High-Resolution Electron Microscopy for Nanocharacterization in Materials Science

1. NNI Interagency WorkshopJanuary 27-29, 2004 Instrumentation and Metrology for Nanotechnology Grand Challenge Workshop National Institute of Standards and Technology, Gaithersburg, MD Track 1- Instrumentation and Metrology for Nanocharacterization Breakout Session: Current State of the Art Sub-Ångstrom Electron Microscopy for Materials Science Michael A. O'Keefe Materials Sciences Division Lawrence Berkeley National Laboratory, Berkeley, CA 94720 and Lawrence F. Allard High-Temperature Materials Laboratory Oak Ridge National Laboratory, Oak Ridge, TN 37831 This work supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science Division, DOE under contract DE-AC03-76SF00098, and Asst. Sec. for EERE, Office of FreedomCAR and Vehicle Tech. for the HTML User Program, ORNL, managed by UT-Battelle, LLC for DOE under contract DE-AC05-00OR22725.

2. NNI Interagency WorkshopJanuary 27-29, 2004 Instrumentation and Metrology for Nanotechnology Grand Challenge Workshop National Institute of Standards and Technology, Gaithersburg, MD The Role of Measurement Theory the circle of nano Measurement Construction The high-resolution electron microscope can provide essential feedback in the nano- theory/construction/measurement loop.

3. Rose (1994) Measurement with the electron microscope OÅM -- 0.78Å (2001) • In 1994, in a paper on aberration correction [1], Harald Rose showed resolution over time. He predicted 0.5Å resolution by 2015. • Better microscope resolution leads to less de-localization of higher spatial frequencies, so better precision in measurement of atomic coordinates. • The OÅM demonstrated sub-Angstrom microscopy to 0.78Å resolution in 2001 [2], using hardware correction of three-fold astigmatism and software correction of spherical aberration. TEAM -- 0.5Å (2006?) • The next-generation TEAM is designed for sub-0.5Å resolution [3], using hardware correction with lens current stability of 0.1ppm (rms) and a mono-chromator to reduce FWHH beam-energy spread below 0.35eV at 300keV or 0.18eV at 200keV. • Better resolution allows characterization in more viewing directions, leading to atomic-resolution 3-D images -- locate every atom in place! [1] “Correction of aberrations, a promising means for improving the spatial and energy resolution of energy-filtering electron microscopes” H. Rose, Ultramicroscopy56 (1994) 11-25. [2] “Sub-Ångstrom resolution of atomistic structures below 0.8Å”, M.A. O’Keefe, E.C. Nelson, Y.C. Wang and A. Thust, Phil. Mag. B81 (2001) 11, 1861-1878. [3] “HRTEM at Half-Ångstrom Resolution: from OÅM to TEAM”, M.A. O’Keefe, Microscopy & Microanalysis 9 (2003) 2: 936-937.

4. 1990: resolution extension by focal series reconstruction. Images of oxygen atoms on JEOL-ARM 1000 O Model 1.6ÅScherzer- focus image 1.4Åreconstruction from 5 images 1.4Å simulation "Resolution of oxygen atoms in staurolite by three-dimensional transmission electron microscopy", Kenneth H. Downing, Hu Meisheng, Hans-Rudolf Wenk, Michael A. O'Keefe, Nature 348 (1990) 525.

5. 1.7Åresolution +1 +1 +1 +1 +1 CM300FEG/UT  = 36Å 0 0 0 0 0 1.07Å info limit -1 -1 -1 -1 -1 OÅM  = 20Å 0.78Å  = 0.25 millirad  1.1Å  n = 2 1.03Å n = 36  0.89Å 0 0 Spatial Frequency (Å-1) Spatial Frequency (Å-1) 1.0 1.0 1.5 1.5 Resolution, information limit, and focal series - CTFs show transfer of spatial frequencies.

