Biomedical subjects

Joonkyung Jang

Publications and source records attributed to Joonkyung Jang.

5 recordsLinked to original sources

PubMed2006

Growth dynamics of self-assembled monolayers in dip-pen nanolithography.

Using molecular dynamics simulations, we studied the growth mechanism of self-assembled monolayers in dip-pen nanolithography. A molecule dropping from the tip kicks out a molecule sitting on the substrate, and the displaced molecule in turn kicks out a molecule next to it. This kicking propagates and finally stops when it hits the periphery of the monolayer. This monolayer growth is faster than predicted from the previous diffusion theory. Increasing the molecule-substrate binding strength enhances the molecular deposition rate and makes the monolayer well-ordered.

Journal Article↗

PubMed2006

Atomic-scale roughness effect on capillary force in atomic force microscopy.

We study the capillary force in atomic force microscopy by using Monte Carlo simulations. Adopting a lattice gas model for water, we simulated water menisci that form between a rough silicon-nitride tip and a mica surface. Unlike its macroscopic counterpart, the water meniscus at the nanoscale gives rise to a capillary force that responds sensitively to the tip roughness. With only a slight change in tip shape, the pull-off force significantly changes its qualitative variation with humidity.

Journal Article↗

PubMed2004

How narrow can a meniscus be?

A water meniscus naturally forms in air between an atomic force microscope (AFM) tip and a substrate. This nanoscale meniscus produces a capillary force on the AFM, and also serves as a molecular transport channel in dip-pen nanolithography (DPN). A stable meniscus is a necessary condition for DPN and for the validity of the Kelvin equation commonly applied to AFM experiments. Lattice gas Monte Carlo simulations show that, due to thermal fluctuation, a stable meniscus has a lower limit in width. We find a minimum width of 5 molecular diameters (1.9 nm) when the tip becomes atomically sharp (terminated by a single atom).

Journal Article↗

PubMed2004

Capillary force in atomic force microscopy.

Under ambient conditions, a water meniscus generally forms between a nanoscale atomic force microscope tip and a hydrophilic surface. Using a lattice gas model for water and thermodynamic integration methods, we calculate the capillary force due to the water meniscus for both hydrophobic and hydrophilic tips at various humidities. As humidity rises, the pull-off force rapidly reaches a plateau value for a hydrophobic tip but monotonically increases for a weakly hydrophilic tip. For a strongly hydrophilic tip, the force increases at low humidities (<30%) and then decreases. We show that mean-field density functional theory reproduces the simulated pull-off force very well.

Journal Article↗

PubMed2003

Capillary force on a nanoscale tip in dip-pen nanolithography.

Monte Carlo simulation has been used to characterize the capillary force due to the condensation of a liquid meniscus between a tip with a nanoscale asperity and a flat surface. To consider both hydrophobic and hydrophilic molecules coating the tip as a model of dip-pen nanolithography, tips with various wettabilities are studied. The capillary force due to the meniscus is calculated for various saturations (humidities). We have implemented a thermodynamic integration technique that can project the force into energetic and entropic contributions. In most cases, the force is mainly energetic in origin. At the snap-off separation where the meniscus disappears, the tip feels a significant entropic force at high saturation. Our calculation shows nonmonotonic behavior of the pull-off force as a function of saturation, which is in qualitative accord with experiments.

Journal Article↗