COMMENTARY INSIGHT precision is that it cannot be allowed by 0.0 a microscope with a 70-pm Rayleigh or 0.02 年年◆季专果果票 0 point resolution2.However,resolution -0.02 华◆*中中蒙单 and precision are two separate physical -0.04 60 terms.Although resolution is defined by the minimum separation of two optically 80■■ 40 ■■ broadened intensity peaks at which these ■ OA can just be separated in the image,the -40 distance between two well-isolated peaks, -80 ■■■■■■■ fitted for example by two-dimensional 0 10 Gaussians,can be measured at a precision Distance (in units of c) more than an order of magnitude higher. It would be a real pity to turn down the Figure 2 Polarization domain wall in ferroelectric PZT.a,Atomic structure.Arrows give the direction extraordinary opportunities offered by of the spontaneous polarization,which can be directly inferred from the local atom displacements.The aberration-corrected electron microscopy shifts of the oxygen atoms (blue circles)out of the Ti/Zr-atom rows (red circles)can be seen directly,as because of an unfortunately common can the change in separation between Ti/Zr and Pb(yellow circles).b,Atomic-resolution measurements misunderstanding. of the shifts of oxygen(),and titanium/zirconium (atoms as a function of distance from the wall centre (units of c-lattice parameter)in a longitudinal-inversion domain wall;c,The value of the local A question to conclude polarization Ps that can be calculated from these data.Adapted from ref.12.2008 NPG. Is the scientific community ready to receive the extraordinary new results achievable through aberration-corrected with atomic resolution means that the and c),which in early work could not be transmission electron microscopy?Readers information must be entirely local on the used for the image formation because of the may find their own individual answer to atomic level.Any change in the position detrimental influence of lens aberrations this question.The everyday life of electron or occupancy of an atomic site in the Over the past three decades,taking microscopists active in this field shows that sample must show up in the image as an so-called 'high-resolution'images has there is still some way to go in alerting the individual signal localized only at the become a common technique in materials scientific community to the opportunities corresponding atomic position.In this science.Some of this work was capable of offered by measuring quantum mechanical stringent sense,apart from a few favourable atomic-scale resolution.In particular this wavefunctions in atomic dimensions. cases,only the images obtainable in applies to studies in which the EPWF was So great is the potential of these new modern aberration-corrected instruments calculated from an image series,even in techniques that it will be well worth match these standards.The concept is non-corrected instruments,when in EPWF engaging in such a process. illustrated in Fig.1. reconstruction the optical aberrations were corrected to a certain extent by software. Knut W.Urban is at the Institute of Solid State It would be a real pity to Nevertheless,the bulk of this earlier work Research and the Ernst Ruska Centre for Microscopy was concerned not with individual atoms and Spectroscopy with Electrons,Research Centre turn down the extraordinary but rather with collective properties such Juilich,D-52425 Jilich,Germany opportunities offered by as crystal structures,lattice parameters and e-mail:k.urban@fz-juelich.de crystal symmetry. abberation-corrected electron In contrast,today's aberration-corrected References microscopy because of an instruments yield images of such quality 1.Urban,K.W.Science 321,506-510(2008). that we can virtually put our finger on 2.Gibson.LM.Nare329,763-764(19871. unfortunately common individual atomic positions.Figure 2a, 3.Huxford,N.P..Eaglesham.D.I.Humphreys,C.I.Natture 329.812-81319871. misunderstanding. for example,shows a ferroelectric domain Coene.W.Janssen.G.Op de Beeck.M.Van Dyck.D. boundary in the microelectronic storage Phrys.Rev.Let69,3743-3746(1992). material PbZro2TisO,(PZT)(ref.12).Not 5.Coene.W.M.Thust,A.Op de Beeck.M.Van Dyck,D. As a matter of fact,the first images Ultramicroscopy 64,109-135 (1996). only are the different atomic positions well 6.Jia,C.