Atomic layer deposition (ALD) can be used to deposit ultra-thin and conformal films on flat substrates, high aspect ratios structures and particles. In this paper, we demonstrate that insulating, multilayered and functionalized ALD coatings can also be deposited conformally on carbon nanotubes. Multilayered coatings consisting of alternating layers of dielectric and conductive materials, such as Al2O3 and W, respectively, are deposited on conductive multi-walled carbon nanotubes. This coated carbon nanotube can function as a nanoscale coaxial cable. Thin layers of Al2O3 ALD are also used as a seed layer to functionalize nanotubes. A carbon nanotube was made highly hydrophobic using an Al2O3 ALD seed layer followed by the attachment of perfluorinated molecules.

Carbon nanotubes have unique properties of interest for applications in aerospace, electronics, and biotechnology. However, the properties of different batches of carbon nanotubes can vary considerably depending on chemical purity and the nanotube types present (e.g., diameter and chirality distribution). Distinguishing the constituents of each nanotube batch is challenging, with many different techniques used in concert. Thermogravimetric analysis (TGA) provides one measure of nanotube purity by assessing the material's thermal stability (i.e., how it oxidizes with temperature). Unfortunately, however, TGA analysis requires a relatively large specimen for each measurement (several milligrams), making it inappropriate for rapid screening of incoming materials. Moreover, the measurement provides only an average purity for the analyzed sample, and variability can occur on a much finer level. As many applications will utilize only a small quantity of nanotubes, new approaches are needed to assess variability for a much smaller specimen size. This paper describes a new analysis method that uses a quartz crystal as a miniature microbalance for determining mass changes at elevated temperature. Thin nanotube coatings are spray deposited onto the crystals, and shifts in a crystal's resonance frequency are directly correlated with changes in coating mass during heating due to volatilization of different carbon species. By monitoring the response of the crystal at one or more temperatures, different nanotube specimens can be directly compared. This paper demonstrates concept feasibility by comparing quartz crystal results with conventional TGA analysis and discusses methods for applying the technique in process and quality control settings.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.