Joonkyung Jang

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.

Gold Nanoparticle Silica Nanopeapods

A subnanometer gap-separated linear chain gold nanoparticle (AuNP) silica nanotube peapod (SNTP) was fabricated by self-assembly. The geometrical configurations of the AuNPs inside the SNTPs were managed in order to pose either a single-line or a double-line nanostructure by controlling the diameters of the AuNPs and the orifice in the silica nanotubes (SNTs). The AuNPs were internalized and self-assembled linearly inside the SNTs by capillary force using a repeated wet-dry process on a rocking plate. Transmission electron microscopy (TEM) images clearly indicated that numerous nanogap junctions with sub-1-nm distances were formed among AuNPs inside SNTs. Finite-dimension time domain (FDTD) calculations were performed to estimate the electric field enhancements. Polarization-dependent surface-enhanced Raman scattering (SERS) spectra of bifunctional aromatic linker p-mercaptobenzoic acid (p-MBA)-coated AuNP-embedded SNTs supported the linearly aligned nanogaps. We could demonstrate a silica wall-protected nanopeapod sensor with single nanotube sensitivity. SNTPs have potential application to intracellular pH sensors after endocytosis in mammalian cells for practical purposes. The TEM images indicated that the nanogaps were preserved inside the cellular constituents. SNTPs exhibited superior quality SERS spectra in vivo due to well-sustained nanogap junctions inside the SNTs, when compared to simply using AuNPs without any silica encapsulation. By using these SNTPs, a robust intracellular optical pH sensor could be developed with the advantage of the sustained nanogaps, due to silica wall-protection.

Blue and Blue-Green Light-Emitting Cationic Iridium Complexes: Synthesis, Characterization, and Optoelectronic Properties

Two new cationic iridium complexes, [Ir(ppy)2(phpzpy)]PF6 (complex 1) and [Ir(dfppy)2(phpzpy)]PF6 (complex 2), bearing a 2-(3-phenyl-1H-pyrazol-1-yl)pyridine (phpzpy) ancillary ligand and either 2-phenylpyridine (Hppy) or 2-(2,4-difluorophenyl)pyridine (Hdfppy) cyclometalating ligands, were synthesized and fully characterized. The photophysical and electrochemical properties of these complexes were investigated by means of UV-visible spectroscopy, emission spectroscopy, and cyclic voltammetry. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations were performed to simulate and study the photophysical and electrochemical properties of both complexes. Light-emitting electrochemical cells (LECs) were fabricated by incorporating complexes 1 and 2, which respectively exhibit blue-green (488 and 516 nm) and blue (463 and 491 nm) emission colors, achieved through the meticulous design of the ancillary ligand. The luminance and current efficiency measurements recorded for the LEC based on complex 1 were 1246 cd m(-2) and 0.46 cd A(-1), respectively, and were higher than those measured for complex 2 because of the superior balanced carrier injection and recombination properties of the former.

Microscopic origin of the humidity dependence of the adhesion force in atomic force microscopy

Water condenses between an atomic force microscope (AFM) tip and a surface to form a nanoscale bridge that produces a significant adhesion force on the tip. As humidity increases, the water bridge always becomes wider but the adhesion force sometimes decreases. The authors show that the humidity dependence of the adhesion force is intimately related to the structural properties of the underlying water bridge. A wide bridge whose width does not vary much with tip-surface distance can increase its volume as distance is increased. In this case, the adhesion force decreases as humidity rises. Narrow bridges whose width decreases rapidly with increasing tip-surface distance give the opposite result. This connection between humidity dependence of the adhesion force and the structural susceptibility of the water bridge is illustrated by performing Monte Carlo simulations for AFM tips with various hydrophilicities.

Dopant induced diameter tuning of Mn-doped CdTe nanorods in aqueous solution

High quality Mn-doped CdTe nanorods with uniform diameter were synthesized in aqueous phase by spontaneous self-organization of Mn-doped CdTe nanoparticles into nanorods. The diameter of the CdTe nanorods increased gradually from 4.1 to 10.2 nm for 0–4.2% Mn incorporations. The intermediate step in the nanorod growth was found to be double or triple chain aggregation. The X-ray diffraction (XRD) patterns of the Mn doped samples retained the same crystal structure as the CdTe nanocrystals, indicating no second phase formation for Mn ion. XPS, ICP-AES and ESR data demonstrate successful incorporation of Mn ions in CdTe nanorods. Superconducting quantum interference device (SQUID) measurements reveal that all of these nanocrystals exhibited ferromagnetic behavior, with a coercive field (Hc) of 2.6 kOe.

