Spectromicroscopy of surfaces by metastable He atoms (He-SMS)

15.12.2010 - 15.12.2011
Globales Einzelvorhaben
Metastable impact electron spectroscopy (MIES) is a modern surface sensitive technique that uses excited noble gas atoms in their metastable state (e.g. He* in the 23S state with E* = 19.82 eV), as a sensitive surface probe. In contrast to electrons or photons, the metastable He* atoms cannot penetrate into a solid, they are de-excited in front of the surface leading to the emission of electrons from the outermost surface layer. These electrons can then be energy analysed, similarly as it occurs in the UV- and X-ray excited electron spectroscopies (UPS, XPS). Besides of the exclusive surface sensitivity, MIES is essentially non-destructive, since the metastable He* atoms have only thermal kinetic energy. We propose such a spectroscopy with high spatial resolution which can be achieved by detecting electrons emitted from a small surface area. Possible applications of a MIES-based spectromicroscopy are studies of the interaction of adsorbed species (e.g. in catalytic processes), examination of radiation-sensitive materials like polymer films, microelectronic structures, phase transitions at surfaces especially with changes in the molecular orientation of adsorbed species, some special questions of the modification of surfaces such as segregation and termination, examination of fragile samples, such as organic layers, biological objects, etc. In the group of the project applicant, in situ imaging of surface processes using electrons emitted by He* impact was realized for the first time, by combining a He* source with the imaging system of a spectroscopic LEEM (Low Energy Electron Microscope). Chemical contrast was obtained and a chemical reaction (hydrogen oxidation) was visualized in situ for the first time. The feasibility of the idea of MIEEM (Metastable Impact Electron Emission Microscopy) was proven in this way. However, the energy resolution of the spectroscopic LEEM is rather poor in comparison with modern hemispherical analysers. Thus, it seems worthwhile to combine emission microscopy by He* atoms with the energy analysis of a hemispherical analyser. Modern hemispherical analysers allow lateral resolution by moving (scanning) the sample position or using lens deflectors which allow a shift of the acceptance area by a certain lateral distance. Applying a two-dimensional electron detecting device (delay-line or a CCD detector) a two-dimensional energy distribution map can be created, with a lateral resolution down to 30 mkm being obtained by this method. Thus, laterally resolved He* Spectromicroscopy of Surfaces (He-SMS) can be realized in this way. Within the proposed project, the new He-SMS with a hemispherical analyser will be combined in a multipurpose system with an already existing PEEM (Photoemission Electron Microscope) that allows real time imaging of surface processes with a lateral resolution in the submicron range. Such a combined PEEM-He-SMS system will allow in situ imaging of surface processes with a high lateral resolution, high energy resolution and with a high video-recording rate (PEEM is equipped by a high-speed CCD-camera that allows up to 500 frames/s), providing access to dynamic surface processes on the time scale of microseconds. The new system will be utilized to examine catalytic reactions on well-defined solid surfaces (single crystals, thin films, model catalytic systems and nanoparticles), microelectronic devices at pressures up to 10-4 mbar. The samples to be studied are prepared by physical methods (evaporation), microelectronic fabrication (lithography, atomic layer deposition (ALD), molecular beam epitaxy (MBE), as well as by the wet-chemical thin film synthesis. Structure characterization on an atomic level will be carried out by AFM, SEM, etc.

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Forschungsschwerpunkte

  • Surfaces and Interfaces: 75%

Schlagwörter

DeutschEnglisch
Elektronenspektroskopieelectron spectroscopy
Metastabile He Atomemetastable He atoms
Spektromikroskopiespectromicroscopy

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