Atom-by-Atom Imaging and Analysis

Ondrej L. Krivanek

Nion Co., www.nion.com

in collaboration with

Niklas Dellby, Neil Bacon, George Corbin, Petr Hrncirik, Nathan Kurz, Tracy Lovejoy, Matt Murfitt, Gwyn Skone and Zoltan Szilagyi, Nion Co., Kirkland, WA ( www.nion.com

) and Phil Batson, Andrew Bleloch, Mick Brown, Matt Chisholm, Christian Colliex, Juan Carlos Idrobo, Vladimir Kolarik, Lena Fitting Kourkoutis, David Muller, Valeria Nicolosi, Steve Pennycook, Tim Pennycook, Quentin Ramasse, John Silcox, Kazu Suenaga, Wu Zhou, and many others

February 2012

Main topics

• Scanning Transmission Electron Microscopy (STEM): - basic principles - a little history • Single atom imaging and spectroscopy • Summary

C and O in BN Si in graphene Si 3 N 4

STEM - an instrument for imaging and analyzing atoms

An electron probe with ~10 10 electron per second

that’s smaller than an atom

is formed and scanned across the sample. Many types of fast electron – single atom interactions can be detected, typically in parallel. Key

primary

signals and

detectors

: 3 2 4 1

1) Elastic scattering

from the atomic nucleus (Rutherford scattering):

high angle ADF

2) Inelastic scattering

from electrons: electron energy loss spectrometer (

EELS)

3)

e- wavefront reconstruction

(holography):

2D camera

4)

Inelastic scattering from the nucleus

:

high resolution EELS

STEM - an instrument for imaging and analyzing atoms

An electron probe with ~10 10 electron per second

that’s smaller than an atom

is formed and scanned across the sample. Many types of fast electron – single atom interactions can be detected, typically in parallel. Key

secondary

signals and

detectors

: 1’)

Secondary electrons (SE)

arising from various scattering processes:

low-energy electron detector

2’)

X-rays

arising from de-excitation of inner shell hole:

X-ray spectrometer (EDXS, WDS)

3’)

Auger electrons

arising from de-excitation of inner shell hole:

low-energy electron detector

2 4 2’ 3’ 1’ 4’)

Optical, infrared + UV photons

arising from various de excitiation processes:

cathodoluminescence (CL) detector

SE detector with energy filtering There are many signals, and this why the STEM approach is very powerful.

CL detector X-ray detector

The father of modern STEM: Albert Crewe

Albert Crewe showing single U atoms in a Z- contrast image of stained DNA (1970) Chicago 40 kV STEM

Washington state, USA: 1

st

EM outside of Europe…

Washington State EM history continued: 1998: Nion Co. started. It makes correctors for VG STEM microscopes.

2007: Nion starts delivering complete STEMs.

… and now the home of a revolutionary new STEM

Nion’s first 200 keV, 0.53 Å resolution STEM is shipped to CNRS Paris-Sud (in Orsay). Members of the Orsay STEM group (Christian Colliex, Odile Spehan, Katia March, Marcel Tence) and Nion’s Niklas Dellby with Orsay’s new 200 kV UltraSTEM

Nion UltraSTEM

™

200

Fully modular and thus very flexible.

Operating voltage range 20-200 kV.

UHV at the sample (<10 -9 torr; <10 -7 Pa).

Ultra-stable, friction-free sample stage Efficiently coupled EELS Aberration corrector 2 Aberration corrector 1 UltraSTEM200* Described in: Krivanek et al. Ultramicroscopy

108

(2008) 179-195 and Dellby et al. EPJAP 2011. More info at www.nion.com.

*instrument shown: CNRS Orsay, France

Other major Nion firsts

2000: first commercial aberration corrector in the world delivered 2001: sub Å electron probe 2007: atomic-resolution EELS elemental mapping 2009: atomic-resolution images of graphene and monolayer BN 2011: EELS fine structure from single light atoms 2012: X-ray spectrum from a single atom

EELS of one Si atom EDXS of one Si atom

C-K Si-K

C and O in BN

STEM probe size in the aberration-corrected era

Graph shows probe size for probe current I p = 0.25 I c

d probe(Cc) ~ (C c δE) 1/2 / E * o 3/4 d probe(C7,8) ~ C 7,8 1/8 / E * o 1/2

uncorrected STEM, C s = 1 mm Area of great current interest, by Matt Chisholm, Juan Carlos Idrobo, David Muller, Quentin Ramase, Kazu Suenaga, Wu Zhou, Jannik Meyer, Ute Kaiser, David Bell and others.

I c = coherent probe current (~0.1-0.5 nA for CFEG) Resolution reached in the Nion 200 keV column (and illustrated in this talk)

STEM probe size in the aberration-corrected era

d probe(Cc) ~ (C c δE) 1/2 / E * o 3/4 d probe(C7,8) ~ C 7,8 1/8 / E * o 1/2

uncorrected STEM, C s = 1 mm For the full expressions describing the above curves, see Krivanek et al.’s chapter in the just-published Pennycook-Nellist STEM volume (Springer).

Graph shows probe size for probe current I p = 0.25 I c I c = coherent probe current (~0.1-1 nA for CFEG) Resolution reached in the Nion 200 keV column (and illustrated in this talk) Area of great current interest: work by Kazu Suenaga, Jannik Meyer, Ute Kaiser, David Bell and others.

