Low Energy Electron Microscopy (LEEM)

The Low Energy Electron Microscope (LEEM) is one of the instruments used in our group to characterize surfaces and interfaces and the physical and chemical processes that take place on them. The Low Energy Electron Microscope was first devised by prof. Ernst Bauer in 1962. The first working LEEM instrument was demonstrated in 1985. Since then the Low Energy Electron Microscope has developed into a diverse instrument (LEEM, SPLEEM, SPE-LEEM, XPEEM, etc.) that can be used to characterize surfaces in both real and reciprocal space. Nowadays, it is a commercially available technique.

The principle of LEEM is based on the deceleration of a beam of electrons using a cathode lens. The electrons are decelerated to an energy of a few eV and interact with the sample surface, yielding an electron diffraction pattern. Because of the potential between the cathode and the sample, the diffracted electrons are accelerated back to their initial energy and travel through the cathode lens a second time, after which they are led through a beam splitter, which is used to separate the incoming and outgoing beams. The diffraction pattern is led to an aperture which is used to select the electrons from the diffraction pattern to form the image. Using a set of projector lenses these electrons are then projected on a set of channel plates and imaged using a CCD camera. The horizontal resolution of the instrument is 7 nm. Vertically, the instrument has a much greater resolution and can be used to image atomic planes and steps.

The instrument that is used in the Solid State Physics group is a so-called SPLEEM, or a Spin-Polarized Low Energy Electron Microscope. The name refers to one of the many modes in which our instrument can operate, in this particular case using a beam of spin-polarized electrons to image magnetic nanostructures. Regular LEEM modes are normally referred to as 'bright field', using the (00) beam, and 'dark field', using any other beam for diffraction contrast. Instead of using an electron beam to probe the sample, a photon source can also be used. An ultraviolet lamp (PEEM) can be used to do threshold emission and probe parameters such as work functions.

For more info on LEEM and it's capabilities we refer to the webpages and review articles by Bauer that have appeared over the years and that describe the ins and outs of the Low Energy Electron Microscope in great detail. If you're new to LEEM you can also check out the LEEM user webpages.