Dresden 2011 – scientific programme
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MI: Fachverband Mikrosonden
MI 3: TEM- and SEM-based Material Analysis
MI 3.2: Invited Talk
Monday, March 14, 2011, 15:15–16:00, BEY 81
The contrast mechanisms of LL-BSE electrons in FE-SEM - Characterization of polymer, single proteins, and oxidization states of elements — •Heiner Jaksch — Carl Zeiss NTS GmbH, 73447 Oberkochen, Germany
Below landing energies of 4 kV, the backscatter coefficient becomes non linear and drops with increasing atomic number stronger than that from elements with low atomic numbers. At a certain landing energy we see equilibrium of backscatter yield and no contrast. Chromium is at 1 kV landing energy brighter than Gold. Carbon will be brighter than Gold around 400 eV landing energy! Due to this fact and the problem, that the mean free path length of BSE electrons from low-density materials, such as proteins or polymers, becomes extremely small, we have introduced new technologies to visualize these low intensity signals coming from electrons with very small energy loss. The low loss BSE electrons are now introduced in SEM. To understand the new contrast mechanisms experiments with hybrids, polymers and all kinds of different oxidization states of elements were made and will be shown. Essential for the contrast at low landing energies is not any more the atomic number or density as contrast mechanism, but only the bonding structure of outer shell electrons or plasmon losses. To get the information from the electrons a double stage filtering is necessary. In the examples these results are explained with the hybridization of carbon as sp2 and sp3 hybrids and shown with imaging examples. These hybrids are responsible for the contrast in all polymer and protein. These hybrids are responsible for the contrast in all polymer and protein. In general we have to consider the bonding-/ ionization energy or plasmon losses and not the nucleus charge as source of the contrast. In the shown examples it will be proven that density rules or z-number contrast fail in explaining the observed contrast. Monte Carlo simulations also are unable to model the fine contrast mechanisms. The sensitivity of the technology is explained with the detection of a single protein (8nm) in virus marked with GFP. The detection concept is verified with quantum dots (GaAlAs) of known band-gap with 2.5eV and 4.8eV. What we see there is the resonance of a more or less free electron, replaced by the primary electrons. Such "free" electrons typically show extreme high contrast due to the very small energy loss when replaced by primary electrons. One can use this contrast mechanism to detect functional groups in polymers or as described above, in fluorophors in live science. As an outlook the technology will be a big step forward for the characterization of anything in live science and material science.
1) Reimer L., Scanning electron microscopy 2nd Edition, Springer-Verlag Berlin Heidelberg New York 1998. 2) Jaksch H., Low Loss BSE imaging in a FE-SEM, Proceedings IMC 2010, Rio de Janeiro.