Direct imaging and parallel-beam diffraction in
an aberration-corrected STEM
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, Chris Own*, Gwyn Skone and Zoltan Szilagyi,
Nion Co., Kirkland, WA (www.nion.com)
Matt Chisholm, ORNL STEM group, Oak Ridge, USA
Tim Pennycook, ORNL STEM group, Oak Ridge, USA
Valeria Nicolosi, Oxford University, UK
Kazu Suenaga, AIST, Tsukuba, Japan
Phil Batson, Mick Brown, Andrew Bleloch, Christian Colliex,
Lena Fitting Kourkoutis, David Muller, Steve Pennycook,
Quentin Ramasse, John Silcox and many others
*now at Halcyon Molecular
2011-06-10
Washington state, USA: 1st EM outside of Europe…
Washington state EM history continued:
1997 - Krivanek and Dellby report on the first working (S)TEM aberration corrector:
Inst. Phys. Conf. Ser. 153 (Proceedings 1997 EMAG meeting) p. 35.
2000 - Nion delivers the first commercial electron-optical aberration corrector in the world;
corrector attains <1 Å directly interpretable resolution in 2001 (Nature 418, 617).
…and
now the place of origin of the most recent STEM in the world
2010 - Nion delivers its first complete electron microscope (a 200 kV STEM)
Main topics
Part I
Scanning transmission electron microscopy: an overview
Aberration correction: why is it important?
Nion UltraSTEM construction and performance
Imaging and elemental mapping
Part II
Parallel-beam diffraction in the STEM:
the pre-requisites
STEM, an instrument for imaging, analysis and diffraction
Scanning Transmission Electron Microscope (STEM):
make a small + intense electron probe, detect and quantitatively measure all the signals that come off the sample.
An aberration-corrected STEM uses sophisticated electron
optics (with ~ 100 independent elements) to produce a very
small electron probe, down to ~0.5 Å Ø.
Three signals are especially interesting:
1)High–angle (Rutherford-scattered) electrons: >90 mr
scattering from atomic nuclei, collected as a mostly
incoherent signal by a high angle annular dark field detector
(HAADF), to show where are the atoms,
2) Inelastically-scattered electrons: scattered by the
sample’s electrons, collected and analyzed by an electron
energy-loss spectrometer (EELS), they show what type
the atoms are, and other sample properties),
3) Low+medium-angle scattered electrons: collected by a
medium-angle detector (MAADF) or a CCD camera, as a
mostly coherent signal, they reveal the crystal structure.
more info at:
http://www.nion.com/resources.html
schematic is from:
Focus on improving transmission electron microscopes
starts to pay off, Physics Today, June 2010, pp. 15-19
Why is aberration correction important?
Aberration-corrected space telescope: a revolution in astronomy
Hubble space telescope, before repair.
Image is blurred by spherical aberration
of incorrectly made primary mirror.
After repair: spherical aberration of
telescope’s mirror is corrected by newly
designed planetary camera optics.
Aberration correction: a revolution in electron microscopy
Graphene before and after aberration correction
Si
N
Si
Uncorrected bright field phase contrast
image, JEOL 2010F, 120 kV
Nature 430 (2004) 870-873 (Fig. 2b)
Image is blurred by the spherical
aberration of the objective lens:
individual atoms cannot be seen.
Aberration-corrected annular dark field
image, Nion UltraSTEM, 60 kV
Matt Chisholm, ORNL (2010)
Individual atoms are clearly visible, and
their type can be distinguished
by their image intensity.
Nion 3rd generation spherical aberration corrector
corrected
(or minimized)
• 16 quadrupole and 3 quadrupole/
aberrations
C1,0
C1,2
octupole stages:
19 layers total
• carefully managed axial and field
trajectories
C
C
2,1
2,3
• designed to correct Cs (a.k.a. C3) while
giving only 0.1 mm increase in Cc,
C
C
C3,4
to3,0set all C3,2
’s
to
0
(including
C5,6),
5
and to give minimized C7’s
• bakeable
and UHV-compatible
C4,1 to 140 C
C4,3
C4,5
C5,0
C5,2
C5,4
The resultant optical instrument needs autotuning and other diagnostic
methods so that it can be set up automatically by computers.
