January 26, 2012 — National Institute of Standards and Technology (NIST) materials scientists, Robert Keller and Roy Geiss, have modified a standard scanning electron microscope (SEM) for a roughly 10-fold improvement in measuring the crystal structure of nanoparticles and extremely thin films. It enables crystal structure study of particles as small as 10nm.
|Figure. At top, a transmission electron diffraction pattern of a 50nm-diameter indium gallium nitride (InGaN) nanowire, taken with an SEM using the new NIST technique, showing a unique pattern associated with crystal diffraction. Bottom: Same pattern with an overlay showing the crystallographic indexing associated with the atomic structure of the material. SOURCE: Geiss/NIST.|
Different crystal phases of a material demonstrate different chemical behavoirs. Understanding the crystalline structure can lead to optimization of thin films in nanoelectronics manufacturing, and other applications in criminal forensics, etc.
In standard SEM-based electron diffraction, the researcher uses an electron back-scatter diffraction (EBSD) detector to analyze patterns formed by electrons bouncing back after striking atoms in the sample. If the sample is a crystalline material, with a regular pattern to the arrangement of atoms, these diffracted electrons form a pattern of lines that reveals the particular crystal structure, or phase and orientation, of the material. "You can determine the crystal structure of an isolated particle down to a size of about 100 to 120nm, but below that the crystals are so small that you’re getting information about the sample holder instead," report the researchers. Transmission electron microscopy (TEM) performs better with samples to about 50nm in size, below which they show very limited diffraction patterns because of the high power of the electron beam.
The two researchers altered the sample position to perform electron diffraction with a SEM in a different way. Keller and Geiss moved the SEM sample holder closer to the beam source and adjusted the angles so that, instead of imaging electrons bouncing back from the sample, the EBSD detector is seeing electrons that scatter forward through the sample in a manner similar to a TEM. A unique sample-holding method contributes to this imaging. This technique produces reliable crystal phase information for nanoparticles as small as 10nm across, as well as for single crystalline grains as small as 15nm in an ultrathin film.
Electron diffraction in an SEM, says Keller, "in general represents the only approach capable of measuring the atomic structure, defect content, or crystallographic phase of single nanoparticles.
Results are scheduled to appear in the Journal of Microscopy, March 2012: R.R. Keller and R.H. Geiss. Transmission EBSD from 10 nm domains in a scanning electron microscope. Journal of Microscopy, 2011. doi: 10.1111/j.1365-2818.2011.03566.x.