SPELEEM microscope

Operating principle and methods

The SPELEEM images a specimen that is illuminated with soft x-rays, ultra-violet (UV) radiation or low energy electrons. The emitted photoelectrons, or the elastically reflected electrons, are accelerated by a strong field in the objective lens, of which the specimen is an integral part. The objective produces a magnified image of the specimen, which is further magnified by several additional lenses in the imaging column of the instrument. The image is energy-filtered by the hemispherical analyser and finally projected onto an imaging MCP detector with phosphorous screen. This is imaged in real-time by a computer controlled CCD camera.

Energy filtered X−PEEM: The PEEM detects electrons emitted from atomic core levels with kinetic energy E_kin = hν - E_bin -φ, where E_bin is the core level binding energy, hν the photon energy and φ the work function. Typically hν is kept fixed, with energies in the range provided by the beamline (50-1000 eV). The energy filter is used to select the kinetic energy E_kin of photoelectrons, which allows measuring the binding energies of emitting atoms or accessing the surface electronic structure, including surface states and resonances. Typical kinetic energies of the photoelectrons are at the minimum of the inelastic mean free path of the electrons in the matter, ensuring good surface sensitivity through the very reduced mean free path of electrons in the matter. Kinetic energies are in the range 50 to 150 eV, typically well beyond the broad peak of secondary emission. Collecting a series of images at different kinetic energies allows obtaining laterally resolved (or angle resolved) spectroscopic information. The intensity of the photoemission signal is proportional to the number of emitters in the topmost layers within their energy-dependent escape depth, and thus provides straightforward and quantitative information about the surface chemical composition. Optimal elemental sensitivity is achieved by tuning the photon energy to maximise photoionisation cross-section.

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, XMC(L)D-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. 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.

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.

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.