Slide 1

Overview of the Modern STEM Matthew J. Kramer November 3, 2009

Overview •

State-of-the-Art Analytical Scanning Transmission Electron Microscope combines: – Conventional TEM • BF, DF and Lattice imaging • SADP, CBED

– Spectroscopy • Point, line and mapping using – Electron Energy Loss Spectroscopy – Energy Dispersive Spectroscopy

– Scanning • BF, DF and HAADF

– Energy Filtered Imaging • Elemental imaging • Thickness imaging • Energy filtered diffraction

– Lorentz Microscopy • Free lens control of the Objective lens – Direct imaging of the magnetic domains – in situ dynamic materials response

Interactions of Electrons with the Sample • What do you ‘see’ in the TEM – Elastically scattered – Inelastically scattered – Characteristic X-rays – Electron Energy Losses – Requires a ‘thin sample’! Williams and Carter, 1996

Imaging Mechanisms • SEM – Secondary or Backscattered electron

• (S)TEM – Contrast! • Diffraction – Two-beam » Phase or strain contrast – Multi-beam » Lattice imaging

• Mass-thickness contrast – Increases with Z and t » Alter Eo and β.

• Z-contrast

Sample Preparation • How the sample is prepared for TEM analysis is of prime importance. – Limits the techniques – Introduce artifacts

Sm2Fe17 J.P. Liu, U Texas

• Form of the sample – Crystalline or amorphous – Inorganic or organic

• Matrix or not? – Dispersed powder – Inclusions • Coherent or incoherent

– Grain Boundary phases

PdTe with nanodot inclusions M. Kanatzidis, Northwestern U and B Cook

Sample Preparation Methods •

Crush and Float –

Quick and easy • • •

•

‘soft matter’ or ductile Sometimes cooling is required

Lose second phases Introduce reaction byproducts

Ion Milling –

•

Can cool

Difficult to perforate at specific localities

Focused Ion Beam (milling) Limited and expensive • •

Precise location Can be very damaging!

CryoPlunge –

(b)

Can introduce artifacts w/ low Z materials –

–

10 µm

Flexible but time consuming •

•

(a)

Need the right chemistry • •

•

Shear band

Electropolishing –

•

20 µm

Relatively easy • •

•

Lose matrix relationships Introduce defects Oxidation

Microtome –

Pt overlayer

Freezing liquids/polymers

Pt overlayer

AlCu DS sample, R. and S. David 2 µmTrivedi Shear band (c) (ORNL) Zr based metallic glass X. Tang and D. Sordelet

Image Formation Electron diffraction patterns

objective aperture

objective aperture objective aperture

specimen

beam

objective lens

TEM-BF

TEM-DF

HRTEM

objective aperture diffraction pattern

diffraction contrast

lattice image

phase contrast

Nanocrystalline phase in amorphous matrix Practical Electron Microscopy in Materials Science, J. W. Edington

High Resolution Transmission Electron Microscopy (HRTEM)

Conical dark field images with the first ring of Fe diffraction to show the distribution and size of Fe grains at different magnifications. The selected area diffraction pattern shows the amorphous SmCo5 and Fe crystalline.

STEM Imaging • Scan a fine probe across the sample

STEM BF

– Resolution increases w/ decreasing probe size • Reduce S:N

– 3 different detector positions STEM DF

• BF, similar to multibeam image • DF, similar to conical DF in TEM • HAADF, higher Z contrast Au on a C film

STEM vs TEM •STEM Co-block - HAADF polymer

TEM - BF

– Weak contrast • Use heavy metal oxides to enhance contrast Co-block Polymer, E Cochran

HRTEM Computer Simulations Computer simulation on HRTEM image: Why simulation? Do the bright spots correspond to the atom positions?

