doi: 10.1098/rsta.2009.0112, 3709-3733 367 2009Phil. Trans. R. Soc. AGarca-Barriocanal, C. Leon, J. Santamara, S. N. Rashkeev and S. T. PantelidesOxley, W. D. Luo, K. van Benthem, S.-H. Oh, D. L. Sales, S. I. Molina, J. S. J. Pennycook, M. F. Chisholm, A. R. Lupini, M. Varela, A. Y. Borisevich, M. P.analysis to solving energy problemselectron microscopy: from atomic imaging and Aberration-corrected scanning transmissionSupplementary data367.1903.3709.DC1.htmlhttp://rsta.royalsocietypublishing.org/content/suppl/2009/08/11/"Audio Supplement"Referencesl.html#ref-list-1http://rsta.royalsocietypublishing.org/content/367/1903/3709.fulThis article cites 67 articles, 11 of which can be accessed freeRapid response1903/3709http://rsta.royalsocietypublishing.org/letters/submit/roypta;367/Respond to this articleSubject collections (14 articles) electron microscopy collectionsArticles on similar topics can be found in the followingEmail alerting servicehere in the box at the top right-hand corner of the article or click Receive free email alerts when new articles cite this article - sign uphttp://rsta.royalsocietypublishing.org/subscriptions go to:Phil. Trans. R. Soc. A To subscribe to This journal is 2009 The Royal Society on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Phil. Trans. R. Soc. A (2009) 367, 37093733doi:10.1098/rsta.2009.0112REVI EWAberration-corrected scanning transmissionelectron microscopy: from atomic imagingand analysis to solving energy problemsBY S. J. PENNYCOOK1,2,*, M. F. CHISHOLM1, A. R. LUPINI1, M. VARELA1,A. Y. BORISEVICH1, M. P. OXLEY1,2, W. D. LUO1,2, K. VAN BENTHEM3,S.-H. OH4, D. L. SALES5, S. I. MOLINA5, J. GARCA-BARRIOCANAL6,C. LEON6, J. SANTAMARA6, S. N. RASHKEEV7AND S. T. PANTELIDES1,21Materials Science and Technology Division, Oak Ridge National Laboratory,Oak Ridge, TN 37831, USA2Department of Physics and Astronomy, Vanderbilt University, Nashville,TN 37235, USA3Department of Chemical Engineering and Materials Science, University ofCalifornia, Davis, CA 95616, USA4Korea Basic Science Institute, Daejeon, Republic of Korea5Departamento de Ciencia de los Materiales e I.M. y Q.I., Universidadde Cdiz, Spain6GFMC, Universidad Complutense de Madrid, Madrid 28040, Spain7Idaho National Laboratory, Idaho Falls, ID 83415, USAThe new possibilities of aberration-corrected scanning transmission electron microscopy(STEM)extendfarbeyondthefactorof 2ormoreinlateral resolutionthatwastheoriginal motivation. The smaller probe also gives enhanced single atom sensitivity, bothfor imaging and for spectroscopy, enabling light elements to be detected in a Z-contrastimage and giving much improved phase contrast imaging using the bright eld detectorwithpixel-by-pixel correlationwiththeZ-contrastimage. Furthermore, theincreasedprobe-forming aperture brings signicant depth sensitivity and the possibility of opticalsectioningtoextractinformationinthreedimensions.Thispaperreviewstheserecentadvances withreference toseveral applications of relevance toenergy, the originofthelow-temperaturecatalyticactivityofnanophaseAu, thenucleationandgrowthofsemiconductingnanowires, andtheoriginof theeight ordersof magnitudeincreasedionic conductivity in oxide superlattices. Possible future directions of aberration-correctedSTEM for solving energy problems are outlined.Keywords: scanning transmission electron microscopy; Z-contrast; electron energyloss spectroscopy*Author for correspondence (pennycooksj@ornl.gov).One contribution of 14 to a Discussion Meeting Issue New possibilities with aberration-correctedelectron microscopy.This journal is 2009 The Royal Society 3709 on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3710 S. J. Pennycook et al.1. IntroductionThe successful implementation of aberration correction in the eld of transmissionelectron microscopy (TEM) (Haider et al. 1998; Batson et al. 2002) hasprecipitatedarateof instrumental advanceunparalleledsincethe1930s. Thebenets for scanning TEM(STEM) have arguably beengreater thanthoseforconventional TEMowingtotheimprovementinsignal-to-noiseratiomadepossible by the availability of higher probe currents. For example, the record forimageresolution,previouslyheldexclusivelybyTEM,hasmovedtoSTEM,asseen by the direct imaging of the dumbbell in Si 1 1 2 at 0.78 , shown in gure1 (Nellist et al. 2004), and more recently by the resolution of 0.63 in wurtziteGaN in the 2 1 1 projection (Sawada et al. 2007; Kisielowski et al. 2008) (see alsogure 2). The detectability of individual atoms has improved markedly, whetheron the surface of a material or inside the bulk; compare, for example, the images inNellist & Pennycook (1996) and Voyles et al. (2002) with the aberration-correctedcounterparts in Lupini & Pennycook (2003) and Sohlberg et al. (2004). We haveseen the spectroscopic identication of a single atom (Varela et al. 2004) and two-dimensional spectrum imaging with good statistics (Bosman et al. 2007; Mulleret al. 2008).The larger probe-formingaperture made possible byaberrationcorrectionhas alsobrought some unanticipatedadvantages. The reduceddepthof eldhas brought the possibility of optical sectioning (van Benthemet al. 2005;Borisevichet al. 2006a,b; vanBenthemet al. 2006; Marinopouloset al. 2008)andeventrueconfocal microscopywithdouble-correctedinstruments (Nellistet al. 2006, 2008). Point defect congurations for heavy atoms can be determinedinsidematerials(Ohetal. 2008; Robertsetal. 