6. 1.0 Resolution (Å) 1.0 Resolution (Å) What does aberration-correction (CS-correction) do? Compare OÅM (CS = 0.6mm) with CS-corrected (0.02mm) OÅM with CS of 0.6mm and Delta of 20Å Info Limit (0.78Å) CS corrected OÅM with CS at 0.02mm and Delta of 20Å With CS corrected, phase reversals are gone. Better mid-range transfer Info Limit (0.78Å)

7. NCEM Materials Sciences Division 1992-2002: the LBNL One Ångstrom Microscope Project Sub-Ångstrom Resolution by Image Reconstruction Principal Investigator: Michael A. O’Keefe 1992 -- 2002 OÅM team: J.-O. Malm 1992 -- 1993 E.C. Nelson 1995 -- 2002 C.J.D. Hetherington 1995 -- 1997 Y.C. Wang 1997 -- 1998 C. Kisielowski 1998 -- 2000 Aim: to produce sub-Ångstrom resolution for NCEM users. *Supported by DOE/SC/BES/DMS

8. 1998: first sub-Ångstrom result from OÅM OÅM image shows 0.89Å spacings in test specimen of diamond 0.89Å Model of diamond structure in [110] orientation. Pairs of C atoms are separated by 0.89Å to form the ‘dumbbells’. OÅM image taken close to alpha-null defocus shows pairs of C atoms separated by 0.89Å in the diamond structure. Y.C. Wang, A. Fitzgerald, E.C. Nelson, C. Song, M.A. O’Keefe et al, Microscopy and Microanalysis 5 (1999) 2: 822-823.

9. 004 (a) |A2| = 2.46m simulated OÅM image averaged 004 OÅM image averaged 1998: aberration correction -- three-fold astigmatism Before correction, diamond image shows effect of 3-fold astigmatism After correction, diamond image shows 0.89Å atom pairs in “dumbbells” (b) |A2| < 0.05m Zemlin tableaux -- O’Keefe, Wang & Pan, 1998 Images -- Wang & O’Keefe, 1998

10. Experimental 0.78Å Transfer at 3kV Electron Gun Extraction Voltage Silicon structure model in [112] orientation. Pairs of Si atoms are separated by 0.78Å in ‘dumbbells’. Si622 (0.82Å) 0.78Å Si444 (0.78Å) Image taken near alpha-null defocus shows pairs of Si atoms separated by 0.78Å. Si531 (0.92Å) Diffractogram confirms transfer of spacings to 0.78Å. M.A. O’Keefe, E.C. Nelson, Y.C. Wang and A. Thust,Philosophical Magazine B81 (2001) 11: 1861-1878.

11. 0.78Å “Last-Century” Cutting-Edge Resolution [112] Si images from STEM and TEM [112] Best possible STEM - HB603U - Best possible TEM - OÅM - 0.78Å [112] Si has become the “de facto” test specimen “Quantitative interpretation and information limits in annular dark-field STEM images”, P.D. Nellist & S.J. Pennycook, Microscopy and Microanalysis6, 2: (2000) 104-105. “Sub-Ångstrom resolution of atomistic structures below 0.8Å”, M.A. O’Keefe, E.C. Nelson, Y.C. Wang and A. Thust, Philosophical Magazine B81 (2001) 11, 1861-1878.

12. Testing Microscope Resolution (the A-OK test series) 1.7 CdTe 1.62Å 1.6 InAs 1.6 1.51Å 1.5 Ge 1.41Å 1.4 Si 1.4 1.36Å 1.3 -InN 1.24Å 1.2 -SiC 1.2 1.1 1.11Å 1.0 CdTe 1.0 diamond AlSb 0.94Å 0.9 0.89Å Ge 0.87Å Si 0.82Å 0.8 0.78Å -InN 0.8 0.72Å 0.7 -SiC 0.64Å 0.6 0.6 diamond 0.5 0.51Å 0.4 3.0 0.4 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Atom-atom spacings for diamond-cubic test specimens from 1.62Å to 0.51Å [110] series Dumbbell Spacing (Å) [112] series Lattice Parameter (Å) [112] silicon [110] diamond 0.89Å 0.78Å OÅM images reconstructed from focal series of 20 component images “A Standard for Sub-Ångstrom Metrology of Resolution in Aberration-Corrected Transmission Electron Microscopes”, Michael A. O’Keefe & Lawrence F. Allard, Microscopy & Microanalysis10 (2004).

