Soonmin Seo

Active Matrix Electronic Skin Strain Sensor Based on Piezopotential‐Powered Graphene Transistors

Dr. Q. Sun, B. J. Kim, Prof. S.-W. Kim, Prof. J. H. Cho SKKU Advanced Institute of Nanotechnology (SAINT) Sungkyunkwan University Suwon 440-746 , South Korea E-mail: kimsw1@skku.edu; jhcho94@skku.edu W. Seung, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. J. H. Cho School of Chemical Engineering Sungkyunkwan University Suwon 440-746 , South Korea Prof. S. Seo College of BioNano Technology Gachon University Seongnam 461-701 , South Korea

Replication of flexible polymer membranes with geometry-controllable nano-apertures via a hierarchical mould-based dewetting

Membranes with nano-apertures are versatile templates that possess a wide range of electronic, optical and biomedical applications. However, such membranes have been limited to silicon-based inorganic materials to utilize standard semiconductor processes. Here we report a new type of flexible and free-standing polymeric membrane with nano-apertures by exploiting high-wettability difference and geometrical reinforcement via multiscale, multilevel architecture. In the method, polymeric membranes with various pore sizes (50-800 nm) and shapes (dots, lines) are fabricated by a hierarchical mould-based dewetting of ultraviolet-curable resins. In particular, the nano-pores are monolithically integrated on a two-level hierarchical supporting layer, allowing for the rapid (<5 min) and robust formation of multiscale and multilevel nano-apertures over large areas (2 × 2 cm(2)).

Solution-based formation of multilayers of small molecules for organic light emitting diodes

We developed an approach for fabricating small molecule organic light emitting diodes by solution-based processing. The approach involves dissolving a small molecule organic in a solvent, spin coating it on a mold, and then transferring the layer onto the existing organic layer on a substrate. This ability to form multilayers of small molecule organics allows one to take advantage of both the efficiency offered by the multilayer structures and the low cost fabrication made possible by the solution processing.

Anisotropic rupture of polymer strips driven by Rayleigh instability

We demonstrate that the separated polymer strips of micro- and sub-micro-length-scales rupture anisotropically along the strip direction, resulting in the formation of distinctly observable, regularly spaced polymer drops. The wavelength of the polymer drops and the surface tension dependence of the rupture behavior are found to be well represented by a relationship derived on the basis of Rayleigh instability. The period is proportional to the square root of the cross-sectional area of the strip and the proportionality constant depends on the contact angle. The rupture of polymer strips into polymer blocks instead of drops, which result when annealed with physically confining walls in place, is found to be well described by the same relationship.

Micropatterning of metal substrate by adhesive force lithography

We introduce adhesive force lithography (AFL), a detachment-based method for patterning metal surface. In this method, all the polymer layer except for the desired pattern gets lifted up from the metal surface. The craze microstructure unique to thin polymer films on the order of 102nm is utilized for this AFL along with a difference in adhesive force at two interfaces. Poly(urethaneacrylate) mold, which has a high enough work of adhesion with polymer, makes AFL effective. This technique is purely additive, fast (∼10s contact time), and applicable to large area patterning (10cm×10cm).

Vertical organic inverter with stacked pentacene thin film transistors

A vertical organic inverter is introduced that consists of two p-channel transistors. The concept of stacking transistors vertically is utilized with the resulting circuit structure is which one pentacene transistor is stacked on top of another pentacene transistor. The two transistors have different polymer dielectrics. Utilization of two different dielectrics enables each of the two transistors to behave as a drive and load transistor for the inverter. The fabrication is simple and allows for a larger scale of integration. The performance of this all p-channel inverter is comparable to that of complementary organic inverters that operate at high voltage.

Multiplex lithography for multilevel multiscale architectures and its application to polymer electrolyte membrane fuel cell

The production of multiscale architectures is of significant interest in materials science, and the integration of those structures could provide a breakthrough for various applications. Here we report a simple yet versatile strategy that allows for the LEGO-like integrations of microscale membranes by quantitatively controlling the oxygen inhibition effects of ultraviolet-curable materials, leading to multilevel multiscale architectures. The spatial control of oxygen concentration induces different curing contrasts in a resin allowing the selective imprinting and bonding at different sides of a membrane, which enables LEGO-like integration together with the multiscale pattern formation. Utilizing the method, the multilevel multiscale Nafion membranes are prepared and applied to polymer electrolyte membrane fuel cell. Our multiscale membrane fuel cell demonstrates significant enhancement of performance while ensuring mechanical robustness. The performance enhancement is caused by the combined effect of the decrease of membrane resistance and the increase of the electrochemical active surface area.