La grain boundary dopants HAADF imaging of ß-Si 3 N 4 0.94 Å Nitrogen columns, separated by only 0.94 Å from Si columns, are clearly visible. Nion UltraSTEM200, 200 kV. Courtesy Tim Pennycok, ORNL.

HAADF imaging of gold particles at 40 and 200 keV

(0.12 nm) -1 40 keV: 1.23 Å lattice planes well resolved (Nion UltraSTEM200, Orsay, France) The image was acquired in the so-called “second zone” OL mode, with 2 beam crossovers in the objective lens. This lowered C c and gave better than the regular imaging mode.

Image recorded by N. Dellby.

Dellby et al, EPJAP (2011), DOI: 10.1051/epjap/2011100429

HAADF imaging of gold particles at 40 and 200 keV

40 keV: 1.23 Å lattice planes well resolved (Nion UltraSTEM200, Orsay, France) regular scan scan rotated by 90 ° 200 keV: 0.53 Å information transfer that’s independent of the scan direction (Nion UltraSTEM200)

Single-wall carbon nanotube imaged at 60 keV

Microscope is housed in a soft steel box, shown here with one of its side doors open. The box makes the microscope relatively insensitive to external disturbances. It also serves as a bake-out enclosure.

MADF image of single wall carbon nanotube, Nion UltraSTEM100.

Masking a set of reflections in the FFT allows the front and the back of the nanotube to be visualized separately.

Image courtesy Matt Chisholm, ORNL.

MAADF images of graphene taken 2 minutes apart

Medium angle annular dark field (MAADF) STEM images of a graphene edge, recorded 2 minutes apart. Nion UltraSTEM, 60 keV primary energy.

Configuration changes at the edge are nicely documented, a single heavier adatom (probably Si) is seen.

Recorded in July 2009.

EELS atomic-resolution chemical mapping (2007)

La (M) Mn (L) Ti (L) RGB 5 Å

Muller et al., Science 319, 1073 –1076 (2008)

EELS chemical maps of La 0.7

Sr 0.3

MnO 3 /SrTiO 3 multilayer structure 40 mr illum. half-angle 0.4 nA beam current ~1.2 Å probe >80% efficient EELS coupling 64x64x1340 voxel spectrum image 7 msec per pixel, i.e. 29 sec total acquisition time 10 sec additional processing time i.e., <1 min total time Nion UltraSTEM100, 100 keV

O-K

Imaging different chemical species separately

1 nm 1 nm EELS chemical mapping: imaging of oxygen and other sub-lattices due to specific chemical elements in LaMnO 3 .

Mn-L 2,3 Octahedral rotations in the O sub-lattice are clearly seen.

La-M 4,5 RGB composite Nion Ultra STEM100, Gatan Enfina EELS, 100 keV.

Courtesy Maria Varela and Steve Pennycook, ORNL.

BN monolayer with impurities imaged by MAADF

Result of DFT calculation overlaid on an experimental image C ring is deformed Cx6 O N B C O Longer bonds C Na adatom

Si substituting for C in monolayer graphene

2 Å Si Si Si Si N Si Si Si Si Si in topologically correct graphene Si at and near topological defects Medium angle annular dark field (MAADF) images.

Nion UltraSTEM100, 60 kV. Image courtesy Matt Chisholm, ORNL, sample courtesy Venna Krisnan and Gerd Duscher, U. of Tennessee.

Si at graphene’s edge

Si substituting for C: 2 structures are possible

2 Å Si Si in defect-free graphene strains (and buckles) the foil. (courtesy Matt Chisholm) Si in defective, but less strained graphene is more stable. (15 images added together, no other processing, courtesy Wu Zhou and Juan-Carlos Idrobo)

Binding of a single Si atom in a stable defect structure

Si-L edge EELS from single Si atom

Exp.

C N Si

Exp.: adding together the signal of the pixels corresponding to the Si atom in the graphene spectrum-image Nion UltraSTEM100, 60 keV. Courtesy Juan-Carlos Idrobo and Wu Zhou, ORNL

Simultaneous EELS and EDXS from a single Si atom EELS of single Si atom on graphene ADF image of 2-3 graphene layers recorded

after

spectra were acquired. Arrow points to a tracked impurity atom. EDXS of single Si atom C-K Si-K EELS and EDXS data I p recorded simultaneously. = 100 pA, 90 s acquisition. E (keV) Nion UltraSTEM100, 60 keV, Daresbury UK. Gatan Enfina EELS, Bruker SDD EDXS. Q. Ramasse, T.C. Lovejoy, O.L. Krivanek et al., to be published.

Summary

• The ability to image and analyze matter atom-by-atom was always inherent to the nature of the electron matter interaction, and it’s now finally available.

• We are able to perform atom-by-atom analysis because we have: ultra-bright electron guns aberration-corrected electron optics ultra-stable electron microscopes ultra-high vacuum at the sample • The ability to analyze matter atom-by-atom has arrived just in time: atom-by atom is how we now make the smallest devices.

• Being small and nimble is an advantage when it comes to creating revolutions.

EDXS of one Si atom

C-K Si-K

EELS of one Si atom Si in graphene