13. Resolution of light atoms -- imaging lithiumYang Shao-Horn & Michael A. O’Keefe • LiCoO2 is the most commonly used positive electrode materials for lithium rechargeable batteries • Energy storage  lithium insertion into and extraction from LixCoO2 • Ultra high resolution is needed to resolve light elements in a heavy matrix • Conventional HRTEMs with resolutions to 1.6Å can routinely image the heavier metal atoms in structures such as oxides. • The OÅM (One-Ångstrom Microscope) at the NCEM has achieved resolutions to 0.8Å and, in addition to heavy atoms, has previously imaged columns of lighter atoms, including O, N, and C. • In this work, we have used the OÅM to image all the component atoms, including columns of Li atoms in a matrix of CoO2. “Atomic resolution of lithium ions in LiCoO2”, Yang Shao-Horn, Laurence Croguennec, Claude Delmas, E. Chris Nelson & Michael A. O’Keefe, Nature Materials2, 464-467 (2003); advance on-line publication 15 June 2003 (doi: 10.1038/nmat922).

14. Schematic of Layered LiCoO2 Structure Single unit cell projected in the [110] orientation Li atoms Co atoms O atoms CoO6 octahedra “Atomic resolution of lithium ions in LiCoO2”, Yang Shao-Horn, Laurence Croguennec, Claude Delmas, E. Chris Nelson & Michael A. O’Keefe, Nature Materials2, 464-467 (2003); advance on-line publication 15 June 2003 (doi: 10.1038/nmat922).

15. Reconstructed Exit-Surface Wave of LiCoO2 Comparison of simulated and experimental ESWs shows that Li atom columns are visible at 0.9Å resolution in the OÅM. Experimental Simulation O Co O Li O is strong Co is “fuzzy” Li is weak The reconstructed exit-surface wave shows that the specimen is tilted away from exact [110] zone axis orientation and also reveals buckling and possible electron beam damage. “Atomic resolution of lithium ions in LiCoO2”, Yang Shao-Horn, Laurence Croguennec, Claude Delmas, E. Chris Nelson & Michael A. O’Keefe, Nature Materials2, 464-467 (2003); advance on-line publication 15 June 2003 (doi: 10.1038/nmat922).

16. a 6 atom column a b 11 atom column b 0.286 radian a b 6 7 8 9 10 11 10 9 8 7 6 # atoms in columns Simulated Pd cube-octahedron analysis -- Line trace shows peaks in ESW phase -- Model ESW phase ESW phase (peak height) is proportional to the number of atoms in the column producing the peak. Line trace shows the one-atom difference between adjacent columns. “Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis9 (2003) 2: 278-279.

17. Analysis of experimental image of 70Å Au nanoparticle Phase shows white atom columns Single image at -2600A underfocus FSR of particle “Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis9 (2003) 2: 278-279.

18. Twinning in ESW phase becomes clearer after application of a high-pass filter Particle image High-pass image “Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis9 (2003) 2: 278-279.

19. Edge Center Analysis of 70Å gold nanoparticle by peak profile 9 7 7 5 Zero? Line trace of ESW phase shows initial increase from outer edge, followed by groups of peaks with very similar heights. “Quantization” of ESW phase peak steps suggests that height differences may be due to different integral numbers of atoms. The technique of profile tracing of phase to measure peak heights suffers from the lack of a well-defined zero level, especially for supported nanoparticles. “Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis9 (2003) 2: 278-279.

