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Choi, Dukhyun,Choi, Min-Yeol,Choi, Won Mook,Shin, Hyeon-Jin,Park, Hyun-Kyu,Seo, Ju-Seok,Park, Jongbong,Yoon, Seon-Mi,Chae, Seung Jin,Lee, Young Hee,Kim, Sang-Woo,Choi, Jae-Young,Lee, Sang Yoon,Kim, Jo WILEY-VCH Verlag 2010 ADVANCED MATERIALS Vol.22 No.19
<B>Graphic Abstract</B> <P>The cover shows an image of fully rollable transparent nanogenerators synthesized using chemical vapor deposition grown large-scale graphene sheets as transparent electrodes and piezoelectric ZnO nanorod arrays. Sang-Woo Kim, Jae-Young Choi, and co-workers report on p. 2187 the electrical and structural stability of the nanogenerators, with excellent charge scavenging performance under external mechanical loads such as bending and rolling. This study shows that graphene-based nanogenerators are very promising for self-powered rollable transparent device applications. <img src='wiley_img_2010/09359648-2010-22-19-ADMA201090066-content.gif' alt='wiley_img_2010/09359648-2010-22-19-ADMA201090066-content'> </P>
Fully Rollable Transparent Nanogenerators Based on Graphene Electrodes
Choi, Dukhyun,Choi, Min-Yeol,Choi, Won Mook,Shin, Hyeon-Jin,Park, Hyun-Kyu,Seo, Ju-Seok,Park, Jongbong,Yoon, Seon-Mi,Chae, Seung Jin,Lee, Young Hee,Kim, Sang-Woo,Choi, Jae-Young,Lee, Sang Yoon,Kim, Jo WILEY-VCH Verlag 2010 Advanced Materials Vol.22 No.19
<B>Graphic Abstract</B> <P>Fully rollable transparent nanogenerators have been developed using chemical vapor deposition-grown large-scale graphene sheets as transparent electrodes and piezoelectric ZnO-nanorod arrays. The electrical and structural stability of the nanogenerators with excellent charge scavenging performance under external mechanical loads such as bending and rolling shows that graphene-based nanogenerators are suitable for self-powered rollable transparent device applications. <img src='wiley_img_2010/09359648-2010-22-19-ADMA200903815-content.gif' alt='wiley_img_2010/09359648-2010-22-19-ADMA200903815-content'> </P>
Choi, Dukhyun,Lee, Keun Young,Jin, Mi-Jin,Ihn, Soo-Ghang,Yun, Sungyoung,Bulliard, Xavier,Choi, Woong,Lee, Sang Yoon,Kim, Sang-Woo,Choi, Jae-Young,Kim, Jong Min,Wang, Zhong Lin Royal Society of Chemistry 2011 ENERGY AND ENVIRONMENTAL SCIENCE Vol.4 No.11
<P>In this paper, we present a simple, low-cost and flexible hybrid cell that converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using piezoelectric zinc oxide (ZnO) in conjunction with organic solar cell design. Since the hybrid cell is designed by coupled piezoelectric and photoconductive properties of ZnO, this is a naturally hybrid architecture without crosstalk and an additional assembling process to create multi-type energy scavengers, thus differing from a simple integration of two different energy generators. It is demonstrated that the behavior of a piezoelectric output is controlled from alternating current (AC) type to direct current (DC)-like type by tailoring mechanical straining processes both in the dark and under light illumination. Based on such controllability of output modes, it is shown that the performance of the hybrid cell is synergistically enhanced by integrating the contribution made by a piezoelectric generator with a solar cell under a normal indoor level of illumination. Our approach clearly demonstrates the potential of the hybrid approach for scavenging multi-type energies whenever and wherever they are available. Furthermore, this work establishes the methodology to harvest solar energy and low-frequency mechanical energies such as body movements, making it possible to produce a promising multi-functional power generator that could be embedded in flexible architectures.</P> <P>Graphic Abstract</P><P>Naturally hybrid flexible energy generator that converts individually or simultaneously low-frequency mechanical energies and photon energy into electricity was developed. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1ee02080c'> </P>
Metal–Insulator–Metal Optical Nanoantenna with Equivalent-Circuit Analysis
Choi, Yeonho,Choi, Dukhyun,Lee, Luke P. WILEY-VCH Verlag 2010 Advanced Materials Vol.22 No.15
<B>Graphic Abstract</B> <P>Selective metal–insulator–metal optical nanoantennas are fabricated with Au deposition on top of anodic aluminum oxide layer (see figure). The complicated nanoantennas are easily analyzed by using an equivalent-circuit system and characterized by surface-enhanced Raman scattering. <img src='wiley_img_2010/09359648-2010-22-15-ADMA200903443-content.gif' alt='wiley_img_2010/09359648-2010-22-15-ADMA200903443-content'> </P>
Improved lateral force calibration based on the angle conversion factor in atomic force microscopy
CHOI, DUKHYUN,HWANG, WOONBONG,YOON, EUISUNG Blackwell Scientific 2007 Journal of Microscopy Vol.228 No.2
<P>Summary</P><P>A novel calibration method is proposed for determining lateral forces in atomic force microscopy (AFM), by introducing an angle conversion factor, which is defined as the ratio of the twist angle of a cantilever to the corresponding lateral signal. This factor greatly simplifies the calibration procedures. Once the angle conversion factor is determined in AFM, the lateral force calibration factors of any rectangular cantilever can be obtained by simple computation without further experiments. To determine the angle conversion factor, this study focuses on the determination of the twist angle of a cantilever during lateral force calibration in AFM. Since the twist angle of a cantilever cannot be directly measured in AFM, the angles are obtained by means of the moment balance equations between a rectangular AFM cantilever and a simple commercially available step grating. To eliminate the effect of the adhesive force, the gradients of the lateral signals and the twist angles as a function of normal force are used in calculating the angle conversion factor. To verify reliability and reproducibility of the method, two step gratings with different heights and two different rectangular cantilevers were used in lateral force calibration in AFM. The results showed good agreement, to within 10%. This method was validated by comparing the coefficient of friction of mica so determined with values in the literature.</P>
