Controlled fabrication of oriented co-doped ZnO clustered nanoassemblies
Graphical abstract
The incorporation of dopants/co-dopants into ZnO clustered nanoassemblies could modulate local electronic structure due to formation/activation of defects which significantly alters their structural, vibrational, optical and magnetic properties.
Introduction
ZnO, with a bandgap of 3.37 eV and a large exciton binding energy (60 meV) at room temperature is a promising material for optoelectronic devices, transparent conducting and piezoelectric material, field emission and surface acoustic wave devices, etc. [1], [2], [3], [4], [5], [6]. Of late, it has also been reported as a ferromagnetic material with a Curie point above room temperature either due to the presence of defects or with doping of Mn, Co or Ni [7], [8], [9], [10], [11]. Furthermore, doping with the ions of group IV and V elements such as Sn, Sb and Bi have been proved to be useful for enhancing optical and electrical properties of ZnO [12], [13], [14]. These cations normally exist in multi valence states and hence, they can serve either as a doubly ionized donor or as an isoelectronic impurity. However, substitutions of these larger size cations [Bi3+ (1.10 Å), Bi5+ (0.90 Å), Sb3+ (0.90 Å), Sb5+ (0.62 Å), Sn2+ (0.93 Å), Sn4+ (0.69 Å)] in ZnO are still an issue of debate [15], [16]. It is well reported in literature of ZnO based varistors that these larger size cations easily segregate at grain boundary in the form of oxides [17]. Recently, a model for large-size mismatched group-V dopants in ZnO has been proposed, in which an SbZn–2VZn complex (Sb occupying Zn site and spontaneously inducing two Zn vacancies) has been reported [18]. In addition, the substitution of Bi and As on the Zn sites and the formation of defect complexes such as BiZn–2VZn and AsZn–2VZn have been reported in Bi-doped ZnO and As-doped ZnO, respectively [14], [19]. The substitution of these larger cations as a co-dopant into ZnO lattice can modulate the local electronic structure due to variation of the defect concentration as well as sp–d coupling. However, there are few reports on the influence of co-doping of these cations in ZnO based dilute magnetic semiconductor. Norton et al. [20] observed ferromagnetism in Mn-implanted ZnO: Sn single crystals. Ji et al. [21] reported the appearance of robust ferromagnetic interaction in Mn and Sb co-doped ZnO films mediated by Sb-induced defects. Recently, Xu et al. [22] demonstrated the origin of ferromagnetism in Bi–Cu co-doped ZnO bicrystal nanowires as a result of the combined effect of structural defects, the substitution of Cu into Zn site and co-doping of Bi. Interestingly in these systems, the origin of ferromagnetism remains an issue of debate, and there is a great deal of controversy over the origin of ferromagnetism. Furthermore, most of the previous investigations have focused on thin films and bulk, and there are much less reports on nanoparticles of transition metal doped ZnO.
We report here a novel process to prepare highly oriented Mn doped ZnO and co-doped ZnO clustered nanoassemblies based on soft chemical approach. In order to investigate the effect of co-dopants, larger cations such as Sn, Sb or Bi were introduced into Mn doped ZnO and their structural, vibrational, optical and magnetic properties were studied. Furthermore, the understanding of defect structure may provide new insight into the activation of defect mediated properties.
Section snippets
Materials and methods
Mn doped ZnO clustered nanoassemblies were prepared by refluxing zinc acetate and manganese acetate in diethylene glycol (DEG) medium as discussed elsewhere [23]. Briefly, 0.03 mol of acetate precursors (95% zinc acetate and 5% manganese acetate) in 300 mL of DEG was heated under reflux, and Mn doped ZnO samples were precipitated out shortly after reaching 170 °C. The reaction mixture was kept under stirring for 30 min at this temperature. The co-doped samples were prepared by additional doping of
Results and discussion
Fig. 1 shows the XRD patterns of (a) Mn doped ZnO, (b) Mn, Sn co-doped ZnO, (c) Mn, Sb co-doped ZnO and (d) Mn, Bi co-doped ZnO samples. The XRD analysis revealed that Mn doped ZnO and co-doped ZnO crystallize into a single phase hexagonal wurtzite structure. The average crystallite sizes of co-doped ZnO are found to be about 9–12 nm whereas that of Mn doped ZnO sample is about 17 nm (σ ⩽ 10%). Furthermore, on doping the intensity of the diffraction peaks decreases acutely and the width broadens
Conclusions
Highly oriented Mn doped ZnO and co-doped ZnO clustered nanoassemblies were successfully prepared by a novel soft chemical process. The XRD analysis confirmed the formation of single phase wurtzite ZnO structure and their nanocrystalline nature. The co-doping elements exist in multi oxidation states (Sb as Sb3+, and Sb5+, Bi as Bi3+and Bi+5, Sn as Sn2+and Sn4+) in samples by forming Zn–O–M (M = Sb, Bi and Sn) bonds. The HRTEM analysis confirmed the oriented attachment of nanocrystals in co-doped
Acknowledgment
The financial support by Nano mission of DST, Govt. of India is gratefully acknowledged. This research is also supported by an Indo-US project [NSF-MWN (Grant No. DMR-0603184) – DST]. K.C. Barick acknowledges AICTE, India for the award of National Doctoral Fellowship and the NSF-MWN program for support for the research at and visit to Northwestern University. M. Aslam would like thank IRCC, IIT Bombay and CSIR, India for financial support. Part of this work was performed in the EPIC/NIFTI
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