2008). PhasecontrastimaginginSTEMhasalsobeneteddramaticallyfromaberrationcorrection, notjustbecause of the larger available probe currents, but because the collector aperturecanbeopenedupanorderof magnitudewhilemaintainingcoherentimagingconditions(Pennycook2006).Simultaneousincoherenthighangleannulardarkeld(ADF), or Z-contrast imaging, andbright eldphase contrast imagingprovide two different channels of information about the specimen.Another benet of the clearer viewavailable with aberration correctionis the more accurate comparison of experiment with theory, whether it bedynamicaldiffractiontheoryforunderstandingissuessuchasbeambroadeningandlocalizationinimagingandspectroscopy(Oxleyetal.2007; LeBeauetal.2008; Oxley&Pennycook2008; Pengetal.2008)ordensityfunctional theoryforunderstandingtheatomicoriginofmaterialsproperties. Inbothcases, thesubstantial advances in computational power over the last few years mean thatsystems requiring hundreds of atoms, such as grain boundaries or nanostructures,can now be tackled.In this paper, we present some of these signicant instrumental advances andthen describe some recent applications to energy problems using a combinationof microscopy and theory, specically the origin of the remarkably high activityof gold nanocatalysts for the oxidation of COto CO2, detection of thenucleation sites for semiconductor nanowires and explanation of their subsequentmorphological evolution and the investigation of oxide superlattices showing eightordersofmagnitudeenhancedoxygenionconductivity, whichsuggestsaroutetowards the realizationof solidoxide low-temperature fuel cells. Finally, wePhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3711discuss possible future directions for investigating energy issues using the abilityof the STEM to correlate imaging and spectroscopy with other signals generatedbytheprobe. Cathodoluminescencedetectionwouldallowcorrelationof lightemission with dislocation core structure and impurity segregation for solid-state lighting applications (Pennycook 2008), electron beam-induced conductivity(EBIC) could provide charge collection maps of solar cell materials and an in situscanning tunnelling microscope could enable bias to be applied to local areas inmaterials to probe the effect of band bending and local electric elds.Many of the results presented here have been previously published, andthe original papers should be consulted for more details and additionalreferences. Some of the material has also appeared in other reviewarticles(Varela et al. 2005; Chisholm&Pennycook2006; Pennycooketal. 2006, 2007,2008a,b; Lupini et al. 2007).2. New possibilities with aberration-corrected scanning transmissionelectron microscopyFigure 1ashowstherstdirectimageof acrystal latticedemonstratingsub-angstrom resolution, Si 1 1 2, obtained on the Oak Ridge National Laboratory(ORNL) VG Microscopes HB603U STEM. A clear dip in intensity is visible in thecentre of every dumbbell, showing the resolution to be 0.78 (Nellist et al. 2004).Least-squares tting to image simulations allowed the experimental parameters ofthickness, defocus and incoherent broadening to be extracted (Peng et al. 2008).Thezero-orderspotwasusedasanadjustableparameter,toavoidissueswithany background intensity or Stobbs factor. The best-t image simulation is showningure 1b, whereas gure 1c shows acomparisonbetweenthe experimentalandbest-ttheoreticalintensityproles.Thecoherentaberrationsalonegiveaprobe full-width half-maximum (FWHM) of 0.46 . The best-t image simulationmodelled the incoherent broadening as a 0.6 Gaussian, which when convolvedwith a probe gives an FWHM of 0.74 , resulting in a dip of 11 per cent betweenthe two columns of the dumbbell. A resolution criterion of a 10 per cent dip wasproposed, sinceall thedumbbellsareseentoberesolvedexperimentally, andso the Rayleigh value of 27 per cent seems unnecessarily severe. Comparing thespotintensitiesintheFouriertransform(gure 1d)withthenoisebackgroundrevealedtheinformationlimit andassociatedcondencelevel. The444spot,which corresponds to the dumbbell separation of 0.784 , gave a condence levelof 97 per cent, while the 084 spot at 0.607 had a condence level of 71 per centrepresenting the effective information cut-off.Figure 2 shows the resolution of Ga dumbbells separated by 0.63 in aZ-contrast image of wurtzite GaNinthe 2 1 1 orientation, obtainedontheFEI Titan 80-300 aberration-corrected STEMat ORNLequipped with theCEOSDCORaberrationcorrector (Mller et al. 2006) andaprototypeFEIhighbrightnessgun. WiththeDCOR, full correctionofall third-, fourth-andfth-order aberrations is achievable to a level that is theoreticallysufcientto allowa probe size of 0.5 . The highbrightness gunis still a thermallyassisted Schottky-type gun that has brightness that is higher by about a factorof 45over the standardgun. This microscope was installedas part of thePhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3712 S. J. Pennycook et al.x ()0 0.5 1.0 1.5 2.0 2.5 3.0normalized intensity0.91.01.154442640841731-1(a) (b)(c) (d)Figure 1. (a) Subsectionof anexperimental ADFimage of Si 1 1 2 obtainedonthe ORNLVGMicroscopes HB603USTEMoperating at 300 kVwith a cold eld emission gun, Nionthird-orderaberrationcorrectorandadetectorspanning90200 mrad. (b)Simulatedimagefort =300 , f =45 , Cs =0.037 mm, C5 =100 mmand probe-forming aperture of 22 mrad.Temporal andspatial incoherencehavebeenaccountedforbyconvolvingwithaGaussianwithFWHM of 0.60 . (c) Line scans averaged over a width of 10 pixels comparing experiment (blackcircle) andtheory(greenline). Theexperimental result has beenaveragedover atotal of 96dumbbells and normalized by the average of the data points shown. (d) Experimental diffractogramwith the objective aperture indicated by the white circle showing information transfer to 0.61 .Adapted from Peng et al. (2008).Transmission Electron Aberration-corrected Microscope (TEAM) Project of theUSDepartmentof Energy(http://ncem.lbl.gov/TEAM-project/). TheTEAM0.5microscopeatLawrenceBerkeleyNational Laboratoryhasrecentlypushedthe resolution limit to 0.47 , with a condence level of 60 per cent using Ge 1 1 4(Erni et al. 2009).Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3713(a)2 0.63 100 (c)908070605000.63 0.2 0.4 0.6 0.8 1.0nm1.2 1.4 1.6 1.8(b)Figure 2. Z-contrast image (a) of wurtzite GaN in the 2 1 1 projection, with the structure showninset, Ga atoms as large spheres, N atoms as small grey spheres, with a line trace from the dashedarea(c)showingresolutionoftheGadumbbells0.63 apart(blackarrows),withtheNatomsalsojustvisible(whitearrows).TheFouriertransform(b)showsinformationtransferto0.63 .Images are raw data from the ORNL FEI Titan 80-300, recorded by A. R. Lupini.The resolution of spectroscopic images also dramatically improved withaberrationcorrection(for arecent review, seePennycooket al. (2009)), andit is possible to investigate experimentally longstanding issues such as thedelocalization of the ionization interaction. Figure 3 shows line proles obtainedfromLaMnO3inthepseudocubic 0 0 1 zoneaxis alongthe 1 1 0 directionacross La and MnO columns (Oxley et al. 2007). La has both a high energy M4,5edge and a low energy N4,5 edge, allowing a direct comparison of image contrast.Furthermore, Mn has a high energy L2,3edge, but differs signicantly in Z fromLa; hence, comparing the thickness dependence of the La and Mn columns allowstheroleofdechannellingtobeexplored. All experimental tracesshowreducedcontrast compared with simulations, but do not show a simple trend with innershell bindingenergy. TheLaN4,5edgeat99 eVstill showsatomicresolution,although the contrastis reduced signicantly compared with that obtained fromtheLaM4,5edgeataround842 eV. TheFWHMof theLaN4,5traceisverysimilar to that of the La M4,5 edge, whereas estimates of ionization delocalizationshowtheLaN4,5excitationtobemuchbroader(e.g. Howie1979; Pennycook1988; Egerton 2007). Such estimates give a good order of magnitude estimate ofthemeaninteractiondistancebutcannotgiveanaccuratepredictionofimagecontrastforwhichquantummechanical simulationsareessential (Oxleyet al.2007; Oxley & Pennycook 2008). The extended tail of the La N4,5response doesnot preclude atomic resolution; instead, it reduces the contrast.Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3714 S. J. Pennycook et al.(b)(c)(e)(d)( f )(a) La columnMn/O columnO column5 ADF2.01.51.00.502.01.51.00.502.01.51.00.50 2 4 6probe position ()8 10 2 4 6probe position ()8 102.01.51.00.50/avg/avg/avg/avg/avg1.51.00.52.00La N4.5La M4.5Mn L2,3ADFOKFigure 3. (a) Z-contrast image of LaMnO3 in the pseudocubic 1 0 0 zone axis, with the pseudocubic1 1 0 direction arrowed, with intensity proles (normalized to the average intensity) of (b) the ADFimage and (cf ) spectroscopic image proles for various core losses. Simulations are shown as solidlines and experimental data as crosses, which always show lower contrast. Note the broad volcano-likefeatureattheOKedge(c), despiteitshighthresholdenergy(532 eV), whichresultsfromtheeffectivenon-local natureoftheinteraction. TheMnL2,3edge(f )atslightlyhigherenergy(642 eV) shows much higher contrast, as does the La L2,3edge at 832 eV. Even the La N4,5edge(d)at99 eVshowsatomicresolution,althoughithasalongdelocalizedtailcomparedwiththeother high energy edges that reduce its contrast. Data obtained with a 100 kV aberration-correctedSTEM with EELS collection angle 12 mrad. The positions of the atomic columns are indicated bythe lled circles. Adapted from Oxley et al. (2007).Manyof thesimulatedimagesshowvolcano-likefeaturesontheatomsites,dips inintensitywhentheprobeis locatedover thesite. Suchfeatures havebeen conrmed experimentally for the O Kand La N4,5edges. Comparing withsimulations for verythincrystals shows that the La N4,5volcano is due todechannellingandscatteringof theprobeas it propagates alongthecolumn(estimatedtobe250 thickexperimentally), whereasintheOcase, itisduetotheeffectivenon-local natureof theinelasticinteraction. Non-local effectsare less pronouncedas the collectionangle into the spectrometer is openedupandthe spectroscopic image approaches the ideal incoherent image thatis obtainedwhenall elasticallyscatteredelectrons are collected(Rose 1976;Ritchie & Howie 1988).Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3715(a) (b)0.5 nm123456 EELS intensity820 850energy loss (eV)880654321Figure4. Spectroscopicidenticationof anindividual atominitsbulkenvironment byEELS.(a)Z-contrastimageofCaTiO3showingtracesoftheCaOandTiO2 {1 0 0}planesassolidanddashed lines, respectively. A single La dopant atom in column 3 causes this column to be slightlybrighter than other Ca columns, and EELS from it shows a clear La M4,5signal. (b) Moving theprobe to adjacent columns gives reduced or undetectable signals. Adapted from Varela et al. (2004).BesidesthedecreasedFWHMof theaberration-correctedprobe, italsohassignicantlyreducedprobetails, meaningthat morecurrent is incident onacolumn of interest and less wasted illuminating surrounding columns, at least insufciently thin crystals. This greatly improves the sensitivity of the analysis tothe column composition, and gure 4 shows the spectroscopic identication of asingleatominsideabulkmaterial, LainCaTiO3(Varelaetal. 2004). Inthisexample, the small residual signal present on the adjacent O column allowed thethicknesstobededucedataround100 incomparisonwithdynamical imagesimulations.STEMbrighteldimagingcanbeopticallyequivalenttoTEMbrighteldimagingviathe principle of reciprocityor time-reversal symmetryof elasticscattering amplitudes (Cowley 1969; Zeitler & Thomson 1970), when the STEMprobe-forming (objective) aperture is equivalent to the TEM objective apertureand the STEM collector aperture is equivalent to the TEM condenser aperture.However, before aberration correction, for phase contrast imaging, a small beamdivergence was necessary to avoid averaging over the oscillations of the transferfunctionanddamping the contrast (gure 5a). This meant that the STEMcollector aperture hadtobe verymuchsmaller thanthe objective aperture,and only a small fraction of the incident probe current could be used toformtheimage. Consequently, STEMphasecontrast imagingwas verynoisyandrarelyused. The situationis quite different after aberrationcorrection.Not onlyis theresolutionextended, as seeningure 5b, but nowthebeamdivergence can be much larger without damping the transfer. The STEM collectoraperturecanbeenlargedanorderof magnitude, givingaroundtwoordersofmagnitude increased signal. Of course, still, only a small fraction of the incidentprobe current is