BYU Physics and Astronomy

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.

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.

Self-assembly of organic thiols is the most common S H way to introduce functional groups onto gold surfaces. Although the gold-sulfur (Au-S) bond is moderately strong (similar to 45 kcal/mol), it is also prone to oxidation, which substantially weakens the Au-S interaction. In this work, we describe the creation of more robust molecular assemblies on gold. As a first step, a thiolated monolayer is prepared on gold with an a,alpha,omega-dithiol. Experiments are also reported for a mercaptosilane monolayer on silicon dioxide. An oligomer of polybutadiene (PBd) was then tethered to these surfaces using thiol-ene chemistry. Residual groups on the PBd are then reacted with thiols, including octadecanethiol (ODT), 1H,1H,2H,2H-perfluoroalkanethiol, and a thiol-terminated 25-mer of DNA. Little nonspecific adsorption of a non-thiolated DNA oligomer was observed. Surface characterization was performed with X-ray photoelectron spectroscopy (XPS), contact angle goniometry, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and spectroscopic ellipsometry. A thiol-gold monolayer and an analogous assembly of the same thiol tethered to gold through PBd on a dithiol monolayer were both exposed to air and light for 2 weeks and then rinsed with water. The monolayer on gold was removed in this process, while the thiol in the assembly appeared unaffected.

Porous graphitic carbon (PGC) particles were functionalized/passivated in situ in packed beds at elevated temperature with neat di-tert-amylperoxide (DTAP) in a column oven. The performance of these particles for high performance liquid chromatography (HPLC) was assayed before and after this chemistry with the following analytes: benzene, toluene, ethyl benzene, n-propyl benzene, n-butyl benzene, p-xylene, phenol, 4-methylphenol, phenetole, 3,5-xylenol, and anisole. After the first functionalization/passivation, the retention factors, k, of these compounds decreased by about 5% and the number of theoretical plates (N) increased by ca. 15%. These values of k then remained roughly constant after a second functionalization/passivation but a further increase in N was noticed. In addition, after each of the reactions, the peak asymmetries decreased by ca. 15%, for a total of ca. 30%. The columns were then subjected twice to methanol at 100 C for 5 h at 1 mL/min. After these stability tests, the values of k remained roughly constant, the number of plates increased, which is favorable, and the asymmetries rose and then declined, where they remained below the initial values for the unfunctionalized columns. Functionalized and unfunctionalized particles were characterized by scanning electron microscopy and BET measurements, which showed no difference between the functionalized and unfunctionalized materials, and X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS), where ToF-SIMS suggested some chemical differences between the functionalized and unfunctionalized materials. In particular ToF-SIMS suggested that the expected five-carbon fragments from DTAP exist at higher concentrations on DTAP-functionalized PGC. First principle calculations on model graphitic surfaces suggest that the first addition of a DTAP radical to the surface proceeds in an approximately isothermal or slightly favorable fashion, but that subsequent DTAP additions are then increasingly thermodynamically favorable. Thus, this analysis suggests that the direct functionalization/passivation of PGC with DTAP is plausible. Chemometric analyses of the chromatographic and ToF-SIMS data are also presented. (C) 2011 Elsevier B.V. All rights reserved.