This work supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science Division, DOE under contract DE-AC03-76SF00098,

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.

Sub-Ångstrom Electron Microscopy

for Materials Science

NNI Interagency Workshop January 27-29, 2004 Instrumentation and Metrology for Nanotechnology Grand Challenge Workshop

National Institute of Standards and Technology, Gaithersburg, MD

Michael A. O'KeefeMaterials Sciences Division

Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Lawrence F. AllardHigh-Temperature Materials Laboratory

Oak Ridge National Laboratory, Oak Ridge, TN 37831

and

Track 1- Instrumentation and Metrology for Nanocharacterization

Breakout Session: Current State of the Art



The high-resolution electron microscope can provide essential feedback in the nano- theory/construction/measurement loop.

The Role of Measurement

Rose (1994)

Measurement with the electron microscope

• Better microscope resolution leads to less de-localization of higher spatial frequencies, so better precision in measurement of atomic coordinates.OÅM -- 0.78Å (2001)

TEAM -- 0.5Å (2006?)

[1] “Correction of aberrations, a promising means for improving the spatial and energy resolution of energy-filtering electron microscopes” H. Rose, Ultramicroscopy 56 (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. B 81 (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.

• Better resolution allows characterization in more viewing directions, leading to atomic-resolution 3-D images -- locate every atom in place!

• 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.

• 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.

• In 1994, in a paper on aberration correction [1], Harald Rose showed resolution over time. He predicted 0.5Å resolution by 2015.

1.4Å simulation 1.4Å reconstructionfrom 5 images

1.6Å Scherzer-focus image

Model

"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.

1990: resolution extension by focal series reconstruction.Images of oxygen atoms on JEOL-ARM 1000

O

1.51.00 Spatial Frequency (Å-1)

1.51.00 Spatial Frequency (Å-1)

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

+1

-1

0

OÅM = 20Å

0.78Å

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

CM300FEG/UT = 36Å+1

-1

0

Resolution, information limit, and focal series - CTFs show transfer of spatial frequencies.

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

= 0.25 millirad

+1

-1

0

1.1Å

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

n = 2 +1

-1

0

1.03Å

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

n = 36 +1

-1

0

0.89Å

1.7Å resolution

1.07Å info limit

Resolution (Å) 1.0

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

151413121110987654321k,(nm-1)

0.0670.0710.0770.0830.0910.1000.1110.1250.1430.1670.2000.2500.3330.5001.000d,(nm)

1

0

-1

1

0

-1

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

Info Limit (0.78Å)

With CS corrected, phase reversals are gone. Better mid-range transfer

Compare OÅM (CS = 0.6mm) with CS-corrected (0.02mm)

Resolution (Å) 1.0

What does aberration-correction (CS-correction) do?

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

1992-2002: the LBNL One Ångstrom Microscope ProjectMaterials Sciences Division

NCEM

OÅM image taken close to alpha-null defocus shows pairs of C atoms separated by 0.89Å in the diamond structure.

Model of diamond structure in [110] orientation. Pairs of C atoms are separated by 0.89Å to form the ‘dumbbells’.

OÅM image shows 0.89Å spacings in test specimen of diamond

Y.C. Wang, A. Fitzgerald, E.C. Nelson, C. Song, M.A. O’Keefe et al, Microscopy and Microanalysis 5 (1999) 2: 822-823.

1998: first sub-Ångstrom result from OÅM

0.89Å

(b)

|A2| = 2.46m

(a)

OÅM image averaged

004

simulated

004

Before correction, diamond image shows effect of 3-fold astigmatism

After correction, diamond image shows 0.89Å atom pairs in “dumbbells”

OÅM image averagedImages -- Wang & O’Keefe, 1998

|A2| < 0.05m

1998: aberration correction -- three-fold astigmatism

Zemlin tableaux -- O’Keefe, Wang & Pan, 1998

Si444 (0.78Å) Si622 (0.82Å)

Si531 (0.92Å)

Image taken near alpha-null defocus shows pairs of Si atoms separated by 0.78Å.

Silicon structure model in [112] orientation. Pairs of Si atoms are separated by 0.78Å in ‘dumbbells’.

Experimental 0.78Å Transfer at 3kV Electron Gun Extraction Voltage

0.78Å

M.A. O’Keefe, E.C. Nelson, Y.C. Wang and A. Thust, Philosophical Magazine B 81 (2001) 11: 1861-1878.

Diffractogram confirms transfer of spacings to 0.78Å.

“Last-Century” Cutting-Edge Resolution [112] Si images from STEM and TEM

Best possible STEM- HB603U -

Best possible TEM- OÅM -

0.78Å

[112]

0.78Å

“Sub-Ångstrom resolution of atomistic structures below 0.8Å”, M.A. O’Keefe, E.C. Nelson, Y.C. Wang and A. Thust, Philosophical Magazine B 81 (2001) 11, 1861-1878.

