Zinc oxide (ZnO), occurring mainly in a hexagonal wurtzite structure,1 is an attractive, non-toxic, chemically stable wide bandgap (Eg) semiconductor (Eg = 3.37 eV at low temperature) owing to its technologically useful opto-electronic properties.2,3 Significantly, ZnO can be easily grown in a variety of bespoke nanostructures2,4,5 which typically provide different but controllable optical and mechanical properties when compared with their bulk counterparts. Consequently, bulk and nanostructured ZnO, in particular ZnO nanorods (NRs), can be used in a broad range of practical electronic devices, such as LEDs, laser diodes, photovoltaics, photodetectors and chemical sensors.6–14 However, many applications require precise control over the dimensions of the ZnO NRs. While this goal can be achieved by a number of different growth techniques,15–18 most of these approaches are expensive and often difficult to scale up. In addition, many of the growth techniques involving physical and chemical deposition methods use high temperatures19 which can often limit or degrade the electrical and optical properties of ZnO NR-based devices. Accordingly, Greene et al.20 proposed a cost-effective and large-scale hydrothermal method for the growth of ZnO NRs at very low temperatures around 90°C with a preferential growth direction along the c-axis.21–23 Significantly, low temperature fabrication also enables ZnO NR fabrication on flexible substrates, such as thermally sensitive organic materials, considerably increasing the utility and functionality of ZnO devices and their applications.24 Due to the distinct advantages of the low temperature hydrothermal technique, this method has been extensively studied to understand the fundamental growth mechanisms to precisely control the morphology and size of the ZnO NRs.25 Despite this effort, there still seems to be little agreement on which growth parameters are principally responsible for determining the spatial dimensions of the ZnO NRs. While some papers report that increasing the growth time results in longer and thicker hexagonal ZnO NRs,20,26−28 others conversely observed that only the NR length increased without either a diameter change29 or a modification in morphology.30 Similar controversy in the literature exists about the use of the growth temperature to tune diameter and/or length of NRs. For example, Xu et al.31 found that the growth temperature had the largest effect on the NR aspect ratio with the optimum aspect ratio occurring at 70°C, while higher temperatures resulted in pyramid-like nanostructures. Nandi et al.,32 however, observed that increasing the growth temperature from 70 to 90°C only affected the NR length but not the diameter, while only a change of morphology was found by Al-lami and Jaber.33 Hydrothermal growth of ZnO NRs conventionally uses a ZnO seed layer on the substrate before growth to promote a higher density and a better coverage of NRs. This ZnO seed layer is generally deposited by a broad range of techniques such as sputter-coating, drop-casting and spin-coating nanoparticles. These techniques collectively produce a wide range of different as-grown pre-treated substrate types.32,34−39 Therefore, it is important and relevant to study and understand the effects that the ZnO seed layer and the other growth parameters have on the degree of control over the morphology and dimensions of ZnO NRs, and to determine the reproducibility of these results.
The great interest in the control over and reproducibility of the morphology, dimension and alignment of ZnO NRs is mainly due the outstanding optical properties of ZnO NRs and their potential use for various opto-electronic applications and devices. Special emphasis has been paid to the enhancement of the excitonic emission in the UV and the reduction/suppression of the defect-related visible emission (DL) or an overall increase of UV-to-DL ratio. It has been repeatedly reported that surface coatings, particularly those consisting of metal thin films or metal nanoparticle (NP), can significantly enhance the UV emission of ZnO NRs (and other nanostructures).40–47 Due to its ease of fabrication and its inertness, Au NP coatings have been studied intensively by many researchers, including us, achieving a moderate excitonic emission enhancement in ZnO NRs. 46,48−50 Although the enhancement mechanism of Au NP-coated ZnO NRs has been attributed to different underlying effects – such as plasmonic coupling, energy- and (hot) electron-transfer, increase of spontaneous emission rate via an additional recombination channel – this article does not attempt to repeat any of the existing studies but rather aims to demonstrate the general advantage of using ZnO NRs with high angular orientation for metal NP coating-mediated enhancement. In the first part of this work, we present a systematic study of the influence of the ZnO seed layer, using a drop-casting deposition and sputter-coating technique, on the morphology of hydrothermally grown ZnO NRs. Additionally, we investigate the tunability of NR length and diameter by systematically varying growth time and temperature within a narrow window of 2 to 4 hours and a temperature ranging from 70 to 100°C.20,32,51 The rationale behind the selection of this small variation in growth parameters is two-fold: (i) the growth of hexagonal, thin ZnO NRs has been repeatedly reported to be within this range of growth parameters; (ii) such a slight change in growth parameters allows to gain insight in the reproducibility and the actual tunability of the dimensions of hydrothermally grown ZnO NRs which is of paramount importance for industrial implementation of this growth method.
We find that both the length and diameter of hydrothermally grown ZnO NRs vary considerably between batches using the exact same growth conditions. The seed layer is found to be important in ensuring homogeneous coverage of the ZnO NRs, however, the size of drop-casted seeds did not significantly influence the diameter of the resulting NRs. Instead, the diameter of the NR was observed to range from 20 to 60 nm with an approximate length of 600 µm regardless of the seed layer properties or the chosen growth parameters. Using a sputter-coated ZnO seed layer, however, provided some control over the diameter of the as-grown ZnO NRs. Furthermore, the coverage and density of ZnO seeds for both methods resulted in a controllable angular/vertical alignment of the hydrothermally grown ZnO NRs. These latter results are useful for application realizations such as wave-guiding, angular distributions of light-emitting profiles or sensing applications.
In the second part of this article, we study the optical properties of the hydrothermally grown ZnO NRs and present a method to enhance the UV emission in ZnO. We show that the as-grown ZnO NRs exhibit exceptional crystallinity with almost exclusive UV emission and close to no defect-related (DL) emission. For both types of excitations – photons and electrons – we achieve a high UV-to-DL ratio with up to 50. Furthermore, we demonstrate an up to 5.1-fold enhanced UV emission by applying an Au nanoparticle (NP) surface coating. This extraordinary optical enhancement is achieved by exploiting the wide angular distribution of the hydrothermally grown ZnO NRs. The random NR network allows for: (i) an excellent surface coverage without shadowing effects using a simple sputter-deposition of Au NPs, and (ii) a more effective (photo- or electron-beam) excitation due to larger surface areas facing the top of the sample, facilitating a larger emission enhancement of the NRs. We confirm (ii) by applying an identical Au NP-coating onto planar ZnO substrates, exhibiting an enhancement factor of only up to 2. ZnO NRs with high angular distribution can therefore be used in conjunction with surface coatings for light emission or light extraction enhancement/management of light-emitting or photovoltaic applications and devices.