Selected Publications

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BYU Authors: Kevin Laughlin, Richard R. Vanfleet, and Robert C. Davis, published in ACS Appl. Nano Mater.

Some of the least reflective materials are vertically aligned carbon nanotubes (VACNTs). However, VACNT films are extremely fragile, restricting the range of applications where they can be used. Here, we have formed VACNT/carbon composite films with greater than 500× stiffness and 50× strength compared to as-grown VACNTs while maintaining ultralow reflectance. This is achieved using an infiltration process that coats the individual nanotubes with a thin layer of nanocrystalline carbon, locking the nanotubes together, and then adding lateral subwavelength surface topography to the tops of the VACNTs with an oxygen plasma etch to reduce the reflectance. While still a delicate material, these composites are robust enough to be exposed to water and handled. We demonstrated that these composites can be transferred from the growth substrate to other surfaces resulting in a reflectance of less than 0.1%.acs

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BYU Authors: Berg Dodson, Guohai Chen, Robert Davis, and Richard Vanfleet, published in J. Vac. Sci. Technol. B

Several electrical devices are formed by growing vertically aligned carbon nanotube (CNT) structures directly on a substrate. In order to attain high aspect ratio CNT forest growths, a support layer for the CNT catalyst, usually alumina, is generally required. In many cases, it has been found that current can pass from a conductive substrate, across the alumina support layer, and through the CNTs with minimal resistance. This is surprising in the cases where alumina is used because alumina has a resistivity of 𝜌>1014ρ>1014 Ω cm. This paper explores the mechanism responsible for current being able to cross the alumina support layer with minimal resistance following CNT growth by using scanning transmission electron microscopy imaging, energy dispersive x-ray spectroscopy, secondary ion mass spectroscopy, and two-point current-voltage (I-V) measurements. Through these methods, it is determined that exposure to the carbonaceous gas used during the CNT growth process is primarily responsible for this phenomenon.

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BYU Authors: Scott Olsen, Richard Vanfleet, David Allred, Steve Turley, and Robert Davis, published in Proc. SPIE

We report on a large-area, high-aspect-ratio, carbon nanotube (CNT) forest structure produced at BYU acting as a window/separator for a hollow cathode EUV lamp. The structure has large-surface-area, high light trans-mission, and differential pumping. CNT fabrication allows for variable dimensions, which allows various EUV distributions and pressure gradients to be possible. Theory is presented for predicting such distributions and gradients. Several structures have been fabricated; their dimensions, properties, and predicted distributions and gradients are given.

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BYU Authors: Derric B. Syme, Jason M. Lund, Brian D. Jensen, Robert C. Davis, Richard R. Vanfleet, and Brian D. Iverson, published in ACS Appl. Nano Mater.

The fabrication and examination of a porous silica thin film, potentially for use as an insulating thin film, were investigated. A vertically aligned carbon nanotube (CNT) forest, created by chemical vapor deposition (CVD), was used as scaffolding to construct the porous film. Silicon was deposited on the CNT forest using low-pressure CVD (LPCVD) and then oxidized to remove the CNTs and convert the silicon to silica for electrical or thermal passivation (e.g., thermal barrier). Thermal conductivity was determined using a 1D heat-transfer analysis that equated radiative heat loss in a vacuum with conduction through the substrate and thin film stack. A comparison of the surface temperature differences between a sample film and a reference of comparable thermal resistance enabled determination of the increase in the thermal resistance and of the thermal conductivity of the films. For film thicknesses of approximately 55 μm, the cross-plane thermal conductivity was found to be 0.054–0.071 W m–1 K–1 over 378–422 K. This thermal conductivity value is in the range of other silica aerogels and consistent with the low gravimetric density of 0.15 g cm–3 for the samples. The film is also relatively smooth and flat, with an average arithmetic mean roughness of 1.04 μm.

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BYU Authors: Parker D. Schnepf, Aaron Davis, Brian D. Iverson, Richard Vanfleet, Robert C. Davis, and Brian D. Jensen, published in J. Microelectromech. Syst.

Combining the resolution of conventional gas chromatography systems with the size factor of microGC systems is important for improving the affordability and portability of high performance gas analysis. Recent work has demonstrated the feasibility of high resolution separation of gases in a benchtop-scale short column system by controlling thermal gradients through the column. This work reports a microfabricated thermally controllable gas chromatographic column with a small footprint (approximately 6.25 cm²). The design of the 20 cm column utilizes 21 individually controllable thin film heaters and conduction cooling to produce a desired temperature profile. The reported device is capable of heating and cooling rates exceeding 8000 °C/min and can reach temperatures of 350 °C. The control methods allow for excellent disturbance rejection and precision to within +/- 1 °ree C. Each length of the column between heaters was demonstrated to be individually controllable and displayed quadratic temperature profiles. This paper focuses on the fabrication process and implementation of the thermal control strategy.

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BYU Authors: David Kane, Robert Davis, and Richard Vanfleet, published in J. Vac. Sci. Technol. A

Atomic layer deposition (ALD) of Al2O3 on tall multiwalled carbon nanotube forests shows concentration variation with depth in discrete steps. While ALD is capable of extremely conformal deposition in high aspect ratio structures, decreasing penetration depth has been observed over multiple thermal ALD cycles on 1.3 mm tall multiwalled carbon nanotube forests. Scanning electron microscopy imaging with energy dispersive x-ray spectroscopy elemental analysis shows steps of decreasing intensity corresponding to decreasing concentrations of Al2O3. A study of these steps suggests that they are produced by a combination of diffusion limited precursor delivery and the increase in precursor adsorption site density due to nuclei growing during the ALD process. This conceptual model has been applied to modify literature models for ALD penetration on high aspect ratio structures, allowing two parameters to be extracted from the experimental data. The Knudsen diffusion constant for trimethylaluminum (TMA) in these carbon nanotube forests has been found to be 0.3 cm2 s−1. From the profile of the Al2O3 concentration, the sticking coefficient of TMA in the TMA/water thermal ALD process was found to be 0.003.