Transmission electron microscopy
A modern transmission electron microscope (TEM) is a very versatile instrument which can be operated in many different modes. In principle, a TEM works similar to a conventional optical microscope but using electrons instead of light. Even the requirements to the sample are somewhat similar; they have to be transparent for the respective kind of probe (light or electrons). Electron transparency is achieved simply by preparation of ultrathin samples, usually less than 100 nm. In addition to the electron transparency, TEM samples need to be stable in ultra high vacuum (UHV) and resistant to electron irradiation.
In principle, the optics of a TEM is composed of the electron emitter, the condenser system, the objective, the projective and the visualization component.
Electron emitter (Gun)
As obvious from the name, the electron emitter or electron gun has the purpose to provide the electrons which are then guided through the column. In commercially TEMs three different types of electron guns are used: tungsten filament, LaB6 cathode or field emission guns (FEG). The latter has the highest luminosity and provides high beam currents. However, tungsten filaments and LaB6 cathodes are still in operation in many TEMs due to their low cost compared to a FEG and as long there are no highly sophisticated applications like STEM-EDX these emitters still do a very good job.
Once the electrons have been generated by the electron gun, they have to be accelerated in order to traverse the column down to the visualization system. This is done by placing an anode close to the emitter. The anode is set to a certain positive potential which is ranging from 80 to 300 kV for most of the commercial TEMs. After passing the hole in the anode, the electrons enter the condenser system of the TEM.
The condenser system of a TEM is usually composed of several electromagnetic lenses and at least one aperture. It´s purpose is to provide a parallel electron beam which illuminates the sample. In practice, the condenser system defines the field of illumination on the sample. Moreover, with the excitation of the condenser lens (illumination knob) one can adjust the brightness and with that the irradiation dose on the sample. Usually, the TEM operator adjusts the field of illumination according to the actual magnification of the objective and projective system. When the TEM is equipped with the respective devices, the condenser system can be adjusted so that the incident doesn´t traverse the sample in a parallel geometry but focused in a small spot on the sample. This illumination is called “convergent beam”. This kind of illumination is used e.g. for convergent beam electron diffraction (CBED) and for scanning TEM applications and it is the common illumination mode in scanning electron microscopes (SEM). After leaving the condenser system the electron beam enters the upper part of the objective lens and transmits the sample.
The objective lens system
The objective lense provides the TEM with the maximum achievable resultion. It is composed of two polepieces placed above and below the specimen. The objective lens finally does an angular spread of the parallel incident electrons from the condenser (illumination) system of the microscope. After the electrons traversed through the sample, the objective spreads the formerly parallel electron trajectory to an angular distribution, just like the objective lens of an optical microscope.
The projective system
The objective lens creates a back focal intermediate image. With the projective system this intermediate image is magnified to observe the desired details in the sample. Moreover, the projective can be focused to the back focal plane of the objective lens in order to receive the electron diffraction pattern. Once the desired image is formed, it has to be visualized in
The visualization system
Electrons per se are invisible for the human eye. Accordingly, in most of the TEMs you can find a viewing screen which is coated with a ZnS layer. Upon irradiation with electrons, the ZnS emits light at a typical wavelength of 450 nm. Usually, the ZnS is modified with impurities in order to shift the emission to approximately 550 nm, which corresponds to green light, which is in the middle of the visible spectrum and thus is most relaxing to the human eye.
In order to document the micrographs, different systems can be used. In former times most microscopists had to use photographic films or plates. More and more these photographic emulsions were replaced by digital recording systems like CCD cameras.
Transmission Electron Microscopes at the MPI-P
At the MPI-P we operate three transmission electron microscopes. These TEMs are equipped to serve all the demands of the scientists in the MPI-P. For our students the JEOL JEM1400 is the perfect TEM to collect first experience in transmission electron microscopy and to “see” their own samples.
The LEO EM912 is equipped with an in column energy filter which is perfect for contrast enhancement of polymer samples and for imaging thick specimens. Furthermore, the in-column filter is ideal to remove the inelastic scattered electrons from the electron diffraction.
The FEI Tecnai F20 is our most versatile instrument and it is used for a broad spectrum of microscopical applications ranging from high resolution, elemental analysis to extreme low dose parallel beam electron diffraction.