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Introduction |
Specifications |
Description |
Operation modes
The illumination of the specimen with x-rays excites a broad electron spectrum consisting of the primary unscattered or elastically scattered photoelectrons, inelastically scattered electrons (secondary electrons), and Auger electrons. Using these electrons, the PEEM can perform laterally resolved versions of most widely used x-ray spectroscopies.
UV−PEEM:
Threshold microscopy can be performed when using a UV light source (such as an Hg lamp) as illunination. The photon energy must be higher then the lowest local work function on the surface. This method is extremely sensitive to small differences in the local work function, which are directly induced by molecules and atoms adsorbed on the surface. Threshold UV-PEEM has been extensively employed to study dynamical processes such as spatio-temporal pattern formation in surface catalytic reactions.
XAS−PEEM
In x-ray absorption spectroscopy PEEM images the secondary electron emission at fixed kinetic energy as a function of the photon energy hν. When hν matches a core level energy a strong increase in the secondary emission intensity is observed. Such resonances arise from transitions from core levels into unoccupied valence states via excitation processes occurring during the filling of the core holes. They are characteristic fingerprints of the emitter chemical state, so that elemental sensitivity is acheived. X-ray absorption near-edge spectroscopy (XANES) provides a wealth of information about the emitter, such as site location and valence state. Due to the very low energy of the secondary electrons (less than a few eV), their mean free path is relatively large. XAS and XANES can thus probe buried interfaces or films up to a depth of ~10nm. The resolving power of the monochromator determines the attainable energy resolution and for this reason energy filter is not required.
XMCD−PEEM and XMLD−PEEM
In combination with magnetic linear and circular dichroism (MLD and MCD respectively) XPEEM can image the magnetic state of surfaces, thin films and buried interfaces.
brightfield and darkfield LEEM
LEEM is a structure sensitive technique which uses elastically backscattered electrons to image a crystalline surface with a lateral resolution of few tens nm. The use of a contrast aperture positioned in the diffraction plane allows employing primary or secondary diffracted beams for imaging. When the primary diffracted beam (or "00" beam) is selected, we perform bright−field LEEM. Here, the contrast is purely structural (diffraction contrast) and depends on the local differences in diffraction for the different surface phases present on the sample. By selecting a secondary diffracted beam, a darkfield image of the surface is produced. Here all areas that contribute to the formation of the selected beam appear bright.
Other methods available in LEEM are phase contrast and quantum size contrast. In the first, the height difference between terraces at different heights on the surface leads to a phase difference in the backscattered waves. Defocusing can convert such phase difference into an amplitude difference, allowing to image steps at surfaces. The second method is based on the interference of waves that are backscattered at the surface and at the interface of a thin film, producing maxima and minima in the backscattered intensity depending on the local thickness of the film.
contrast methods in LEEM.
MEM
In mirror electron microscopy (MEM) the surface is illuminated with electrons at very low energy. The sample bias is adjusted so that the electrons interact very weakly with the surface (this occurs at the transition MEM−LEEM). Under these conditions the contrast is due to work function differeces and topography variations. MEM allows non-crystalline samples to be imaged.
µ−LEED
The SPELEEM is operated as a LEED instrument. Reflection of the e-beam by a crystalline surface results in the formation of a diffraction pattern in the back-focal plane of the objective lens. The beam energy is varied by changing the bias voltage between sample and electron emitter. The probed area can be restricted to 2µm either by inserting an aperture in the image plane in the input or exit side of the beam separator, respectively. LEED data yields information about the surface structure.
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