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, USATim Pennycook, ORNL STEM group, Oak Ridge, USA

Valeria Nicolosi, Oxford University, UKKazu 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

2011-06-10*now at Halcyon Molecular

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).2010 - Nion delivers its first complete electron microscope (a 200 kV STEM) …and now the place of origin of the most recent STEM in the world

Main topics

Part IScanning transmission electron microscopy: an overview

Aberration correction: why is it important?

Nion UltraSTEM construction and performance

Imaging and elemental mapping

Part IIParallel-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 quanti-tatively 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,

schematic is from:Focus on improving transmission electron microscopes

starts to pay off, Physics Today, June 2010, pp. 15-19

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.

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),

more info at:http://www.nion.com/resources.html

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-corrected space telescope: a revolution in astronomy

Why is aberration correction important?

Aberration correction: a revolution in electron microscopy

Uncorrected bright field phase contrast image, JEOL 2010F, 120 kVNature 430 (2004) 870-873 (Fig. 2b)Image is blurred by the spherical aberration of the objective lens: individual atoms cannot be seen.

Si

Si

N

Aberration-corrected annular dark fieldimage, Nion UltraSTEM, 60 kVMatt Chisholm, ORNL (2010)Individual atoms are clearly visible, and their type can be distinguished by their image intensity.

Graphene before and after aberration correction

Nion 3rd generation spherical aberration corrector

• 16 quadrupole and 3 quadrupole/ octupole stages: 19 layers total

• carefully managed axial and field trajectories

• designed to correct Cs (a.k.a. C3) while giving only 0.1 mm increase in Cc, to set all C5’s to 0 (including C5,6), and to give minimized C7’s

• bakeable to 140 C and UHV-compatible

C1,0 C1,2

C2,1 C2,3

C4,1 C4,3 C4,5

C3,0 C3,2 C3,4

C5,0 C5,2 C5,4 C5,6

corrected (or minimized)aberrations

The resultant optical instrument needs autotuning and other diagnostic methods so that it can be set up automatically by computers.

STEM probe size in the aberration-corrected era

Ic = coherent probe current (~0.1-1 nA for CFEG)

Graph shows probe size for probe current Ip = 0.25 Ic

uncorrected STEM,Cs = 1 mm

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

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)

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 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.

The gun is a three-part design:

the HT generationthe HT measurement, the beam acceleration

are done in separate volumes.

This makes it possible to stabilize the HT more effectively.

200 kV imaging of a gold particle at low current

HAADF image and FFT of a gold particle. UltraSTEM200, 200 kV, 15 pA beam current.

Making sure the small spacing reflections are real: rotate the scan direction and record the image again.

Resolving 1.23 Å at 40 keV

HAADF image and FFT of a gold particle at 40 kV. Nion UltraSTEM200, 60 pA beam current.

(0.123 nm)

-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

raw data processed

Krivanek et al. Ultramicroscopy 110 (2010) 935-945

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

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.

dose ~ 109 e- / nm2

Atom-by-atom crystallography I: single layer BN, with O and C impurities

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.

1.4 Å

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.

BN monolayer with impurities: histogram analysis

Dose required: ~107 e- / Å2

BN monolayer with impurities: the final result

Result of DFT calculation overlaid on the experimental image

Cx6

Na adatom

O

N

Longer bonds

C ring is deformed

BC

C

O

Atom-by-atom crystallography II: Si and N in graphene

MAADF images of graphene. Nion UltraSTEM100, 60 kV. Image courtesy Matt Chisholm, ORNL, sample courtesy V. Krisnan and G. Duscher, U. of Tennessee

Si in topologically correct graphene (but with longer Si-C

bonds than C-C bonds)

Si

Si at graphene’sedge

Si(5x)

Si and N at and near topological defects (rings other than 6-fold are

labeled, note that net departure from 6-fold = 0)

Si

Si

N

55

55

5

7

7

9

Species-sensitive crystallography:EEL spectrum from 3 Er atoms, and Spectrum-Images

C-KEr-N4,5

Spectrum-images

C

HAADF image

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

1.4 nm

MAADF

Er-N4,5 C-K

Single Er

atoms

Er

Dose required: ~108 - 109 e- / Å2

EELS atomic-resolution chemical mapping (2007)

La0.7Sr0.3MnO3/SrTiO3 multilayer

40 mr illum. half-angle0.4 nA beam current~1.2 Å probe>70% efficiency EELS coupling

64x64 pixel map7 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

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

5 Å

Mn (L)

Ti (L)La (M)

RGB

Imaging different chemical species separately

Imaging of oxygen octahedral rotations in LaMnO3. Nion Ultra-STEM100, Gatan Enfina EELS, 100 keV. Courtesy Maria Varela and Steve Pennycook, ORNL.

