nature INSIGHT I REVIEW ARTICLES materials PUBLISHED ONLINE:24 MARCH 2009|DOI:10.1038/NMAT2406 Electron tomography and holography in materials science Paul A.Midgley1*and Rafal E.Dunin-Borkowski2* The rapid development of electron tomography,in particular the introduction of novel tomographic imaging modes,has led to the visualization and analysis of three-dimensional structural and chemical information from materials at the nanometre level. In addition,the phase information revealed in electron holograms allows electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and,when combined with tomography,in three dimensions.Here we present an overview of the techniques of electron tomography and electron holography and demonstrate their capabilities with the aid of case studies that span materials science and the interface between the physical sciences and the life sciences. ver the past few years,transmission electron microscopy projection".The second paper showed how asymmetrical objects (TEM)has been revolutionized,not onlyby the introduction of can be reconstructed from a sufficient number of projections'.The new hardware such as field-emission electron guns,aberration third paper demonstrated how the signal-to-noise ratio could be correctors and monochromators,which are described elsewhere in improved by using an average re-projection calculated from a tilt this Insight edition2,but also by the development of new techniques, series of images.Three methods evolved:(1)electron crystallo- algorithms and software that take advantage of increased computa- graphy,in which diffraction patterns and/or high-resolution images tional speed and the ability to control and automate modern electron are recorded from biological systems,such as proteins,for which microscopes.Two techniques that have benefited from the intro- crystals can be grown;(2)single-particle analysis,in which images duction of digital image acquisition and the ability to record images of the same 'particle'(for example a virus)are recorded at differ- under different electron optical or specimen conditions are electron ent,often random,orientations;(3)electron tomography,in which tomography and electron holography.Electron tomography has been images of a single object are recorded about a tilt axis adopted rapidly by materials scientists as an important microscopy Although electron tomography was first applied in materials tool for the three-dimensional (3D)study of the morphologies and science in the late 1980s,its popularity has increased only in the chemical compositions of nanostructures,and electron holography last decade,with the introduction of novel tomographic imaging offers unique insights into the magnetic and electrostatic properties modes,automation of microscope control,new reconstruction of materials.For each technique,multiple images must be acquired algorithms and the increased speed and ease of the computation to reveal quantitative 3D,magnetic or electrical information,in com- involved.It is worth recalling,however,that electron tomography bination with automation and analysis software,resulting in the need is not the only 3D imaging mode available to the materials scientist. for challenging and often lengthy experiments. X-ray tomography is a routine tool in many laboratories,with desk- top instruments achieving a 3D resolution of a few micrometres. Electron tomography By using a synchrotron source,X-ray tomograms can be produced Although many forms of microscopy can be used to provide remark- with sub-100-nm resolution,and a 2D spatial resolution of~15 nm able images of materials across a range of length scales,the majority is possible using zone plates An X-ray approach based on'diffrac- of these techniques are used to record two-dimensional (2D)pro- tive imaging,which involves recording a tilt series of coherent dif- jections of 3D structures.However,the complexity of both natural fraction patterns and using phase-retrieval methods to reconstruct and artificial materials,such as device architectures in modern inte- real-space tomograms,allows a transverse resolution of-10 nm to grated circuits,highlights the need to develop tools and techniques be achieved.Scanning probe microscopy has been used by record- to explore the morphologies,compositions and physical proper- ing images of fresh surfaces revealed sequentially using a calibrated ties of materials in three dimensions.Such 3D imaging techniques chemical etch'.Atom probe tomography offers,in principle,atomic are encompassed by the field of tomography,which originates in a resolution in three dimensions's.Although recent developments 1917 paper on the projection of an object into a lower-dimensional have allowed problems with sample preparation and suitability to space3.Nearly 50 years later,tomographic X-ray scanning for 3D be overcome by using focused-ion-beam milling and laser-assisted medical imaging was proposed These ideas were picked up by staff field ionization,interpretation and the presence of artefacts remain at EMI,who built the first X-ray computed tomography scanner in a challenge. 1971 (ref.5).(It is often claimed that EMI were able to fund work The scanning electron microscope also provides an excellent on the computed tomography project only because of the enormous platform for 3D imaging at the 'mesoscale(20 nm to 20 um).To revenue generated by sales of the Beatles records in the 1960s.)Since achieve 3D imaging,new surfaces must be exposed in a sequential then the use and type of tomographic scanners for medical imaging and controlled fashion.Modern 'dual-beam'instruments,which have has proliferated. both electron optical and ion optical columns,enable a focused gal- The first examples of 3D reconstructions using TEM were pub- lium ion beam to mill thin slices sequentially and the electron beam lished in 1968 in three seminal papers.The first paper described the to image each exposed surface using secondary or backscattered determination of the structure of a biological macromolecule-the electrons.The in-plane resolution(typically ~5 nm)is finer than the helical symmetry of which allowed reconstruction from a single slice thickness(typically~100 nm),so the 3D resolution function is Department of Materials Science&Metallurgy,University of Cambridge,Pembroke Street,Cambridge CB2 3QZ,UK.Center for Electron Nanoscopy, Technical University of Denmark,DK-2800 Kongens Lyngby,Denmark."e-mail:pam33@cam.ac.uk;rdb@cen.dtu.dk. NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 271 2009 Macmillan Publishers Limited.All rights