Type: 
SSO Seminar
Date-Time: 
Wednesday, April 20, 2011 - 16:00
Location: 
Weniger 304
Event Speaker: 
Peter Moeck, PSU
Local Contact: 
Oksana
Abstract: 

It has been said that applied crystallographers have a “special approach” to problem solving [1]. This talk illustrates this approach on three applications involving two different kinds of microscopes. All of our applications rely on crystallographic reference data and open-access sources of such data are mentioned. The microscopes are a generic traditional scanning probe microscope (SPM) and transmission electron microscopes (TEMs).
Traditional SPM images of 2D-periodic arrays are processed by us crystallographically in order to quantify their deviations from the 16 higher symmetric plane symmetry groups. This information is then used to remove from the SPM image all kinds of geometric distortions that are due to the “less than perfect” imaging process. The combined effects of these distortions result in a point spread function that gives a quantitative measure of the microscope’s performance for a certain set of experimental parameters. On the basis of highly symmetric 2D periodic calibration samples, the point spread function of the microscope can be extracted and utilized for the correction of SPM images of unknowns that were recorded under essentially the same experimental conditions [2].
Nanocrystals cannot be fingerprinted structurally from powder X-ray diffraction patterns. Three strategies for the structural identification of nanocrystals in a TEM have, therefore, been developed by us over recent years. Either a single High-Resolution TEM (HRTEM) image [3] or a single Precession Electron Diffraction (PED) spot pattern [4] can be employed. The structural identification information is in both cases collected from an individual nanocrystal. PED ring patterns from fine-grained crystal powders may also be utilized.
An automated technique for the mapping of crystallite phases and orientations of polycrystalline materials in a TEM has recently been commercialized by NanoMEGAS. When the TEM is equipped with a field emission gun, this technique delivers (in the quasi-parallel illumination nano-probe mode) a significantly higher spatial resolution than the well known electron backscatter Kikuchi diffraction technique in a scanning electron microscope. It is also much less sensitive to both plastic deformations and surface contaminations of the crystallites. The new technique is based on template matching of experi-mental PED spot patterns (with typical precession angles of 0.1º to 1º) to their pre-calculated theoretical counterparts [5]. Our work in this newly emerging research & development field focuses on improvements of the fidelity of the technique (and is preformed in close collaboration with NanoMEGAS).
There are now comprehensive data for more than 180,000 crystal structures in open access. The Crystallography Open Database (COD) is rapidly growing and features currently more than 140,000 entries at its main web site and its four mirror sites in Europe and the USA [6]. All of the entries of the COD can be visualized interactively in 3D over our web portal. An about 20,000 entry mainly inorganic subset of the COD that possesses the same functionality and supports our structural fingerprinting strategies [3,4] is hosted at Portland State University [7].

[1] D. Schwarzenbach, Zeits. Kristallogr. 217 (2002) 366-367
[2] P. Moeck, in: “Microscopy: Science Technology, Applications and Education”, Microscopy Book Series No. 4,
Vol. 3, pp. 1951-1962, Formatex Research Center, 2010, open access:
http://www.formatex.info/microscopy4/1951-1962.pdf
[3] P. Moeck and P. Fraundorf, Zeits. Kristallogr. 222 (2007) 634-645
[4] P. Moeck and S. Rouvimov, Zeits. Kristallogr. 225 (2010) 110-124
[5] P. Moeck et al., Cryst. Res. Technol. special Issue “New developments in electron diffraction”,
DOI 10.1002/crat.201000676 (2010)
[6] cod.ibt.lt; mirrors: crystallography.net, http://cod.ensicaen.fr/, nanocrystallography.org;
web-portal: nanocrystallography.net; S. Gražulis et al., J. Appl. Cryst. 42 (2009) 726-729
[7] http://nanocrystallography.research.pdx.edu