REVIEW ARTICLES INSIGHT NATURE MATERIALS DOL:10.1038/NMAT2406 images,and approaches such as in situ annealing of specimens in the TEM"and simulations of the effect of the presence of the speci- men surfaces"and the high-energy electron beam on the electrical properties of the specimen are being studied.An interesting vari- ant of off-axis electron holography,which can be used to provide 2D information about strain distributions in semiconductors,has recently been developed.This approach involves using the electro- W,depletion width static biprism to form a dark-field electron hologram by interfering with each other diffracted electrons that have been scattered in strained and unstrained regions of the specimen'00. b Potential,V(x) Electron holographic tomography A particularly exciting prospect is the combination of electron holography with electron tomography to characterize electrostatic and magnetic fields inside nanostructured materials with nano- metre spatial resolution in three dimensions,rather than simply in projection,by acquiring ultrahigh-tilt series of electron holo- grams.Both the electrostatic phase shift and the magnetic phase gradient recorded using electron holography satisfy the projection requirement for electron tomographic reconstruction.Figure 8d shows the extremely promising result of an experiment performed to characterize the 3D electrostatic potential of a semiconductor p-n junction in a thin TEM specimen examined under an applied reverse bias,in which the effect of the surfaces of the thin specimen on the internal potential has been measured in three dimensions0 The prospect of characterizing magnetic vector fields inside nano- crystals in three dimensions by combining electron tomography with electron holography is also of great interest This approach has previously been used to image magnetic fringing fields outside 25m materials in three dimensions,by acquiring two ultrahigh-tilt series 25 nm of differential phase-contrast images or electron holograms about orthogonal specimen tilt axes'04.Although the theory underlying such measurements is well establisheds,their application to the 280nm characterization of magnetic fields inside nanostructured materi- als is complicated by the fact that the (often dominant)contribu- tion of the mean inner potential to the measured phase shift must be removed at each sample tilt angle.Four tilt series of holograms may then be required.In addition,the need to acquire electron holograms at high specimen tilt angles about two axes imposes 1,000nm additional requirements on the specimen geometry. y Concluding remarks Electron tomographic acquisition and reconstruction,coupled Crystalline but electrically ■n-type silicon with the remarkably large number of imaging modes available damaged silicon in the electron microscope,ensures that a diversity of 3D struc- Amorphous surface layers ■p-type silicon tural and chemical information can be obtained from a variety of materials.There is great scope to develop the technique further as a Figure 8|Electron holographic tomography.a,Diagram showing the cross reliable method for quantitative measurements of the physical and sectional geometry of a TEM specimen of uniform thickness that contains a chemical properties of nanoscale structures in three dimensions, symmetrical semiconductor p-n junction.The thickness of the 'electrically to move towards genuine 3D nanometrology.Off-axis electron active'specimen is denoted by to The layers at the top and bottom surfaces of holography provides quantitative information about electrostatic the specimen represent electrically passivated or depleted layers,the physical and magnetic fields in materials,as well as allowing fundamental and electrical nature of which is affected by TEM specimen preparation. studies of the physics of electromagnetic fields in nanoscale struc- b,Diagram showing the electrostatic potential profile across the p-n junction. tures.Considerable further work is possible to develop,optimize The built-in voltage is denoted by V,and W is the width of the depletion and automate the technique,including the measurement of weak region over which the potential changes.The sign convention for the potential fields(towards detecting single magnetic spins),the development of is consistent with the mean inner potential of the specimen being positive approaches for improving its time resolution(for studying chemical relative to vacuum.c,Representative phase image reconstructed from an reactions,biological samples and beam-sensitive materials such as off-axis electron hologram of a silicon p-n junction sample.The sample edge zeolites)and overcoming the effects of specimen preparation and is at the lower right of the image,and no attempt has been made to remove electron irradiation on measurements of electrostatic fields. phase 'wraps'lying along this edge.The sign convention for the potential As well as individually providing unique high-spatial-resolution is as in b.d,Tomographic reconstruction of the electrostatic potential in a information,electron tomography and electron holography can focused-ion-beam-milled specimen containing an electrically biased silicon be combined with other capabilities available in modern elec- p-n junction.Contours spaced every 0.2 V have been superimposed onto the tron microscopes,including aberration-corrected imaging,in situ reconstructed tomogram.