David D. Allred received Alumni Professorship Award

Selected Publications

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Ozone priming of patterned carbon nanotube forests for subsequent atomic layer deposition-like deposition of SiO2 for the preparation of microfabricated thin layer chromatography plates
David S. Jensen, Supriya S. Kanyal, Nitesh Madaan, Robert C. Davis, Richard Vanfleet, and Matthew R. Linford (et al.)
The authors report the ozonation of patterned, vertically aligned carbon nanotube (CNT) forests as a method of priming them for subsequent pseudo atomic layer deposition (psi-ALD) (alternating layer deposition) of silica to produce microfabricated, CNT-templated thin layer chromatography (TLC) plates. Gas phase ozonation simplifies our deposition scheme by replacing two steps in our previous fabrication process: chemical vapor deposition of carbon and ALD of Al2O3, with this much more straightforward priming step. As shown by x-ray photoelectron spectroscopy (XPS), ozonation appears to prime/increase the number of nucleation sites on the CNTs by oxidizing them, thereby facilitating conformal growth of silica by psi-ALD, where some form of priming appears to be necessary for this growth. (As shown previously, psi-ALD of SiO2 onto unprimed CNTs is ineffective and leads to poor quality depositions.) In conjunction with a discussion of the challenges of good peak fitting of complex C 1s XPS narrow scans, the authors present an analysis of their C 1s data that suggests an increase in oxidized carbon, particularly the C=O group, with increasing oxygen content of the CNT forests. After coating with SiO2, the CNTs are removed by elevated temperature air oxidation, the SiO2 is rehydrated, and the plates are coated with 3-aminopropyltriethoxysilane (APTES). The resulting APTES-coated plates separate various fluorescent dyes giving results that are generally at least as good as those the authors reported previously with their more complicated fabrication/priming scheme. TLC plates with different geometries are microfabricated, where plates with narrower channels show longer run times (lower mobile phase velocities) and plates with narrower features appear to give higher efficiencies. (C) 2013 American Vacuum Society.
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Effects of catalyst thickness on the fabrication and performance of carbon nanotube-templated thin layer chromatography plates
Supriya S. Kanyal, Rebecca Olsen, Richard R. Vanfleet, Robert C. Davis, and Matthew R. Linford (et al.)
The effects of iron catalyst thickness on the fabrication and performance of microfabricated, binder-free, carbon nanotube (CNT)-templated, thin layer chromatography (TLC) plates are demonstrated. The iron catalyst was deposited at thicknesses ranging from 4 to 18 nm in increments of 2 nm. Its thickness plays a key role in governing the integrity and separation capabilities of microfabricated TLC plates, as determined using a test dye mixture. Atomic force microscopy and scanning electron microscopy show that smaller and more numerous catalyst nanoparticles are formed from thinner Fe layers, which in turn govern the diameters and densities of the CNTs. The average diameter of the Fe nanoparticles, D-p, is approximately six times the initial Fe film thickness, t(Fe): D-p approximate to t(Fe). After deposition of relatively thick silicon layers on CNTs made with different Fe thicknesses, followed by oxidation, all of the resulting CNT-templated SiO2 wires had nearly the same diameter. Consequently, their surface areas were very similar, although their areal densities on the TLC plates were not because thinner catalyst layers produce denser CNT forests. For t(Fe)=6 nm, nanotube growth appears to be base growth, not tip growth. Best TLC separations of a test dye mixture were obtained with plates prepared with 6 or 4 nm of catalyst. Calculations suggest a loss of surface area for TLC plates made with thicker Fe layers as a result of fewer, thicker CNTs, where the density of silica nanotubes (device surface area) goes approximately as 1/t(Fe)(2). While the focus of this paper is toward a greater understanding of the processing conditions that lead to the best TLC plates, a baseline separation of three analgesics (caffeine, phenacetine, and propyphenazone) is shown on a normal phase TLC plate grown with 6 nm of iron. (C) 2013 American Vacuum Society.
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Synthesis of V, Pt and Pt-V colloidal nanoparticles
Bimetallic platinum-vanadium nanoparticles have been successfully synthesised by high temperature thermal decomposition of metal precursor salts. Vanadium is a highly reactive metal and poses considerable difficulty in synthesising metallic nanoparticles, while platinum has low reactivity where nanoparticles are easily synthesised. The difficulties associated with the reduction of vanadium salt were circumvented by the use of high boiling point solvents and selected surfactants. Co-reduction using platinum precursors compatible with the high temperature processes was required for bimetallic nanoparticle synthesis. The chemical synthesis route described is novel, robust and highly reproducible. Microstructural characterisation of nanoparticles synthesised as described, using transmission electron microscopy, reveals single-crystal particles.
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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.