Facile Patterning of Periodic Arrays of Metal Oxides

Highly ordered arrays of periodic nanostructures show interesting characteristics for applications in photonics, electronics, optoelectronics, sensing, biochips, and catalysis. The array properties are determined by the choice of material and can be tuned further by varying the geometry, chemical composition, periodicity, and the size of the nanostructures. Such controlled and elaborate arrays are commonly fabricated by X-ray or electron-beam lithography. However, there are limitations to patterning over large areas using these lithographical techniques due to long processing times, which result in higher costs. Relatively simple and costeffective nanopatterning processes have been developed to address these inherent limitations, including interference lithography, nanosphere lithography, and soft lithography. In order to pattern metals or metal oxides, however, the non-conventional methods still require multistep processing as does conventional photoresist lithography. In this paper, we demonstrate a novel but simple methodology for the fabrication of periodic structures comprising metal oxides at the sub-100 nm scale using polymeric templates, colloids, and common laboratory equipment. Thus, this nanopatterning technique does not require high-vacuum deposition, etching, or photoresists, which are common in other processes. The patterns produced with this method were periodic and composed of 1D or 2D arrays of nanostructures over large areas. Fabrication of these metal oxide nanostructures required only a few hours once the nanometer-sized colloid of the metal oxide and polymeric templates were available. The structural resolution of the resulting pattern was determined by the amount of colloidal deposition onto the grooves (or grating) of the templates used. Although this process utilized microstructured templates, it is possible to fabricate features of metal oxide at the 100 nm scale. The geometry and periodicity of the structure can be well defined and controlled through selection from a wide variety of polymeric templates fabricated by numerous techniques. This unique strategy to achieve controlled nanopatterning can utilize a wide variety of inorganic colloids and/or intricate templates (or masks) available from various established lithographic methods. Microstructured polymeric templates have been prepared using a single-step holographic patterning process by exposing films of azobenzene-functionalized polymers (AFPs) on substrates to an argon-ion laser (488 nm) interference pattern. The interference pattern produced photoinduced mass transfer in the polymer film giving rise to a surface-relief structure without additional processing. Such surface modulation leads to 1D or 2D periodic arrays of surface-relief-grating (SRG) structures on the polymeric films. A variety of holographic surface structures can be designed and fabricated with good control over the modulation and periodicity of the patterns. In this work, SRG structures from thin films of AFP on quartz and glass substrates were prepared by exposing the films to a 488 nm laser interference pattern. Titania nanoparticles, with a mean diameter of less than 10 nm, were then deposited selectively on the surface of the SRG using sequential electrostatic layer-by-layer deposition (ELbL) with the polyelectrolyte, poly(4-styrene sulfonate) (SPS). The nanolayered assemblies of SPS and titania (SPS/titania) were then heated to a suitable temperature in air to aggregate the colloids further along the grooves of the SRG. At higher temperatures, the colloidal particles sinter together to form 1D or 2D arrays of metal oxide and the polymeric template eventually degrades. The resulting line features were composed of titania and extended over several tens of micrometers or longer with line widths of 100 nm. This quick and cost-effective process took only a few hours to pattern more than one square centimeter using ordinary laboratory equipment under ambient conditions. Scheme 1 illustrates the stepwise process used to prepare these titanium dioxide nanopatterns on the surface of a glass substrate. Figure 1A and B show 3D views of the SRG patterns of the AFP films that were typically used in this approach. The AFP used in this work was a copolymer of bisphenol A diglycidyl ether and disperse orange 3 (PDO3). Both structures were used as the templates for the ELbL layering of SPS/titania but C O M M U N IC A TI O N S