The synthesis of nanoporous membranes based on different concepts and materials is a field of active research. This review focuses on the synthesis strategies, mesophase evolution mechanisms and potential applications of mesoporous materials confined within anodic alumina membranes (AAM). Following a rapid evolution of synthetic techniques, a significant number of different mesoporous materials (e.g., silica, titania, and carbon) with highly regular structures can now be prepared within these membranes. In recent years, efforts have also been made to understand the formation mechanisms of these hierarchical mesophases. The resulting organized nanoporous membranes open up a wide range of potential applications in fields such as templating oriented nanowires and controlled separation and release of molecules. For example, while various synthesis strategies can be used for the preparation of membrane-embedded nanowires, the latter can also be obtained as isolated objects after dissolution of the alumina host matrix. The review also discusses issues such as control of structural defects or integrity of interfaces that should be addressed in future research in order to fully exploit the potential of these hierarchical mesoporous channel structures.

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated with in situ small-angle X-ray scattering (SAXS) measurements: thermally induced self-assembly. Unlike the well-established evaporation-induced self-assembly (EISA), the structure formation for organic-organic self-assembly of an oligomeric resol precursor and the block-copolymer templates Pluronic P123 and F127 does not occur during evaporation but only by following a thermopolymerization step at temperatures above 100 °C. The systems investigated here were cubic (Im3m), orthorhombic Fmmm) and 2D-hexagonal (plane group p6mm) mesoporous carbon phases in confined environments, as thin films and within the pores of anodic alumina membranes (AAMs), respectively. The thin films were prepared by spin-coating mixtures of the resol precursor and the surfactants in ethanol followed by thermopolymerization of the precursor oligomers. The carbon phases within the pores of AAMs were made by imbibition of the latter solutions followed by solvent evaporation and thermopolymerization within the solid template. This thermopolymerization step was investigated in detail with in situ grazing incidence small-angle X-ray scattering (GISAXS, for films) and in situ SAXS (for AAMs). It was found that the structural evolution strongly depends on the chosen temperature, which controls both the rate of the mesostructure formation and the spatial dimensions of the resulting mesophase. Therefore the process of structure formation differs significantly from the known EISA process and may rather be viewed as thermally induced self-assembly. The complete process of structure formation, template removal, and shrinkage during carbonization up to 1100 °C was monitored in this in situ SAXS study.

Two-dimensional (2D)-hexagonal (plane group, p6mm) mesoporous polymer resin thin films were obtained through evaporation-induced organic-organic self-assembly of a preformed oligomeric resol precursor and the triblock copolymer template pluronic P123. The polymer resin films were prepared on silicon wafers by spin-coating a mixture of precursor and surfactant in ethanol. Evaporation-induced self-assembly is followed by the formation of a condensed-wall material through thermopolymerization of the precursor oligomers, resulting in mesostructured phenolic resin films. Subsequent decomposition of the surfactant and partial carbonization were achieved through thermal treatment in an inert atmosphere. The films are crack-free with tunable homogeneous thicknesses and show either a 2D-hexagonal or lamellar mesostructure. An additional, yet unknown, three-dimensional (3D) mesostructure was also found. The mesoporous polymer resin films can serve as precursors for various mesoporous carbon structures.

Copper, silver, and tellurium nanowires were electrodeposited into a hierarchical channel structure formed from columnar silica mesophases inside of anodic alumina membranes. The resulting wires were structurally and spectroscopically characterized within the host matrix, in the partially dissolved matrix, and completely removed from the matrix with electron microscopy methods. Plan view images of wires featuring 10 nm diameter within the intact matrix showed the successful replication of the hexagonal arrangement of the columnar mesoporous system. Dissolving only the alumina while leaving the silica mesophase still intact, the long-range organization of the mesoporous system could be visualized. Finally, by completely dissolving the matrix (both alumina and silica), silver wires in the form of bundles of individual wires of about 10 nm diameter could be obtained.

The synthesis of horizontal porous anodic alumina (PAA) structures with individually addressable channel systems is demonstrated. This was achieved by developing a multicontact design of aluminum finger structures (two to five contacts) on silicon wafers. These aluminum contacts were electrically isolated from each other, allowing the individual anodization of each contact at different conditions. This way it is possible to synthesize different pore diameters, pore densities, and channel lengths on a single chip. Scanning electron microscopy (SEM) characterization revealed that the neighboring contacts are not significantly altered during the anodization procedure. After successful barrier-layer thinning, the individual finger structures of each contact were filled by electrodeposition and thermal chemical vapor deposition. The resulting metal (Au, Cu, Ni, Co) and semiconductor (Te, Si) nanowires embedded within the porous anodic alumina mold were characterized by SEM and energy dispersive X-ray measurements. The multicontact fabrication results open a new route toward complex nanoelectronic and sensing applications.