Fabrication and optical properties of gold nanotube arrays

Arrays of gold nanotubes with polypyrrole cores were grown on glass substrates by electrodeposition into thin film porous alumina templates. Measurements of optical transmission revealed strong extinction peaks related to plasmonic resonances, which were sensitive to the polarization state and angle of incidence. On prolonging the electrodeposition of gold, the polypyrrole core became fully encapsulated and this had a dramatic effect on the optical properties of the arrays, which was rationalized by finite element simulation of the local field intensities resulting from plasmon excitation. (Some figures in this article are in colour only in the electronic version) Metal nanoparticles exhibit distinctive optical properties that are highly sensitive to their size, shape, orientation and environment. Underlying these properties are surface plasmons, collective oscillations of the conduction electrons of the metal. Plasmonic nanoparticles have increasing technological relevance and are finding diverse applications in areas such as cancer treatment [1, 2], sensors [3–5], metamaterials [6–8], optoelectronics [9–11] and sub-wavelength photonics [12–14]. In spite of the high level of interest in plasmonic nanoparticles of various shapes and sizes there have been relatively few reports on nanotubes. Yet they have potential applications as electrodes for nanoscale devices [15], in metal enhanced luminescence [16–18] and as SERS substrates [19]. Most methods for making metal nanotubes employ a porous alumina or polycarbonate membrane as a template. A typical approach is to chemically pretreat the template to promote plating of the pore walls in an electrochemical or electroless reaction [20–22]. Thus nanotube length is determined by template thickness and the inner diameter depends on deposition time. Recently, alternative approaches 3 Author to whom any correspondence should be addressed. that avoid chemical modification of the template and give independent control over nanotube length have been developed and used successfully [15, 16, 19, 23–27]. These either involve very specific deposition conditions [19, 23–26], or coaxial composite nanowires with sacrificial cores [15, 27], which once removed leave tubular shells. This paper reports the fabrication of vertically aligned arrays of gold nanotubes on glass substrates, which exhibit distinct plasmonic optical behaviour. The method of manufacture extends proven techniques originally developed to produce substrate supported nanowire arrays for optical studies [28–30]. The method for fabrication of gold nanotube arrays is summarized in figure 1. An anodic alumina (AAO) template was prepared by anodizing a specially developed thin film multilayer structure, incorporating an electrode and a 400 nm thick aluminium layer, in 0.3 M sulfuric acid [28]. Polypyrrole was deposited into the AAO template using a standard three electrode cell at a relatively high electropolymerization voltage of +1.5 V versus a Pt wire reference. The electrolyte was an acetonitrile solution containing 0.5 M pyrrole monomer and 0.3 M tetraethylammonium-p-toluene sulfonate [31]. After 0953-8984/08/362203+05$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1 J. Phys.: Condens. Matter 20 (2008) 362203 Fast Track Communication Figure 1. The process for the fabrication of gold nanotube arrays. Figure 2. SEM micrograph of a gold nanotube array before plasma etching. Polypyrrole cores can be seen emerging from inside the nanotubes. electropolymerization, the AAO pores were widened in sodium hydroxide to create space around the polypyrrole nanowires. Gold was electrodeposited from a thiosulfate–sulfite bath [32] to form gold nanotubes. To expose the nanotubes, the AAO template was dissolved in sodium hydroxide. The polypyrrole nanowires were removed by plasma etching in a 3:1 argon– oxygen atmosphere. The manufacturing route enables some control over the nanotube spacing (40–70 nm), length (<400 nm), inside diameter (10–40 nm) and outside diameter (30–60 nm). Figure 2 shows gold nanotubes with growth terminated at approximately half the polypyrrole wire length. The figure clearly demonstrates that the tubes grow from the substrate and not directly onto the polypyrrole. If the gold nanotube growth is not terminated before it reaches the top of the polypyrrole wires, overgrowth will occur and a new capped structure is formed. This is illustrated in figure 3, which shows capped and uncapped nanotubes after the removal of the alumina template and plasma etching. Figure 3. SEM micrographs of (a) an uncapped nanotube array (b) a capped nanotube array, after template removal and plasma etch. Nanotube array dimensions estimated from the images are as follows: inside diameter 16 nm, outside diameter 35 nm and spacing 52 nm. The optical properties of the arrays were investigated using measurements of transmission. In figures 4(a) and (b), the results for uncapped nanotubes are expressed as extinction spectra for sand p-polarized light at several angles of incidence. A broad peak extends over most of the observable range and extinction increases with angle of incidence for both polarization states; however, the effect is more pronounced for