-L Thust,A.Phys.Rev.Lett. showing contrast resembling that of atomic resolved,but we can even measure the 82,5052-5055(19991. structures were published more than individual lateral atomic shifts (Fig.2b),of 7.Kisielowski,C.etal Ulramicroscopy 89,243-263(2001) 50 years ago.These images were obtained the order of 40 pm,of the oxygen,zirconium 8.Haider.M.tal.atre392,768-769(1998). and titanium atoms out of their symmetry 9.Jia,C.L.Lentzen,M.Urban,K.Science by placing an aperture in the back focal 299.870-87320031 plane of the objective lens so that only a positions,inducing electronic lattice 10.Ja.CL.&Urban.K.Science303,2001-2004(2000. few low-angle reflections were selected polarization(Fig.2c). 11.Houben,L.Thust,A.Urban,K.Ultramicroscopy to form the image.This is equivalent to An investigation using Gaussian 106,200-214(2006). 12.l.C-L et al Nature Moter.7.57-61 (2008) constructing an image taking into account regression analyses"revealed that such 13.Lentzen,M.Urban,K.Acta Cryst.A 56,235-247 (2000) the basic Fourier components only(Fig.la). position measurements can be carried out 14.Tang.C.Y.Chen.I H.Zandbergen.H.W.Li.F.H. Although such an image shows a periodic at a precision of better than +5 pm (at a 95% Utramicroscopy106,539-546(2006). structure consisting of black and white areas confidence level).Such a precision is far 15.Scherzer,O.I Appl.Phys.20,20-29(1949) 16.Lentzen,M.Microsc.Microunal 12,191-205(2006). or dots,which superficially resembles an superior to that of any other microscopic 17.Menter,L W.Proc.R.Soc.Lond.A 236,119-135 (1956) atomic lattice,it is entirely inadequate with technique,including the scanning 1&.Smith.D.1Rcp.ogP%x60,1513-1580(1997. respect to atomic resolution in the sense transmission electron microscope or even 19.Williams,D.B.Carter,C.B.Transmission Elec (Plenum,1996). just defined.Atomic 'individuality'requires the scanning tunnelling microscope.The 20.Spence,C.H.High Resolution Electron Microscopy,3dedn high-order Fourier components(Fig.1b standard objection to such extremely high (Oxford Univ.Press,2007). 262 NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 2009 Macmillan Publishers Limited.All rights reserved262 nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials commentary | insight with atomic resolution means that the information must be entirely local on the atomic level. Any change in the position or occupancy of an atomic site in the sample must show up in the image as an individual signal localized only at the corresponding atomic position. In this stringent sense, apart from a few favourable cases, only the images obtainable in modern aberration-corrected instruments match these standards. The concept is illustrated in Fig. 1. As a matter of fact, the first images showing contrast resembling that of atomic structures were published more than 50 years ago17. These images were obtained by placing an aperture in the back focal plane of the objective lens so that only a few low-angle reflections were selected to form the image. This is equivalent to constructing an image taking into account the basic Fourier components only (Fig. 1a). Although such an image shows a periodic structure consisting of black and white areas or dots, which superficially resembles an atomic lattice, it is entirely inadequate with respect to atomic resolution in the sense just defined. Atomic ‘individuality’ requires high-order Fourier components (Fig. 1b and c), which in early work could not be used for the image formation because of the detrimental influence of lens aberrations. Over the past three decades, taking so-called ‘high-resolution’ images has become a common technique in materials science18. Some of this work was capable of atomic-scale resolution. In particular this applies to studies in which the EPWF was calculated from an image series, even in non-corrected instruments, when in EPWF reconstruction the optical aberrations were corrected to a certain extent by software. Nevertheless, the bulk of this earlier work was concerned not with individual atoms but rather with collective properties such as crystal structures, lattice parameters and crystal symmetry. In contrast, today’s aberration-corrected instruments yield images of such quality that we can virtually put our finger on individual atomic positions. Figure 2a, for example, shows a ferroelectric domain boundary in the microelectronic storage material PbZr0.2Ti0.8O3 (PZT) (ref. 12). Not only are the different atomic positions well resolved, but we can even measure the individual lateral atomic shifts (Fig. 2b), of the order of 40 pm, of the