Effect of Co doping on the structural, optical and magnetic properties of ZnO nanoparticles

Abstract

We report the results of a detailed investigation of sol-gel-synthesized nanoscale Zn(1-x)Co(x)O powders processed at 350 °C with 0≤x≤0.12 to understand how the structural, morphological, optical and magnetic properties of ZnO are modified by Co doping, in addition to searching for the theoretically predicted ferromagnetism. With x increasing to 0.03, both lattice parameters a and c of the hexagonal ZnO decreased, suggesting substitutional doping of Co at the tetrahedral Zn(2+) sites. For x>0.03, these trends reversed and the lattice showed a gradual expansion as x approached 0.12, probably due to additional interstitial incorporation of Co. Raman spectroscopy measurements showed a rapid change in the ZnO peak positions for x>0.03, suggesting significant disorder and changes in the ZnO structure, in support of additional interstitial Co doping possibility. Combined x-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, photoluminescence spectroscopy and diffuse reflectance spectroscopy showed clear evidence for tetrahedrally coordinated high-spin Co(2+) ions occupying the lattice sites of ZnO host system, which became saturated for x>0.03. Magnetic measurements showed a paramagnetic behaviour in Zn(1-x)Co(x)O with increasing antiferromagnetic interactions as x increased to 0.10. Surprisingly, a weak ferromagnetic behaviour was observed for the sample with x = 0.12 with a characteristic hysteresis loop showing a coercivity H(c)∼350 Oe, 25% remanence M(r), a low saturation magnetization M(s)∼0.04 emu g(-1) and with a Curie temperature T(c)∼540 K. The XPS data collected from Zn(1-x)Co(x)O samples showed a gradual increase in the oxygen concentration, changing the oxygen-deficient undoped ZnO to an excess oxygen state for x = 0.12. This indicates that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co(2+) ions.

Solution-based synthesis of cobalt-doped ZnO thin films

“…Considering next the visible spectral range, we note that while it has been reported that PL decreases with increasing Co doping [37] significant PL persists in Co-doped ZnO samples of various types [14,23,29,38]. Starting at the short-wavelength end of the visible, a violet band, centered at 2.96 eV, due to zinc interstitials (Zn i ) appears in the case of the ZnO film.…”

“…Hays et al [37] observed a progressive attenuation of the E 2 mode with increasing Co concentration in ZnO which they attributed to increased strain induced by an increase in the density of Co occupying interstitial sites. Here, however, with the less heavily doped samples (ZC1 and ZC2) displaying attenuation of the 439 cm -1 peak, we suggest that in these cases the origin of the strain…”

Surfactant Assisted Sonochemical Synthesis and Characterization of Gadolinium Doped Zinc Oxide Nanoparticles

“…Doping induces drastic changes in its optical, electrical and magnetic properties by modifying its electronic structure. [11][12][13] Different methods have been used for the synthesis of doped ZnO such as laser ablation, 14 magneton sputtering, 15 high temperature calcinations, 16 sol-gel, 17 hydrothermal 18 and sonochemical methods 19 . Among these techniques, sonochemical method has attracted much attention.…”

Co doped ZnO semiconductor materials: structural, morphological and magnetic properties

“…The peak at 490 cm −1 is assigned to the most probable impurity phase, Co 3 O 4 [22]. This impurity phase wasn't observed in XRD measurements, possibly due to its relatively low concentration, which makes it difficult to detect.…”

“…Co 2+ exchange pairs were observed by EPR in singlecrystalline Zn 1−x Co x O thin films in the very low doping regime, x = 0 03 [33]. If Co pairs could be formed at higher doping concentrations, those Co ions would not be expected to contribute to the EPR spectrum representing the isolated impurities [34]. A careful analysis of our EPR spectra does not show any signature corresponding to Co pairs and therefore, we did not include in Eq.…”