Harry W. Deckman

Three-Dimensional X-ray Microtomography

The new technique of x-ray microtomography nondestructively generates three-dimensional maps of the x-ray attenuation coefficient inside small samples with approximately 1 percent accuracy and with resolution approaching 1 micrometer. Spatially resolved elemental maps can be produced with synchrotron x-ray sources by scanning samples at energies just above and below characteristic atomic absorption edges. The system consists of a high-resolution imaging x-ray detector and high-speed algorithms for tomographic image reconstruction. The design and operation of the microtomography device are described, and tomographic images that illustrate its performance with both synchrotron and laboratory x-ray sources are presented.

High-flux MFI membranes

The synthesis and evaluation of high performance MFI-type membranes is described. These systems exhibit fluxes that are one to two orders of magnitude higher than previous literature reports, with ...

Optically enhanced amorphous silicon solar cells

We describe the first application of optical enhancement to thin‐film (∼0.75 μm thick) amorphous silicon solar cells and define cell geometries which maximize enhancement effects. We observed that due to the improved infrared absorption the external AM1 short circuit current increases by 3.0 mA/cm2 in cells constructed in accordance with the principles of optical enhancement.

Maximum statistical increase of optical absorption in textured semiconductor films

Complete statistical randomization of the direction of propagation of light trapped in semiconductor films can result in a large absorption enhancement. We have employed a calorimetric technique, photothermal deflection spectroscopy, to monitor the absorption of alpha-SiH(x) films textured by the natural lithography process. The observed enhancement factors, as high as 11.5, are consistent with full internal phase-space randomization of the incoming light.

Edge Surfaces in Lithographically Textured Molybdenum Disulfide

Lithographic techniques were used to expose edge surfaces in layered molybdenum disulfide single crystals. This microstructuring produced ideal samples for the study of the surface morphology and electronic structure of this catalytically important material. The optical absorption that was measured at mid-gap increased by two orders of magnitude after texturing. This increase resulted from reduced molybdenum at surface defects that are located on edge planes, as shown by photoemission spectroscopy. This information cannot easily be obtained on conventional crystals with predominantly basal plane surfaces.

Reverse osmosis molecular differentiation of organic liquids using carbon molecular sieve membranes

Carbon sieving to separate the similar Separating organic molecules, particularly those with almost equal sizes and similar physical properties, can be challenging and may require energy-intensive techniques such as freeze fractionation. Taking inspiration from reverse osmosis of aqueous fluids, Koh et al. describe the synthesis, characterization, and mass transport performance of carbon molecular sieve membranes for the separation of liquid-phase organic molecules at room temperature. This technique is capable of separating very similar isomers, such as ortho- and para-xylene, on an industrial scale. Science, this issue p. 804 Carbon membranes efficiently separate similarly sized organic liquid molecules and isomers. Liquid-phase separations of similarly sized organic molecules using membranes is a major challenge for energy-intensive industrial separation processes. We created free-standing carbon molecular sieve membranes that translate the advantages of reverse osmosis for aqueous separations to the separation of organic liquids. Polymer precursors were cross-linked with a one-pot technique that protected the porous morphology of the membranes from thermally induced structural rearrangement during carbonization. Permeation studies using benzene derivatives whose kinetic diameters differ by less than an angstrom show kinetically selective organic liquid reverse osmosis. Ratios of single-component fluxes for para- and ortho-xylene exceeding 25 were observed and para- and ortho- liquid mixtures were efficiently separated, with an equimolar feed enriched to 81 mole % para-xylene, without phase change and at ambient temperature.

Submicron x‐ray lithography using laser‐produced plasma as a source

X‐ray lithography was studied, using laser‐produced plasma as a source. A single target shot of a frequency‐tripled Nd:glass laser (λ=0.35 μm, 35 J in 1 ns) was found to be sufficient for submicron x‐ray lithography in poly(butene‐1‐sulfone) (PBS) or poly(glycidyl‐methacrylate‐ethyl acrylate) (COP) resists. The incident x‐ray flux is about an order of a magnitude smaller than that normally required. This behavior could be the result of the transient character of the exposure and an abrupt rise in the resist temperature.

Transmission electron microscopy of hydrogenated amorphous semiconductor superlattices

Transmission electron microscopy (TEM) of amorphous semiconductor superlattices grown by plasma assisted chemical vapor deposition demonstrates that sequential layers can be deposited without cumulative roughening and with atomically abrupt interfaces. Absence of cumulative roughening effects allows layers to be grown flat and smooth to within 5 A on a∼100‐A lateral scale length, even after deposition of several hundred layers. In highly localized regions unusual defect structures appear as bifurcations of individual layers. The TEM sections used to investigate amorphous superlattice structure were prepared by a novel microfabrication technique.

Microfabrication of molecular scale microstructures

Microfabrication techniques have been used to prepare a new class of molecular scale microporous materials. These materials are formed by chemically etching slots into alternate layers of a lithographically exposed amorphous superlattice cross section. The slot width is accurately controllable from 10 A to more than 500 A.

X‐ray microtomography with monochromatic synchrotron radiation (invited)

We review results obtained with the Exxon Microtomography apparatus. The technique is based on tomographic methods widely used in medicine and nondestructive evaluation. When used with a tunable x‐ray source, it is a powerful diagnostic and research tool for a wide variety of materials problems. It is capable of producing maps of the interior structure and chemical composition of samples approximately 0.5–1.0 mm in size, with spatial resolution in the map of the density variations approaching 10.0 μm.