oxygen, zirconium and titanium atoms out of their symmetry positions, inducing electronic lattice polarization (Fig. 2c). An investigation using Gaussian regression analyses11 revealed that such position measurements can be carried out at a precision of better than ±5 pm (at a 95% confidence level). Such a precision is far superior to that of any other microscopic technique, including the scanning transmission electron microscope or even the scanning tunnelling microscope. The standard objection to such extremely high precision is that it cannot be allowed by a microscope with a 70-pm Rayleigh or point resolution19,20. However, resolution and precision are two separate physical terms. Although resolution is defined by the minimum separation of two optically broadened intensity peaks at which these can just be separated in the image, the distance between two well-isolated peaks, fitted for example by two-dimensional Gaussians, can be measured at a precision more than an order of magnitude higher. It would be a real pity to turn down the extraordinary opportunities offered by aberration-corrected electron microscopy because of an unfortunately common misunderstanding. a question to conclude Is the scientific community ready to receive the extraordinary new results achievable through aberration-corrected transmission electron microscopy? Readers may find their own individual answer to this question. The everyday life of electron microscopists active in this field shows that there is still some way to go in alerting the scientific community to the opportunities offered by measuring quantum mechanical wavefunctions in atomic dimensions. So great is the potential of these new techniques that it will be well worth engaging in such a process. ❐ Knut W. Urban is at the Institute of Solid State Research and the Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons, Research Centre Jülich, D‑52425 Jülich, Germany. e‑mail: k.urban@fz‑juelich.de references 1. Urban, K. W. Science 321, 506–510 (2008). 2. Gibson, J. M. Nature 329, 763–764 (1987). 3. Huxford, N. P., Eaglesham, D. J. & Humphreys, C. J. Nature 329, 812–813 (1987). 4. Coene, W., Janssen, G., Op de Beeck, M. & Van Dyck, D. Phys. Rev. Lett. 69, 3743–3746 (1992). 5. Coene, W. M. J., Thust, A., Op de Beeck, M. & Van Dyck, D. Ultramicroscopy 64, 109–135 (1996). 6. Jia, C.-L. & Thust, A. Phys. Rev. Lett. 82, 5052–5055 (1999). 7. Kisielowski, C. et al. Ultramicroscopy 89, 243–263 (2001). 8. Haider, M. et al. Nature 392, 768–769 (1998). 9. Jia, C. L., Lentzen, M. & Urban, K. Science 299, 870–873 (2003). 10. Jia, C. L. & Urban, K. Science 303, 2001–2004 (2004). 11. Houben, L., Thust, A. & Urban, K. Ultramicroscopy 106, 200–214 (2006). 12. Jia, C.-L. et al. Nature Μater. 7, 57–61 (2008). 13. Lentzen, M. & Urban, K. Acta Cryst. A 56, 235–247 (2000). 14. Tang, C. Y., Chen, J. H., Zandbergen, H. W. & Li, F. H. Ultramicroscopy 106, 539–546 (2006). 15. Scherzer, O. J. Appl. Phys. 20, 20–29 (1949). 16. Lentzen, M. Microsc. Microanal. 12, 191–205 (2006). 17. Menter, J. W. Proc. R. Soc. Lond. A 236, 119–135 (1956). 18. Smith, D. J. Rep. Prog. Phys. 60, 1513–1580 (1997). 19. Williams, D. B. & Carter, C. B. Transmission Electron Microscopy (Plenum, 1996). 20. Spence, J. C. H. High Resolution Electron Microscopy, 3rd edn (Oxford Univ. Press, 2007). 1 nm Ps Ps Distance (in units of c) –5 0 1 5 0 P –80 s (µC cm–2) δO δ (nm) δZr/Ti –0.04 –0.02 0.02 0.04 0 –40 40 80 0 a b c Figure 2 | Polarization domain wall in ferroelectric PZT. a, Atomic structure. Arrows give the direction of the spontaneous polarization, which can be directly inferred from the local atom displacements. The shifts of the oxygen atoms (blue circles) out of the Ti/Zr-atom rows (red circles) can be seen directly, as can the change in separation between Ti/Zr and Pb (yellow circles). b, Atomic-resolution measurements of the shifts of oxygen (δO), and titanium/zirconium (δTi/Zr) atoms as a function of distance from the wall centre (units of c-lattice parameter) in a longitudinal-inversion domain wall; c, The value of the local polarization PS that can be calculated from these data. Adapted from ref. 12. © 2008 NPG. It would be a real pity to turn down the extraordinary opportunities offered by abberation-corrected electron microscopy because of an unfortunately common misunderstanding. nmat_2407_APR09.indd 262 11/3/09 11:17:48 © 2009 Macmillan Publishers Limited. All rights reserved