ter of the nanopillars was quite noticeable and would be useful in fabricating nanostructures with feature sizes smaller than those of the original master. The aspect ratio was as high as 5 for the nanostructures produced using SPS, and might be increased further with structures of smaller feature sizes. The black spots next to the pillars in the figures are dimples formed by pressure buildup.In summary, we have presented the observation of several intriguing nanostructures, such as mushroom-like nanopillars, vertical nanopillars, and nanospheres, using capillary lithography with a UV-curable, polyurethane acrylate mold. It has been shown that air permeation during capillary rise plays an important role in pattern replication, which has not been previously observed in any type of nanofabrication involving PDMS molds. Depending on the film thickness of the polymer solution or the wetting conditions at the time of contact, nanopillars or nanospheres were observed for two different step heights of the mold used in the experiment. Furthermore, the step height could be adjusted to obtain well-defined vertical nanopillars with diameters less than that of the step height. This simple method would be potentially useful in fabricating unique nanostructures without resorting to other complicated, multistep methods.
ExperimentalFabrication of UV-Curable Mold: The UV-curable mold material consisted of a functionalized prepolymer with acrylate groups for crosslinking, a monomeric modulator, a photoinitiator, and a radiation-curable releasing agent for surface activity. Details on the synthesis and characterization of the polymer have been published elsewhere [26]. The UV-curable mold used in the experiment was a thin sheet with a thickness ranging from 0.3 to 1 mm (see Fig. 1d).Polymers: We used a PEG-based random copolymer, poly(3-trimethoxysilyl)propyl methacrylate-r-polyethylene glycol methyl ether) (poly(TMSMA-r-PEGMA)) that has potential for use in biological applications. This polymer contains surface-reactive trimethoxysilyl groups as part of its backbone, which allows for the formation of multivalent bonds onto oxide surfaces, as well as multiple PEG chains. Detailed information on the synthesis and characterization of the polymer has been published elsewhere [28]. For comparison, we also used poly(sodium 4-styrenesulfonate) with a molecular weight of 200 000 (30 wt.-% in water, Aldrich). The polymer solution was diluted prior to use.Capillary Lithography: A few drops of the polymer solution, of varying concentration (1±10 wt.-%), were placed on a silicon substrate and thin films were obtained by spin coating (Model CB 15, Headaway Research, Inc.) at 1000 rpm for 10 s. To make conformal contact, the polyurethane acrylate molds were carefully placed onto the surface and then the samples were stored overnight at room temperature to allow for evaporation of the solvent. The molds were peeled off using a sharp tweezer after complete evaporation of the solvent.Scanning Electron Microscopy (SEM): Images were taken using a h...
