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Conferences

Investigation of emission capability of reduced graphene oxide film cathode for ...

IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves

Bhattacharya R., Baek I.-K., Lee J.S., Barik R.K., Kim S., Hong D., Kwon O., Sattorov M.A., Kim Y.H., Park G.-S.

Abstract
Terahertz vacuum sources with high power are in immediate need for several applications like medical, security, communication, etc. The power and performance of these devices mainly depends on cathode. As structure become smaller, it is very much difficult to obtain high power at terahertz frequency, using conventional low current density thermionic cathodes. As a result development of non-conventional field emission cathode is in progress, which may produce a very high current density with comparatively high current and can help in terahertz research and application. In this work our main aim is to develop and analyze high current density (>103A/cm2) sheet beam film cathode using reduced graphene oxide (rGO)-nano particle composite. © 2015 IEEE.
Conditioning of vertically aligned reduced graphene oxide film electron emitter ...

IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves

Baek I.-K., Bhattacharya R., Lee J.S., Kim S., Hong D., Ohjoon K., Kim Y.H., Park G.-S.

Abstract
The field emission from the edge of vertically aligned reduced graphene oxide (rGO) film was examined experimentally. High current (I > 5 mA) and high emission current density (J > 100 A/cm2) was obtained using diode configuration. The thermal runaway of sharp protrusions on emission edge are considered to be source for the destructive vacuum breakdowns due to high local emission current. A remarkable field emission stability was achieved using processed field induced evaporation, by removing the protrusions and recovering the totally field emission attributed I-V characteristics. © 2015 IEEE.
Accelerated terahertz water dynamics under osmotic interaction of lipid bilay...

IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves

Eom K., Son H., Jeong J.-Y., Jang J., Park I.-K., Kim S., Park G.-S.

Abstract
Intracelluar water is expected to have different physical properties from those of bulk water. Using polyethylene glycol, we induce model intracellular environment on DMPC lipid solution and we shows the accelerated movement of non-hydrogen bonding (NHB) water molecules interacting with DMPC in polyethylene glycol (PEG) solution from the dielectric response in the terahertz range. With the measurement in the GHz range, detailed behavior of hydration water is also analyzed. © 2015 IEEE.
Spatiotemporal features of nucleic acid hydration and their changes during denat...

IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves

Son H., Jeong J., Kim S., Park J., Jang J., Eom K., Park I., Park G.-S.

Abstract
Dynamics and structure of water network around nucleic acid have been subjects of intensive studies due to their contributions to biological process. Here, we identified spatiotemporal features of DNA hydration through examining fully-hydrated DNA solutions by THz spectroscopy. The results suggest the presence of weakly bound water beyond tight-binding hydration layer with the timescale of 11.6ps and the thickness of 7.6Å. Also, we will present changes in the hydration water during DNA denaturation at the conference. © 2015 IEEE.
Correlation between salt-induced change in lipid hydration water and inter-space...

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Choi D.-H., Son H., Jang J.-M., Jeong J.-Y., Eom K.-H., Jin K.S., Park J., Park G.-S.

Abstract
Salt-induced change in water dynamics inside multi-lamellar vesicles (MLV) is experimentally studied with terahertz (THz) time-domain spectroscopy (TDS). The dielectric constants of lipids dispersed in NaCl solutions with different salt concentrations are measured. We find that the water molecules with fast relaxation time (∼100 fs) significantly increase with NaCl concentrations. Furthermore, by combining the THz TDS results with small-angle X-ray scattering (SAXS), we find a strong correlation between multi-lamellar spacing and the fast water fraction. This suggests that long-range hydration might be critical in membrane-membrane communications. © 2016 IEEE.
Dependence of water relaxation dynamics on lipid structure

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Eom K., Jang J., Jeong J.-Y., Yang Y., Choi D.-H., Son H., Park I.-K., Kim S., Park G.-S.

Abstract
The water relaxation dynamics is measured for DMPC vesicle solution and LMPC micelle solution from the dielectric spectroscopy. Both lipid have a same headgroup, which is interacting parts with the surrounding water molecules, but the change of the water relaxation time with increasing lipid concentration shows opposite trend. The result suggests that one needs to consider the lipid structure in water when the hydration property is determined. © 2016 IEEE.
Precise determination of optical parameter of highly absorptive liquid in THz re...

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Son H., Choi D.-H., Jeong J.-Y., Jang J.-M., Park G.-S.

Abstract
A new approach for precise complex optical parameter of highly absorptive liquid is proposed for transmission THz time-domain spectroscopy. Due to the nature of liquid, indirect measurement of sample thickness increases inaccuracy in optical parameter. Here, we reduced uncertainty in optical parameter through an algorithm optimizing sample thickness by Kramers-Kronig relations. Less than 2% error in absorption coefficient of water at 1THz is achieved in proof-of-concept experiment. This analysis technique can be of use in studies of water-based liquid such as biomolecular solution. © 2016 IEEE.
High-Q electronic fano resonances in metallic subwavelength slits

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Kim S., Baek I.-K., Bhattacharya R., Hong D., Matlabjon S., Park G.-S.

Abstract
We have experimentally investigated electronical excitation of Fano resonances in subwavelength slits. These resonances can provide strong concentration of electromagnetic waves under subwavelength conditions in microwave and terahertz regime as analogous to plasmonic Fano resonances in optics regime. It is observed that the quality factors of the resonances become exceedingly high by introducing the structural asymmetry. High-Q subwavelength slits which also show Rayleigh-like scattering, can be employed in various electro-optical devices of microwave, terahertz and infrared region. © 2017 IEEE.
Is fast relaxation water really a free water?

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz

Jeong J.-Y., Jang J., Patra A., Eom K., Park I., Yang Y., Kim S., Di Tommaso D., Park G.-S.

Abstract
The origin of fast relaxation water in the terahertz frequency range have been elusive for years. In this contribution we have experimentally investigated the change in dynamical structure of water in salt solution using broadband dielectric relaxation spectroscopy (DRS) in the GHz to THz region to study the fast water. Numerical simulations have been conducted to quantify the dynamical properties in the hydration shell of the ions, including rotational and translational residence times. As the amount of the fast water increases with salt solution concentration, the fraction of water molecules with zero or one hydrogen-bond (HB) is also increased. We can conclude that the fast water is composed of the free water and one hydrogen-bonded water. © 2017 IEEE.
Improved field emission from tungsten doped reduced graphene oxide

IVEC 2017 - 18th International Vacuum Electronics Conference

Bhattacharya R., Baek I.-K., Barik R.K., Kim S., Hong D., Kwon O., Sattorov M.A., Park G.-S.

Abstract
The emission capability of as prepared reduced graphene oxide (rGO) and tungsten doped reduced graphene oxide (W-rGO) film field emitter (FE) cathodes are investigated experimentally. It is observed that W-rGO films showed a highest emission current density of ∼ 800 A/cm2 at an applied DC electric field of 6.6 V/μm, which is far superior than the current density of ∼ 390 A/cm2 from an as prepared rGO film. Short stability test also showed that emission current is much more stable than the rGO films. These excellent improvements can be attributed to the doping of tungsten into the rGO film, which helped to repair the broken bonds thus lowering the turn on field. Also, tungsten gives the film an excellent mechanical stability and higher thermal load handling capability, which is very important to achieve a stable, high performance operation and long life. © 2017 IEEE.

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