NATURE MATERIALS DOL:10.1038/NMAT2406 INSIGHT I REVIEW ARTICLES from such EFTEM series,as illustrated in Fig.5b2.More recently, Electron holography image spectroscopy was extended to volume spectroscopy by Electron holography was originally proposedes as a means of recording a large energy series at every tilt angless.A low-loss series correcting for electron microscope lens aberrations,substantially of a nanocomposite composed of a multiwalled carbon nanotube before the advent of the laser and the use of holography in light encased in nylon was recorded every 3 eV over a wide range of tilts. optics.The technique is based on the formation of an interference The different plasmon excitation energies of the nylon(~22 eV)and pattern or 'hologram'in the TEM.Its development followed from the nanotube (~27 ev)enabled the two components of the com- earlier experiments in electron interferometry64-66,many of which posite to be distinguished.By reconstructing tomograms at indi- took place at the University of Tubingen,and relied on the develop- vidual energy losses,it was possible to identify a voxel or subvolume ment and availability of high-brightness electron sources.The tech- common to all of the energy-loss tomograms and,by plotting the nique overcomes the important limitation of most TEM imaging intensity of the voxel as a function of energy loss,to extract spec- modes,namely that only spatial distributions of image intensity are tral information from within the tomogram.In conventional EELS, recorded.All information about the phase shift of the high-energy spectral information is always projected through the structure,but electron wave that passes through the specimen is then lost now it is possible to extract spectral information from a subvolume By contrast,electron holography allows the phase shift of the without any projection artefact. electron wave to be recovered.As the phase shift is sensitive to local Early attempts to map chemical information using EDX were variations in magnetic and electrostatic potential,the technique complicated by the directionality and inefficiency of the sample- can be used to obtain quantitative information about magnetic detector geometry,by the need to tilt away from the detector and and electric fields in materials and devices with a spatial resolution by the consequent shadowing in half of the tilt seriess.Recent that can approach the nanometre scale.This capability is of great work has taken advantage of needle specimens,where shadowing is importance for the study of a wide variety of material properties. eliminated and the detector geometry is not such a problemss.EELS, such as the characterization of magnetic domain walls in spintronic EFTEM and EDX images are all prone to diffraction effects through devices?and the factors that affect the coercive fields of individual the coupling of elastic and inelastic signals.These can be mini- magnetic nanostructures8 mized by forming jump-ratio images or dividing elemental maps The original work described the reconstruction of an image by by low-loss (or zero-loss)images.However,care must be taken if illuminating an 'in-line'electron hologram with a parallel beam of such images are used for tomography as the resultant signal may not light and using a spherical-aberration-correcting plate and an astig- satisfy the projection requirement. matism corrector,but the image reconstructed in this way is dis- Electron tomography has also been developed to study crystal- turbed by a 'ghost'or 'conjugate'twin image.The mode of electron line defects,and especially dislocation networks,in three dimen- holography that is most often used for tackling problems in materials sions.By recording a tilt series of weak-beam dark-field images, science is instead the off-axis,or 'sideband,mode,which is available it was possible to reveal a dislocation network in a gallium nitride on many modern electron microscopes and has been applied to the epitaxial layers.However,to do so it is critical that the diffraction characterization of materials as diverse as quantum well structures, conditions do not change significantly as the tilt series is recorded magnetoresistive films,nanowires and semiconductor devices9 (a difficult practical task)and that extraneous contrast,such as The electron microscope geometry for the TEM mode of off-axis thickness fringe contrast,is minimized.Weak-beam dark-field electron holography is shown schematically in Fig.6a.A field- tomography has also been used to investigate secondary phases in emission electron gun is used to provide a highly coherent source of metallic alloys where ordered phases grow from the matrix.In con- electrons.In reality,the source is never perfectly coherent,but the junction with EFTEM tomography,the shapes and compositions of degree of coherence must be such that an interference fringe pattern y'precipitates were determined in a nickel-aluminium-titanium of sufficient quality can be recorded within a reasonable acquisition superalloys.The practical difficulties of weak-beam dark-field time,during which specimen and/or beam drift must be negligible. tomography led to the development of a STEM analogue using a Although electron holograms have historically been recorded on low-angle annular dark-field imaging mode in which a number of photographic film,digital acquisition using charge-coupled-device dark-field beams contribute to the image.The advantages of this cameras is now common practice.To acquire an off-axis electron method for dislocation tomography are that the image is less sensi- hologram,the specimen is positioned so that it covers approxi- tive to changes in diffraction conditions,the image is effectively a mately half the field of view.A voltage is then applied to an elec- sum of many dark-field images,which tends to average out thick- trostatic 'biprism',which is usually located in place of one of ness contrast but enhance (albeit slightly blur)dislocation contrast, the conventional selected-area apertures in the microscope.The and data collection is easily automatedss biprism is analogous to a glass prism in light optics,but takes the As well as being able to map morphology and composition,it form of a fine(<1-um diameter)wire that is often made from gold- is also possible to map physical properties in three dimensions coated quartz.The voltage applied to the biprism acts to tilt a'ref- using a combination of electron holography,which is sensitive to erence'electron wave that passes through vacuum with respect to changes in electrostatic potentials and magnetic fields,and electron the electron wave that passes through the specimen.The two waves tomography.Such 3D potential and field mapping will be discussed are allowed to overlap and interfere.If the electron source is suf- below.A future goal is to be able to visualize atoms in three dimen- ficiently coherent then,in addition to a bright-field image of the sions.True atomic-resolution tomography may become possible specimen,an interference fringe pattern is formed on the detector either using new aberration-corrected instruments39.40 in combi- in the overlap region.Just as in a textbook 'double-slit experiment; nation with the discrete constraint that the object is composed of electrons are emitted one by one from the field-emission electron atoms,or by using an aberration-corrected STEM to reduce the gun in the microscope.After being deflected by the biprism,they depth of field and recording a series of 'confocal'images to build reach the detector and are detected individually as particles.When a up a 3D atomic lattice.Suggestions have been made to combine the large number of electrons has accumulated,their wave-like proper- confocal approach with a limited tilt series and use iterative con- ties become apparent and an interference fringe pattern is built up. straints and discrete tomography algorithms to build up a best-fit The amplitude and the phase shift of the electron wave that 3D lattice.Electron diffractive imaging,analogous to the synchro- leaves the specimen are recorded in the intensities and the posi- tron X-ray technique,may also be able to help in the quest for 3D tions of the interference fringes in the hologram,respectively. lattice imaging. The phase shift is sensitive to the in-plane component of the NATURE MATERIALS|VOL 8|APRIL 2009 www.nature.com/naturematerials 275 2009 Macmillan Publishers Limited.All rights reservednature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 275 NaTure maTerIals doi: 10.1038/nmat2406 insight | review articles from such EFTEM series, as illustrated in Fig. 5b52. More recently, image spectroscopy was extended to volume spectroscopy by recording a large energy series at every tilt angle53. A low-loss series of a nanocomposite composed of a multiwalled carbon nanotube encased in nylon was recorded every 3 eV over a wide range of tilts. The different plasmon excitation energies of the nylon (~22 eV) and the nanotube (~27 eV) enabled the two components of the com￾posite to be distinguished. By reconstructing tomograms at indi￾vidual energy losses, it was possible to identify a voxel or subvolume common to all of the energy-loss tomograms and, by plotting the intensity of the voxel as a function of energy loss, to extract spec￾tral information from within the tomogram. In conventional EELS, spectral information is always projected through the structure, but now it is possible to extract spectral information from a subvolume without any projection artefact. Early attempts to map chemical information using EDX were complicated by the directionality and inefficiency of the sample– detector geometry, by the need to tilt away from the detector and by the consequent shadowing in half of the tilt series54. Recent work has taken advantage of needle specimens, where shadowing is eliminated and the detector geometry is not such a problem55. EELS, EFTEM and EDX images are all prone to diffraction effects through the coupling of elastic and inelastic signals. These can be mini￾mized by forming jump-ratio images or dividing elemental maps by low-loss (or zero-loss) images. However, care must be taken if such images are used for tomography as the resultant signal may not satisfy the projection requirement. Electron tomography has also been developed to study crystal￾line defects, and especially dislocation networks, in three dimen￾sions. By recording a tilt series of weak-beam dark-field images, it was possible to reveal a dislocation network in a gallium nitride epitaxial layer56. However, to do so it is critical that the diffraction conditions do not change significantly as the tilt series is recorded (a difficult practical task) and that extraneous contrast, such as thickness fringe contrast, is minimized. Weak-beam dark-field tomography has also been used to investigate secondary phases in metallic alloys where ordered phases grow from the matrix. In con￾junction with EFTEM tomography, the shapes and compositions of γ′ precipitates were determined in a nickel–aluminium–titanium superalloy57. The practical difficulties of weak-beam dark-field tomography led to the development of a STEM analogue using a low-angle annular dark-field imaging mode in which a number of dark-field beams contribute to the image. The advantages of this method for dislocation tomography are that the image is less sensi￾tive to changes in diffraction conditions, the image is effectively a sum of many dark-field images, which tends to average out thick￾ness contrast but enhance (albeit slightly blur) dislocation contrast, and data collection is easily automated58. As well as being able to map morphology and composition, it is also possible to map physical properties in three dimensions using a combination of electron holography, which is sensitive to changes in electrostatic potentials and magnetic fields, and electron tomography. Such 3D potential and field mapping will be discussed below. A future goal is to be able to visualize atoms in three dimen￾sions. True atomic-resolution tomography may become possible either using new aberration-corrected instruments59,60 in combi￾nation with the discrete constraint that the object is composed of atoms, or by using an aberration-corrected STEM to reduce the depth of field and recording a series of ‘confocal’ images to build up a 3D atomic lattice. Suggestions have been made to combine the confocal approach with a limited tilt series and use iterative con￾straints and discrete tomography algorithms to build up a best-fit 3D lattice. Electron diffractive imaging61, analogous to the synchro￾tron X-ray technique, may also be able to help in the quest for 3D lattice imaging. electron holography Electron holography62 was originally proposed63 as a means of correcting for electron microscope lens aberrations, substantially before the advent of the laser and the use of holography in light optics. The technique is based on the formation of an interference pattern or ‘hologram’ in the TEM. Its development followed from earlier experiments in electron interferometry64–66, many of which took place at the University of Tübingen, and relied on the develop￾ment and availability of high-brightness electron sources. The tech￾nique overcomes the important limitation of most TEM imaging modes, namely that only spatial distributions of image intensity are recorded. All information about the phase shift of the high-energy electron wave that passes through the specimen is then lost. By contrast, electron holography allows the phase shift of the electron wave to be recovered. As the phase shift is sensitive to local variations in magnetic and electrostatic potential, the technique can be used to obtain quantitative information about magnetic and electric fields in materials and devices with a spatial resolution that can approach the nanometre scale. This capability is of great importance for the study of a wide variety of material properties, such as the characterization of magnetic domain walls in spintronic devices67 and the factors that affect the coercive fields of individual magnetic nanostructures68. The original work63 described the reconstruction of an image by illuminating an ‘in-line’ electron hologram with a parallel beam of light and using a spherical-aberration-correcting plate and an astig￾matism corrector, but the image reconstructed in this way is dis￾turbed by a ‘ghost’ or ‘conjugate’ twin image. The mode of electron holography that is most often used for tackling problems in materials science is instead the off-axis, or ‘sideband’, mode, which is available on many modern electron microscopes and has been applied to the characterization of materials as diverse as quantum well structures, magnetoresistive films, nanowires and semiconductor devices69. The electron microscope geometry for the TEM mode of off-axis electron holography is shown schematically in Fig. 6a. A field￾emission electron gun is used to provide a highly coherent source of electrons. In reality, the source is never perfectly coherent, but the degree of coherence must be such that an interference fringe pattern of sufficient quality can be recorded within a reasonable acquisition time, during which specimen and/or beam drift must be negligible. Although electron holograms have historically been recorded on photographic film, digital acquisition using charge-coupled-device cameras is now common practice. To acquire an off-axis electron hologram, the specimen is positioned so that it covers approxi￾mately half the field of view. A voltage is then applied to an elec￾trostatic ‘biprism’70, which is usually located in place of one of the conventional selected-area apertures in the microscope. The biprism is analogous to a glass prism in light optics, but takes the form of a fine (<1-μm diameter) wire that is often made from gold￾coated quartz. The voltage applied to the biprism acts to tilt a ‘ref￾erence’ electron wave that passes through vacuum with respect to the electron wave that passes through the specimen. The two waves are allowed to overlap and interfere. If the electron source is suf￾ficiently coherent then, in addition to a bright-field image of the specimen, an interference fringe pattern is formed on the detector in the overlap region. Just as in a textbook ‘double-slit experiment’, electrons are emitted one by one from the field-emission electron gun in the microscope. After being deflected by the biprism, they reach the detector and are detected individually as particles. When a large number of electrons has accumulated, their wave-like proper￾ties become apparent and an interference fringe pattern is built up. The amplitude and the phase shift of the electron wave that leaves the specimen are recorded in the intensities and the posi￾tions of the interference fringes in the hologram, respectively. The phase shift is sensitive to the in-plane component of the nmat_2406_APR09.indd 275 13/3/09 12:08:33 © 2009 Macmillan Publishers Limited. All rights reserved