Surface Modification of Etched Ion-Track Polymer Membranes by Atomic Layer Deposition

Inorganic nanochannels integrated in solid state membranes as well as nanotubes are of high relevance for fundamental research and industrial applications in catalysis, filtration, sensorics, solar energy harvesting, biomedicine, and nanofluidics. Currently, lots of effects are being devoted to develop reproducible and efficient fabrication techniques that enable the precise tailoring of geometry, dimensions, and properties of the nanochannels. This thesis presents the combination of ion-track technology with low-temperature atomic layer deposition (ALD). Cylindrical and conical nanotubes as well as highly-ordered nanotube networks exhibiting aspect ratios above 3000 were synthesized. 30-μm thick polycarbonate membranes were irradiated with ∼GeV swift heavy ions at the UNILAC accelerator of GSI under normal or tilted beam incidence. By subsequent wet-chemical etching, each individual ion track was dissolved and thus converted into an open nanochannel. The length of the nanochannel was determined by the thickness of the polymer, whereas geometry and diameter (≥ 18 nm) were controlled by the etching parameters. To reduce the channel diameter further and modify the surface of the channel walls without affecting the channel shape, titania (TiO2), silicon dioxide (SiO2), and alumina (Al2O3) were deposited onto the templates by ALD. This sequential and self-limiting surface modification technique provided the precise control of the deposited thickness due to layer-by-layer growth. From small angle X-ray scattering (SAXS) analysis average channel diameter, diameter distribution before and after ALD as well as coating thickness were deduced. The results demonstrated homogeneous and conformal deposition along the entire cylindrical nanochannels down to inner diameters below 10 nm. For these samples, X-ray photoelectron spectroscopy (XPS) evidenced almost stoichiometric composition of the ALD layers deposited onto the membrane surface. For all investigated nanostructures, the dissolution of the supporting polymer template by wet-chemical methods and the following visualization of the resulting structures by electron microscope (SEM) in scanning and transmission mode revealed exactly defined geometries, diameters, and wall thicknesses. The preparation of arrays of free-standing conical nanotubes from multichannel membranes and the novel alike preparation of free-standing single tubular nanocones from single channel membranes enabled the comparison between the asymmetric etching process in single and multichannel membranes that resulted in the agreement of the radial base etching rates whereas the tip etching was by a factor of ∼ 2 faster for single channels. In addition, the base diameter was determined at the replica of the single channel itself, which enabled precise computation of the tip diameter. The homogeneity of the ALD processes inside the nanochannels was confirmed by energy dispersive X-ray spectroscopy (EDX) of the released nanotubes. In addition, ionic conductance (I-V) studies of cylindrical and conical single nanochannels before and after ALD demonstrated conformal deposition processes inside the single channels in polycarbonate. Surface charges induced by variation of the pH value of the electrolyte influenced the recorded ionic currents in agreement with the theory of nanofludics for channel diameters up to 100 nm. Furthermore, the gating of single conical nanochannels in polycarbonate membranes surface modified with a 5 nm thick TiO2 film was enabled by a straight-forward set-up comparable to n-type JFETs