Going with the grain: Raman takes a look

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  • Published: Jan 7, 2016
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Going with the grain: Raman takes a look

Well oriented

Orientation-distribution mapping of polycrystalline materials by Raman microspectroscopy,

Raman microspectroscopy can reveal the orientation distributions of individual grain domains within a crystalline material as well as scanning electron microscopy.

Many solids are polycrystalline and the orientation of individual grains within their structure ultimately affects functional properties to a greater or lesser degree. Scanning electron microscopy is commonly employed to examine the grain across large specimen areas. Unfortunately, preparing such a surface for probing under vacuum with an electron beam and analysis using electron backscatter diffraction (EBSD) is a complicated, time-consuming process.

Now, a team at HZB(Helmholtz-Zentrum Berlin) headed by Daniel Abou-Ras and Thomas Schmid from BAM (Bundesanstalt für Materialforschung und -prüfung, Federal Institute for Materials Research and Testing, Berlin), and their colleagues Norbert Schäfer, Sergiu Levcenko and Thorsten Rissom, has shown that equivalent orientation distribution maps can be obtained using Raman microspectroscopy. This method, they explains requires only an optical microprobe setup, no time-consuming specimen preparation, and can be conducted under ambient conditions. It has been known since the 1930s that the direction of light incidence and signal observation affect the Raman scattering intensity and so should be readily applicable to grain observations.

HEADING

As proof of principle, the team worked with thin film layers of copper indium selenide, CuInSe2, as a model for investigation; the material crystallizes with a tetragonal, chalcopyrite-type structure. They demonstrated that the experimental Raman intensities correspond well with the theoretical intensities calculated by using the local orientations from the EBSD map, an important validation given the well-established use of EBSD. "The sample area was scanned by a laser beam using step sizes of 200 nanometres. For such measurement conditions, the sample environment needs to be controlled carefully and kept stable for several hours," explains Abou-Ras.

Local grain

The team reports, that "Raman microspectroscopy provides the means to obtain local orientations on polycrystalline materials at the submicrometre level. The present work demonstrates how orientation-distribution maps composed of Raman intensity distributions can be acquired on large areas of several hundreds of square micrometres." Importantly, for acceptance of the technique by the wider materials science community, the team emphasises that Raman microspectroscopy for orientation distribution analysis will be applicable in principle to any polycrystalline material, inorganic or organic, provide the substrate is Raman active and the grain sizes are appropriate to scattering.

Related Links

Sci Rep 2015, online: "Orientation-distribution mapping of polycrystalline materials by Raman microspectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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