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Paper: IR Spectra of Nano- and Macro-Crystals: the Overriding Importance of Optical Path
Volume: 196, Thermal Emission Spectroscopy and Analysis of Dust, Disks, and Regoliths
Page: 291
Authors: Hofmeister, A. M.; Rosen, L. J.; Speck, A. K.
Abstract: To clarify the effect of optical path, infrared (IR) absorption spectra were collected from nanocrystals of SiC and structurally related AlN and TiB2, and from commercial bulk samples using thin film, powder dispersion and single crystal methods. Crystals of 5-10 nm that were individually encapsulated in salt and subsequently pressed into thin films give identical features to those from thin films of the bulk sample (grain size >1 μm), which removes all possibility of a matrix effect, and negates the importance of grain size below a micron. Shifts in peak position for SiC samples are shown to relate to optical path, and are such that the β- and α- polymorphs can be distinguished. The shifts arise because peaks have finite widths and hence small frequency increments can have widely different absorption coefficient for the intense Si-C stretch. The high absorption coefficient serves as a filter for particulate dimensions perpendicular to the propagation of light.
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