Monday, March 16, 2009

Raman Spectroscopy of Carbon — More Information Than You Would Think

What is curious about the Raman spectrum of carbon is that even though the spectra of these materials is actually quite simple, they have been found to be quite useful. To quote Ferrari in his review (1), "The Raman spectra of all carbon systems show only a few prominent features, no matter the final structure, be it a conjugated polymer or a fullerene. The spectra appear deceivingly simple: just a couple of very intense bands in the 1000–2000 cm–1 region and few other second-order modulations. However, their shape, intensity, and positions allow us to distinguish a hard, amorphous carbon from a metallic nanotube, giving as much information as that obtained by a combination of other lengthy and destructuive approaches. The peculiar dispersion of the π electrons in graphene is the fundamental reason why Raman spectroscopy in carbons is always resonant and, thus, a powerful and efficient probe of their electronic properties, not only of their vibrations."

Principles of Raman Spectroscopy Principles of RamanDifferent carbon materials are used in various engineering applications, from high tech to low tech. Films are used as tribological coatings; fibers are used in composites for their strength; and nanotubes and graphene are being explored for use in microelectronics. Carbon composites have wide-ranging uses from aerospace to athletic gear. The ability of Raman spectroscopy to characterize these materials, in situ, with high spatial resolution (better than 1 μm) can be, and is being, exploited during development and QC.

The Raman spectra of the various allotropes of carbon are best understood in the framework of solid-state physics.

Diamond has the same structure as silicon and germanium, with two atoms in the unit cell. All C–C bonds are tetrahedral (sp3 ) and the lattice is cubic, so there is one triply degenerate optical phonon at the center of the Brillouin zone. As expected, the phonon frequency scales inversely with the mass of the element — 1332 cm–1 for carbon (diamond), 521 cm–1 for silicon, and 300 cm–1 for germanium.

Graphite is composed of stacks of planes of sp2 -bonded carbon with the layers staggered (some atoms of one layer sit atop the centers of the hexagonal rings of adjacent layers) in an ABAB arrangement. There are two planes in the unit cell, and the symmetry is D46h. There are two doubly degenerate, Raman-active, in-plane, E2g modes, with frequencies of 1582 and 42 cm–1, the former being known as the "G" mode. The lower frequency mode corresponds to shear motion of the two planes and can only be observed in instruments capable of recording frequencies well below 100 cm–1.

A continuum of structures of sp2 -bonded carbon has been observed, manufactured, and characterized. If one starts with graphite and grinds it into smaller crystallites, a "disorder" or D band appears (2) somewhere in the vicinity of 1280 and 1400 cm–1, depending upon the excitation wavelength (3). This band originally was assigned to scattering from the normally forbidden edge of the Brillouin zone, but was allowed because of the limited size of the crystallites. However, boron-doped crystals, in which the boron atoms are substitutional, also exhibit this behavior, which argues for symmetry breaking, rather than disorder or reduced crystallite size. In disordered materials, there is also a band at about 1620 cm–1, which often is called the "D'" band. A careful analysis of all the observations, including the inequivalence of the Stokes and anti-Stokes frequencies, indicates that a more coherent explanation invokes "double resonances" with electronic transitions (4).

The spectra of carbon fibers (made, for example, by pyrolyzing polyacrylonitrile) show the features described previously. Carbon fiber composites often are glued together with pyrolyzed organic resin that was impregnated in carbon fiber cloth previously, or by carbon deposited by chemical vapor deposition (CVD). The Raman spectrum of the deposited carbon also can be characterized by recording the G and D bands.

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