1 nm

O-K

La-M4,5

Mn-L2,3

RGB composite

1 nm

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.

Eu elemental map showing a reduced Eu concentration

at the interface

Part of simul-taneously recorded HAADF image

Evolution of the horizontally

averaged Eu-M edge fine structure across the interface

The three components

extracted using MCR

methods

Three-component fit to the full SI demonstrating 2D

mapping of bonding changes with atomic

resolution

Eu3+

Eu2+

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

Parallel-beam diffraction pattern from the same [110] Si sample area

(=nanodiffraction 1)

Coherent convergent-beam diffractionpattern (Ronchigram) With aberration

correction, the fringes are straight.

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 conver-gence is done very simply, by changing the focal lengths of the condenser lenses.

Note that tracing out the field trajectories shows that

the image of aperture moves around when the magnification is changed

beam direction

source magnified 0.5x,

angular range magnified 2x

source magnified 2x,

angular range magnified 0.5x

Conservation of brightness means that:

crossover sizetimes

angular range=

constant

€

αo

€

d

Two types of trajectories are

needed to understand the optics: axial and

field

Axial trajectories cross the axis in the image (and object) planes, field trajectories traverse the image plane away from the optic axis, and cross the axis at the angular range-defining aperture.

Two types of principal planes in the illumination column

image of source

image of source

image of source

image of aperture

image of aperture

(sin(x) / x

The two planes:

A – image of sourceB – 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.

mixed image

In all other planes, there is a de-focused image of the aperture.

Practical implementation

beam direction

A (source image):~50x demagnifi-cation by the obje-ctive lens gives:

source size projected onto the sample: <0.1 nm

convergence semi-angle: > 30 mr

microdiffraction

FFP

BFPB (Köhler ill.):beam semi-angle at sample: αo = dffp / 2fo

convergence semi-angles 0.1-0.01 mr are easily obtained (= 40 nm / (2x2mm))

Probe size at sample is then:dp = 0.61 / αo

=200 nm - 2 µm

fo

CFEG

C1

C2aperture

C3

corrector

scancoils

sample

OL

convergent beam

scattered electrons

FFP

BFP

CFEG source size ~ 3nm

crossover size in condenser section

~ few nmconvergence

~ 1 mr

Five practical ways of illuminating the sample in STEM

beam direction

regular imaging mode (±30 mr)

change to 1 mr semi-angle

1 mr semi-angle using mini-lens

0.1 mr semi-angle, OL on

0.1 mr semi-angle, OL off

Probe size vs. convergence angle

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dd = 0.61λ

α odiffraction-limited probe diameter:

The above is only valid with zero source size, i.e. zero beam current.

For non-zero probe currents, the probe size broadens as:

€

dp = 1 + IpIc

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 2

Ip … probe currentIc … coherent current (of the source)

if Ip = Ic , 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:

Some typical Ic values: Ic (pA) Bn (A/ (m2 sr V)cold field emission (CFE): 150-500 1-3 x 108

Schottky guns: 30-150 0.2-1 x 108

LaB6 guns: ~1 ~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)

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Ic = π 20.612h2 / 8mee( )[ ]Bn =1.4 ×10−18Bnwhen Ip < Ic, the probe can be said to be largely coherent

when Ip > Ic, the probe is largely incoherent

Three ways of scanning/rocking the beam in the STEM

beam direction

scanned probe moves on the sample nearly parallel-

like, 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.

(3) has not been done yet. It consists of computing the required scans and loading them into the computer memory.

(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).

Interested students please see me.

Conclusions

• The STEM is a very flexible instrument (but some STEMs are more flexible than others).

• The full power of the new techniques is yet to be applied across the whole of physics, materials science and biology.

• Parallel-beam difraction with nm-sized coherent probes in the STEM promises to be very powerful, but it has not yet been fully explored.

• 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.

• An Erice school on aberration-corrected (S)TEM might be a very good idea!