used, meaning that it is less efcient than the equivalentTEMimaging conditions. However, the STEMbright eldimage is nowofPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3716 S. J. Pennycook et al.1.00.500.51.0 (a)(b)CTF1.00.500.51.00 0.5 1.0 1.5K (1)CTFFigure5. (a)Contrasttransferfunctionsforanuncorrected300 kVmicroscope, greyline, withacoefcientofthird-orderspherical aberrationCs =1.0 mm, chromaticaberrationCc =1.6 mm,anddefocus f =44 nm, withthe damping envelopes introducedbya beamdivergence of0.25 mrad(dottedline)andanenergyspreadof 0.6 eVFWHMtypical of aSchottky(thermal)eldemissionsource(dashedline). (b) Transfer for acorrected300 kVmicroscope(solidline)(Cs =37 m, C5 =100 mm, Cc =1.6 mm,f =5 nm), with the damping envelopes introduced bya beam divergence of 2.5 mrad (dotted line) and an energy spread of 0.3 eV typical of a cold eldemission source (dashed line). Reproduced from Pennycook et al. (2007).highqualityandis available simultaneouslywiththe Z-contrast image withpixel-to-pixel correlation. Anexampleof thesimultaneousphasecontrastandZ-contrastimagingofSrTiO3 1 1 0isshowningure 6.TheOatomcolumnsare just visible in the Z-contrast image in between the Ti columns. If they werespurious features produced by probe tails, then there would be brighter featuresinbetweentheSr columns, but, sincetheyareabsent, theobservedfeaturesmust indeedbe Ocolumns. The optimumdefocus andthird-order sphericalaberrationaretunedfortheZ-contrastimage, buttheyalsoresultinagoodphase contrast image, although the optimum focus is slightly different in the twocases. The phase contrast images show the oxygen columns with stronger contrast(Jia et al. 2003; Jia 2004, 2005, 2006).A less anticipated result of the much reduced depth of focus of the aberration-corrected probe is the ability to perform optical sectioning in a light amorphousor randomly oriented matrix. A through-focal series of images becomes a depthPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3717BF(a)(b)ADF+2 +6 +10defocus (nm)ADF, +2 nm defocus+14TiSrOBF, +6 nm defocus1 nmTiSrOFigure 6. Comparison of coherent and incoherent imaging of SrTiO3 in the 1 1 0 projection using a300 kV VG Microscopes HB603U STEM with Nion aberration corrector. (a) A through-focal serieswith 4 nm defocus steps, in which the phase contrast images show complex variations in contrastwhiletheincoherentADFimagecontrastslowlyblurs. All imagesarerawdataandshowsomeinstabilities. (b) Under optimum conditions, the ADF image shows the Sr columns brightest, the Ticolumns less bright and the O columns barely visible, while the phase contrast BF image shows theO columns with high contrast. Microscope parameters are optimized for the smallest probe, with,nominally, a probe-forming aperture of approximately 22 mrad, Cs 0.037 mm and C5 100 mm.Courtesy of M. F. Chisholm, A. R. Lupini and A. Y. Borisevich, adapted from Pennycook (2006).sequence of images as shown in gure 7 (Borisevich et al. 2006a). In this example,the atoms are well separated in depth and provide an excellent test of the expecteddepth resolution of 78 nm. The incremental intensity as a function of focus hasanFWHMof12 nm,higherthanexpectedperhapsduetobeambroadeningorresidual aberrations. Because of the high signal-to-noise ratio, the peak intensitycanbelocatedtoaprecisionof 2 with95per cent condence. This is arelativelyideal case, andthepresenceof alargerbackgroundcansignicantlyreduce the range of visibility for single atoms. In a through-focal series of images ofPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3718 S. J. Pennycook et al.(a)(c)2 nm(d)(b)relative incremental intensity048 44 40defocus (nm)36 320.51.0Figure 7. Three frames from a through-focal series of Z-contrast images from a Pt/Ru catalyst on a -alumina support, at defocus of (a) 12 nm, (b) 16 nm and (c) 40 nm from the initial defocussetting. Arrows point to regions in focus, and in (c), a single atom is seen in focus on the carbonsupport lm. (d) Integrated intensity of the Pt atom seen in (c) above the level of the carbon lmasafunctionofdefocus,comparedwithaGaussiant.TheFWHMofthetis12 nm,buttheprecisionof thelocationof thepeakintensityis0.2 nmwith95%condence. ReproducedfromBorisevich et al. (2006a).individual Hf atoms in a high dielectric constant semiconductor device structure,the atoms were visible onlyover arange of approximately1 nmor so(vanBenthem et al. 2005, 2006).The situation is more complicated in a crystal aligned to a zone axis. Opticalsectioning is nowopposed by the tendency of the columns to channel theelectrons. Beams incident near the optic axis couple efciently into the 1 s Blochstate,whichbeforeaberrationcorrectionwasthedominantcontributiontotheADFimage(Pennycook&Jesson1990,1991;Jesson&Pennycook1993,1995;Nellist & Pennycook 1999). After aberration correction, the outer beams passingthroughtheouter regions of theobjectiveaperturearenowsofar fromthePhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 37194 1053 1052 1054 1053 1052 1054 1053 1052 1054 105(a)(b)(c)(d)3 1052 1050.50.5 nmdSi cba1.0 1.5distance (nm)2.0Figure8. HAADFimagesof aSi nanowirein 1 1 0zone-axisorientation(leftpanel). Aslightimage distortion caused during the scan was unwarped. Boxes show the regions used for intensityproles, with Au atoms in various congurations arrowed; (a) substitutional; (b) tetrahedral; (c)hexagonal; (d) buckled SiAuSi chain congurations. The intensity proles across the Si dumbbellscorrespond to a width of 18 pixels. Adapted from Oh et al. (2008).zoneaxis that theydonot channel stronglyandthereforestill tendtocometoafocusasifpropagatinginfreespace. ThiseffectwasrstseeninaBlochwave analysis of ADF STEM imaging with large probe-forming apertures (Penget al. 2004). The competitionof the twoprobe components depends onthechannelling strength of the column. Calculations for Si 1 1 0-containing Bi atomsat various depths predicted that individual Bi atoms at different depths could beselectively brought into focus (Borisevich et al. 2006b; see