C5,6
STEM probe size in the aberration-corrected era
Graph shows probe
size for probe
current Ip = 0.25 Ic
uncorrected STEM,
Cs = 1 mm
Ic = coherent probe
current (~0.1-1 nA
for CFEG)
Resolution
reached in the
Nion 200 keV
column
For the expressions describing the above curves,
see Krivanek et al.’s chapter in the just-published
Pennycook-Nellist STEM volume (Springer).
Nion UltraSTEM™ 200
Fully modular and
thus very flexible
true UHV at the
sample (~5x10-10 torr)
ultra-precise stage
with 0.5 nm minimum
mechanical motion
Aberration
corrector 2
computer-controlled
sample exchange
3rd and 5th order
correction for probe
3rd order aberration
correction for EELS
Ultra-stable (probe
jitter <0.1 Å rms)
Aberration
corrector 1
Described in: Krivanek et al. Ultramicroscopy 108 (2008) 179-195
and Dellby et al. EPJAP in press. More info at www.nion.com.
instrument shown:
CNRS Orsay, France
200 kV UltraSTEM: new CFEG & EELS ZL peaks
The gun is a three-part
design:
the HT generation
the HT measurement,
the beam acceleration
are done in separate
volumes.
This makes it possible
to stabilize the HT
more effectively.
The gun accelerates the electrons as rapidly as possible, using a shortened
accelerator. This minimizes electron-electron Coulomb interactions (Boersch
effect) and gives less energy spread and higher gun brightness at useful
emission currents.
It is designed to operate at any kV between 20 and 200.
200 kV imaging of a gold particle at low current
Making sure the small spacing
reflections are real: rotate the
scan direction and record the
image again.
HAADF image and FFT of a gold particle.
UltraSTEM200, 200 kV, 15 pA beam current.
Resolving 1.23 Å at 40 keV
(0.123 nm)
HAADF image and FFT of a gold particle at
40 kV. Nion UltraSTEM200, 60 pA beam current.
-1
Second zone operation of a 4 mm gap objective lens: the
optical properties approximate those of a 2 mm gap OL.
Real-space crystallography: MAADF STEM of graphene
Medium-angle
annular dark field
(MAADF) STEM
imaging gives about
1.1 Å resolution at
60 kV (below the
knock-on threshold),
and is very
quantitative.
Images become
clearer if they are
Fourier-filtered to
remove high
frequency shot
noise and probe
tails.
Image intensity
scales as ~Z1.64
raw data
processed
Krivanek et al.
Ultramicroscopy
110 (2010) 935945
Carbon nanotube imaged at 60 keV
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.
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.
Image courtesy Matt
Chisholm, ORNL.
dose ~ 109 e- / nm2
Atom-by-atom crystallography I: single layer BN, with O
and C impurities
1.4 Å
60 kV MAADF image
B and N atoms are readily identifiable by their MAADF intensities.
C and O substitutional impurities are also identifiable in the line profiles.
BN monolayer with impurities: histogram analysis
Histogram analysis of image shows
that B, C, N and O can be identified
unambiguously in monolayer BN.
The experimentally worked out
dependence of image intensity on Z
goes as Z1.64.
Nature 464 (2010), 571-574.
