Spring 2010 Experimental Methods in Physics Marco Cantoni 1

Advanced Techniques

1. High-Resolution TEM

2. Analytical EM

3. 3D Microscopy, Special Techniques, Trends

Spring 2010 Experimental Methods in Physics Marco Cantoni

Field emission (electron, ion) microscopy

+/- HV FEM: vacuumHV<0

FIM: low pressure rare gasHV>0

Sharp tip(R radius of curvature):Electric field E=V/R

(E≈109 V/m) V≈1 kVR≈0.1 μm

Magnification M=D/Rβ(M≈ …106) D≈0.3 m

β≈1.5 (tip shape)

D

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Field Ion MicroscopyGas Field Ionisation Source (GFIS)

•• atoms (molecules) are trapped by atoms (molecules) are trapped by polarizations forcespolarizations forces

•• Trapped atoms hop on the surface until Trapped atoms hop on the surface until they are ionisedthey are ionisedIonisation: tunnelling process withprobability D:

I : Ionisation potentialΦ : Work function of emitterV : El. Potentialc : constant

•• Ions are ejectedIons are ejectedfrom the surfacefrom the surface

-c(I- )

VD eαΦ

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Field ion micrograph from a [100] orientedspecimen of Pt 17 at.% Rh catalyst materialimaged in Ne gas with an applied voltage of approx. 10 kV at 80K. Each of the bright spots in the left picture is the image of an individual atomon the specimen surface.

Ball model . The prominent sites, representing kink site atoms on the surface are shown in white. These forma serie of concentric rings similar to those seen in the field-ion picture

Field emission (electron, ion) microscopy FEM, FIM

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Atom probe Tomography

• Use the sample atoms as imaging ions….!– APFIM: Atom Probe Field Ion Microscopy

• Measure the Time Of Flight (TOF), to determine the mass of the ion…!– elemental analysis on atomic level

• Use Laser to assist ablation of ions– LAWATAP: Laser Assisted Wide Angle

Tomographic Atom Probe– Insulating samples become possible

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Wide Angle position sensitive detector

Sample needs to have tip shape: metals: etching, insulators: FIB

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Annu. Rev. Mater. Res. 2007.37:127-158

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Detection speed

• Electrical pulsingThe pulse repetition rate is variable in discrete steps from 1 kHz to 250 kHz, and a detection rate of up to 2×106 ion min−1 (120×106 ion h−1) can be achieved. This implies that a data set containing 109 atoms can be obtained in 8 1/3 h from a single cooperative specimen. For electrical pulsing, the full-width half-maximum (FWHM) value of m/Δm is 500.

• Laser (picosecond) PulsingFor the LEAP 3000X Si XTM, the laser pulse repetition rate is variable in steps from 1 kHz to 500 kHz, and a detection rate of up to 5×106 ion min−1 (300×106

ion h−1) can be obtained. Therefore, a data set containing 109 atoms is attainable from a single very cooperative specimen in 3 1/3 h, which is a factor of 2.5 faster than with electrical pulsing.

Spring 2010 Experimental Methods in Physics Marco Cantoni 9

Al–2.2 at.% Mg–0.12 at.% Sc alloy

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Introduction to Tomography

• Tomography is imaging by sections or sectioning. A device usedin tomography is called a tomograph, while the image produced isa tomogram.

• The method is used in medicine, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other sciences.

• In most cases it is based on the mathematical procedure calledtomographic reconstruction.

• The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).

Wickipedia

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Introduction to Tomography

• Tomography is a method in which a 3-D structure isreconstructed from a series of 2-D projections (images) acquired at successive tilts (Radon 1917).

• First developed for use in medical imaging (1963, Nobel Prize for Medicine in 1979) using X-rays, ultrasound and magnetic resonance (e.g. ‘cat-scans’)..

• Found further application in geology, astronomy, materials science, etc…

P. Midgley, tomo workshop in Berlin

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Introduction to Tomography

Recording

• Series of 2D images• Destructive: serial

sectioning, FIB, LEAP• Non-destructive:

X-rays, TEM

Reconstruction and « viewing »• Registration (alignment of

images)• Back-projection, reconstruct

(tilt-series)• Tomogram• Segmentation (image

processing), extraction of the desired information

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3D imaging in medicine

• Non-invasive methods are preferred!

