• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.101-101
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• 2010

ZnO shows a direct band gap of 3.37eV, large exciton binding energy (~60 meV), high oxidation ability, high sensitivity to many gases, and low cost, and it has been used in various applications such as transparent electrodes, light emitting diodes (LEDs), gas sensors and photocatalysts. Among these applications ZnO as photocatalyst has considerably attracted attention over the past few years because of its high activities in removing organic contaminants generated from industrial activities. In this research, ZnO nanoparticles were synthesized by spray-pyrolysis method using the zinc acetate dihydrate as starting material at synthesis temperature of $900^{\circ}C$ with concentration varied from 0.01 to 1.0M. The physical and chemical properties of the synthesized ZnO nanoparticles were examined by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transformation Infrared (FT-IR), and UV-vis spectroscopy. The Miller indices of XRD patterns indicate that the synthesized ZnO nanoparticles showed a hexagonal wurtzite structure. With increased precursor concentration, a primary, secondary particle sizes of ZnO nanoparticles increased by 0.8 to $1.5{\mu}m$ and 15 to 35nm, and their crystallinity was improved. Methyleneblue (MB) solution ($1{\mu}M$) as a test comtaminant was prepared for evaluating the photocatalytic activities of ZnO nanoparticles synthesized in different precursor concentration. The results show that the photocatalytic efficiency of ZnO nanoparticles was gradually enhanced by increased precursor concentration.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.174-174
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• 2010

Gallium Nitride(GaN) attracts great attention due to their wide band gap energy (3.4eV), high thermal stability to the solid state lighting devices like LED, Laser diode, UV photo detector, spintronic devices, solar cells, sensors etc. Recently, researchers are interested in synthesis of polycrystalline and amorphous GaN which has also attracted towards optoelectronic device applications significantly. One of the alternatives to deposit GaN at low temperature is to use Single Source Molecular Percursor (SSP) which provides preformed Ga-N bonding. Moreover, our group succeeds in hybridization of SSP synthesized GaN with Single wall carbon nanotube which could be applicable in field emitting devices, hybrid LEDs and sensors. In this work, the GaN thin films were deposited on c-axis oriented sapphire substrate by MBE (Molecular Beam Epitaxy) using novel single source precursor of dimethyl gallium azido-tert-butylamine($Me_2Ga(N_3)NH_2C(CH_3)_3$) with additional source of ammonia. The surface morphology, structural and optical properties of GaN thin films were analyzed for the deposition in the temperature range of $600^{\circ}C$ to $750^{\circ}C$. Electrical properties of deposited thin films were carried out by four point probe technique and home made Hall effect measurement. The effect of ammonia on the crystallinity, microstructure and optical properties of as-deposited thin films are discussed briefly. The crystalline quality of GaN thin film was improved with substrate temperature as indicated by XRD rocking curve measurement. Photoluminescence measurement shows broad emission around 350nm-650nm which could be related to impurities or defects.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.281.2-281.2
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• 2013

Since heavy metal ions included in water or food resources have critical effects on human health, highly sensitive, rapid and selective analysis for heavy metal detection has been extensively explored by means of electrochemical, optical and colorimetric methods. For example, quantum dots (QDs), such as semiconductor QDs, have received enormous attention due to extraordinary optical properties including high fluorescence intensity and its narrow emission peaks, and have been utilized for heavy metal ion detection. However, the semiconductor QDs have a drawback of serious toxicity derived from cadmium, lead and other lethal elements, thereby limiting its application in the environmental screening system. On the other hand, Graphene oxide (GO) has proven its superlative properties of biocompatibility, unique photoluminescence (PL), good quenching efficiency and facile surface modification. Recently, the size of GO was controlled to a few nanometers, enhancing its optical properties to be applied for biological or chemical sensors. Interestingly, the presence of various oxygenous functional groups of GO contributes to opening the band gap of graphene, resulting in a unique PL emission pattern, and the control of the sp2 domain in the sp3 matrix of GO can tune the PL intensity as well as the PL emission wavelength. Herein, we reported a photoluminescent GO array on which heavy metal ion-specific DNA aptamers were immobilized, and sensitive and multiplex heavy metal ion detection was performed utilizing fluorescence resonance energy transfer (FRET) between the photoluminescent monolayered GO and the captured metal ion.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.245.2-245.2
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• 2013