Octahedral rotations in strained LaAlO3/SrTiO3 (001) heterostructures

Many complex oxides display an array of structural instabilities often tied to altered electronic behavior. For oxide heterostructures, several different interfacial effects can dramatically change the nature of these instabilities. Here, we investigate LaAlO3/SrTiO3 (001) heterostructures using synchrotron x-ray scattering. We find that when cooling from high temperature, LaAlO3 transforms from the Pm3¯m to the Imma phase due to strain. Furthermore, the first 4 unit cells of the film adjacent to the substrate exhibit a gradient in rotation angle that can couple with polar displacements in films thinner than that necessary for 2D electron gas formation.

Optical Lithography with Printed Metal Mask and a Simple Superhydrophobic Surface

Transfer printing or patterning has been used to form multilayer structures, to fabricate semiconductor nanowires, and to produce three-dimensional electronics. In this work, we utilized a metal pattern that was transfer printed onto a photoresist as a photomask for the patterning of the resist, with flood illumination, directly onto the printed mask. We utilized the optical lithography to produce a simply structured superhydrophobic surface. This optical lithography in essence combined elements of photolithography, the elements that can be realized without specialized equipment, with an unconventional patterning technique, the one involving transfer of a metal pattern onto a substrate. This combination has led to useful patterning capabilities. Techniques involving embossed photoresists and elastomeric phase masks that are both near-field optical methods provided means by which a significant reduction in feature size can be realized. The latter led to edge transfer lithography involving self-assembled monolayer (SAM) materials. The procedure that was involved in the optical lithography presented here is illustrated in Scheme 1. A mold with the desired pattern is first coated with a material of low surface energy and then with a metal by thermal evaporation, as shown in Scheme 1a. The material of low surface energy that was used in this work was fluorinated ethylene propylene (FEP) copolymer, which lowered the work needed for adhesion at the FEP–metal interface for easy release of the metal from the mold in the transfer printing. The mold material chosen was poly(urethaneacrylate) (PUA). This mold was rigid enough to withstand the external pressure applied for the transfer printing, unlike poly(dimenthylsiloxane) (PDMS) that deforms when subjected to a load. It was also flexible enough in its film form, unlike the hard molds such as those made with silicon wafer, to be applicable to large areas. The coated mold was brought into contact with a substrate coated with a negative photoresist and pressure was applied to the back side of the mold for a period of time, typically 15 min, at an elevated temperature of 608C. The mold was then simply removed, resulting in the transfer of the metal pattern, on the protruding parts of the mold, onto the photoresist. As shown in Scheme 1b, the photoresist between the metal lines was embossed by the external pressure applied for the metal transfer, leading to a hemispherically protruding resist pattern. This resist pattern could essentially act as resist lenses. The radii of curvature of these lenses could be controlled, within a certain range, by manipulating the applied pressure. The radius became smaller with increasing pressure. When the transferred metal mask with embossed-resist lenses is flood illuminated by an ultraviolet (UV) beam, the light only passed through the resist lenses and because it was focused by the lenses, only a small portion of the center of the resist lenses became crosslinked by the photons. This crosslinked region is schematically shown as bars in Scheme 1c. Therefore, as shown in Scheme 1c and d, the resist pattern that remained after developing was smaller in size than that of the photomask resist pattern. The metal pattern was lifted off in the development process. Because of the presence of the metal photomask, the depth of the developed resist could be almost as large as the resist thickness, unlike the shallow depth that results with near-field optical methods. Simulation results, based on EM-Explorer program, for the distribution of light intensity revealed how the reduction in feature size is realized. A ray of light traveling through an interface refracts according to Snell;s law:

Tunable and flexible solvent-free liquid organic distributed feedback lasers

We report on optically pumped blue, green, and red liquid organic distributed feedback (DFB) lasers based on solvent-free fluidic organic semiconductors, and prepared on highly flexible corrugated polymeric patterns. By the appropriate selection of laser dyes doping a liquid 9-(2-ethylhexyl)carbazole host, the lasing wavelength is effectively tuned across the visible spectrum via a cascade energy transfer scheme. We also demonstrate a mechanical tunability of the flexible liquid DFB laser emission, which is due to the deformation of the high-aspect ratio DFB grating under bending. Overall, this work provides an important step in the development of flexible liquid organic optoelectronic devices.