Low-Energy Electron Microscopy

At CMU, the research group of Prof. Feenstra utilizes a low-energy electron microscope (LEEM) to study two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDs) such as MoS2, and others. The LEEM is an Elmitec III instrument, capable of bright- and dark-field imaging with 10 nm resolution, selected-area low-energy electron diffraction (μLEED), and spectroscopic low-energy electron reflectivity (LEER) measurements. The is primarily used for characterizing epitaxial films of 2D materials, in which case the spatial extent and crystal orientation of the grains is determined (from imaging the μLEED), along with the number of 2D layers in each grain (from LEER measurements).

A photo of the Elmitec LEEM system is shown below (along with graduate student Nishtha Srivastava), together with a layout of the system.

Electrons are produced in an electron gun containing a thermionic LaB6 emitter, which is biased at typically -20 keV. Once the electron beam has left the emitter, it is accelerated to high energy by a grounded extractor into the illumination column, after which the beam is deflected towards the sample surface by a magnetic deflector. Passing through the grounded objective lens, the beam is rapidly decelerated to low energy due to the large potential difference between the objective and the sample, which is also close to -20 keV. A potential difference, Vs (start voltage or sample voltage), can be applied between the sample surface and the gun filament to alter the incident electron energy. Typically incident energies of 0 - 50 eV are employed. The electrons are then reflected, or diffracted, from the sample surface. They pass through the magnetic deflector again, and then are imaged (either as a diffraction pattern or a real-space image) on the micro-channel plate. A contrast aperture is used to select particular diffraction spots for imaging. Bright-field images are formed using the reflected (0,0) spot, whereas dark-field images are formed using other, specifically selected diffraction spots. Due to their low energies, the only electrons to leave the surface are those that originate from the top few atomic layers of the sample. Hence LEEM is a very surface sensitive technique.

Click here for list of reprints and description of research activities in the group of Prof. Feenstra