Microscopical Characterisation of Solid and Biological Materials KOO 065

Also known as “Microanalysis Course” “Okända prov.”

Reine Wallenberg, (SEM, TEM)

Crispin Hetherington (TEM, EELS) Axel Persson (SEM, TEM)

Reza Zamani (TEM)

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Contents

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JSM-6700F JEM-3000F JEM-2200FS Training in SEM, TEM, EDX, EELS

Books TEM (Williams and Carter, 2009) SEM (Stokes, 2008, chapter 2)

group presentation of results and conclusions individual exam

7 lectures And hands-on investigation of 3 unknown samples

Lectures Library at Materials Chemistry, Monday afternoons

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lecture 1 Introduction and TEM basics lecture 2 SEM lecture 3 EDX (2 week break) lecture 4 EELS and EFTEM lecture 5, 6, 7 TEM (spec prep, HREM, CTF...) possible extras Environmental TEM Image simulation in JEMS Image processing in DigitalMicrograph

What is Microanalysis and what will you learn?

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How to get information about the structure and chemical composition at the microscopic level - operate the instrument, - interpret the data - understand some theory What do we do with this structural information - develop new materials - link growth (synthesis) to properties - understand how biological materials function.

Lecture 1 - TEM

• Electrons, • TEM instrument, • Image contrast, • STEM and microanalysis

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Why do we use electrons for our microscopes? what are the advantages compared to light or X-rays or neutrons?

• charged particles interact strongly with the sample

• charged particles so we can use magnetic lenses to form an image

• short wavelength (0.002nm)

allows imaging at high resolution

do = 0.66 Cs1/4λ3/4

The National Center for HREM, Lund University, Reine Wallenberg

Hur stor är en nm? Verktyg:

- ögat - Svep-elektron-mikroskop

SEM

SPM

- Svep-probs-mikroskop

TEM

- Trans-missions-elektron-mikroskop

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SEM

SEM

SEM

TEM

STEM

STEM STEM TEM

Signals “mainly” used in different techniques

illumination

specimen

lens

camera

Light Microscope Electron Microscope

illumination

specimen

lens

camera

How do we generate electron beam? • thermal emission (W or LaB6 crystal) or

field emission

• Accelerate the electrons by 60-300kV – electrons behave like waves – high kV for resolution – low kV to minimise beam damage

• ~1eV energy spread

– required for imaging and spectroscopy

W

How we configure the beam for imaging or microanalysis?

wide parallel beam or small probe

How big is a TEM specimen • x = 3mm across • thin all over, y = 0.1mm

• important bit must be

electron transparent z =10-100nm

• Specimen holders are necessary (and fragile too)

3 examples of specimen types

metal foil

nanoparticles (on carbon film and copper grid)

FIB lamella (on copper support grid)

electron beam

The objective lens: ”heart of the microscope”

• Quality of this lens determines image resolution

• An immersion lens - so astigmatism must be corrected for each sample

• Lens strength usually held in narrow range – alignment and calibrations preserved – change sample height for coarse focus – change lens current for fine focus

resolution limit do = 0.66Cs1/4λ3/4

The objective lens and its aberrations

perfect lens spherical aberration Cs chromatic aberration Cc

To paraphrase Ernst Abbe (1849 – 1905) : ”if you want to understand the microscope, consider the specimen as a diffraction grating and ask, does the diffracted beam get through the lens ok”

resolution limit do = 0.66Cs1/4λ3/4

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Diffraction and Bragg’s Law

2dhklsinθ = nλ dhkl is crystal lattice spacing n denotes fractional lattice spacings θ is Bragg angle

θ θ

unscattered

scattered

incident

And the rest of the lenses.... • we can change

magnification of the image

..or switch between imaging and diffraction

camera

Transmission Electron Microscope

accelerator

condenser lens

specimen holder

electron gun

beam deflectors (eg scan coils)

objective lens

objective aperture

projector lens

Xray spectrometer

intermediate aperture

condenser aperture

EELS spectrometer

thin ”electron transparent” sample

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In TEM, we illuminate the sample uniformly and we use thin samples that transmit the fast electrons. A good lens focusses all the scattered and unscattered electrons from point A on the sample onto the point B on the camera... Err!?

sample

lens

camera

incident beam

A

B

Mass-thickness contrast • The objective aperture

is placed around the forward-scattered beam, excluding the scattered electrons.

sample

image

Mass-thickness contrast

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Overview of Image Contrast

Mass/thickness “absorption”

(aperture in diffraction plane)

Diffraction contrast

spec

imen

im

age

(aperture in diffraction plane & energy filter)

elec

tron

be

am

phase contrast

(contrast transfer function in diffraction plane)

Diffraction contrast at twin bands in a nanowire

100nm

GaAs nanowire with micro twins

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diffraction contrast to reveal line defects in an interface

dark field micrographs can identify type of defect

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phase object

incident plane wave

transmitted wave note: phase of wavefront has changed - but amplitude unaffected

q(x) = exp{-iσϕ(x)} σ = interaction constant ϕ = projected potential

atoms

HREM – phase contrast

atomic columns change the phase of the electron beam . objective lens ”defects” (focus, Cs) generate phase contrast.

100nm

Schematic diagram of the objective lens area showing the positions of the objective- and selected

area apertures

your turn to draw simple version

TEM mode STEM mode

condenser lens

objective lens

TEM and STEM - beam alignment, lens astigmatism, focus...

sample

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STEM with analytical electron microscopy (AEM)

HAADF, high angle annular dark field detector to record signal for STEM image

electron spectrometer for electron energy loss (STEM) and energy filtered images (TEM)

Xray spectrometer for elemental analysis (STEM)

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STEM image with scattered eletrons

Elastic scattering

•The electrons do not lose energy

•At a distance it looks like elastic collision

•Wave description also works – phase change only

•Electrons passing close to nucleus – stronger scattering

•Electrons are scattered by the potential (Coulomb interaction) as opposed to x-rays (electron density)

Unscattered (HREM)

Forward scattered <10 mrad (HREM)

High-angle Scattered (HAADF)

Rutherford Backscatter (SEM)

e-

Inelastic scattering

The chemists microscope! Incoming electrons ionize the atom (core-shell excitation). Ionization energies depend on which element.

To lose the extra energy, the atom emits an x-ray (XEDS)

The energy loss of the transmitted electron can be measured (EELS)

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A quick word about looking after your TEM • Good environment

– stable floors – quiet room

• Good vacuum – clean specimens – clean holder

• Safety

– X rays, high kV, Be-coated holders, liquid nitrogen

Good practice for hands-on working

• good sharp tweezers to handle grids

• pre-inspect 3mm sample in light microscope/i-phone

• take good notes in your lab book

• understand alignment, but don’t overdo it

• do insert objective aperture for standard images

• take low magnification image of general area

notebook