Low-temperature, self-nucleated growth of indium–tin oxide nanostructures by pulsed laser deposition on amorphous substrates Raluca Savu a, * and Ednan Joanni a,b a UOSE, INESC-Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal b Departamento de Fı ´sica, Universidade de Tra ´ s-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal Received 22 June 2006; revised 10 August 2006; accepted 14 August 2006 Available online 18 September 2006 Indium–tin oxide nanostructures were deposited by excimer laser ablation in a nitrogen atmosphere using catalyst-free oxidized silicon substrates at 500 °C. Up to 1 mbar, nanowires grew by the vapor–liquid–solid (VLS) mechanism, with the amount of liquid material decreasing as the deposition pressure increased. The nanowires present the single-crystalline cubic bixbyite structure, oriented h100i. For the highest pressure used, pyramids were formed and no sign of liquid material could be observed, indicating that these structures grew by a vapor–solid mechanism. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Laser deposition; Nanocrystalline materials; Nanostructure; VLS Recent developments in nanotechnology have led to the synthesis and characterization of a variety of nano- structures, such as nanowires, nanorods, nanotubes and nanopyramids. These nanometer-scale structures pos- sess enhanced optical and electrical characteristics due to quantum confinement effects, as well as high sur- face-to-volume ratios [1–3]. One of the crucial factors in the synthesis of nanowires is the precise control of the size uniformity, dimensionality, growth direction and dopant distribution within the nanostructures, as these structural parameters will ultimately dictate their functionality [1]. Tin-doped indium oxide is a well-known metal oxide degenerate semiconductor that has attractive properties for electronic and optoelectronic applications. More recently, research has been conducted on pure or tin- doped indium oxide nanostructures (predominantly nanowires) by both physical [4–6] and chemical methods [3,7], for potential applications in high-sensitivity sen- sor, optoelectronic, field-emission, electronic and mem- ory devices [3–8]. In this letter we report that indium–tin oxide (ITO) nanowires and nanopyramids can be prepared by the laser ablation process on catalyst-free amorphous silica surfaces at the relatively low-temperature of 500 °C. The influence of deposition pressure on the morphology and growth mechanism of the nanostructures was inves- tigated. Self-catalytic growth of zinc oxide nanowires by laser ablation has been previously reported, but in those articles the depositions were performed at higher tem- peratures and pressures on monocrystalline substrates or template layers [4,9]. Pulsed laser ablation was performed in order to gener- ate ITO nanostructures using a ceramic target with a composition of (In 2 O 3 ) 0.9 (SnO 2 ) 0.1 and catalyst-free oxidized silicon substrates. The target–substrate distance and the deposition time were kept constant at 4 cm and 20 min, respectively. A KrF laser beam with an energy of 130 mJ and a frequency of 10 Hz was focused on the target to a spot size of 0.2 cm 2 for all depositions. Before each deposition, the chamber was evacuated to least 10 7 mbar in order to avoid contamination by residual gases. After the substrate temperature reached 500 °C, pure nitrogen was introduced in the chamber to the desired pressure (0.1, 0.5, 1 or 2 mbar). The samples were cooled in the same atmosphere and pressure used for the deposition until the substrate reached a temperature below 100 °C. The morphology of the nanostructures and their dimensions were analyzed using a field-emission scan- ning electron microscope (SEM, Hitachi, S-4100). Figure 1 shows high-resolution SEM surface images of 1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.08.022 * Corresponding author. Address: Universidade Estadual Paulista – UNESP, Instituto de Quı ´mica, Rua Francisco Degni s/n, 14800-900 Araraquara – SP, Brazil. Tel.: +55 16 33016600; fax: +55 16 33227932; e-mail: raluk1978@yahoo.com Scripta Materialia 55 (2006) 979–981 www.actamat-journals.com