“Quantitative interpretation and information limits in annular dark-field STEM images”, P.D. Nellist & S.J. Pennycook,

Microscopy and Microanalysis 6, 2: (2000) 104-105.

[112] Si has become the “de facto” test specimen

Atom-atom spacings for diamond-cubic test specimens from 1.62Å to 0.51ÅD

um

bb

ell

Sp

acin

g (

Å) [110] series

[112] series

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

3.0

0.89Å

[110] diamond [112] silicon

0.78Å

Testing Microscope Resolution (the A-OK test series)

3.0 4.0 5.0 6.0 6.55.54.5 7.03.5

1.4

1.2

1.0

0.8

0.6

0.4

1.6

Lattice Parameter (Å)

diamond

-SiC

-InN

SiGe

AlSbCdTe

0.51Å

0.64Å

0.72Å0.78Å

0.82Å0.87Å

0.94Ådiamond

-SiC

-InN

SiGe

InAs

CdTe

0.89Å

1.11Å

1.24Å

1.36Å

1.41Å

1.51Å

1.62Å

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 & Microanalysis 10 (2004).

• 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.

Resolution of light atoms -- imaging lithiumYang Shao-Horn & Michael A. O’Keefe

“Atomic resolution of lithium ions in LiCoO2”, Yang Shao-Horn, Laurence Croguennec, Claude Delmas, E. Chris Nelson & Michael A. O’Keefe, Nature Materials 2, 464-467 (2003); advance on-line publication 15 June 2003 (doi: 10.1038/nmat922).

Schematic of Layered LiCoO2 Structure

Li atoms

CoO6 octahedra

Single unit cell projected in the [110] orientation

Co atoms

O atoms


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.

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.

CoO

OLi

Experimental

Co is “fuzzy” O is strong

Li is weak

Simulation


Model

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.

Simulated Pd cube-octahedron analysis -- Line trace shows peaks in ESW phase --

ESW phaseb

aa

b

0.286 radian

6 atom column

11 atom column

# atoms in columns

6 7 8 9 10 11 10 9 8 7 6a b

“Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis 9 (2003) 2: 278-279.

Analysis of experimental image of 70Å Au nanoparticle

Single image at -2600A underfocus Phase shows white atom columns

FSR of particle“Focal-Series Reconstruction of Nanoparticle Exit-Surface Electron Wave”, M.A. O’Keefe, E.C. Nelson & L.F. Allard, Microscopy & Microanalysis 9 (2003) 2: 278-279.

Twinning in ESW phase becomes clearer after application of a high-pass filter

Particle image High-pass image


Analysis of 70Å gold nanoparticle by peak profile

Line trace of ESW phase shows initial increase from outer edge, followed by groups of peaks with very similar heights.

Edge Center

“Quantization” of ESW phase peak steps suggests that height differences may be due to different integral numbers of atoms.

Zero?

57 7

9

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.


Z-Contrast Microscopy

• Atomic structure

Detector

0.2 nm

Sr Sr

Ti Ti

Spectrometer

1

54

6

2

3

and electronic structure

550 600 6500

2

4

6

8

10

12

14

Energy Loss (eV)

Mn L II/IIIO-K

1

2

3

45

6

Courtesy of S. Pennycook

STEM Probe Size is Limited by Spherical Aberration

No spherical aberration

FWHM ~ 0.8 Å

Current density is concentrated into central maximum

FWHM ~2 Å

Significant current is

lost in probe

“tails”

Aberration limited

Aberration correction can achieve the smaller brighter probe

VG Microscope’s HB501UX, 100 kV


Electron Microscopy in 2003 -- aberration-corrected STEM

Single Atom Spectroscopy

5 Å Spectroscopic identification of a single atom within a bulk material.

8% collection

efficiency

820 850 880

Inte

nsi

ty

Energy (eV)

La M4/5

La in CaTiO3 grown by MBE


Linetrace of STEM Intensities

Au to Au spacing 2.88 Å

Single Au

Single Au

First Column

Carbon film background


Measurement of gate-oxide widthwith TEM and STEM

“Thin Dielectric Film Thickness Determination by Advanced Transmission Electron Microscopy”, A.C. Diebold et al., Microscopy & Microanalysis 9 (2003) 493–508.

Electron Microscopy in 2003

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

Advanced TEM

Diebold et al. (2003).

3-D STEM

Work by

P.A. Midgley and M. Weyland Cambridge U.

Electron Microscopy in 2003

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.

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

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.

Object Reconstruction

WeightedDirect

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”.



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.

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.



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.

Conclusion

Documents

This work supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science Division, DOE under contract DE-AC03-76SF00098,