Joseph W. Tringe

Diamond Ablators for Inertial Confinement Fusion

Abstract Diamond has a unique combination of physical properties for the inertial confinement fusion ablator application, such as appropriate optical properties, high atomic density, high yield strength, and high thermal conductivity. Here, we present a feasible concept for fabrication of diamond ablator shells. The fabrication of diamond capsules is a multi-step process which involves diamond chemical vapor deposition on silicon mandrels followed by polishing, microfabrication of holes, and removing of the silicon mandrel by an etch process. We also discuss the pros and cons of coarse-grained optical quality and nanocrystalline chemical vapor deposition diamond films for the ablator application.

Crystallographic Anisotropy of Wear on a Polycrystalline Diamond Surface

We correlate topography and diffraction measurements to demonstrate that grain orientation profoundly influences polishing rates in polycrystalline diamond synthesized by chemical vapor deposition. Grains oriented with {111} or {100} planes perpendicular to the surface normal polish at significantly lower rates compared with grains of all other orientations when the surface is polished in continuously varying in-plane directions. These observations agree with predictions of the periodic bond chain vector model, developed previously for single crystals, and indicate that the polishing rate depends strongly on the number of periodic bond chain vectors that are within 10° of the exposed surface plane.

Line Edge Detection and Characterization in SEM Images Using Wavelets

Edge characterization has become increasingly important in nanotechnology due to the growing demand for precise nanoscale structure fabrication and assembly. Edge detection and assembly. Edge detection is often performed by thresholding the spatial information of a top-down image obtained by scanning electron microscopy or other surface characterization techniques. Results are highly dependent on an arbitrary threshold value, which makes it difficult to reveal the nature of the real surface and to compare results among images. In this paper, we present an alternative edge boundary detection technique based on the wavelet framework. Our results indicate that the method facilitates nanoscale edge detection and characterization by providing a systematic threshold determination step.

Fabrication of functional silicon-based nanoporous membranes

Macroscopic porous membranes with pore diameter uniformity approaching the nanometer scale have great potential to significantly increase the speed, selectivity, and efficiency of molecular separations. We present fabrication, characterization, and molecular transport evaluation of nanoporous thin silicon-based sieves created by laser interferometric lithography (LIL). This fabrication approach is ideally suited for the integration of nanostructured pore arrays into larger microfluidic processing systems, using a simple all-silicon lithographic process. Submilli-meter-scale planar arrays of uniform cylindrical and pyramidal nanopores are created in silicon nitride and silicon, respectively, with average pore diameters below 250 nm and significantly smaller standard error than commercial polycarbonate track etched (PCTE) membranes. Molecular transport properties of short cylindrical pores fabricated by LIL are compared to those of thicker commercial PCTE membranes for the first time. A 10-fold increase in pyridine pore flux is achieved with thin membranes relative to commercial sieves, without any modification of the membrane surface.

Mesoscale evolution of voids and microstructural changes in HMX-based explosives during heating through the β-δ phase transition

HMX-based explosives LX-10 and PBX-9501 were heated through the β-δ phase transition. Ultra-small angle x-ray scattering (USAXS) and molecular diffraction were simultaneously recorded as the HMX was heated. Mesoscale voids and structure dramatically change promptly with the β-δ phase transition, rather than with other thermal effects. Also, x-ray induced damage, observed in the USAXS, occurs more readily at elevated temperatures; as such, the dose was reduced to mitigate this effect. Optical microscopy performed during a similar heating cycle gives an indication of changes on longer length scales, while x-ray microtomography, performed before and after heating, shows the character of extensive microstructural damage resulting from the temperature cycle and solid-state phase transition.