(Adapted from refs 95 and 101.) environmental cells and the ability to examine working devices 278 NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 2009 Macmillan Publishers Limited.All rights reserved278 nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials review articles | insight NaTure maTerIals doi: 10.1038/nmat2406 images95,96, and approaches such as in situ annealing of specimens in the TEM97 and simulations of the effect of the presence of the speci￾men surfaces98 and the high-energy electron beam99 on the electrical properties of the specimen are being studied. An interesting vari￾ant of off-axis electron holography, which can be used to provide 2D information about strain distributions in semiconductors, has recently been developed. This approach involves using the electro￾static biprism to form a dark-field electron hologram by interfering with each other diffracted electrons that have been scattered in strained and unstrained regions of the specimen100. electron holographic tomography A particularly exciting prospect is the combination of electron holography with electron tomography to characterize electrostatic and magnetic fields inside nanostructured materials with nano￾metre spatial resolution in three dimensions, rather than simply in projection, by acquiring ultrahigh-tilt series of electron holo￾grams. Both the electrostatic phase shift and the magnetic phase gradient recorded using electron holography satisfy the projection requirement for electron tomographic reconstruction. Figure 8d shows the extremely promising result of an experiment performed to characterize the 3D electrostatic potential of a semiconductor p–n junction in a thin TEM specimen examined under an applied reverse bias, in which the effect of the surfaces of the thin specimen on the internal potential has been measured in three dimensions101. The prospect of characterizing magnetic vector fields inside nano￾crystals in three dimensions by combining electron tomography with electron holography is also of great interest102,103. This approach has previously been used to image magnetic fringing fields outside materials in three dimensions, by acquiring two ultrahigh-tilt series of differential phase-contrast images or electron holograms about orthogonal specimen tilt axes104. Although the theory underlying such measurements is well established105, their application to the characterization of magnetic fields inside nanostructured materi￾als is complicated by the fact that the (often dominant) contribu￾tion of the mean inner potential to the measured phase shift must be removed at each sample tilt angle. Four tilt series of holograms may then be required. In addition, the need to acquire electron holograms at high specimen tilt angles about two axes imposes additional requirements on the specimen geometry. concluding remarks Electron tomographic acquisition and reconstruction, coupled with the remarkably large number of imaging modes available in the electron microscope, ensures that a diversity of 3D struc￾tural and chemical information can be obtained from a variety of materials. There is great scope to develop the technique further as a reliable method for quantitative measurements of the physical and chemical properties of nanoscale structures in three dimensions, to move towards genuine 3D nanometrology. Off-axis electron holography provides quantitative information about electrostatic and magnetic fields in materials, as well as allowing fundamental studies of the physics of electromagnetic fields in nanoscale struc￾tures. Considerable further work is possible to develop, optimize and automate the technique, including the measurement of weak fields (towards detecting single magnetic spins), the development of approaches for improving its time resolution (for studying chemical reactions, biological samples and beam-sensitive materials such as zeolites) and overcoming the effects of specimen preparation and electron irradiation on measurements of electrostatic fields. As well as individually providing unique high-spatial-resolution information, electron tomography and electron holography can be combined with other capabilities available in modern elec￾tron microscopes, including aberration-corrected imaging, in situ environmental cells and the ability to examine working devices e– tel Vbi Potential, V(x) W, depletion width x p n p n 200 nm 25 nm 25 nm 25 nm 25 nm 280 nm 1,000 nm z x y Crystalline but electrically damaged silicon Amorphous surface layers n-type silicon p-type silicon a b c d Figure 8 | electron holographic tomography. a, Diagram showing the cross￾sectional geometry of a TEM specimen of uniform thickness that contains a symmetrical semiconductor p–n junction. The thickness of the ‘electrically active’ specimen is denoted by tel. The layers at the top and bottom surfaces of the specimen represent electrically passivated or depleted layers, the physical and electrical nature of which is affected by TEM specimen preparation. b, Diagram showing the electrostatic potential profile across the p–n junction. The built-in voltage is denoted by Vbi and W is the width of the depletion region over which the potential changes. The sign convention for the potential is consistent with the mean inner potential of the specimen being positive relative to vacuum. c, Representative phase image reconstructed from an off-axis electron hologram of a silicon p–n junction sample. The sample edge is at the lower right of the image, and no attempt has been made to remove phase ‘wraps’ lying along this edge. The sign convention for the potential is as in b. d, Tomographic reconstruction of the electrostatic potential in a focused-ion-beam-milled specimen containing an electrically biased silicon p–n junction. Contours spaced every 0.2 V have been superimposed onto the reconstructed tomogram. (Adapted from refs 95 and 101.) nmat_2406_APR09.indd 278 13/3/09 12:08:35 © 2009 Macmillan Publishers Limited. All rights reserved