David D. Allred received Alumni Professorship Award

Abstract: This work examines the anisotropic electroless plating of DNA-functionalized gold nanorods attached to a DNA origami template to fabricate continuous metal structures of rectanglar, square, and T shapes. DNA origami, a versatile method for assembling a variety of 2- and 3-D nanostructures, is utilized to construct the DNA breadboard template used for this study. Staple strands on selective sites of the breadboard template are extended with an additional nucleotide sequence for the attachment of DNA-functionalized gold nanorods to the template via base pairing. The nanorod-seeded DNA templates are then introduced into an electroless gold plating solution to determine the extent to which the anisotropic growth of the nanorods is able to fill the gaps between seeds to create continuous structures. Our results show that the DNA-functionalized nanorods grow anisotropically during plating at a rate that is approximately 4 times faster in the length direction than in the width direction to effectively fill gaps of up to 11–13 nm in length. The feasibility of using this directional growth at specific sites to enable the fabrication of continuous metal nanostructures with diameters as thin as 10 nm is demonstrated and represents important progress toward the creation of devices and systems based on self-assembled biological templates.

Abstract: 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.

Abstract: 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.

Abstract: Carbon nanotube (CNT)/polymer composite materials can be high strength, stiff, and lightweight, which makes them attractive for fabrication of micromechanical structures. Here we demonstrate a method whereby smooth, thin, high CNT concentration composite sheets can be fabricated and patterned on the microscale using a process of photolithography and plasma etching. Two types of CNT/polymer composite sheets were fabricated: one made from CNTs grown on patterned supported catalyst and one made from CNTs grown with floating catalyst; these had thicknesses of 6 µm and 26 µm respectively and a roughness of less than 60 nm.

Abstract: In this study, silicon-coated vertically aligned carbon nanotube (Si-VACNT) electrodes were used to examine the impact of encapsulation, which effectively reduced the surface area exposed to the electrolyte. This system is ideal for examining the influence of an electrolyte-blocking layer due to its well-defined geometry and high aspect ratio. The morphology, composition and electrochemical performance of electrodes cycled at different rates were characterized for a range of silicon loadings. Significant differences were observed in the morphology and composition of the electrodes. However, the electrochemical performance was similar, and capacity fading was still observed for the encapsulated electrodes. The impact of the encapsulation layer on lithium transport was examined. Two different transport directions and length scales are relevant–1) radial transport of Li in/out of each silicon-coated nanotube (∼50 nm diameter) and 2) lithium transport along the length of the nanotubes (∼100 μm height). Experimental results indicate that the height of the Si-VACNT electrodes does not limit Li transport, even though that height was orders of magnitude greater than the diameter of the tubes. These results have important implications for a variety of encapsulation strategies.

Abstract: The two microphone transfer function technique is used to measure sound transmission properties of porous screens or membranes in a plane wave tube. This paper will compare sound transmission of porous screens from several manufacturers. Measurements are made with two different plane wave tubes, one of diameter 10.2 cm to measure frequencies between 100 Hz and 2 kHz, and the other of diameter 1.3 cm to measure frequencies between 2 kHz and 16 kHz. Multiple methods of transmission loss measurement and analysis are presented. Special considerations are made to account for the intrinsic losses in the smaller diameter tube.