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Lee, Yongsu,Lee, Hyeonwoo,Jang, Jaeeun,Lee, Jihee,Kim, Minseo,Lee, Jaehyuk,Kim, Hyunki,Yoo, Seunghyup,Yoo, Hoi-Jun IEEE 2017 IEEE journal on emerging and selected topics in ci Vol.7 No.1
<P>A sticker-type system with hybrid integration of CMOS IC and organic optical sensors is proposed to monitor photoplethysmogram (PPG) signals. To solve problems with the previous solely organic sensor-based works, CMOS IC is implemented in 180 nm technology under 5 V/1.5 V dual power supply. The silver-wire printed planar-fashionable circuit board (P-FCB) is used to connect the CMOS IC with organic sensors. The proposed hybrid system has the five following key features: 1) Power-efficient structure of organic sensor; 2) Integrated analog front-end and digital processor; 3) Degradation compensation scheme; 4) Large parasitic elements optimized design; and 5) Motion artifact rejection scheme. The stickertype PPG monitoring system has mass of only 2g, including the batteries, and consumes only 233 mu W to operate. The PPG signal could be acquired from various body parts (finger, wrist, and neck). The peripheral oxygen saturation level (SpO(2)) extraction results are verified by comparison with a commercial sensor device.</P>
Lee, Seunghyup,Lee, Junghan,Park, Jinjoo,Choi, Youngwoo,Yong, Kijung WILEY‐VCH Verlag 2012 ADVANCED MATERIALS Vol.24 No.18
<P><B>The resistive switching (RS) characteristics of a tungsten oxide (WO<SUB>x</SUB>)‐Au core‐shell nanowire device array</B> is demonstrated for the first time. In addition to the stable bipolar RS characteristics, the nanowire structure of our RS devices provides superhydrophobic properties. The superhydrophobic RS nanowires repelled water that was poured over, such that the device was protected from failure by water contact‐driven leakage currents. Moreover, surprisingly, the devices still work even with when the device is submerged underwater.</P>
Lee, Keunsoo,Lee, Jonghee,Kim, Eunhye,Lee, Jeong-Ik,Cho, Doo-Hee,Lim, Jong Tae,Joo, Chul Woong,Kim, Joo Yeon,Yoo, Seunghyup,Ju, Byeong-Kwon,Moon, Jaehyun IOP 2016 Nanotechnology Vol.27 No.7
<P>An optical functional film applicable to various lighting devices is demonstrated in this study. The phase separation of two immiscible polymers in a common solvent was used to fabricate the film. In this paper, a self-organized lens-like structure is realized in this manner with optical OLED functional film. For an OLED, there are a few optical drawbacks, including light confinement or viewing angle distortion. By applying the optical film to an OLED, the angular spectra distortion resulting from the designed organic stack which produced the highest efficiency was successfully stabilized, simultaneously enhancing the efficiency of the OLED. We prove the effect of the film on the efficiency of OLEDs through an optical simulation. With the capability to overcome the main drawbacks of OLEDs, we contend that the proposed film can be applied to various lighting devices.</P>
Lee, Seunghyup,Yun, Dong-Jin,Rhee, Shi-Woo,Yong, Kijung Royal Society of Chemistry 2009 Journal of materials chemistry Vol.19 No.37
<P>The performance of pentacene thin film transistors (TFTs) was improved using a hafnium silicate (Hf<SUB><I>x</I></SUB>Si<SUB>1−<I>x</I></SUB>O<SUB>2</SUB>) thin film as a high-<I>k</I> dielectric layer. For growth of the Hf<SUB><I>x</I></SUB>Si<SUB>1−<I>x</I></SUB>O<SUB>2</SUB> thin films, an atomic layer chemical vapor deposition (ALCVD) process was optimized using silicon alkoxide and hafnium amido as precursors. The self-limiting surface reactions of each precursor were observed, indicating the ALCVD growth characteristics. The film thickness linearly increased depending on the number of process cycles, with a remarkably high growth rate of 2.3 Å per cycle. The chemical binding states, thermal stability and electrical characteristics of the films grown were investigated using XPS, XRD and capacitance–voltage and leakage current–voltage analysis. The pentacene TFTs fabricated with the ALCVD-grown