Scripta Materialia 55 (2006) 979–981

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Low-temperature, self-nucleated growth of indium–tin oxidenanostructures by pulsed laser deposition on amorphous substrates

Raluca Savua,* and Ednan Joannia,b

aUOSE, INESC-Porto, Rua do Campo Alegre 687, 4169-007 Porto, PortugalbDepartamento de Fısica, Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal

Received 22 June 2006; revised 10 August 2006; accepted 14 August 2006Available online 18 September 2006

Indium–tin oxide nanostructures were deposited by excimer laser ablation in a nitrogen atmosphere using catalyst-free oxidizedsilicon substrates at 500 �C. Up to 1 mbar, nanowires grew by the vapor–liquid–solid (VLS) mechanism, with the amount of liquidmaterial 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, indicatingthat these structures grew by a vapor–solid mechanism.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Laser deposition; Nanocrystalline materials; Nanostructure; VLS

Recent developments in nanotechnology have led tothe synthesis and characterization of a variety of nano-structures, such as nanowires, nanorods, nanotubes andnanopyramids. These nanometer-scale structures pos-sess enhanced optical and electrical characteristics dueto quantum confinement effects, as well as high sur-face-to-volume ratios [1–3]. One of the crucial factorsin the synthesis of nanowires is the precise control ofthe size uniformity, dimensionality, growth directionand dopant distribution within the nanostructures, asthese structural parameters will ultimately dictate theirfunctionality [1].

Tin-doped indium oxide is a well-known metal oxidedegenerate semiconductor that has attractive propertiesfor electronic and optoelectronic applications. Morerecently, research has been conducted on pure or tin-doped indium oxide nanostructures (predominantlynanowires) 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

1359-6462/$ - see front matter � 2006 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2006.08.022

* Corresponding author. Address: Universidade Estadual Paulista –UNESP, Instituto de Quımica, Rua Francisco Degni s/n, 14800-900Araraquara – SP, Brazil. Tel.: +55 16 33016600; fax: +55 1633227932; e-mail: raluk1978@yahoo.com

laser ablation process on catalyst-free amorphous silicasurfaces at the relatively low-temperature of 500 �C.The influence of deposition pressure on the morphologyand growth mechanism of the nanostructures was inves-tigated. Self-catalytic growth of zinc oxide nanowires bylaser ablation has been previously reported, but in thosearticles the depositions were performed at higher tem-peratures and pressures on monocrystalline substratesor template layers [4,9].

Pulsed laser ablation was performed in order to gener-ate ITO nanostructures using a ceramic target with acomposition of (In2O3)0.9(SnO2)0.1 and catalyst-freeoxidized silicon substrates. The target–substrate distanceand the deposition time were kept constant at 4 cm and20 min, respectively. A KrF laser beam with an energyof 130 mJ and a frequency of 10 Hz was focused on thetarget to a spot size of 0.2 cm2 for all depositions. Beforeeach deposition, the chamber was evacuated to least10�7 mbar in order to avoid contamination by residualgases. After the substrate temperature reached 500 �C,pure nitrogen was introduced in the chamber to thedesired pressure (0.1, 0.5, 1 or 2 mbar). The samples werecooled in the same atmosphere and pressure used forthe deposition until the substrate reached a temperaturebelow 100 �C.

The morphology of the nanostructures and theirdimensions were analyzed using a field-emission scan-ning electron microscope (SEM, Hitachi, S-4100).Figure 1 shows high-resolution SEM surface images of

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Figure 1. Surface images of nanostructured films deposited at 0.1 mbar(a, b), 0.5 mbar (c, d), 1 mbar (e, f) and 2 mbar (g, h).

Figure 2. Cross sections (left column) and tilted surface images (rightcolumn) of nanostructured films deposited at 0.1 mbar (a, b), 0.5 mbar(c, d), 1 mbar (e, f) and 2 mbar (g, h).

980 R. Savu, E. Joanni / Scripta Materialia 55 (2006) 979–981

the nanostructures grown at 0.1, 0.5, 1 and 2 mbar depo-sition pressures, respectively. Figure 2 shows the crosssection and tilted surface images of the same samples.In order to allow cross-sectional observation, the sam-ples were snapped, generating a number of misaligned,detached or broken needles, as can be observed inFigure 2a, c and d.

The nitrogen pressure during deposition had a strongeffect on the morphology of the nanostructures. For0.1 mbar deposition pressure (Figs. 1a and b and 2aand b) the SEM images display big structures, withdiameters between 500 nm and 1 lm, dispersed in a ma-trix of very thin and short wires. The nanowires are notpreferentially oriented in relation to the substrate, areapproximately 40 nm thick and 700 nm long, and havebranches growing perpendicular and aligned along theirlength. Both the wires and the branches have balls onthe tips, indicating that they were formed by thevapor–liquid–solid (VLS) growth mechanism. More evi-dence for the existence of a large amount of liquid dur-ing deposition is provided by the shape of the bigstructures, which appear to have been once composedof molten material. Metallic In–Sn alloy droplets areformed in the oxygen-deficient deposition atmosphereat high temperatures, and act as catalysts for unidimen-sional crystal growth; similar results were obtained by

Chen et al. [10] using thermal evaporation of metallicIn and SnO powders at a much higher deposition tem-perature (920 �C).

