Correlation between the structural and optical properties of Mn-doped ZnO nanoparticles

doi:10.1016/j.jallcom.2012.01.116

Journal of Alloys and Compounds

Volume 522, 5 May 2012, Pages 114-117

https://doi.org/10.1016/j.jallcom.2012.01.116 Get rights and content

Abstract

The crystallographic and optical properties of Mn-doped ZnO nanoparticles prepared by a sol–gel process have been investigated by X-ray diffraction, UV-visible absorption spectroscopy and cathodoluminescence microanalysis. X-ray diffraction reveals that the nanoparticles have hexagonal wurtzite crystal structure, with the lattice constants along the a- and c-axes increasing with increasing Mn concentration from 0 to 2.4 at%. For all Mn concentrations in this range, the nanoparticles are essentially free of native point defects so that they exhibit only band-edge luminescence. The optical bandgap and band-edge emission energies for Mn-doped ZnO were found to increase in proportion to the lattice constants. The direct correlation between the bandgap and crystal structure suggests that the band-edge optical properties of Mn-doped ZnO is predominantly influenced by the amount of Mn atoms substituting Zn on the lattice sites.

Highlights

► Mn-doped ZnO nanoparticles with wurzite structure are prepared by sol–gel. ► Lattice constants along a- and c-axes increase with increasing Mn concentration. ► Optical band gap and band-edge emission energy increase in proportion to a and c. ► Optical properties are mainly influenced by Mn atoms substituting Zn lattice sites.

Introduction

The possibility of bandgap engineering and influencing physical and magnetic properties by alloying wide bandgap semiconductors has provided a strong impetus to study doping effects on electronic compounds [1]. Advances in the synthesis of high-quality ZnO nanostructures are enabling device applications, in particular, light emitters with nanoscale dimensions and ferromagnetic semiconductors, yet the effect of transition metal doping on near-band-edge optical emission of ZnO is still a subject of considerable debate. Several studies have focused on Mn doping of ZnO; however, the results in the open literature are contradictory. For example, the relationship between the bandgap and Mn content has been reported to be linear [2], [3], a second-order polynomial [4], [5], [6] and non-monotonic [7], [8]. This makes the reproducibility of ferromagnetism in Mn-doped ZnO a challenging problem since the magnetic behaviour of the material is highly sensitive to its electronic structure [9], [10].

A key issue with the growth of transition metal (TM)-doped ZnO bulk and nanocrystals is the possible existence of secondary phases [11], [12], especially for specimens prepared by high temperature processes. Previous attempts of doping nanocrystals have been fraught with problems because dopants are frequently expelled to the surface by the intrinsic process of self-annealing [13]. This makes TM-doped ZnO exhibit interesting properties but also contribute to wide discrepancies in reported optical properties of ZnO nanoparticles. One way to overcome this issue is to quantify relationships between the core structure of nanoparticles and their optical data. For this purpose, we have prepared ZnO specimens doped with Mn in the concentration range from 0 to 2.4 at% and evaluated the relationship between the optical bandgap and crystal structure of the host material. It will be shown that the bandgap of Mn-doped ZnO is directly correlated with the lattice constants of the host material.

Section snippets

Synthesis

The starting materials, Zn(CH₃COO)₂·2H₂O (Aldrich, >99.0%), Na₂CO₃ (Aldrich, >99.5%) and Mn(CH₃COO)₂·4H₂O (Aldrich, >99%) were used without further purification. The synthesis of undoped ZnO particulates was conducted via the following reaction.Zn(CH₃COO)₂·2H₂O + Na₂CO₃ → ZnCO₃ + 2Na(CH₃COO)

In a typical synthesis of undoped ZnO, 13.5 g of Zn(CH₃COO)₂·2H₂O and 6.5 g of Na₂CO₃ were separately dissolved in 50 ml of deionised water. The Na₂CO₃ solution was added into the Zn(CH₃COO)₂·2H₂O solution to form

Results and discussion

SEM images of pure and doped ZnO nanoparticles reveal nearly spherical shape with dimensions in the range 100–250 nm (inset, Fig. 1). XRD patterns of the pure and Mn-doped nanoparticles reveal a single hexagonal wurtzite structure, with two strong peaks ZnO (0 0 2) and (1 0 1) at approximately 34.5° and 36.3°, respectively. The presence of Mn dopants (up to 2.4 at%) does not influence the shape and crystal structure of the nanoparticles; however, XRD peak shifts are discernible. The concentration

Acknowledgements

We are grateful to M. Berkahn and R. Wuhrer, both from the Microstructural Analysis Unit, for technical assistance. This work was supported by the Australian Synchrotron Research Program.

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Correlation between the structural and optical properties of Mn-doped ZnO nanoparticles

Abstract

Highlights

Introduction

Section snippets

Synthesis

Results and discussion

Acknowledgements

Solid State Commun.

Superlattices Microstruct.

Solid State Commun.

J. Inorg. Nucl. Chem.

Appl. Phys. Lett.

Mater. Lett.

J. Lumines.

Mater. Lett.

J. Eur. Ceram. Soc.

J. Mater. Sci. – Mater. Electron.

J. Appl. Phys.