Su-Hwan Lee

Fabrication and electrical characteristics of carbon nanotube-based microcathodes for use in a parallel electron-beam lithography system

This article presents an overview of the “Nanolith” parallel electron-beam (e-beam) lithography approach. The e-beam writing head consists of an array of microguns independently driven by an active matrix complementary metal–oxide–semiconductor circuit. At the heart of each microgun is a field-emission microcathode comprised of an extraction gate and vertical carbon nanotube emitter, whose mutual alignment is critical in order to achieve highly focused electron beams. Thus, in this work, a single-mask, self-aligned technique is developed to pattern the extraction gate, insulator, and nanotubes in the microcathode. The microcathode examined here (150×150 gates, 2 μm gate diameter, with multiple nanotubes per gate) exhibited a peak current of 10.5 μA at 48 V when operated with a duty cycle of 0.5%. The self-aligned process was extended to demonstrate the fabrication of single nanotube-based microcathodes with submicron gates.

Fabrication of carbon nanotube lateral field emitters

We report on the fabrication and field emission of carbon nanotube lateral field emitters. Due to its high aspect ratio and mechanical strength, we use vertically aligned multi-wall carbon nanotubes prepared by plasma-enhanced chemical vapour deposition as cathodes, which makes the fabrication of cantilever type lateral field emitters possible. The emission characteristics show that the field emission initiates at 11–17 V. The device has high geometrical enhancement factors (9.3 × 106 cm−1) compared to standard Spindt tips, which may be due to increased field concentration at the nanotube tip and the close proximity of the anode (<1μm). The relative ease of fabrication compared to vertical field emitters and enhanced field emission characteristics observed makes the carbon nanotube lateral field emitter a good candidate for future integrated nano-electronic devices.

Characteristics of multiwalled carbon nanotube nanobridges fabricated by poly(methylmethacrylate) suspended dispersion

We report on the transport characteristics of individual multiwalled carbon nanotube/nanofibers (MWCNTs) grown by plasma-enhanced chemical vapor deposition (PECVD). The measurements were performed on individual MWCNT nanobridges suspended by sputtered Nb contacts. Temperature dependent measurements of conductance revealed that the conductance is dominated by a contribution from thermally activated carriers. High-field measurements show that the PECVD grown MWCNTs are able to carry high current densities (∼108 A/cm2) and after reaching a critical limit, break down in segments of nanotube shells while still being electrically stable. The high-density current transport and reliability make PECVD grown MWCNTs good candidates for applications as field emission cathodes and nanoelectronic interconnects.

Fabrication of multiwalled carbon nanotube bridges by poly-methylmethacrylate suspended dispersion

We report on the fabrication of multiwalled carbon nanotube (MWCNT) bridges using poly-methylmethacrylate (PMMA) suspended dispersion. This method makes it possible to suspend nanotubes between metal electrodes, without any chemical etching of the substrate, and to remove unwanted nanotubes from the substrate. Using a spacer layer of PMMA with a known thickness, it is also possible to control the suspended height of the MWCNT bridges. The electrical measurement results on suspended MWCNT bridges reveals that the room temperature resistance ranges from under a kΩ to a few MΩ, with the majority around 2–4 kΩ. It was shown that a plasma-enhanced chemical vapor deposition grown MWCNT with a diameter ∼55 nm can sustain current densities of ∼108 A/cm2, which will make them suitable for applications as integrated field emission cathodes.

Fabrication of self-aligned side gates to carbon nanotubes

We have fabricated self-aligned, side-gated suspended multi-walled carbon nanotubes (MWCNTs), with nanotube-to-gate spacing of less than 10 nm. Evaporated metal forms an island on a suspended MWCNT, the island and the nanotube act as a mask shielding the substrate, and lift-off then removes the metal island, leaving a set of self-aligned side gates. Al, Cr, Au, and Ti were investigated and the best results were obtained with Cr, at a yield of over 90%.

The mechanism of antibacterial activity of tetrandrine against Staphylococcus aureus.