• The disadvantage of conventional X-radiographs is its inability to discriminatebetween organs of close absorptivity or overlapping organs in the viewingdirection.

• X-ray computed tomography overcomesthat limitation:

• X-radiographs are made in many differentdirections and combined mathematicallyto to reconstruct cross-sectional maps.

• reconstruction tomography or computer assisted tomography.

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Tomograph

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Radon 1917

Ber. Sächs. Akad. Wiss. Leipzig, Math. Phys. Kl. 69, 262 (1917)

English translation in: Deans, S.R. (1983) The Radon transform and its applications. John Wiley & Sons, NY)

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Radon Transform

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Radon Transform

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Radon Transform

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Back projection

Projectionrecording

Back projectionreconstructing

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Back projection

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back projection

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Tomography in medicine

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3D imaging in materials science360degree X-ray tomographyMilan FelberbaumSTI-IMX-LSMX

Cylinder of an Al-Cu Alloy

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3D imaging in materials science

Tomogram

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3D imaging in materials science

Reconstructed pore

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Tomography with electrons

Element(specific weight)

4-Be1.84 g/cm3

13-Al2.7 g/cm3

29-Cu8.93 g/cm3

82-Pb11.3 g/cm3

X-raysCu-Kα λ=1.54 ÅMo-Kα λ=0.71 Å

16 mm83 mm

0.35 mm 3.3 mm

0.10 mm0.10 mm

0.017 mm 0.034 mm

Neutrons λ≈1.08 Å89 m 6 m 0.26 m 14 m

Électronsλ=0.037 Å à 100 kV λ=0.020 Å à 300 kV

39 µm 42 µm~330 µm

11 µm 0.6 µm

Stopping range for electrons (99% absorbed)

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Bio-EM, Tomography

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Tilt series, -60 … +60 degree tilt

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Tomogram

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Tomo workflow

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resolution

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geometrical limit, the missing wedge

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Missing wedge

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Weighted back projection WBP

Lim

ited

num

ber

of p

roje

ctio

ns

Lim

ited

tilt

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• projection requirement: monotonically varying function of a physical property: mass-thickness dominant in biological samples !

Si-Ge multiple quantum well structure

projection requirementprojection requirement

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Tomography in Electron Microscopy

Fro

mP

. Mid

gle

y

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Tomography with HAADF (z-contrast)

nanoparticle bimetallic catalystssupported on mesoporous silica

Dogan Ozkaya,Paul Midgley;Catalysis Letters 60 (1999) 113–120

STEM HAADF: heterogeneous catalystcomposed of Pd6Ru6 nanoparticles (~ 1 nm)on mesoporous silica support withmesopores of ~ 3 nm diameter.

Pd6Ru6 nanoparticlesanchored to the wall

of mesopore

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Electron Tomography in materials science

• Acquisition and alignment of 2x71 tilted images• Back-projection: missing wedge• Projection requirement (image contrast)• Thickness limitation (samples rather thin)• …• …

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The future of scanning (electron) microscopy…?

An Introduction to Helium Ion Microscopy and its Nanotechnology Applications

John A. Notte*, Lou Farkas, Raymond Hill, Bill Ward*ALIS Corporation, 10 Technology Drive, Peabody, MA 01960, USA,

jnotte@aliscorporation.com

Microscopy today (2006)

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The principle: field ion microscope as a source

Pyramidal tip

round tip

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30kV Electrons vs He Ions

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5kV Electrons vs 30kV He

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(top) Secondary electron SEM image of alignment cross.(bottom) Secondary electron ALIS image of same alignment cross

•• smallsmall interaction volume for SEinteraction volume for SE--II

•• almostalmost no SEno SE--IIII

•• highhigh SESE--YieldYield ((currentscurrents in the in the fAfA--pApA range)range)

•• SESE--YieldYield dependsdepends on on atomicatomicnumbernumber z, z, materialmaterial contrastcontrast

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Rutherford back-scattered ions

(left) Secondary electron image of a solder bump showingtopography, but not muchmaterial difference.(right) RBI image of same solder bump clearly showing the difference between areas of tin (dark) and lead (bright).

Laser shot on coated material

SESE RBIRBI

SESE RBIRBI

RBI are not affected by charging effects

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ALIS RBI Images: No Charging Artifacts even on Uncoated Biological Samples

RBI images show no signs of charging on insulating materials. Sample is Uncoated Benthic Foraminifera courtesy of WHOI.