Hf<SUB>0.67</SUB>Si<SUB>0.33</SUB>O<SUB>2</SUB> dielectric layer were characterized and the results were compared to the pentacene TFTs using Al<SUB>2</SUB>O<SUB>3</SUB> and SiO<SUB>2</SUB> film as dielectric layers. The pentacene/Hf<SUB>0.67</SUB>Si<SUB>0.33</SUB>O<SUB>2</SUB> TFT showed a three-fold and five-fold higher mobility than a pentacene/Al<SUB>2</SUB>O<SUB>3</SUB> TFT and a pentacene/SiO<SUB>2</SUB> TFT, respectively. With additional treatments to enhance the characteristics of the OTFT, pentacene/Hf<SUB><I>x</I></SUB>Si<SUB>1−<I>x</I></SUB>O<SUB>2</SUB> TFTs have great potential as high mobility devices with low operational voltage.</P> <P>Graphic Abstract</P><P>A high growth rate ALCVD process for hafnium silicate thin films was developed using a Si-alkoxide as both Si and O precursor. As an application, pentacene/hafnium silicate TFTs were fabricated, showing enhanced characteristics compared to SiO<SUB>2</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB> gate dielectrics. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b908216f'> </P>
Electroforming-less and multi-level resistive switching characteristics in tungsten oxide thin film
Lee, Seunghyup,Yun, Ki Hoon,Kim, Doeun Elsevier 2019 THIN SOLID FILMS - Vol.674 No.-
<P><B>Abstract</B></P> <P>Thermally grown tungsten oxide (WOx) thin film on a tungsten electrode is studied as a potential memristor material. Due to the dislocation formation inside the film, the device could present stable memristive characteristics without electroforming process. In this research, the opportunity of applying the WOx memristor to the multi-level resistive switching behavior based multi-level memory or artificial synaptic device was presented. In particular, the flexibility of the device shows the possibility of a future flexible memory device. Interpreting the memristive characteristics, the resistance switching mechanism was discussed based on the valance change at the device structure interface. Based on the simple fabrication process and electroforming-less operation, the results present the possibility of applying WOx based memristors in current electronic devices at an early stage. This work reveals a useful proposal of WOx based memristors for emerging device applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Memristive characteristics studies of thermally grown tungsten oxide thin film. </LI> <LI> Investigation of electroforming-less, multi-level resistive switching. </LI> <LI> Flexible memristive characteristics of tungsten oxide thin film device. </LI> <LI> Interpretation of interfacial valance change based resistive switching behavior. </LI> </UL> </P>
Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes.
Lee, Jonghee,Cho, Hyunsu,Koh, Tae-Wook,Yun, Changhun,Hofmann, Simone,Lee, Jae-Hyun,Kim, Yong Hyun,L?ssem, Bj?rn,Lee, Jeong-Ik,Leo, Karl,Gather, Malte C,Yoo, Seunghyup Optical Society of America 2013 Optics express Vol.21 No.23
<P>We report on the characteristics of enhanced and balanced white-light emission from bi-directional organic light-emitting diodes (BiOLEDs) enabled by the introduction of micro-cavity effects. The insertion of an additional metal layer between the indium tin oxide anode and the hole transporting layer results in similar light output of our BiOLEDs in both top and bottom direction and in reduced distortion of the electroluminescence spectrum. Furthermore, we find that by utilizing MC effects, the overall current efficiency can be improved by 26.2% compared to that of a conventional device.</P>
Lee, Seunghyup,Kim, Wooseok,Yong, Kijung WILEY‐VCH Verlag 2011 ADVANCED MATERIALS Vol.23 No.38
<P>Water‐resistant electronic devices are developed based on superhydrophobic nanostructures. A ZnO resistive switching device is tested as a model system. As reported on page 4398 by Kijung Yong and co‐workers, the application of superhydrophobic nanostructures on device surfaces efficiently blocks the direct contact of water with electronic components and the devices work even when water is poured on the surface of the device. </P>