reservednature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 271 insight | review articles Published online: 24 march 2009 | doi: 10.1038/nmat2406 Over the past few years, transmission electron microscopy (TEM) has been revolutionized, not only by the introduction of new hardware such as field-emission electron guns, aberration correctors and monochromators, which are described elsewhere in this Insight edition1,2, but also by the development of new techniques, algorithms and software that take advantage of increased computa￾tional speed and the ability to control and automate modern electron microscopes. Two techniques that have benefited from the intro￾duction of digital image acquisition and the ability to record images under different electron optical or specimen conditions are electron tomography and electron holography. Electron tomography has been adopted rapidly by materials scientists as an important microscopy tool for the three-dimensional (3D) study of the morphologies and chemical compositions of nanostructures, and electron holography offers unique insights into the magnetic and electrostatic properties of materials. For each technique, multiple images must be acquired to reveal quantitative 3D, magnetic or electrical information, in com￾bination with automation and analysis software, resulting in the need for challenging and often lengthy experiments. electron tomography Although many forms of microscopy can be used to provide remark￾able images of materials across a range of length scales, the majority of these techniques are used to record two-dimensional (2D) pro￾jections of 3D structures. However, the complexity of both natural and artificial materials, such as device architectures in modern inte￾grated circuits, highlights the need to develop tools and techniques to explore the morphologies, compositions and physical proper￾ties of materials in three dimensions. Such 3D imaging techniques are encompassed by the field of tomography, which originates in a 1917 paper on the projection of an object into a lower-dimensional space3 . Nearly 50 years later, tomographic X-ray scanning for 3D medical imaging was proposed4 . These ideas were picked up by staff at EMI, who built the first X-ray computed tomography scanner in 1971 (ref. 5). (It is often claimed that EMI were able to fund work on the computed tomography project only because of the enormous revenue generated by sales of the Beatles records in the 1960s.) Since then the use and type of tomographic scanners for medical imaging has proliferated. The first examples of 3D reconstructions using TEM were pub￾lished in 1968 in three seminal papers. The first paper described the determination of the structure of a biological macromolecule — the helical symmetry of which allowed reconstruction from a single electron tomography and holography in materials science Paul a. midgley1 * and rafal e. dunin-borkowski2 * The rapid development of electron tomography, in particular the introduction of novel tomographic imaging modes, has led to the visualization and analysis of three-dimensional structural and chemical information from materials at the nanometre level. In addition, the phase information revealed in electron holograms allows electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography, in three dimensions. Here we present an overview of the techniques of electron tomography and electron holography and demonstrate their capabilities with the aid of case studies that span materials science and the interface between the physical sciences and the life sciences. projection6 . The second paper showed how asymmetrical objects can be reconstructed from a sufficient number of projections7 . The third paper demonstrated how the signal-to-noise ratio could be improved by using an average re-projection calculated from a tilt series of images8 . Three methods evolved: (1) electron crystallo￾graphy, in which diffraction patterns and/or high-resolution images are recorded from biological systems, such as proteins, for which crystals can be grown9 ; (2) single-particle analysis, in which images of the same ‘particle’ (for example a virus) are recorded at differ￾ent, often random, orientations10; (3) electron tomography, in which images of a single object are recorded about a tilt axis11. Although electron tomography was first applied in materials science in the late 1980s, its popularity has increased only in the last decade, with the introduction of novel tomographic imaging modes, automation of microscope control, new reconstruction algorithms and the increased speed and ease of the computation involved. It is worth recalling, however, that electron tomography is not the only 3D imaging mode available to the materials scientist. X-ray tomography is a routine tool in many laboratories, with desk￾top instruments achieving a 3D resolution of a few micrometres. By using a synchrotron source, X-ray tomograms can be produced with sub-100-nm resolution, and a 2D spatial resolution of ~15 nm is possible using zone plates12. An X-ray approach based on ‘diffrac￾tive imaging’, which involves recording a tilt series of coherent dif￾fraction patterns and using phase-retrieval methods to reconstruct real-space tomograms, allows a transverse resolution of ~10 nm to be achieved13. Scanning probe microscopy has been used by record￾ing images of fresh surfaces revealed sequentially using a calibrated chemical etch14. Atom probe tomography offers, in principle, atomic resolution in three dimensions15. Although recent developments have allowed problems with sample preparation and suitability to be overcome by using focused-ion-beam milling and laser-assisted field ionization, interpretation and the presence of artefacts remain a challenge. The scanning electron microscope also provides an excellent platform for 3D imaging at the ‘mesoscale’ (20 nm to 20 μm)16. To achieve 3D imaging, new surfaces must be exposed in a sequential and controlled fashion. Modern ‘dual-beam’ instruments, which have both electron optical and ion optical columns, enable a focused gal￾lium ion beam to mill thin slices sequentially and the electron beam to image each exposed surface using secondary or backscattered electrons. The in-plane resolution (typically ~5 nm) is finer than the slice thickness (typically ~100 nm), so the 3D resolution function is 1 Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. 2 Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. *e-mail: pam33@cam.ac.uk; rdb@cen.dtu.dk. nmat_2406_APR09.indd 271 13/3/09 12:08:29 © 2009 Macmillan Publishers Limited. All rights reserved