Sung Hwan Heo

Korea Advanced Institute of Science and Technology , Sŏul, Seoul, South Korea

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Publications (12)26.64 Total impact

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    ABSTRACT: A vacuum-sealed miniature X-ray tube based on a carbon nanotube field-emission electron source has been demonstrated. The diameter of the X-ray tube is 10 mm; the total length of the tube is 50 mm, and no external vacuum pump is required for the operation. The maximum tube voltage reaches up to 70 kV, and the X-ray tube generates intense X-rays with the air kerma strength of 108 Gy * cm2 min1. In addition, X-rays produced from the miniature X-ray tube have a comparatively uniform spatial dose distribution.
    Nanoscale Research Letters 05/2012; 7(1):258. · 2.52 Impact Factor
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    ABSTRACT: The photoelectrochemical (PEC) responses of electron-irradiated [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/TiO2 electrodes were evaluated in a PEC cell. By coating PCBM on TiO2 nanoparticle film, the light absorption of PCBM/TiO2 electrode has expanded to the visible light region and improved the PEC responses compared to bare TiO2 electrode. The PEC responses were further improved by irradiating an electron beam on PCBM/TiO2 electrodes. Compared to non-irradiated PCBM/TiO2 electrodes, electron irradiation increased the photocurrent density and the open-circuit potential of PEC cells by approximately 90% and approximately 36%, respectively at an optimum electron irradiation condition. The PEC responses are carefully evaluated correlating with the optical and electronic properties of electron-irradiated PCBM/TiO2 electrodes.
    Nanoscale Research Letters 03/2012; 7(142). · 2.52 Impact Factor
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    ABSTRACT: Small tip-type CNT emitters with the diameter of 0.8 mm were fabricated for miniaturized X-ray tubes. The CNT emitters were prepared by dropping CNTs and silver nanoparticles on a flat surface of a W metal tip followed by annealing at 800°C for 2 h under vacuum. The CNT emitters exhibit good field emission properties with the threshold electric field of 1.15 V/μm and the field enhancement factor of 12,050. CNTs were well attached to a flat W tip surface without coating on the side plane of the tip, and thus beam divergence could be minimized. Consequently, a miniaturized X-ray tube with the inner diameter of 5 mm was successfully demonstrated using the tip-type CNT emitter.
    Journal of Nanomaterials 01/2012; 2012. · 1.55 Impact Factor
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    ABSTRACT: We present a novel design of an X-ray target to deliver uniform dose from an electronic brachytherapy system (EBS). This design comprises of a combination of both the reflection- and transmission-type target geometries. Monte-Carlo simulation code MCNP5 has been employed for the calculation of angular distribution of the X-ray intensity produced from various morphologies of X-ray targets. The simulation results reveal that the combinatorial target-assembly is promising and effective in achieving uniformity of X-ray emission over the entire space of solid angle of 4π in comparison to a transmission-type target that produces X-rays mainly in the forward direction and a reflection-type target that generates X-rays mostly in the backward direction. As a direct consequence of the uniformity of X-ray emission, the combinatorial target-assembly can impart a uniform dose distribution which makes it suitable as a target of an X-ray tube for EBS.Highlights► A novel X-ray target design is proposed. ► The target consists of a truncated conical reflection-type target and a transmission-type hemispherical cap. ► This combinatorial target assembly can produce X-rays with a good spatial uniformity over the entire solid angles.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2011; 269(10):1053-1057. · 1.19 Impact Factor
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    ABSTRACT: We present the synthesis and visible-light-induced catalytic activity of Ag(2)S-coupled TiO(2) nanoparticles (NPs) and TiO(2) nanowires (NWs). Through a simple wet chemical process from a mixture of peroxo titanic acid (PTA) solution, thiourea and AgAc, a composite of Ag(2)S NPs and TiO(2) NPs with sizes of less than 7 nm was formed. When the NP composite was further treated with NaOH solution followed by annealing at ambient conditions, a new nanocomposite material comprising Ag(2)S NPs on TiO(2) NWs was created. Due to the coupling with such a low bandgap material as Ag(2)S, the TiO(2) nanocomposites could have a visible-light absorption capability much higher than that of pure TiO(2). As a result, the synthesized Ag(2)S/TiO(2) nanocomposites exhibited much higher catalytic efficiency for the decomposition of methyl orange than commercial TiO(2) (Degussa P25, Germany) under visible light.
    Nanotechnology 01/2010; 21(1):015703. · 3.84 Impact Factor
  • Aamir Ihsan, Sung Hwan Heo, Sung Oh Cho
