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Jongtaek Lee,Taehee Park,Jungwoo Lee,Whikun Yi IEEE 2015 IEEE SENSORS JOURNAL Vol.15 No.1
<P>We fabricated a gas ionization sensor based on 3-D single-walled carbon nanotube (SWNT) networks suspended between ZnO nanorods using a chemical vapor deposition method. ZnO nanorods were grown on the pyramid-like protrusions of a textured Si substrate. The breakdown voltages and stability of the 3-D SWNT networks were measured and compared with those of a SWNT film synthesized on Si substrate. Our results show that 3-D SWNT networks on ZnO nanorods enable ionization gas sensors to be sensitive, selective, and highly stable devices.</P>
A Scheme for Location-Based Internet Broadcasting and Its Applications
Jongtaek Oh,Haas, Z.J. IEEE 2007 IEEE communications magazine Vol.45 No.11
<P>In this article, a new Internet broadcasting scheme that combines the existing network-wide unicast protocol and a subnet-wide broadcast protocol is proposed. The proposed broadcasting scheme is appropriate for location-based push services and requires only a minor functional addition to the existing edge routers, base stations, or access points. To eliminate unnecessary broadcasting traffic, an efficient terminal-originated triggering algorithm for broadcast initiation is introduced. The scheme provides location-based information to mobile users without the need for protocol reconfigurations or prior knowledge of a user's location. We also discuss some potential applications of the scheme. Performance of the proposed scheme has been evaluated and selected results are presented in this article. The evaluation shows that the broadcasting scheme significantly reduces the network traffic and the server load.</P>
Frame error reduction method by using vatiable span CRC coding
Oh, Jongtaek 漢城大學校 2000 論文集 Vol.24 No.2
In this proposed method, CRC coding is only applied to data unit, not to padded zeros that are to fill up the fixed length of data unit. So improved frame error rate and protocol efficiency are possible instead of increment of receiver complexity. According to the channel characteristics, negotiation of proper data unit size between transceivers will improve the protocol efficiency. In this paper, frame error reduction technique by using variable span CRC coding is proposed, and the FER and protocol efficiency for the technique are analyzed.
Phonon Modes of Organic Electro-Optic Molecular Crystals for Terahertz Photonics
Kim, Jongtaek,Kwon, O-Pil,Brunner, Fabian D. J.,Jazbinsek, Mojca,Lee, Seung-Heon,Gü,nter, Peter American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.18
<P>We investigate theoretically and experimentally the phonon modes of organic electro-optic crystals for THz-wave photonics. THz phonon modes significantly influence the characteristics of THz-wave generation and detection through the THz absorption and refractive-index dispersion. For a model as THz generation material, electro-optic PNP (2-(<I>N</I>-prolinol)-5-nitropyridine) molecular crystal containing a relatively small numbers of atoms in the unit cell is chosen for phonon-mode calculation. THz phonon modes with their frequencies and intensities are calculated by periodic density functional theory, and the resulting spectrum exhibits a good agreement with the experimental absorption spectrum measured by THz time-domain spectroscopy. THz-wave generation experiment is performed with PNP as THz generator and the organic electro-optic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal as THz detector. Considering the phase matching condition and THz absorption, which are both influenced by the THz phonon modes of THz generation and detection materials, the calculated THz generation spectrum also matches well with the experimental THz spectrum obtained by optical rectification at the pump wavelength of 1300 nm. Therefore, the phonon mode identification is very important for optimizing and designing new THz source and detection materials as well as for THz spectroscopy.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-18/acs.jpcc.5b02445/production/images/medium/jp-2015-02445p_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b02445'>ACS Electronic Supporting Info</A></P>
Kim, Jongtaek,Yoo, Eunjung,Chang, Sukbok,Lee, Yoon Sup Wiley Subscription Services, Inc., A Wiley Company 2011 Journal of computational chemistry Vol.32 No.9
<P><B>Abstract</B></P><P>Density functional theory calculations with the B3LYP functional were performed for the title ring‐opening reaction to understand the intrinsic activating and directing effects of the <I>N</I>‐substituents, as well as the electron donating effect of the para‐substituted (Y = Cl, H, Me) phenyl group at the more hindered benzylic C2 atom. The <I>N</I>‐tosyl group (i.e., <I>N</I>‐Tos) or the <I>N</I>‐(2‐pyridyl)sulfonyl group (i.e., <I>N</I>‐Py) was introduced to activate the ring nitrogen atom (N1) and the para‐substituted (Y = Cl, H, Me) phenyl group for the activation of the C2 atom. Conformational searches and geometry optimizations were performed for the <I>N</I>‐(<I>para</I>‐substituted)phenylaziridines (<B>1</B>∼<B>6</B>). Calculations indicate that the aziridine <B>6</B> (i.e., Py/Me) has the most elongated C2N1 bond intrinsically due to the electronic activating effects, implying the aziridine <B>6</B> to be the most potent candidate for the more‐hindered C2 opening. Transition states (TSs) were investigated for the prospective ring‐opening paths (I∼IV), considering the types of intermolecular push–pull interactions between the <I>N</I>‐activated phenylaziridines and the cuprate. The <I>N</I>‐Py group provides an unique C2‐favored TS along the path IV, which the <I>N</I>‐Tos group cannot afford, due to the less charge transfer from the nucleophilic CH 3δ−<IMG src='/wiley-blackwell_img/equation/tex2gif-stack-1.gif' alt ='math image'/> of the cuprate into the electrophilic C2 atom. Furthermore, the e‐donating effect of the para‐substituents (Y = Cl, H, Me) enhances the C2 opening for the path IV. This study enables us to understand the unusual ring‐opening phenomena in terms of electronic and directing effects and hence may serve as a tool to design substrates for highly regioselective ring openings. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011</P>
Kim, Jongtaek,Kwon, O-Pil,Jazbinsek, Mojca,Park, Young Choon,Seo, Jung-In,Lee, Yoon Sup American Chemical Society 2011 The Journal of Physical Chemistry Part C Vol.115 No.47
<P>The direction and the magnitude of the first hyperpolarizability are investigated by the finite field (FF) method for organic nonlinear optical ionic chromophores and crystals with an isotropic counteranion, either a point charge or hexafluorophosphate. Simple model compound 4-aminopyridinium and DAPSH (<I>N</I>,<I>N</I>-dimethylamino-<I>N</I>′-phenyl-4-stilbazolium hexafluorophosphate) crystal with the highest quadratic optical nonlinearities measured to date in organic crystals, as well as stilbazolium derivatives having heteroaromatic and/or bulky aromatic rings, are investigated. Calculations indicate that the extrinsic isotropic point-charges or anions almost have no effect on the direction of the intrinsic first hyperpolarizability of the cation chromophores, and induce a limited increase of the magnitude smaller than ∼15% due to their asymmetric distribution with respect to the pyridinium nitrogen atom. This finding enables one to simply approximate the first hyperpolarizability of ionic crystalline systems having 1D intramolecular charge transfer character by considering only the cation. In addition, the heteroaromatic or bulky aromatic substituents at the end of the pyridinium acceptor intrinsically do not affect the direction and only moderately affect the magnitude of the first hyperpolarizability of stilbazolium salts. These results are potentially useful for molecular design and crystal engineering of ionic organic materials, where the magnitude and the direction of first hyperpolarizability as well as the shape of the ionic chromophore are of great importance.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2011/jpccck.2011.115.issue-47/jp204218t/production/images/medium/jp-2011-04218t_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp204218t'>ACS Electronic Supporting Info</A></P>