Dose required: ~107 e- / Å2
BN monolayer with impurities: the final result
Result of DFT calculation overlaid on the experimental image
C ring is
deformed
N
Cx6
O
B
C
Longer
bonds
C
Na
adatom
O
Atom-by-atom crystallography II: Si and N in graphene
Si
Si
5
5
9
Si
7
5
5
N
Si
(5x)
5
7
Si in topologically correct
graphene (but with longer Si-C
bonds than C-C bonds)
Si and N at and near topological
defects (rings other than 6-fold are
labeled, note that net departure from
6-fold = 0)
Si at graphene’s
edge
MAADF images of graphene. Nion UltraSTEM100, 60 kV. Image courtesy Matt
Chisholm, ORNL, sample courtesy V. Krisnan and G. Duscher, U. of Tennessee
Species-sensitive crystallography:
EEL spectrum from 3 Er atoms, and Spectrum-Images
HAADF
MAADF
image
Single
Er
atoms
Spectrum-images
1.4
nm
Er-N4,5
Er-N4,5
C-K
C-K
Er
C
The size of the atoms in the Er N4,5
image is only about 3 Å, and
nanopods can be seen in the C map
Dose required: ~108 - 109 e- / Å2
EELS atomic-resolution chemical mapping (2007)
La (M)
Ti (L)
La0.7Sr0.3MnO3/SrTiO3
multilayer
40 mr illum. half-angle
0.4 nA beam current
~1.2 Å probe
>70% efficiency EELS coupling
Mn (L)
RGB
64x64 pixel map
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
5Å
Muller et al., Science 319, 1073–1076 (2008)
Imaging different chemical species separately
1 nm
1 nm
O-K
Mn-L2,3
La-M4,5
RGB composite
Imaging of oxygen
octahedral rotations
in LaMnO3.
Nion Ultra-STEM100,
Gatan Enfina EELS,
100 keV. Courtesy
Maria Varela and
Steve Pennycook,
ORNL.
Mapping atomic bonding in EuTiO3/DyScO3
Increased Eu valence is found in a single atomic layer at the interface. Nion UltraSTEM100, 100 kV.
Courtesy Lena Fitting-Kourkoutis and David Muller, Cornell U. Proceedings IMC17.
Eu2+
Eu3+
Eu elemental map Part of simul- Evolution of the
The three Three-component fit to the
showing a reduced
taneously
components full SI demonstrating 2D
horizontally
Eu concentration
recorded
extracted
mapping of bonding
averaged Eu-M
at the
changes with atomic
HAADF
edge fine structure using MCR
interface
resolution
image
across the interface methods
Part II: classical (reciprocal space) crystallography in the
STEM
Two possibilities for recording reciprocal space data:
1) Leave the beam as it is set up for imaging, record and
analyze convergent beam diffraction patterns. Depending on
how the illumination (and the sample) are set up, the patterns
can be either coherent (fringes are seen in Bragg disk
overlap regions) or incoherent (no fringes).
2) Make the beam parallel, record and analyze point-like
diffraction patterns.
Convergent beam to nanodiffraction: one mouse-click
Coherent convergent-beam diffraction
pattern (Ronchigram) With aberration
correction, the fringes are straight.
Parallel-beam diffraction pattern
from the same [110] Si sample area
(=nanodiffraction 1)
Going from one mode to the other takes about 3 s, the probe stays on the same area.
Going from a convergent to a parallel probe in the STEM
Changing the beam convergence is done very simply, by
changing
thetypes
focal lengths
of
Two
of
the condenser
lenses.
trajectories
are
needed to understand
theConservation
optics: axial of
and
brightness
means that:
field
beam
direction
crossover size
times
angular range
=
Note that tracing out the
constant
field trajectories shows that
o
the image of aperture
moves around when the
magnification is changed
d

magnified
source(and
magnified
Axial trajectories crosssource
the axis
in the image
object) planes,
2x,from the optic axis, and cross the
field trajectories traverse the0.5x,
image plane away
angular range
angular range
axis at the angular range-defining
aperture.
magnified 2x
magnified 0.5x
Two types of principal planes in the illumination column
The two planes:
image of
source
image of
aperture
mixed
image
image of
source
(sin(x) / x)2
image of
source
image of
aperture
A – image of source
B – focused image of aperture
alternate throughout the
illumination column.
