Aberration-Corrected Low Energy Electron Microscope
In 2010 an ELMITEC Aberration Corrected Low Energy Electron Microscope (AC-LEEM) and Photo Electron Emission Microscope (PEEM) was installed in the labs of PGI-3. The microscope is one of the best LEEM/PEEM microscopes available in terms of spatial and energy-resolution as well as transmission. When it came in operation in winter 2010/2011, there were only about 5 comparable microscopes available worldwide.
Since 2013 the microscope is equipped with a high-flux He-lamp producing UV radiation with 21.4 eV photon energy (beside the Hg-Lamp having 4.9 eV already in place before). The new lamp does not only extend the energy range of the radiation for PEEM measurements, it also enables the full spectroscopic capabilities of the Spectro-Microscope. Full 3d k-space mapping is possible (angular resolved UPS measurements), which allows the analysis of the data by orbital tomography, a rather new technique to disentangle the spectroscopic signals of different molecules adsorbed on a substrate.
More deatails on the LEEM/PEEM technique:
The LEEM/PEEM technique uses electrons emitted from a sample surface for imaging. Different contrast mechanism are possible: Back-scattered or diffracted electrons in the case of LEEM (bright- or dark-field), or photo-emitted electrons when ultraviolet or (soft) x-rays are used for illumination of the sample (PEEM). The LEEM method, originally developed by E. Bauer in the 1960’s, is widely used in surface science and booming recently due to the development of aberration correctors which improve the spatial resolution of the microscope to approx. 2 nm in AC-LEEM mode and approx. 5 nm for AC-PEEM. Additionally, and in many cases even more importantly, the transmission of the microscope is improved by almost one order of magnitude. This enables experiments with radiation-sensitive samples like organic thin films as they are performed in our institute, since radiation damage can be reduced by using lower illumination intensities.
Fig. 3 shows a test experiment using aberration corrected dark-field LEEM on a (2x1) reconstructed Si(001) surface. The black and white surface areas are different rotational domains of the surface which represent separated by a height-step of one atomic layer. Upper and lower terraces are (2x1) and (1x2) reconstructed and can hence be easily distinguished in dark-field LEEM by selecting (e.g.) the (½0) diffraction spot for imaging. The step edge appears to be approx. 2.5 nm wide (16-84% criterion for the contrast change across the edge), a value which can be considered as instrumental resolution in the present configuration. The corresponding movie (first movie shown below) shows the movement of the step edges due to diffusion of Si atoms at approx. 980°C.
The second movie shows the in situ growth of PTCDA molecules on a clean Ag(111) surface observed in PEEM. The darkening of the surface in the beginning indicates the growth of the first three layers of PTCDA which reduce the work function of the surface and hence appear dark. No PTCDA islands can be resolved due to small terrace widths and the rather limited resolution in this movie. After approx. 8 sec. (2½ min in real time) the growth of higher, three-dimensional islands starts. They appear black.