Synthesis of aligned single-walled nanotubes using catalysts defined by nanosphere lithography.

Synthesis of highly aligned single-walled carbon nanotubes (SWNTs) with controlled positions is an important step toward manufacturable ultradense carbon nanotube integrated circuits. One of the most promising synthesis methods is chemical vapor deposition (CVD), in which small catalyst particles with diameters of approximately a few nanometers determine the position and diameter of SWNTs.1 Significant advance has been made in the preparation of catalyst nanoparticles, including chemical synthesis and e-beam patterning.2 In parallel, aligned nanotube growth has been achieved using sapphire3 and quartz substrates4 using randomly distributed ferritin or evaporated metal particles. Novel techniques that combine innovative catalyst preparation and aligned nanotube growth will be essential for further progress in the nanotube field. Here we report the development of a nanosphere lithography (NSL) technique5 for the preparation of catalyst nanoparticles for the synthesis of aligned single-walled carbon nanotubes on quartz. This technique, also known as colloidal crystal lithography for catalyst nanoparticles formation, uses close-packed monolayer or double layer of nanospheres as a shadow mask for metal evaporation, where the size and shape of resulting nanoparticles depend on (1) the size of template nanospheres, (2) incidence angle of metal evaporation, and (3) thickness of deposited metal layer. Previously, NSL utilized mostly large nanospheres (e.g., 895 nm in ref 6) and was shown to produce regular arrays of particles with uniform size (ca. several tens of nm) that have been successfully implemented to produce arrays of vertical nanowires6 or multiwalled carbon nanotubes.7 Packing of smaller nanospheres and synthesis of SWNTs has been achieved with the assistance of confinement defined by e-beam lithography,8 which has the drawback of being a serial, slow, and expensive technique. In this paper we demonstrate that ordered arrays of nanospheres as small as 50, 100, and 200 nm can be obtained via a simple and reliable spin-coat technique, which subsequently led to highly ordered catalyst nanoparticles with narrow diameter distributions suitable for singlewalled nanotube growth. In addition, we have combined photolithography and nanosphere lithography to gain simultaneous control over the packing and location of the nanospheres and catalysts. This technique has led to the successful synthesis of highly aligned and defect-free single-walled carbon nanotubes on quartz and sapphire, as revealed by scanning electron microscopy (SEM) and Raman characterization. Figure 1a shows the schematic diagram of nanosphere lithography for aligned nanotube growth. Quartz or sapphire substrates were first cleaned in piranha (H2SO4/H2O2 ) 3:1) followed by base treatment (H2O/NH4OH/H2O2 ) 5:1:1) with sonication for 1 h to render the surface hydrophilic.5 We subsequently optimized the recipe for the deposition and packing of polystyrene spheres by comparing different deposition techniques (spin-coat vs drop and drying), and using polystyrene from different vendors, of different sizes, and with different concentrations. The optimum packing has been achieved by spin coat of 200, 100, and 50 nm polystyrene nanospheres (from Duke Scientific Co. and Alfa Aesar) at concentrations of 2%, 2%, and 1% and at spin rates of 4000, 4000, and 5000 rpm, respectively. This process resulted in the selfassembly of nanospheres into a close-packed monolayer or bilayer structures. Figure 1b shows the photograph of a three-inch quartz wafer with spin-coated 200 nm polystyrene nanospheres. Figure 1 panels c-e display the SEM images of highly ordered monolayers of 50, 100, and 200 nm, respectively, with domain size up to several μm2. Figure 1e inset shows the SEM image of a 200 nm polystyrene bilayer structure. We note that our work represents significant extension of nanosphere lithography toward the deep submicrometer regime, as most previous work dealt with rather large nanospheres around several hundred nanometers.6-7 We subsequently deposited 5-10 Å of catalyst metal films made of Fe, Ni, or Co onto the substrates through the nanosphere shadow mask at either normal incidence or at controlled angles with respect to the substrate surface for fine-tuning of the catalyst size. The substrates coated with the catalyst metal films were then soaked into dichloremethane solution and sonicated for 3-5 min to remove nanospheres, leaving metal films deposited through the openings of the NSL mask on top of the substrate (Figure 2a). After annealing the substrates at 700∼900 °C in H2 ambient for 10 min, the remaining metal films aggregated into spherical catalyst particles, as shown in Figure 2b. By assuming all volume of the metal film Figure 1. (a) Schematic diagram of nanosphere lithography for aligned nanotube growth; (b) photograph of a three-inch quartz wafer with spincoated polystyrene nanospheres; (c-e) SEM images of ordered monolayers of 50, 100, and 200 nm polystyrene nanospheres, respectively; (e, inset) SEM image of an ordered bilayer of 200 nm nanospheres (The bottom layer is visible through the missing nanosphere in the top layer). Published on Web 08/01/2007