Luminescence and EPR study of ZnO:Mn:Cu nanowire array
Introduction
Transition metals-doped ZnO semiconductor has stimulated considerable research due to prediction of their high-temperature ferromagnetism [1], [2], [3], [4], [5]. Recently, nanoscale Mn-doped ZnO semiconductors have been widely investigated for their potential applications in photonics and spintronics. However, controversial magnetic and optical properties have been reported in Mn-doped ZnO system; both the existence and absence of ferromagnetism [5] have been shown and the luminescence of Mn-doped ZnO nanocrystals is very different for varied morphologies and Mn doping contents [6].
Co-doping with Cu+ ions may facilitate the control of charge carriers responsible for the exchange interaction, contributing in this way to the increase of the Curie temperature for the studied structures. Doping Cu in ZnO quantum dots, which have OH radicals on its surface, is responsible for quenching green luminescence at 530 nm [7]. On the other hand, photoluminescence spectrum of ZnO:Cu (0.1 at% Cu) nanowires show a broad band from 386 to 650 nm [8].
In this paper, we report the doping effect on the photoluminescence and electron paramagnetic resonance (EPR) spectra of ZnO:Mn:Cu nanowires fabricated by cathodic electrodeposition from aqueous solution.
Section snippets
Experimental
Mn and Cu-doped ZnO have been grown both as film and fibers on Pt electrode and inside a polycarbonate membrane, respectively. Doped-ZnO electrodeposition was performed potentiostatically at 70 °C by cathodic polarization at potentials of −0.7 to −1.0 V referenced to saturated calomel electrode (SCE). For preparation, we used aqueous solutions containing 5×10−2 M Zn(NO3)2 and 7.5×10−3 M lactic acid with addition of Mn(NO3)2 and Cu(NO3)2 in different concentrations. Polycarbonate membranes were used
Characterizations
The morphology of ZnO:Mn:Cu nanowire arrays is illustrated in Fig. 1, Fig. 2; it depends on the electrochemical bath composition which is used to prepare the samples. Both samples, having the composition presented in Table 1 were obtained as arrays of tubes with a granular structure. ZnO:Mn:Cu tubules with small walls thickness were prepared from a solution with higher Mn(NO3)2 concentration (Fig. 2).
The Mn content of ZnO:Mn:Cu nanotube array increased (Table 2) concomitantly with the increase
Conclusions
Manganese and copper-doped ZnO micro and nanotubes with hexagonal structure were prepared by template method. Doping of copper was confirmed by optical reflection spectra, EDX and EPR spectra. Presence of manganese ions was demonstrated by EPR and EDX measurements. Copper was incorporated both as Cu2+ ions and Cu+ ions.
The dependence of EPR signal of orientations of magnetic field, parallel or perpendicular to nanotube direction, can be observed.
EPR spectra suggest the existence of a
Acknowledgments
We acknowledge the financial support of the Romanian Education and Research Ministry (Contract CEEX no. 46/2005) and of Swiss National Science Foundation (Scopes, Project no. 110869).
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