Controlled fabrication of oriented co-doped ZnO clustered nanoassemblies

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Abstract

Clustered nanoassemblies of Mn doped ZnO and co-doped ZnO (Mn, Sn co-doped ZnO; Mn, Sb co-doped ZnO; and Mn, Bi co-doped ZnO) were prepared by refluxing their respective precursors in diethylene glycol medium. The co-doping elements, Sn, Sb and Bi exist in multi oxidation states by forming Zn–O–M (M = Sb, Bi and Sn) bonds in hexagonal wurtzite nanostructure. The analyses of detailed structural characterization performed by XRD, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM), show that co-doping ions are successfully incorporated into the ZnO nanostructure and do not appear as precipitates or secondary phases. HRTEM analysis also confirmed the oriented attachment of nanocrystals as well as their defect structures. The formation/activation of higher amount of intrinsic host defects, for instance, oxygen vacancies in co-doped ZnO as compared to Mn doped ZnO sample is evident from Raman spectra. The doped and co-doped samples exhibit ferromagnetic like behavior at room temperature presumably due to the presence of defects. Specifically, it has been observed that the incorporation of dopant and co-dopants into ZnO structure can modulate the local electronic structure due to the formation/activation of defects and hence, cause significant changes in their structural, vibrational, optical and magnetic properties.

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.

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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

References (38)

  • D. Wu et al.

    J. Coll. Interf. Sci.

    (2009)
  • H. Chen et al.

    Acta Phys. -Chim. Sin.

    (2007)
  • H. Miao et al.

    Microelectron. Eng.

    (2003)
  • Y. Huang et al.

    Mater. Sci. Eng. B

    (2003)
  • B.L. Zhu et al.

    Mater. Chem. Phys.

    (2006)
  • L. Balan et al.

    Mater. Lett.

    (2005)
  • Z.B. Gu et al.

    Thin Solid Films

    (2006)
  • Z.H. Wang et al.

    Solid State Commun.

    (2009)
  • N. Saito et al.

    Adv. Mater.

    (2002)
  • V. Bhosle et al.

    J. Appl. Phys.

    (2006)
  • B.A. Buchine et al.

    Nano Lett.

    (2006)
  • V. Srikant et al.

    J. Appl. Phys.

    (1998)
  • J. Cheng et al.

    Appl. Phys. Lett.

    (2004)
  • X.Y. Du et al.

    J. Phys.: Conf. Ser.

    (2007)
  • A. Sundaresan et al.

    Phys. Rev. B

    (2006)
  • K.C. Barick et al.

    J. Phys. Chem. C

    (2008)
  • H. Ohno

    Science

    (1998)
  • P. Sharma et al.

    Nat. Mater.

    (2003)
  • Y. Yang et al.

    J. Phys. Chem. C

    (2008)
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