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MM: Fachverband Metall- und Materialphysik

MM 14: Topical Session TEM IV

MM 14.5: Vortrag

Dienstag, 15. März 2011, 12:30–12:45, IFW A

Optimum TEM imaging conditions for graphene at low and medium voltages — •Zhongbo Lee¹, Ute Kaiser¹, Jannik Meyer², and Harald Rose¹ — ¹Central Facility of Electron Microscopy, Group of Materials Science Electron Microscopy, University of Ulm, 89069 Ulm, Germany — ²University of Vienna, Department of Physics, 1090 Vienna, Austria

In standard electron microscopy graphene is usually considered as a weak phase object (WPO) for which the contribution of the non-linear terms to the image contrast is negligibly small. Here we investigate whether this argument holds true for low voltages by means of calculating optimum positive and negative image contrast for an aberration-corrected TEM operating at acceleration voltages of 80kV and 20kV, respectively. The results in the case of achromatic imaging conditions show that graphene cannot be treated as a WPO for low voltages. Even at 80kV the difference between optimum positive and negative contrast is significant. At 20kV this difference becomes very large for the same imaging conditions demonstrating that in this case graphene acts as a strong phase object. In the presence of chromatic aberration, however, the finite energy width of the electron beam largely nullifies this difference. This behavior is shown by the calculations and verified by the experiment for 80kV. Moreover, for chromatic imaging conditions at 80kV the contrast can be increased significantly by means of a monochromator which reduces the energy width to about 0.1eV. In the case of 20kV, however, a significant improvement of contrast can only be achieved by the additional correction of chromatic aberration.