Hyung Ju Park

Electric property evolution of structurally defected multilayer graphene.

We report on the influence of structural disorder on the electrical properties of multilayer graphene (MLG). Exponential decreases in the conductance and transconductance with increase of defects in the MLG were observed, which could be explained by the percolation and the variable range hopping conduction. An enhancement of p-type nature with increasing disorders was considered to be the result of oxygen doping in the graphene sheets introduced by oxygen plasma. The rapid increase of low-frequency noise was attributed to the formation of conductive network through the continuum percolation, as the low-frequency noise could be increased by the enhanced carrier scattering at the defect sites. We hope that our result should suggest a simple method of tuning the electrical properties of graphene.

Temperature-induced control of aspect ratio of gold nanorods

Aspect ratio of gold nanorods can be controlled by simply adjusting the reaction temperature in the seed-mediated synthesis of the nanorods. The gold nanorods were synthesized by the injection of gold nanoparticle seeds of around 4nm in diameter into a reaction mixture containing hydrogen tetrachloroaurate, hexadecyltrimethylammonium bromide, and ascorbic acid. Average aspect ratio of the resulting nanorod increases from 1 to about 40 with decreasing the reaction temperature from 315to276K, which can be attributed to the temperature-induced change in the shape of the micellar templates. For further understanding of the growth mechanism, silver nanoparticles were also used as seeds in the preparation of the gold nanorod.

Controlled synthesis and characterization of the enhanced local field of octahedral Au nanocrystals

Octahedral Au nanocrystals with localized surface plasmon-assisted enhancing optical properties can be prepared in aqueous solution via the forced reduction of Au ions by ascorbic acid through the addition of NaOH.

Fabrication of integrated nanogap electrodes by surface-catalyzed chemical deposition

Integrated nanogap electrodes with separations of several nanometers were fabricated by a simple and highly reproducible method of surface-catalyzed chemical deposition. By this method, multifingered nanogap electrodes of a few nanometers in separation were fabricated with a good yield (over 90%). The fabrication was achieved by immersing the initial gap electrodes obtained by conventional e-beam lithography into a stock solution containing Au ions and a mild reducing agent. After the surface-catalyzed chemical deposition, a rather wide initial gap distance of 18–52nm was decreased to a few nanometers, showing a much narrower distribution (centered at 3.3nm).

Fabrication of versatile nanocomponents using single-crystalline Au nanoplates

We suggest an approach to the fabrication of versatile nanocomponents designed deliberately by selective Ga+ focused-ion-beam etching or Ar+ ion milling of single-crystalline Au nanoplates synthesized by the chemical reaction. The nanocomponents have various shapes like gear, wheel, dumbbell, square and letter “A” with in-plane size of about 400nm and thickness of 40–50nm. They can be picked up or moved freely one by one to be assembled into sophisticated nanodevices or micromachines. The applicability of our approach both to the fundamental research and to the applied research is discussed.

Strain distribution and interface modulation of highly lattice-mismatched InN/GaN heterostructure nanowires

The structural properties of InN/GaN heterostructure nanowires (NWs) were studied using transmission electron microscope techniques to determine strain behavior. A great quantity of the misfit strain between InN and GaN was relaxed through the introduction of misfit dislocations along the interface. Geometric phase analysis revealed a strain-concentration phenomenon in the strain map of the out-of-plane components and a gradual lattice recovery in that of the in-plane components over the InN/GaN interface. Interface structures that were modulated at the atomic-scale were observed in several InN/GaN heterostructure NWs. Complex strain distributions were identified in both InN and GaN.

Plasmon resonance changes of gold nanoparticle arrays upon modification

Periodic Au nanoparticle arrays, fabricated using electron beam lithography, have been modified by chemical reaction in solutions having various concentrations of a reducing agent. As the nanoparticles enlarge due to the formation of additional Au nanolumps on the surface, both the position and intensity of plasmon absorbance of Au nanoparticle arrays change in proportion to the concentration of the reducing agent. Moreover, the plasmon absorbance is split into dipole and quadrupole modes as conductive connections form between the particles. Our results demonstrate that the changes in both the position and intensity of plasmon absorbance can be employed together as complementary readout values of nanosensors.

High voltage-derived enhancement of electric conduction in nanogap devices for detection of prostate-specific antigen

We report a simple method of enhancing electric conductance in nanogap devices without any additional treatments, such as silver-enhancing process. The low electric conductance after selective immobilization of biofunctionalized gold nanoparticles in the gap region was greatly enhanced by repeated I-V scans at relatively high voltage ranges of −5 to 5 V, which was attributed to the formation of a new conduction pathway across the gap. The higher conduction state of the nanogap device showed a very stable I-V curve, which was used as an excellent measure of the existence of prostate-specific antigen.

Synthesis of Hollow Iron Oxide Nanospheres and Their Application to Gas Sensors

Hollow nanomaterials have attracted great interest because of their many applications in catalysis, nanoreactors, drug delivery systems, for lubrication and in gas sensors. Here, carbon sphere templates were prepared from glucose under hydrothermal conditions to facilitate the synthesis of hollow Fe2O3 nanospheres. Thermal decomposition of an iron precursor in benzylalcohol with the carbon spheres resulted in the deposition of Fe3O4 nanoparticles on the carbon sphere templates. The nanoparticles on the carbon surface naturally agglomerate and form a dense oxide shell during the calcination step, which produces typical Fe2O3 hollow structures. The gas sensing performance of the hollow Fe2O3 nanospheres was investigated at an operating temperature of 300 °C. The hollow Fe2O3 nanospheres showed high sensitivity (R = 10.766 at 1 ppm formaldehyde) with a linear response to formaldehyde gas concentration in the range of 0.8~2.4 ppm, and good selectivity to formaldehyde gas in volatile organic compounds, compared to commercial Fe2O3 nanoparticles.