"If, in some cataclysm, all scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or atomic fact; or whatever you wish to call it) that all things are made seeing atoms of atoms." Mhairi Gass & Andrew Bleloch It is not an unreasonable paraphrase of this famous statement by Richard Feynman (1964) to say that "to understand the properties of all material things we need only to know which atoms are where". Electron microscopy is a technique that in the last decade has been able to image and analyse the atomic structure of a range of materials with atom by atom sensitivity and has led to a greater understanding of materials properties, from nanotubes and nanowires through to grain boundaries in metal alloys and ferritin particles in cells. The specimen is a 70 nm thick ultra-microtomed section of lung cell from a mouse. The specimen was osmicated and the section poststained with uranyl acetate and lead citrate. The Z-contrast STEM image shows detailed cross sections of a number of cilia consisting of an outer ring of nine microtubial doublets and two central microtubial doublets. The primary function of cilia is to sweep dirt and mucus out of the lung and they are found in the lining of the trachea. The field of view for the image is 3 microns. 70 Issue 18 JuNe 2010 71 Forty years ago, Albert Crewe (Crewe et al. (1970)) showed images of individual atoms acquired from new equipment under development in his laboratory at the University of Chicago, Illinois; the machine was the scanning transmission electron microscope (STEM). Using a novel annular dark field detector (rather than the conventional bright field imaging method which was popular in Fig. 1. Spherical aberration, a) all the rays that pass through a perfect electron microscopy), Crewe demonstrated, both lens focus at the same point, b) in an imperfect lens the rays that pass furthest from the optic axis are more strongly focused than the rays that pass through the centre of the lens; these aberrations result in a STEM has been a loss of resolution. quadrupole-octupole corrector for the scanning technique that has played electron microscope, Zach and Haider (1995). In an important role in 1997, at the Electron Microscopy and Analysis Group (EMAG) conference in Cambridge, Krivanek materials research and et al. presented results from their prototype has helped solve many spherical aberration electromagnetic quadrupole – octupole corrector for a STEM and in 2002 Batson materials questions over et al. demonstrated sub-angstrom resolution using the years. such an aberration corrector in a STEM. There Fig. 3. Nickel disilicide. The topmost images are high resolution Z-contrast images of the interface for variant 1 and variant 2; the position of the are now in excess of 100 aberration corrected outer most layers of Ni atoms in the silicide are marked by thin black lines. The bottom images are averaged images of the interface structure and overlaid ball-and-stick display of the deduced model in [-110] (left) and [110] (right) projections. Reprinted Figure with permission from Falke et theoretically and experimentally, that by acquiring STEM machines worldwide, as well as numerous al. ‘Atomic Structure of a (2 x 1) Reconstructed NiSi2/Si(001) Interface’, Copyright (2004), by the American Physical Society. (http://prl.aps.org/ abstract/PRL/v92/i11/e116103). the electrons that had been elastically scattered to aberration-corrected conventional transmission astigmatism that are corrected. The corrector that high angles, individual heavy atoms such as uranium electron microscopes. In 2001 the first dedicated So what is an aberration corrector, what information was used for the first SuperSTEM machine was a and thorium could be imaged on a thin carbon film. AC-STEM facility, SuperSTEM, was opened in can we now achieve that was previously inaccessible Daresbury, UK, with the aim of making aberration and which materials problems can AC-STEM help quadrupole-octupole design by Nion Co. based on a Ever since, STEM has been a technique that has correction available to academic researchers. solve? derivative of Scherzer’s 1947 design. The corrector played an important role in materials research and was retrofitted into an existing VG HB501 STEM, What is a spherical aberration has helped solve many materials questions over as shown in Figure 2a, improving the resolution to corrector? the years. However, until recently its resolution, as 1Å; the machine was also fitted with an electron Spherical aberration is a defect which afflicts all round with other electron microscopes, has been limited energy loss spectrometer to allow detailed electromagnetic lenses. The effect of this aberration by lens aberrations and thus little improvement in elemental information to be extracted. The second on a round lens is shown in Figure 