Alumina Thin-Film Deposition on Rough Topographies Comprising Vertically Aligned Carbon Nanotubes: Implications for Membranes, Sensors, and Electrodes

Abstract

In this article, the limits of thin-film deposition on very rough topographies are demonstrated by depositing alumina on vertically aligned carbon nanotubes (VACNTs). Vapor deposition techniques are the enabling platforms of the thin-film industry, offering high material versatility and good coverage ability on relatively flat surfaces, leading to frequent use in a large array of applications, especially nanoscale electronic devices such as sensors and electrodes. However, when surface topography exhibits high roughness, even depositions that are not limited to line-of-sight show only partial coverage, significantly hindering performances. Our manufacturing process of VACNT/Al2O3 nanocomposites has three vaporous steps: CNT growth by chemical vapor deposition (CVD), functionalization via controlled thermal oxidation, and atomic layer deposition (ALD) of alumina. The same limited accessibility hinders each of these three steps. Morphological analyses show different CNT heights throughout the sample, with shorter CNTs in the middle, having less access to gases. As height differences between the center and peripheries escalate, sample centers may collapse under the tension. The limited accessibility of the center is manifested also in inhomogeneous oxygen contents, between sample centers and peripheries. Finally, a sharp transition in deposition quality occurs during the deposition process of Al2O3, from homogeneous to inconsistent, which is also linked to the accessibility differences between. Adjusting process parameters, we have successfully coated 1.8 mm-tall VACNT arrays with a homogeneous thin (few nm) Al2O3 layer and were able to increase the depth at which, thick (few dozens of nm) Al2O3 coating is uniform from 20 to 350 μm. However, when VACNTs were functionalized, the penetration depth was found to correlate negatively with center oxygen content. These results, indicating diffusion as a rate-setting step in complex topography coatings, can significantly improve deposition quality and enhance the performance of thin-film applications such as membranes, sensors, and electrodes for energy harvesting and storage.

Frictional Behavior of Alumina-Coated Vertically Aligned Carbon Nanotube Forests: Implications for Micro and Nano Electromechanical Devices

“…The coating thickness across the depth of the CNT forest may not be homogeneous due to highly dense CNT forest structure. 25 The investigated cross-sectional morphology of the alumina-coated VACNT forest shows non-uniform coating of alumina across the height, as shown in Figure 5. Figure 5a shows the 20 nm coated VACNT forest sample, which was cut in half to expose the middle section.…”

Atomic Layer Deposition of Ultrathin Al2O3 Layer on Carbon Nanotube Anodes for Potassium Ion Batteries with High Initial Coulombic Efficiency and Electrochemical Performance

“…[24] Furthermore, the CNT powders tend to agglomerate due to the large van der Waals forces, resulting in poor coverage of the metal oxide. [24,25] The chemical and/or physical pretreatments of CNTs are frequently used to increase coverage by introducing new functional groups and/or defects. [26] These defects increase the number of electron and hole recombination sites, which is detrimental to the original electrical characteristics.…”