上海交通大学：《探索物质微观世界》课程教学资源（文献资料）Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials

nature INSIGHTI REVIEW ARTICLES materials PUBLISHED ONLINE:24 MARCH 2009|DOI:10.1038/NMAT2399 Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials Harald Ade*and Herman Stoll2 Many high-performance materials and novel devices consist of multiple components and are naturally or intentionally nano- structured for optimal properties and performance.To understand their structure-property relationships fully,quantitative compositional analysis at length scales below 100 nm is required,a need that is often uniquely addressed using soft X-ray microscopy.Similarly,the interaction of X-rays with magnetic materials provides unique element-specific contrast that allows the determination of magnetic properties in multi-element antiferromagnetic and ferromagnetic materials.Pump-probe-type experiments can even investigate magnetic domain dynamics.Here we review and exemplify the ability of soft X-ray micro- scopy to provide information that is otherwise inaccessible,and discuss a perspective on future developments. ithout contrast,any inherent spatial resolution from bioapplications to materials characterization occurred for of a microscope is useless.It is thus the contrast photoelectron emission microscopy (PEEM)15.The main step mechanism that often imparts unique capabilities to a towards materials characterization was the combination of soft microscopy technique.Such uniqueness can be provided by the X-ray absorption spectroscopy techniques with transmission and specific interaction of soft X-rays with matter.Most fruitful is the surface instruments25.The resulting NEXAFS microscopy provides exploitation of the absorption process of a soft X-ray photon near an a unique combination of contrast mechanism,interaction cross- absorption edge,that is,near-edge X-ray absorption fine-structure section,spatial resolution and relatively low sample damage56. (NEXAFS)spectroscopy,as the detailed dependence of absorption on photon energy reveals much of the electronic structure-core Contrast mechanism and instrument parameters and unoccupied valence states-of the material investigated 2.This Examples of NEXAFS spectroscopy that illustrate its ability to provides excellent quantitative elemental and functional sensitivity characterize composition and magnetic properties are displayed in and,if coupled to the polarization state of the photons,can also Fig.1.NEXAFS spectroscopy is particularly rich in spectral features characterize bond orientation'and magnetic domain structures. when acquired from organic materials and is especially reveal- Pump-probe-type experiments even allow for very high time ing for magnetic materials when photon polarization effects and resolution and the investigation of magnetic-domain dynamics37. element specificity are used2.In organic materials,NEXAFS shows NEXAFS microscopy,and soft X-ray microscopy in general,is very detailed correlations with specific chemical moieties that can a comparatively young discipline that is still rapidly evolving and often be understood quite well from theoretical calculations or developing.The field is in the transition to an 'industrial stage,in through use of molecular analogues.In conjunction with the polari- which commercial companies emerge to provide for the needs of zation state of the X-rays,the element-specific NEXAFS of magnetic a wide range of users.This led to the very recent decision at the domains shows pronounced dichroism that can be used to infer the Ninth International Conference for X-ray Microscopy (Zurich, domain orientation as well as the specific contribution of an ele- 20-25 July 2008)to shorten the conference cycle of this series from ment to the magnetization.In addition,NEXAFS is sensitive to the three years to two.In this Review,we seek to provide a flavour of orientation of specific chemical orbitals in organic materials this growing field,illustrate capabilities and applications with Zone-plate-based microscopes are of two main types:scanning soft X-rays,and speculate on some future developments.A short transmission X-ray microscopes(STXMs)and transmission X-ray historical context is provided,the physics underlying the contrast microscopes(TXMs).The basic operating principles are similar to mechanism is discussed,and a few applications are selected to the optical or electron microscopy analogues.In the STXM,the illustrate some of the capabilities.Throughout,the focus will be on sample is mechanically raster-scanned through the focal spot pro- characterizing materials with NEXAFS microscopy,most notably vided by a zone plate.In the TXM,a zone plate magnifies the sample organic and magnetic materials. onto a two-dimensional detector.STXMs are routinely operated on grazing-incidence grating monochromators at synchrotron facili- Historical notes ties,and the implementation of NEXAFS microscopy was relatively The development of modern,high-spatial-resolution soft X-ray easy even though zone plates are highly chromatic.Special acqui- microscopes based on zone-plate technology is about three decades sition or analysis tools were developed to avoid potential degrada- olds.Although initially envisioned and developed with the goal of tion of the spatial resolution caused by the need to refocus during imaging live biological samples in their natural,that is,hydrated, changes in energy202.Historically,TXMs have been installed on unfixed and unfrozen,state with suboptical resolution,radia- inflexible zone-plate-based monochromators,and NEXAFS micro- tion damage during exposure necessitated the development of scopy applications have been much more limited.In a PEEM, cryofixation techniques for biological applicationst.Unsurpris- implementation of NEXAFS microscopy is straightforward as ingly,the technology developed has proved to be very useful over only the photon energy needs to be scannedss.Scanning instru- the past 15 years in characterizing a much wider range of materials ments have also been developed to perform spatially resolved than biological samples2.A similar evolutionary broadening X-ray photoemission spectroscopy(XPS)from surfaces 22.XPS Department of Physics,NCSU,Raleigh,North Carolina 27695,USA;2Max-Planck-Institut fur Metallforschung,HeisenbergstraBe 3,70569 Stuttgart, Germany.e-mail:Harald_Ade@ncsu.edu NATURE MATERIALS VOL 8 APRIL 2009 www.nature.com/naturematerials 281 2009 Macmillan Publishers Limited.All rights reserved

nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 281 insight | review articles Published online: 24 march 2009 | doi: 10.1038/nmat2399 Without contrast, any inherent spatial resolution of a microscope is useless. It is thus the contrast mechanism that often imparts unique capabilities to a microscopy technique. Such uniqueness can be provided by the specific interaction of soft X-rays with matter. Most fruitful is the exploitation of the absorption process of a soft X-ray photon near an absorption edge, that is, near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy, as the detailed dependence of absorption on photon energy reveals much of the electronic structure — core and unoccupied valence states — of the material investigated1,2. This provides excellent quantitative elemental and functional sensitivity3 and, if coupled to the polarization state of the photons, can also characterize bond orientation4 and magnetic domain structure5 . Pump–probe-type experiments even allow for very high time resolution and the investigation of magnetic-domain dynamics6,7. NEXAFS microscopy, and soft X-ray microscopy in general, is a comparatively young discipline that is still rapidly evolving and developing. The field is in the transition to an ‘industrial stage’, in which commercial companies emerge to provide for the needs of a wide range of users. This led to the very recent decision at the Ninth International Conference for X-ray Microscopy (Zurich, 20–25 July 2008) to shorten the conference cycle of this series from three years to two. In this Review, we seek to provide a flavour of this growing field, illustrate capabilities and applications with soft X-rays, and speculate on some future developments. A short historical context is provided, the physics underlying the contrast mechanism is discussed, and a few applications are selected to illustrate some of the capabilities. Throughout, the focus will be on characterizing materials with NEXAFS microscopy, most notably organic and magnetic materials. historical notes The development of modern, high-spatial-resolution soft X-ray microscopes based on zone-plate technology is about three decades old8,9. Although initially envisioned and developed with the goal of imaging live biological samples in their natural, that is, hydrated, unfixed and unfrozen, state with suboptical resolution8,9, radia￾tion damage during exposure necessitated the development of cryofixation techniques for biological applications10. Unsurpris￾ingly, the technology developed has proved to be very useful over the past 15 years in characterizing a much wider range of materials than biological samples3,4,11,12. A similar evolutionary broadening near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials harald ade1 * and herman stoll2 Many high-performance materials and novel devices consist of multiple components and are naturally or intentionally nano￾structured for optimal properties and performance. To understand their structure–property relationships fully, quantitative compositional analysis at length scales below 100 nm is required, a need that is often uniquely addressed using soft X-ray microscopy. Similarly, the interaction of X-rays with magnetic materials provides unique element-specific contrast that allows the determination of magnetic properties in multi-element antiferromagnetic and ferromagnetic materials. Pump–probe-type experiments can even investigate magnetic domain dynamics. Here we review and exemplify the ability of soft X-ray micro￾scopy to provide information that is otherwise inaccessible, and discuss a perspective on future developments. from bioapplications to materials characterization occurred for photoelectron emission microscopy (PEEM)5,13–15. The main step towards materials characterization was the combination of soft X-ray absorption spectroscopy techniques with transmission and surface instruments3,15. The resulting NEXAFS microscopy provides a unique combination of contrast mechanism, interaction cross￾section, spatial resolution and relatively low sample damage3,15,16. contrast mechanism and instrument parameters Examples of NEXAFS spectroscopy that illustrate its ability to characterize composition and magnetic properties are displayed in Fig. 1. NEXAFS spectroscopy is particularly rich in spectral features when acquired from organic materials1,17,18 and is especially reveal￾ing for magnetic materials when photon polarization effects and element specificity are used2 . In organic materials, NEXAFS shows very detailed correlations with specific chemical moieties that can often be understood quite well from theoretical calculations or through use of molecular analogues. In conjunction with the polari￾zation state of the X-rays, the element-specific NEXAFS of magnetic domains shows pronounced dichroism that can be used to infer the domain orientation as well as the specific contribution of an ele￾ment to the magnetization. In addition, NEXAFS is sensitive to the orientation of specific chemical orbitals in organic materials1,4,19. Zone-plate-based microscopes are of two main types: scanning transmission X-ray microscopes (STXMs) and transmission X-ray microscopes (TXMs). The basic operating principles are similar to the optical or electron microscopy analogues. In the STXM, the sample is mechanically raster-scanned through the focal spot pro￾vided by a zone plate. In the TXM, a zone plate magnifies the sample onto a two-dimensional detector. STXMs are routinely operated on grazing-incidence grating monochromators at synchrotron facili￾ties, and the implementation of NEXAFS microscopy was relatively easy even though zone plates are highly chromatic3,4. Special acqui￾sition or analysis tools were developed to avoid potential degrada￾tion of the spatial resolution caused by the need to refocus during changes in energy20,21. Historically, TXMs have been installed on inflexible zone-plate-based monochromators, and NEXAFS micro￾scopy applications have been much more limited. In a PEEM, implementation of NEXAFS microscopy is straightforward as only the photon energy needs to be scanned5,15. Scanning instru￾ments have also been developed to perform spatially resolved X-ray photoemission spectroscopy (XPS) from surfaces 22,23. XPS 1 Department of Physics, NCSU, Raleigh, North Carolina 27695, USA; 2 Max-Planck-Institut für Metallforschung, Heisenbergstraβe 3, 70569 Stuttgart, Germany. *e-mail: Harald_Ade@ncsu.edu nmat_2399_APR09.indd 281 13/3/09 11:54:35 © 2009 Macmillan Publishers Limited. All rights reserved

