Geun Chul Park

Hydrothermally Grown In-doped ZnO Nanorods on p-GaN Films for Color-tunable Heterojunction Light-emitting-diodes

The incorporation of doping elements in ZnO nanostructures plays an important role in adjusting the optical and electrical properties in optoelectronic devices. In the present study, we fabricated 1-D ZnO nanorods (NRs) doped with different In contents (0% ~ 5%) on p-GaN films using a facile hydrothermal method, and investigated the effect of the In doping on the morphology and electronic structure of the NRs and the electrical and optical performances of the n-ZnO NRs/p-GaN heterojunction light emitting diodes (LEDs). As the In content increased, the size (diameter and length) of the NRs increased, and the electrical performance of the LEDs improved. From the electroluminescence (EL) spectra, it was found that the broad green-yellow-orange emission band significantly increased with increasing In content due to the increased defect states (oxygen vacancies) in the ZnO NRs, and consequently, the superposition of the emission bands centered at 415 nm and 570 nm led to the generation of white-light. These results suggest that In doping is an effective way to tailor the morphology and the optical, electronic, and electrical properties of ZnO NRs, as well as the EL emission property of heterojunction LEDs.

Synergistic effect of Indium and Gallium co-doping on growth behavior and physical properties of hydrothermally grown ZnO nanorods

We synthesized ZnO nanorods (NRs) using simple hydrothermal method, with the simultaneous incorporation of gallium (Ga) and indium (In), in addition, investigated the co-doping effect on the morphology, microstructure, electronic structure, and electrical/optical properties. The growth behavior of the doped NRs was affected by the nuclei density and polarity of the (001) plane. The c-axis parameter of the co-doped NRs was similar to that of undoped NRs due to the compensated lattice distortion caused by the presence of dopants that are both larger (In3+) and smaller (Ga3+) than the host Zn2+ cations. Red shifts in the ultraviolet emission peaks were observed in all doped NRs, owing to the combined effects of NR size, band gap renormalization, and the presence of stacking faults created by the dopant-induced lattice distortions. In addition, the NR/p-GaN diodes using co-doped NRs exhibited superior electrical conductivity compared to the other specimens due to the increase in the charge carrier density of NRs and the relatively large effective contact area of (001) planes. The simultaneous doping of In and Ga is therefore anticipated to provide a broader range of optical, physical, and electrical properties of ZnO NRs for a variety of opto-electronic applications.

Facet Control of Subunits in Anatase TiO2 Mesocrystal by Diethanolamine for Enhanced Photocatalytic Activity

Controllable TiO2 architectures with different facets play a key role in the adjustment of the surface area and charge recombination efficiency in photocatalysts. In this study, we synthesized anatase TiO2 mesocrystals with diethanolamine (DEA, 0-5 mM) as a nontoxic capping agent to control the facet of subunits using hydrothermal method, and determine the effect of DEA on subunits, by-products, and consequent photocatalytic performance of TiO2 mesocrystals. The pure TiO2 was spindle-like shaped mesocrystal having bipyramidal subunits with high portion of {101} facets. As DEA concentration was increased, the percentage of {001} exposed facet of subunits increased, but the number of subunits was gradually reduced. When a large amount of DEA (≥3 mM) was added, TiO2 exhibited single crystals rather than mesocrystal, probably because the long chain of DEA interfered with the oriented attachment of subunits. The photocatalytic activity of the TiO2 improved at 0.5 mM of DEA due to low charge recombination rate by increasing {001} facet. On the other hand, subsequent deterioration with further increase of DEA was attributed to the increased by-products and reduced the number of subunits despite the increase in specific surface area. These results imply that charge separation efficiency, rather than specific surface area, is highly responsible for the photocatalytic activity of TiO2.

Surface-tunable bioluminescence resonance energy transfer via geometry-controlled ZnO nanorod coordination

The use of ZnO nanorods (NRs) as an effective coordinator and biosensing platform to create bioluminescence resonance energy transfer (BRET) is reported. Herein, a hydrothermal approach is applied to obtain morphologically controlled ZnO NRs, which are directly bound to luciferase (Luc) and carboxy-modified quantum dot (QD) acting as a donor-acceptor pair for BRET. BRET efficiency varies significantly with the geometry of ZnO NRs, which modulates the coordination between hexahistidine-tagged Luc (Luc-His6 ) and QD, owing to the combined effect of the total surface area consisting of (001) and (100) planes and their surface polarities. Unlike typical QD-BRET reactions with metal ions (e.g., zinc ions), a geometry-controlled ZnO NR platform can facilitate the design of surface-initiated BRET sensors without being supplemented by copious metal ions: the geometry-controlled ZnO NR platform can therefore pave the way for nanostructure-based biosensors with enhanced analytical performance.

