Yong Kwan Kim

Photoconductance of aligned SnO2 nanowire field effect transistors

We report on the optoelectronic properties of the aligned SnO2 nanowire (NW) field effect transistors (FETs) fabricated via a sliding transfer of NWs grown by chemical vapor deposition. Photocurrent measurements with polarized UV light confirmed a well aligned NWs along the channels. UV photosensitivity of ∼107 at the gate voltage Vg=−40u2002V was obtained due to a small dark-current at the turn-off state of FET. The dynamic response of the photocurrent became faster for the higher mobility SnO2 NW FETs. We expect our aligned SnO2 NW FETs will be useful as polarized UV detectors with a high sensitivity.

Control of adsorption and alignment of V2O5 nanowires via chemically functionalized patterns

We developed a method to precisely control the locations and orientations of deposited divanadium pentoxide (V2O5) nanowires on SiO2 surfaces using chemically functionalized patterns. The nanowires were deposited onto the substrates either from solution or via the edge transfer mechanism of micro-contact printing. In both cases, negatively charged V2O5 nanowires showed a strong adsorption selectivity onto 3-aminopropyltriethoxysilane (APS) self-assembled monolayers (SAMs) with positively charged functional groups, whereas the SAMs with non-polar and neutral terminal groups of octadecyltrichlorosilane (OTS) worked as perfect passivation layers. In particular, directional alignment of nanowires inside the chemically patterned area, depending upon the shapes of the patterns, was observed. The wires were aligned along the long axis of the pattern when the width of the APS pattern was as small as 1xa0µm. This strategy allows us to control the adsorption and alignment of V2O5 nanowires on the substrates, which could be used for various nano-device applications such as interconnection of electronic circuits.

Fabrication of Nanowire Channels with Unidirectional Alignment and Controlled Length by a Simple, Gas‐Blowing‐Assisted, Selective‐Transfer‐Printing Technique

A printing-based lithographic technique for the patterning of V(2)O(5) nanowire channels with unidirectional orientation and controlled length is introduced. The simple, directional blowing of a patterned polymer stamp with N(2) gas, inked with randomly distributed V(2)O(5) nanowires, induces alignment of the nanowires perpendicular to the long axis of the line patterns. Subsequent stamping on the amine-terminated surface results in the selective transfer of the aligned nanowires with a controlled length corresponding to the width of the relief region of the polymer stamp. By employing such a gas-blowing-assisted, selective-transfer-printing technique, two kinds of device structures consisting of nanowire channels and two metal electrodes with top contact, whereby the nanowires were aligned either parallel (parallel device) or perpendicular (serial device) to the current flow in the conduction channel, are fabricated. The electrical properties demonstrate a noticeable difference between the two devices, with a large hysteresis in the parallel device but none in the serial device. Systematic analysis of the hysteresis and the electrical stability account for the observed hysteresis in terms of the proton diffusion in the water layer of the V(2)O(5) nanowires, induced by the application of an external bias voltage higher than a certain threshold voltage.

Controlled direct patterning of V2O5 nanowires onto SiO2 substrates by a microcontact printing technique

Vanadium pentoxide (V2O5) nanowires were directly transferred to desired patterns on SiO2 substrates using the microcontact printing (MCP) technique. The hydrophilicity of the poly(dimethylsiloxane) (PDMS) stamp exerted a strong influence on the mechanism of transfer of polar V2O5 nanowires onto the substrate. The V2O5 nanowires were transferred from the relief side of the hydrophilic stamp, whereas they were transferred from the recess edges of the hydrophobic one forming agglomerated nanowire patterns on the substrate. When the hydrophobic stamp was used, the width of the agglomerated nanowire patterns could be controlled by the concentration of the nanowire solution as well as by the width of the recess area of the PDMS stamp. This method allows us to generate nanowire patterns with a submicrometre line width, which is much smaller than a the few-micrometre sizes of PDMS stamp patterns. When the hydrophilic stamp with a small-sized (≤ the average length of V2O5 nanowires) pattern was used, alignment of individual nanowires in the direction of the boundary of the line pattern was obtained. These results suggest that the transfer mechanism in the MCP process strongly depends on the wetting interaction between the stamp and the nanowire ink.