REVIEW ARTICLES INSIGHT NATURE MATERIALS DOL:10.1038/NMAT2399 Polyvinyl 十CH2-CH N~CC -methylketone C=0 CHg C-H Nylon-6 Functional group Polymethyl -methacrylate HC XY =C.N,O Polyurea N 8 Polyurethane Polycarbonate tO-2+ 284 286 288 290 292 Photon energy (eV) X-ray magnetic linear dichroism X-ray magnetic circular dichroism Antiferromagnets Ferromagnets LaFeO, 5- cos-e 720722 724 700 720 740 Photon energy (eV) Photon energy(eV) Figure 1 Illustration of compositional,linear and circular magnetic dichroic contrast mechanisms using NEXAFS spectroscopy.a,NEXAFS spectral features corresponding to the carbonyl group (shown in red)as function of progressive functional group environment (ketone to carbonate)show a systematic shift towards higher energies for a more electronegative environment'.b,Linearly polarized X-rays produce spectral differences,that is,linear dichroism,for parallel or perpendicularly aligned antiferromagnetic domains.c,Circularly polarized light produces spectral differences,that is,circular dichroism,for parallel,perpendicular and antiparallel aligned ferromagnetic domains.Figure courtesy of J.Stohr. microscopy can also be accomplished in a PEEM2.Generally,XPS They are particularly radiation sensitive(chemical changes and mass microscopy has been little used for the characterization of organic loss)owing to the ability of damaging radicals to diffuse rapidly. and magnetic materials. Thus,these materials are an area of application where NEXAFS For organic matter,soft X-rays not only provide excellent com- microscopy has unique advantages when suboptical resolution is positional contrast,but are also about three orders of magnitude required.The first direct,real-space characterization of a new type less damaging than electron energy-loss spectroscopy in trans- of polyvinyl-alcohol-based micro-balloon in aqueous environment mission electron microscopes'.Not surprisingly,a large number (Fig.2)is a good example2s.A six-month-old suspension of the of X-ray microscopy applications involve organic matter in some micro-balloons that contained at least partially degraded balloons form or another.Similarly,numerous magnetic materials have was investigated.Imaging below and above the oxygen K edge,as been investigated as a result of the special contrast mechanism of well as complete NEXAFS analysis of individual micro-balloons, element-specific linear and circular dichroism,and the possibility could provide unique information on the composition of the micro- of studying dynamical phenomena.Material classes,such as tradi- balloons in water.From the oxygen NEXAFS of the micro-balloon tional semiconductors or superconductors and other oxides,have interiors,a distinction between water-and air-filled particles could not been investigated extensively with NEXAFS microscopy so far. be made unambiguously.For example,the NEXAFS spectrum of micro-balloon A in Fig.2a showed only the resonances typical Organics in aqueous environments for the telechelic polyvinyl-alcohol shell,strongly suggesting that Some of the most challenging materials to characterize are organic micro-balloon A is air filled.Furthermore,the micro-balloons A, materials suspended or dispersed in the hydrated,unfrozen state. B and C shown in Fig.2a had total shell thicknesses of 0.38 um, 282 NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 2009 Macmillan Publishers Limited.All rights reserved

282 nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials review articles | insight NaTure MaTerialS doi: 10.1038/nmat2399 microscopy can also be accomplished in a PEEM24. Generally, XPS microscopy has been little used for the characterization of organic and magnetic materials. For organic matter, soft X-rays not only provide excellent com￾positional contrast, but are also about three orders of magnitude less damaging than electron energy-loss spectroscopy in trans￾mission electron microscopes16. Not surprisingly, a large number of X-ray microscopy applications involve organic matter in some form or another. Similarly, numerous magnetic materials have been investigated as a result of the special contrast mechanism of element-specific linear and circular dichroism, and the possibility of studying dynamical phenomena. Material classes, such as tradi￾tional semiconductors or superconductors and other oxides, have not been investigated extensively with NEXAFS microscopy so far. organics in aqueous environments Some of the most challenging materials to characterize are organic materials suspended or dispersed in the hydrated, unfrozen state. They are particularly radiation sensitive (chemical changes and mass loss) owing to the ability of damaging radicals to diffuse rapidly. Thus, these materials are an area of application where NEXAFS microscopy has unique advantages when suboptical resolution is required. The first direct, real-space characterization of a new type of polyvinyl-alcohol-based micro-balloon in aqueous environment (Fig. 2) is a good example25. A six-month-old suspension of the micro-balloons that contained at least partially degraded balloons was investigated. Imaging below and above the oxygen K edge, as well as complete NEXAFS analysis of individual micro-balloons, could provide unique information on the composition of the micro￾balloons in water. From the oxygen NEXAFS of the micro-balloon interiors, a distinction between water- and air-filled particles could be made unambiguously. For example, the NEXAFS spectrum of micro-balloon A in Fig. 2a showed only the resonances typical for the telechelic polyvinyl-alcohol shell, strongly suggesting that micro-balloon A is air filled. Furthermore, the micro-balloons A, B and C shown in Fig. 2a had total shell thicknesses of 0.38 μm, E X-ray magnetic linear dichroism Antiferromagnets 720 722 724 Photon energy (eV) 4 1 2 3 LaFeO3 L2 Normalized electron yield I | cos2Ѳ Ѳ X-ray magnetic circular dichroism Ferromagnets 700 720 740 0 4 8 Photon energy (eV) Fe L3 L2 Absorption coefficient I | cosѲ Ѳ Photon energy (eV) Polyvinyl -methylketone Nylon-6 Polymethyl -methacrylate Polyurea Polyurethane Polycarbonate C-H Intensity 284 286 288 290 292 C n O O O CH3 CH3 CH2 n CH3 H3C C O C O C n C O * N * H n H CH3 H O C N N C n O O C O N O H N H CH3 CH2 CH3 CH n C O C O C C N C O C C O O C N C O N N C O O C O O N O O Functional group X,Y = C,N,O X O C Y a b c Figure 1 | illustration of compositional, linear and circular magnetic dichroic contrast mechanisms using NeXaFS spectroscopy. a, NEXAFS spectral features corresponding to the carbonyl group (shown in red) as function of progressive functional group environment (ketone to carbonate) show a systematic shift towards higher energies for a more electronegative environment18. b, Linearly polarized X-rays produce spectral differences, that is, linear dichroism, for parallel or perpendicularly aligned antiferromagnetic domains. c, Circularly polarized light produces spectral differences, that is, circular dichroism, for parallel, perpendicular and antiparallel aligned ferromagnetic domains. Figure courtesy of J. Stöhr. nmat_2399_APR09.indd 282 13/3/09 11:54:35 © 2009 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOL:10.1038/NMAT2399 INSIGHT I REVIEW ARTICLES Distance(um) 400nm um Figure 2 NEXAFS imaging of latex particles and microballoons dispersed in water.a,STXM transmission image of microballoons recorded at 520 eV, that is,below the oxygen absorption edge.b,Radial transmittance profiles of micro-balloons A,B and C,labelled as in a.c,Expanded views of micro- balloon shells.d,Three-dimensional representations of the polyacrylate (green)and polystyrene (greyscale)distributions of water-suspended latex particles.Figures reproduced with permission from:a-c,ref.25 2008 RSC:d,ref.28@2007 IUCr. 0.56 um and 0.63 um,respectively.The difference between these water,and each component could be selectively mapped in 3D.As results and the shell thickness of 0.90 um derived from confocal there is no inefficient X-ray optics between the sample and the detec- laser scanning microscopy was explained by the limited resolution tor in a STXM,the dose used to acquire the STXM tomographs was of about 180 nm for this technique,compared with 40-nm reso- only~200 MGy,which is considerably lower than the dose required lution for X-rays.Closer inspection of the shells revealed further for TXM tomography(typically 1-10 GGy).Even so,tomography details(Fig.2c).The increase of the shell thickness is correlated is a relatively high-dose method and the ultimate spatial resolution with the appearance of a fuzzy contour around the polymeric shell might be limited by damage.Unfortunately,cryogenic methods and the loss of air content.Degradation of the polymeric micro- only prevent mass loss and the spectroscopic changes due to chemi- balloons upon aging leads to leakage of the gas filling and is asso- cal damage are essentially unmitigated by cryogenic techniques*. ciated with an increase of the shell thickness.The appearance of a Other studies ofhydrated soft matter include the in situ,real-space fuzzy contour around the polymeric shell of the corrupted micro- determination of the charge state in stimulus-responsive micro- balloons is presumably the result of the partial destruction of the gels",the determination of crosslink density in super-absorbent polymeric network,but further investigations are needed to reveal polymers",density determinations of polyelectrolyte assemblies" the degradation mechanism definitively.The X-ray microscopy and the correlation of metal distributions in organic biofilms3536.The results directly showed that the particular micro-balloon system studies of microgels,crosslink density in super-absorbent polymers, investigated has remarkably high stability.These balloons are able and biofilms made explicit use of NEXAFS spectral differences and to contain gases for six months,which makes them well suited to features,whereas the studies of the polyelectrolyte assemblies relied biomedical applications. primarily on the 'water-window contrast'(at fixed photon energy) As complex materials often have three-dimensional(3D)morph- due to the elemental differences in the cross-sections of carbon ologies,a number of new methods have been developed recently to and oxygen. perform 3D chemical mapping24-2 or 3D morphological mapping of thick,frozen biological samples29.However,of these methods, Organic electronic materials only NEXAFS tomography has high spectroscopic resolution and Owing to their processability from solvents,organic electronic sensitivity,and the ability to analyse wet specimens28.To illustrate materials may yield cheap light-emitting diodes and photovoltaic the present state of the art (achieved with a STXM),the 3D chemical devices.Furthermore,flexible devices can be constructed.Organic composition of structured,hollow latex spheres in an aqueous solu- devices are thus extensively studied by a large number of research tion is presented in Fig.2d.A NEXAFS feature of the acrylate com- groups around the world.Generally,devices are composed of donor ponent just below the onset of the oxygen 1s edge of water was used and acceptor materials that are used in layered thin-film geometries to provide selective contrast between the acrylate,the styrene and that often include one or more buffer layers and the electrodes. NATURE MATERIALS|VOL 8|APRIL 2009 www.nature.com/naturematerials 283 2009 Macmillan Publishers Limited.All rights reserved

nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 283 NaTure MaTerialS doi: 10.1038/nmat2399 insight | review articles 0.56 μm and 0.63 μm, respectively. The difference between these results and the shell thickness of 0.90 μm derived from confocal laser scanning microscopy was explained by the limited resolution of about 180 nm for this technique, compared with 40-nm reso￾lution for X-rays. Closer inspection of the shells revealed further details (Fig. 2c). The increase of the shell thickness is correlated with the appearance of a fuzzy contour around the polymeric shell and the loss of air content. Degradation of the polymeric micro￾balloons upon aging leads to leakage of the gas filling and is asso￾ciated with an increase of the shell thickness. The appearance of a fuzzy contour around the polymeric shell of the corrupted micro￾balloons is presumably the result of the partial destruction of the polymeric network, but further investigations are needed to reveal the degradation mechanism definitively. The X-ray microscopy results directly showed that the particular micro-balloon system investigated has remarkably high stability. These balloons are able to contain gases for six months, which makes them well suited to biomedical applications. As complex materials often have three-dimensional (3D) morph￾ologies, a number of new methods have been developed recently to perform 3D chemical mapping26–28 or 3D morphological mapping of thick, frozen biological samples29. However, of these methods, only NEXAFS tomography has high spectroscopic resolution and sensitivity, and the ability to analyse wet specimens28. To illustrate the present state of the art (achieved with a STXM), the 3D chemical composition of structured, hollow latex spheres in an aqueous solu￾tion is presented in Fig. 2d. A NEXAFS feature of the acrylate com￾ponent just below the onset of the oxygen 1s edge of water was used to provide selective contrast between the acrylate, the styrene and water, and each component could be selectively mapped in 3D. As there is no inefficient X-ray optics between the sample and the detec￾tor in a STXM, the dose used to acquire the STXM tomographs was only ~200 MGy, which is considerably lower than the dose required for TXM tomography (typically 1–10 GGy). Even so, tomography is a relatively high-dose method and the ultimate spatial resolution might be limited by damage. Unfortunately, cryogenic methods only prevent mass loss and the spectroscopic changes due to chemi￾cal damage are essentially unmitigated by cryogenic techniques30. Other studies of hydrated soft matter include the in situ, real-space determination of the charge state in stimulus-responsive micro￾gels31, the determination of crosslink density in super-absorbent polymers32, density determinations of polyelectrolyte assemblies33,34 and the correlation of metal distributions in organic biofilms35,36. The studies of microgels, crosslink density in super-absorbent polymers, and biofilms made explicit use of NEXAFS spectral differences and features, whereas the studies of the polyelectrolyte assemblies relied primarily on the ‘water-window contrast’ (at fixed photon energy) due to the elemental differences in the cross-sections of carbon and oxygen. organic electronic materials Owing to their processability from solvents, organic electronic materials may yield cheap light-emitting diodes and photovoltaic devices. Furthermore, flexible devices can be constructed. Organic devices are thus extensively studied by a large number of research groups around the world. Generally, devices are composed of donor and acceptor materials that are used in layered thin-film geometries that often include one or more buffer layers and the electrodes. a c 2 µm A B C 1 µm 1 µm 1 µm A B C 400 nm –3 –2 –1 0 1 2 3 Distance (µm) A B C Transmittance b d Figure 2 | NeXaFS imaging of latex particles and microballoons dispersed in water. a, STXM transmission image of microballoons recorded at 520 eV, that is, below the oxygen absorption edge. b, Radial transmittance profiles of micro-balloons A, B and C, labelled as in a. c, Expanded views of micro￾balloon shells. d, Three-dimensional representations of the polyacrylate (green) and polystyrene (greyscale) distributions of water-suspended latex particles. Figures reproduced with permission from: a-c, ref. 25 © 2008 RSC; d, ref. 28 © 2007 IUCr. nmat_2399_APR09.indd 283 13/3/09 11:54:37 © 2009 Macmillan Publishers Limited. All rights reserved

REVIEW ARTICLES INSIGHT NATURE MATERIALS DOL:10.1038/NMAT2399 (typically less than 10 nm),efficient photovoltaic devices require an 50 nm intimately mixed morphology with phase separation on the scale of 10 nm,yet at the same time have a bi-continuous network or perco- lation pathways for efficient charge collection. 0 nm Mixtures of the donor polymer poly(9,9'-dioctylfluorene-co- % 11 PFB:F88T bis-N,N-(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) 50 50 nm (PFB)with the acceptor polymer poly(9,9'-dioctylfluorene-co- benzothiadiazole)(F8BT)have been used as an all-polymer model system for the study of morphology effects4 and charge generation 0 nm and transport dynamics".Blends of PFB and F8BT spin-cast from a low-boiling-point solvent,such as chloroform,show asmooth surface topography".Films spin-cast from a high-boiling-point solvent,such 15nm as xylene,show a more complicated morphology and surface topog- raphy characterized by micrometre-size domains".Understanding the charge generation mechanism within these domains requires 60 mapping the composition of these blend structures.Although an 1:5 PFB:F8 array of advanced techniques has been used to probe the chemistry F8BT percentage PFB percentage Height and structure of PFB:F8BT blends,only NEXAFS microscopy has composition composition the required high chemical sensitivity and spatial resolution(<um) This capability has been exploited to characterize PFB:F8BT blends cast from xylenes and chloroform".Quantitative compositional maps of xylene-cast PFB:F8BT blends with different composition ratios are shown in Fig.3.The micrometre-scale domains contain significant proportions of both polymers.In particular,the minor- ity phases of 1:5 and 5:1 PFB:F8BT xylene-blend films can contain each polymer in a proportion up to 50%.By analysing the quantita- tive composition,the superior photovoltaic performance of the 1:5 PFB:F8BT blend over the 5:1 blend could be explained:a higher pro- portion of the area of the 1:5 PFB:F8BT film has the optimum blend composition of 60 to 80 wt%F8BT.The local composition and its distribution in 1:1 PFB:F8BT films,however,was unable to explain the lower photovoltaic performance of the 1:1 PFB:F8BT blend.This was attributed to a presumed vertical structure that is difficult to characterize at present.Overall,the lower performance of xylene- 12×103 processed films relative to chloroform-processed films could not be 1.0×10 accounted for by the local blend composition at length scales above the resolution limit of~45 nm and a nanoscale phase separation at a 283284285286287 8.0×10-4 length scale below the spatial resolution has been postulated. Similarly,compositionaland morphologicalanalysisofPFB:F8BT 6.0×10-4 blends cast from chloroform provided much insight into the nano- scale morphological evolution of these blends upon annealing" 4.0×10-4 The coarsening and evolution towards purer domains has been 20×10-4 _F8BT quantified for the more aggressively annealed films.However,the ----FBT comparison with photoluminescence data of as-cast or minimally 0. annealed samples suggested the presence of a nanomorphology that 280285290295300305310315320 was below the resolution limit of the X-ray microscope used.Char- Energy (eV) acterization of PFB:F8BT blends thus simultaneously illustrates the power of compositional imaging using NEXAFS microscopy and Figure 3 NEXAFS compositional mapping applied to two-component the need to improve the spatial resolution further.Given that the organic electronic devices that show macroscopic phase separation. complex morphology