Piezoelectric touch-sensitive flexible hybrid energy harvesting nanoarchitectures
Choi, Dukhyun,Lee, Keun Young,Lee, Kang Hyuck,Kim, Eok Su,Kim, Tae Sang,Lee, Sang Yoon,Kim, Sang-Woo,Choi, Jae-Young,Kim, Jong Min IOP Pub 2010 Nanotechnology Vol.21 No.40
<P>In this work, we report a flexible hybrid nanoarchitecture that can be utilized as both an energy harvester and a touch sensor on a single platform without any cross-talk problems. Based on the electron transport and piezoelectric properties of a zinc oxide (ZnO) nanostructured thin film, a hybrid cell was designed and the total thickness was below 500 nm on a plastic substrate. Piezoelectric touch signals were demonstrated under independent and simultaneous operations with respect to photo-induced charges. Different levels of piezoelectric output signals from different magnitudes of touching pressures suggest new user-interface functions from our hybrid cell. From a signal controller, the decoupled performance of a hybrid cell as an energy harvester and a touch sensor was confirmed. Our hybrid approach does not require additional assembly processes for such multiplex systems of an energy harvester and a touch sensor since we utilize the coupled material properties of ZnO and output signal processing. Furthermore, the hybrid cell can provide a multi-type energy harvester by both solar and mechanical touching energies. </P>
Choi, Min-Yeol,Choi, Dukhyun,Jin, Mi-Jin,Kim, Insoo,Kim, Sang-Hyeob,Choi, Jae-Young,Lee, Sang Yoon,Kim, Jong Min,Kim, Sang-Woo WILEY-VCH Verlag 2009 ADVANCED MATERIALS Vol.21 No.21
<B>Graphic Abstract</B> <P>Transparent flexible charge-generating piezoelectric nanodevices are developed. The resulting integrated nanodevice generates a noticeable current when it is pushed by application of an external load. Piezoelectric ZnO nanorod-based nanodevices with embossed PdAu top electrodes produce the highest output current density of approximately 10 μA cm<SUP>−2</SUP> at a load of 0.9 kgf. <img src='wiley_img/09359648-2009-21-21-ADMA200803605-content.gif' alt='wiley_img/09359648-2009-21-21-ADMA200803605-content'> </P>
Additional amplifications of SERS <i>via</i> an optofluidic CD-based platform
Choi, Dukhyun,Kang, Taewook,Cho, Hansang,Choi, Yeonho,Lee, Luke P. Royal Society of Chemistry 2009 Lab on a chip Vol.9 No.2
<P>In this paper, signal amplifications of surface-enhanced Raman scattering (SERS) are realized by an optofluidic compact disc (CD)-based preconcentration method for effective label-free environmental and biomolecular detections. The preconcentration of target molecules is accomplished through the accumulation of adsorbed molecules on SERS-active sites by repeating a ‘filling–drying’ cycle of the assay solution in the optofluidic CD platform. After 30 cycles, the clear and high SERS signal of rhodamine 6G of 1 nM is readily detected. In addition to the preconcentration-based signal amplification by the optofluidic SERS system on the CD platform, we introduce a controlled precipitation of gold nanoparticles by CuSO<SUB>4</SUB> for SERS substrates. This method provides high-throughput, high-sensitive and large-area uniform SERS substrates on the optofluidic CD platform. The uniform SERS signals from different positions in spots of 3 mm<SUP>2</SUP> on the different CDs gives us confidence in the reliability and stability of our SERS substrates.</P> <P>Graphic Abstract</P><P>We present optofluidic SERS on a compact disk (CD) platform which is designed to preconcentrate the molecule of interest <I>via</I> simply repeating ‘filling–drying’ cycles of molecular solutions for SERS signal amplification. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b812067f'> </P>
Dependence of the mechanical properties of nanohoneycomb structures on porosity
Choi, Dukhyun,Lee, Sangmin,Lee, Changwoo,Lee, Pyungsoo,Lee, Junghyun,Lee, Kunhong,Park, Hyunchul,Hwang, Woonbong IOP 2007 Journal of micromechanics and microengineering Vol.17 No.3
<P>Mechanical properties of nanohoneycomb structures are measured for varying porosity (or pore diameter) of the nanohoneycomb structure. The indentation modulus and hardness in the pore direction (or thickness direction) are obtained from indentation tests using a nano-indenter. The bending modulus of the nanohoneycomb structures in the vertical direction relative to the pore (generally along the beam length) is determined from bending tests in AFM. To determine the bending modulus of the nanohoneycomb structures, the area moment of inertia of the nanohoneycomb structure is determined according to the arrangement of the pores. The indentation moduli and the hardness are found to decrease nonlinearly with increasing porosity. The bending moduli of the nanohoneycomb structures also decrease nonlinearly as a function of porosity over a large range. It is made clear that the elastic modulus of a homogenous material can be controlled by changing the pore diameter.</P>