Observation of High-Aspect-Ratio Nanostructures Using Capillary Lithography, by K. Y. Suh, S.-J. Choi, S. J. Baek, T. W. Kim, and R. Langer, Adv. Mater. 2005, 17, No. 5, March 8

Agenda Periodic nanostructure Capillary lithography Results Nanostructure for 300 nm High Mold for Different Film Thickness Nanostructure for 500 nm High Mold for Different Film Thickness Nanostructure for 450 nm High Mold for Different Polymer Conclusion 5/13/20152

Periodic Nanostructure Potential application Photonic crystals Data storage Nanometer-scale biological sensor 5/13/20153 Photonic crystals, from S. M. Yang, G. A. Ozin, Chem. Commun. 2000, 2507 Data storage, from Y.-W. Chen, Y.-H. Tang, L.-Z Pei, and C. Guo,Adv. Mater 2005, 17, No. 5, March 8 Nanometer-scale biological sensor, from K.-B. Lee, S.-J. Park, C.-A. Mirkin, J. C. Smith, and M. Mrksich, Science 2002, 295, 1702.

Capillary Lithography (I) Temperature-Induced Capillarity Solvent-Induced Capillarity A small gap is present between the mold and the polymer. No gap is present since the solvent wet the entire mold surface. 5/13/20154 Place the mold on the polymer surface Heating Cooling and mold remove Slight pressing Solvent evaporate Remove the model Solvent polymer Mold

Capillary Lithography (II) Material for capillary lithography Mold Polyurethane acrylate Polymer Poly(ethylene glycol)-based (PEG-based) random copolymer Capillary rise is not expected but happens. It provides several unexpected nanostructure as a result of gas permeation and different wetting condition. Permeability issue comes into play in the case of the polyurethane acrylate mold, especially when solvent-induced capillary was utilized. 5/13/20155

Capillary Lithography (III) Depending on the etching conditions of the original silicon master, the height of the voids of the mode could be controlled. The voids are of a truncated cone shape, with dimensions of nm at the base and nm at the top. a) SEM images of the positive mold with step heights of 300 nm. b) SEM images of the positive mold with step heights of 500 nm. c) SEM image of the replicated negative mold. d) An example of the sheet-type polyurethane acrylate mold. 5/13/20156

Nanostructure for 300 nm High Mold for Different Film Thickness The max height of the structure is (2/3) of the mold height. Be achieved by Maintain conformal contact over the entire surface without many trapped bubbles To avoid air trapped in the voids to escape Air permeability of the mode is very small No air escape through the mode The solubility of air in the polymer solution is smaller Valid for the water is the solvent Two different nanostructures Nanopillar has an average height of 300 nm (for thin films < 500 nm). Nanosphere has an average height of nm (for thick films >800 nm). SEM images of the two types of nanostructures formed when the mold with a step height of 300 nm is used. a) Formation of nanopillars of reduced diameter. A dimple next to a nanopillar is indicated by an arrow in the inset. b) Formation of anao- hemispheres. Some hemisphere that are peeled off a shown as voids in the inset. dimple 5/13/20157 Peel off

Nanostructure for 500 nm High Mold for Different Film Thickness (I) Thin film forms mushroom-like nanopillar Mushroom-like nanopillar A structure in which the head is larger than the body. Caused by the polymer solution wets the adjacent dry region in the mold void after reaching the ceiling. The orientation varies for each nanopillar because of different wetting path Has a reduced diameter. Dimples are formed next the nanopillar A schematic illustration for the formation of the two kinds of nanostructure, depending on wetting condition 5/13/20158

Nanostructure for 500 nm High Mold for Different Film Thickness (II) Thin film forms mushroom-like nanopillar Mushroom-like nanopillar Has a reduced diameter. Dimples are formed next the nanopillar. Thick film forms nanospheres The diameter of the sphere is nm. The spaces between nanospheres is 500 nm. Agrees with the mold. SEM images of the two types of nanostructure formed using the mold with a step height of 500 nm. a, c) Formation of mushroom-like nanopillar with a reduced diameter; large- scale (a) and magnified (c) view are shown. A dimple next to a nanopillar is indicated by an arrow in (c). b, d) Formation of nanospheres; larger scale (b) and magnified (d) views are shown. dimple 5/13/20159

Nanostructure for 450 nm High Mold for Different Polymer (II) Use 450 nm high mold with different polymer to form nanopillar SPS has a reduced 90 nm diameter PEG has a reduced 110 nm diameter The formation of vertical sidewalls is quite reproducible. The black spots next to the pillars in the figures are dimples formed by pressure buildup. 5/13/ SEM images of well-defined, high-aspect-ratio nanopillars using a) SPS and b) PEG copolymer. The diameters at the base are 90 nm and 110 nm, respectively.

Conclusion Present the observation of several nanostructures, such as mushroom-like nanopillars, vertical nanopillars, and nanospheres, using capillary lithography with UV-curable, polyurethane acrylate mold. Air permeation during capillary plays an important role in pattern replication. Depends on the thickness of the film, a nanopillar or nanosphere will be produced. The step height of the mold could be adjusted to obtain well- defined vertical nanopillars less than the step height of mold. 5/13/201511