Extreme high apspect ratio nanotubes in polymer membranes produced by catalytic ALD

German scientists from Darmstadt and Hamburg has shown that the combination of ion-track technology and ALD provides unique opportunities for highly homogeneous and conformal coatings of extremely long nanochannels. The results clearly demonstrate successful conformal coating of cylindrical 30 μm long nanochannels with initial diameter between 55 and 18 nm by three different inorganic materials (TiO2, SiO2, and Al2O3). The ALD process was carefully adjusted to temperatures low enough to avoid damage to the ion-track etched polymer membranes.

 

(a) Flexible SiO₂ nanotubes exhibiting an outer diameter of ~50 nm and a wall thickness of ~20 nm. (b) Al₂O₃ nanotubes (outer diameter ~50 nm, wall thickness ~15 nm), which are broken due to their rather high brittleness. They are attached to the flat Al₂O₃-film deposited on the polycarbonate surface. (c) TiO₂ nanotubes (outer diameter ~100 nm due to 240 s of etching time, wall thickness ~10 nm) with a length corresponding to the template thickness.

Interestingly they have used the pyridine catalyzed process for H2O low temperature SiO2 (SiCl4) and for TiO2 (titanium isopropoxide). I have never seen it used for other than SiCl4 and HCDS for growing SiO2. This got me curious to know if there is any work done with other metal chlorides - you know the usual suspects - Zr, Hf, Ta, ...

Check out all the details and the experimental part especially in the OPEN ACCESS paper below!

TiO2, SiO2, and Al2O3 coated nanopores and nanotubes produced by ALD in etched ion-track membranes for transport measurements [OPEN ACCESS]

Anne Spende, Nicolas Sobel, Manuela Lukas, Robert Zierold, Jesse C Riedl, Leonard Gura, Ina Schubert, Josep M Montero Moreno, Kornelius Nielsch, Bernd Stühn, Christian Hess, Christina Trautmann and Maria E Toimil-Molares

Published 30 July 2015 • © 2015 IOP Publishing Ltd, Nanotechnology, Volume 26, Number 33

Scheme of fabrication of TiO₂, SiO₂, and Al₂O₃ coated track-etched membranes. (a) Polycarbonate foils are irradiated with high-energy heavy ions; each projectile creates an individual ion track; (b) chemical etching converts ion tracks into cylindrical nanochannels of well-defined diameter; (c) ALD of TiO₂, SiO₂, and Al₂O₃ produces conformal homogeneous coatings.

Low-temperature atomic layer deposition (ALD) of TiO2, SiO2, and Al2O3 was applied to modify the surface and to tailor the diameter of nanochannels in etched ion-track polycarbonate membranes. The homogeneity, conformity, and composition of the coating inside the nanochannels are investigated for different channel diameters (18–55 nm) and film thicknesses (5–22 nm). Small angle x-ray scattering before and after ALD demonstrates conformal coating along the full channel length. X-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy provide evidence of nearly stoichiometric composition of the different coatings. By wet-chemical methods, the ALD-deposited film is released from the supporting polymer templates providing 30 μm long self-supporting nanotubes with walls as thin as 5 nm. Electrolytic ion-conductivity measurements provide proof-of-concept that combining ALD coating with ion-track nanotechnology offers promising perspectives for single-pore applications by controlled shrinking of an oversized pore to a preferred smaller diameter and fine-tuning of the chemical and physical nature of the inner channel surface.