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news digest ♦ Equipment and Materials


structure or “phase” and orientation of the material.


SEMs are often outfitted with an electron back- scatter diffraction (EBSD) detector for just this task. The problem, they say, is that below a certain size, the usual setup just doesn’t work. “You can determine the crystal structure of an isolated particle down to a size of about 100 to 120 nm, but below that the crystals are so small that you’re getting information about the sample holder instead.” A somewhat more exotic instrument, the transmission electron microscope (TEM), does much better , but samples below about 50 nm in size show very limited diffraction patterns because the higher- powered electron beam of the TEM just blasts through them.


The novel tweak developed by Keller and Geiss combines a little of each. They moved the SEM sample holder closer to the beam source and adjust the angles so that instead of imaging electrons bouncing back from the sample, the EBSD detector is actually seeing electrons that scatter forward through the sample in a manner similar to a TEM. (They also came up with a unique method of holding samples to obtain these results.)


Top: Transmission electron diffraction pattern from a segment of an InGaN nanowire about 50 nm in diameter taken with an SEM using the new NIST technique clearly shows a unique pattern associated with crystal diffraction. Bottom: Same pattern but with an overlay showing the crystallographic indexing associated with the atomic structure of the material. (Credit: Geiss / NIST)


The information, say NIST’s Robert Keller and Roy Geiss, can be critical. “A common example is titanium dioxide, which can exist in a couple of different crystal phases. That difference significantly affects how the material behaves chemically, how reactive it is. You need to add crystallographic identification to the chemical composition to completely characterise the material.”


216 www.compoundsemiconductor.net January / February 2012


They have shown that their technique produces reliable crystal phase information for nanoparticles as small as 10 nm across, as well as for single crystalline grains as small as 15 nm 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. This is a critical need in cases of extremely limited sampling of unknown particles. This work pushes electron diffraction to a new frontier by providing spatial resolution that rivals that possible in a TEM, and makes it available to anyone with an SEM. And that’s an ubiquitous tool in virtually all fields that require characterisation of solids.”


Typical applications, the researchers say, include pinpointing ammunition sources from gunshot residue at crime scenes; determining


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