NATURE MATERIALS DOL:10.1038/NMAT2406 INSIGHT I REVIEW ARTICLES Tilt axis 2 Tilt axis 1 20nm Figure 3 Tomographic reconstruction of a heterogeneous catalyst. 100nm 100nm Surface-rendered representation of a tomographic reconstruction of a heterogeneous catalyst based on disordered mesoporous silica supporting bimetallic ruthenium-platinum nanoparticles.The surface has been colour- coded according to the Gaussian curvature of the surface,with blue regions delineating saddle points.The nanoparticles(red)appear to prefer to anchor themselves at the (blue)saddle points. atomic number.These properties make the STEM HAADF signal ideal for tomographic applications".The earliest example of STEM HAADF tomography was in the study of heterogeneous catalysts based on metallic nanoparticles distributed within highly porous siliceous and carbonaceous support structures".There the STEM HAADF signal was able to discriminate nanometre-sized particles 100nr 25 nm from the background support,whereas in bright-field TEM the contrast from the particles was very weak More recent work%on similar catalyst structures,as shown in Fig.3,has revealed the dis- Figure 2|Dual-axis electron tomography.a,Illustration showing how tribution of particles on and within a porous framework and the a dual-axis tilt series collapses a missing wedge into a missing pyramid fractal nature of the internal surface.Theoretical work has shown of information.b,c,d,e,Reconstructions of cadmium telluride tetrapods from a dual-axis tilt series,reconstructed individually (b,c)and then as that when two parallel chemical reactions are taking place on a a dual-axis series (d).The tetrapod shown boxed in d is magnified in e. fractal surface,the slower,often undesired,reaction can be sup- The arrows indicate regions where the missing wedge has had its greatest pressed.Figure 3 also shows that STEM tomography can be used to relate the distribution of particles to the underlying surface curva- effect on the individual reconstructions.Each leg'of each tetrapod is better ture,showing in this case the strong preference of the particles(red) reconstructed in the dual-axis reconstruction.(Adapted from ref.29.) for saddle-shaped anchor points(blue). The suppression of unwanted diffraction contrast in STEM and the same tip can be imaged using both techniques,to provide HAADF tomography has led to the study of faceting and crystal complementary information32. morphology.Figure 4 reveals the faceting of magnetite crystals that The TEM is a remarkably versatile instrument,and the strong make up the backbone'of one strain of magnetotactic bacteria27.38 interaction of the electron beam with the specimen leads to a host of Similar studies have now been completed on a number of nano- possible imaging modes that can be used,in principle,for electron crystals,especially in catalyst systems where different facets can tomography.Bright-field TEM,which is used so prevalently in bio- have different catalytic properties.STEM tomography has also been logical tomography,is not in general suited to the study of crystalline used to determine the real-space crystallography of mesoporous materials.Diffraction contrast and Fresnel fringes do not satisfy the structures".For example,in MCM-48 mesoporous silica,which has projection requirement and can lead to serious artefacts in recon- a double-gyroid form,electron diffraction and 2D high-resolution structions.The image signal seen in the scanning TEM(STEM), electron microscopy(HREM)studies had concluded that an addi- using high-angle annular dark-field(HAADF)imaging,offers an tional pore system was present in the system.STEM tomography excellent alternative.As described elsewhere in this Insight edition', was able to visualize this directly in three dimensions and confirm STEM HAADF imaging can be considered incoherent,almost com- the space group symmetry40. pletely eliminating diffraction and phase contrast.The contrast is In metallurgy,STEM tomography is especially useful for inves- then,to a good approximation,monotonic with thickness,and is tigating the morphologies and distributions of precipitates in steels also sensitive to changes in composition;for a typical geometry and alloys.Figure 5a shows an example of a surface-rendered recon- and material,it is approximately proportional to 2,where Z is the struction of germanium precipitates in an aluminium-germanium NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 273 2009 Macmillan Publishers Limited.All rights reservednature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 273 NaTure