Spectroscopic Imaging

Since the pioneering work in the 1980s [1], X-ray excited Photoemission Electron Microscopy has become a powerful and versatile technique for the investigation of surfaces. In particular, materials or nanostructures with heterogeneous element distributions, polycrystalline materials or chemical surface reactions, segregation etc. are accessible using X-ray excitation. Two different experimental techniques can be distinguished:

X-ray absorption near-edge structure (XANES-PEEM) is the most common approach. The high brilliance and easy tuneability of Synchrotron radiation sources makes XANES-PEEM a relatively simple method for spectroscopic imaging. Since the secondary electrons are used for image formation, this method does not require any additional equipment but a standard PEEM. However, tuneable X-rays in the spectral region of strong X-ray absorption lines of the elements of interest are required. Furthermore, the XANES features are relatively broad because they contain information on the core level as well as the unoccupied part of the density of states above the Fermi energy. Hence, special algorithms in the imaging software (like the "spectral unmixing" algorithm) are needed for a spatially-resolved analysis of different chemical states of an element further news.

Imaging X-ray photoelectron spectroscopy (XPEEM) constitutes an alternative very powerful approach that provides spectromicroscopic imaging by implementing an electron spectrometer into the microscopy column. A new instrument – the NanoESCA- consists of a PEEM optic as entrance lens and a tandem hemispherical energy filter giving an achromatic image that is corrected for the aberration of a single hemispherical analyzer. The microscope allows imaging with high chemical contrast by energy filtering of photoelectron images at kinetic energies up to 1.6 keV, which are typical for XPS further news. The NanoESCA does not require tuneable radiation and can thus be operated using laboratory X-ray sources [2]. The energy discrimination can also be made by a high-pass energy filter (further news and further news) or by a time-of-flight technique (further news and further news).

[1] B.P. Tonner and G.R. Harp, Rev. Sci. Instrum. 59 (1988) 853
[2] M. Escher et al., J. Electron Spectrosc. Relat. Phenom. 144-147 (2005) 1179-1182




Last update: Thursday, 17-Nov-2011 12:05:35 CET Email D. Panzer Impressum