DOI: 10.1002/ejic.201501342
Microreview
Electron Microscopy
What Can Electron Microscopy Tell Us Beyond Crystal Structures? Wuzong Zhou*[a] and Heather F. Greer[a] Abstract: Transmission electron microscopy is a powerful tool to directly image crystal structures. Not only that, it is often used to reveal crystal size and morphology, crystal orientation, crystal defects, surface structures, superstructures, etc. However, due to the 2D nature of TEM images, it is easy to make mistakes when we try to recover a 3D structure from them. Scanning electron microscopy is able to provide information on the parti-
cle size, morphology and surface topography. However, obtaining information on crystallinity of particles using SEM is difficult. In this microreview article, some practical cases of transmission and scanning electron microscopy investigations of inorganic crystals are reviewed. Commonly occurring uncertainties, imperfection and misunderstandings are discussed.
1. Introduction
PXRD pattern is easy, the data can often be difficult to interpret due to a phase problem, broadening or overlap of reflections with similar diffraction angles, especially when the structure has a large unit cell or low symmetry. Typically, in order to determine a complex structure, PXRD intensity data is combined with structure factor phase information from HRTEM images,[3] rotation electron diffraction (RED)[4] or precession electron diffraction (PED).[2,5] The most significant characteristic of electron microscopy is that it can directly show local structures in an atomic scale or a nanometer scale.[6] For nanomaterials, e.g. nanoparticle catalysts, nanowires and nanotubes of metal oxides, HRTEM is particularly useful, since the dimensions of the nanomaterials are often too small to give sufficient structural information in conventional diffraction experiments, such as X-ray diffraction (XRD) and neutron diffraction.[7] HRTEM images of thick samples often present complicated contrast patterns, which are not simply dependent on the crystal structures, due to multiple scattering of the electron beams. Computer simulation is the
Although electron microscopy related techniques, such as transmission electron microscopic (TEM) imaging, high resolution TEM (HRTEM) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray microanalysis (EDX), can reveal crystal structures, X-ray and neutron diffraction methods are much more accurate in determination of average crystal structures.[1] Nevertheless, electron microscopy techniques are often used to complement powder X-ray diffraction data (PXRD) to solve complex crystal structures.[2] Although collection of a [a] School of Chemistry, University of St Andrews St Andrews KY16 9ST, United Kingdom E-mail:
[email protected] http://www.st-andrews.ac.uk/chemistry/contact/academic/#wzhou © 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. · This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Wuzong Zhou is Professor of Chemistry at the University of St Andrews. He obtained his BSc in 1982 (Fudan University) and PhD in 1988 (University of Cambridge). His interests are in solid state chemistry in general, HRTEM investigations of defects in solids and non-classical mechanisms of crystal growth.
Heather F Greer is a Research Fellow at the University of St Andrews. She received her B.Sc in Chemistry and Mathematics at the University of St Andrews in 2009, and her PhD there in 2013. Her current research interests are the electron microscopic investigation of early stage crystal growth and reversed crystal growth of various solid state materials.
Eur. J. Inorg. Chem. 2016, 941–950
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© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Microreview common trial and error method to assess the proposed structural models. For nanomaterials, on the other hand, this multiple scattering effect can be ignored, and the explanation of the HRTEM images is relatively simple. In many cases, determination of average crystal structures of bulk materials is often not enough for us to understand the physico-chemical properties. When individual particles are examined using electron microscopy, much more information beyond average crystal structures can be obtained, including particle size and shape, crystal orientation, defects, crystallinity, surface structures, superstructures, etc.[8] These microstructures play important roles in exhibiting material properties. However, the explanations of the experimental data are not always straightforward. In this microreview, some practical cases of electron microscopic investigations of inorganic crystals are discussed. Common misunderstandings are explained and rectified, and the limits of the techniques are clearly delineated.
2. Crystal Size and Morphology Among all the extra structural features detectable by electron microscopy, the easiest target is to determine crystal size and morphology. Particle size of a powder sample can be directly observed from TEM and SEM images. However, the inherent disadvantage of electron microscopy is that the number of particles examined is always limited. When we have recorded a TEM/SEM image of particles, a common question for ourselves is that whether this image is typical for the sample. To convince ourselves, we normally check the sample preparation method for TEM and make sure that the particles deposited on a specimen grid is not size selective. We also observe and record a large number of particles, at both low and high magnifications although only one or two images may be used in a final presentation. This will increase the likelihood of an impartial representation of the sample, although this can be quite time consuming and expensive. When the number of measured particle sizes is large enough (say 300 to 500), the size distribution should show a smooth log-normal distribution curve.[9] With advances in synthesis capabilities the size of nanoparticles is steadily decreasing and moving towards a level where nanoparticles can consist from a small, predetermined number of atoms.[10] This leaves an even greater demand for accurate nanoparticle size distribution analysis. TEM, out of all the existing experimental techniques capable of measuring the particle size, is one of few techniques that allows direct (real space) visualisation of the nanoparticles. Using electron tomography, 3-dimensional images of nanoparticles can be shown. Pyrz and Buttrey[10] outlined a selection of topics which should be considered to avoid over-interpretation or the improper use of the information provided in TEM micrographs in regards to particle size characterisation. The issues include magnification, analysis method (manual vs. automated) and imaging type, (bright vs. dark field) and (TEM vs. STEM). Comparison of size measurements of nanoparticles > 5 nm in diameter by HRTEM and annular dark field scanning transmission electron microscopy (ADF-STEM) have been reported to agree very well. In contrast, similar measurements for smaller particles (
