Selected Publications
Lauren R. Richey, K.C. Erb, Conrad W. Rosenbrock, Lance J. Nelson, Richard R. Vanfleet, Harold T. Stokes, Branton J. Campbell, and Gus L. W. Hart (et al.)
Experimentally and computationally, the structure of Pt–Cu at 1:3 stoichiometry has a convoluted history. The L13 structure has been predicted to occur in binary alloy systems, but has not been linked to experimental observations. Using a combination of electron diffraction, synchrotron X-ray powder diffraction, and Monte Carlo simulations, we demonstrate that it is present in the Cu–Pt system at 1:3 stoichiometry. We also find that the 4-atom, fcc superstructure L13 is equivalent to the large 32-atom orthorhombic superstructure reported in older literature, resolving much of the confusion surrounding this composition. Quantitative Rietveld analysis of the X-ray data and qualitative trends in the electron-diffraction patterns reveal that the secondary X1+(a,0,0) order parameter of the L13 phase is unexpectedly weak relative to the primary L1+(a,a,0,0) order parameter, resulting in a partially-ordered L13 ordering, which we conclude to be the result of kinetic limitations. Monte Carlo simulations confirm the formation of a large cubic superstructure at high temperatures, and its eventual transformation to the L13 structure at lower temperature, but also provide evidence of other transitional orderings.
Supriya S. Kanyal, Richard R. Vanfleet, Robert C. Davis, and Matthew R. Linford (et al.)
We describe the direct, conformal, atomic layer deposition (ALD) of silica onto carbon nanotubes (CNTs) in the microfabrication of thinlayer chromatography (TLC) plates. As before, these plates were prepared with zig-zag hedge and channel microstructures, with high aspect ratio, porous hedges. After ALD, scanning electron microscopy (SEM) showed an increase in the radius of the CNTs of 8–40 nm. X-ray photoelectron spectroscopy (XPS) showed that the plates were composed almost entirely of silicon and oxygen, without contamination of metals or other elements that might compromise chromatographic performance, e.g., aluminum. Time-of-flight secondary ion mass spectrometry confirmed the extremely low level of aluminum in the plates. The final TLC layer thickness was ca. 50 μm. Separations of a test mixture of dyes from CAMAG (Muttenz, Switzerland) on an uncoated silica plate under traditional, normal phase conditions gave efficiencies of 40,000–140,000 plates m−1 with migration distances ranging from 2 to 36 mm. A separation of two fluorescent dyes, eosin Y disodium salt and sulforhodamine B, on an amino silane-coated plate gave efficiencies of ca. 170,000 and 200,000 plates m−1, with hRF values of 76 and 88, respectively. Run times on these new plates were much faster than on conventional TLC plates.
Anthony D. Willey, Richard R. Vanfleet, and Robert C. Davis (et al.)
Described is a method for ultrasonically spraying thin films of carbon nanotubes that have been suspended in organic solvents. Nanotubes were sonicated in N -methyl-2-pyrrolidone or N -cyclohexyl-2-pyrrolidone (CHP) and then sprayed onto a heated substrate using an ultrasonic spray nozzle. The solvent was quickly evaporated, leaving a thin film of randomly oriented nanotubes. Unlike other methods of spraying nanotube films, this does not require removal of surfactant after spraying and is compatible with creating films of functionalized nanotubes. Film thickness was controlled by the spray time and films were sprayed with thicknesses between 10 and 500 nm. Single-walled, multiwalled, and functionalized multiwalled nanotubes were sprayed. Transparent conducting thin films prepared by spraying single-walled carbon nanotubes dispersed in CHP demonstrated similar sheet resistance (for a given optical transmittance) as those prepared by spraying aqueous polymer-based dispersions that required postdeposition polymer removal.
David S. Jensen, Supriya S. Kanyal, Nitesh Madaan, Jared M. Hancock, Richard Vanfleet, and Matthew R. Linford (et al.)
We apply a suite of analytical tools to characterize materials created in the production of microfabricated thin layer chromatography plates. Techniques used include X-ray photoelectron spectroscopy (XPS), valence band spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS) in both positive and negative ion modes, Rutherford backscattering spectroscopy (RBS), and helium ion microscopy. Materials characterized include: the Si(100) substrate with native oxide: Si/SiO2, alumina (35nm) deposited as a diffusion barrier on the Si/SiO2: Si/SiO2/Al2O3, iron (6nm) thermally evaporated on the Al2O3: Si/SiO2/Al2O3/Fe, the iron film annealed in H-2 to make Fe catalyst nanoparticles: Si/SiO2/Al2O3/Fe(NP), and carbon nanotubes (CNTs) grown from the Fe nanoparticles: Si/SiO2/Al2O3/Fe(NP)/CNT. The Fe films and nanoparticles appear in an oxidized state. Some of the analyses of the CNTs/CNT forests appear to be unique: (i) the CNT forest appears to exhibit an interesting channeling' phenomenon by RBS, (ii) we observe an odd-even effect in the SIMS spectra of C-n(-) species for n=1 - 6, with the n6 ions showing a steady decrease in intensity, and (iii) valence band characterization of CNTs using X-radiation is reported. Initial analysis of the CNT forest by XPS shows that it is 100at.% carbon. After one year, only ca. 0.25at.% oxygen is observed. The information obtained from the combination of the different analytical tools provides a more complete understanding of our materials than a single technique, which is analogous to the story of The Blind Men and the Elephant'. The raw XPS and ToF-SIMS spectra from this study will be submitted to Surface Science Spectra for archiving. Copyright (c) 2013 John Wiley & Sons, Ltd.
Transmission electron microscopy (TEM) and focused ion beam (FIB) are proven tools to produce site-specific samples in which to study devices from initial processing to causes for failure, as well as investigating the quality, defects, interface layers, etc. However, the use of polymer substrates presents new challenges, in the preparation of suitable site-specific TEM samples, which include sample warping, heating, charging, and melting. In addition to current options that address some of these problems such as cryo FIB, we add an alternative method and FIB sample geometry that address these challenges and produce viable samples suitable for TEM elemental analysis. The key feature to this approach is a larger than usual lift-out block into which small viewing windows are thinned. Significant largely unthinned regions of the block are left between and at the base of the thinned windows. These large unthinned regions supply structural support and thermal reservoirs during the thinning process. As proof-of-concept of this sample preparation method, we also present TEM elemental analysis of various thin metallic films deposited on patterned polycarbonate, lacquer, and poly-di-methyl-siloxane substrates where the pattern (from low-to high-aspect ratio) is preserved.
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.