David D. Allred received Alumni Professorship Award

Abstract: High-aspect-ratio metallic microstructures have a variety of potential applications in sensing and actuation. However, fabrication remains a challenge. We have fabricated nickel microstructures with over 20:1 aspect ratios by electroplating patterned carbon-coated carbon-nanotube forests using a nickel chloride bath. Pulse plating allows nickel ions to diffuse into the interior of the forest during off portions of the cycle. Done properly, this solves the problem of the formation of an external crust, which otherwise blocks nickel deposition in the interior of the structures. Thus, densities of 86 ± 3% of bulk Ni for the composite structures are achieved. Cantilever structures do not yield under load, but break. Measurements of the material properties of this composite material indicate an elastic modulus of ~42 GPa and a strength of 400 MPa. We demonstrate the utility of this method with an external field magnetic actuator consisting of a proof mass and two flexures. We achieved 1-mN actuation forces. [2014-0274]

Abstract: Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U-O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting methods as well as theoretical calculations are combined to examine the intrinsic electronic structure and the corresponding band gap of uranium oxides to determine the chemical speciation in a,102 nm thick reactively sputtered uranium oxide film. The SE results reveal that the UOx film exhibits two absorption edges, a primary absorption edge slightly above 2.6 eV and a secondary absorption at 1.7-1.8 eV. The optical band gaps compared with the theoretical calculations performed on UO2, U4O9, U3O7, alpha-U3O8, alpha-UO3, delta-UO3, and gamma-UO3 suggest that the UOx film is composed of at least two components; the primary absorption is caused by the alpha-UO3 sublayer, which is superimposed on top of an adjacent alpha-U3O8 sublayer that is hypothesized to be heteroepitaxial growth of alpha-U3O8 along the UOx/substrate interface. Comparison to the ellipsometry measurements shows that the DFT+U and hybrid (HSE) calculations predict the correct trend for band gaps as a function of oxidation state and crystallography but they fail to capture the exact gaps. However, they provide important information for interpretation of the experimental results and highlight some of the structural complexity that prevails in the UOx compounds. The combination of theoretical and experimental methods to examine the intrinsic electronic structure and the band gap of the corresponding uranium oxides could benefit from the development of new methods for better distinguishing chemical speciation in uranium oxides. In addition, the experimental measurement of the indirect band gap of alpha-U3O8, is, to our knowledge, reported for the first time.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract:

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.