maTerIals doi: 10.1038/nmat2406 insight | review articles and the same tip can be imaged using both techniques, to provide complementary information32. The TEM is a remarkably versatile instrument, and the strong interaction of the electron beam with the specimen leads to a host of possible imaging modes that can be used, in principle, for electron tomography. Bright-field TEM, which is used so prevalently in bio￾logical tomography, is not in general suited to the study of crystalline materials. Diffraction contrast and Fresnel fringes do not satisfy the projection requirement and can lead to serious artefacts in recon￾structions. The image signal seen in the scanning TEM (STEM), using high-angle annular dark-field (HAADF) imaging, offers an excellent alternative. As described elsewhere in this Insight edition1 , STEM HAADF imaging can be considered incoherent, almost com￾pletely eliminating diffraction and phase contrast. The contrast is then, to a good approximation, monotonic with thickness, and is also sensitive to changes in composition; for a typical geometry and material, it is approximately proportional to Z1.8, where Z is the atomic number. These properties make the STEM HAADF signal ideal for tomographic applications33. The earliest example of STEM HAADF tomography was in the study of heterogeneous catalysts based on metallic nanoparticles distributed within highly porous siliceous and carbonaceous support structures34. There the STEM HAADF signal was able to discriminate nanometre-sized particles from the background support, whereas in bright-field TEM the contrast from the particles was very weak35. More recent work36 on similar catalyst structures, as shown in Fig. 3, has revealed the dis￾tribution of particles on and within a porous framework and the fractal nature of the internal surface. Theoretical work has shown that when two parallel chemical reactions are taking place on a fractal surface, the slower, often undesired, reaction can be sup￾pressed. Figure 3 also shows that STEM tomography can be used to relate the distribution of particles to the underlying surface curva￾ture, showing in this case the strong preference of the particles (red) for saddle-shaped anchor points (blue). The suppression of unwanted diffraction contrast in STEM HAADF tomography has led to the study of faceting and crystal morphology. Figure 4 reveals the faceting of magnetite crystals that make up the ‘backbone’ of one strain of magnetotactic bacteria37,38. Similar studies have now been completed on a number of nano￾crystals, especially in catalyst systems where different facets can have different catalytic properties. STEM tomography has also been used to determine the real-space crystallography of mesoporous structures39. For example, in MCM-48 mesoporous silica, which has a double-gyroid form, electron diffraction and 2D high-resolution electron microscopy (HREM) studies had concluded that an addi￾tional pore system was present in the system. STEM tomography was able to visualize this directly in three dimensions and confirm the space group symmetry40. In metallurgy, STEM tomography is especially useful for inves￾tigating the morphologies and distributions of precipitates in steels and alloys. Figure 5a shows an example of a surface-rendered recon￾struction of germanium precipitates in an aluminium–germanium b a c d e 100 nm 100 nm 25 nm 100 nm Tilt axis 1 Tilt axis 2 20 nm Figure 3 | Tomographic reconstruction of a heterogeneous catalyst. Surface-rendered representation of a tomographic reconstruction of a heterogeneous catalyst based on disordered mesoporous silica supporting bimetallic ruthenium–platinum nanoparticles. The surface has been colour￾coded according to the Gaussian curvature of the surface, with blue regions delineating saddle points. The nanoparticles (red) appear to prefer to anchor themselves at the (blue) saddle points. Figure 2 | Dual-axis electron tomography. a, Illustration showing how a dual-axis tilt series collapses a missing wedge into a missing pyramid of information. b, c, d, e, Reconstructions of cadmium telluride tetrapods from a dual-axis tilt series, reconstructed individually (b, c) and then as a dual-axis series (d). The tetrapod shown boxed in d is magnified in e. The arrows indicate regions where the missing wedge has had its greatest effect on the individual reconstructions. Each ‘leg’ of each tetrapod is better reconstructed in the dual-axis reconstruction. (Adapted from ref. 29.) nmat_2406_APR09.indd 273 13/3/09 12:08:31 © 2009 Macmillan Publishers Limited. All rights reserved