Extremely elongated, conducting dust particles (also known as metallic "needles" or "whiskers") are seen in carbonaceous chondrites and in samples brought back from the Itokawa asteroid. Their formation in protostellar nebulae and subsequent injection into the interstellar medium have been demonstrated, both experimentally and theoretically. Metallic needles have been suggested to explain a wide variety of astrophysical phenomena, ranging from the mid-infrared interstellar extinction at ~3--8 micron to the thermalization of starlight to generate the cosmic microwave background. To validate (or invalidate) these suggestions, an accurate knowledge of the optics (e.g., the amplitude and the wavelength dependence of the absorption cross sections) of metallic needles is crucial. Here we calculate the absorption cross sections of iron needles of various aspect ratios over a wide wavelength range, by exploiting the discrete dipole approximation, the most powerful technique for rigorously calculating the optics of irregular or nonspherical grains. Our calculations support the earlier findings that the antenna theory and the Rayleigh approximation, which are often taken to approximate the optical properties of metallic needles are indeed inapplicable.

Optical properties of elongated conducting grains

The most recent data on experimentally undiscovered carbon allotropes, predicted by the Density Function Theory (DFT), are reviewed. Classic carbon allotropes, graphenes, carbon nanotubes, fullerenes and their hybrids are being studied using DFT and related methods, resulting a host of potentially existing forms based on 5-, 6-, 7-member cycles and their distinct combinations. Also known are cyclocarbons and large carbon clusters, nanobelts and nanoribbons, liquid, metallic, semiconductive and superhard carbons, which could exist under high or low pressures. These carbon allotropes can contain C atoms in the same hybridization state or be as their mixture, for example sp2+sp3. For several carbon allotropes, possible synthesis methods, properties and applications are proposed. 

Density Function Theory Predicted Carbon Allotropes: Recent Developments

This atomistic modeling study is companion to new experimental work on carbon vapor slow-cooled in (a) the laboratory and (b) the atmosphere of ancient carbon-synthesizing stars. It specifically follows up on TEM clues about the nucleation of unlayered graphene sheets in a solidifying carbon liquid, to show that 5 atom loops may help explain evidence for faceted pentacones in a slow-cooled melt. This is also first in a series of modeling studies that may open the door to laboratory studies of (i) condensation in cool-giant star atmospheres and (ii) liquid carbon at low pressures.

/pdf/dft-study-of-unlayered-graphene-solid-formation-in-liquid-10bbswkj0a.pdf

DFT study of “unlayered graphene solid” formation, in liquid carbon droplets at low pressures

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DFT study of “unlayered graphene solid” formation, in liquid carbon droplets at low pressures