Absence of free carrier and paramagnetism in cobalt-doped ZnO nanoparticles synthesized at low temperature using citrate sol–gel route

Abstract

Cobalt-doped ZnO nanoparticles have been synthesized using a simple citrate sol-gel auto-combustion method. The XRD confirms nano-single phase and Wurtzite structure. Increased cobalt solubility from 15 to 20 % was observed in ZnO matrix due to low temperature synthesis. Room temperature paramagnetic contribution is observed for all the samples, since cobalt is a neutral dopant and no free carriers are produced. Normally, ferromagnetism is observed in ZnO due to carrier mediated interaction between transition metal ions and free carriers. Hence no ferromagnetism was observed and only paramagnetism was observed due to non-availability of free carriers for long range ferromagnetic interaction in our system. At higher doping, peak broadening of the highly intense XRD peak (101) was observed indicating formation of cobalt cluster (metal-metal) which reduces paramagnetism due to antiferromagnetic interaction and the magnetization value decreases to 0.00456 from 0.0076 emu/g. Absence of photoluminescence peak at 520 nm due to oxygen related defects also supports, the presence of paramagnetism in our samples, since oxygen defects are the another source of ferromagnetism in ZnO. Indirect evidence for the presence of cobalt clustering is also obtained from the photoluminescence studies which lead to concentration quenching of peaks. Photoluminescence studies exhibit NBE peak at 412 nm and defect peaks at 471 and 672 nm. The intensity of red emission peak at 672 nm remains constant whereas the intensity of the peaks at 412 and 471 nm increases and then decreases due to doping induced disorder leading to concentration quenching.

Effect of ionic size compensation by Ag+ incorporation in homogeneous Fe-substituted ZnO: studies on structural, mechanical, optical, and magnetic properties

“…Zn 0.95 Co 0.05 O nanoparticles prepared by sol-gel method demonstrate paramagnetism at room temperature, while thin lms of same composition deposited by pulsed laser deposition method showed superparamagnetic behavior at room temperature. 67,68 These observations and proposed explanations mentioned in the literature suggest that the magnetic properties of substituted ZnO are extremely sensitive to defects that may vary with growth conditions. Even in absence of defects, a long-range ferromagnetic coupling may happen from interactions mediated by delocalized Zn-s states resulting in super-and double-exchange.…”

“…Room temperature paramagnetism has been observed in Co-doped ZnO nanoparticles. 68 The reason for this observation was attributed to the unavailability of free carriers necessary for ferromagnetic interactions. The absence of RTF in our ZFO and ZFAO samples may be attributed to the same reason.…”

Effect of La Doping on ZnO Thin Films by Spray Pyrolysis

“…6 shows FTIR spectra of 0 wt%,2.0 wt% and 4wt% La-doped ZnO products. The strong absorption bands at 426 and 565 cm -1 are attributed to the Zn-O stretching vibration of wurtzite hexagonal type ZnO crystal [21,22], belonging to the oxygen sublattice (E2H) vibration and oxygen vacancies of wurtzite ZnO crystal, respectively [23]. The broad absorption bands at 3013-3633 cm -1 are the O-H stretching vibration of adsorbed water on the ZnO surface [24].…”