Radioluminescence in ZnO

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Abstract

ZnO powders were obtained by proteic sol–gel process using zinc sulfate as the precursor. They have an energy gap of 3.3 eV and display a radioluminescence signal centered in 550 nm, which corresponds to 2.6 eV. This signal has appeared due to defects in the gap (energy sublevels) caused by the distortions in its crystalline structure, which result from the loss of sulfur at high temperature. These powders were characterized by radioluminescence, XRD, SEM, EDS and DTA/TG.

Introduction

ZnO is a very well known and intensively studied semiconductor. There are many classic papers about the material, such as the determination of the lattice parameter by Bunn (1935), studies of vibrational properties with Raman scattering by Damen et al. (1966), detailed optical studies by Mollwo (1954), growth by chemical-vapor deposition by Galli and Coker (1970), etc. It is a rare material because it has a large family of growth morphologies that possess properties with possible applications in optoelectronics, sensors, transducers and the biomedical sciences (Wang, 2004). It is also an excellent matrix for doping with 2+ ions, which can make it a material with magnetic and optical properties. Its properties are almost all well known. Özgür et al. (2005) compiled a complete review of ZnO, detailing its properties and applications.

The structure of zinc oxide can be described in terms of planes composed of tetrahedrally coordinated Zn2+ and O2− ions stacked alternately along the c axis. This tetrahedral coordination results in a structure without central symmetry and, consequently, gives rise to piezoelectric and pyroelectric properties. The ions produce positively and negatively loaded surfaces resulting in a moment of normal dipole and a spontaneous polarization along the c axis as well as a divergence in the surface energy (Wang, 2004). In ambient conditions, ZnO has wurtzite structures (hexagonal) with space group P63mc, in which the atoms of Zn (or O) are in a tetrahedral coordination with four atoms of O (or Zn). Application of pressure above 9 GPa leads to a transition from the wurtzite structure to the rock salt (cubic) structure with space group Fm3m and an increase in the coordination number from 4 to 6 (Liu et al., 2006).

We present, in this work, a new route for obtaining pure ZnO powders with radioluminescent properties caused by defects in the crystal lattice.

Section snippets

Experimental

The samples were prepared through the proteic sol–gel process (Brito et al., 2006). Initially, 5 g of zinc sulfate heptahydrated (ZnSO4·7H2O) was diluted in 4 ml of coconut water. This solution was heated at 100 °C for 24 h for forming the xerogel (Brinker and Scherer, 1990). Soon after, quantities of the xerogel were calcined in the air at 500, 600, 800, 1000, 1200 and 1400 °C during 1 h for forming the ZnO powders. The powders were characterized with measurements of radioluminescence, XRD, SEM, EDS

Results and discussion

The X-ray diffraction results are shown in Fig. 1. It was observed that at 500 and 600 °C, the ZnO has not yet been formed. In the treated sample, at 800 °C, the ZnO phase appears with an unidentified impurity in small quantities (detached with the arrows), and from 1000 °C onward only the ZnO phase exists. The ZnO crystallite formed is of the hexagonal wurtzite type with space group P63mc. Fig. 2 shows the radioluminescence results normalized by the exposure time. It was observed that the

Conclusion

Zinc oxide powders yield a radioluminescence signal centered at 550 nm, which originates from the defects in the band gap (energy sublevels) caused by distortions in the crystalline structure due to the removal of sulfur at high temperature. We think that the new route presented here helps considerably in new applications of ZnO as a device for the detection of ionizing radiation because it is very efficient, simple and low in cost.

Acknowledgement

The authors would like to acknowledge the support from RENAMI-CNPq and FINEP-CTPetro.

References (13)

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