Myoung-Kyu Oh

Recently, the investigation of the dielectric properties of glasses in the GHz frequency range has attracted great interest for use in printed circuit boards (PCBs) as a reinforcing material in the application of high-speed 5G/6G communications. In particular, glasses with low dielectric properties are a prerequisite for high-frequency applications. In this study, the GHz dielectric properties of alkali-free aluminoborosilicate glasses without and with La₂O₃ were analyzed using the Clausius–Mossotti equation where both the electronic and ionic polarizabilities contribute to the dielectric constant. The dielectric polarizability (α_D) and oxide ion polarizability (α_O2−) were calculated from the measured dielectric constant (ε_GHz) at 1 GHz and the glass density. The dielectric constants (ε_opt) at the optical frequencies and electronic polarizabilities (α_e) of the glasses were calculated from the refractive index measured at 633 nm and the glass density. The ε_GHz values were found to be significantly higher than the ε_opt values in both series of glasses, due to the ionic polarizability (α_i), which contributes additionally to the ε_GHz. The lower dielectric constants of the La₂O₃-incoporated glasses than that of the reference glass without La₂O₃ may be due to the lower ionic polarizability originated from the incorporation of the high cation field strength of the La³⁺ ions.

A highly sensitive surface-enhanced Raman spectroscopy (SERS) film was fabricated based on anodic aluminum oxide template and Ag coating techniques. The array of alumina nanowires of ∼5 μm length and 26 nm thickness was fabricated and then made collapse to build a micro-scale funnel array, where each cell consists of hundreds of nanowires. Ag film was deposited on the nanowire structure by thermal evaporation. Due to numerous tapered nanogaps and efficient illumination of incident light in the micro-funnel structures, Raman signal enhancement factor (E.F.) as high as ∼1.8 × 10⁷ could be obtained at 785 nm. The SERS film functionalized by 1-propanethiol was used for benzene gas detection, where the sensitivity of σ) was realized within 120 s. This nanostructure is expected to be widely used for highly sensitive chemical and bio-sensors and have higher E.F. when the morphology is optimized further.

To develop highly sensitive surface-enhanced Raman spectroscopy (SERS) films, various types of aggregated Ag nanowire (NW) and nanoparticle (NP) complex structures were fabricated using anodic aluminum oxide (AAO) templates and thermal evaporation. Aggregated AgNW structures with numerous tapered nanogaps were fabricated via Ag deposition on aggregated thin alumina nanowires of different lengths. AgNP complex structures were obtained by collapsing vertically aligned thin alumina nanowires 1 μm in length and depositing AgNPs on their tops and sides using surface tension during ethanol drying after functionalization. The Raman signal enhancement factors (EFs) of the samples were evaluated by comparing the SERS signal of the thiophenol (TP) self-assembled monolayer (SAM) on the nanostructures with the Raman signal of neat TP. EFs as high as ~2.3 × 10⁷ were obtained for the optimized aggregated AgNW structure (NW length of 1 μm) and ~3.5 × 10⁷ for the optimized AgNP complex structure. The large EF of the AgNP complex film is attributed mainly to the AgNPs dispersed in three dimensions on the sides of the thin alumina nanowires, strongly implying some important, relevant physics yet to be discovered and also a very promising nanostructure scheme for developing ultrahighly sensitive SERS films with EF > 10⁸.

Microplasma is a promising source for optical emission spectroscopy (OES) capable of multiple elemental analysis with high sensitivity owing to its high density and stability. Compared with solids or liquids, the application of OES to gas samples has been rare until now owing to the limited detection sensitivity. In this work, a bright and stable microplasma with a long life span was generated by employing high AC drive voltage and used as a source for a home-built compact optical emission spectrometer with high sensitivity. A compact dual-channel spectrometer of 15 cm focal length was developed by employing custom-fabricated spectroscopy fiber bundles having a 700 μm-diameter circular input and 100 μm-width slit output. The spectral resolution of each channel was 2 and Ar in ambient O₂, where 358 and 813 nm emission lines were used for N₂ and Ar, the detection sensitivities for N₂ and Ar were found to be <1 and <0.2 ppm (3-σ), respectively, within 10 s, which implied that ppb-level detection is possible within a few minutes.

