Cryst. Res. Technol. 49, No. 8, 558–563 (2014) / DOI 10.1002/crat.201300422 Original Paper Controllable vapor phase growth of vertically aligned ZnO nanorods on TCO/Glass substrates Sathish Chander Dhanabalan, John Paul Garcia, Davide Calestani ∗ , Francesco Pattini, F. Bissoli, Marco Villani, Stefano Rampino, and Andrea Zappettini Received 17 December 2013, revised 21 February 2014, accepted 27 May 2014 Published online 8 July 2014 Solution-free and catalyst-free vertically aligned ZnO nanorods have been synthesized by thermal CVD reactor at relatively low temperature (<500 °C) to produce high- surface 3D photoanode on glass substrate. Diferent TCOs flms such as Al doped ZnO flms deposited by PED, RF- sputtering techniques and ITO were considered for the growth as starting seeding layer for the nanorods. The aim of the paper is mainly focused to control the thickness and length of these nanostructures by varying not only the growth parameters, such as amount of Zn evaporation, but also substrate characteristics, such as grain size of Al doped ZnO and ITO seeding flms. The morphology of the difer- ent TCO substrates and also the grown ZnO nanorods have been analyzed with the help of atomic force microscopy and scanning electron microscopy. The study revealed that size and orientation of ZnO nanorods are mainly related to TCO’s grain morphology and crystallinity, while their length can be controlled by varying Zn evaporation parameters. 1 Introduction Arrays of one dimensional aligned nanostructures of- fer the concurrence of high efficiencies of charge car- rier transport and large interface areas for charge sep- aration and/or transfer with the surrounding species, material or electrolyte. So that, nanowires (NWs) and nanotube based electrodes are broadly used to improve the efficiency of a plenty of applications where both charge carrier separation and transfer are fundamen- tal [1]. Zinc oxide is one of the most interesting wide- gap semiconductor material because of its physical and chemical properties, direct band gap of 3.37 eV and a large exiton binding energy of 60 meV at room temper- ature. Moreover, it has a polar crystal structure and ex- cellent piezoelectric properties. ZnO is easy to obtain in form of nanostructures with different shapes due to its peculiar properties, allowing researchers to design complex 3D-structures for ZnO based power generators and transparent conducting oxides (TCOs), with smarter light and more efficient short range charge collection in excitonic solar cells. More specifically, vertical aligned ZnO nanowires hold the promise to drive the revolu- tion in different applications such as dye-sensitized so- lar cells (DSSCs) [2, 3], piezoelectric energy harvesting [4, 5], light-emitting diodes [6–8], water splitting, [9–11] sensor arrays, field electron emitter arrays, and verti- cal field-effect transistors, [12] etc. Synthesis of vertically aligned ZnO nanowires has been always very challeng- ing for device fabrications. It was already suggested [13, 14] that control over the dimension, composition, po- sition, orientation, reproducible and large-area growth processes of the ZnO nanostructures plays a very cru- cial role in the development of novel devices. Among the various available techniques to synthesize aligned ZnO nanorods, the use of thermal evaporation reactor is surely one of the simpler and cheaper vapour phase pro- cesses, as it does not require expensive precursors and catalysts for the growth of metal oxide nanostructures and it relies just on a metallic Zn source. The growth of vertically oriented ZnO nanorods on TCOs by this method at relatively low temperature (< 500 °C) was pre- viously reported by our group [15, 16]. It is an alterna- tive solution-free and catalyst-free synthesis method to chemical approach and it has been explored for the re- alization of 3D nanostructured ZnO-based TCOs. That studies concluded that small Zn nanoclusters on the polar surface of a (001)-oriented ZnO film can gen- erate preferential, energetically favoured and oriented ∗ Corresponding author: e-mail: calle@imem.cnr.it; Phone: +39.0521.269202; Fax: +39.0521.269206 IMEM-CNR, Parma, Italy 558 C  2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim