NATURE MATERIALS DOL:10.1038/NMAT2400 INSIGHT I REVIEW ARTICLES Figure 4 New results from CXD experiments ongoing at APS beamline 34-ID-C.a,Phase map of a slice through a 200-nm Au nanocrystal obtained by hybrid input-output inversion of its CXD pattern measured with the {111}Bragg peak using a focused X-ray beam.b,Cross-section showing the phase within a hexagonal-prism ZnO crystal imaged by CXD using its 100 reflection,roughly perpendicular to one of the hexagon edges.The density isosurface superimposed as a contour line,shows a missing region of density that may have resulted from handling with a micromanipulator.An SEM image of the crystal,4 um in length,is shown as an inset (scale bar is 2 um). subject of the investigation.The 200-nm Au nanocrystals like that X-ray methods are especially well suited to in situ measurement shown in Fig.4a were intimately attached to a Si,substrate,by of materials within a working environment.Because the interesting growing them in place,but were apparently not strained by the mechanical properties of nanoscale materials are more associated substrate in this case.These patterns were completely stable and with their unusual strengths,it is important to examine them under found to invert reliably.A useful method of attaching crystals that working conditions,especially those that lead to their breakdown.It is are stable at high temperature to Si wafers is to overgrow a layer of therefore expected that these X-ray strain imaging methods will find amorphous SiO by heating to 900 C in oxygen.This method was widespread application to nanomaterials under extreme tempera- used for the ZnO crystal shown in Fig.4b. tures and pressure,as components of electronic or micromechanical As mentioned in the introduction,an important goal of the CXD devices,under deformation,and in extreme chemical environments. method is to solve the registration problem of placing the beam on a The methods will be used to great effect to study the pattern formation specific crystal,instead of picking one at random.This would allow that is associated with materials synthesis on the nanometre scale.The not only the parallel use of other diagnostic methods,but also meas- self-limiting growth associated with self-assembly in the presence of urement of multiple Bragg peaks from the same crystal;the latter surfactants is an obvious examples.The most exciting direction will would allow spatial mapping of all components of the displacement probably be ultrafast imaging of shock waves in nanocrystals when field (and,hence,strain tensor)by resolving them along different Q the new X-ray free-electron lasers become operational. vectors.This has been attempted for the ZnO crystal whose cross- sectional phase map is shown in Fig.4b.The crystal was dropped onto References the substrate and moved with a micromanipulator to place it next to 1.Pauling,L The Nature of the Chemical Bond,and the Structure of Molecules and a marker cross scribed onto the substrate.In this way,a scanning Crystals(Cornell Univ.Press,1960). electron microscope(SEM)image of the same crystal was obtained 2. Buffat,P.Borel,J.-P.Size effect on the melting temperature of gold particles. (Fig.4b).This manipulation must have inadvertently damaged the Ps.ReA13,2287-2297(1975). 3. Madey,T.E.,Chen,W.,Wang.H.,Kaghazchi,P.Jacob,T.Nanoscale surface crystal.A gash can be seen in the density contour shown and there is chemistry over faceted substrates:structure,reactivity and nanotemplates. an associated pattern of strain localized at the tip of the damage. Chem.Soc.Re.37,2310-2327(2008). 4.Yu,H.,Li,I.B.,Loomis,R.A.,Wang,L.W.Buhro,W.E.Two-versus Future prospects three-dimensional quantum confinement in indium phosphide wires and dots. X-ray and electron microscopy methods complement each other in Nature Mater.2,517-520(2003). 5. Hodge,A.M.et al.Scaling equation for yield strength of nanoporous open-cell the range of sample sizes they cover best,with electrons typically foams..Acta Math.55,1343-1349(2007). giving a resolution an order of magnitude higher and a correspond- 6.Fritz,D.M.et al.Ultrafast bond softening in bismuth:Mapping a solid's ing smaller field of view.The need to prepare thin samples has always interatomic potential with X-rays.Science 315,633-636(2007). been a limitation of TEM because the structures under investigation 7. Vartanyants,I.A.etal.Coherent X-ray scattering and lensless imaging at the can be disturbed by the sample preparation.Traditionally,X-rays European XFEL facility.J.Synchrotron Radiat.14,453-470(2007). 8. McCartney,M.R.Smith,D.J.Electron holography:Phase imaging with have been used only in reciprocal space,but the recent introduction nanometer resolution.Anmu.Rev.Mater.Res.37,729-767(2007) of coherence-based methods at the newer synchrotron radiation 9. Midgley.P.A.&Dunin-Borkowski,R.E.Electron tomography and holography sources has enabled diffraction patterns to be phased and inverted in materials science.Nature Mater.8,271-280 (2009). to form 'lensless'images,as we have described.Electron diffraction 10.Kim,M.,Zuo,J.M.Park,G.S.High-resolution strain measurement in patterns can also be interpreteds2 or invertedss to obtain images shallow trench isolation structures using dynamic electron diffraction. Appl.Phs.Lett.84,2181-2183(2004). of strain,again on a length scale smaller than the corresponding 11.Urban,K.W.Studying atomic structures by aberration-corrected transmission X-ray work. electron microscopy.Science 321,506-511 (2008). NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 297 2009 Macmillan Publishers Limited.All rights reservednature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 297 NaTure maTerials doi: 10.1038/nmat2400 insight | review articles