A nanoporous silver film (npAgF), a promising structure for surface-enhanced Raman spectroscopy (SERS), can be fabricated by using successive O-2 and Ar plasma treatments on a planar silver film. The common dealloying method for producing an npAgF involves annealing at high temperatures to produce an alloy film, as well as harsh etching using corrosive chemicals. By contrast, the plasma-based method can be applied directly to various functional substrates to produce more sophisticated npAgF structures. Herein, we report a facile fabrication method for a wrinkled npAgF (w-npAgF) for SERS applications using a thermally contractible polystyrene substrate. The w-npAgF had 3D wrinkles of the nanoporous structure and showed approximately 8 times higher SERS enhancement than did the flat npAgF. Moreover, the w-npAgF could be reused for multiple SERS measurements of different molecules by mild O-2 and Ar plasma treatments after each use, in which the O-2 plasma effectively removed the adsorbed organic molecules and the Ar plasma reduced silver oxide to pristine silver for subsequent SERS measurements. The wrinkled nanoporous structure was maintained after multiple mild plasma treatments for reuse. The simplicity of plasma-based fabrication and high sensitivity of w-npAgFs are promising features for the green production of low-cost and reusable 3D SERS substrates.

Measurement of particulate matter, i.e. fine/ultrafine dust is important for monitoring our surrounding environment. We demonstrate the fabrication of lenticular fiber based fine/ultrafine dust measurement devices for monitoring the density and size of dust. The plano-convex lenticular fibers were fabricated by CO ₂ laser polishing technique. The optical lens, used in the conventional light scattering based fine/ultrafine dust measurement devices, was replaced by a plano-convex lenticular optical fiber to reduce the size and complexity of the dust measurement device. We utilized light emitting diode as the light source of the fine/ultrafine dust density measurement device. The fabricated device could indicate the dust density level (good, normal, bad, and very bad) according to Korean standard. To estimate the size of the dust particulates, we fabricated a dust size measurement device that consists a compact laser diode, a lenticular fiber, and a photo detector. We are successful in estimating the size of both fine and ultrafine particulate matters by detecting the power of the scattered light coming from the dust particulates. Dust size down to 0.9 μm was detected using our dust measurement device. Our proposed dust measurement devices are simple, small, and cost effective and can be considered as a candidate for portable applications.

Trace-level ammonia gas in air was analyzed using a fiber-based compact cavity ring-down spectrometer (CRDS). For the compact spectrometer, a 20 cm linear cavity with two high reflectivity ( > 99.999 % ) mirrors was employed. The laser frequency was locked to the cavity resonance by using frequency shifted (160 MHz) optical feedback. For N H 3 detection, a strong absorption band at 1513.98 nm with an absorption cross section of 3 . 3 ⋅ 1 0 − 21 c m / m o l e c u l e was used. As a result, a detection sensitivity of ∼ 0.1 p p b ( 3 - σ ) was achieved for N H 3 within 10 s.

A trace level of N₂ and Ar gases in O₂ ambience were quantitatively analyzed for the first time by glow discharge optical emission spectroscopy. Microplasma was generated in a compact gas cell by a metal (Au) electrode pair of 1 mm diameter and ∼1 mm separation with the driving voltage of ∼0.9 kV and the current of ∼5 mA in ∼60 Torr. 358 and 813 nm optical emission bands were used for N₂ and Ar detection, respectively. A spectroscopy fiber bundle having a circular input cross section of ∼700 µm in diameter and an output of 100 µm in width and 3 mm in height was employed to promote the signal collection efficiency while preserving high spectral resolution. As a result, the detection sensitivities 2 and Ar within 10 s. The detection accuracy was also promoted by correcting the N₂ and Ar band signal intensities using the neighboring O emission band intensities, which led to detection errors 2 and Ar.

ZnO nanomaterials are promising building blocks for an efficient UV photodetector; however, their slow sensing behavior and undesired response to visible light, which are attributed to surface defects, such as oxygen or zinc vacancies, are challenges that remain to be addressed. Here, we transformed the ZnO nanorod surface into a zeolitic imidazolate framework-8 (ZIF-8) to eliminate ZnO surface defects. Vertical-type photodetectors were fabricated incorporating a Schottky junction at the ZIF-8/gold (Au) top electrode and could respond to UV light with a rapid response and recovery (1–2 s) and demonstrated a UV-to-visible rejection ratio in the order of 10³, qualifying them as efficient visible-blind UV photodetectors. It is noteworthy that the ZIF-8 layer effectively separated the photogenerated electron–hole pairs, and thus reduced their recombination probability. The enhanced photodetector displayed excellent figures-of-merit: a responsivity of 291 A W⁻¹ and a detectivity of 5.9 × 10¹³ cm Hz^1/2 W⁻¹ under illumination at 295 nm.