(similar to the way image and
diffraction planes alternate in an
imaging column)
Either plane can be projected
onto the sample as the
illumination. A is typically used
for forming small probes, B for
broad, Köhler-type illumination.
In all other planes, there is a defocused image of the aperture.
Practical implementation
scattered
electrons
A (source image):
BFP
~50x demagnification by the objeOL
ctive lens gives:
source size
projected onto the
sample: <0.1 nm
sample
FFP
scan
coils
corrector
convergence semiangle: > 30 mr
C3
crossover size in
condenser section
~ few nm
convergence
~ 1 mr
C1
beam
direction
CFEG
C2
aperture
B (Köhler ill.):
beam semi-angle
at sample:
o = dffp / 2fo
BFP
fo
convergence
semi-angles 0.10.01 mr are easily
obtained
(= 40 nm /
(2x2mm))
FFP
Probe size at
sample is then:
dp = 0.61 l/ o
=200 nm - 2 µm
CFEG source size
~ 3nm
convergent beam
microdiffraction
Five practical ways of illuminating the sample in STEM
beam
direction
regular imaging
mode (±30 mr)
change to 1 mr 1 mr semi-angle 0.1 mr semi- 0.1 mr semisemi-angle
using mini-lens angle, OL on angle, OL off

Probe size vs. convergence angle
diffraction-limited probe diameter:
l
dd  0.61
o
The above is only valid with zero source size, i.e. zero beam current.
For non-zero probe currents, the probe size broadens as:
1/ 2


I p 
dp  1 + 
Ic 


if
Ip = Ic ,
Ip … probe current
Ic … coherent current (of the source)
dp = √2 dd
Coherent probe current
The coherent current is a characteristic property of the source. It is
independent of the accelerating voltage, aberrations and aperture size used.
Its value is related to normalized (reduced) brightness Bn as:


Ic   2 0.612 h 2 /8mee) Bn 1.4 1018 Bn
when Ip < Ic, the probe can be said to be largely coherent
when Ip > Ic, the probe is largely incoherent
Some typical Ic values:
cold field emission (CFE):
Schottky guns:
LaB6 guns:
Ic (pA)
150-500
30-150
~1
Bn (A/ (m2 sr V)
1-3 x 108
0.2-1 x 108
~106
(more detailed explanation, including a discussion of why Schottky guns should not be
called FEGs, is in Krivanek et al.’s chapter in the Pennycook-Nellist STEM book)
Three ways of scanning/rocking the beam in the STEM
beam
direction
scanned probe moves on
the sample nearly parallellike, but not quite
without Cs compensation,
probe rocking works best
for angles < 20 mr
with Cs compensation, >50
mr should be possible
Compensated rocking: 3 ingredients needed
1) Complete scan-descan coil system, preferably symmetric about the OL.
2) Electronics control which makes the currents supplied to the 4 layers of
scan coils completely programmable.
3) Software that computes and implements the scan ramps required for
compensated beam rocking.
(1+2) are available in the Nion UltraSTEM column. (2) is implemented by
pre-computing a table of deflections to be done by all the scan layers,
and then reading it out and implementing it at pixel advance rates of up to
1 pixel / 50 µs (=20k scan points per second).
(3) has not been done yet. It consists of computing the required scans
and loading them into the computer memory.
Interested students please see me.
Conclusions
• The STEM is a very flexible instrument (but some STEMs are more
flexible than others).
• Imaging single atoms is not difficult with an aberration-corrected
STEM. It’s also possible to identify the type of the individual atoms,
by ADF imaging, EELS, and also EDXS.
• Parallel-beam difraction with nm-sized coherent probes in the STEM
promises to be very powerful, but it has not yet been fully explored.
• The full power of the new techniques is yet to be applied across the
whole of physics, materials science and biology.
• An Erice school on aberration-corrected (S)TEM might be a very good
idea!