Xiangmeng Li

Fabrication of high-aspect-ratio microstructures using dielectrophoresis-electrocapillary force-driven UV-imprinting

We propose a novel method for fabricating high-aspect-ratio micro-/nano-structures by dielectrophoresis-electrocapillary force (DEP-ECF)-driven UV-imprinting. The force of DEP-ECF, acting on an air–liquid interface and an air–liquid–solid three-phase contact line, is generated by applying voltage between an electrically conductive mold and a substrate, and tends to pull the dielectric liquid (a UV-curable pre-polymer) into the mold micro-cavities. The existence of DEP-ECF is explained theoretically and demonstrated experimentally by the electrically induced reduction of the contact angle. Furthermore, DEP-ECF is proven to play a critical role in forcing the polymer to fill into the mold cavities by the real-time observation of the dynamic filling process. Using the DEP-ECF-driven UV-imprinting process, high-aspect-ratio polymer micro-/nano-structures (more than 10:1) are fabricated with high consistency. This patterning method can overcome the drawbacks of the mechanically induced mold deformation and position shift in conventional imprinting lithography and maximize the pattern uniformity which is usually poor in capillary force lithography.

Shape-controllable plano-convex lenses with enhanced transmittance via electrowetting on a nanotextured dielectric

Millimetre-scale plano-convex lenses with controllable focal lengths are presented via electrowetting UV-curable polymer droplets on a nanotextured dielectric. A large range of numerical aperture is achieved. Such plano-convex lenses can be peeled off and transferred to other substrates. The lenses with nanocavities on the bottom surfaces exhibit enhanced transmittance.

Electrowetting of liquid polymer on petal-mimetic microbowl-array surfaces for formation of microlens array with varying focus on a single substrate

In this paper, microlens array with varying focal lengths were fabricated on a single microbowl-array textured substrate. The solid microbowl-arrayed NOA61 (kind of polyurethane-based polymer with UV curablity) surface was resulted from nanoimprinting by polydimethylsiloxane (PDMS) mold. The PDMS mold was replicated from an SU-8 master which was generated by electron beam lithography. Such microbowl-arrayed surfaces demonstrate petal-mimetic highly adhesive hydrophobic wetting properties, which can promote an irreversible electrowetting (EW) effect and a dereased contact angle of water droplets as well as other liquid droplets by applying direct current (DC) voltage. To fabricate a microlens array with varying focal-lengths, liquid NOA61 was supplied from a syringe on the solid NOA61 microtextured film and DC voltage was applied succesively. After removing the DC voltage, these liquid NOA61 microdrops deposited on the solid microtextured NOA61 surface on tin-indium-oxide coated substrate could be solidified via UV irradiation, thus leading to microlens array with uneven numerical apertures on a single substrate. Numerical simulation was also done to verify the EW effect. Finally, optical imaging characterization was performed to confirm the varied focus of the NOA61 microdrops.

Microstructure Formation of Functional Polymers by Evaporative Self-Assembly under Flexible Geometric Confinement

Polymer microstructures are widely used in optics, flexible electronics, and so forth. We demonstrate a cost-effective bottom-up manner for patterning polymer microstructures by evaporative self-assembly under a flexible geometric confinement at a high temperature. Two-parallel-plates confinement would become curve-to-flat shape geometric confinement as the polydimethylsiloxane (PDMS) cover plate deformed during solvent swelling. We found that a flexible cover plate would be favorable for the formation of gradient microstructures, with various periodicities and widths obtained at varied heights of clearance. After thermal annealing, the edge of the PMMA (Poly-methylmethacrylate) microstructures would become smooth, while the RR-P3HT (regioregular-poly(3-hexylthiophene)) might generate nanocrystals. The morphologies of RR-P3HT structures included thick films, straight lines, hierarchical stripes, incomplete stripes, and regular dots. Finally, a simple field-effect transistor (FET) device was demonstrated with the RR-P3HT micropattern as an active layer.

Silver Nanowires Deposited on Grooved Surface as Bendable Resistance Sensor

In this paper, we present a simple bendable resistance sensor with silver nanowires (AgNWs) stripe coated on microgroove surfaces formed on a polyimide (PI) film. We adopted an anisotropic wetting manner to form a narrow stripe in order to improve the sensitivity of the resistance sensor. The ultraviolet (UV)-curable polymer NOA61 is firstly drop-coated on the substrate, and then imprinted by a groove-textured poly (dimethylsiloxane) (PDMS) template. After UV irradiation and de-molding, the surface is further treated with a UV-ozone (UVO) lamp, so that the grooved surface becomes more hydrophilic, which will promote the anisotropic wettability of AgNWs suspension and thereby deposition of AgNWs into uniform multi-layer stripe pattern. Moreover, the conductance with respect to the bending motion is measured. The most variation of conductance ΔG/G0 can reach about 200 % with bending radius of 2 mm.