Facile Transfer of Thickness Controllable Poly(methyl methacrylate) Patterns on a Nanometer Scale onto SiO2 Substrates via Microcontact Printing Combined with Simplified Langmuir-Schaefer Technique

We report on the facile patterning of poly(methyl methacrylate) (PMMA) layers onto SiO2 substrates via microcontact printing combined with the simplified Langmuir-Schaefer (LS) technique. Langmuir film of PMMA was formed just by dropping a dilute PMMA solution onto the air/water surface in a glass Petri dish via self-assembly, and it was used as an ink for the patterned poly(dimethylsilioxane) (PDMS) stamp. The transferred film properties were systematically investigated with variation of postannealing temperature, molecular weight of PMMA, and the inking number. The patterned PMMA film surface was smooth with no vacancy defect in a few micrometers scale AFM images over the whole film area after post-annealing process. The thickness of the PMMA patterns was controlled on the nanometer scale by the number of inkings of the LS layer of PMMA on the PDMS stamp. By using the PMMA patterns as a barrier and a sacrificial layer against the chemical etching and metal deposition, SiO2 and metal patterns were fabricated, respectively. The PMMA layers also worked as a passivation layer against the patterning of V2O5 nanowires and the selective adsorption of single-walled carbon nanotubes (SWCNTs). We also fabricated thin film transistors using patterned SWCNTs with different percolation states and investigated the electrical properties.

Structural Studies of V2O5 Nanowires by Ultrahigh Vacuum-Scanning Tunneling Microscope and Atomic Force Microscope

We have investigated the structures and electronic properties of vanadium pentoxide (V2O5) nanowires synthesized by a sol–gel process. The time-dependent evolution of the V2O5 nanowires at different temperatures was systematically studied by atomic force microscopy. The structural dimension and the current–voltage (I–V) characteristics were measured by scanning tunneling microscopy/spectroscopy. V2O5 nanowires with a cross section of 10×1.5 nm2, whose length varied with the duration time in sol, were synthesized. The V2O5 nanowires adsorbed on a self-assembled monolayer of aminothiophenol (ATP) on a Au(111)/mica substrate showed semiconducting I–V characteristics. The height of the V2O5 nanowires decreased from 1.5 to 0.8 nm with prolonged annealing at temperatures above 100 °C, implying the existence of a water interlayer in the V2O5 double-layer structure.

Angle-resolved ultraviolet photoelectron spectroscopy study on the α-Si(1 1 1)3×3-Bi surface

Abstract We have investigated the electronic structure of the Si(1xa01xa01) 3 × 3 -Bi surface with 1/3 ML coverage (the α- 3 -Bi surface) using angle-resolved photoelectron spectroscopy and low-energy electron diffraction. Three occupied surface localized states (SL1, SL2, and SL3) and some other surface related states (DS and SR) were observed. The SL1 band of three prominent surface localized state bands is found to be located in the bulk band gap over entire k ∥ with band width of 0.7 eV and does not cross the Fermi level. This band behavior is in accord with the S1 band of the 3 -Sn surface. It is found that the α- 3 -Bi surface is semiconducting and has the band gap of 1.1 eV. The SL2 and SL3 bands appear only in the restricted k ∥ region and the most parts are superposed on the bulk band. The energy separation of the SL2 and SL3 bands is observed to be very small, near the M ′ point unlike the 3 -III and IV surfaces.

Nanofabrication via direct transfer of BOE treated PDMS stamp patterns onto SiO2 surfaces

Various PDMS patterns with a few microns to sub-micron size and thickness of 20~30 nanometers were successfully transferred on the substrate via simple printing process of a buffered oxide etchant-treated PDMS stamp on the SiO2 substrate. The patterned PDMS layer acted as sacrificial layer for metal-film patterning and as chemical passivation layer for the selective adsorption of V2O5 nanowires. In particular, the electrical measurement of the patterned V2O5 nanowire network showed the semiconducting non-ohmic behavior in the channel.