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Structural and physical properties of ZnO on lithium niobates with two domains
장서형 한국물리학회 2017 Current Applied Physics Vol.17 No.5
In this paper, we investigated epitaxial ZnO thin films grown on LiNbO3 {0001} substrates with positive (þc) and negative (-c) polarization charges terminated surfaces by using pulsed laser deposition. The polarizations of LiNbO3 substrates can play an important role on modulating structural and physical properties of ZnO thin films. X-ray diffraction studies revealed that the growth behavior of ZnO films was significantly dependent on the temperature and oxygen partial pressure. At the optimal deposition condition, ZnO thin films were grown on two substrates, LiNbO3 (0001) and (000-1) substrates, which have þc andec domain, respectively. ZnO/LiNiO3(-c) exhibited a sharp rocking curve with highlyoriented out-of-plane than that of ZnO/LiNbO3(þc). The polarization of the substrates was attributed to the carrier concentration and resistance of ZnO layers. After thermal cycles, the resistance between two samples became clearly different due to the elimination of adsorbates, indicating the possibility of polarization-induced charge accumulation in ZnO/LiNbO3 systems.
이신범,장서형,유향근,윤문지,양상모,강보수 한국물리학회 2012 Current Applied Physics Vol.12 No.6
We find that resistance switching (RS) phenomena change reversibly between bipolar RS (BRS) and unipolar RS (URS) in a Pt/SrTiOx/Pt cell. For an asymmetric electrode configuration of Ti/SrTiOx/Pt cells whose top and bottom interfaces are Ohmic and Schottky-like rectifying, we determine that BRS only occurs when a positive voltage is applied to the bottom Pt electrode at the forming process. During the set process of BRS in a Pt/SrTiOx/Pt cell, O2 bubbles develop on the top Pt electrode. From the experimental results for a single sample in which both BRS and URS occur, O2- ion movement and consequent interfacial resistance modification might play an important role in BRS but not URS.
김태준,박신애,장서형,전호환,김용태 대한화학회 2015 Bulletin of the Korean Chemical Society Vol.36 No.7
It is widely recognized that RuO2 is the best electrocatalysts for oxygen evolution reaction (OER) in water splitting in acidic media. In this study, we investigated the effect of a surface area and a d-band oxidation state on the activity and the stability of RuO x electrocatalysts for OER. We synthesized RuO x thin film on glassy carbon electrode by the radio frequency magnetic sputtering and controlled the deposition temperature to vary the surface morphology and the electronic structure. In particular, we obtained a precise d-band valence structure by using the synchrotron beam. A higher surface area and a more stable Ru4+ state were obtained at a lower deposition temperature owing to a smaller grain size and a lower oxidation state, respectively. As a result, the order of activity and stability is clearly consistent with that of surface area and stable Ru4+ state (RuO x > RuO x 200 > RuO x 400 > RuO x 600). Hence, we suggest that the surface area and the Ru d-band oxidation state should be carefully controlled in the synthesis process in order to enhance the activity and the stability of RuO x electrocatalysts for OER.
Resonant inelastic X-ray scattering of tantalum double perovskite structures
오주현,김정호,정중현,장서형 한국물리학회 2018 Current Applied Physics Vol.18 No.11
In this paper, we investigated the electronic structures and defect states of SrLaMgTaO6 (SLMTO) double perovskite structures by using resonant inelastic x-ray scattering. Recently, Eu3+ doped SLMTO red phosphors have been vigorously investigated due to their higher red emission efficiency compared to commercial white light emitting diodes (W-LED). However, a comprehensive understanding on the electronic structures and defect states of host SLMTO compounds, which are specifically related to the W-LED and photoluminescence (PL), is far from complete. Here, we found that the PL spectra of SLMTO powder compounds sintered at a higher temperature, 1400 °C, were weaker in the blue emission regions (at around 400 nm) and became enhanced in near infrared (NIR) regions compared to those sintered at 1200 °C. To elucidate the difference of the PL spectra, we performed resonant inelastic x-ray spectroscopy (RIXS) at Ta L-edge. Our RIXS result implies that the microscopic origin of different PL spectra is not relevant to the Ta-related defects and oxygen vacancies.
