BYU Physics and Astronomy

Microscale porous carbon mechanical resonators were formed using carbon nanotube templated microfabrication. These cantilever resonators exhibited nanoscale porosity resulting in a high surface area to volume ratio which could enable sensitive analyte detection in air. These resonators were shown to be mechanically robust and the porosity could be controllably varied resulting in densities from 102 to 103 kg m−3, with pore diameters on the order of hundreds of nanometers. Cantilevers with lengths ranging from 500 μm to 5 mm were clamped in a fixture for mechanical resonance testing where quality factors from 102 to 103 were observed at atmospheric pressure in air.

Test disk electrodes were fabricated from carbon nanotubes (CNT) using the Carbon Nanotube Templated Microfabrication (CNT-M) technique. The CNT-M process uses patterned growth of carbon nanotube forests from surfaces to form complex patterns, enabling electrode sizing and shaping. The additional carbon infiltration process stabilizes these structures for further processing and handling. At a macroscopic scale, the electrochemical, electrical and magnetic properties, and magnetic resonance imaging (MRI) characteristics of the disk electrodes were investigated; their microstructure was also assessed. CNT disk electrodes showed electrical resistivity around 1 Ω·cm, charge storage capacity between 3.4 and 38.4 mC/cm2, low electrochemical impedance and magnetic susceptibility of −5.9 to −8.1 ppm, closely matched to that of tissue (∼−9 ppm). Phantom MR imaging experiments showed almost no distortion caused by these electrodes compared with Cu and Pt-Ir reference electrodes, indicating the potential for significant improvement in accurate tip visualization.

Microelectrode arrays of carbon nanotube (CNT)/carbon composite posts with high aspect ratio and millimeter-length were fabricated using carbon-nanotube-templated microfabrication with a sacrificial “hedge”. The high aspect ratio, mechanical robustness, and electrical conductivity of these electrodes make them a potential candidate for next-generation neural interfacing. Electrochemical measurements were also demonstrated using an individual CNT post microelectrode with a diameter of 25 μm and a length of 1 mm to perform cyclic voltammetry on both methyl viologen and dopamine in a phosphate-buffered saline solution. In addition to detection of the characteristic peaks, the CNT post microelectrodes show a fast electrochemical response, which may be enabling for in vivo and/or in vitro measurements. The CNT post electrode fabrication process was also integrated with other microfabrication techniques, resulting in individually addressable electrodes.

A nanoporous carbon monolith structure has been developed for use as a scaffold for silicon anodes for lithium batteries. This scaffold was fabricated by coating vertically aligned carbon nanotubes in a highly conformal coating of nanocrystalline carbon, applied via atmospheric pressure chemical vapor deposition. The coating increases the mechanical stability of the nanotube structure, which provides electrically conductive pathways through the anode. Silicon anodes were fabricated with the monoliths by low pressure chemical vapor infiltration of silicon. This platform allows the carbon and silicon volume fractions to be independently varied in the anode. Anodes with a low silicon content (less than 5% by volume) showed high stability in cycling against lithium with a capacity retention of 89.7% between cycles 2 and 185. Anodes with a high silicon content (∼25% by volume) showed poor capacity retention when the carbon content was low (<40% by volume), and transmission electron microscopy analysis indicated that the anodes failed due to the destruction of the nanocrystalline carbon coating during cycling. However, by increasing the carbon content to ∼60% volume percent in the monolith, capacity retention was substantially stabilized even for anodes with very high silicon loadings. These stabilized electrodes exhibited volumetric capacities as high as ∼1000 mA h/ml and retained over 725 mA h/ml by cycle 100.

In this study, we have fabricated nanofuses from thin-film, arc-deposited carbon for use in permanent data storage. Thin-film carbon fuses have fewer fabrication barriers and retain the required resistivity and structural stability to act as a data-storage medium. Carbon thin films were characterized for their electrical, microstructural, and chemical bonding properties. Annealing these films in an argon environment at 400 °C reduced the resistivity from about 4 × 10–2 Ω cm as deposited to about 5 × 10–4 Ω cm, allowing a lower blowing voltage. Nanofuses with widths ranging from 200 to 60 nm were fabricated and tested. They blow with voltages between 2 and 5.5 V, and the nanofuses remain stable in both “1” and “0” states under a constantly applied read voltage of 1 V for over 90 h.