Ultrathick, low-stress nanostructured diamond films

We describe a hot-filament chemical vapor deposition process for growing freestanding nanostructured diamond films, ∼80μm thick, with residual tensile stress levels ≲90MPa. We characterize the film microstructure, mechanical properties, chemical bond distribution, and elemental composition. Results show that our films are nanostructured with columnar grain diameters of ≲150nm and a highly variable grain length along the growth direction of ∼50–1500nm. These films have a rms surface roughness of ≲200nm for a 300×400μm2 scan, which is about one order of magnitude lower than the roughness of typical microcrystalline diamond films of comparable thickness. Soft x-ray absorption near-edge structure (XANES) spectroscopy indicates a large percentage of sp3 bonding in the films, consistent with a high hardness of 66GPa. Nanoindentation and XANES results are also consistent with a high phase and elemental purity of the films, directly measured by x-ray and electron diffraction, Rutherford backscattering spectrometr...

Deeply etched grating structures for enhanced absorption in thin c-Si solar cells

Sub-wavelength periodic structures in crystalline-silicon (c-Si) for solar cell applications can be designed for maximizing optical absorption in thin films. We have investigated optical response of deeply etched c-Si grating structures using rigorous modeling, hemispherical reflectance, one-sun LIV, and internal quantum efficiency measurements. Model calculations predict that almost /spl sim/ 100 % optical absorption can be achieved in subwavelength 2D structures etched to a depth of /spl sim/ 15 /spl mu/m. Using advanced reactive ion etching techniques, subwavelength deeply etched grating structures have been fabricated and integrated into solar cells. Preliminary one-sun solar cell measurements from /spl sim/ 10-/spl mu/m 2D period structures have demonstrated short-circuit current enhancement of /spl sim/ 10 mA. The cell efficiencies were poor due to the lack of surface passivation and emitter optimization. Subwavelength grating solar cells failed to provide any performance boost probably due to the lack of surface passivation. Optimization of emitter formation on these types of deeply etched grating surfaces is expected to lead to high-efficiency, thin-film c-Si solar cells.

Efficient Nanoporous Silicon Membranes for Integrated Microfluidic Separation and Sensing Systems

Nanoporous devices constitute emerging platforms for selective molecule separation and sensing, with great potential for high throughput and economy in manufacturing and operation. Acting as mass transfer diodes similar to a solid-state device based on electron conduction, conical pores are shown to have superior performance characteristics compared to traditional cylindrical pores. Such phenomena, however, remain to be exploited for molecular separation. Here we present performance results from silicon membranes created by a new synthesis technique based on interferometric lithography. This method creates millimeter sized planar arrays of uniformly tapered nanopores in silicon with pore diameter 100 nm or smaller, ideally-suited for integration into a multi-scale microfluidic processing system. Molecular transport properties of these devices are compared against state-of-the-art polycarbonate track etched (PCTE) membranes. Mass transfer rates of up to fifteen-fold greater than commercial sieve technology are obtained. Complementary results from molecular dynamics simulations on molecular transport are reported.

Radiation damage mechanisms for luminescence in Eu-doped GaN

Thin films of Eu-doped GaN were irradiated with 500keV He+ ions to understand radiation damage mechanisms and to quantify luminescence efficiency. The dependence of ion-beam-induced luminescence intensity on ion fluence was consistent with the simultaneous creation of nonradiative defects and the destruction of luminescent centers associated with 4f-4f core-level transitions in Eu3+. This model contrasts with a previous description which takes into account only nonradiative defect generation in GaN:Eu. Based on light from a BaF2 scintillator standard, the luminescent energy generation efficiency of GaN:Eu films doped to ∼3×1018cm−3 Eu is estimated to be ∼0.1%.

Using atomic layer deposited tungsten to increase thermal conductivity of a packed bed

This study investigated the effective thermal conductivity (keff) of packed-beds that contained porous particles with nanoscale tungsten (W) films of different thicknesses formed by atomic layer deposition (ALD). A continuous film on the particles is vital towards increasing keff of the packed beds. For example, the keff of an alumina packed bed was increased by three times after an ∼8-nm continuous W film with 20 cycles of W ALD, whereas keff was decreased on a polymer packed bed with discontinuous, evenly dispersed W-islands due to nanoparticle scattering of phonons. For catalysts, understanding the thermal properties of these packed beds is essential for developing thermally conductive supports as alternatives to structured supports.