David D. Allred received Alumni Professorship Award

Selected Publications

D. D. Allred (et al.)

Over the last few years many ion beam techniques have been reported for the profiling of hydrogen in materials. We have evaluated nine of these using similar samples of hydrogen ion-implanted into silicon. When possible the samples were analysed using two or more techniques to confirm the ion-implanted accuracy. We report the results of this work which has produced a consensus profile of H in silicon which is useful as a calibration standard. The analytical techniques used have capabilities ranging from very high depth resolution (≈50Å) and high sensitivity (< 1 ppm) to deep probes for hydrogen which can sample throughout thin sheets (up to 0.2 mm thick).

D. D. Allred (et al.)
The nuclear reaction 19F(1H, αγ) 16O has been used to determine the hydrogen concentration in natural and synthetic quartz samples. The depth-profile of the hydrogen concentration in these samples has been determined in detail for the smoky and X o quartzes. These profiles exhibit a region of high hydrogen concentration in the near surface region (down to a depth of ∼2000Å), with a lower concentration in the bulk of the sample. The results provide a plausible explanation for the substantial disagreement between previous hydrogen analysis in these quartzes by other techniques. Evidence for hydrogen mobility in crystalline quartz under ion beam bombardment is presented and discussed.
D. D. Allred (et al.)
Depth profiles for hydrogen in amorphous silicon have been determined by the use of resonantnuclear reactions [1H(15N,αγ)12C and 1H(19F,αγ)16O] and by secondary ion mass spectroscopy(SIMS). Independent calibration procedures were used for the two techniques. Measurements were made on the same amorphous silicon film to provide a direct comparison of the two hydrogen analysis techniques. The hydrogen concentration in the bulk of the film was determined to be about 9 at.% H. The SIMS results agree with the resonantnuclear reaction results to within 10%, which demonstrates that quantitative hydrogen depth profiles can be obtained by SIMSanalysis for materials such as amorphous silicon.
David Allred (et al.)
The delayed-proton spectrum following the β decay of C9 (τ12=126.5±1.0 msec) was found to consist primarily of a continuum extending from 13 to 1.5 MeV, the latter being the lowest energy observed. In addition to the previously observed peaks at 9.28 and 12.30 MeV (c.m.), possible peaks between 3 and 7 MeV have been tentatively identified.
Spencer M. Roberts, Joseph S. Carter, Brian D. Jensen, Richard R. Vanfleet, and Robert C. Davis

Introduction: This study aims to explore the mechanical properties of porous microelectrodes formed from vertically aligned carbon nanotube (CNT) forests. Specifically, we investigate the range of effective CNT-based microelectrode (ME) moduli that can be fabricated and identify moduli within that range that significantly reduce strain on brain tissue during micromotion.Materials and methods: To address these questions, we developed a micromechanical measurement method, known as the dual deflection (DD) test, which is compatible with microelectrode array (MEA) form factors and can measure a wide range of moduli with a 30% uncertainty. Using the DD test with small deflections, we measured the effective Young’s modulus of freestanding CNT microelectrodes (MEs) fabricated with different carbon infiltration times (0, 15, and 30 s) at 900°C. We also developed a static 10 μm deflection finite element analysis (FEA) model to compare the brain tissue strain induced by probes with the maximum (1.7 GPa), median (72 MPa), and minimum (3.9 MPa) measured CNT moduli, along with the modulus of silicon (165 GPa) for comparison.Results: The DD test results showed mean effective moduli of 19.6 ± 14.5 MPa, 67.7 ± 22.7 MPa, and 168 ± 62.3 MPa for arrays fabricated with 0, 15, and 30 s infiltrations, respectively. The FEA model revealed that probes with the maximum CNT modulus induced similar strain to the silicon probes at the tip, while probes with the minimum and median CNT moduli showed minimal strain at the tip.Discussion: These findings suggest that CNT microelectrodes with moduli in the tens of MPa range, achievable through 15 s of carbon infiltration, can significantly reduce brain tissue strain. Additionally, we consistently observed that microelectrodes with 15 s of infiltration were apparently undamaged after deflection, making them mechanically promising candidates for neural probe arrays.

Ben VanDyke, Joshua D. Hancock, David Van Gundy, and Richard R. Vanfleet (et al.)

The Extended Murphy Good (EMG) mathematical theory proposed by Richard G. Forbes for Large Area Field Emitters is applied to Patterned As-Grown Carbon Nanotube field emitters as well as existing research for comparison. Patterned As-Grown Carbon Nanotubes (developed by Brigham Young University in collaboration with Varex Imaging) were transferred to titanium substrates by means of a precise thermal process which creates a unique titanium carbide nanostructure as an adhesion mechanism for Patterned As-Grown Carbon Nanotubes. This deposition process is called the VanDyke Deposition Method. Field emission IV curve data was measured from 10 mA to 100 mA and was placed in the EMG calculator. As the EMG data more correctly estimates the vertical axis intercept the EMG results show that As-grown CNTs adhered to titanium can have Formal Emission Areas of 318.9u2 & 684.5u2 .