When the deposition pressure is increased to 0.5 mbarthe only structures present in the film are nanowires, ascan be observed in Figures 1c and d and 2c and d. Thesmaller number and size of the spherical particles in thefilm indicates a decrease in the amount of liquid materialpresent during growth. The nanowires and theirbranches are longer than the ones formed at lower pres-sure, since the material that was present in the big struc-tures is now used to form the wires. This seems to bereasonable if we assume that the amount of materialreaching the substrate is approximately the same forboth depositions. The nanowires are more aligned tothe substrate and are relatively uniform in size (length�2.5 lm; diameter �60 nm). Increasing the pressure to1 mbar decreases even further the material found in li-quid state, and as a consequence the nanowires presentfewer branches and have a better alignment, perpendic-ular to the substrate, as shown on Figures 1e and f and2e and f.

A deposition pressure of 2 mbar results in a com-pletely different morphology of the nanostructures.The open structure of the previous films composed of

Figure 4. TEM image of an individual nanowire from the film grownat 1 mbar deposition pressure; the selected area electron diffractionpattern was taken perpendicular to the long axis of the wire, showingthe (400) growth direction.

R. Savu, E. Joanni / Scripta Materialia 55 (2006) 979–981 981

nanowires is replaced by a dense array of prisms andpyramids grown in close contact, as can be observedin Figures 1g and h and 2h. The cross section shownin Figure 2g confirms the dense columnar growth. Notraces of molten material can be seen in the pictures,suggesting that for high deposition pressures the nano-structures grow by a vapor–solid mechanism.

X-ray diffraction scans were performed in order toassess the crystal structure of the nanostructures andtheir orientation (Rigaku D/max, Siemens D5000).Figure 3 shows the diffraction patterns of the targetas well as of the films whose SEM images are displayedin Figures 1 and 2. Even though the X-ray diffrac-tion pattern of the target shows that the two strongestlines for tin oxide are present, the films exhibit peaksfrom only the cubic bixbyite structure of indium oxide.The samples made at 0.1 and 0.5 mbar depositionpressures present a diffraction pattern, similar to theone for the target, showing that for this range of depo-sition pressures the needles are randomly arranged, aspreviously observed by SEM. For the highest pressures(1 and 2 mbar) the films exhibit (400) orientation,confirming the SEM observations that for these deposi-tion conditions there is a good alignment of the nano-wires and nanopyramids, respectively. The strong(400) orientation of the nanowires obtained at 1 mbarindicates a highly anisotropic crystalline growth anda good degree of alignment, perpendicular to thesubstrate.

Further detailed structural characterization of thenanowires was performed on a high-resolution transmis-sion electron microscope (TEM, Hitachi H-9000).Figure 4 shows a TEM image as well as the selectedarea electron diffraction pattern (SAED) of a needletaken from the sample grown at 1 mbar. The ITO nano-wires present a good homogeneity, without grainboundaries, indicating their single-crystalline structure.Energy-dispersive X-ray analyses performed on the ballsat the tips of the nanowires indicated a higher concen-tration of tin and a lower concentration of oxygen com-pared to the body of the wires; the decreased oxygenconcentration at the wire tips was expected, given the

Figure 3. X-ray diffraction patterns of the target and of the sampleswhose SEM images appeared in Figure 1 (the peak marked with * isfrom the substrate).

presence of an indium–tin liquid metal alloy duringVLS crystal growth. The SAED pattern taken perpen-dicular to the long axis of the nanowire conforms tothe indexed patterns from the cubic (bixbyite) indiumoxide structure, showing that the growth direction ish100i.

In conclusion, we succeeded in synthesizing ITOnanowires by the laser ablation method onto catalyst-free silica substrates at a low-temperature of 500 �Cusing an oxide target and an oxygen-free depositionatmosphere. The deposition pressure had a strong influ-ence on the morphology and dimensions of the nano-structures. At low pressure a large amount of liquid isformed during deposition, promoting the VLS growthof thin, branched nanowires. As the deposition pressureincreases, the amount of liquid phase decreases and thenanowires have fewer branches, until at 1 mbar the wiresare almost perpendicular to the substrate and free ofbranches. At 2 mbar the growth mode is no longer uni-dimensional and a columnar dense film is formed withpyramidal and triangular forms ((4 00) and (222)orientations) protruding from the surface. These nano-structures have potential applications in field-emissiondisplays, gas sensors, solar cells, photodetectors andother nanoscale devices.

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