1, where the rays resolution was achieved during the 30 year period machine to be installed at SuperSTEM was the that pass furthest from the optical axis are more following Crewe’s landmark experiments. Although first dedicated STEM designed for the aberration strongly focused than the rays that pass along the approaches to correct the aberrations had been correction era by Nion Co. and incorporated a fifth optical axis; this results in a range of focal points suggested starting with the fundamental work order aberration corrector and significantly greater and thus the resolution is limited. The addition of by Scherzer in 1936, experimental success was experimental flexibility, Figure 2b. In this article, we a spherical aberration corrector compensates for elusive until the mid 1990s when a combination demonstrate the adaptability of these machines to this effect and brings the focal crossover of the rays of scientific ingenuity, stable electronics, machining a range of materials and provide examples of the to the same point, thus improving the resolution, capabilities and computing power resulted in several variety of information that can be extracted. as illustrated by the “perfect lens” in Figure 1a. successful attempts at aberration correction; e.g. Fig. 2. Aberration corrected scanning transmission electron This is analogous to the way spectacles are added Initial work at SuperSTEM the electromagnetic sextupole corrector for the microscopes at the SuperSTEM facility, a) SuperSTEM 1 is a VG HB501 retro fitted with a Nion 2nd generation aberration corrector, to our eyes to correct aberrations in our vision The germ of the SuperSTEM facility was planted at conventional transmission electron microscope b) SuperSTEM 2 is a Nion UltraSTEM. The electron gun for both though in this case it is usually only defocus and the same 1997 EMAG meeting at which Krivanek by Haider et al. (1995) and the electrostatic machines is beneath the column for stability. 72 Issue 18 JuNe 2010 73 Z-contrast images of the interfaces (variant 1 and Atomic resolution focus” with the aim to infer some 3-dimensional variant 2) where the NiSi2 appears brighter than information from the specimen. An example of the Si due to the heavier atomic species present. Z-contrast images have this is shown in Figure 5 where the locations of The position of the outermost layer of Ni atoms in gold catalyst atoms within a silicon nanowire were the silicide is marked by a thin black line. Averaged revolutionised microscopy investigated, Allen et al. The inclusion of gold within images of the interface structure and an overlaid over recent years. the nanowire would be detrimental to its electrical ball-and-stick display of the deduced model in the properties. A series of Z-contrast images with 1nm [-110] (left) and [110] (right) projection are shown focus steps were acquired of an intrinsic Si nanowire. below. These indicate that variant 1 and variant 2 Figure 5a presents the sum of 7 aligned images from are orthogonal projections of one and the same the series; it shows impurities trapped at a (111) structure which must then be of a 2 x 1 type. twin defect e.g. d1,d2, and ‘bulk’ impurities e.g. b1,b2. The excess intensity of Au atoms is plotted as a The Z-contrast image is also sensitive to the function of focal depth in Figure 5b indicating that amount of material being imaged; for small the peak associated with atom b1 lies between the nanoparticles consisting of only one element the defect atom peaks and is therefore located within relationship is linear. This relationship has been used the nanowire as indicated in the schematic diagram, Fig. 4. Gold 309 nanoparticles. a) – c) Z-contrast images of Au309 to calculate the number of atoms in gold clusters by Figure 5c. clusters on a carbon film demonstrating various outline shapes: Li et al. Z-contrast images of Au309 clusters on a pentagon, square and hexagon. The intensity variation within the clusters clearly demonstrates atomic column resolution. d) Three- carbon film are shown in Figures 4a-c. The intensity Extracting elemental dimensional atom density profile of Au309 was derived from a) and a hard-sphere model for an Ino-decahedral structure is shown with variation within the clusters clearly demonstrates information the electron beam (arrow) parallel to the five-fold axis. Experimental atomic column resolution. A three-dimensional Atomic resolution Z-contrast images have e) and simulated f) intensity line profiles are taken from the central atom column of the cluster to one of the corners (indicated in insets revolutionised microscopy over recent years, with red line) for comparison. Figure first published by Li et al. in Nature, Copyright (2008). presented his initial results by a paper presented Fig. 6. Automically resolved EELS mapping.