is in many cases most probably 3D in nature, Quantitative (wt%)F8BT(a,d,g)and PFB(b,e,h)composition maps optimal characterization would require NEXAFS tomography2 at and AFM images (c,f,i)of PFB-F8BT blend films of variable composition. higher spatial resolution than has been possible so far. a,b,c,1:1 PFB:F8BT;d,e,f,5:1 PFB:F8BT:g.h,i,1:5 PFB:F8BT.j,Chemical structures of both polymers and their respective NEXAFS spectra,which Biofibres provide the basis of the quantitative analysis.Figure reproduced with Biofibres are some of nature's most remarkable materials.Some permission from ref.43@2007 ACS. fibres can achieve combinations of strength and toughness that are still unmatched by high-performance synthetic fibres.One of the Frequently,devices contain an active layer that is composed of fibres produced by spiders,dragline silk,is such a fibre,with an complex non-equilibrium structures of a two-component donor- unusual combination of stiffness and extensibility.Numerous mod- acceptor blend that shows phase separation on multiple length els to explain the fibres'properties exist.They are thought to be due scales,created during spin-casting and subsequent processing to alternating alanine-rich hard segments containing B-sheets and The influence of local composition and distribution on device per- glycine-rich soft segments,but the fundamental structure-property formance is of particular interest,both for light-emitting diodes* relationships in spider silk are not yet fully understood.They are and for 'bulk heterojunction technology'in photovoltaic devices4. of intrinsic scientific interest,but improved knowledge would Owing to the small exciton diffusion length in conjugated polymers also aid industrial efforts to produce biomimetic synthetic fibres. 284 NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 2009 Macmillan Publishers Limited.All rights reserved

284 nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials review articles | insight NaTure MaTerialS doi: 10.1038/nmat2399 Frequently, devices contain an active layer that is composed of complex non-equilibrium structures of a two-component donor– acceptor blend that shows phase separation on multiple length scales, created during spin-casting and subsequent processing37,38. The influence of local composition and distribution on device per￾formance is of particular interest, both for light-emitting diodes39 and for ‘bulk heterojunction technology’ in photovoltaic devices40. Owing to the small exciton diffusion length in conjugated polymers (typically less than 10 nm), efficient photovoltaic devices require an intimately mixed morphology with phase separation on the scale of 10 nm, yet at the same time have a bi-continuous network or perco￾lation pathways for efficient charge collection. Mixtures of the donor polymer poly(9,9′-dioctylfluorene-co￾bis-N,N′-(4,butylphenyl)-bis-N,N′-phenyl-1,4-phenylene-diamine) (PFB) with the acceptor polymer poly(9,9′-dioctylfluorene-co￾benzothiadiazole) (F8BT) have been used as an all-polymer model system for the study of morphology effects41 and charge generation and transport dynamics42. Blends of PFB and F8BT spin-cast from a low-boiling-point solvent, such as chloroform, show a smooth surface topography41. Films spin-cast from a high-boiling-point solvent, such as xylene, show a more complicated morphology and surface topog￾raphy characterized by micrometre-size domains41. Understanding the charge generation mechanism within these domains requires mapping the composition of these blend structures. Although an array of advanced techniques has been used to probe the chemistry and structure of PFB:F8BT blends, only NEXAFS microscopy has the required high chemical sensitivity and spatial resolution (<1 μm). This capability has been exploited to characterize PFB:F8BT blends cast from xylene43 and chloroform44. Quantitative compositional maps of xylene-cast PFB:F8BT blends with different composition ratios are shown in Fig. 3. The micrometre-scale domains contain significant proportions of both polymers. In particular, the minor￾ity phases of 1:5 and 5:1 PFB:F8BT xylene-blend films can contain each polymer in a proportion up to 50%. By analysing the quantita￾tive composition, the superior photovoltaic performance of the 1:5 PFB:F8BT blend over the 5:1 blend could be explained: a higher pro￾portion of the area of the 1:5 PFB:F8BT film has the optimum blend composition of 60 to 80 wt% F8BT. The local composition and its distribution in 1:1 PFB:F8BT films, however, was unable to explain the lower photovoltaic performance of the 1:1 PFB:F8BT blend. This was attributed to a presumed vertical structure that is difficult to characterize at present. Overall, the lower performance of xylene￾processed films relative to chloroform-processed films could not be accounted for by the local blend composition at length scales above the resolution limit of ~45 nm and a nanoscale phase separation at a length scale below the spatial resolution has been postulated. Similarly, compositional and morphological analysis of PFB:F8BT blends cast from chloroform provided much insight into the nano￾scale morphological evolution of these blends upon annealing44. The coarsening and evolution towards purer domains has been quantified for the more aggressively annealed films. However, the comparison with photoluminescence data of as-cast or minimally annealed samples suggested the presence of a nanomorphology that was below the resolution limit of the X-ray microscope used. Char￾acterization of PFB:F8BT blends thus simultaneously illustrates the power of compositional imaging using NEXAFS microscopy and the need to improve the spatial resolution further. Given that the complex morphology is in many cases most probably 3D in nature, optimal characterization would require NEXAFS tomography28 at higher spatial resolution than has been possible so far. biofibres Biofibres are some of nature’s most remarkable materials. Some fibres can achieve combinations of strength and toughness that are still unmatched by high-performance synthetic fibres. One of the fibres produced by spiders, dragline silk, is such a fibre, with an unusual combination of stiffness and extensibility. Numerous mod￾els to explain the fibres’ properties exist. They are thought to be due to alternating alanine-rich hard segments containing β-sheets and glycine-rich soft segments, but the fundamental structure–property relationships in spider silk are not yet fully understood. They are of