Characterization of ZnO:Co particles prepared by hydrothermal method for room temperature magnetism

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Abstract

ZnO based diluted magnetic semiconductor particles (ZnO:Co) have been grown using a hydrothermal method with good crystallinity. The atomic percentage of Co presented in the specimen is about 0.01. Based on the x-ray diffraction and high-resolution transition electron, Co is found to be incorporated into ZnO lattice without evidence of obvious Co precipitates. However, from photoluminescence (PL) spectra in the range of 1.94 –3.45 eV, a strong broad emission centered around 600 nm (2.07 eV) in the visible range as well as a relatively weak peak at 2.81 eV are observed, indicating the presence of Co impurities. Moreover, intrinsic emissions such as DOX suggest that at least some Co have been doped into ZnO lattice, substituting for Zn2+ ions. The PL results further confirm the substitution of Zn2+ ions by Co, which leads to the changes of the electronic band structures. Magnetism could be realized at room temperature for the ZnO:Co nanoparticles under our experimental conditions although with low coercivity. The field-cooled and zero-field-cooled curves can be explained as a result of competition between the ferromagnetic and the antiferromagnetic ordering in the ZnO:Co nanoparticles. Combining the results from PL and magnetism characterization, it is reasonable to think that both doped Co in the ZnO lattice and Co impurities contribute to magnetism in ZnO:Co nanoparticles at room temperature.

Highlights

▸ Nanoparticle (ZnO:Co) grown using a hydrothermal method. ▸ Magnetic mechanism discussions for ZnO based diluted magnetic semiconductors. ▸ Magnetism realized at room temperature although with low coercivity. ▸ Both the doped Co in the ZnO lattice and Co impurities contribute to the magnetism at room temperature.

Introduction

Transition metals (TM) doped ZnO diluted magnetic semiconductors (DMSs) have attracted much scientific attention as both charge and spin degrees of freedom can be manipulated in these materials [1], [2]. From the feasible application point of view, at least room temperature ferromagnetism (FM) is required, if impractical refrigeration is to be avoided. It is well known that DMSs based on TM doped II–VI semiconductors, including ZnO based DMSs, are predicted to have a Curie temperature that can be raised above 300 K [3]. Hence, such materials are potential candidates for room temperature ferromagnetism semiconductors for spintronics applications. There have been many studies on this kind of materials since then [4], [5], and Co-doped ZnO DMSs have particularly attracted strong interest [6].

DMSs usually have in common that they are deposited on substrates and many experimental results are obtained based on the data, which have been subtracted from the magnetic response due to the substrate. Many fabrication methods including magnetron sputtering, pulsed laser deposition, and molecular-beam epitaxy have been employed. However for very sensitive magnetic signals, great care should be exercised in the subtraction of the response due to the substrates. Recently, particles were synthesized to study their magnetic behaviors. The nanoparticles of ZnO:Ni synthesized by means of low temperature sol–gel method [7] exhibited room temperature ferromagnetic phase. The ferromagnetism observed in Zn1−xCoxO single-crystalline nanorods with x<0.1 synthesized by the solvothermal method was intrinsic and can be understood within the framework of the percolation of bound magnetic polarons [8]. In the meantime, Mn doped ZnO nano-crystallites were synthesized by sol–gel derived auto-combustion technique. Room temperature magnetic properties determined by vibrating sample magnetometer revealed the presence of ferromagnetic and diamagnetic contributions in Mn doped ZnO [9]. Although many experimental results have been reported, there has not been a consensus on the physics underlying these magnetic behaviors. There is not a physical theory as yet, which is able to explain the differences in conflicting reports. Recent interest in ZnO based DMSs is no longer due solely to their promises as room temperature ferromagnetic semiconductors. Increasingly scientific interest has turned towards understanding the physics underlying their magnetic behaviors. In fact, this has become the most important motivation in recent years [10].

As we know, photoluminescence (PL) emissions are related to excitation states, which can be used to study the correlation between density of states and impurities. Temperature-dependent PL spectroscopy is a sensitive and powerful tool for the characterization of active impurities. Hence, it may be helpful towards uncovering the origin of magnetism. The temperature-dependence of spontaneous magnetization (MT) possesses many characteristics, such as concavity/convexity. Thus the study of MT might potentially lead to the elucidation of the physics behind DMS ferromagnetism.

In this study, ZnO based DMS particles were prepared using a simple hydrothermal method. The structural, optical and magnetic properties are studied. It is noted that to elucidate the possible effect from impurities, temperature-dependent PL was used. The purpose of this study is to investigate the underlying magnetic mechanisms of ZnO-based DMSs, hence ZnO:Co particles with a low Co concentration were prepared.

Section snippets

Experimental details

ZnO based DMS particles (ZnO:Co) have been grown using a hydrothermal method, which is similar to that of Ref. [11]. In brief, a solution with zinc chloride and cobalt chloride, in which the mole ratio of Co is about 2% was prepared and stirred. Then, an appropriate quantity of ammonia was introduced into the solution (28%, Chemicals) to adjust the pH range (10–11). The solution was transferred to the Teflon lined stainless steel autoclave, which was then screwed up and sealed. Next the

Results and discussion

Fig. 1(a) shows the XRD pattern for ZnO:Co particles synthesized at 100 °C, 24 h. There are many peaks presented in the XRD pattern. All the diffraction peaks in the pattern can be assigned to the hexagonal structure, that is, wurtzite structure of ZnO:Co particles with the lattice constants a=0.3258 and c=0.5200 nm. The diffraction peaks appeared at 2θ=31.69°, 34.35°, 36.18°, 47.44°, 56.47°, 62.71°, 66.23°, 67.83°, 68.97°, 72.44° and 76.83°, which correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0),

Conclusions

ZnO based diluted magnetic semiconductor particles (ZnO:Co) have been grown using a hydrothermal method with good crystallinity. The atomic percentage of Co presented in the specimen is about 0.01, which is smaller than the mole ratio for Co in the solution prepared. Based on XRD and HRTEM, Co is found to be incorporated into ZnO lattice without obvious evidence of Co precipitates. However, from the photoluminescence (PL) spectra in the range of 1.94–3.45 eV, a strong broad emission centered

Acknowledgments

This project was supported by the funds from State Key Lab of Silicon Materials, Zhejiang University (Grant no. SKL2008-5), and the fund KYF09150603 from Hangzhou Dianzi University. The authors are grateful to Dr. Qian Tao for his help in magnetism characterizations. The authors would like to thank Research Center of Analysis and Measurement in Zhejiang University of Technology for their help in performing the HRTEM observations.

References (19)

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