Electron diffraction
Electron diffraction is the influencing of the propagation of electrons by elastic scattering on an ensemble of scattering objects ( atoms ). The De Broglie wavelengths are assigned to the electrons via the wave-particle dualism , so that microscopy can be carried out with electrons , analogous to the light microscope .
functionality
Electron diffraction works in a similar way to X-ray diffraction . A typical diffraction pattern is created by diffraction of the electron beam at the individual scatterers . From this diffraction pattern, fundamental statements can be made about the arrangement of the atoms (periodic, non-periodic, distances, thermal oscillations, etc.). However, electrons interact much more strongly with atoms than X-rays, which is one of the reasons why electron diffraction processes are usually carried out in a vacuum.
Since electron diffraction can only analyze the intensity distribution of the diffracted electron waves, but not their phase, a direct analysis of the structure of the scattering material is not possible. Often a certain structure is assumed, the diffraction pattern of which is calculated and compared with the measurement. By successively adapting such a “model”, atomic distances can then be determined down to one Ångström (10 −10 m).
There are several subtypes of electron diffraction. The important ones are LEED , RHEED and TED . LEED and RHEED are methods for surface analysis, TED is a volume method, i.e. suitable for examining the inside of the material (with the limitation of material thickness for electron transmission methods, see below). A special variant of the TED is GED , in which the apparent contradiction between carrying out an experiment in a vacuum and diffraction on gases as a test object is cleverly resolved.
Compared to X-ray diffraction, in which wavelengths in the order of magnitude in the range of approximately one Angstrom , the order of magnitude of the atomic diameter , are used, the wavelengths in electron diffraction are significantly lower, depending on the method, with electron energies of 100 keV e.g. At about 0.037 Å.
LEED ( low-energy electron diffraction )
LEED ( low-energy electron diffraction ) is mainly used to analyze material surfaces. For this purpose, a surface is bombarded with slow, low-energy (10–200 eV ) electrons in an ultra-high vacuum (UHV) and these are bent in a characteristic way at the top layers.
Due to the short mean free path of the low-energy electrons in the crystal (approx. 1 monolayer ), this process is very surface-sensitive.
In order to be able to examine the dynamics of structural changes, the LEED image can be recorded with a camera . This method is known as video LEED. This allows the structure-determining parameters to be varied during the LEED measurement and their influence to be determined.
MEED ( medium-energy electron diffraction )
With MEED, the multilayer surface growth is observed as a function of time with electron diffraction. If the layers grow monolayer by monolayer on the surface ( Frank van der Merve growth ), the degree of order of the surface changes periodically. In completely closed layers, the long-range order is greatest, including the intensity of the diffraction reflex. As a result, more or less diffraction reflections are obtained at certain intervals, which indicate the monolayer growth as a function of time.
RHEED ( reflection-high-energy electron diffraction )
RHEED is also used to analyze surfaces. A similar depth of interaction with a very shallow angle of reflection, which allows the combination with other analytical or preparative systems, is achieved through a higher electron energy, in the range of about 10 to 50 keV.
TED ( transmission electron diffraction )
TED is used to analyze the inside of the material. Electrons with energies of a few 10 keV to a few 100 keV are shot through a sufficiently thin material sample (a few 10 nm to a few 100 nm). This method is a standard method of transmission electron microscopy . The combination of imaging and diffraction in a transmission electron microscope is particularly useful.
In order to determine the distances and angles of the diffractive structure, the diffraction reflections in the diffraction image should be as "sharp" as possible, i.e. H. be punctiform. Sharp reflections correspond to the smallest possible angular range for which constructive interference occurs. The width of this angle depends directly on the width of the angular range from which the structure is illuminated. This width is called the angle of convergence of the lighting. With parallel illumination this convergence angle becomes infinitely small, which is why the samples are always illuminated in parallel with conventional diffraction methods.
In a modern TEM with a Schottky or field emitter, the illumination can be focused to around 20 nm due to the coherence of the electron beam without the beam noticeably losing parallelism, so that diffraction studies of very small areas are possible. If the electron beam is to be focused on an even smaller diameter, its angle of convergence must inevitably increase. One then speaks of " Micro Diffraction ". Here the diffraction reflexes are small discs instead of points in the diffraction image.
A very wide angle of convergence of the electron beam is called convergent beam electron diffraction (CBED; engl. Convergent beam electron diffraction ) of crystals. The convergent illumination turns the diffraction reflections into wide disks, but since with crystals only structural interference occurs for certain angles (distances in the diffraction pattern), the illumination angle can be set up in such a way that these diffraction disks do not overlap one another. The advantage of CBED is that the diffraction patterns for different angles of incidence of the electron beam can be observed "simultaneously". Therefore, patterns can be seen in the diffraction disks, which result from the differences in (dynamic) diffraction in the crystal for the various entrance angles. From these patterns, many properties of the structure under investigation can be determined, such as B. polarity, three-dimensional symmetry and electron density between atoms.
GED ( gas electron diffraction )
GED (German also abbreviated to GEB) is used to elucidate the structure of small and medium-sized molecules. For this purpose, a gaseous substance sample (or, for example, vaporized by heating) is introduced through a fine nozzle into a high vacuum chamber. There it hits an electron beam directly at the nozzle exit, which typically has an energy of 40–60 keV. The resulting diffraction image consists of concentric rings and can be recorded with different techniques (photo plate, image plate, CCD camera). The diffraction pattern contains the information about all atom-atom distances within the examined molecule. Gas-phase electron diffraction is the most important method for determining the structure of small molecules and is particularly used as a reference for quantum mechanical and molecular mechanical calculations. One of the important milestones achieved with this method was the conformational elucidation of cyclohexane by Odd Hassel (Oslo), for which he was awarded the 1969 Nobel Prize in Chemistry. More recent developments aim at the direct elucidation of reaction mechanisms ( Ahmed H. Zewail , Nobel Prize for Chemistry 1999).
Electron backscatter diffraction (EBSD )
This method is used to examine crystalline materials. Electrons are shot at about 10-30 keV onto a sample surface that has been prepared with as few defects as possible, backscattered coherently at the crystal lattice and the diffraction pattern is made visible by means of a fluorescent screen. After subtracting the subsoil, so-called Kikuchi bands can be seen, from whose arrangement the crystalline phase and the orientation of the crystal can be inferred. The information volume of a single measurement (within a few milliseconds) must inevitably be monocrystalline. By mapping multi-phase structures, the orientation and phase distribution can be assessed.
literature
- Antje Vollmer: Growth and structure of thin silver and gold films on a Re (10-10) surface . Dissertation, Free University of Berlin. 1999, urn : nbn: de: kobv: 188-1999000363 .
Web links
- Electron diffraction - remotely controllable real experiment (see there under "RCLs")