Jinsub Choi

Highly ordered monocrystalline silver nanowire arrays

Highly ordered silver nanowire arrays have been obtained by pulsed electrodeposition in self-ordered porous alumina templates. Homogeneous filling of all the pores of the alumina template is achieved. The interwire distance is about 110 nm corresponding to a density of silver nanowires of 61×109 in.−2 and the diameter can be varied between 30 and 70 nm. The silver wires are monocrystalline with some twin lamella defects and grow perpendicular to the 〈110〉 direction. The previously encountered difficulty to obtain 100% filling of the alumina pores is discussed in the framework of electrostatic instabilities taking into account the different potential contributions during electrodeposition. To obtain homogeneously filled pore membranes, a highly conductive metal containing electrolyte, a homogeneous aluminum oxide barrier layer, and pulsed electrodeposition are a prerequisite.

Perfect two-dimensional porous alumina photonic crystals with duplex oxide layers

A perfect two-dimensional porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of 4 cm2 via imprint methods and subsequent electrochemical anodization. By comparing measured reflectivity with theory, the refractive indices in the oxide layers were determined. The results indicate that the porous alumina structure is composed of a duplex oxide layer: an inner oxide layer consisting of pure alumina oxide of 50 nm in thickness, and an outer oxide layer of a nonuniform refractive index. We suggest that the nonuniform refractive index of the outer oxide arises from an inhomogeneous distribution of anion species concentrated in the intermediate part of the outer oxide.

Fabrication of monodomain alumina pore arrays with an interpore distance smaller than the lattice constant of the imprint stamp

Large-area monodomain porous alumina arrays with an interpore distance of 500 nm are fabricated by imprint lithography. A 4 in. imprint master fully compatible with silicon technology was developed, which allows imprint pressures as low as 5 kN/cm2 for direct imprint on aluminum. Due to the self-ordering phenomenon of porous alumina growth, we were able to reduce the interpore distance of the pore array to 60% of the lattice constant of the master stamp. Three lithographically defined pores are sufficient to guide anodization of a new pore in the center.

Preparation and Characterization of Chemically Functionalized Silica-Coated Magnetic Nanoparticles as a DNA Separator

The work describes a simple and convenient process for highly efficient and direct DNA separation with functionalized silica-coated magnetic nanoparticles. Iron oxide magnetic nanoparticles and silica-coated magnetic nanoparticles were prepared uniformly, and the silica coating thickness could be easily controlled in a range from 10 to 50 nm by changing the concentration of silica precursor (TEOS) including controlled magnetic strength and particle size. A change in the surface modification on the nanoparticles was introduced by aminosilanization to enhance the selective DNA separation resulting from electrostatic interaction. The efficiency of the DNA separation was explored via the function of the amino-group numbers, particle size, the amount of the nanoparticles used, and the concentration of NaCl salt. The DNA adsorption yields were high in terms of the amount of triamino-functionalized nanoparticles used, and the average particle size was 25 nm. The adsorption efficiency of aminofunctionalized nanoparticles was the 4-5 times (80-100%) higher compared to silica-coated nanoparticles only (10-20%). DNA desorption efficiency showed an optimum level of over 0.7 M of the NaCl concentration. To elucidate the agglomeration of nanoparticles after electrostatic DNA binding, the Guinier plots were calculated from small-angle X-ray diffractions in a comparison of the results of energy diffraction TEM and confocal laser scanning microscopy. Additionally, the direct separation of human genomic DNA was achieved from human saliva and whole blood with high efficiency.

In-situ synthesis of reactive hydroxyapatite nano-crystals for a novel approach of surface grafting polymerization

A novel approach to surface modification of hydroxyapatite (HAp) nano-crystals was described based on in-situ synthesis of surface thiol-functionalized HAp (HAp-SH) and subsequent grafting polymerization of ethylene glycol methacrylate phosphate (EGMP). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses showed that thiol groups were introduced on HAp surfaces by adding 3-mercaptopropionic acid during hydrothermal synthesis of HAp nano-crystals. The radical chain transfer to surface thiol groups generated the sulfur-centered radicals on HAp nano-surfaces, which initiated the surface grafting polymerization of EGMP. Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD) analyses confirmed the grafting reaction on HAp surfaces. Zeta potentials of control HAp, thiol-functionalized HAp (HAp-SH), and PolyEGMP-grafted HAp in phosphate buffered saline (PBS) solutions (pH 7.4) were negative and decreased with increasing the amount of grafted PolyEGMP. TEM measurements and time-dependent phase monitoring suggested that the colloidal stability of PolyEGMP-grafted HAp over synthesized HAp nano-crystals in water dramatically increased without inter-crystal aggregation.

