Anodic Aluminum Oxide Nanochannels Research Articles

Focused‐Ion‐Beam‐Based Selective Closing and Opening of Anodic Alumina Nanochannels for the Growth of Nanowire Arrays Comprising Multiple Elements

A lithographic process based on focused ion beam bombardment is developed for selectively closing and opening nanochannels on a porous anodic alumina film. This resist-free process is based on the use of focused ion beams with different energies that strike a balance between material sputtering and material relocation. This process is used to selectively grow nanowire patterns of different elements.

Enhanced growth of anodic alumina nanochannels on Ga-ion pre-irradiated aluminum

Exposure of an Al substrate to energetic Ga ions is found to result in enhanced growth rate of its nanochannels during a subsequent anodization process. Depending on the ion dose, the nanochannels in the pre-irradiated area exhibit different lengths. This interesting phenomenon is exploited by scanning a Ga focused ion beam over the desired area to facilitate the fabrication of arrays of anodic alumina nanochannels with custom-designed super-structure, which is based on the variation of channel length in different areas.

Electrochemically replicated smooth aluminum foils for anodic alumina nanochannelarrays

A fast electrochemical replication technique has been developed to fabricate large-scaleultra-smooth aluminum foils by exploiting readily available large-scale smooth siliconwafers as the masters. Since the adhesion of aluminum on silicon depends on the time ofsurface pretreatment in water, it is possible to either detach the replicated aluminum fromthe silicon master without damaging the replicated aluminum and master or integrate thealuminum film to the silicon substrate. Replicated ultra-smooth aluminum foils areused for the growth of both self-organized and lithographically guided long-rangeordered arrays of anodic alumina nanochannels without any polishing pretreatment.

Chemical-free synthesis of electrically connected gold nanotubes/nanoparticles from solution-infiltrated anodized aluminum oxide template

Gold nanotubes/nanoparticles are synthesized by two-step thermal decomposition of HAuCl4 infiltrated into anodized aluminum oxide (AAO) membrane without any catalysts, reducing agents, or surfactants. The Au nanotubes deposited in AAO nanochannel (60nm in diameter) are composed of single crystalline Au nanoparticles (10±2nm in diameter). The Au nanotubes are electrically conducting with an apparent electrical resistivity of 3.9Ωcm, which was about 106 times larger than that of the bulk metal, 2.2×10−6Ωcm, at room temperature.

Long-range ordered nanoaperture array with uniform diameter and interpore spacing

In application of focused-ion-beam lithography and grazing Ar+ milling on the U-shape barrier layer of anodic alumina nanochannels, we fabricated a hexagonally symmetry aperture array with nominal diameter of 12±2nm and interspacing of 100±2nm. Besides long-range spatial ordering, the focused-ion-beam guided-grown process has also significantly improved uniformity of both the interpore spacing and the aperture size. This aperture array membrane can be applied to the fabrication of nanostructures, such as a lithographic contact mask for ordered quantum-dot array.

Synthesis of carbon nanotube arrays using ethanol in porous anodic aluminum oxide template

Carbon nanotube (CNT) arrays confined by porous anodic aluminum oxide (AAO) template were synthesized using ethanol as reactant carbon source at low pressure. Images by scanning electron microscope (SEM) and low magnification transmission electron microscopy (TEM) show that these CNTs have highly uniform outer diameter and length, absolutely controlled by the diameter and depth of nano-channel arrays of the AAO. High resolution transmission electron microscopy (HRTEM) imaging indicates that the graphitization of the CNT walls is better than the results reported on this kind of template-based CNT arrays, although it is not so good as that of multiwalled carbon nanotubes (MWCNTs) synthesized by catalysis. CNTs synthesized using acetylene as reactant gas show much less graphitization than those prepared using ethanol by comparing the results of HRTEM and Raman spectroscopy. The etching effects of decomposed OH radicals on the amorphous carbon and the roughness of AAO nano-channel arrays on the CNTs growth were employed to explain the graphitization and growth of the CNTs.

Order–disorder transition of anodic alumina nanochannel arrays grown under the guidance of focused-ion-beam patterning

By anodizing an aluminum surface that has been patterned with different guiding lattices using a focused ion beam, the resulting anodic alumina nanochannel arrays exhibit different degree of order arrangement. Long-range order is achieved only when the guiding lattice is carefully matched to that of the self-organized hcp array formed locally in an unguided area. Lattice mismatch between the guiding and locally self-organized lattice leads to an order–disorder transition via the creation and annihilation of nanochannels. The driving force of the transition is attributed to the unbalance in the stress among the nanochannels.

High-speed focused-ion-beam patterning for guiding the growth of anodic alumina nanochannel arrays

Long-range ordered arrays of anodic alumina nanochannels are grown by anodizing an aluminum covered with a patterned layer of polymethylmethacrylate (PMMA) resist. The two-dimensional hexagonal-closed-packed pattern is created by focused ion beam (FIB) exposure of the PMMA and transferred onto the aluminum surface by phosphoric acid etching. The required exposure time per channel is only ∼20 μs, more than two orders of magnitude reduction in comparison with the previous method employing FIB direct sputtering of the aluminum surface.

Synthesis of Single-Crystalline TiO2 Nanotubes

In this work, we demonstrate experimentally that the classification of “inorganic fullerene-like structures” can be extended from transition metal sulfides to include metal oxides. Single-crystalline TiO2 nanotubes have been synthesized for the first time with an unconstrained solution growth method by hydrolyzing TiF4 under acidic condition at 60 °C. With SEM/HRTEM/XRD methods, it is found that the walls of TiO2 tubes are formed with stacking of {101} planes (anatase polymorph). Both HRTEM and XRD indicate an interplanar spacing of d101 = 0.36 ± 0.01 nm in the wall structure. Although the {101} planes can be curved into concentric cylinders, straight segments of {101} planes are also observed. The inner diameters of the tubes are in the range of 2.5−5 nm, and outer diameters are in the range of 20−40 nm with lengths up to a few hundred nanometers. Similar to the well-known fullerene-like nanotubes formed from layered materials (graphite, MoS2, and WS2), spherical and polyhedral closings for the TiO2 nanotubes are observed. Anodic aluminum oxide nanochannels and alumina nanoparticles have been used to differentiate growth modes and to address the role of solid substances in TiF4 hydrolysis. It is revealed that TiO2 can still be synthesized in the form of fullerene-like inorganic nanotubes, although it is not a layered compound. Future directions in this research area are also discussed.

Ordered anodic alumina nanochannels on focused-ion-beam-prepatterned aluminum surfaces

A combined process of electropolishing, focused-ion-beam lithography, and controlled anodization is used to fabricate anodic alumina films with ordered nanochannels. The ion beam is used to create a hexagonally close-packed lattice of concaves on a polished aluminum surface and the concaves act as pinning points for guiding the growth of nanochannels in the following anodization step. By carefully matching the lattice constant (100 nm) with the anodization voltage, ordered nanochannels with aspect ratio of ∼100 are fabricated. The effects of the ion dose and its corresponding depth of the concaves on the ordering of the nanochannel array are investigated and a minimum depth of 3 nm is found to be necessary for effective guidance of the growth of ordered nanochannels.