Kyung Woo Choi

Fabrication of 3D nano-structures using reverse imprint lithography

In spite of the fact that the fabrication process of three-dimensional nano-structures is complicated and expensive, it can be applied to a range of devices to increase their efficiency and sensitivity. Simple and inexpensive fabrication of three-dimensional nano-structures is necessary. In this study, reverse imprint lithography (RIL) with UV-curable benzylmethacrylate, methacryloxypropyl terminated poly-dimethylsiloxane (M-PDMS) resin and ZnO-nano-particle-dispersed resin was used to fabricate three-dimensional nano-structures.UV-curable resins were placed between a silicon stamp and a PVA transfer template, followed by a UV curing process. Then, the silicon stamp was detached and a 2D pattern layer was transferred to the substrate using diluted UV-curable glue. Consequently, three-dimensional nano-structures were formed by stacking the two-dimensional nano-patterned layers. RIL was applied to a light-emitting diode (LED) to evaluate the optical effects of a nano-patterned layer. As a result, the light extraction of the patterned LED was increased by about 12% compared to an unpatterned LED.

Fabrication of sub-100 nm sized patterns on curved acryl substrate using a flexible stamp

As small as 100 nm patterns were successfully transferred onto a non-planar acryl substrate using both UV nanoimprinting and hot embossing techniques. Two different types of flexible imprint stamps, electroformed nickel foil stamp and molded water-soluble poly(vinyl alcohol) (PVA) stamp, were used. 100 nm line and space pattern of Si master was successfully transferred to nickel foil stamp and PVA stamp and their patterns were also transferred to the surface of curved acryl substrate using either UV nanoimprint lithography or hot embossing lithography.

Fabrication of ZnO nano-structures using UV nanoimprint lithography of a ZnO nano-particle dispersion resin

Recently, nanoimprint lithography (NIL) has gained great attention as an effective patterning technology in the fields of light emitting diodes (LEDs), solar cells, and other optical devices, because of its simplicity and cost effectiveness. The aim of this study is the development of an imprint resin containing dispersed zinc oxide (ZnO) nano-particles that is applicable in the UV NIL process. UV NIL uses conventional monomer-based resins, which contain a UV initiator, but restricts the use of imprinted structures in optical devices due to their relatively low refractive index. In order to resolve this problem, an imprint resin containing dispersed ZnO nano-particles was prepared, using which submicron-scale structures were fabricated by the UV NIL process. The haziness of submicron-scale ZnO nano-particle resin structures and the refractive index of the ZnO nano-particle dispersion resin were measured to analyze the optical properties of the ZnO nano-particle dispersion resin and the resulting structures.

Various Metallic Nano-Sized Patterns Fabricated Using an Ag Ink Printing Technique

This paper presents a new simple metal patterning technique, which is based on soft nanoimprint lithography. By using this method with a commercial Ag nano particle ink, a nano-sized metal pattern was successfully fabricated. The problem of the residual layer of patterned Ag layer was minimized by controlling the concentration of the solution and the process conditions. By using this method, we could easily fabricate various patterns without reference to any shape. Furthermore, we fabricated an Ag mesh type pattern for the application of conducting transparent glass.

Development of a natural analogue database to support the safety case of the Korean radioactive waste disposal program

In this study, the status of natural analogue studies in Korea is briefly summarized and applicability of existing natural analogue information to the Korean safety case has been evaluated. To enable effective application of natural analogue information to the overall evaluation of long-term safety (the “safety case”) for the geological disposal of radioactive wastes, a natural analogue database has been developed by collecting, classifying, and evaluating relevant data. The natural analogue data collected were classified into categories based on site information, components/processes of the disposal system, properties/phenomena, reference, safety case application, application method, and suitability to a safety case. Suitability of the natural analogue data to a specific safety case was evaluated based upon the importance and the applicability to the Korean safety case. As a result, 75 natural analogue datasets were selected as important for the Korean safety case. The database developed can now be utilized in the RD&D (Research, Development, and Demonstration) program development for natural analogue studies. In addition, the methodology developed and the database compiled in this study may assist in the development of safety case including safety assessment for high-level radioactive waste disposal in Korea as well as in other countries.

Fabrication of Diverse-Scale Patterned Layer on Organic Photovoltaics

In order to increase the conversion efficiency of organic photovoltaics (OPV), diverse-scale patterns were formed on a glass substrate using the direct printing technique. The optical properties of the patterns depended on the size, shape, height, and pitch of the patterns. Randomly distributed nano- and micro-patterns caused light scattering, which increased the diffusion transmittance. The other pattern, which was a nano-sized anti-reflective pattern comprising a 300 nm sized hexagonal array, decreased the reflectance of light on the surface. The optical properties of these patterns, can be used to improve solar cell efficiency by increasing the light allowed onto the light-absorbing layer. We used a direct printing method with a poly(dimethylsiloxane) (PDMS) mold to fabricate these patterns on glass substrates. These patterns were transferred from PDMS to the surface of a glass substrate. Hydrogen silsesquioxane (HSQ) was used as a resin since its properties include volatility and allow for spin-coating; it also has a reflective index similar to that of glass. This method has low cost and a simple process compared to optic-based lithography. After these patterns were formed outside the glass substrate, a conductive polymer layer and the active layer were formed by spin-coating, and the cathode was deposited by a thermal evaporator. The electrical properties of solar cells fabricated with these patterns on their surfaces were measured with a solar simulator. The conversion efficiency of the solar cells with these surface patterns showed an increase of up to 6.8% in comparison with conventional cells.