also Cosgriff & Nellist2007). However, thiswasnotthecaseforLaatomsinCaTiO3. Similardepth-dependent effects arepredictedfor electronenergyloss spectroscopy(EELS),where the ability to elementally select a particular species is anadvantage(DAlfonsoet al. 2007). Adisadvantageof theoptical sectioningapproachisthat thedepthresolutionfor extendedobjects is muchworse, givenbyd/,where d is the object diameter and is the objective aperture semiangle.Thisisovercomewithatrueconfocal modeinadouble-correctedinstrument(Nellist et al. 2006, 2008).Figure 8 shows the images of individual Au atoms inside a Si nanowire in the1 1 0 orientation obtained by optical sectioning. The beam is brought to a focusinsidethethickness of thewiresothat surfaceatoms arenolonger infocus(Oh et al. 2008). Hence, the Au atoms visible in the image are denitely locatedPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3720 S. J. Pennycook et al.(a)(b)(c)(d)Figure9. Local views of defect congurations of AuinSi. (a) Substitutional; (b) tetrahedral;(c) hexagonal; (d) buckled SiAuSi chain congurations. The HAADF images (left panels) wereprocessed by maximum entropy image deconvolution (HREM Inc.) using a Lorentzian-type probefunction. The corresponding atomic models (right panels) calculated by density functional theorymatch the images closely. Adapted from Oh et al. (2008).inside the bulk of the wire, although they may well be injected there by knock-onprocesses since they are observed to be quite mobile, moving between successiveimages. Nevertheless, asubstitutional andthree interstitial congurationsatetrahedral, a hexagonal and a buckled SiAuSi chain congurationare seen.Density functional calculations conrm that these are stable sites for Au atoms(gure 9), andcalculatedformationenergies are inaccordwiththe numberdensities observed in the images. However, the number densities imply an effectivetemperature of around1000C, another indicationthat theyare metastablecongurations induced by the electron beam.Optical sectioning combinedwithdensityfunctional theoryhas also beenappliedtohighdielectricconstantsemiconductordevicestructures. Thethree-dimensional distribution of stray Hf atoms was determined by optical sectioning(van Benthem et al. 2005, 2006). A relatively high density was seen in the 1 nmthick SiO2layer, but, in this case, the Hf atoms were not affected by the 300 kVbeam. No Hf atoms were found at the critical Si/SiO2 interface, a result explainedby density functional calculations that showed interfacial Hf atoms to have 1.4 eVenergyhigherthanthoseawayfromtheinterface(Marinopouloset al. 2008).Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3721(b)V(a)1 nm110001Figure 10. (a) Z-contrast STEMimage of Pt onthe surface of -Al2O3close to the 1 1 0orientation. Two Pt3 trimer structures are circled. (b) Schematic of the conguration for the Pt3OHunit on the {1 1 0} surface of -Al2O3, determined by rst-principles calculations; Pt atoms showncircled. ThecircledVmarksavacant tetrahedral cationsite; trianglesmarkpossibleHatompositions in the vacancy. Adapted from Sohlberg et al. (2004).Calculations further showed that signicant leakage currents could occur acrossthedeviceattheobservedHfdensities,aswasindeedfoundexperimentallyinthis particular sample.3. Application to catalystsCatalysts often comprise heavy transition metals on a light support, which is anideal situationforADFSTEMimaging. Whilesingleatomscouldbedetectedpriortoaberrationcorrection(Nellist&Pennycook1996), aftercorrectiontheimage quality is signicantly enhanced, allowing accurate extraction of Pt atompositionsasshowningure 10(Sohlberget al. 2004). Inthecaseof thePt3trimers, twoof thespacings werefoundtobetoolargefor ametallicbond,andthe image couldonly be matchedto density functional calculations byassuming the trimer to be capped by an OH group. The combination of densityfunctional theoryandimaging has provedquite illuminating concerning thenatureandcatalyticactivityof several transitionmetal systems. Inthecaseof the Rh/ -alumina catalytic system, the raft-like structures observed with themicroscopewereidentiedbycomparisonwithsimulationsofthehigh-pressurephase of Rh sesquioxide. The denser structure of the high-pressure Rh2O3phasegives a better lattice match to the Al2O3structure (Sohlberg et al. 2008). -Alumina is a common catalytic support, which is often stabilized thermallyby the addition of La. However, controversy existed over the mechanism involved.Images of single La atoms on the -alumina support showed them to be widelyseparatedandlocatedover AlOpositions onthe {1 0 0} surface, as opposedto the {1 1 0} surface that is stable in the undoped alumina. Calculationsshowedthat theoriginof stabilizationwas amuchhigher bindingenergyonPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3722 S. J. Pennycook et al. -alumina (7.59 eV) than on the -alumina (45 eV), which is the product of theundesirablehigh-temperaturephasetransformation.TheLaatomsincreasetheenergycostof thephasetransformation, increasingthetransitiontemperature(Wang et al. 2004). Inthe case of chromia/aluminacatalytic systems, it wasfoundthattherateof degradationwitha -aluminasupportwasveryrapid,but replacing the support with-alumina greatly improves stability. The effect isquite remarkable because the two structures differ only in the distribution of thestructural vacancies. In -Al2O3, most of the vacancies are located in tetrahedralsites, whereas, in-Al2O3, theyaredistributedmainlyintheoctahedral sites(Zhou&Snyder1991). However, thedifferencesignicantlyaffectsthesurfacerelaxationprocesses inthese polytypes that result intheir different surfacereactivity(Sohlberg et al. 1999). ThedifferencemeansthatCrclustersonthe -aluminasurfacebutremainshighlydispersedonthe-aluminasurface, andhence catalytically active (Borisevich et al. 2007).Finally, weshowhowaberration-correctedSTEMandrst-principlestheoryelucidatedthe mechanismfor the low-temperature oxidationof COto CO2byAunanoparticles(Rashkeevet al. 2007). Auisinactiveinthebulkform,but nanoscale Aucatalyses the reactionevenbelowroomtemperature, andmanyexplanationshavebeenproposed(Bamwendaet al. 1997; Valdenet al.1998; Yoonet al. 2003). Figure 11shows images of thecatalyst preparedbydeposition/precipitationontonanocrystallineanatase(Zhuetal. 