Login  |  Legal Disclosure / Privacy policy  |  Contact   |  

Transmission Electron Microscopy (TEM)

The services offered by M.A.S… in transmission electron microscopy (TEM) include the preparation of the sample, examination with a potential analysis (EFTEM or EDX) and the acquisition of digital images.

M.A.S… offers the following methods:
  • Conventional TEM
  • EDX
  • Cryo-TEM for aqueous samples
  • Diffraction
  • Image analysis

A Zeiss 912 with a Cryo-extension and digital image processing is available for the transmission-electron-microscopical analyses.

Even the smallest details are revealed!

Functionality, scattering processes, contrast generation

Transmission electron microscopy was developed by Ruska and Knoll in 1931. It has been continually improved upon, so that a resolution at an atomic level can be attained today. Thick samples can be radiated through by increasing the accelerating voltage. Instrumentation improvements in combination with the extended exploitation of analytical data, that go beyond structural- and image information, create new opportunities for the contrast generation and enhancement.

The structural components of the transmission electron microscope correspond to those of a light microscope. The light bulb is replaced by the electron gun with a Wehnelt cylinder. Collector, condenser, lens and ocular are replaced by electron lenses. A screen and a photography equipment assume the role of the human eye.

The electron beam is shot onto the specimen. This results in interactions between the beam electrons and the atoms of the sample, which is commonly known as scattering. A distinction is generally drawn between elastic and inelastic scattering, whose mechanisms lie at the atomic level. The majority of the electrons penetrate the ultra-thin sample sections scattered. Elastically scattered electrons change their direction but not their energy upon penetration. Inelastically scattered electrons experience an energy loss in addition to the change in direction so that part of that energy is transferred to the sample’s atoms. These electrons provide information about the state and nature of the sample’s atoms. The resulting signals can be used for the imaging or physico-chemical analysis of tiny structures.

The elastic scattering takes place due to coulomb-interactions between the electrons and the relatively large atomic nucleus. The scattered electron is hyperbolically accelerated towards the nucleus. Thereby, acceleration in the direction of motion is experienced up to the point nearest to the nucleus, and then in the opposite direction. Since both acceleration components are even, the velocity before and after the scattering process is the same, that is, the electron loses no energy. The electron orbit can be described by the Rutherford scattering formula. From this simple equation, it can be deduced that the scattering angle increases with increasing accelerating voltage.

The inelastic scattering is based on coulomb-interactions between the electrons of the beams and samples. Energy exchange takes place here. Due to the approximate mass equality of both electrons, the sample’s electrons do not remain motionless, as one might assume for the atom nucleus, but are accelerated. The energy transmitted from the beam electrons suffices to lift the sample’s electron to a higher energy level or eject it from its shell. If the sample’s atom is ionized, the shell electron collects the energy, which has the magnitude of 5-5000 electron volts (eV). The primary electron then has a smaller amount of energy corresponding exactly to that of the ionization potential as it exits the sample again. Furthermore, it also contains information about the state and nature of the sample’s atom. The ionization potential of the shell electrons are characteristic of the individual elements and depend on their atomic number and shell of origin.

With the aid of an installed energy-loss-spectrometer, the intensity of the inelastically scattered electrons is plotted against the energy loss. Consequently, conclusions can be drawn about the chemical and physical structure. The spectrum is divided into three areas: the “Zero Loss”-, the “Plasmon”- and the “Higher Energy Loss” area with the element-specific energy loss edges, which are of great importance for the electron energy loss spectroscopy (EELS).

The processes of inelastic scattering are also responsible for radiation damage in the sample. The supplied energy causes chemical changes, such as the breaking down of bonds which leads to structural changes.

The excitation of the inner electron shells by means of inelastic scattering is the cause for other phenomena, like the emission of electromagnetic radiation (VIS-, X-Ray), secondary electrons or Auger electrons. Backscattered- and secondary electrons serve as the basis for raster electron microscopy.

All electrons passing through the sample are used for the imaging on the screen in TEM. Atoms with higher atomic numbers scatter the electrons to a greater extent, meaning that the intensity of the electron beam decreases. These atoms are therefore depicted more darkly. Unscattered and elastically scattered electrons with a small scattering angle generate a bright background.

Additional links