Lattice-Fringe Fingerprinting: Structural identification of nanocrystals by HRTEM
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Lattice-Fringe Fingerprinting: Structural identification of nanocrystals by HRTEM Moeck Peter, and Ruben Bjorge Physics, Portland State University, P.O.Box 751, Portland, OR, 97207-0751 ABSTRACT A novel method for the structurally identification of a nanocrystal from a single high resolution (HR) transmission electron microscopy (TEM) micrograph is described. Components of this method are demonstrated on both experimental and simulated HRTEM images. On the experimental side, the structural information that can be extracted from a HRTEM image is the projected reciprocal lattice geometry, the plane symmetry group, a few structure factor amplitudes and phases, and an outline of the projected atomic structure to the limited resolution of the HRTEM (via a Fourier synthesis of the structure factors). Searching for this information in a comprehensive database and matching it with high figures of merit to that of candidate structures should allow for highly discriminatory identifications of nanocrystals, even without additional chemical information as obtainable in analytical TEMs. INTRODUCTION Nanocrystals are appealing for their novel and improved physical, chemical, and biocompatible properties relative to those of the materials in their bulk form. Any future large-scale commercial “nanocrystal powder-based industry” will need to be supported by structural assessment methods [1]. The “quite ubiquitous method” of identifying crystal structures is (Cu-tube based) powder X-ray diffraction [2]. That method works best for micrometer-sized crystals and becomes less useful to useless for crystals in the nanometer range due to peak broadening [3,4]. X-ray powder patterns are also made significantly less characteristic by surface relaxation of nanocrystals [5]. New methods for the structural identification of nanocrystals need, therefore, to be developed. There is structural information in high-resolution (HR) transmission electron microscopy (TEM) images of nanocrystals that can be employed for their structural identification [6-8]. Extracting this information from HRTEM images of “unknowns” and comparing it to structural information that is contained for a range of candidate structures in a crystallographic database, e.g. the open access, approximately 10,000 entries, mainly inorganic subset [9] of the Crystallography Open Database [10], constitutes structural fingerprinting in the TEM. Because atomic detail HRTEM images are in essence composed of crossing lattice fringes, i.e. dark and bright periodic intensity variations, we call our method “lattice-fringe fingerprinting”. This paper demonstrates the extraction of structural information from HRTEM images. Initially, this information is the projected geometry of the reciprocal lattice. As many different crystal structures possess similar projected reciprocal lattice geometries, one can, in following steps extract information on the accessible structure factors. Lattice fringe fingerprinting can, thus, be conceptually divided into a geometric component and a struct
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