Effects of electrode polarity on filament ruptures during unipolar resistance switchings
Jay Hyun Jo,이신범,Jonathan Hanjin Lee,장서형,채승철,정창욱 한국물리학회 2010 Current Applied Physics Vol.10 No.3
It has been argued that in unipolar resistance switching of Pt/TiO2/Pt capacitors, conducting filaments should propagate from the cathode interface, and the rupture and recovery of the filaments should occur near the anode interface [Kim et al., Appl. Phys. Lett. 91 (2007) 012907]. We investigated whether this ‘‘anode-interface localized filamentary mechanism” holds for Pt/NiO/Pt capacitors. We examined how the electrode polarity affects the rupturing of the conducting filaments in unipolar resistance switching by performing resistance switching measurements on 246 NiO capacitors. We found some dependencies on electrode polarity; however, they were not consistent with the anode-interface localized filamentary mechanism. We also found that Joule heating affects the rupturing process, suggesting that the weakest link of conducting filaments in NiO thin films plays an important role during the rupturing process.
정민기(Mingi Jeong),김지수(Jisu Kim),장서형(Seohyeong Jang),이태재(Tae-Jae Lee),심형보(Hyungbo Shim),고형호(Hyoungho Ko),조규진(Kyu-Jin Cho),조동일(Dong-Il “Dan” Cho) 제어로봇시스템학회 2016 제어·로봇·시스템학회 논문지 Vol.22 No.9
Micro-electro-mechanical systems (MEMS) gyroscopes are widely used in various robot applications. However, these conventional gyroscopes need to vibrate the proof mass using a built-in actuator at a fixed resonance frequency to sense the Coriolis force. When a robot is not moving, the meaningless vibration of the gyroscope wastes power. In addition, this continuous vibration makes the sensor vulnerable to external sound waves with a frequency close to the proof-mass resonance frequency. In this paper, a feasibility study of a new type of gyroscope inspired by insect halteres is presented. In dipterous insects, halteres are a biological gyroscope that measures the Coriolis force. Wing muscles and halteres are mechanically linked, and the halteres oscillate simultaneously with wing beats. The vibrating haltere experiences the Coriolis force if the insect is going through a rotational motion. Inspired by this haltere structure, a gyroscope using a thin mast integrated with a robot actuation mechanism is proposed. The mast vibrates only when the robot is moving without requiring a separate actuator. The Coriolis force of the mast can be measured with an accelerometer installed at the tip of the mast. However, the signal from the accelerometer has multiple frequency components and also can be highly corrupted with noise, such that raw data are not meaningful. This paper also presents a suitable signal processing technique using the amplitude modulation method. The feasibility of the proposed haltere-inspired gyroscope is also experimentally evaluated.
Structural properties of ferroelectric heterostructures using coherent bragg rod analysis
이경준,신영재,최태양,노태원,Hua Zhou,장서형 한국물리학회 2020 Current Applied Physics Vol.20 No.4
We investigated the crystal structure of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructures grown on a SrTiO3 (001) substrate using surface X-ray scattering. The interface structure of SRO/BTO/SRO heterostructures can decrease the critical thickness of ferroelectric BTO films reduced to 3.5 unit cells. Owing to weak intensity of ultrathin films, lab-source-based X-ray measurement cannot determine the atomic arrangement of the heterostructures and their lattice parameters. We introduced synchrotron-based X-ray scattering techniques combined with coherent Bragg rod analysis (COBRA) to resolve the details of the heterostructures. We obtained detailed crystal structural information based on the electron density profiles of the SRO/BTO/SRO heterostructures acquired from COBRA results, e.g., lattice parameters and atomic arrangements.