Hea-Yeon Lee

Formation of Co nanoclusters in epitaxial Ti0.96Co0.04O2 thin films and their ferromagnetism

Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films.

Control of electrical conduction in DNA using oxygen hole doping

Using oxygen adsorption experiments on poly (dG)-poly (dC) DNA molecules, we found that their conductance can be easily controlled by several orders of magnitudes using oxygen hole doping, which is a characteristic behavior of a p-type semiconductor. It also suggests that the conductance of the DNA under doping results from charge carrier transport, not from an ionic conduction. On the other hand, we will also show that the poly (dA)-poly (dT) DNA molecules behave as an n-type semiconductor. This letter demonstrates that the concentration and the type of carriers in the DNA molecules could be controlled using proper doping methods.

Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will res...

Influence of Humidity on the Electrical Conductivity of Synthesized DNA Film on Nanogap Electrode

We have studied the electrical conductivity of DNA film using nanogap electrodes. Current–voltage measurements and alternating current measurements were performed for analysis of conductivity. The electrical conductivity of the DNA films of poly(dG)poly(dC) are found to depend strongly on the humidity. The resistance of poly(dG)poly(dC) decreases dramatically with increasing relative humidity. The contact resistance between DNA film and Au electrodes is also examined by the conventional four-probe technique.

Conductance measurement of a DNA network in nanoscale by point contact current imaging atomic force microscopy

We measured the electrical conductivity of a DNA network as a spatially-resolved current-image using point-contact current imaging atomic force microscopy (PCI-AFM) under various humidity conditions. The simultaneous observation of topography and current image by PCI-AFM can provide information pertaining to the electrical properties of biological and∕or soft materials in the nano-scale range. Under dry condition (0% humidity), no difference was observed for the electrical current both of the DNA network and mica surface, whereas the electrical current along the DNA network was larger than that of the mica surface by 20pA at a bias voltage of 5V under high humidity conditions of 60%.

Well-oriented nanowell array metrics for integrated digital nanobiosensors

Nanofabrication technologies should be useful for developing highly sensitive, reproducible nanobiosensors. This letter presents a nanometric system that is composed of well-oriented nanowells. The geometry permits only one or a few biomolecules to enter and become attached to nanosized gold dots. This nanoarray is easily fabricated using current nanolithography technology. When the authors applied this array to highly sensitive electrochemical DNA detection, they obtained a two-orders-of-magnitude enhancement in sensitivity. This nanometric system could be applied to numerous other integrated digital biosensors.

Ferroelectric behavior of epitaxial Bi2VO5.5 thin films on Si(100) formed by pulsed-laser deposition

Thin films of Bi2VO5.5 have been prepared epitaxially using a pulsed-laser deposition method on a Si(100) substrate using TiN as a buffer layer and SrTiO3 as a seed layer. The films have smooth surface morphology with atomically flat terraces and steps of 4 A in height. The ferroelectric characterization shows a spontaneous polarization of 2.2 μC/cm2 and a coercive field (Ec) of 22 kV/cm. The leakage current obtained is about 5×10−6 A/cm2 at a drive voltage of ±2 V.

Highly dense protein layers confirmed by atomic force microscopy and quartz crystal microbalance.

Protein adsorption on a gold surface is investigated by comparing the results of quartz crystal microbalance method and atomic force microscopy. The adsorption of streptavidin on functional gold surfaces is directly monitored by a quartz crystal microbalance, and confirmed by atomic force microscopy. For this investigation, a modified gold substrate is fabricated to obtain a topographic image of streptavidin molecules. Both methods show a correlation in terms of the highly dense protein single-layer formation, and the modified gold electrode shows a slightly denser protein layer formation because of the difference in substrate geometry as compared with that of a mica surface.

Electrochemical detection of nonlabeled oligonucleotide DNA using biotin-modified DNA(ss) on a streptavidin-modified gold electrode

In the electrochemical detection of nonlabeled DNA, it is important to control the bonding at the interface between the DNA and the electrode. Atomic force microscope (AFM) was taken for the commonly used thiol-modified DNA on a gold surface. It was found that the coverage of the DNA was very low. On the other hand, a streptavidin-modified gold electrode provided a much better alternative where DNA hybridization resulted in large changes in the electrochemical reaction responses. This work demonstrates that streptavidin-modified gold electrodes could be used in the development of a new electrochemical protocol for the detection of nonlabeled DNA.

Electrical Properties of Poly(dA)·Poly(dT) and Poly(dG)·Poly(dC) DNA Doped with Iodine Molecules

Electrical resistivity of Poly(dA)·Poly(dT) and Poly(dG)·Poly(dC) DNA were measured in a vacuum on nano-gap electrodes separated by 30–50 nm. The DNAs showed very low conductivity of less than 0.1 pA at 1 V. On the other hand, I2-doped Poly(dA)·Poly(dT) and Poly(dG)·Poly(dC) DNA form conducting DNAs, the current of which increased with an increase in the doping time. Doped Poly(dG)·Poly(dC) DNA shows higher current than doped Poly(dA)·Poly(dT) DNA. Time dependence of resistivity of Poly(dG)·Poly(dC) DNA doped with iodine molecules exhibited that the total current composes of both hole current and ion current.