Tetrandrine (TET) is a bis-benzylisoquinoline alkaloid derived from the radix of Stephania tetrandra S. Moore. TET performs a wide spectrum of biological activities. The radix of S. tetrandrae has been used traditionally in Asia, including Korea, to treat congestive circulatory disorders and inflammatory diseases. The aim of this study was to examine the mechanism of antibacterial activity of tetrandrine against Staphylococcus aureus. The mechanism was investigated by studying the effects of TET in combination with detergent or membrane potential un-couplers. In addition, the direct involvement of peptidoglycan (PGN) was assessed in titration assays. TET activity against S. aureus was 125-250 μg/mL, and the minimum inhibitory concentration (MIC) of the two reference strains was 250 μg/mL. The OD(600) of each suspension treated with a combination of ethylenediaminetetraacetic acid (EDTA), tris(hydroxymethyl) aminomethane (TRIS), and Triton X-100 (TX) with TET (0.25×MIC) had been reduced from 43% to 96%. Additional structure-function studies on the antibacterial activity of TET in combination with other agents may lead to the discovery of more effective antibacterial agents.

The effect of donor layer thickness on the power conversion efficiency of organic photovoltaic devices fabricated with a double small-molecular layer

In organic photovoltaic (OPV) devices fabricated with a double small-molecular layer, the power conversion efficiency strongly depends on the thickness of the organic donor layer (here, copper phthalocyanine). In other words, the power conversion efficiency increases with the donor layer thickness up to a specific thickness ( approximately 12.7 nm) and then decreases beyond that thickness. This trend is associated with the light absorption and carrier transport resistance of the small-molecular donor layer, both of which strongly depend on the layer thickness. Experimental and calculated results showed that the short-circuit current due to light absorption increased with the donor layer thickness, while that due to current through the donor layer decreased with 1/R. Since the total short-circuit current is the product of the light absorption current and current through the donor layer, there is a trade-off, and the maximum power conversion efficiency occurs at a specific organic donor layer thickness (e.g. approximately 12.7 nm in this experiment).

Characteristics of a free-standing superconducting nanobridge with an integrated heater fabricated using a self-aligned technique

We report on a novel method of controlling the supercurrent flow in a superconducting weaklink. We have fabricated a free-standing superconducting nanobridge with an integrated NiCr heater. This NiCr layer acts as a self-aligned mask for reactive ion etching and also as the normal metal heating element of the nanobridge. The heater is used to control the critical current of the superconducting weaklink part of the nanobridge; the free-standing nature of the nanobridge reduces the heater power required. We discuss the dependence of the current–voltage characteristics of the superconducting weaklink on heater current. This device has advantages over existing controllable weaklink devices.

The fabrication and characterization of organic light-emitting diodes using transparent single-crystal Si membranes

For applications such as solar cells and displays, transparent single-crystal Si membranes were fabricated on a silicon-on-insulator (SOI) wafer. The SOI wafer included a buried layer of SiO2 and Si3N4 as an etch-stop layer. The etch-stop layer enabled fabrication of transparent single-crystal Si membranes with various thicknesses, and the thinning technology is described. For membranes with thicknesses of 18, 72 and 5000 nm, the respective optical transparent were 96.9%, 93.7% and 9% for R (red, lambda = 660 nm), 96.9%, 91.4% and 1% for G (green, lambda = 525 nm), and 97.0%, 93.2% and 0% for B (blue, lambda = 470 nm). Organic light-emitting diodes (OLEDs) were then fabricated on transparent single-crystal Si membranes with various top Si thicknesses. OLEDs fabricated on 18, 72 and 5000 nm thick membranes and operated at 6 V demonstrated a luminance of 1350, 443 and 27 cd m(-2) at the current densities of 148, 131 and 1.5 mA cm(-2), respectively.

Plasma Enhanced Chemical Vapour Deposited Carbon Nanotubes for Field Emission Applications

Plasma Enhanced Chemical Vapour Deposition is an extremely versatile technique for directly growing multiwalled carbon nanotubes onto various substrates. We will demonstrate the deposition of vertically aligned nanotube arrays, sparsely or densely populated nanotube forests, and precisely patterned arrays of nanotubes. The high-aspect ratio nanotubes (~50 nm in diameter and 5 microns long) produced are metallic in nature and direct contact electrical measurements reveal that each nanotube has a current carrying capacity of 10 7 -10 8 A/cm 2 , making them excellent candidates as field emission sources. We examined the field emission characteristics of dense nanotube forests as well as sparse nanotube forests and found that the sparse forests had significantly lower turn-on fields and higher emission currents. This is due to a reduction in the field enhancement of the nanotubes due to electric field shielding from adjacent nanotubes in the dense nanotube arrays. We thus fabricated a uniform array of single nanotubes to attempt to overcome these issues and will present the field emission characteristics of this.