Elsevier

Applied Surface Science

Volume 379, 30 August 2016, Pages 440-445
Applied Surface Science

Role of ZnO thin film in the vertically aligned growth of ZnO nanorods by chemical bath deposition

https://doi.org/10.1016/j.apsusc.2016.04.107Get rights and content

Highlights

  • ZnO thin films were deposited on the Si substrate by RF sputtering.

  • The ZnO films showed granular surface profiles dependent on film thickness.

  • ZnO nanorods were grown vertically from the ZnO films using a CBD method.

  • The dimensions of ZnO nanorods were closely correlated with the features of seed layers.

Abstract

The effect of ZnO thin film on the growth of ZnO nanorods was investigated. ZnO thin films were sputter-deposited on Si substrate with varying the thickness. ZnO nanorods were grown on the thin film using a chemical bath deposition (CBD) method at 90 °C. The ZnO thin films showed granular structure and vertical roughness on the surface, which facilitated the vertical growth of ZnO nanorods. The average grain size and the surface roughness of ZnO film increased with an increase in film thickness, and this led to the increase in both the average diameter and the average length of vertically grown ZnO nanorods. In particular, it was found that the average diameter of ZnO nanorods was very close to the average grain size of ZnO thin film, confirming the role of ZnO film as a seed layer for the vertical growth of ZnO nanorods. The CBD growth on ZnO seed layers may provide a facile route to engineering vertically aligned ZnO nanorod arrays.

Graphical abstract

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Size-controlled ZnO nanorod arrays were vertically grown on ZnO thin films by a chemical bath deposition method at 90 °C using hexamethylenetetramine as a reducing agent.

Introduction

Zinc oxide (ZnO) nanostructures have long been gaining attention due to their intriguing properties such as wide and direct band gap (3.37 eV) at room temperature and simultaneous revelation of high electrical, optical, and piezoelectric performance. ZnO nanostructures have found applications in the fields of piezoelectric transducers, photovoltaic devices, gas sensors, biosensors, transistors, and optoelectronic devices [1], [2], [3], [4], [5], [6], [7]. In particular, highly-oriented ZnO nanowire or nanorod arrays are required for high-performance optoelectronic devices, and their growth method are of critical importance.

Several groups have grown vertically aligned ZnO nanowire/nanorod arrays on silicon or glass substrates without the use of any textured thin film as a seed layer. These arrays were synthesized at a temperature range of 400–600 °C, using metal-organic chemical vapor deposition (MOCVD) [8], [9], [10], pulsed laser deposition (PLD) [11], or chemical vapor transport (CVT) [12]. However, all of these methods employ complex and expensive processes at high temperatures. As an alternative of these physical vapor methods, solution-based ZnO nanorod growth methods were emerged, which do not need high temperature or vacuum [13], [14], [15]. Chemical bath deposition (CBD) has particularly received a great interest because of its simple experimental setup, low cost, and good potential for scaling up. As far as the CBD growth of ZnO nanorods is concerned, using hexamethylenetetramine (HMTA) ensures high crystallinity and good morphological property of ZnO nanorods, as compared with other reducing agents [16], [17].

Vertically aligned ZnO nanorod arrays have also been grown on various substrates with the support of textured ZnO seed layers such as ZnO colloids, ZnO nanocrystal layers, and ZnO thin films [18], [19], [20]. For instance, Greene et al. prepared ZnO colloids and nanocrystals in aqueous solution by the hydrolysis of zinc salts on Si substrate at 200–350 °C [19]. Solis-Pomar et al. deposited ZnO thin films with different thicknesses by atomic layer deposition [20]. However, these methods also require high temperature or costly processes. As previously demonstrated for various applications [21], [22], [23], [24], [25], [26], [27], sputtering ZnO thin films on a substrate may be a much simpler way to prepare a ZnO seed layer for the subsequent growth of vertically aligned ZnO nanorods. Till now, no systematic study has been reported on a correlation between the property of sputtered ZnO seed film and the features of ZnO nanorods grown from that.

In this work, ZnO seed layers are deposited on Si substrate by RF magnetron sputtering at room temperature, with varying the layer thickness. Thickness-dependent surface morphology of the seed layer and its effect on the vertical growth of ZnO nanorod arrays are investigated. The CBD technique is employed to grow the ZnO nanorod arrays using HMTA as a reducing agent.

Section snippets

Experimental

The fabrication process of vertically aligned ZnO nanorod arrays includes two steps: preparation of textured ZnO thin films using RF sputtering and ZnO nanorod growth using CBD method.

Results and discussion

Fig. 1 shows the AFM images of ZnO seed layers prepared by RF sputtering. The growth rate was kept constant at 8 nm/min for all samples, and the sputtering time was altered to obtain ZnO seed layers of 80, 120, 160, and 200 nm, respectively, in thickness. It is found from the AFM images that ZnO thin films are composed of small grains and the grain size depends on the sputtering time. For the 10 min-sputtered ZnO thin film, small ZnO grains are observed on the surface (Fig. 1(a) and (b)). The

Conclusions

ZnO thin films were deposited on Si substrate at room temperature by RF sputtering. The film thickness was adjusted in the range of 80–200 nm by controlling the sputtering time. Both the surface roughness and the average grain size of ZnO film turned out to increase with an increase in the film thickness. ZnO nanorods were grown on these ZnO thin films on Si substrate, using a CBD technique. It was disclosed that dense, hexagonal-shaped ZnO nanorods were almost vertically aligned, and their

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2010630).

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