Molecular simulation of the nanoscale water confined between an atomic force microscope tip and a surface

Under ambient humidity, water condenses as a nanometre meniscus between an atomic force microscope (AFM) tip and a surface, giving rise to a strong capillary force on the tip. To examine the molecular features of the meniscus, we performed an all-atom molecular dynamics simulation. By varying the tip–surface distance, we have simulated the formation, thinning and snap-off of the water meniscus. The meniscus is several nanometres wide and substantially fluctuates in its periphery when its neck is narrow. The density profile of the meniscus shows that its periphery is not sharp but has a fuzzy boundary whose thickness ranges from 0.4 to 0.9 nm. We obtained the neck radius of the meniscus and the radius of curvature of its periphery. Due to the sharp asperity of the AFM tip, these two structural parameters are comparable in size, in contrast to the case of a macroscopic tip, where the neck radius is much greater. We found that the meniscus periphery is often far from a circle in shape. With the structural parameters of the meniscus, we calculated the capillary force by using the Laplace–Kelvin equation. Our calculation reproduces the typical behaviour of the force–distance curve in the AFM experiment.

Molecular simulation of the water meniscus in dip-pen nanolithography

We report a molecular dynamics simulation of the nanometer water meniscus formed in dip-pen nanolithography (DPN). When an atomic force microscope tip is in contact with a surface, the meniscus is significantly asymmetric around the tip axis. The meniscus as a whole can move away from the tip axis due to surface diffusion. The structure of the meniscus fluctuates and its periphery has a finite thickness as large as 25% of its width. We simulated the transport of nonpolar hydrophobic molecules through a water meniscus. Molecules move on the surface of, not dissolving into the interior of, the meniscus. As a result, an annular pattern forms in DPN. Even if the meniscus is cylindrically symmetric, the molecular flow from the tip and the subsequent pattern growth on the surface are anisotropic at the nanosecond timescale.

Molecular Dynamics Study of the Hydrophilic-to-Hydrophobic Switching in the Wettability of a Gold Surface Corrugated with Spherical Cavities

This paper reports a large scale molecular dynamics (MD) simulation study of the wettability of a gold surface engraved with (hemi)spherical cavities. By increasing the depth of cavities, the contact angle (CA) of a water droplet on the surface was varied from a hydrophilic (69°) to a hydrophobic value (>109°). The nonmonotonic behavior of the CA vs the depth of the cavities was consistent with the Cassie-Baxter theory, as found in the experiment by Abdelsalam et al. (Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Wetting of Regularly Structured Gold Surfaces. Langmuir 2005, 21, 1753-1757). Depending on the depth of cavities, however, the droplet existed not only in the Cassie-Baxter state, but also in the Wenzel or an intermediate state, where the cavities were penetrated partially by the droplet.

Density Functional Theory Study on the Cross-Linking of Mussel Adhesive Proteins

The water-resistant adhesion of mussel adhesive proteins (MAPs) to a wet surface requires a cross-linking step, where the catecholic ligands of MAPs coordinate to various transition-metal ions. Fe(III), among the range of metal ions, induces particularly strong cross-linking. The molecular details underlying this cross-linking mediated by transition-metal ions are largely unknown. Of particular interest is the metal-ligand binding energy, which is the molecular origin of the mechanical properties of cross-linked MAPs. Using density functional theory, this study examined the structures and binding energies of various trivalent metal ions (Ti-Ga) forming coordination complexes with a polymeric ligand similar to a MAP. These binding energies were 1 order of magnitude larger than the physisorption energy of a catechol molecule on a metallic surface. On the other hand, the coordination strength of Fe(III) with the ligand was not particularly strong compared to the other metal ions studied. Therefore, the strong cross-linking in the presence of Fe(III) is ascribed to its additional ability as an oxidant to induce covalent cross-linking of the catecholic groups of MAPs.

Serial Pushing Model for the Self-Assembly in Dip-Pen Nanolithography †

Based on the findings of molecular dynamics simulations, we propose a novel diffusion model for the self-assembly in dip-pen nanolithography. A central question in such modeling is how a nascent droplet created below an AFM tip spreads out to form a self-assembled monolayer (SAM) on a substrate later. In the present model, a molecule dropping from the tip pushes a molecule on the substrate out of its original position, and the molecule pushed out in turn pushes its own neighbor. A SAM grows through such a series of push-induced movements. The initial pushing propagates all the way to the periphery where there is no molecule to push out. By contrast, according to the previous hopping-down model, a molecule moves by stepping over molecules trapped on the substrate and occasionally hops down to the substrate. By implementing our model in random walk simulations, we study the structure and growth dynamics of the SAM generated by a fixed tip and the lines and characters created by a moving tip. We investigate how the SAM is influenced by the molecular dripping rate and tip scan speed. Compared to the hopping model, the present model gives a SAM growing faster and more fluctuating in its periphery. A salient feature of our model is its ability to generate various SAMs by changing the directional coherence length of the push-induced displacement. If we choose the coherence length to be zero, each push-induced displacement is random in direction to give a compact circular SAM. As the directional coherence length increases, the SAM becomes a noncircular pattern with distinct branches. In the limit of an infinite coherence length, the SAM becomes a long narrow cross due to the substrate anisotropy.