Atomic-Layer Deposition into 2- versus 3‑Dimensionally Ordered Nanoporous Media: Pore Size or Connectivity?

Atomic-layer deposition (ALD) is now being recognized as a powerful, general tool for modifying the surfaces of nanomaterials in applications for many energy conversion devices. However, ALD involves slow processes particularly when it is subjected to nanoporous media with high-aspect ratios. Predicting the exact experimental conditions of the desired reactions for coating inside deep pores by ALD is not available because of the lack of complete understanding of diffusion in nanoporous media. Here, we report a comparative study of the ALD coating onto two distinctive templates having nanopores, i.e., 2- and 3-dimensionally ordered media (DOM), of similar porosity and pore dimension. Self-supporting, crack-free templates were carefully prepared in centimeters for both 2- and 3-DOM and thus avoid any possible sources of uncontrollable diffusion of precursor gas molecules through unwanted microvoids and cracks. Comparison of the ALD coating profiles across the thickness of both templates reveals a fundamentally distinct coating mechanism. While a uniform growth zone develops along the pores of the 2-DOM (i.e., 1-D diffusion path), a gradual decrease in the deposition is observed in those of the 3-DOM (i.e., 3-D diffusion path) as ALD pulse time increases. This observation suggests an essential role of the pore connectivity, rather than individual pore sizes, in the gas diffusion dynamics inside nanoporous media. The present model can universally predict the ALD behaviors in nanoporous media even with different types of pore connectivity

Spatial Charge Separation in Asymmetric Structure of Au Nanoparticle on TiO₂ Nanotube by Light-Induced Surface Potential Imaging

Both enhancing the excitons’ lifetime and ingeniously controlling the spatial charge transfer are the key to the realization of efficiently photocatalytic and artificially photosynthetic devices. Nanostructured metal/metal-oxide interfaces often exhibit improved energy conversion efficiency. Understanding the surface potential changes of nano-objects under light illumination is crucial in photoelectrochemical cells. Under ultraviolet (UV) illumination, here, we directly observed the charge separation phenomena at the Au-nanoparticle/TiO₂-nanotube interfaces by using Kelvin probe force microscopy. The surface potential maps of TiO₂ nanotubes with and without Au nanoparticles were compared on the effect of different substrates. We observed that in a steady state, approximately 0.3 electron per Au particle of about 4 nm in diameter is effectively charged and consequently screens the surface potential of the underlying TiO₂ nanotubes. Our observations should help design improved photoelectrochemical devices for energy conversion applications

Initial Self-Ordering of Porous Anodic Alumina: Transition from Polydispersity to Monodispersity

Self-ordered porous anodic alumina (PAA) membranes have been widely employed as a scaffold for fabricating various nanomaterials and functional nanostructures with an excellent uniformity. The self-organization processes are only found in narrow experimental windows even in PAA, and their formation mechanisms have not been fully understood yet and might allow us to access a hint that generally extends into other material systems. Here, we revisit the self-organization process of PAA by experimentally observing its initial stage in great detail. Surface morphologies of PAA were carefully monitored which have been imprinted upon the first anodization in the solutions of oxalic acid around the inflection point in the current–time curves. The physical dimensions were analyzed by electron microscopy, and the degree of ordering was evaluated using the radial power spectral density method. We found that the inflection point reflects the occurrence of a uniform pore diameter as well as interpore distance which is crucial for the self-organization phenomena resulting from the minimization of surface free energy. The proposed model was further supported by electric field simulation near the inflection point

X-ray scattering data analysis.

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X-ray scattering analysis of lipid films.

Reconfiguration of SLM films as a function of compression. A) 2D GIWAXS diffraction patterns of DPPC films before and after compression by 10%. The brackets denote a family of planes hkl. The inset are schematics of the changes in layer alignment corresponding with changes in correlation length ξ induced in the DPPC SLM by compression. B) Linear integration of the GIWAXS data. C) Schematic of GIWAXS peak shift and widening related to the effect of compression at the hydrocarbon chain length scale. D) Schematic of DPPC hexagonal phase from GIWAXS data which displays changes in lattice spacing a and increased disorder of lipid tails.</p

Effect of compression on lipid tilt.

A) Color coded representation of the simulated DPPC systems at three compressive strains. The color represents lipid orientation (tilt) angle with respect to z-axis. The box represents the simulation unit cell. B) The tilt angle distributions of the Lβ phase DPPC lipids. The inset shows the average order parameter values.</p

Single-Droplet Multiplex Bioassay on a Robust and Stretchable Extreme Wetting Substrate through Vacuum-Based Droplet Manipulation

Herein, a droplet manipulation system with a superamphiphobic (SPO)–superamphiphilic (SPI) patterned polydimethylsiloxane (PDMS) substrate is developed for a multiplex bioassay from single-droplet samples. The SPO substrate is fabricated by sequential spraying of adhesive and fluorinated silica nanoparticles onto a PDMS substrate. It is subsequently subjected to oxygen plasma with a patterned mask to form SPI patterns. The SPO layer exhibits extreme liquid repellency with a high contact angle (>150°) toward low surface tension and viscous biofluidic droplets (<i>e.g.</i>, ethylene glycol, blood, dimethyl sulfoxide, and alginate hydrogel). In contrast, the SPI exhibits liquid adhesion with a near zero contact angle. Using the droplet manipulation system, various liquid droplets can be precisely manipulated and dispensed onto the predefined SPI patterns on the SPO PDMS substrate. This system enables a multiplex colorimetric bioassay, capable of detecting multiple analytes, including glucose, uric acid, and lactate, from a single sample droplet. In addition, the detection of glucose concentrations in a plasma droplet of diabetic and healthy mice are performed to demonstrate the feasibility of the proposed system for efficient clinical diagnostic applications

Simulations.

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Derivation of adhesion energy between supported lipid films and PDMS.

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Micro and nano scale deformation of DPPC SLM films after compression.

Micro (A) and sub-micro (B) scale topography maps of uncompressed and compressed lipid films. C) Schematic of wrinkling and delamination buckle of lipids film after compression where t and λ denote the thickness and width of the lipid film. D) Relationship between compression strain and λ/t1.5. The colored lines on the graph correspond to the theoretical relationship based on continuum elastic model with different adhesion energies between DPPC SLM film and PDMS whereas the squares correspond to the experimental data measured in twelve different delamination buckles. The red shade indicates no observable delamination buckle onto lipid films after compression.</p