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
Si3N4
STEM - an instrument for imaging and analyzing atoms
An electron probe with ~1010 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:
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
3
2
4
1
STEM - an instrument for imaging and analyzing atoms
An electron probe with ~1010 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)
2
4
1
4’
CL
detector
2’
3’
1’
3’) Auger electrons arising from
de-excitation of inner shell hole:
low-energy electron detector
X-ray detector
SE detector
with energy filtering
4’) Optical, infrared + UV photons arising from various deexcitiation processes: cathodoluminescence (CL) detector
There are many signals, and this why the STEM approach is very powerful.
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: 1st 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
Aberration
corrector 2
Efficiently coupled
EELS
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 Ip
= 0.25 Ic
dprobe(Cc) ~ (Cc δE) 1/2 / E*o3/4
uncorrected STEM,
Cs = 1 mm
dprobe(C7,8) ~
C7,81/8
/
E*o1/2
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.
Ic = 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
Graph shows probe
size for probe
current Ip = 0.25 Ic
dprobe(Cc) ~ (Cc δE) 1/2 / E*o3/4
uncorrected STEM,
Cs = 1 mm
dprobe(C7,8) ~ C7,81/8 / E*o1/2
For the full expressions describing the
above curves, see Krivanek et al.’s chapter
in the just-published Pennycook-Nellist
STEM volume (Springer).
Ic = 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.
HAADF imaging of ß-Si3N4
La grain
boundary
dopants
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
-1
(0.12 nm)
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 Cc 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)
200 keV: 0.53 Å information
transfer that’s independent
of the scan direction
(Nion UltraSTEM200)
regular scan
scan rotated by 90°
Single-wall 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.
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)
Ti (L)
EELS chemical maps of
La0.7Sr0.3MnO3/SrTiO3
multilayer structure
40 mr illum. half-angle
0.4 nA beam current
~1.2 Å probe
>80% efficient EELS coupling
Mn (L)
RGB
64x64x1340 voxel spectrumimage
7 msec per pixel, i.e. 29 sec
total acquisition time
10 sec additional processing
time
i.e., <1 min total time
5Å
Muller et al., Science 319, 1073–1076 (2008)
Nion UltraSTEM100, 100 keV
Imaging different chemical species separately
1 nm
1 nm
O-K
La-M4,5
Mn-L2,3
RGB composite
Courtesy Maria Varela and Steve Pennycook, ORNL.
EELS chemical
mapping: imaging
of oxygen and other
sub-lattices due to
specific chemical
elements in
LaMnO3.
Octahedral
rotations in the O
sub-lattice are
clearly seen.
Nion UltraSTEM100, Gatan
Enfina EELS, 100
keV.
BN monolayer with impurities imaged by MAADF
Result of DFT calculation overlaid on an experimental image
C ring is
deformed
N
Cx6
O
B
C
Longer
bonds
C
Na
adatom
O
Si substituting for C in monolayer graphene
2Å
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
2Å
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
Si-K
C-K
EELS and EDXS data
recorded simultaneously.
Ip = 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-byatom 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