Electrical, dielectric, photoluminescence and magnetic properties of ZnO nanoparticles co-doped with Co and Cu

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Highlights

  • Hexagonal structure of Zn0.96−xCu0.04CoxO was not changed by Co-doping.

  • Higher ac conductivity at Zn0.94Cu0.04Co0.02O is due to nano-dipoles.

  • The minimum Igreen/Iblue at Co=1% depicted the low defects sites and vacancies.

  • Suppressed magnetization by Co-doping is due to the AF interaction between Cu–Cu ions.

Abstract

X-ray diffraction spectra of Zn0.96−xCu0.04CoxO (0≤x≤0.04) nanoparticles synthesized by co-precipitation method confirmed the hexagonal wurtzite structure without any secondary phase formation. The dielectric dispersion was high at lower frequencies and almost frequency independent at higher frequencies. The observed higher dielectric constant, dielectric loss and ac conductivity in Co=2% doped Zn0.96Cu0.04O samples was explained in terms of average crystalline size and number of nano-dipoles. Photoluminescence spectra of undoped and Co-doped Zn0.96Cu0.04O samples showed four distinct bands, (i) ultra violet emission bands around 382–391 nm, (ii) violet emission band centered at 417 nm, (iii) blue emission bands centered at 478 nm and (iv) green emission bands centered at 523 nm. The observed minimum of Igreen/Iblue revealed that Co=1% doped Zn0.96Cu0.04O sample had minimum defects sites and vacancies and it saturated after Co=3% doping. Undoped Zn0.96Cu0.04O sample had higher magnetization and it was suppressed by Co-doping due to the enhanced antiferromagnetic interaction between neighbouring Cu–Cu ion.

Introduction

Recent years, establishment of semiconductor spin electronics (spintronics) have attracted researchers due to the charge and spin degree of freedoms in a single substance. Ferromagnetism with Curie temperature (Tc) above room temperature and large magnetization natures of ZnO based DMSs makes it to be a promising candidate material for next generation spintronics applications. Dietl et al. theoretically predicted that the transition metal doped ZnO and GaN have ferromagnetic nature in room temperature [1]. Several first principle calculations showed that ZnO doped with Cu possesses a ferromagnetic ground state [2], [3]. Experimentally, the room temperature ferromagnetism was discovered in Cu doped ZnO nanoparticles [4], [5]. Moreover, at higher doping concentration the ferromagnetic ordering was decreased due to the formation of secondary phase like CuO [4]. The secondary phase formation led to decrease the carrier concentration in the Cu doped ZnO samples. Therefore, the Cu doping concentration of present investigation is limited to 4%. Two or more favor metals co-doping is one of the key way to increase the carrier concentration without any secondary phase. Jayakumar et al. showed that the co-doping of Cu with Co increases the carrier concentration, considerably and the ferromagnetism was enhanced by additional carriers [6].

The knowledge on ac conductivity and dielectric properties of semiconductors are very important to understand the nature of conduction and defects centers which are present in the crystalline materials [7], [8]. The electrical transport in semiconductor was explained by polaron hopping theory [9]. The small polarons are created around defect states by the localized charge carrier [10]. The doping of TM creates more distortion and defects in the ZnO lattice, which can largely modify the electrical conduction of ZnO semiconductors. Therefore, it is important to study the dielectric and ac conducting properties to know about interior of material, since the defects play crucial role on RTFM, the complete knowledge on interior of the material is important. In the present investigation, the effect of Co co-doping concentration on electrical properties, room temperature ferromagnetism and photoluminescence are studied and discussed in detail.

Section snippets

Experimental procedure

The Zn0.96−xCu0.04CoxO nanoparticles were synthesized by co-precipitation method and the Co concentration is varied from 0 to 4%. The detailed preparation procedure was given in our previous literature [11]. The crystal structure of Zn0.96−xCu0.04CoxO (0≤x≤0.04) nanoparticles was determined by powder X-ray diffraction (XRD). XRD patterns were recorded by RigaKuC/max-2500 diffractometer using Cu Kα radiation (λ=1.5406 Å) at 40 kV and 30 mA from 2θ=30° to 70°. Dielectric and ac conductivity

X-ray diffraction (XRD)—Structural study

The XRD spectra of Zn0.96Cu0.04O and Co-doped Zn0.96Cu0.04O (Co=0.02 and 0.04) nanoparticles are shown in Fig. 1. The standard diffraction peaks for (1 0 0), (0 0 2) and (1 0 1) planes show the hexagonal wurtzite crystal structure and very close to the standard data of pure ZnO (a=3.2488 Å, c=5.2061 Å, space group P63mc, 186, JCPDS data card no. 36-1451) with preferred orientation along (1 0 1) plane. No secondary phases corresponding to Cu/Co or oxides of Cu/Co or Cu/Co related secondary and impurity

Conclusions

Undoped and Co-doped Zn0.96Cu0.04O nanoparticles with different Co concentrations from 0 to 4% were successfully synthesized by co-precipitation method. X-ray diffraction spectra of Zn0.96−xCu0.04CoxO (0≤x≤0.04) nanoparticles synthesized by co-precipitation method confirmed the hexagonal wurtzite structure without any secondary phase formation. Co-doped Zn0.96Cu0.04O samples had high dielectric constant, dielectric loss and ac conductivity than undoped Zn0.96Cu0.04O samples. The observed higher

Acknowledgments

The authors are thankful to the University Grant Commission (UGC), New Delhi, India, for financial support under the project [File no.: 41-968/2012 (SR)].

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