Structural fingerprinting of a cubic iron-oxide nanocrystal mixture: A case study Peter Moeck 1 , Sergei Rouvimov 1 , Stavros Nicolopoulos 2 and Peter Oleynikov 3 1 Nano-Crystallography Group, Department of Physics, Portland State University, P.O. Box 751, Portland, OR 97207-0751, USA; 2 NanoMEGAS SPRL, Boulevard Edmond Machterns No 79, Sint Jean Molenbeek, Brussels, B-1080, Belgium http://nanomegas.com ; 3 AnaliTEX, http://www.analitex.com ABSTRACT Two novel strategies for the structurally identification of a nanocrystal from either a single high resolution (HR) transmission electron microscopy (TEM) image or a single precession electron diffractogram (PED) are proposed and their advantages discussed in comparison to structural fingerprinting from powder X-ray diffraction pattern. Simulations for cubic maghemite and magnetite nanocrystals are used as case study examples. Keywords: structure, nanocrystals, transmission electron microscopy, structural fingerprinting INTRODUCTION Nanocrystals possess size [1] and morphology [2] dependent properties that are frequently superior to those of the same materials in their bulk form. Any future large- scale commercial “nanocrystal powder-based industry” will need to be supported by structural assessment methods [3]. The quite ubiquitous method of identifying crystal structures is (Cu-tube based) powder X-ray diffraction (XRD) [4], e.g. Fig. 1. That method works best for micrometer-sized crystals and becomes due to peak broadening and (isotropic or anisotropic) shifting less useful to useless for crystals in the nanometer range [5, 6]. XRD patterns of nanocrystals are also made significantly less characteristic by surface relaxation effects [7]. Two novel strategies for the structural identification of nanocrystals in the TEM are, therefore, proposed. Both of these methods are applicable to nanocrystal thicknesses for which the scattering of fast electron can be considered as essentially (quasi-)kinematic. This thickness range is for HRTEM imaging 1 to about 10 nm and for PEDs 10 to 50 nm. In the dynamic scattering limit, these methods become analogous to the well known structural identification methods for single crystals in the TEM that only use information on the projected reciprocal lattice geometry. For a recent review of those methods and more information on the two novel strategies, see ref. [8]. Because cubic maghemite and magnetite possess almost the same lattice constant and “rather similar” atomic arrangements (i. e. nearly cubic densest packings of oxygen with differences in the iron occupancies of the intersites), the XRD patterns are very similar, Fig. 1. Allowing for peak broadening, peak shifting, and surface relaxation, nanometer sized crystals of these two cubic iron-oxide minerals can hardly be told apart and their mixtures can not be quantified by XRD. Quite independent on the nanocrystal size, there is, however, structure information at the atomic level in a (single) HRTEM image and a (single) precession* electron diffractogram (PED) of a (single) nanocrystal that can be advantageously employed for its structural identification [8- 13]. Figure 1: Calculated powder X-ray (Cu-Ka) diffraction patterns for micrometer-sized cubic maghemite, γ-Fe 2 O 3 , and magnetite, Fe 3 O 4 , out to the 400 reflections. The space group symbols and their numbers are also given. This atomic-structure-level structure information is in the case of TEM images after crystallographic image processing [8, 14, 15] structure factor amplitudes and phases out to the point resolution of the microscope, e.g. at least out to 5 nm -1 for dedicated (but non-aberration corrected) HRTEMs. In the case of PEDs, this atomic- structure-level information is the structure factor amplitudes and extends to at least twice as far in reciprocal space. Extracting this kind of information for unknowns, combining it with the extractable projected reciprocal lattice geometry, and comparing it to structural information that is contained for a range of candidate structures in a Fe 3 O 4 γ-Fe 2 O 3 32 4 1 P m Fd 3 No. 227 No. 213 912 NSTI-Nanotech 2008, www.nsti.org, ISBN 978-1-4200-8503-7 Vol. 1