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Spray-Coated Liquid Metal Reflectors for Transparent Hydrogel Atomic Force Microscope Cantilevers
Kim, Kwangseok,Kim, Seokbeom,Kim, Taehong,Kim, Wonjung,Lee, Jungchul Institute of Electrical and Electronics Engineers 2016 Journal of microelectromechanical systems Vol. No.
<P>This letter reports optical-lever detection with spray-coated liquid metal reflectors for transparent hydrogel atomic force microscope (AFM) cantilevers. Eutectic gallium-indium (EGaIn) filled in an air brush was locally sprayed over poly(ethylene) glycol-diacrylate (PEG-DA) AFM cantilevers with a shadow mask. Oxygen plasma treatment for bare hydrogel cantilevers prior to the EGaIn spray was shown to improve the wetting characteristics between EGaIn and PEG-DA, thus enabled relatively thin coating of similar to 800 nm within similar to 100 ms. PEG-DA AFM cantilevers coated with EGaIn were underwent mechanical characterization, and were employed for AFM imaging in both air and deionized (DI) water. Except the reflectivity degraded down to 40% of the initial value after 1-h dwell in DI water, their overall performance was comparable with those of PEG-DA AFM cantilevers with solid metal reflectors.</P>
Micropatterning of Liquid Metal by Dewetting
Kim, Taehong,Kim, Kwangseok,Kim, Seokbeom,Lee, Jungchul,Kim, Wonjung IEEE 2017 Journal of microelectromechanical systems Vol.26 No.6
<P>Although gallium-based liquid metals are attracting growing interest thanks to its potential applications in deformable, flexible electronic devices, challenges in fabrication associated with the high surface tension and oxide skin remain to be overcome. We report a novel fabrication technique for liquid metal circuits using dewetting. An excessively thin liquid film spontaneously shrinks on a substrate to reduce surface free energy. In the case of a thin liquid metal film on a substrate with microgrooves, the oxide on the microgroove wall and the additional viscous resistance delay the shrinkage in the grooves, which separates the liquid volume inside the microgrooves from the external volume. Utilizing this mechanism, we successfully produced 20-mu m-thick conductive lines of eutectic gallium indium (EGaIn) on polydimethylsiloxane. This fabrication technique is simple, fast, and cost-effective and requires no top covering layer. The resultant metal lines show potentially applicable electrical resistance to flexible and stretchable electronic devices. [2017-0082]</P>
VoIP Capacity Analysis in Full Duplex WLANs
Kim, Wonjung,Song, Taewon,Kim, Taeyoon,Park, Hyunhee,Pack, Sangheon IEEE 2017 IEEE Transactions on Vehicular Technology VT Vol.66 No.12
<P>With the increase of the popularity of voice over wireless local area networks (VoWLANs), how to improve the capacity of VoWLANs is being actively investigated. A recent technology to allow simultaneous transmission and reception, i.e., full duplex (FD) communications, can be a promising candidate for improving the capacity of VoWLANs. In this paper, we develop an analytical model of an FD medium access control protocol for VoWLANs, and study its capacity. Extensive simulation results are given to show the effects of FD communications, different voice over IP (VoIP) codecs, and data connections on the VoIP capacity.</P>
Ligand-dependent blinking of zinc-blende CdSe/ZnS core/shell nanocrystals
Kim, Yongwook,Song, Nam Woong,Yu, Hyunung,Moon, Dae Won,Lim, Sung Jun,Kim, Wonjung,Yoon, Hye-Joo,Koo Shin, Seung Royal Society of Chemistry 2009 Physical chemistry chemical physics Vol.11 No.18
<P>Blinking of zinc-blende CdSe/ZnS core/shell nanocrystals are studied as a function of surface passivating ligands. Organic-soluble CdSe/ZnS core/shell nanocrystals are prepared by colloidal synthesis free of trioctylphosphine oxide and converted into water-soluble ones by ligand exchange with three different hydrophilic thiols, 2-aminoethanethiol, 3-mercapto-1-propanol, and 3-mercaptopropionic acid. The zinc-blende lattice structure is confirmed by powder X-ray diffraction, the size and shape distributions are visualized by high-resolution transmission electron microscopy, and hydrodynamic size distributions of water-soluble nanocrystals are determined by dynamic light scattering. Ligand-dependent optical properties, such as the absorption and emission spectra as well as the photoluminescence lifetime, are obtained in both solution and glass substrate to characterize the effects of ligand on the bright state of nanocrystals. The time trace of blinking is recorded for single nanocrystals in polymer film. Both on- and off-time distributions are fit to a power law. The off-time exponents are clustered at 1.67 ± 0.05, whereas the on-time exponents are scattered in the range of 1.71–2.25. The thiolate conjugation on the surface zinc atom greatly reduces the on-time duration, suggesting that the rate of photoinduced charge separation from the bright (on) to the dark (off) state increases as the number of surface hole-trap states increases.</P> <P>Graphic Abstract</P><P>The on-time distributions in the blinking of single zinc-blende CdSe/ZnS nanocrystals vary with the density of hole-trap states. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b822351c'> </P>