This semiconductor oxide has a direct band gap of 3.3 eV at room temperature and an excitation binding energy of 60 meV [1]. Several theoretical and experimental studies have suggested that doping of ZnO with transition metals or rare earth ions could induce room-temperature ferromagnetic and opto-magnetic properties in the so-called dilute magnetic semiconductor, DMS [2,3]. Although the results are very promising, there is still some controversy regarding the intrinsic or extrinsic nature of the observed ferromagnetism. ZnO-based DMS materials exhibiting room-temperature ferromagnetism would allow the development of spintronic devices that would operate on a quantum mechanical property of electrons called “spin” rather than on the electron electrical charge [4]. ZnO thin films have been synthesized using different techniques, including sputtering [5], spray pyrolysis [6], chemical vapor deposition [7], pulsed laser deposition
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