Effect of gas concentration on structural and optical characteristics of ZnO nanorods

https://doi.org/10.1016/j.physe.2010.04.010Get rights and content

Abstract

ZnO nanorods have been synthesized using vapor–liquid–solid (VLS) growth technique under different flow ratios of Ar/O2. The effects of different gas concentrations on the crystal structure, surface morphology, and optical properties of ZnO nanorods are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), photoluminescence (PL), and Raman spectra measurements. The PL measurements exhibit two peaks, one around 383 nm and the other around 502 nm, which correspond to ultraviolet (UV) and green emission. With decrease in flow ratio of Ar/O2, the blue-shift of UV PL spectra is observed in the nanorods. In addition, a nanogenerator with ZnO nanorods is fabricated. The output current is generated in the device and showed the Schottky-like current–voltage characteristic.

Introduction

ZnO nanorod arrays have attracted much attention due to their physical and optoelectrical properties and have promising potentials in wide applications such as lasers [1], solar cell [2], transistors [3], and nanogenerator [4]. ZnO nanorods can be synthesized by various techniques such as metal–organic chemical vapor deposition [5], chemical solution method [6], and vapor–liquid–solid (VLS) process [7].

The ZnO nanorods with excellent quality can be synthesized by the VLS technique. Recently, some researchers have studied the physical properties of ZnO nanostructures fabricated by the VLS process. For example, Zhao et al. [8] studied ZnO nanorods, which were grown on Si and SiC substrates with a VLS growth using Au nano-particles as catalyst. They found that the formation of ZnO nanostructures is strongly influenced by the growth conditions and used substrates. Hejazi et al. [9] investigated the role of reactants and droplet interfaces on nucleation and growth of ZnO nanorods synthesized by the VLS mechanism. Yang et al. [10] carried out optical analysis of ZnO nanowires deposited on Si(1 0 0) via the VLS process and estimated the biaxial compressive stress within the nanowires.

Optical properties of ZnO nanostructures are sensitive to its crystal perfection. A high crystal perfection of ZnO in growth process can be affected by gas concentration. In this study, the effects of different gas concentration ratios of Ar/O2 on the optical characteristics of ZnO nanorods synthesized by the VLS mechanism are studied. The morphology and structure of the ZnO nanorods are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). In addition, photoluminescence (PL), absorption spectrum, and Raman spectrum are used to study the optical properties of nanorods. Finally, a nanogenerator with ZnO nanorods is fabricated and its current–voltage characteristics are studied.

Section snippets

Experimental

The ZnO nanorods are synthesized by the vapor–liquid–solid mechanism using a tube furnace. The source material is pure Zn powder. Sputtered Au catalyst on Si substrates is placed at the downstream side of the Ar flow, 1–2 cm away from the source materials. The reaction temperature is 650 °C. Different flow rates of Ar/O2 are introduced in the quartz tube. In the process of preparing the samples, Ar was applied as carrying gas and O2 was used as reactant gas. The crystalline structure and the

Results and discussion

Fig. 1 shows the XRD peak patterns of ZnO rods with various Ar/O2 flow ratios of 2:1, 8:1, 10:1, and 12:1. The (1 0 0), (0 0 2), (1 0 1), and (1 0 2) orientations of ZnO rods are observed and the most intense is (0 0 2) orientation. The intensity of the (0 0 2) peak increases when the Ar/O2 ratio increased. It is found that there is a ZnO (1 0 1) peak when the ZnO film is deposited at an Ar/O2 ratio of 12:1. This shows that superfluous Ar has great effect on ZnO orientation growth. In addition, in the

Conclusion

In this paper, the effect of different Ar/O2 flow ratios on crystalline, optical, and piezoelectric characteristics of ZnO nanorods prepared by the VLS method was investigated using XRD, SEM, EDS, and PL measurements. Result shows from the XRD analysis that the intensity of the (0 0 2) peak increases when the Ar/O2 ratio increases. The better ZnO nanorod was obtained at the flow ratio of 8:1. Furthermore, from the SEM images, some nanorods of about 30–50 nm diameters were observed at the flow

Acknowledgements

This work was supported by the National Science Council of Taiwan under Grant no. NSC 96-2628-E150-005-MY3.

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