Cylindrical Alumina Nanopores Research Articles

How thermal stress alters the confinement of polymers vitrificated in nanopores.

Understanding and controlling the glass transition temperature (Tg) and dynamics of polymers in confined geometries are of significance in both academia and industry. Here, we investigate how the thermal stress induced by a mismatch in the coefficient of thermal expansion affects the Tg behavior of polystyrene (PS) nanorods located inside cylindrical alumina nanopores. The size effects and molecular weight dependence of the Tg are also studied. A multi-step relaxation process was employed to study the relationship between thermal stress and cooling rate. At fast cooling rates, the imparted thermal stress would overcome the yield stress of PS and peel chains off the pore walls, while at slow cooling rates, chains are kept in contact with the pore walls due to timely dissipation of the produced thermal stress during vitrification. In smaller nanopores, more PS chains closely contact with pore walls, then stronger internal thermal stress would be generated between core and shell of PS nanorod, which results in a larger deviation between two Tgs. The core part of PS shows lower Tg than bulk value, which can induce faster dynamics in the center region. A complex and important role stress plays is supposed in complex confinement condition, e.g., in nanopores, during vitrification.

Effect of geometric curvature on vitrification behavior for polymer nanotubes confined in anodic aluminum oxide templates.

The glass transition behavior of polystyrene (PS) nanotubes confined in cylindrical alumina nanopores was studied as a function of pore diameter (d) and polymer tube thickness (δ). Both the calorimetric glass transition temperature and the microstructure measured by a nonradiative energy transfer method indicated that the polymer nanotube, or concave polymer thin film, exhibited significant differences in vitrification behavior compared to the planar one. A closer interchain proximity and an increased T_{g} were observed for polymer nanotubes with respect to the bulk polymer. T_{g} for polymer nanotubes was primarily dependent on the curvature radius d of the template, while it was less dependent on the thickness δ of the PS tube wall in the range of 11-23 nm. For small nanotubes (d=55nm), the T_{g} increased as high as 18°C above the bulk value. This vitrified property reverted back to the bulk value when the substrate was chemically removed, which indicated the crucial importance of the interfacial effect imposed by the hard wall with a concave geometry.

Inclusion Complexation of γ-Cyclodextrin and Coumarin Dye inside Alumina Nanopores over a Temperature Range of 303–233 K

In the present study, we aimed to demonstrate that complexation reactions inside an inorganic water-filled nanopore can be enhanced at below the homogeneous nucleation temperature of bulk water (273 K). Using fluorescence experiments, we examined the inclusion complexation of propylamide coumarin 343 (PAC343) with γ-cyclodextrin (γ-CD) inside cylindrical alumina nanopores over a temperature range of 303–233 K. A PAC343-immobilized porous anodic alumina membrane (PAC343-PAA) with a pore diameter of ca. 70 nm was fabricated, and its fluorescence spectra were measured after introduction of aqueous γ-CD solution into the alumina nanopores. Our results confirmed that the inclusion complexation reaction was gradually enhanced with decreasing temperature, particularly at temperatures between 273 and 233 K. The inclusion complexation reaction was confirmed to be exothermic from van’t Hoff analysis of adsorption constants of γ-CD obtained at temperatures between 288 and 283 K. These results confirm low-temperature...

Highly organised and dense vertical silicon nanowire arrays grown in porous alumina template on <100> silicon wafers

In this work, nanoimprint lithography combined with standard anodization etching is used to make perfectly organised triangular arrays of vertical cylindrical alumina nanopores onto standard <100>−oriented silicon wafers. Both the pore diameter and the period of alumina porous array are well controlled and can be tuned: the periods vary from 80 to 460 nm, and the diameters vary from 15 nm to any required diameter. These porous thin layers are then successfully used as templates for the guided epitaxial growth of organised mono-crystalline silicon nanowire arrays in a chemical vapour deposition chamber. We report the densities of silicon nanowires up to 9 × 109 cm−2 organised in highly regular arrays with excellent diameter distribution. All process steps are demonstrated on surfaces up to 2 × 2 cm2. Specific emphasis was made to select techniques compatible with microelectronic fabrication standards, adaptable to large surface samples and with a reasonable cost. Achievements made in the quality of the porous alumina array, therefore on the silicon nanowire array, widen the number of potential applications for this technology, such as optical detectors or biological sensors.

Multisegmented nanotubes by surface-selective atomic layer deposition

We describe a general strategy for fabricating multisegmented nanotubes and nanopores via sequential, surface-selective modification with organic and inorganic layers combined with in situ formation of nanopores by electrochemical anodization. We found that cylindrical alumina nanopores can be continuously anodized upon coating thin organic and/or inorganic layers such as octadecyltrichlorosilane (OTS)-self-assembled monolayers (SAMs), and atomic layer deposition (ALD)-grown TiO2, ZnO, and ZrO2, allowing for three-dimensionally site-selective ALD. As model systems, we show that (1) isolated metal oxide nanotubes can be prepared as-grown with controlled opening at the distal ends of tubes, (2) inner surfaces of nanopores can be chemically and physically modified in a segmented manner, and (3) a nanotube core–multisegmented shell geometry can be achieved. We believe that the present approach will open up new avenues for realizing complex nanodevices such as nanofluidic diodes, photovoltaic junctions, and transistors by adding a degree of freedom in the synthesis of nanotubes/pores in their axis direction.

