Electron Microcopy

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Useful info – many websites.Images here from

www.microscopy.ethz.ch/elmi-home.htm

De Brogle wavelength of electrons

100 KeV electron has 3.7 pm wavelength!

Electron-matter interactions

Electron-matter interactions

EDXS spectrum

Elastic interactionsNo energy is transferred fromthe electron to the sample.

The electron either passes withoutany interaction (direct beam) oris scattered by the positivepotential inside the electroncloud.

These signals are mainlyexploited in TEM and electrondiffraction.

Inelastic InteractionsEnergy is transferred from theincident electrons to the sample:

secondary electrons, phonons,UV quanta or cathodoluminescenceare produced;

shooting out inner shell electronsleads to the emission of X-raysor Auger electrons.

These signalsare used in analytical electronmicroscopy.

Excellent manuscript: www.microscopy.ethz.ch/downloads/Interactions.pdf

Interaction volumes

Energy dependence

Energy dependence II

Some SEM images

Backscattered electrons

Material contrast

Basic TEM and SEM

Which EM to use?The method that is needed is determined by the question to be solved:

Structure (High-Resolution) Transmission Electron Microscopy Scanning Transmission Electron Microscopy (STEM)

Electron diffraction (ED)Composition Energy-dispersive X-ray spectroscopy (EDXS)

Electron Energy Loss Spectroscopy (EELS)Morphology Scanning Electron Microscopy (SEM)Elemental mapping

Electron Spectroscopic Imaging (ESI)STEM + X-ray spectroscopy / EELSSEM + X-ray spectroscopy

De Brogle wavelength of electrons

100 KeV electron has 3.7 pm wavelength!

Is this the limit?

Scattering and diffraction

• Experiment• Basic structures and how to label them• X-ray diffraction

Experiment

Bragg’s Law

constructive interference destructive interference of waves

Crystal symmetries: finite number!

7 crystal systems: The crystal systems are a grouping of crystal structures according to the axial system used to describe their lattice. Each

crystal system consists of a set of three axes in a particular geometrical arrangement.

Cubic, hexagonal, tetragonal, rhombohedral (also known as trigonal)orthorhombic, monoclinic and triclinic.

14 Bravais lattices: When the crystal systems are combined with the various possible lattice centerings, we arrive at the Bravais lattices. They describe the geometric arrangement of the lattice points, and thereby the translational symmetry of the crystal.

Unit cells of a cubic crystal 3 different Bravais lattices

Directions in a cubic crystal

Miller indices of crystal planes

Examples of low index planes

Important results for cubic systems

Important results for cubic systems