2004). Intheas-depositedstate,manyindividualAuatomscanbeseen,butafterreductionto the active form, the atoms are much less visible. The melting point ofAu nanoparticles reduces with diameter and reaches roomtemperature atapproximately the size range observed (Buffat & Borel 1976). Therefore, it seemslikely that the smallest nanoparticles may be in a quasi-liquid state. Most of thereduced nanoparticles are 12 nm in diameter, and quantifying the thicknesses bycomparing with image simulations revealed that a high fraction is just one or twomonolayers thick.First-principles calculations were carried out on a whole sequence ofnanoparticles with different sizes and atomic congurations on anatase. The keyfeature for the activity of the nanoparticles relative to bulk Au is the presence oflow-coordination sites. As shown in gure 12, with reducing coordination number,the energy required to desorb O2exceeds the activation barrier for the reactionfor acoordinationnumber less than5. This means that anO2moleculecanresideonthesurfacelongenoughtoreactwithanadsorbedCOmolecule. Atsiteswithcoordinationnumber6orgreater, desorptionwill occuronaveragebefore the reaction proceeds, and the surface of the Au nanoparticle will becomeinactive for the reaction. In this situation, only perimeter sites can still participatesignicantly in the reaction. This is the underlying reason for the vast change incatalytic behaviour for nanoscale Au (see Rashkeev et al. (2007) for details andfurtherreferences).ThelowactivationbarrierontheAunanoparticleispartlyduetotheloosenessoftheAuAubonds,asproposedbyYoonetal.(2003),which allows the molecules to move during the reaction and nd a congurationthat minimizes the activation barrier, as shown in gure 12b. First-principles CarParrinellomolecular dynamics calculations of aAu10cluster onanataseat atemperature of 200 K indicated atomic displacements in the region of 0.60.7 ,sufciently high to preclude any atomic resolution imaging.Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 372310.1frequency0.20.30.10.20.3(c) (d)0.42 3particle thickness (layers) particle diameter (nm)4 5 6 0 0.5 1.0 1.5 2.0 2.5 3.0(a) (b)1 nm 1 nmFigure 11. Z-contrast images of Au nanoparticles deposited on nanocrystalline anatase bydeposition/precipitation, (a) asdeposited, withAuatomsresolved, and(b) afterreductiontotheactiveform. Theaverageparticlesizeisaround1 nm(c)andmanyparticlesareonlyonemonolayer in thickness (d). Reproduced from Rashkeev et al. (2007).4. Semiconductor quantum wiresControlled growth of semiconductor quantum nano-object arrays is important forreproducible fabrication of optoelectronic devices to achieve high efciency. Thesublattice sensitivity of aberration-corrected Z-contrast imaging, combined withelasticity calculations, has revealed the nucleation sites for nanowires and allowedquantitative understandingof the growthof stacked, strainedlayer nanowirearrays. Figure 13showsacross-sectional viewof anInAsxP1xnanowirearraysandwiched between InP layers, grown by molecular beam epitaxy (Fuster et al.2004). It is clear that thearrayis not vertical but cantedtotheright. Theorigin of this effect can be traced to the nucleation process. Atomic forcemicroscopy examination of the nanowires deposited on an InP substrate showedthetendencyforthenanowirestobelocatednearstepedges. Whilemanyofthestepswerewavy,somewiresnucleatednearstraightsegmentsofsteps,andthesegaveaclearsharpimageincross-sectionZ-contrastimages, asshowningure 14(Molinaet al. 2007). Inthis 1 1 0 projection, thecubiczinc-blendestructure projects as dumbbells with the In column towards the left of the gureandthePorAsxP1xcolumntowardstheright. TheimagehasitsthresholdPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3724 S. J. Pennycook et al.101231.01.2energy (eV)0.80.60.400.20 0.2 0.4 0.6reaction coordinate0.8 1.0 3 4 5 6coordination number7 8 9(a) (b)Figure12. (a)Calculateddesorptionenergies(Ed)for(O2)moleculesboundtoAunanoparticlesites as a function of coordination number, and the activation barrier (Er) for conversion of CO toCO2(blue line, Ed; green line, Ed,p; red line; Er; black line, Er,p). The desorption energy exceedsthe reaction barrier below a coordination number of 5, which allows the reaction to proceed on thenanoparticle surface. Above this coordination number, only perimeter sites are important. (b) AnexampleoftheactivationbarrierfortheCOtoCO2reaction,inwhichtheexibilityoftheAunanocluster lowers the activation barrier at the transition state marked by the red arrow, a featurecritical to achieving high activity at room temperature. The schematic shows the nanocluster in itstransition state, with Ti in grey, O in red and Au in gold. Reproduced from Rashkeev et al. (2007).30 nmInP (001)InP bufferFigure13. LowmagnicationTEMdiffractioncontrastimageof anInAsxP1xnanowirearraysandwichedbetweenInPlayers,showingthatthealignmentofthenanowirestackisoffsetfromthe growth direction. Adapted from Fuster et al. (2004).at the intensity level corresponding to the Pcolumn, so that the interfacebetweentheInPlayer andthenanowirecanbelocatedtoatomicprecision.ItisseenthatthetransitionfromthesubstrateInPlayertothenanowireisatomicallyabrupt.Thelocationofthestepisslightlyoffsetfromthecentreofthe nanowire, indicating that the initial nucleation took place on the upper terraceof the step.Nucleation on the upper terrace is entirely in accord with expectations based onthe 3.2 per cent lattice mismatch between InAs and InP. Because of the existenceof thestep, afewunit cells next tothestepontheupper terracecanrelaxsideways a little, reducing the lattice mismatch, and therefore making this areathe preferential nucleation site. Finite-element elasticity calculations showed thatPhil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 37252 nmInAsxP1x(a)(b)Figure14. (a)Z-contrastimageof theInAsxP1xnanowire. Theimagehasbeencolouredandhas its threshold at the intensity level of the P columns to highlight the AsxP1xcolumns in thenanowires. (b) Higher magnication view showing the atomically abrupt transition from the InPto the InAsxP1xnanowire, with an interface step present under the nanowire. Reproduced fromMolina et al. (2007).the magnitude of the relaxation was indeed signicant. With the nucleation takingplace on the upper terrace, the surface of the growing lm will roughen, so thisrepresents the onset of the StranskiKrastanov transition from a two-dimensionalto a three-dimensional growth mode.Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3726 S. J. Pennycook et al.Itcanalsobeseeningure 14athatthereisaslightasymmetryintheAsdistribution within the wire, the region above the upper terrace showing brighterandthereforecontainingmoreAs. This is typical andwas alsoconrmedbyquantitative EELS, using the P edge intensity in the barrier layers as calibration.TheresultingAsconcentrationmapswereusedasinputintoanite-elementelasticity calculation (Molina et al. 2006). Owing to the microscopic asymmetryof the As distribution, an anisotropy appeared in the stress on the surface of thesubsequent InP barrier layer, with the peak stress emanating from the nanowireappearingslightlyoffsetfromthegeometriccentreof thewire. Therefore, thefavourable nucleation site for the next nanowire is slightly offset from the centreof the underlying wire. With carefully controlled conditions, all nanowires line upat a specic angle to the growth direction and the observed angle is in quantitativeagreement with the nite-element calculations.5. Colossal ionic conductivity in Y2O3-stabilized ZrO2/SrTiO3 superlatticesSolidoxidefuelcells(SOFCs)areapromisingnon-pollutingtechnologyforthesubstitution of fossil fuels, but the conversion efciency of chemical into electricalenergy is limited by the transport of oxygen anions through an electrolyte(Steele&Heinzel 2001). Yttria-stabilizedzirconia(Y2O3)x(ZrO2)1x(YSZ) isthematerial mostcommonlyusedinSOFCsowingtoitsmechanical stability,chemical compatibilitywithelectrodesanditshighoxygenionicconductivity.Amaximumvalue of 0.1 S cm1at 1000Cis observed for 89 mol%yttriacontent.However,aseverelimitationofthismaterialisitsrelativelylowroomtemperatureionicconductivity, whichimposeshighoperational temperatures,around800C. Thereisamajorresearcheffortdevotedtothesearchfornewmaterials with greatly enhanced ionic conductivity (often referred to as superionicconductors), but it has not yet beensuccessful inreachingthe conductivityvalue desired for applications, at least 0.01 S cm1, with operation close toroom temperature.Superlattices comprising strained layers of YSZ between SrTiO3 (STO) spacershaverecentlybeenfoundtoshowuptoeight orders of magnitudeenhancedOionconductivitynear roomtemperature (Garcia-Barriocanal et al. 2008),creatingintense interest for their potential toachieve highefciencySOFCsatlowoperatingtemperatures. Theconductivitywasfoundtoscalewiththenumberof interfaces, butwasindependentof thethicknessof theYSZlayer,except when the layer thickness exceeded six to eight unit cells when relaxationset inandtheeffect disappeared. Thecontributionof electronicconductivitythroughthe STOwas foundtobe three tofour orders of magnitude lower,demonstrating the ionic nature of the conductivity. Furthermore, experiments alsoruled out a protonic contribution to the conductance, proving the mobile speciestobeoxygenions(seeelectronicsupplementarymaterialinGarcia-Barriocanaletal.2008).BothX-rayandSTEManalysisofthestrainedthinlayersshowedthemtobe highlycoherent withthe YSZlattice rotated45tothat of theSTO. Figure 15a shows alowmagnicationZ-contrast image of anine-layersuperlattice of 1 nm (two unit cells) thick YSZ layers between STO barriers andahighmagnicationviewshowingthecationlatticetobecontinuous acrossthe YSZ layer.Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 3727Insight into the nature of the interface is providedby EELS. Figure 15bcomparesspectrafromtheinterfaceplanewithspectrafromthecentreof theSTO layer. No detectable change is seen in the Ti ne structure, indicating thata Ti 4+ conguration is predominant at the interface plane, which is consistentwith the lack of electronic conductivity. However, changes are seen in the O nestructure, indicating a change in coordination, which would be expected since theO sublattices of YSZ and STO are different and incompatible across the interface.Determination of the STO-terminating plane at the interface is provided by thespectrumimageshowningure 16a.ComparingtheSrM3andTiL2,3prolesacrosstheinterfaceshowsthatonbothsidestheSredgedropsbeforetheTiedge, sothattheSTOisterminatedattheTiO2plane. Nowfromthecationpositions visibleintheimage(gure 15), it is apparent that thestructureisasshowningure 16b. WhilethedistancebetweentheTiO2planeandtheZrplane is sufcient for O ions to be present, the perovskite and uorite structuresdemanddifferent Opositions. This maygiverisetoanenhancednumber ofoxygen vacancies (shown shaded in the model) and structural disorder necessaryto explain the origin of the colossal ionic conductivity. Perhaps this plane couldbe better thought of as more like a vacancy liquid than a solid oxide, exhibitingliquid-like diffusivities.6. Future possibilities for aberration-corrected scanning transmissionelectron microscopyAberration correction allows the pole piece gap to be opened up without a majorcostinresolution,sothatthereismorespaceforspecialholdersforheatingorcoolingorforinsituobservations. Particularlyappealingisthepossibilityofusing thin SiN windows for imaging in high-pressure or corrosive environments.With a 300 kV aberration-corrected incident beam, a 20 nm thick window wouldgive minimal broadening of the probe and should still allow atomic resolution tobe achieved for objects near the