Low energy electron microscope
The low-energy electron microscope ( english low-energy electron microscopy , LEEM) is a device for the investigation of surface structures by means of electron described by G. Ernst Bauer has been invented in 1962, but fully developed until the 1985th With its help, atomically smooth surfaces, atom-surface interactions and thin (crystalline) films can be microscoped .
Working principle
The principle of the low-energy electron microscope is comparable to that of light microscopy. The sample is illuminated over a large area (diameter up to approx. 50 µm) with an electron beam. In electron optics, these electrons have an energy of a few thousand electron volts . In front of the sample, the electrons are slowed down to a few eV and therefore only penetrate a few angstroms into the sample. Low-energy electron microscopy is therefore a surface-sensitive method for a few atomic layers. The reflected (more precisely: backscattered as with LEED ) electrons are guided through imaging optics, which, like a light microscope, generate an image that is made visible by an electron detector and recorded with a camera.
In this way dynamic surface processes can be observed.
With LEEM, conductive, crystalline samples can be examined, which can be aligned in a defined way to the incident electron beam. Enhancements include aberration correction on the imaging side (AC-LEEM) and Spin Polarized LEEM (SPLEEM).
Imaging system
Electrons are emitted from an electron gun with 15-20 keV, focused using a condenser lens and sent through a magnetic beam splitter (60 ° or 90 °). The fast electrons fly through an objective lens and are decelerated in the direction of the sample surface, which is at a potential similar to that of the electron gun. As a result, the electrons are now surface sensitive (1–100 eV) and the penetration depth can be changed by varying the electron energy (difference between sample potential / electron gun potential minus work function of the sample). The backscattered electrons fly through the condenser lens (whose potential is at ground), accelerate to the electron gun and pass through the beam splitter again. Now the electrons move away from the capacitor optics and fly into the projector lens. The projection of the focal plane of the objective lens through an intermediate lens into the object plane of the projector lens results in a diffraction pattern ( LEED - Low Energy Electron Diffraction ) in the image plane, which can be recorded in various ways. The intensity distribution of the diffraction pattern depends on the sample surface and is a direct result of the wave nature of the electron. The individual intensities of the diffraction spots can be measured by switching off the intermediate lens and inserting a contrast aperture .
Differences from ordinary electron microscopy
The sample must be illuminated from the side of the imaging optics, as materials for low-energy electrons are not transparent. A magnetic electron prism is used to split the incident and scattered beam, which focuses electrons both out and into the electron beam plane in order to avoid disturbances in the image.
Furthermore, the potential of the sample is not kept at ground, but brought close to that of the electron gun through an electronic immersion objective lens in order to slow down the electrons. In addition, the device usually has to operate under ultra-high vacuum (UHV). However, there are also "near-ambient pressure" (NAP-LEEM) instruments that allow pressures of up to 10 −1 mbar by means of differential pumps and a special high-pressure cell .
Individual evidence
- ↑ Torsten Franz, Bernhard von Boehn, Helder Marchetto, Benjamin Borkenhagen, Gerhard Lilienkamp, Winfried Daum, Ronald Imbihl: Catalytic CO oxidation on Pt under near ambient pressure: A NAP-LEEM study . In: Elsevier BV (Ed.): Ultramicroscopy . 200, 2019, ISSN 0304-3991 , pp. 73-78. doi : 10.1016 / j.ultramic.2019.02.024 .