Def = -200Å -100Å 100Å 0Å

Computer Simulations Do the bright spots correspond to the atom positions? a

z

S.G.: Fm3m (225); a = 3.615 Å

[001]

z

x

y

y

b

b

By thickness

20 Å

x

a

c

c

Simulation of [001] f.c.c. Cu (2x2 cells):

40 Å

60 Å

80 Å

100 Å

-100 Å

0Å

100 Å

200 Å

By defocus

-200 Å

Computer Simulations Computer simulation on HRTEM image: Why simulation? Do the bright spots correspond to the atom positions? As a result of the strong scattering and of the transfer of information by the microscope, HRTEM images, that are interference images, depend mainly on the thickness of the crystal and the transfer function of the microscope (defocus).

¾ Lattice image and structure image. ¾ For known structure, to interpret the image with atom configuration. ¾ For unknown structure, to test the proposed structure model.

Image Interpretation • Gd2Te5

ac

y zx

I. Fisher, Stanford

b

– Crystal perfection – Check lattice over a wide region – Zoom into a thin region and model structure

HRTEM VS STEM Imaging •

HRTEM – Planer illumination – Multi-beam scattering – Image contrast • Thickness • defocus

•

Z-contrast – Scans a fine probe – Electrons are scattered to an annular detector – Strength of the scattering ~ Z

From Eiji Abe and An Pang Tsai

Interfaces and Defects • HRTEM – Delocalization effects due to strongly scattered electrons introduces further complications in image interpretation Si twins in a directionally solidified Al-Si alloy Ralph Napolitano and Halim Meco

Grain Boundaries HRTEM STEM-BF

STEM-HAADF

I. Anderson, R.W. McCallum, Y Wu and W. Tang

Chemistry and Structure – Allows for better control of the fine probe • EDS or EELS can be done – Point – Line – Area

• Convergent beam pattern collection

80

-■- Cr -■- Co -▲- Sm -●- Pd

Atomic %

• STEM Imaging

100

60 40 20 0 0 

50 

100 

Position (nm)

150 

200 

Electron Energy Loss Spectroscopy EELS – Zero Loss

Fe

• Sample thickness = ∑Io/ ∑ I(eV)

Co

– Low Loss Region < 100 eV • Plasmons

Sm

– longitudinal oscillations of free electrons, which decay either in photons or phonons

– High Loss Region ~ > 100 eV • ionization energy of the inner-shell1.0 e’s • More sensitive to lighter elements • Balances EDS 0.8

Hex BN stnd Cubic BN

–

Has different energy spectrum that X-rays – In addition to composition can be 0.6 used to determine I/Io

•

• Thickness • Binding/oxidation states

0.4

– Harder to quantify – More sensitive to thickness effects

0.2 170

190

210

230

energy loss (eV)

250

270

EFTEM Imaging

EFTEM Imaging

• Uses quadropole to bend the image and select out a narrow energy window

carbon

oxygen

Mesoporous SiO2 spheres on a Lacey C grid V. Lin

EFTEM maps for different elements of the same area with Fig. 5 showing the distribution. There are small Sm rich areas.

Holography • Recover phase objects in the TEM – Electrostatic potentials – Magnetic fields

Holography

Rafal E. Dunin-Borkowski, Martha R. McCartney, Richard B. Frankel, Dennis A. Bazylinski, Mihaly Posfai,* Peter R. Buseck, 4 DECEMBER 1998 VOL 282 SCIENCE

Holography • Fields from 2 permanent magnet particles Interference image

Phase reconstruction

Tomography • Collect a series of images over a wide range of tilts • Bring the series into registry • Assemble into a movie or deconstruct into 3D slices

Summary • (S)TEM – Powerful tool for • Structure – HRTEM – Z-Contrast

• Chemistry – EDS/EELS » Point » Line » Mapping – EFTEM

– Must know your system • Sample preparation • Contrast mechanisms – Amorphous, low Z materials – Matrix, particles and defects

Concluding Remarks •

200 keV FEG source provides – High brightness, < 0.2 nm probe – Narrow energy spread, < 0.7 ev – Point-to-point resolution, < 0.25 nm

•

Configured as an analytical tool with minimal compromising HRTEM – Scanning capabilities • HAADF (Z-contrast) • EELS • EDS

– Imaging • BF/DF • Energy filtered imaging • Fluctuation Electron Microscopy

– Diffraction • SA, energy filtered SA • Convergent beam

– Lorentz – Tomography – Holography