Dae Ho Lee

Switchable Transparency and Wetting of Elastomeric Smart Windows

Smart windows with switchable light transmittance properties have recently attracted signifi cant attention because of their many applications, such as architectural or vehicle windows, skylights, and internal partitions. To date, reversible switching has been achieved either by a change in molecular arrange-ments or by the oxidation–reduction reaction of chromogenic materials activated by light, electrical voltage, or tempera-ture.

Hierarchical gecko-inspired nanohairs with a high aspect ratio induced by nanoyielding

We present a simple method for fabricating hierarchical polymeric nanohairs using a multi-branched anodic aluminum oxide (AAO) template prepared by two-step anodization and barrier layer thinning processes. Combined with nanohair yielding of a polymeric material during peeling-off from the hydrophobically modified AAO template, elongated hierarchical nanohairs with a high aspect ratio were fabricated without fiber collapse, which showed excellent adhesive and frictional properties compared to other structures such as single-level nanohairs and hierarchical nanohairs with a low aspect ratio. Furthermore, this surface showed remarkable superhydrophobic properties similar to those of natural gecko foot hairs.

Solubility-driven polythiophene nanowires and their electrical characteristics

We demonstrated that interconnected nanofibrillar networks of poly(3-hexylthiophene) (P3HT) thin films with improved crystallinity can be easily fabricated by aging the precursor solution with marginal solvent. Structural analysis revealed that these benefits arise from the improvements in the crystallinity of P3HT in the precursor solution. At dilute concentrations, P3HT molecules grew into near-spherical particles during the aging time. As the aging time increased further, P3HT molecules exhibited one-dimensional growth into rod-like structures. At higher P3HT concentrations and longer P3HT solution aging times, dense nanowires were observed to form gradually, thereby improving the electronic properties of field-effect transistors (FETs) based on these films. This improvement was due to the change in P3HT organization in the precursor solution from a random-coil conformation to an ordered aggregate as a result of aging in a marginal solvent, methylene chloride. At high temperatures, the P3HT molecules were completely solvated and adopted a random-coil conformation, as is observed in good solvents. Whereas upon aging the solution at room temperature, methylene chloride poorly solvated the P3HT molecules such that ordered aggregates of P3HT grew in solution, which improved the molecular ordering of P3HT thin films produced from these solutions. The field-effect mobility of the thin films was, therefore, enhanced without the need for post-treatments.

Chemical and geometrical criteria for the release of elastomeric 1D nanoarrays from porous nanotemplates.

Releasing 1D nanoarrays from nanotemplates is a significant challenge for the integration of mechanically soft materials in a variety of newly emerging technological areas. To fabricate nanoarrays without defects, the combined effects of the surface energy and the geometric features of the nanotemplate should be considered. A previously reported approach based on the correlation between the adhesion energy and the real contact area was not satisfactory to describe the rupture conditions of the nanofibers while they were being peeled off from the porous template. Here we demonstrate that the aspect ratio rather than the contact area of the nanoporous template is the key factor determining the upper limit of the pore length of the nanotemplate with respect to the rupture of the nanoarray during separation. We propose that the value of alpha(c)*, which is calculated with a simple expression in which the adhesion energy is multiplied by the aspect ratio, can be used as an excellent criterion for the fabrication of 1D nanoarrays without defects with a simple peel-off processes. Our approach opens up new applications for unconventional lithographic techniques, such as soft lithography, imprint lithography, and others.