Temperature-dependent subsurface growth during atomic layer deposition on polypropylene and cellulose fibers.

Nucleation and subsequent growth of aluminum oxide by atomic layer deposition (ALD) on polypropylene fiber substrates is strongly dependent on processing temperature and polymer backbone structure. Deposition on cellulose cotton, which contains ample hydroxyl sites for ALD nucleation and growth on the polymer backbone, readily produces a uniform and conformal coating. However, similar ALD processing on polypropylene, which contains no readily available active sites for growth initiation, results in a graded and intermixed polymer/inorganic interface layer. The structure of the polymer/inorganic layer depends strongly on the process temperature, where lower temperature (60 degrees C) produced a more abrupt transition. Cross-sectional transmission electron microscopy images of polypropylene fibers coated at higher temperature (90 degrees C) show that non-coalesced particles form in the near-surface region of the polymer, and the particles grow in size and coalesce into a film as the number of ALD cycles increases. Quartz crystal microbalance analysis on polypropylene films confirms enhanced mass uptake at higher processing temperatures, and X-ray photoelectron spectroscopy data also confirm heterogeneous mixing between the aluminum oxide and the polypropylene during deposition at higher temperatures. The strong temperature dependence of film nucleation and subsurface growth is ascribed to a relatively large increase in bulk species diffusivity that occurs upon the temperature-driven free volume expansion of the polypropylene. These results provide helpful insight into mechanisms for controlled organic/inorganic thin film and fiber materials integration.

Atomic layer deposition and abrupt wetting transitions on nonwoven polypropylene and woven cotton fabrics.

Atomic layer deposition (ALD) of aluminum oxide on nonwoven polypropylene and woven cotton fabric materials can be used to transform and control fiber surface wetting properties. Infrared analysis shows that ALD can produce a uniform coating throughout the nonwoven polypropylene fiber matrix, and the amount of coating can be controlled by the number of ALD cycles. Upon coating by ALD aluminum oxide, nonwetting hydrophobic polypropylene fibers transition to either a metastable hydrophobic or a fully wetting hydrophilic state, consistent with well-known Cassie-Baxter and Wenzel models of surface wetting of roughened surfaces. The observed nonwetting/wetting transition depends on ALD process variables such as the number of ALD coating cycles and deposition temperature. Cotton fabrics coated with ALD aluminum oxide at moderate temperatures were also observed to transition from a natural wetting state to a metastable hydrophobic state and back to wetting depending on the number of ALD cycles. The transitions on cotton appear to be less sensitive to deposition temperature. The results provide insight into the effect of ALD film growth mechanisms on hydrophobic and hydrophilic polymers and fibrous structures. The ability to adjust and control surface energy, surface reactivity, and wettability of polymer and natural fiber systems using atomic layer deposition may enable a wide range of new applications for functional fiber-based systems.

Bi-directional Kirkendall effect in coaxial microtube nanolaminate assemblies fabricated by atomic layer deposition.

The solid-state reaction within a coaxial Al2O3/ZnO/Al2O3 multilayered microtubular structure can be used to prepare discrete microtube-in-microtube ZnAl2O4 spinel assemblies through a Kirkendall void production mechanism at 700 degrees C. In contrast with previous studies of the nanoscale Kirkendall effect, the reaction observed here proceeds through a bi-directional vacancy diffusion mechanism wherein ZnO species diffuse into inner- and outer-Al2O3 concentric layers, thereby resulting in vacancy supersaturation and void production between two isolated spinel microtubes. Low-temperature atomic layer deposition (ALD) of Al2O3 and ZnO enables the fabrication of complex coaxial multilayered microtubes with precise control of the starting film thicknesses and relative composition. When a molar excess of ZnO is present between two Al2O3 layers, electron microscopy images reveal incomplete ZnO consumption after annealing at 700 degrees C. At higher initial Al2O3 concentrations, however, complete reaction with ZnO is observed, and the size of the Kirkendall gap between isolated spinel microtubes appears to be directly influenced by the thickness of the intermediate ZnO layer.

Atomic layer deposition on electrospun polymer fibers as a direct route to AL2O3 microtubes with precise wall thickness control.

Atomic layer deposition (ALD) of Al2O3 on electrospun poly(vinyl alcohol) microfiber templates is demonstrated as an effective and robust strategy by which to fabricate long and uniform metal-oxide microtubes. The wall thickness is shown to be precisely controlled within a molecular layer or so by adjusting the number of ALD cycles utilized. By judicious selection of the electrospinning and ALD parameters, designer tubes of various sizes and inorganic materials can be synthesized.