Overview

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Learn about the fundamentals of transmission electron microscopy in materials sciences: you will be able to understand papers where TEM has been used and have the necessary theoretical basis for taking a practical training on the TEM. This course provides a comprehensive introduction to transmission electron microscopy (TEM) in the field of materials science. For an instrument operated by a single user, modern TEM provides an analytical platform with unsurpassed versatility, giving access to structural and chemical information from the micrometer to the sub-angstrom scale. In a thin, electron-transparent sample one can measure the crystallinity, grain structure, size, and defects, and the chemical composition. The crystal lattice can be imaged with atomic resolution, allowing observation of grain boundaries and interfaces. It is the only direct structural analysis method for studying nanoparticles. With this course you will gain a deep understanding of modern TEM and the connection between: - the optics and operation of the instrument; - the physics of electron-matter interactions; - insights into the materials properties of the sample. This gives the background to: - identify TEM techniques suitable to solving specific scientific problems; - interpret TEM data presented in articles; appreciate the impact of technological advances that have, for instance, led to sub-angstrom resolution by aberration correction. It can also be the basis for subsequent practical training on this remarkable instrument, and a stepping stone towards learning very advanced techniques with magical names like “dark field holography” or “angular resolved electron energy-loss spectroscopy”. Recommended background: Basics of crystallography and diffraction, college optics (construction of ray diagrams) are absolutely mandatory prerequisites; Fourier optics, more advanced crystallography and solid state physics are of great advantage.

Syllabus

Introduction

This week will be devoted to an introduction to the instrument, with some historical notes, as well as a review of the building blocks of a transmission electron microscope. In a second part, we will review the main lens aberrations relevant in transmission electron microscopy.

Introduction (II)

This week, we will see how to build the microscope from its individual components: lenses and aperture. Then we will have a review of the operating modes of the microscope.

Diffraction basics (I): Ewald sphere / Reciprocal lattice

In this week on the basics of electron diffraction we discuss the case of 2-beam diffraction at the Bragg angle in TEM and then show how it can be represented by the Ewald sphere/reciprocal lattice construction.

Diffraction basics (II): Multi-beam / Kinematical scattering

In this week we finish on the basics of electron diffraction, by first taking a look at zone axis or multi-beam diffraction where we have scattering from many different crystal planes at the same time. Next, we explain this by a relaxation of the Bragg condition in the Ewald sphere/reciprocal lattice construction resulting from the TEM sample size and shape. We then see how this affects diffraction spot intensity when slightly deviated from the perfect Bragg condition in the 2-beam case.

Diffraction and imaging: Dynamical effects (I)

In this week we will tackle the subject of dynamical scattering in TEM. Dynamical scattering is multiple elastic scattering; it effectively involves the diffraction and rediffraction of electrons as they transmit through a sample. In the first lecture, we look at the basic theory of dynamical scattering in the 2-beam case and use the theoretical expressions to calculate plots of beam intensity versus excitation error for different specimen conditions. In the next lecture, we use this theory to explain the dynamical scattering phenomenon of thickness fringes.

Diffraction and imaging: Dynamical effects (II)

This week we look at more effects of dynamical scattering, on both TEM images and diffraction pattern formation. First we look at how dynamical scattering produces bend contours in bright-field and dark-field images when the crystal lattice is bent across an imaged region of TEM sample. Secondly, a special case of dynamical scattering called double diffraction is introduced, in which multiple elastic scattering leads to the formation of diffraction spots for crystal planes which are systematic absences.

Phase contrast (I)

In this first week about phase contrast, we will define the contrast transfer function of the objective lens. In a second part we will consider an object that affects only weakly the phase of the electron wave and not its amplitude. This will lead us to the definition of the phase contrast transfer function.

Phase contrast (II)

In this week we will analyse more in details the Phase Contrast Transfer Function, and see how it can be used to understand the contrast in the image of a thin amorphous film. In a second part, we will address the high resolution images of crystalline specimens.