Effects of Current Density and Frequency on Microstructure and Mechanical Properties of Ni Stencil Masks Fabricated by Pulse Electroforming

We fabricated Ni stencil masks using a pulse electroforming and investigated the effects of current density and frequency on the microstructure and mechanical property. In the electroforming process, the current densities were 2.5 and 5 A/dm2 and the frequency varied from 0 (DC) to 1000 Hz at a duty cycle of 50%. Texture, microstructure, and mechanical properties varied with the current density and frequency. The preferred orientation of (220) at 2.5 A/dm2 changed to (200) as the current density increased to 5 A/dm2. Grain size decreased with decreasing current density or increasing frequency, probably due to a sufficient supply of Ni ions and the presence of inhibitor species. This decrease in grain size resulted in increase of hardness and wear resistance. However, with increase in current density and frequency (5 A/dm2 and 1000 Hz), the grain size became large, as a result of faradic current during the off-time.

Study on the Development of Sensors for Distance Measure Using Ultrasonic

Abstract In this paper, we report a novel algorithm based on phase displacement, which supplements conventional TOF methods for distancemeasurement using an ultrasonic wave. The proposed algorithm roughly measures the distance between the transmission part and thereceiving part by using the initial TOF. Thereafter, the precise distance is determined by measuring the phase displacement valuebetween the synchronizing transmission signal and the signal obtained at the receiving end. A distance measurement experiment usinga micrometer was performed to verify the accuracy of the ultrasonic wave sensor system. We found that the mean errors from the oneadopting the distance measurement algorithm based on phase displacement varied from a minimum of 0.03 mm to a maximum of 0.09mm. In addition, the standard deviation varied from a minimum of 0.04 mm to a maximum of 0.07 mm, thus giving a precision of0.1 mm. Keywords : Ultrasonic, Distance measure 1. 서론 초음파는 주파수가 20 kHz 이상으로 사람의 귀로 들을 수 없는 음파이다. 초음파의 고유한 성질은 가청 범위의 음파와 같다.그러나 주파수가 높고 파장이 짧기 때문에 상당히 강한 진동이발생되는 특징을 지니고 있다. 초음파는 물체 인식[1,2], 거리 측정[3,4], 질병 진단과 치료[5] 등 거리 정보에 기반한 응용에 많이 이용되고 있다.초음파를 이용해서 거리 정보를 알아내기 위해서는 초음파를발생 시켜 송신하는 부분과 송신된 초음파를 수신하는 부분으로 구성하고 펄스파 또는 연속파를 사용한다. 연속파를 사용한거리 측정은 송신부에서 초음파를 전송한 순간부터 수신부에 그파가 도착한 순간의 시간 경과를 계산하여 수행한다[6-8]. 이 거리 측정 방법을 일반적으로 TOF(Time-of-Flight)법이라고 한다.TOF를 측정하는 방법은 임계값 검출 기법[9], 포락선 추정 기법[10], 주파수 인식 기법[11] 등이 적용되고 있다. 임계값 검출기법은 초음파 신호의 세기가 주위 환경에 의해 발생되는 소음보다 높아야 한다는 전제 조건을 만족해야 한다. 포락선 추정기법은 포락선이 측정 거리에 따라 감쇠되는 단점이 존재한다.주파수 인식 기법에서는 수신되는 초음파 신호에 잡음과 여러주파수들이 혼합되어 수신된다. 따라서 잡음 제거를 위한 필터와 추가적인 신호처리 기법을 적용해야 하는 단점이 있다. TOF방법의 단점을 제거하기 위해 초음파 신호를 진폭 변조하여 송수신하고, 송신부와 수신부 사이의 위상 차이를 이용하여 거리를 측정하는 방법이 소개 되었다[12]. 이 방법은 변조를 위해 송신부에 변조기, 수신부에 복조기와 증폭기를 추가해야 하는 단점이 있다.본 논문에서는 TOF 방법의 단점을 배제시킬 수 있고 간단하게 구성할 수 있는 위상 변위 기반+ 거리 측정 알고리즘을 제안한다. 제안하는 위상 변위 기반 거리 측정 알고리즘은 초기 TOF를 사용하여 송신부와 수신부의 거리를 개략적으로 측정한 후,송신 동기 신호와 초음파 센서의 수신부에서 얻은 신호 사이의위상 변위 값을 측정하여 정밀하게 거리를 측정한다. 그리고 본논문에서 제안하는 알고리즘을 기반으로 거리를 측정할 수 있