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    ABSTRACT: We present a novel concept to develop a microfocus X-ray tube based on a microstructured X-ray target that is irradiated with a nonfocused electron beam. X-ray emissions from the microstructured targets with various morphologies were studied using Monte-Carlo simulation code MCNP5. The calculations revealed that the microstructured targets are quite capable of minimizing the effective X-ray spot size compared with those of conventional transmission-type X-ray targets. Based on the simulation results of X-ray brightness, optimum geometric parameters were derived for the microstructured targets with different morphologies. Moreover, the stability of the microstructured target against heat loads delivered by an electron beam was also investigated under both the continuous and pulsed operation modes. From the analysis, the limitations of the maximum allowable electron beam currents for the stable operation of the X-ray targets are presented. The combination of the microstructured targets and nonfocused electron beam allows the miniaturization of a microfocus X-ray tube by eliminating the needs for massive and complex focusing devices.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2009; 267(21):3566-3573. · 1.19 Impact Factor
  • Aamir Ihsan, Sung Hwan Heo
    Brachytherapy 01/2009; 8(2):151-151. · 1.22 Impact Factor
  • Aamir Ihsan, Sung Hwan Heo, Sung Oh Cho
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    ABSTRACT: Parameters of a transmission-type X-ray target have been optimized for a microfocus X-ray tube with an emphasis on maximizing the intensity of X-rays generated from the target while reducing the X-ray focal spot size. Monte Carlo simulation code MCNPX was used for the calculation of X-ray intensity generated from the X-ray tube. The calculation shows that the target thickness plays a dominant role in the enhancement of X-ray intensity. The optimum thickness to get maximum X-ray intensity for a given beam current was derived from the calculation for a broad range of electron energies from 30keV to 150keV. In addition, stability of a transmission-type target against localized thermal loading, which is attributed to the electron beam impinging on a micron-sized target region, was also investigated by the calculation of the temperature distribution within the target. From the analysis, we present the maximum allowable electron-beam power loading for a stable operation of the transmission-type target at different incident electron beam energies.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 11/2007; 264(2):371-377. · 1.19 Impact Factor
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    Sung Hwan Heo, Aamir Ihsan, Sung Oh Cho
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    ABSTRACT: A microfocus x-ray tube that can generate x rays with the focal spot size less than 5 μm has been demonstrated using carbon nanotube (CNT) field emitters. A CNT cathode on a sharp tungsten tip, a magnetic solenoid lens, and a transmission-type x-ray target were adopted for the microfocus x-ray tube. The design characteristics and the operation performance of the microfocus x-ray tube are presented. Due to the small focal spot size, clear x-ray radiographic images of 6 μm bars and x-ray images with the magnification factor of higher than 230 were obtained.
    Applied Physics Letters 04/2007; 90(18):183109-183109-3. · 3.52 Impact Factor
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    ABSTRACT: Superhydrophobic bionic surfaces with hierarchical micro/nano structures were synthesized by decorating single-walled or multiwalled carbon nanotubes (CNTs) on monolayer polystyrene colloidal crystals using a wet chemical self-assembly technique and subsequent surface treatment with a low surface-energy material of fluoroalkylsilane. The bionic surfaces are based on the regularly ordered colloidal crystals, and thus the surfaces have a uniform superhydrophobic property on the whole surface. Moreover, the wettability of the bionic surface can be well controlled by changing the distribution density of CNTs or the size of polystyrene microspheres. The morphologies of the synthesized bionic surfaces bear much resemblance to natural lotus leaves, and the wettability exhibited remarkable superhydrophobicity with a water contact angle of about 165 degrees and a sliding angle of 5 degrees.
    Langmuir 03/2007; 23(4):2169-74. · 4.38 Impact Factor
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    ABSTRACT: A simple seed-layer assisted electrochemical deposition (ECD) route has been successfully developed for preparation of different ZnO nanostructures, and their optical and field emission properties are also studied. ZnO films, nanowires, and nanosheets could be prepared in a rational way by just controlling the ECD current density. The corresponding growth mechanisms are also discussed on the basis of the characteristics of the ZnO crystal structure and the influences of the seed-layer and ECD current density. Except for ZnO nanosheets, both the room-temperature and low-temperature photoluminescence measurements of the ZnO films and nanowire arrays show strong ultraviolet excitonic emission, which proves their good crystal quality. Detailed analysis of the field emission (FE) properties indicates that the hierarchical ZnO nanowire array shows good FE property due to their high aspect ratio, small radius curvature, and proper density.
    Journal of Physical Chemistry C - J PHYS CHEM C. 01/2007; 111(6).
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    ABSTRACT: Large-scale, well-aligned and oriented wurtzite ZnS nanobelt arrays are fabricated by a simple solvothermal reaction and subsequent heat treatment. The nanobelts grow along the [0001] direction perpendicularly on a zinc substrate, which are about 30 nm in thickness, several hundreds of nanometers in width, and up to 4 mum in length. Importantly, such nanostructured arrays show a good field emission property with low turn-on field (3.8 V mum-1) and high field enhancement factor (~1800), which is ascribed to the top edges and corners of the freestanding and well aligned nanobelts, and good electric contact with the conducting substrate where they grow.
    Applied Physics Letters 01/2006; 89. · 3.52 Impact Factor