David D. Allred received Alumni Professorship Award

Selected Publications

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Stable, microfabricated thin layer chromatography plates without volume distortion on patterned, carbon and Al2O3-primed carbon nanotube forests
David S. Jensen, Supriya S. Kanyal, Vipul Gupta, Richard Vanfleet, Robert C. Davis, and Matthew R. Linford (et al.)
Some of us recently described the fabrication of thin layer chromatography (TLC) plates from patterned carbon nanotube (CNT) forests via direct infiltration/coating of the CNTs by low pressure chemical vapor deposition (LPCVD) of silicon from SiH4, followed by high temperature oxidation of the CNTs and Si. Herein we present an improved microfabrication process for the preparation of these TLC plates. First, a few nanometers of carbon and/or a thin film of Al2O3 is deposited on the CNTs. This method of priming the CNTs for subsequent depositions appears to be new. X-ray photoelectron spectroscopy confirms the presence of additional oxygen after carbon deposition. After priming, the plates are coated by rapid, conformal deposition of an inorganic material that does not require subsequent oxidation, i.e., by a fast pseudo atomic layer deposition (psi-ALD) of SiO2 from trimethylaluminum and tris(tert-butoxy)silanol. Unlike devices described previously, faithful reproduction of the features in the masks is still observed after oxidation. A bonded, amino phase on the resulting plates shows fast, highly efficient separations of fluorescent dyes (plate heights in the range of 1.6-7.7 mu m). Extensive characterization of the new materials by TEM. SEM, EDAX, DRIFT, and XPS is reported. A substantially lower process temperature for the removal of the CNT scaffold is possible as a result of the already oxidized materials used. (C) 2012 Elsevier B.V. All rights reserved.
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Shock Testing of Free Standing Silicon-Nitride and Beryllium Membranes
David Brough, Lawrence Barrett, Richard Vanfleet, Sterling Cornaby, and Robert C. Davis (et al.)

As micro and nanotechnology continue to advice into products, durability, reliability and robustness become important factors. One application where micro technology needs such qualities is X-ray windows. X-ray windows consist of free standing thin film membranes made from low Z elements. An ideal X-ray window is thin enough to allow for soft X-ray transmission and yet is strong enough to maintain a vacuum. X-ray windows are used to analyze samples in microscopes and hand held devices for mining and other applications. These membranes in hand held devices need to be able to withstand impacts due to dropping or jarring of the device. Shock test studies have been performed on electronics and membranes related to biological system, but literature showing the robustness of free standing membranes is not ready found. In this study free standing thin film membranes’ ability to withstand repeated shocks created by using a bar contact pendulum shock apparatus is investigated. A comparison of shock resistance of X-ray window membrane materials specifically silicon nitride and beryllium will be presented.

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Low-Z, Chemically Resistant, Microfabricated Carbon Composite Transmission Electron Microscope Grids
K. Zufelt, R. Vanfleet, and R. C. Davis

An issue that often impacts x-ray and electron analysis of transmission electron microscopy (TEM) samples is the presence of high-Z atoms in the sample substrate. In many cases, it is also desirable that the chosen substrate be resistant to chemicals and various processing methods. We present an improved TEM grid made by carbon-infiltrated carbon nanotube templated microfabrication (CNT-M). These grids provide a significant advantage in analytical TEM applications due to the absence of high-Z atoms and the improved chemical resistivity which allows for a wider range of sample preparation and processing techniques. We have refined the CNT-M process by developing a method for preventing delamination of highly carbon-infiltrated CNT-M structures from the growth substrate. We further present a scalable method for suspending thin films (<30 nm) across large gaps (>100 um) between CNT-M defined features. Several membranes were deposited on the grids including amorphous carbon, boron carbide, silicon dioxide, and alumina. These results are of significance to CNT-M MEMS design and production.

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Ultrasonic Spraying of Carbon Nanotubes using NMP
Anthony D. Willey, Josh M. Holt, Richard R. Vanfleet, and Robert C. Davis (et al.)

The objective of this study was to test the mechanical durability of a polyurethane/organoclay nanocomposite modified with perfluoroalkyl methacrylic copolymer in conditions that replicate extended rain impact. Samples were impacted with 1.4 mm droplets at a velocity of 24 m/s and a flow rate of 0.78 gpm for a period of 5 hours by an axial full cone nozzle. The cases of the spray nozzle being placed vertically above a sample as well as at a 40° angle were examined. After the spray period, samples were heated at 100° C to allow saturated liquid to evaporate from the surface. Contact angle and sliding of the superhydrophobic surfaces were measured before and after the spray period. A decrease in performance for both samples was observed, with the vertically sprayed sample seeing greater degradation. SEM images of superhydrophobic samples before and after spray impact revealed large amounts of circular indentations on the surfaces caused by the impacting droplets which, along with leaching of the fluoroacrylic copolymer, was likely the cause of the decrease in performance.

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Effect of iron catalyst thickness on vertically aligned carbon nanotube forest straightness for CNT-MEMS
Kellen Moulton, Nicholas B. Morrill, Adam M. Konneker, Brian D. Jensen, Richard R. Vanfleet, David D. Allred, and Robert C. Davis
This paper examines the effect of iron catalyst thickness on the straightness of growth of carbon nanotubes (CNTs) for microelectromechanical systems fabricated using the CNT-templated-microfabrication (CNT-M) process. SEM images of samples grown using various iron catalyst thicknesses show that both straight sidewalls and good edge definition are achieved using an iron thickness between 7 and 8 nm. Below this thickness, individual CNTs are well aligned, but the sidewalls of CNT forests formed into posts and long walls are not always straight. Above this thickness, the CNT forest sidewalls are relatively straight, but edge definition is poor, with significantly increased sidewall roughness. The proximity of a device or feature to other regions of iron catalyst also affects CNT growth. By using an iron catalyst thickness appropriate for straight growth, and by adding borders of iron around features or devices, a designer can greatly improve straightness of growth for CNT-MEMS.
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Electron back-scattered diffraction of crystallized vanadium dioxide thin films on amorphous silicon dioxide
Crystalline films and isolated particles of vanadium dioxide (VO2) were obtained through solid phase crystallization of amorphous vanadium oxide thin films sputtered on silicon dioxide. Electron back-scattered diffraction (EBSD) was used to study the crystals obtained in the thin films, to differentiate them from different vanadium oxide stoichiometries that may have formed during the annealing process, and to study their phase and orientation. EBSD showed that the crystallization process yielded crystalline vanadium dioxide thin films, semi-continuous thin films, and films of isolated particles, and did not show evidence of other vanadium oxide stoichiometries present. Indexing of the crystals for the orientation study was performed using EBSD patterns for the tetragonal phase of vanadium dioxide, since it was observed that EBSD patterns for the monoclinic and tetragonal phases of vanadium dioxide are not distinguishable by computer automated indexing. Using the EBSD patterns for the tetragonal phase of vanadium dioxide, orientation maps showed that all VO2 crystals that were measurable (approximately the thickness of the film) had a preferred orientation with the c-axis of the tetragonal phase parallel to the plane of the specimen. (C) 2011 Elsevier B.V. All rights reserved.