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While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively ''low'' energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg 2 SiO 4 ) amorphizes when irradiated by 10 MeV Xe ions at large enough fluences. Using modeling, we extrapolate these results to show that 0.1Y5.0 GeV heavy-ion cosmic rays can rapidly ($70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium.

process surfaces as smoothly as possible, although there are potential applications where roughened surfaces are desirable. For example, ion beam "rippling" [4] and/or quantum dot formation are being considered as self-assembly templates [5]. Rippling is often considered in the presence of a broad, static ion beam, and for its potential application to a full wafer substrate. However, devices requiring site-specific ripples will require FIB processing. If broad beam ripples are processed in conjunction with photolithography patterns, the ripples within these patterned pits will have similar boundary conditions as FIB-processed ripples. The FIB also provides a versatile platform for changing experimental variables. A FIB can produce several different sets of ripples in seconds, where a static beam may take days; and the SEM in the DualBeam provides instantaneous diagnostics of the process. Thus studies of processing parameters that provide faster etch rates, also identify the parameters that influence the ripple formation. Figure 3a presents an SEM image of a broken diamond, which has been exposed to modest ion sputter etching. (Fig. 3c depicts a sharpened tip after hours of FIB processing.) A close-up view (Fig. 3b) shows "ripples" develop simultaneously on all of these inclined slopes. At a lower magnification these ripples resemble terraced slopes on hillsides such as Machu Picchu, or erosion steps on the great pyramids, or even cow-grazing paths (Fig. 3d). Such macroscopic terracing occurs due to "erosion" and/or "redeposition". Erosion, redeposition, and the formation of ripples are also observed in fusion reactors studies [3]. . The present data (overlaid color for the no-water condition) indicates yield can be more, or less. Figure 2. Although yield (etch rate) varies with angle and chemistry, yield always decays with increased aspect ratio (depth/length) and more so with angle and chemistry. (As yield drops, the redeposition is increasing.) Figure 3. A single crystal diamond is FIB-micromachined to make a sharp tip (Fig. 3c). However, the many facets and inclinations make an ideal platform for testing ripple formation (Fig. 3b) from different angles and crystallography. Ripples appear similar to erosion and redeposition patterns in nature, such as cow paths on a steep hillside (Fig. 3d). Surface Rippling & Ion Etch Yields of Diamond Using a Focused I) Motivation & Background: Micromachining & RipplesThe Focused Ion Beam (FIB) instrument, originally designed for semiconductor circuit modification and repair, has found considerable utility as a tool for specimen preparation in several microscopy disciplines and for micromachining small parts. Essentially, a FIB makes very small and precise cuts into a target sample, which implies well-controlled etch rates and close tolerances of surface finish. However, redeposition can affect etch rate and final surface topographies. This work quantifies this redeposition as it modifies yields for different parameters of etching; models the influence of redeposition as ap...

The electronic structure of ultralow density nanoporous chromia monoliths (aerogels) is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. These aerogels are prepared by the epoxide sol-gel method. High-resolution O K edge and Cr L2,3 edge XANES spectra of aerogels (before and after thermal annealing at 550°C) are compared with spectra of microcrystalline α-Cr2O3 and orthorhombic CrO3 powders, α-Cr2O3 (0001) and (11¯02) single crystals, and an unrelaxed amorphous phase of full-density Cr2O3. Spectra are interpreted based on the results of crystal-field multiplet calculations (for Cr L2,3 edge) and ab initio spin-polarized density functional theory calculations with and without a Hubbard-type on-site Coulomb repulsion (for O K edge). Results show that the electronic structure of as-prepared aerogels, with a large fraction of undercoordinated surface atoms, is significantly different from that of bulk α-Cr2O3, CrO3, or amorphous Cr2O3. Thermal treatment transforms the as-prepared amorphous aerogel into α-Cr2O3 nanopowder.

Wurtzite GaN films bombarded with 40keV C ions to high doses (5×1017 and 1×1018cm−2) are studied by a combination of Rutherford backscattering/channeling spectrometry, transmission electron microscopy, and soft x-ray absorption spectroscopy. Results show that, contrary to other ion species, implanted C forms nitrilelike carbon-nitride bonds (CN) and suppresses ion-beam-induced material decomposition involving the formation and agglomeration of ≳5-nm-large N2 gas bubbles.