Solution processed organic photovoltaic devices have relatively less attention compared to polymer photovoltaic devices even though they have high possibility to be developed because they have both advantages of polymer and organic, such as solution processable, no synthetic batch dependence of photovoltaic performance, high purity and high charge carrier mobility as well as relatively high efficiency (~7%). In addition, solution processed organic photovoltaic devices have an advantage of easiness to study the relationship between the molecular structure and photovoltaic performance due to its simple structure. In this work, five isoindigo based low band gap donor-acceptor-donor (D-A-D) small molecules with different electron donating strength were synthesized for investigating the relationship between the molecular structure and photovoltaic performance, especially, investigating the effects of different electron donating effect of donor group in isoindigo backbone to photovoltaic device performance. The variation of electron donating strength of donor group strongly affected the optical, thermal, electrochemical and photovoltaic device performances of isoindigo organic materials. The highest power conversion efficiency of ~3.2% was realized in bulk heterojuction photovoltaic device consisted of the ID3T as donor and PC70BM as acceptor. This work demonstrates the great potential of isoindigo moieties as electron deficient units as well as guideline for synthesis of donor-acceptor-donor (D-A-D) small molecules for realizing highly efficient solution processed organic photovoltaic devices.

• Korean Journal of Materials Research
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• v.25 no.3
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• pp.138-143
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• 2015

Aluminum nitride, a compound semiconductor, has a Wurtzite structure; good material properties such as high thermal conductivity, great electric conductivity, high dielectric breakdown strength, a wide energy band gap (6.2eV), a fast elastic wave speed; and excellent in thermal and chemical stability. Furthermore, the thermal expansion coefficient of the aluminum nitride is similar to those of Si and GaAs. Due to these characteristics, aluminum nitride can be applied to electric packaging components, dielectric materials, SAW (surface acoustic wave) devices, and photoelectric devices. In this study, we surveyed the crystallization and preferred orientation of AlN thin films with an X-ray diffractometer. To fabricate the AlN thin film, we used the magnetron sputtering method with $N_2$, NH3 and Ar. According to an increase in the partial pressures of $N_2$ and $NH_3$, Al was nitrified and deposited onto a substrate in a molecular form. When AlN was fabricated with $N_2$, it showed a c-axis orientation and tended toward a high orientation with an increase in the temperature. On the other hand, when AlN was fabricated with $NH_3$, it showed a-axis orientation. This result is coincident with the proposed mechanism. We fabricated AlN thin films with an a-axis orientation by controlling the sputtering electric power, $NH_3$ pressure, deposition speed, and substrate temperature. According to the proposed mechanism, we also fabricated AlN thin films which demonstrated high a-axis and c-axis orientations.

• Korean Journal of Materials Research
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• v.22 no.10
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• pp.504-507
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• 2012

Synthesis of RGO (reduced graphene oxide)-CdS composite material was performed through CBD (chemical bath deposition) method in which graphene oxide served as the support and Cadmium Sulfate Hydrate as the starting material. Graphene-based semiconductor photocatalysts have attracted extensive attention due to their usefulness for environmental and energy applications. The band gap (2.4 eV) of CdS corresponds well with the spectrum of sunlight because the crystalline phase, size, morphology, specic surface area and defects, etc., of CdS can affect its photocatalytic activity. The specific surface structure (morphology) of the photocatalyst can be effective for the suppression of recombination between photogenerated electrons and holes. Graphene (GN) has unique properties such as a high value of Young's modulus, large theoretical specific surface area, excellent thermal conductivity, high mobility of charge carriers, and good optical transmittance. These excellent properties make GN an ideal building block in nanocomposites. It can act as an excellent electron-acceptor/transport material. Therefore, the morphology, structural characterization and crystal structure were observed using various analytical tools, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. From this analysis, it is shown that CdS particles were well dispersed uniformly in the RGO sheet. Furthermore, the photocatalytic property of the resulting RGO-CdS composite is also discussed in relation to environmental applications such as the photocatalytic degradation of pollutants. It was found that the prepared RGO-CdS nanocomposites exhibited enhanced photocatalytic activity as compared with that of CdS nanoparticles. Therefore, better efficiency of photodegradation was found for water purification applications using RGO-CdS composite.

