Low Energy Electron Microscopy

Besides imaging with photo-emitted electron, our electron microscopy can also work as a LEEM as it is equipped with a conventional electron cathode. LEEM images surfaces with elastically backscattered low energy electrons. Due to large backscattering cross sections of low energy electrons and non-scanning imaging principle of the cathode immersion lens design, LEEM imaging can be carried out routinely in real-time at video frame rate. It is an ideal instrument for monitoring in situ dynamic processes, such as growth and self-organization of nanostructures and thin films. Lateral resolution of our LEEM is better than 10nm. In the future, we are also planning push this resolution down to 2nm with aberration correction optics.

Schematic of the LEEM/PEEM

In LEEM, imaging is done with two major contrast mechanisms, namely diffraction contrast and phase contrast. For diffraction contrast, bright field imaging is done using backscattered electrons that undergo no momentum transfer parallel to the surface. In this mode, contrast comes from local variation of the structure factor on the sample surface. On the other hand, dark field imaging using diffracted electrons with non-zero parallel momentum transfer is also possible and it is useful in identification of coexisting phases of different rotational variants. The other contrast mechanism, phase contrast, can be further divided up into geometry phase contrast and quantum size contrast. In geometry phase contrast, the height different of steps on surface give raise to phase different in the backscattered electron wave, interference of these backscattered electron waves lead to amplitude differences and it allows imaging of steps at surfaces. Quantum size contrast is based on interference of electron wave backscattered from the surface and from the thin film/substrate interface, which produce intensity maxima and minima depending on thickness of the thin film and the electron energy. This mechanism allows quantitative measurement of film thickness on surfaces.

Besides direct imaging of surfaces, we can also image the back focal plane of the objective lens which gives access to low energy electron diffraction (LEED). LEED is one of the most important technique for studying atomic structure in material surfaces. In our electron microscope, structural information from very small selected area as small as 250nm can be obtained by insertion of a small aperture in the electron beam path. In one of our work, we applied this technique to investigate how substrate can induce structural deformation in atomically thin 2D materials [1]. With our electron microscope, we can look at submicron sized regions of molybdenum disulfide (MoS2) flakes with different thickness. We see that by inserting a monolayer of hBN as a buffer layer, we can bring vast improvement in the flatness in MoS2 and restore the other key optical properties of MoS2.

[1] M. K. L. Man, et al., Scientific Reports 6, 20890 (2016).

Contrast mechanisms in LEEM


Page last updated on September 3, 2017 (MM/CP).