ZnO nanostructures growth with silver catalyst—Effect of annealing

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Abstract

Zinc oxide (ZnO) nanostructures have been grown on both the as-deposited silver thin-film templates over silicon substrates and annealed silver thin-film templates on silicon substrates. ZnO was grown by evaporation of metallic zinc over the silver templates followed by thermal annealing in air. Sword-like ZnO nanostructures grew densely throughout the surface of the annealed silver template sample. A small number of ZnO swords embedded in a porous surface were found for the as-deposited silver template sample. It is observed that the annealing treatment of the Ag thin-film is the key factor in controlling the formation of ZnO nanostructures. XRD study shows that the nanostructures have very good crystallinity and have the hexagonal wurtzite ZnO structure. The room-temperature photoluminescence spectrum indicates that the nanostructures grown on annealed Ag template are less defective and have high optical quality. On the other hand, a very weak UV emission peak and the blue emission doublet band reveal that the ZnO sample grown on the as-deposited silver template are highly defective. The micro-Raman spectra of the ZnO nanostructures grown on both types of Ag templates show enhanced Raman scattering which is related to surface enhanced Raman scattering (SERS).

Introduction

Recently, zinc oxide (ZnO) has attracted much attention within the scientific community as a ‘key technological material’. Since 1960, the growth of ZnO thin films has been studied actively because of its application in sensors, transducers and optoelectronic devices. Nanostructured ZnO materials have also received attention due to their attractive performance in electronics, optics and photonics. One-dimensional ZnO structures can be used as gas sensors [1], [2], [3], ultraviolet (UV) and blue light-emitting devices (LEDs) [4], laser diodes [5], etc. ZnO nanostructures are suitable materials for field emission devices, too [6]. Furthermore, the piezoelectric property of ZnO enables it to be used in nanoscale mechanical devices [7].

ZnO is a versatile functional material that has various types of growth morphologies, such as nanorods [8], nanobelts [9], nanowires [10], nanotetrapods [11], nanomultipods [12], nanocages [13], nanocombs [14], nanorings [15], nanosprings [16] and nanohelixes [17]. A variety of techniques have been used to grow ZnO nanostructures. Solid–vapor and high-temperature vapor–liquid–solid (VLS) are often used to synthesize them. Some catalysts, such as Au, Ag, Ni and Co nanoparticles, influence growth [18] and structural/electronic properties of ZnO nanowires. In this paper, the effect of thermal annealing of silver catalyst for the growth of ZnO nanostructures has been discussed and the quality of the nanostructures has been characterized by different techniques.

Section snippets

Experimental details

A thin film of silver (thickness ∼1.4 μm) was deposited on p-type Si (1 1 1) substrates by thermal evaporation of metallic silver (purity ∼99.99%) without intentional heating of the substrate. Before deposition, cleaning of the silicon wafers was done by a 1:1 H2O2 and H2SO4 mixture followed by 5% HF etching. The samples were then rinsed with DI water and dried in flowing dry nitrogen. Few of the as-deposited silver films were annealed at 200 °C for 1 h in air to form silver island structures. ZnO

Results and discussion

Fig. 1a and b shows AFM surface profiles and corresponding particle distribution histograms for the as-deposited and annealed silver films on Si (1 1 1) substrates. The as-deposited silver film on Si substrate (Fig. 1a) is continuous with a finite number of particles scattered over the surface. The corresponding histogram indicates that the as-deposited silver film is smooth (RMS roughness: 4 nm) and hardly has particles with height larger than 25 nm. The annealed film is rough (RMS roughness: 45 

Conclusions

The annealing treatment of the Ag thin film is the key factor for the dense growth of sword-like ZnO nanostructures. The growth of ZnO nanostructures took place by the oxidation of precipitated Zn particles from Ag/Zn liquid alloy. The room-temperature photoluminescence study shows that the nanorods in both the samples are defective (O vacancy or Zn interstitials) in nature and the ZnO films grown on annealed silver templates are of high optical quality compared with ZnO films grown on

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