20. Atomic structure O-K Mn L 14 II/III 12 1 Ti Ti 10 2 8 3 6 Sr Sr 4 4 5 6 0.2 nm 2 0 550 600 650 Energy Loss (eV) Z-Contrast Microscopy and electronic structure 1 2 4 5 Detector 3 6 Spectrometer Courtesy of S. Pennycook

21. No spherical aberration FWHM ~ 0.8 Å Current density is concentrated into central maximum Electron Microscopy in 2003 -- aberration-corrected STEM STEM Probe Size is Limited by Spherical Aberration VG Microscope’s HB501UX, 100 kV Aberration limited Significant current is lost in probe “tails” FWHM ~2 Å Aberration correction can achieve the smaller brighter probe Courtesy of S. Pennycook

22. Spectroscopic identification of a single atom within a bulk material. 8% collection efficiency La M4/5 Intensity 820 850 880 Energy (eV) Single Atom Spectroscopy 5 Å La in CaTiO3 grown by MBE Courtesy of S. Pennycook

23. First Column Single Au Single Au Au to Au spacing 2.88 Å Carbon film background Linetrace of STEM Intensities Courtesy of S. Pennycook

24. Electron Microscopy in 2003 Advanced TEM Diebold et al. (2003). Measurement of gate-oxide width with TEM and STEM Diebold et al. (2003) have compared measurements of gate-oxide width using TEM and STEM. a. OÅM (TEM) image shows silicon [110] dumbbells (left) up to nitrided gate oxide, then oxide, then polysilicon. b. STEM (HAADF) with 10 millirad aperture agrees with OÅM oxide width c. STEM with 13 millirad aperture shows oxide as wider d. STEM with larger aperture shows even “wider” oxide “Thin Dielectric Film Thickness Determination by Advanced Transmission Electron Microscopy”, A.C. Diebold et al., Microscopy & Microanalysis 9 (2003) 493–508.

25. Electron Microscopy in 2003 3-D STEM Work by P.A. Midgley and M. Weyland Cambridge U.

26. P.A. Midgley and M. Weyland, Cambridge U. Fig. 2. Non-uniform sampling of Fourier space over-emphasizes lower frequencies, giving a blurred reconstruction. The greater density of low-frequency data is compensated by using weightedback-projection reconstruction. 2-D test object for simulation Fig. 3a. Result of adding successively more projections to the reconstruction, using direct (left) and weighted (right) back-projection over a tilt range of 90.

27. Object Reconstruction Direct Weighted P.A. Midgley and M. Weyland, Cambridge U. Fig. 3b. Effect of tilt range. Limited tilt produces a missing wedge in Fourier space. Missing data limit the reconstruction resolution in the vertical direction, causing streaking. Figure shows tilt ranges from 10 to 60. Tilt axis is into the plane of the figure. Recent advances in tomographic specimen holders allow tilts to70 around two axes within the 2.2mm polepiece gap of modern ultra-high-resolution electron microscopes. With a tilt series in x and one in y, the “missing wedge” becomes a 20 “missing pyramid”.

28. P.A. Midgley and M. Weyland, Cambridge U. 3-D image of nanoparticles. Reconstructed using weighted back projection from 55 STEM HAADF images of Pd6Ru6–MCM 41 catalysts. Tilts from +60 to -48 in 2 steps at 300kV. Metal particles have been colored red for clarity. An individual nanoparticle in the reconstructed data set can be isolated to show that it is anchored to the wall of a 3nm-diameter mesopore. The particle is about 1nm in diameter.

29. NNI Interagency WorkshopJanuary 27-29, 2004 Instrumentation and Metrology for Nanotechnology Grand Challenge Workshop National Institute of Standards and Technology, Gaithersburg, MD Conclusion Theory the circle of nano Measurement Construction The electron microscope will continue to evolve (with higher resolution and 3-D capability) and to provide essential feedback in the nano- theory/construction/measurement loop.