• Journal of the Korean institute of surface engineering
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• v.47 no.5
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• pp.239-243
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• 2014

The electrical and optical properties of amorphous In-Tin-Zinc-Oxide(ITZO) deposited at room temperature using rf-magnetron sputtering were investigated. The amorphous ITZO thin films were obtained at the composition of In:Sn:Zn = 6:2:2, 4:3:3, and 2:4:4, but the ITZO (8:1:1) showed a crystalline phase of bixbyite structure of In2O3. The resistivity of ITZO could be controlled by oxygen pressure in the sputtering ambient. The resistivity of post-annealed ITZO thin films exhibited the dependence on the amount of Indium. Optical energy band gap and transmittance increased as the amount of indium in ITZO increased. For the device application with ITZO, the bottom-gated thin-film transistor using ITZO as a active channel layer was fabricated. It showed a threshold voltage of 1.42V and an on/off ratio of $5.63{\times}10^7$ operated with saturation field-effect mobility of $14.2cm^2/V{\cdot}s$.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.233-233
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• 2010

CdS thin films were deposited on glass substrates by R.F. magnetron sputtering method and some of the samples were treated by rapid thermal annealing (RTA) process. Effects of thermal annealing on structural and optical properties were investigated at different temperatures ranging from 100 to $600^{\circ}C$. The crystallographic structure of the films and the size of the crystallites in the films were studied by X-ray diffraction. The crystallite sizes were found to increase, and the X-ray diffraction patterns were seen to sharpen by annealing. Optical properties of the films were calculated using the envelope method and the photoluminescence measurements. The optical properties of the films were seen to be dependent on the film thicknesses. The energy gap of the films was found to decrease by annealing. The band edge sharpness of the optical absorption was seen to oscillate by thermal annealing. Annealing over $400^{\circ}C$ was seen to degrade the optical properties of the film. The best annealing temperature for the films was found to be $400^{\circ}C$ from the optical properties. It is observed that the CdS film annealed at $400^{\circ}C$ reveals the strongest UV emission intensity and narrowest full width at half maximum among the temperature ranges studied. The enhanced UV emission from the film annealed at $400^{\circ}C$ is attributed to the improved crystalline quality of CdS thin film due to the effective relaxation of residual compressive stress and achieving maximum grain size. The results show that heat treatments under optimal annealing condition can provide significant improvements in the properties of CdS thin films.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.232-232
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• 2010

Recently light emitting diodes (LEDs) have been expected as the new generation light sources because of their advantages such as small size, long lifetime and energy-saving. GaN, as a wide band gap material, is widely used as a material of LEDs and GaN nanorods are the one of the most widely investigated nanostructure which has advantages for the light extraction of LEDs and increasing the active area by making the cylindrical core-shell structure. Lately GaN nanorods are fabricated by various techniques, such as selective area growth, vapor-liquid-solid (VLS) technique. But these techniques have some disadvantages. Selective area growth technique is too complicated and expensive to grow the rods. And in the case of VLS technique, GaN nanorods are not vertically aligned well and the metal catalyst may act as the impurity. So we just tried to grow the GaN nanorods on Si substrate without catalyst to get the vertically well aligned nanorods without impurity. First we deposited the AlN buffer layer on Si substrate which shows more vertical growth mode than sapphire substrate. After the buffer growth, we flew trimethylgallium (TMGa) as the III group source and ammonia as the V group source. And during the GaN growth, we kept the ammonia flow stable and periodically changed the flow rate of TMGa to change the growth mode of the nanorods. Finally, as the optimization, we changed the various growth conditions such as the growth temperature, the working pressure, V/III ratio and the doping level. And we are still in the process to reduce the diameter of the nanorods and to extend the length of the nanorods simultaneously. In this study, we focused on the shape changing of GaN nanorods with different growth conditions. So we confirmed the shape of the nanorods by scanning electron microscope (SEM) and carried out the Photoluminescence (PL) measurement and x-ray diffraction (XRD) to examine the crystal quality difference between samples. Detailed results will be discussed.

• Journal of Adhesion and Interface
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• v.13 no.4
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• pp.188-192
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• 2012

Herein, CdS-coated PMMA nanoparticles were prepared by in-situ surfactant-free emulsion copolymerization and subsequent CdS coating process. As-prepared CdS/PMMA hybrid particles had 201.7 nm in diameter. The amount of CdS nanocrystals in the hybrid particles was 10.37 wt% determined by TGA and elemental analysis. The size of CdS crystals was 3.55 nm preferentially grown in (111) plane. UV-vis spectrum of PMMA/CdS nanoparticles showed the significant blue-shift in optical illumination. The reason was found because the synthesized CdS nanocrystals on PMMA particles had a different band gap energy of 2.70 eV which was significantly higher than that of known-value of bulk CdS (2.41 eV) due to a quantum confinement effect.