Polymer nanofibers by controllable infiltration of vapour swollen polymers into cylindrical nanopores

We report on a new method for the fabrication of polymer nanofibers based on wetting of cylindrical alumina nanopores with solvent vapour swollen polymers. Solvent annealing is used to swell and soften polymers at room temperature, so that they are allowed to infiltrate into cylindrical nanopores by capillary force. The method of swelling assisted infiltration is capable of eliminating the problem of thermal degradation from the widely used “melt wetting”; in addition, the thus formed nanofibers display well aligned one-dimensional structures and controllable aspect ratios, in contrast to those from “solution wetting”, the morphologies of which are difficult to be controlled. By using the proposed method, several homopolymers including thermo-sensitive polyvinyl chloride (PVC) and conducting poly(3-hexylthiophene) (P3HT) were found to form nanofibers with high aspect ratios. In particular, we have studied the microphase separation behaviour of vapour swollen block copolymers (BCPs) under cylindrical confinement, and found that the intrinsic ordered pattern of BCPs (i.e., in the melt state) would transform into a different structure due to solvent swelling under cylindrical confinement.

Electrophoretic Migration and Axial Diffusion of Individual Nanoparticles in Cylindrical Nanopores

Membranes with straight, vertical nanopores have found widespread applications in chemical and biological sciences, including separation, detection, catalysis, and drug delivery. They can also serve as a model system to understand molecular behavior and fundamental mechanisms of separation, bridging the gap between conventional model systems such as flat surfaces and real chromatographic stationary phases such as micrometer-sized porous particles. We recently found that the axial motion of individual biomolecules inside nanopores can be significantly slower than in bulk solution. This suggests that either chromatographic adsorption was present and/or the viscosity inside the nanopores was unusually high. In this study, we measured the electrophoretic motion as well as the axial diffusion of individual nanoparticles in cylindrical alumina nanopores. We found that the electrophoretic mobilities and the diffusion coefficients of polystyrene nanoparticles were both substantially smaller compared to bulk solution independent of particle size or pore diameter. The results imply that the apparent solution viscosity in nanodomains is anomalous.

Synthesis of polymer nanograss and nanotubes by surface-initiated photopolymerization in cylindrical alumina nanopores

Functional polymer nanograss and nanotubes with active epoxy groups were synthesized by a surface-initiated photopolymerization pattern transfer method in cylindrical alumina nanopores. The nanograss structures were obtained when the neat glycidyl methacrylate (GMA) monomer or the mixture of GMA monomer and ethylene glycol dimethacrylate (EGDMA) crosslinker solution was injected into the photoinitiator-immobilized template. In contrast, uniform polymer nanotubes that completely follow the contour of the template were generated with the mixture of GMA monomer, EGDMA crosslinker and pre-synthesized poly(glycidyl methacrylate) (PGMA) polymer as the precursor material. The mechanism of the formation of polymer nanograss and nanotubes was discussed in terms of mechanical strength of polymer and polymerization shrinkage. The incorporation of the pre-synthesized polymer into the monomer solution as the precursor material allows for precompensation of the structure shrinkage induced by photopolymerization reactions. Effects of the pre-synthesized PGMA polymer percentage and the amount of the EGDMA crosslinker on the morphology of the formed polymer nanostructures were investigated respectively. FT-IR spectra confirmed the chemically immobilized photoinitiators, and chemical component, poly(GMA-co-EGDMA), of the polymer nanotubes inside the cylindrical alumina nanopores respectively. Finally, an intriguing dendritic-shaped fractal pattern was formed from concentrated crosslinked polymer nanoneedles by self-assembly and droplet evaporation.

Spontaneous Formation of Nanoscale Polymer Spheres, Capsules, or Rods by Evaporation of Polymer Solutions in Cylindrical Alumina Nanopores

In this study, we report for the first time the fabrication of polymer nanospheres, nanocapsules, and hemispherically capped nanorods simply through the wetting of anodic aluminum oxide (AAO) membranes with polymer solutions. For the wetting with polystyrene solutions, we have demonstrated that the formation of nanorods is dependent upon the solvents used (e.g., tetrahydrofuran and methyl ethyl ketone) which have strongly adsorbent nature toward alumina surfaces, displaying no correlation to molecular weight. More importantly, we have shown that there is a coarsening process from spheres to capsules to rods in AAO cylindrical nanopores during solvent evaporation. The coarsening process could serve as the formation mechanism for nanorods which have the similar diameter as the AAO nanopores. Our observation also provides a facile approach for fabricating polymer nanospheres and nanocapsules that can be directed by cylindrical nanopores into ordered arrays.

Electrodeposited Nickel Nanodots Array on the Silicon Wafer

Nanoporous anodic aluminum oxide (AAO) membrane has many advantages as a host material for a variety of magnetic materials. The magnetic materials can be embedded into the host pores by electrodeposition, sputtering, and infiltration routes. This work shows that the entire fabrication process to the magnetic nanomaterial through the AAO templating method can be thoroughly integrated with silicon technology. Thin aluminum film is directly deposited on the titanium precoated silicon wafer by dc sputtering. Subsequent twostep anodization of the aluminum film results in uniform morphology of the long-range ordered array of cylindrical alumina nanopores, which is used as a template for the electrochemical growth of the Ni nanodots array. The observed magnetic hysteresis loops at 10 K and room temperature show unique ferromagnetic coupling for the Ni nanodot array depending on field directions.

Wetting Transition in Cylindrical Alumina Nanopores with Polymer Melts

We report a systematic study of the wetting of cylindrical alumina nanopores with polystyrene melts. A transition from partial to complete wetting was observed with increasing annealing temperature, resulting in the formation of very different one-dimensional polymeric nanostructures (nanorods and nanotubes) within cylindrical alumina nanopores. The wetting transition temperature is dependent on the polymer molecular weight. The large difference in the wetting rate between partial and complete wetting was successfully used to fractionate polymers with different molecular weights.