Seul Ki Youn

Metal‐Dielectric‐CNT Nanowires for Femtomolar Chemical Detection by Surface Enhanced Raman Spectroscopy

A highly sensitive substrate for surface enhanced Raman spectroscopy (SERS) is formed by arrays of gold-coated metallic carbon nanotubes having a nanoinsert of high-k dielectric (hafnia) as an energy coupling barrier. Repeated femtomolar detection of 1,2 bis-(4-pyridyl)-ethylene in solution demonstrates the critical contribution of this plasmonic energy coupling barrier to the enhanced chemical sensitivity.

Facile diameter control of vertically aligned, narrow single-walled carbon nanotubes

Here, we report a diameter-controlled synthesis of vertically aligned (VA) single-walled carbon nanotubes (SWNTs) via catalytic chemical vapor deposition (CVD), enabled by ultrathin iron (Fe) catalysts on alumina (Al2O3) and low acetylene (C2H2) partial pressure. A long forest of sub-3-nm SWNTs up to one millimeter in height could be obtained without addition of hydrogen or moisture, and precise control of the SWNT diameters was successfully established. Key for the efficient growth of such arrays of narrow SWNTs is threefold: (a) growth temperature low enough to suppress catalyst agglomeration and Ostwald ripening, (b) C2H2 partial pressure below a certain level to extend the catalyst lifetime, and (c) size-matching at nanometer scale between Fe catalyst seeds and Al2O3 support asperities in order to mitigate the surface migration and undesirable enlargement of catalyst particles. Our findings can contribute to the facile achievement of uniform, dense arrays of high quality VA-SWNTs with narrow diameter distributions desirable for advanced nanofiltration and electronic applications.

Enhanced charge transport kinetics in anisotropic, stratified photoanodes.

The kinetics of charge transport in mesoporous photoanodes strongly constrains the design and power conversion efficiencies of dye sensitized solar cells (DSSCs). Here, we report a stratified photoanode design with enhanced kinetics achieved through the incorporation of a fast charge transport intermediary between the titania and charge collector. Proof of concept photoanodes demonstrate that the inclusion of the intermediary not only enhances effective diffusion coefficients but also significantly suppresses charge recombination, leading to diffusion lengths two orders of magnitude greater than in standard mesoporous titania photoanodes. The intermediary concept holds promise for higher-efficiency DSSCs.

A Forest of Sub-1.5-nm-wide Single-Walled Carbon Nanotubes over an Engineered Alumina Support

A precise control of the dimension of carbon nanotubes (CNTs) in their vertical array could enable many promising applications in various fields. Here, we demonstrate the growth of vertically aligned, single-walled CNTs (VA-SWCNTs) with diameters in the sub-1.5-nm range (0.98 ± 0.24 nm), by engineering a catalyst support layer of alumina via thermal annealing followed by ion beam treatment. We find out that the ion beam bombardment on the alumina allows the growth of ultra-narrow nanotubes, whereas the thermal annealing promotes the vertical alignment at the expense of enlarged diameters; in an optimal combination, these two effects can cooperate to produce the ultra-narrow VA-SWCNTs. According to micro- and spectroscopic characterizations, ion beam bombardment amorphizes the alumina surface to increase the porosity, defects, and oxygen-laden functional groups on it to inhibit Ostwald ripening of catalytic Fe nanoparticles effectively, while thermal annealing can densify bulk alumina to prevent subsurface diffusion of the catalyst particles. Our findings contribute to the current efforts of precise diameter control of VA-SWCNTs, essential for applications such as membranes and energy storage devices.

Confined Water in Carbon Nanotubes and Its Applications

Unique nanoscale transport properties of carbon nanotubes (CNTs) have inspired researchers for over a decade, initially with their analogies to various biological pores and later with the potential impact on water purification. Water can permeate through a nanometer-wide pipe of the CNT interior at rates far exceeding those predicted by Hagen-Poiseuille formulation and measured in nano conduits of different material, attributed to nano confinement, hydrophobicity, and smooth potential energy landscape. Also, chemical addition to the nanotube ends was found effective in electrostatic exclusion of ions without much loss of water permeability, suggesting the emergence of CNT membranes for desalination and purification of water resources. This article introduces Carbon Nanotube Nanofluidics by capturing important findings and progresses made in the early developments of the area.

Carbon micronymphaea: graphene on vertically aligned carbon nanotubes

This paper describes themorphology of carbon nanomaterials such as carbon nanotube (CNT), graphene, and their hybrid structure under various operating conditions during a one-step synthesis via plasma-enhanced chemical vapor deposition (PECVD). We focus on the synthetic aspects of carbon hybrid material composed of heteroepitaxially grown graphene on top of a vertical array of carbon nanotubes, called carbon micronymphaea. We characterize the structural features of this unique nanocomposite by uses of electron microscopy and micro-Raman spectroscopy. We observe carbon nanofibers, poorly aligned and well-aligned vertical arrays of CNT sequentially as the growth temperature increases, while we always discover the carbon hybrids, called carbon micronymphaea, at specific cooling rate of 15°C/s, which is optimal for the carbon precipitation from the Ni nanoparticles in this study. We expect one-pot synthesized graphene-on-nanotube hybrid structure poses great potential for applications that demand ultrahigh surface-to-volume ratios with intact graphitic nature and directional electronic and thermal transports.

Osmotic Transport across Surface Functionalized Carbon Nanotube Membrane

Osmosis plays a central role in many chemical separation processes. Among various biological and artificial channels, carbon nanotubes (CNTs) stand out due to their exceptional water transport efficiency and variability of pore-size, down to molecular dimensions, thereby approaching ideal semipermeability. We report osmotically driven water and salt transport across a membrane of vertically aligned CNTs in a titania matrix whose surface is functionalized with a self-assembled monolayer of octadecylphosphonic acid. The increased steric hindrance and hydrophobicity at the pore entrance of CNTs improved salt rejection while maintaining enhanced osmotic water transport, thanks to an atomically smooth surface of the nanotubes. In addition to the experimental demonstration of osmosis, we observed a net negative osmotic water flow at lower salt concentration gradient and non-Fickian behavior of the reverse salt flux. This observation is attributable to the interface-driven fluidic phenomenon known as diffusio-osmosis that drives water flow in the direction opposite to osmotic flow. The ion-CNT interactions are responsible for the simultaneous occurrence of the two osmotic transport mechanisms and the salt-specific osmotic transport characteristics.

Enhanced Chemical Separation by Freestanding CNT–Polyamide/Imide Nanofilm Synthesized at the Vapor–Liquid Interface

In chemical separation, thin membranes exhibit high selectivity, but often require a support at the expense of permeance. Here, we report a pinhole-free polymeric layer synthesized within freestanding carbon nanotube buckypaper through vapor-liquid interfacial polymerization (VLIP). The VLIP process results in thin, smooth and uniform polyamide and imide films. The scaffold reinforces the nanofilm, defines the membrane thickness, and introduces an additional transport mechanism. Our membranes exhibit superior gas selectivity and osmotic semipermeability. Plasticization resistance and high permeance in hydrocarbon separation together with a considerable improvement in water-salt permselectivity highlight their potential as new membrane architecture for chemical separation.