intrinsic scientific interest, but improved knowledge would also aid industrial efforts to produce biomimetic synthetic fibres. Figure 3 | NeXaFS compositional mapping applied to two-component organic electronic devices that show macroscopic phase separation. Quantitative (wt%) F8BT (a, d, g) and PFB (b, e, h) composition maps and AFM images (c, f, i) of PFB–F8BT blend films of variable composition. a, b, c, 1:1 PFB:F8BT; d, e, f, 5:1 PFB:F8BT; g, h, i, 1:5 PFB:F8BT. j, Chemical structures of both polymers and their respective NEXAFS spectra, which provide the basis of the quantitative analysis. Figure reproduced with permission from ref. 43 © 2007 ACS. N S N N N n PFB F8BT [ n [ [ [ j Mass absorption coefficient (cm–2g) F8BT FBT 283 284 285 286 287 1.2 × 10–5 1.0 × 10–5 8.0 × 10–4 6.0 × 10–4 4.0 × 10–4 2.0 × 10–4 0.0 280 285 290 295 300 305 310 315 320 Energy (eV) 0 nm 15 nm 1:5 PFB:F8BT 0 nm 50 nm 0 nm 50 nm 5:1 PFB:F8BT i f c 1:1 PFB:F8BT 0 50 wt% 50 90 wt% h e b 0 wt% 100 g d a 60 100 wt% 0 wt% 100 0 50 wt% 1 um 1 um 1 um PFB percentage composition F8BT percentage Height composition nmat_2399_APR09.indd 284 13/3/09 11:54:37 © 2009 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOL:10.1038/NMAT2399 INSIGHT I REVIEW ARTICLES The microstructure of the silk filament is controlled in part by the technological application such as sensor technology (for example, conditions under which the fibre is spun and in part by the block giant-magnetoresistance and magnetic-tunnel-junction reading copolymer structure determined by its amino-acid sequence.The heads)and magnetic data storage devices(for example,magnetic alignment of the B-sheets and the size of the crystallites and crys- random-access memory).The X-ray magnetic circular dichroism talline regions within the fibre are thought to be important factors (XMCD)49 shown in Fig.1 can be used as a contrast mechanism responsible for the tensile strength of dragline silk.This alignment in PEEM,full-fields and scannings!X-ray microscopes and for and the fibre texture can be characterized using NEXAFS micro- magnetic investigations with coherent scatterings2.In comparison scopy and systematic quantitative maps of the orientation of the with magneto-optical Kerr microscopys,the synchrotron-based B-sheets in dragline silk of the spider Nephila clavipes have been techniques show element-specific contrast (by tuning the X-ray produced and compared with the Bombyx mori cocoon fibre s.New energy to the absorption edges of the magnetically relevant ele- discoveries that help explain why spider silk has such noteworthy ments)and lateral resolutions of~25 nm,which is an improvement mechanical properties include the observation that the previously of about one order of magnitude.Although the X-ray techniques observed skin has enhanced orientation.An apparently random cannot achieve the atomic resolution possible with the spin- distribution of more-oriented B-sheet crystallites interspersed with polarized scanning tunnelling microscopes4ss,the unique advan- amorphous regions was also observed.This type of stiff reinforce- tages of soft X-ray microscopy for studying magnetic micro-and ment within a flexible matrix is a common feature of high-strength, nanostructures are the element specificity of the XMCD contrast yet flexible,materials. mechanism and its direct correlation with local spin and orbital The properties of spider dragline silk are known to depend moments,which can even be separated by applying magneto- strongly on moisture levels,with the initial stiffness dropping by optical sum ruless6s7.In addition to using the XMCD effect for three orders of magnitude upon immersion of unsupported fibres investigation of ferromagnetic samples,linearly polarized light in in water4647.Very recent initial results on the effects of moisture on conjunction with X-ray magnetic linear dichroism has been used the microstructure of the fibre are shown in Fig.4,where we com- in X-ray microscopy for the study of antiferromagnetic systemss. pare the orientation maps of the carbonyl groups for a N.clavipes The unique contrast mechanism of NEXAFS microscopy allowed fibre in dry and wet states (Fig.4a).The order parameter used(P,) ferromagnetic domains to be correlated with antiferromagnetic is equal to 1 for perfect parallel orientation,-0.5 for perfect per- domains in a number of complex magnetic thin films.For example pendicular orientation and 0 for random orientation.The dry fibre the domains of a bilayer of 1.0 nm of platinum on 1.2 nm of cobalt shows moderately oriented domains and a homogeneous micro- on a LaFeO,(100)substrate are shown in Fig.5(ref.59).The con- structure of small domains.The influence of water strongly affects tributions of the various elements to the magnetic properties could the microstructure of the fibre,resulting in a much coarser texture be distinguished because each element has its absorption edge at a and a broader range of(P)values(Fig.4b).Both (P2)distributions different photon energy and the antiferromagnetic domains of the displayed in Fig.4b have an approximately Gaussian shape and are substrate could be clearly correlated with the cobalt-specific ferro- centred at a (P,)value of about -0.1,but the distribution is much magnetic domains above it.In effect,the element-specific NEXAFS broader for the fibre in the wet,and hence low-stiffness,state. provided layer-specific depth resolution.Similar experiments The first linear X-ray dichroic measurements on fibres revealed allowed the details of ferromagnetic-antiferromagnetic coupling the qualitative radial alignment of various functional groups in to be investigated in a number of systems. Kevlar fibres',and a quantitative comparison of the carbon 1s and As the incoming and outgoing photons are not influenced by nitrogen Is signals from different Kevlar fibre grades provided a magnetic fields,transmission X-ray microscopy allows the study of quantitative comparison of the degrees oforientation in the different magnetic properties on small length scales as a function of applied fibre grades. field;for example,local hysteresis loops and magnetization profiles of cobalt-platinum multilayers deposited on self-assembled arrays Magnetic soft X-ray microscopy of polystyrene nanospheres could be assessed on the 100-nm scale62. Magnetism in confined structurests has become an important Furthermore,XMCD is very sensitive,as was demonstrated by topic for theoretical and experimental studies as well as studying magnetic carbons a Dry 0.1 6 (000 10 Dry Wet um Wet -03 -02 -01 0.0 0.1 -0.3 P》 Figure 4 NEXAFS orientational maps for spider silk in the dry and wet states.a,Maps (3 um x 7 um)of the order parameter (P2)of N.clavipes in thin sections of dragline silk in the dry and wet states.b,(P)distribution for the maps shown in a.Figure reproduced with permission from ref.46,courtesy of A.P.Hitchcock and M.Pezolet. NATURE MATERIALS VOL 8|APRIL 2009 www.nature.com/naturematerials 285 2009 Macmillan Publishers Limited.All rights reserved

nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials 285 NaTure MaTerialS doi: 10.1038/nmat2399 insight | review articles The microstructure of the silk filament is controlled in part by the conditions under which the fibre is spun and in part by the block copolymer structure determined by its amino-acid sequence. The alignment of the β-sheets and the size of the crystallites and crys￾talline regions within the fibre are thought to be important factors responsible for the tensile strength of dragline silk. This alignment and the fibre texture can be characterized using NEXAFS micro￾scopy and systematic quantitative maps of the orientation of the β-sheets in dragline silk of the spider Nephila clavipes have been produced and compared with the Bombyx mori cocoon fibre45. New discoveries that help explain why spider silk has such noteworthy mechanical properties include the observation that the previously observed skin has enhanced orientation. An apparently random distribution of more-oriented β-sheet crystallites interspersed with amorphous regions was also observed. This type of stiff reinforce￾ment within a flexible matrix is a common feature of high-strength, yet flexible, materials. The properties of spider dragline silk are known to depend strongly on moisture levels, with the initial stiffness dropping by three orders of magnitude upon immersion of unsupported fibres in water46,47. Very recent initial results on the effects of moisture on the microstructure of the fibre are shown in Fig. 4, where we com￾pare the orientation maps of the carbonyl groups for a N. clavipes fibre in dry and wet states (Fig. 4a). The order parameter used 〈P2〉 is equal to 1 for perfect parallel orientation, −0.5 for perfect per￾pendicular orientation and 0 for random orientation. The dry fibre shows moderately oriented domains and a homogeneous micro￾structure of small domains. The influence of water strongly affects the microstructure of the fibre, resulting in a much coarser texture and a broader range of 〈P2〉 values (Fig. 4b). Both 〈P2〉 distributions displayed in Fig. 4b have an approximately Gaussian shape and are centred at a 〈P2〉 value of about −0.1, but the distribution is much broader for the fibre in the wet, and hence low-stiffness, state. The first linear X-ray dichroic measurements on fibres revealed the qualitative radial alignment of various functional groups in Kevlar fibres4 , and a quantitative comparison of the carbon 1s and nitrogen 1s signals from different Kevlar fibre grades provided a quantitative comparison of the degrees of orientation in the different fibre grades19. magnetic soft X-ray microscopy Magnetism in confined structures48 has become an important topic for theoretical and experimental studies as well as technological application such as sensor technology (for example, giant-magnetoresistance and magnetic-tunnel-junction reading heads) and magnetic data storage devices (for example, magnetic random-access memory). The X-ray magnetic circular dichroism (XMCD)49 shown in Fig. 1 can be used as a contrast mechanism in PEEM5 , full-field50 and scanning51 X-ray microscopes and for magnetic investigations with coherent scattering52. In comparison with magneto-optical Kerr microscopy53, the synchrotron-based techniques show element-specific contrast (by tuning the X-ray energy to the absorption edges of the magnetically relevant ele￾ments) and lateral resolutions of ~25 nm, which is an improvement of about one order of magnitude. Although the X-ray techniques cannot achieve the atomic resolution possible with the spin￾polarized scanning tunnelling microscope54,55, the unique advan￾tages of soft X-ray microscopy for studying magnetic micro- and nanostructures are the element specificity of the XMCD contrast mechanism and its direct correlation with local spin and orbital moments, which can even be separated by applying magneto￾optical sum rules56,57. In addition to using the XMCD effect for investigation of ferromagnetic samples, linearly polarized light in conjunction with X-ray magnetic linear dichroism has been used in X-ray microscopy for the study of antiferromagnetic systems58–61. The unique contrast mechanism of NEXAFS microscopy allowed ferromagnetic domains to be correlated with antiferromagnetic domains in a number of complex magnetic thin films. For example, the domains of a bilayer of 1.0 nm of platinum on 1.2 nm of cobalt on a LaFeO3(100) substrate are shown in Fig. 5 (ref. 59). The con￾tributions of the various elements to the magnetic properties could be distinguished because each element has its absorption edge at a different photon energy and the antiferromagnetic domains of the substrate could be clearly correlated with the cobalt-specific ferro￾magnetic domains above it. In effect, the element-specific NEXAFS provided layer-specific depth resolution. Similar experiments allowed the details of ferromagnetic–antiferromagnetic coupling to be investigated in a number of systems60,61. As the incoming and outgoing photons are not influenced by magnetic fields, transmission X-ray microscopy allows the study of magnetic properties on small length scales as a function of applied field; for example, local hysteresis loops and magnetization profiles of cobalt–platinum multilayers deposited on self-assembled arrays of polystyrene nanospheres could be assessed on the 100-nm scale62. Furthermore, XMCD is very sensitive, as was demonstrated by studying magnetic carbon63. Dry Wet –0.3 –0.2 –0.1 0.0 0.1 P2œ Fraction of pixels (×100) 10 5 0 Dry Wet 1 μm 0.1 –0.3 a b Figure 4 | NeXaFS orientational maps for spider silk in the dry and wet states. a, Maps (3 μm × 7 μm) of the order parameter 〈P2〉 of N. clavipes in thin sections of dragline silk in the dry and wet states. b, 〈P2〉 distribution for the maps shown in a. Figure reproduced with permission from ref. 46, courtesy of A.P. Hitchcock and M. Pézolet. nmat_2399_APR09.indd 285 13/3/09 11:54:38 © 2009 Macmillan Publishers Limited. All rights reserved