subject of the investigation. The 200-nm Au nanocrystals like that shown in Fig. 4a were intimately attached to a SiO2 substrate, by growing them in place, but were apparently not strained by the substrate in this case. These patterns were completely stable and found to invert reliably. A useful method of attaching crystals that are stable at high temperature to Si wafers is to overgrow a layer of amorphous SiO2 by heating to 900 °C in oxygen. This method was used for the ZnO crystal shown in Fig. 4b. As mentioned in the introduction, an important goal of the CXD method is to solve the registration problem of placing the beam on a specific crystal, instead of picking one at random. This would allow not only the parallel use of other diagnostic methods, but also meas￾urement of multiple Bragg peaks from the same crystal; the latter would allow spatial mapping of all components of the displacement field (and, hence, strain tensor) by resolving them along different Q vectors. This has been attempted for the ZnO crystal whose cross￾sectional phase map is shown in Fig. 4b. The crystal was dropped onto the substrate and moved with a micromanipulator to place it next to a marker cross scribed onto the substrate. In this way, a scanning electron microscope (SEM) image of the same crystal was obtained (Fig. 4b). This manipulation must have inadvertently damaged the crystal. A gash can be seen in the density contour shown and there is an associated pattern of strain localized at the tip of the damage. Future prospects X-ray and electron microscopy methods complement each other in the range of sample sizes they cover best, with electrons typically giving a resolution an order of magnitude higher and a correspond￾ing smaller field of view. The need to prepare thin samples has always been a limitation of TEM because the structures under investigation can be disturbed by the sample preparation. Traditionally, X-rays have been used only in reciprocal space, but the recent introduction of coherence-based methods at the newer synchrotron radiation sources has enabled diffraction patterns to be phased and inverted to form ‘lensless’ images, as we have described. Electron diffraction patterns can also be interpreted52 or inverted53 to obtain images of strain, again on a length scale smaller than the corresponding X-ray work. X-ray methods are especially well suited to in situ measurement of materials within a working environment. Because the interesting mechanical properties of nanoscale materials are more associated with their unusual strength5 , it is important to examine them under working conditions, especially those that lead to their breakdown. It is therefore expected that these X-ray strain imaging methods will find widespread application to nanomaterials under extreme tempera￾tures and pressure, as components of electronic or micromechanical devices, under deformation, and in extreme chemical environments. The methods will be used to great effect to study the pattern formation that is associated with materials synthesis on the nanometre scale. The self-limiting growth associated with self-assembly in the presence of surfactants is an obvious example54. The most exciting direction will probably be ultrafast imaging of shock waves in nanocrystals when the new X-ray free-electron lasers become operational. references 1. Pauling, L. The Nature of the Chemical Bond, and the Structure of Molecules and Crystals (Cornell Univ. Press, 1960). 2. Buffat, P. & Borel, J.-P. Size effect on the melting temperature of gold particles. Phys. Rev. A 13, 2287–2297 (1975). 3. Madey, T. E., Chen, W., Wang, H., Kaghazchi, P. & Jacob, T. Nanoscale surface chemistry over faceted substrates: structure, reactivity and nanotemplates. Chem. Soc. Rev. 37, 2310–2327 (2008). 4. Yu, H., Li, J. B., Loomis, R. A., Wang, L. W. & Buhro, W. E. Two- versus three-dimensional quantum confinement in indium phosphide wires and dots. Nature Mater. 2, 517–520 (2003). 5. Hodge, A. M. et al. Scaling equation for yield strength of nanoporous open-cell foams. Acta Math. 55, 1343–1349 (2007). 6. Fritz, D. M. et al. Ultrafast bond softening in bismuth: Mapping a solid’s interatomic potential with X-rays. Science 315, 633–636 (2007). 7. Vartanyants, I. A. et al. Coherent X-ray scattering and lensless imaging at the European XFEL facility. J. Synchrotron Radiat. 14, 453–470 (2007). 8. McCartney, M. R. & Smith, D. J. Electron holography: Phase imaging with nanometer resolution. Annu. Rev. Mater. Res. 37, 729–767 (2007). 9. Midgley, P. A. & Dunin-Borkowski, R. E. Electron tomography and holography in materials science. Nature Mater. 8, 271–280 (2009). 10. Kim, M., Zuo, J. M. & Park, G. S. High-resolution strain measurement in shallow trench isolation structures using dynamic electron diffraction. Appl. Phys. Lett. 84, 2181–2183 (2004). 11. Urban, K. W. Studying atomic structures by aberration-corrected transmission electron microscopy. Science 321, 506–511 (2008). a b Figure 4 | New results from CXD experiments ongoing at aPs beamline 34-iD-C. a, Phase map of a slice through a 200-nm Au nanocrystal obtained by hybrid input–output inversion of its CXD pattern measured with the {111} Bragg peak using a focused X-ray beam. b, Cross-section showing the phase within a hexagonal-prism ZnO crystal imaged by CXD using its 100 reflection, roughly perpendicular to one of the hexagon edges. The density isosurface, superimposed as a contour line, shows a missing region of density that may have resulted from handling with a micromanipulator. An SEM image of the crystal, 4 μm in length, is shown as an inset (scale bar is 2 μm). nmat_2400_APR09.indd 297 13/3/09 12:04:33 © 2009 Macmillan Publishers Limited. All rights reserved