Kibeom Kim

Biomimetic Microfingerprints for Anti‐Counterfeiting Strategies

An unclonable, fingerprint-mimicking anti-counterfeiting strategy is presented that encrypts polymeric particles with randomly generated silica film wrinkles. The generated wrinkle codes are as highly unique as human fingerprints and are technically irreproducible. Superior to previous physical unclonable functions, codes are tunable on demand and generable on various geometries. Reliable authentication of real-world products that have these microfingerprints is demonstrated using optical decoding methods.

A well-aligned simple cubic blue phase for a liquid crystal laser

Here, a well-aligned dye-doped simple cubic blue phase (BPII) sample is fabricated that is stable over the temperature range of 4 °C and has a photonic bandgap (PBG) equivalent to a visible optical wavelength. We demonstrate the maximum widest tunability range of 30 nm as a function of temperature for the well-aligned dye-doped BPII sample. Moreover, it is found that the emission threshold energy for the laser action can be dramatically reduced.

Fine-tuned grayscale optofluidic maskless lithography for three-dimensional freeform shape microstructure fabrication

This article presents free-floating three-dimensional (3D) microstructure fabrication in a microfluidic channel using direct fine-tuned grayscale image lithography. The image is designed as a freeform shape and is composed of gray shades as light-absorbing features. Gray shade levels are modulated through multiple reflections of light in a digital micromirror device (DMD) to produce different height formations. Whereas conventional photolithography has several limitations in producing grayscale colors on photomask features, our method focuses on a maskless, single-shot process for fabrication of freeform 3D micro-scale shapes. The fine-tuned gray image is designed using an 8-bit grayscale color; thus, each pixel is capable of displaying 256 gray shades. The pattern of the UV light reflecting on the DMD is transferred to a photocurable resin flowing through a microfluidic channel. Here, we demonstrate diverse free-floating 3D microstructure fabrication using fine-tuned grayscale image lithography. Additionally, we produce polymeric microstructures with locally embedded gray encoding patterns, such as grayscale-encoded microtags. This functional microstructure can be applied to a biophysical detection system combined with 3D microstructures. This method would be suitable for fabricating 3D microstructures that have a specific morphology to be used for particular biological or medical applications.

A monodomain-like liquid-crystalline simple cubic blue phase II

In this work, a systematic approach to controlling the orientation of a liquid-crystalline simple cubic blue phase (BPII) was carried out using a conventional surface rubbing treatment method. A more uniform lattice plane alignment in BPII was produced than in the liquid-crystalline-body-centered blue phase (BPI) using rubbed polyimide (PI) films on glass substrates. A single-color monodomain-like BPII was achieved regardless of the rubbed PI substrate pretilt angle. Additionally, single-color monodomain-like BPIIs with three elementary colors (red, green, and blue) were prepared, simply by changing the percentage of the chiral dopant.

Photomodulated Supramolecular Chirality in Achiral Photoresponsive Rodlike Compounds Nanosegregated from the Helical Nanofilaments of Achiral Bent-Core Molecules

We prepared a nonchiral mixture of achiral bent-core molecules and photoresponsive rodlike liquid crystalline (LC) molecules. With the help of the isothermal photochemical nematic (N)-isotropic (Iso) phase transition of the photoresponsive rodlike LC molecules, the corresponding phase transition from a dark conglomerate BX phase to another distinguishable dark conglomerate B4 phase took place in the mixture. A large circular dichroism (CD) signal originating from supramolecular chirality was detected in the initial BX phase. On the other hand, the detected CD signal was decreased in the B4 phase after UV irradiation. Interestingly, the decreased CD signal could be reverted to the initial CD signal with visible irradiation. This chiroptical process revealed in this work was stable and reversible and thus opens up the possibility of practical applications such as rewritable optical storage.

Uniform Alignment of Liquid Crystalline Cubic Blue Phase II via Rubbing Treatment

The uniform alignment of liquid crystalline cubic blue phase II (BPII) has been achieved using a conventional surface rubbing treatment. Through comparison of thick and thin cells of assembled rubbed glass, which were covered with commercially available polyimide (PI), we confirmed that rubbed surfaces play an important role in the fabrication of monodomain PBII. These well-aligned cubic BPII materials can be promising for photonics devices.

ITO-free transparent conductive films based on carbon nanomaterials with metal grid for liquid crystal displays

Carbon nanomaterials with metal grids were used as transparent conductive electrodes for liquid crystal displays (LCDs) to develop an indium tin oxide (ITO)-free device. We prepared LCDs with CNTs and graphene electrodes; however, the working voltage of the device with the CNT electrodes was high. The device with graphene electrodes had good performance, but not as good as devices with ITO electrodes. To improve the device performance, we applied a metal grid to the carbon nanomaterial to create low sheet-resistance transparent electrodes. The device with the graphene and metal grid transparent electrodes had a threshold voltage as low as 0.23 V/µm, which is similar to that of typical LCDs with ITO electrodes. The results show that a hybrid transparent conductive film with graphene and metal grid could be an alternative to ITO for developing ITO-free LCDs.

Reflective composite sheet design for LCD backlight recycling

We have designed a reflective composite sheet consisting of a birefringent polymer matrix and isolated isotropic or minimally birefringent fibers. The optical properties of the sheet have been investigated in terms of the width, spacing, and thickness of the individual fibers. Commercial software (FDTD Solution) was used to simulate the reflectance of the proposed sheet, and conventional processes such as cast-film extrusion in combination with solid-state drawing were used to manufacture the multilayer composite sheet. The measured and simulated reflectance spectra confirm the feasibility of employing the sheet as a reflective polarizer.

Self-organization of maze-like structures via guided wrinkling

A novel self-organization approach for constructing maze-like structures is presented by controlling wrinkling on a microparticle. Sophisticated three-dimensional (3D) structures found in nature are self-organized by bottom-up natural processes. To artificially construct these complex systems, various bottom-up fabrication methods, designed to transform 2D structures into 3D structures, have been developed as alternatives to conventional top-down lithography processes. We present a different self-organization approach, where we construct microstructures with periodic and ordered, but with random architecture, like mazes. For this purpose, we transformed planar surfaces using wrinkling to directly use randomly generated ridges as maze walls. Highly regular maze structures, consisting of several tessellations with customized designs, were fabricated by precisely controlling wrinkling with the ridge-guiding structure, analogous to the creases in origami. The method presented here could have widespread applications in various material systems with multiple length scales.

Lithographic resolution enhancement of a maskless lithography system based on a wobulation technique for flow lithography

We present a method for improving the lithographic resolution of digital micromirror devices for flow-lithography using a wobulation technique. While maintaining the area of UV exposure, the lithographic resolution was improved using a wobulation technique, which is a large screen display technique that enhances resolution via overlapping pixels by half a pixel. The edges of a diagonal pattern in a microstructure were smoothly generated with additional sub-patterns compared to conventional single pattern-exposure. In addition, the surface roughness of the microstructure was improved because the gaps between pixels were filled by the overlapping patterns.