Hyung-Man Lee

Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network

A highly stretchable metal electrode is developed via the solution-processing of very long (>100 μm) metallic nanowires and subsequent percolation network formation via low-temperature nanowelding. The stretchable metal electrode from very long metal nanowires demonstrated high electrical conductivity (~9 ohm sq(-1) ) and mechanical compliance (strain > 460%) at the same time. This method is expected to overcome the performance limitation of the current stretchable electronics such as graphene, carbon nanotubes, and buckled nanoribbons.

Nonvacuum, Maskless Fabrication of a Flexible Metal Grid Transparent Conductor by Low-Temperature Selective Laser Sintering of Nanoparticle Ink

We introduce a facile approach to fabricate a metallic grid transparent conductor on a flexible substrate using selective laser sintering of metal nanoparticle ink. The metallic grid transparent conductors with high transmittance (>85%) and low sheet resistance (30 Ω/sq) are readily produced on glass and polymer substrates at large scale without any vacuum or high-temperature environment. Being a maskless direct writing method, the shape and the parameters of the grid can be easily changed by CAD data. The resultant metallic grid also showed a superior stability in terms of adhesion and bending. This transparent conductor is further applied to the touch screen panel, and it is confirmed that the final device operates firmly under continuous mechanical stress.

Selective Sintering of Metal Nanoparticle Ink for Maskless Fabrication of an Electrode Micropattern Using a Spatially Modulated Laser Beam by a Digital Micromirror Device

We demonstrate selective laser sintering of silver (Ag) nanoparticle (NP) ink using a digital micromirror device (DMD) for the facile fabrication of 2D electrode pattern without any conventional lithographic means or scanning procedure. An arbitrary 2D pattern at the lateral size of 25 μm × 25 μm with 160 nm height is readily produced on a glass substrate by a short exposure of 532 nm Nd:YAG continuous wave laser. The resultant metal pattern exhibits low electrical resistivity of 10.8 uΩ · cm and also shows a fine edge sharpness by the virtue of low thermal conductivity of Ag NP ink. Furthermore, 10 × 10 star-shaped micropattern arrays are fabricated through a step-and-repeat scheme to ensure the potential of this process for the large-area metal pattern fabrication.

Silver nanoparticle piezoresistive sensors fabricated by roll-to-roll slot-die coating and laser direct writing

In this study, we propose new fusion technology to overcome the limitations of the current printing process for printed electronics. The combinative slot-die coating and laser direct writing on a roll-to-roll (R2R) basis was studied and applied to the fabrication of piezoresistive strain gauges. Piezoresistive sensors were fabricated for the first time by this developed fusion technology. A parametric study was performed for the R2R process, followed by a comparison of the fabricated functional devices with commercial products, which confirmed a 40% increase in the gauge factor of the fabricated sensors over the commercial product. The combinative manufacturing process for functional device fabrication will open a new chapter in the future of printed electronics due to its large area capacity at low cost.

Integrated optical modulator for signal up-conversion over radio-on-fiber link.

An integrated optical modulator, which consists of a dual-sideband suppressed carrier (DSB-SC) modulator cascaded with a single-sideband (SSB) modulator, is proposed for signal up-conversion over Radio-on-Fiber. Utilizing a single-drive domain inverted structure in both modulators, balanced modulations were obtained without complicated radio frequency (RF) driving circuits and delicate RF phase adjustments. Intermediate frequency (IF) band signal was up-conversed to 60GHz band by using the fabricated device and was transmitted over optical fiber. Experiment results show that the proposed device enables millimeter wave generation and signal transmission without any power penalty caused by chromatic dispersion.

Birefringent waveguide sensor using a polarizer rotating technique

A birefringence measurement system is introduced to get high phase resolution for detection of low contents of biochemicals. By using a fixed quarter-wave plate and a rotating polarizer, the phase difference between two orthogonal polarizations is transformed into phase delay of output sinusoidal signal. Analyzing the output phase, birefringence change could be detected with a phase noise of 0.14 degrees. As well as the birefringence measurement system, an optical evanescent waveguide sensor was developed. A rib-type silica waveguide overlaid with TiO2 film was fabricated, and a developed birefringence measurement technique was employed in evaluating a refractive index change on waveguide surface. For the fabricated waveguide with a 40-nm-thick TiO2 film, experiment results showed that the minimum detectable index change was 5.9x10(-7).

60 GHz Optical Carrier Generation Using a Domain Reversed LiNbO

A 30 GHz bandpass modulator was fabricated on a domain reversed LiNbO3 substrate. The circuit effect on current waveforms during wafer-scale poling was analyzed, and the domain wall movement was precisely controlled in the poling process. A complementary polarization reversal technique enabled the device with a single electrode structure to perform balanced modulation. A 60 GHz optical carrier was generated by double sideband modulation with the suppressed carrier. The fabricated device showed that carrier suppression was better than 45 dB and that the power ratio of a 60 GHz spaced two-tone lightwave signal to the suppressed carrier was 35 dB.

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A refractive index sensor based on a three-layered polymeric waveguide was proposed and demonstrated. A high-index thin film in TiO2 was placed on top of the waveguide in the sensing region, playing the role of strengthening the evanescent field to enhance the sensitivity of the sensor. The refractive index of the analyte applied to the surface of the sensor was estimated by observing the count of the polarimetric interference between the TE and TM polarizations, which is manifested as a periodic variation in the optical output of the sensor. For a fabricated sensor involving a 20 nm thick TiO2 film, the sensitivity was found to be equivalent to 1.8x10-3 RIU. It was found to be enhanced by increasing the thickness of the high-index overlay to a certain degree.

Optical Modulator

We fabricate a low-concentration photovoltaic (LCPV) module with a reflective compound parabolic concentrator (CPC) using roll-to-roll (R2R) slot-die coating and 3D printing technologies. A highly reflective silver thin-film is coated on a flexible plastic substrate, and the CPC frame is manufactured via 3D printing. The slot-die-coated silver film with thickness of more than 100 nm stably exhibits 95% reflectivity at 550 nm. Further, CPC concentrators with concentration ratios of 4X and 3X are assembled into silicon solar cells and characterized. Although the fill factor and maximum voltage slightly decrease, power and efficiency increase by factors of 3.51 and 2.63 with respect to the no-CPC-module case. Our approach can be used to optimize the design of various engineering products.