Mesoscopic ferroelectric cell arrays prepared by imprint lithography

Arrays of mesoscopic ferroelectric (Pb,Zr)TiO3 cells with lateral sizes from several micrometers down to below 300 nm were prepared applying nanoimprint lithography. The ferroelectric properties of the mesoscopic cells were investigated by scanning force microscopy in piezoresponse mode. The best chemical route to obtain ferroelectric cells was found to be the sol-gel method. Using Nb-doped SrTiO3 single crystals as bottom electrodes, the crystallization into the ferroelectric phase was uniform with grain sizes in the 35 nm range.

Monodisperse metal nanowire arrays on Si by integration of template synthesis with silicon technology

Thin films of monodomain porous alumina with 2 µm thickness and adjustable monodisperse pore diameter are obtained via anodization of nanoimprinted Al evaporated on a Si wafer. By using these templates, hexagonally arranged monodisperse silver nanowires on Si are prepared by electrochemical deposition with a commercially available bath. In addition, free standing Ag nanowires on Si are prepared by selective dissolution of the alumina matrix.

In situ surface-enhanced Raman spectroscopy of monodisperse silver nanowire arrays

Highly ordered two-dimensional arrays of monodisperse coinage metal nanowires embedded in an alumina matrix have been prepared. When light is propagating in the direction of the long axis of the nanowire, plasmon-enhanced absorption and light guidance of the nanowire were observed by optical microspectroscopy and scanning near-field optical spectroscopy and compared to Mie scattering theory. By selectively dissolving the matrix at a constant etching rate, we detected in situ and ex situ the surface-enhanced Raman scattering (SERS) of organic dyes. In contrast to earlier publications, we find that the SERS signal is linearly proportional to the free-surface area of the nanowires that are in contact with the dye. We cannot detect any change in the enhancement factor due to the releasing of the nanowires from the host structure.

Designing a highly bioactive 3D bone-regenerative scaffold by surface immobilization of nano-hydroxyapatite

A novel approach to the fabrication of porous scaffolds with surface-immobilized nano-hydroxyapatite (N-HAp) is developed for effective bone tissue engineering. The discrete nano-level anchoring of N-HAp on the pore surface of chitosan scaffolds is achieved using surface-repellent stable colloidal N-HAp with surface phosphate functionality. Field-emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) confirm pronounced exposure of N-HAp on the surfaces of chitosan scaffolds at the nano-level, which can not be accomplished with the conventional polymer/N-HAp composite scaffolds. This rational surface engineering enables surface-anchored N-HAp to express its overall intrinsic bioactivity, since N-HAp is not phase-mixed with the polymers. The porous chitosan scaffolds with surface-immobilized N-HAp provide more favorable environments than conventional bulk phase-mixed chitosan/N-HAp scaffolds in terms of cellular interaction and growth. In vitro biological evaluation using alkaline phosphatase activity assay supports that immobilized N-HAp on pore surfaces of chitosan scaffolds contributed to the more enhanced in vitro osteogenic potential. In addition, scaffolds with surface-exposed N-HAp provide favorable environments for enhanced in vivo bone tissue growth, estimated by characteristic biomarkers of bone formation such as collagen. The results suggest that the newly developed hybrid scaffolds with surface-immobilized N-HAp may serve as useful 3D substrates with pore surfaces featuring excellent bone tissue-regenerative properties.

Nickel oxalate nanostructures for supercapacitors

Herein, we describe a facile method to produce nickel oxalate nanostructures by chemical reaction of oxalic acid and a nickel foil in various organic solvents and water. Grass-like structures consisting of nickel oxalate are produced by the chemical reaction within 30 min for all solvents. Interestingly, nickel oxalate nanowires can be produced by the addition of a small amount of water in certain solvents. Annealing of nickel oxalate structures leads to formation of nickel oxide structures with a slight morphological change. Compositions of the nanostructures are investigated by TEM and FT-IR analyses. In addition, the supercapacitance of the nickel oxalate nanostructures is characterized, and the results show that they are superior to that of nickel oxide nanostructures.