entrance window.Manymorepossibilitiesexistformappingfunctionalityinenergyconversiondevices such as solar cells or materials for solid-state lighting. Acathodo-luminescence detector in an aberration-corrected STEMwould allow lightemissiontobe correlatedtodislocationcore structure andimpuritycontent(Pennycook 2008) andperhaps enable the actual non-radiative pathways tobedeterminedandeliminated. Similarly, EBICcouldprovidemapsof chargecollection efciency in solar cells. Operating devices have already been observedin STEM at low resolution using cathodoluminescence (Bunker et al. 2005) andEBIC(Progl et al. 2008). Extendingthesestudiestotheaberration-correctedSTEM, inconjunctionwiththeory, couldprovidekeyinsightsintothecarrierdynamics in the vicinity of defects and howthey change as a function ofoperatingcurrent. Additionof ascanningtunnellingmicroscopetotheSTEMwouldallowbiastobeappliedtolocal regionsofsample, inducinglocal eldsand band bending and the effect on local electronic and optical propertiesto be investigated.It is apparent that the era of aberration-corrected microscopy will provide newmeans to connect structure to functionality. In operandi imaging of catalysts willbecomefeasible, althoughcareshouldbetakentoavoidbeam-inducedeffects,Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from 3728 S. J. Pennycook et al.STO100 nm 1.2 nm0460E loss (eV)4702 1041 104intensity (arbitrary units)normalizedintensity540 530E loss (eV)550STOYSZSTO(1 0 0)(a) (b)Figure15. (a)Z-contrastSTEMimageof theSTO/YSZinterfaceof the(YSZ1 nm/STO10 nm)9superlattice(withninerepeats),obtainedintheVGMicroscopesHB603Umicroscope.Ayellowarrowmarksthepositionof theYSZlayer. Theinsetisalowmagnicationimageobtainedinthe VG Microscopes HB501UX. In both cases, a white arrow shows the growth direction. (b) EELspectra showing the O Kedge obtained from the STO unit cell at the interface plane (red circles)and4.5 nmintotheSTOlayer(blacksquares). TheinsetshowstheTi L2,3edgesforthesamepositions, same colour code. Reproduced from Garcia-Barriocanal et al. (2008).andtheabilitytostudyenergyconversionprocessesinmaterialsanddeviceswill be greatly enhanced. Electron microscopy, in conjunction with theory, couldtherefore play a critical role insolving the pressing energy problems facingus today.Theauthors wouldliketothanktheir collaborators intheworkreviewedhere, P. D. Nellist,O. L. Krivanek, N. Dellby, M. F. Murtt, Z. S. Szilagyi, Y. Peng, S. Travaglini, H. M. Christen,T. J. Pennycook, S. F. Findlay, A. J. DAlfonso, L. J. Allen, P. Werner, N. D. Zakharov,D. Kumar, K. Sohlberg, S. H. Overbury, T. Ben, D. L. Sales, J. Pizarro, P. L. Galindo,D. Fuster, Y. Gonzalez, L. Gonzalez, A. Rivera-Calzada, Z. Sefrioui, E. Iborra, W. H. Sidesand J. T. Luck, which was supported by the Ofce of Basic Energy Sciences, Divisions ofMaterials Sciences andEngineering (S.J.P., M.F.C., A.R.L., M.V., A.Y.B., M.P.O., W.D.L.)andScienticUserFacilities(A.Y.B., K.v.B.), anNSF-NIRTproject (DMR-0403480, S.H.O.),the NODE and SANDiE European Networks of Excellence (contract no. NMP4-CT-2004-500101), theSpanishMEC(TEC2005-05781-C03-02andTEC2008-06756-C03-02)andtheJuntade Andalucia (PAI research group TEP-120; projects PAI05-TEP-383 and TEP-3516, SIM),theSpanishMinistryforScienceandInnovation(MAT200506024, MAT200762162andMAT2008) and the Madrid Regional Government Communidad Autnoma de Madrid throughUniversidad Complutense de Madrid Groups Programme (J.C.B., C.L., J.S.), the McMinnEndowmentatVanderbiltUniversity(S.T.P.)andinpartbytheLaboratoryDirectedResearchandDevelopmentProgramofORNL(S.J.P. andA.Y.B.). Someoftheinstrumentationusedinthis researchwas providedas part of theTEAMproject, fundedbytheDivisionof ScienticUserFacilities, Ofceof Science, USDepartmentof Energy. Thisworkwassupportedinpartby a grant of computer time fromthe DODHigh Performance Computing ModernizationProgramat the Naval Oceanographic Ofce (NAVO), the USArmy Engineer ResearchandDevelopment Center (ERDC) andthe DOENational Energy ResearchScientic ComputingCenter (NERSC).Phil. Trans. R. Soc. A (2009) on 18 August 2009 rsta.royalsocietypublishing.org Downloaded from Review. Aberration-corrected STEM 37292.2 1041.6 1041.9 104(iii)(ii)1.00.5normalized integratedintensitysimultaneous ADF0 5 10 15d (nm)20 25(a)(b)(i)spectrum imageTiSrZrOFigure16. (a)EELSchemical maps. TheADFimageintheupperpanel showstheareausedfor EELS mapping (spectrum image, marked with a green rectangle) in the (YSZ1 nm/STO10 nm)9superlattice. The middle panel shows the averaged ADF signal acquired simultaneously with theEEL spectrum image, showing the STO (low-intensity regions) and YSZ (higher intensity) layers.The lower panel shows the Ti (red) and Sr (dark yellow) EELS line traces across several consecutiveinterfaces. These line traces are averaged from the elemental two-dimensional images shown in theinsets, eachframedwiththesamecolourcode(redforTi, darkyellowforSr). DataobtainedintheVGMicroscopesHB501UX. AstheSTEMspecimenwasrelativelythick, several tensofnanometres, the wide chemical interface proles are most likely due to beam broadening. (b) Solidspheres model of theYSZ/STOinterfaceshowing: (i) thecompatibilityof theperovskiteanduorite(rotated)structures; (ii)asideviewoftheinterfacebetweenSTO(atthebottom)andYSZ(ontop)withrealisticionicradius; theshadedoxygenpositionsintheinterfaceplanearepresumedabsentordisplacedowingtovolumeconstraints,enablingthehighionicconductivity;and (iii) a three-dimensional view of the interface, with the ionic radius reduced by half to bettervisualize the plane of oxygen vacancies introduced in the interface. Note that the square symbol inthelegendrepresentstheemptypositionsavailableforoxygenionsattheinterface.Reproducedfrom Garcia-Barriocanal et al. (2008).ReferencesBamwenda, G. R., Tsubota, S., Nakamura, T. & Haruta, M. 1997 The inuence of the preparationmethodsonthecatalyticactivityofplatinumandgoldsupportedonTiO2forCOoxidation.Catal. Lett. 44, 8387. 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