Photoemission electron microscopy
The photoemission electron microscopy ( English photoemission electron microscopy , PEEM) is a by photoelectron spectroscopy (PES) and the electron related analytical method for the study of surface phenomena. It is characterized by the recording of a two-dimensional intensity distribution of the photoelectrons and thus results in an enlarged image of the sample surface. It is therefore, characteristic of microscopes , an imaging measuring technique.
Measuring principle
Similar to photoelectron spectroscopy (PES), in PEEM electrons are released from the sample by the photoelectric effect . In contrast to the PES, the number of electrons of a kinetic energy selected by the analyzer is not measured, but rather the intensity distribution of the photoelectrons of a two-dimensional area of the sample is interested. For this purpose, the emitted photoelectrons are collected by means of a strong electrostatic field between the sample and the imaging column, with the electron image subsequently being enlarged with several coaxial electron lenses. The electrons selected in this way hit a luminescent screen and generate an image, which in turn can be displayed using a CCD camera . The more direct way of illustration is a microchannel plate (Engl. Channel plate ): large number of two-dimensionally arranged channel electron multipliers (Engl. Channeltrons ).
Standard laboratory light sources in the UV range, such as mercury (hν = 4.9 eV) and deuterium lamps (hν = 6.4 eV), or synchrotron radiation, are used as light sources . Due to its high energy and intensity, the latter also enables photoelectrons of the inner shells to be displayed.
Evaluation of the measurement
The variation in the intensity of the electron yield is shown directly on the screen in real time during the measurement. In the case of PEEM, it is generally based on the different work functions of the examined topographical areas of the sample. In this way, various properties such as B. topography of the surface, differences in the work function of different substances, chemical composition or, using polarized light (especially synchrotron radiation), magnetic properties can be made visible through a contrast gradation in real time with a resolution of up to 20 nm.
XPS microscope or μ-ESCA
By installing a microanalyser in the beam path that selects the kinetic energy of the photoelectrons (analogous to normal PES) and by using narrow-band and short-wave excitation light sources such as. B. synchrotron radiation, it is possible to carry out laterally resolved XPS. The term μ-ESCA (ESCA stands for Electron Spectroscopy for Chemical Analysis , English for photoelectron spectroscopy for chemical analysis ) chemical analysis describes a micrometer-sized area of the sample. This enables both the determination of the elemental composition of the sample and the investigation of local differences in the electronic properties.