Selected Publications
D. D. Allred (et al.)
A unique approach to detect chemical speciation and distribution on nanometer-scale nuclear materials has been achieved by the combination of neutron reflectometry and shell-isolated surface-enhanced Raman spectroscopy. Both surface and underlying layers of the uranium oxide materials were determined with angstrom-level resolution. Our results reveal that the UOx film is composed of three sublayers: an, similar to 38 angstrom thick layer of U3O8 formed along the UOx/substrate interface; the adjacent sublayer consists of an similar to 900 angstrom thick single phase of alpha-UO3, and the top layer is gamma-UO3 with a thickness of similar to 115 angstrom.
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.
J. A. Brame, J. E. Goodsell, and D. D. Allred (et al.)
A test structure for a prototype nanoscale magnetometer exploiting the strain sensitivity of single-walled carbon nanotubes (SWCNTs) has been fabricated. The nanotube magnetometer would boast reduced dimensions, mass, and power requirements compared with a Fluxgate magnetometer. Dramatic resistance increase with strain has been previously reported for individual nanotubes, and this magnetometer design concept seeks to extend this strainresistance property to an "asgrown" ensemble of SWCNTs. Measurements of a test structure show a correlation between applied magnetic field and device conductivity. This correlation indicates an increase in conductivity with strain to the network of nanotubes; candidate mechanisms for this behavior are discussed.
Heidi M. Dumais, R. Steven Turley, and David D. Allred
We measured the transmittance and reflectance of two reactively sputtered diodes prepared with approximately 20 nm of UOx in the extreme ultraviolet (XUV) at the Advanced Light Source at Lawrence Berkeley National Laboratory. Fitting the reflectance data to the Parratt model yielded the thickness of the UOx film. This thickness combined with a simple analysis of the transmission measurements provides estimates for the imaginary part of the index of refraction for UOx at approximately every tenth of a nanometer from about 3 nm to 30 nm with emphasis in the 12- to 13-nm range. The analysis discussed in this paper yields only the imaginary part of the complex index but will lead to a more robust analysis to find both the real and imaginary parts of the index of refraction. These values provide researchers with information for modeling, design, and fabrication of optical systems in the extreme ultraviolet.
Lei Pei, Amy Balls, Cary Tippets, Jonnathan Abbott, Matthew R. Linford, David D. Allred, Richard R. Vanfleet, and Robert C. Davis (et al.)
Here, the authors report the fabrication of transparent polymer templates for nanostructured amorphous siliconphotovoltaics using low-cost nanoimprint lithography of polydimethylsiloxane. The template contains a square two-dimensional array of high-aspect-ratio nanoholes (300 nm diameter by 1 μm deep holes) on a 500×500 nm2 pitch. A 100 nm thick layer of a-Si:H was deposited on the template surface resulting in a periodically nanostructured film. The optical characterization of the nanopatterned film showed lower light transmission at 600–850 nm wavelengths and lower light reflection at 400–650 nm wavelengths, resulting in 20% higher optical absorbance at AM 1.5 spectral irradiance versus a nonpatterned film.
Bryan Hicks and David Allred (et al.)
Single-walled carbon nanotubes (SWCNT) were prepared using iron catalysts deposited by indirect evaporation on silicon substrate covered with 500 nm-thick thermal oxide. Diode SWCNT devices have been fabricated using Au and Al, as the asymmetric metal contacts, and a random network of metallic and semiconducting nanotubes as the device channel. No effort was made to align the SWCNTs or to eliminate metallic nanotubes in our devices. Asymmetric voltage-current behavior was seen. Current rectification was observed in the source-drain bias range of -3V to +3 V. Rectification was somewhat surprising since, although metallic tubes are in the minority (similar to 1/3), they could potentially act as shunts and mask the electric properties of the semiconducting majority. No correlation between electrode spacing and current rectification was observed. The lowest leakage current measured was 1% of the maximum current carrying capacity. Maximum forward-biased current capacities range between 8 mu A and 841 mu A. (C) 2009 Elsevier B.V. All rights reserved.