Effects of substrate annealing on the gold-catalyzed growth of ZnO nanostructures
© Weigand et al; licensee Springer. 2011
Received: 23 June 2011
Accepted: 26 October 2011
Published: 26 October 2011
The effects of thermal substrate pretreatment on the growth of Au-catalyzed ZnO nanostructures by pulsed laser deposition are investigated. C-plane sapphire substrates are annealed prior to deposition of a thin Au layer. Subsequent ZnO growths on substrates annealed above 1,200°C resulted in a high density of nanosheets and nanowires, whereas lower temperatures led to low nanostructure densities. Separate Au film annealing experiments at 700°C showed little variation in the size and density of the Au catalyst droplets with substrate annealing temperature. The observed variation in the density of nanostructures is attributed to the number of surface nucleation sites on the substrate, leading to a competition between nucleation promoted by the Au catalyst and surface nucleation sites on the rougher surfaces annealed below 1,200°C.
Keywordszinc oxide laser ablation atomic force microscopy thermal annealing vapourliquid-solid growth nanostructures; surface roughness surface defects
Semiconductor nanostructures have attracted great interest in the past decade due to a wide range of potential applications, e.g., in solar cells, lasers and sensors, and as building blocks of integrated systems [1–3]. Fabrication of different types of nanostructures, such as nanowires, nanorods, nanobelts and nanosheets can be achieved from a variety of methods, including solution-based, chemical and physical vapor deposition techniques, with and without the use of a metal catalyst [1, 4–7]. The morphology and orientation of nanostructures can be controlled by tuning growth parameters such as the substrate temperature, background pressure and precursor flux, as well as the substrate material. Furthermore, substrate treatments like chemical etching and thermal annealing have been shown to have significant impact on nanostructure growth for solution-based and catalyst-free vapor deposition techniques [8–11]. For catalyst-assisted nanostructure growth, however, little information exists on the effects of substrate pretreatment prior to catalyst metal deposition . Moreover, reports on pretreatment by thermal annealing often refer to substrates coated with a thin layer of the catalyst metal, resulting in alloying and formation of catalyst droplets, which serve to guide the subsequent nanostructure growth [13–16]. Thermal annealing of clean substrates, however, is often reported to cause smooth and well-defined step-and-terrace substrate surfaces [17, 18].
In this report, we show how thermal annealing of the substrate prior to catalyst metalization can significantly impact catalyst-assisted nanostructure growth. This is demonstrated for ZnO nanostructures grown on c-plane sapphire substrates by pulsed laser deposition (PLD) using gold as a catalyst.
The nanostructures were grown by ablation from a raster-scanned ZnO target using a 248-nm KrF excimer laser at 10 Hz repetition rate and a fluency of ~1.33 J/cm2. The substrates were heated to 700°C in a 0.5 mbar ambient of 5% oxygen/95% argon and ZnO was deposited for 30 min. Prior to growth, the "epi-ready" c-plane sapphire substrates (Valley Design Corp., 0° ± 0.25° miscut) were annealed in oxygen at 1,000, 1,200 and 1,400°C for 1 h, followed by deposition of a 1-nm-thin layer of Au using e-beam evaporation. No further annealing of the Au layer took place before introduction into the PLD chamber.
In order to investigate the effects of substrate annealing on the formation of Au catalyst droplets, separate experiments were carried out in the PLD chamber with a Au layer only. C-plane sapphire substrates coated with a 1-nm-thin film of Au were annealed for 5 min at the same growth temperature and ambient as for the growth of ZnO nanostructures.
The clean and Au-coated substrates were examined using atomic force microscopy (AFM), and the PLD-grown ZnO nanostructures were studied with scanning electron microscopy (SEM).
The size and density of the catalyst droplets resulting from the Au layer annealing experiments were determined using the image processing software "ImageJ" . The analysis procedure adopting image contrast enhancement, noise removal and particle separation by threshold and "watershed" methods was applied to 2μm × 2μm AFM images of the Au droplets. From subsequent automated particle measurements, the area, circularity, diameter and number density of the Au droplets were calculated.
Results and discussion
Characteristic features of substrates annealed at different temperatures and nanostructures grown on these substrates
Substrate anneal T (°C)
RMS roughness (nm)
Terrace width (nm)
Gold droplet size (nm)
Gold droplet density (cm-2)
Nanostructure density (cm-2)
24.5 ± 10.8
7.3 · 1010
2.53 · 108
22.3 ± 10.2
7.0 · 1010
4.21 · 108
24.0 ± 9.3
6.8 · 1010
1.19 · 109
19.4 ± 6.5
1.4 · 1011
9.87 · 108
In the ideal scenario of catalyst-assisted nanostructure growth, the growth species are all incorporated into the nanostructure lattice via the catalyst-nanostructure interface. These species can reach the catalyst droplet either by direct impingement from the vapor or by surface diffusion from the substrate. In reality, however, the catalyst-nanostructure interface competes with nucleation at low-energy surface sites such as pits, craters and grain boundaries, as well as other surface defects abundant in rough surfaces [22–24]. At these surface sites, catalyst-free growth of ZnO is promoted, leading to reduced incorporation of growth species at the catalyst-nanostructure interface. Simultaneously, these surface sites also provide increased energy barriers for surface diffusion and thus imply reduced diffusion lengths of the adsorbed growth species .
Furthermore, it is obvious from Figure 4 that the rate of catalyst-free ZnO growth is significantly higher on the as-received substrate than on sapphire annealed at 1,200°C. It has been previously reported that high rates of catalyst-free, vapor-solid (VS) growth can obstruct the catalyst-assisted growth of oxide nanostructures due to competition between the two growth modes . This provides further support for the scenario proposed in the present work.
In this study, we have shown that thermal annealing of the substrate prior to the Au catalyst deposition affects the density of ZnO nanostructures grown on c-plane sapphire. However, this substrate annealing does not seem to have a significant impact on the nanostructure morphology or the size and location of the Au catalyst droplets. The observed difference in nanostructure size and density can be explained by the competition between nucleation at the Au-ZnO interface and nucleation at low-energy surface sites associated with defects on rough substrate surfaces. The atomically flat surfaces obtained by high-temperature annealing promote formation of high densities of ZnO nanostructures through a significant reduction in surface nucleation sites, thus demonstrating the importance of smooth surfaces for catalyst-assisted nanostructure growth. We believe these findings will help improve control and understanding of catalyst-assisted nanostructure growth, also beyond the ZnO material system.
This report is based on work supported by the "NANOMAT" program of the Research Council of Norway under grant no. 182092/S10.
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