Precession‐assisted Quasi‐Parallel Illumination
STEM
on three condenser lenses
TEMs
Authors: Plana Sergi , Portillo Joaquim , Estradé Sònia , Mendoza Joan , Peiró Francesca 
Published: December 2016

The analytical mapping applications for which the STEM illumination mode in TEM columns is mostly used imply a high convergence angle of the electron beam focused onto a nanometric probe on the specimen, so that high electron doses are obtained. This design then enables high lateral resolution for energy dispersive or electron energy loss spectrum maps [1]. In view of the recent fast growth of quantitative electron diffraction work [2] the natural extension of STEM illumination mode in “TEM/STEM columns” would be towards diffractive recording for either low dose work or electron diffraction tomography because of the stable condition of the projector system, which stays solely in diffraction mode. The main problem for these applications is that even when using small condenser apertures of 10 μm, the obtained diffraction patterns consist of large discs of 3–4 mrad spread instead of small spots of only 0.5–1 mrad, the latter values being required for structure determination from crystals with large unit cells, such as zeolites. Expanding the approach used in multifunctional dedicated “STEM columns” [3], we have developed a stable method for working under Precessed Quasi‐Parallel illumination condition in TEM/STEM columns, using their internal scanning unit. The pre‐requisites – usually found in modern microscopes – are that the column should be digitally controllable, equipped with a 3‐lens condenser system and a condenser aperture of 10 μm, and have at least a set of deflecting coils past the objective lens, as well as the usual double set above the specimen plane. The key factor for enabling the Quasi‐Parallel illumination lays in decreasing the excitation of precisely the condenser lens which controls beam convergence, the second one in the 3 lens condenser system. Naturally, a refocusing of the beam is needed and an accurate curve of beam spread versus convergence angle must be produced, but, once calibrated, the resulting desired configurations are conveniently stored in computer memory for later recall and use. In this work, we have furthermore added to such Quasi‐Parallel Illumination STEM mode, the precession of the beam at 100 Hz, in order to obtain quasi‐kinematical diffraction patterns. The challenge has been to adjust the Precessed Quasi‐Parallel STEM HAADF image [4] [5] and a specific alignment method has proved to be specially suited, at least up to 0.6 degrees, providing almost non‐distorted scanned images (see images bellow, which include Precessed Diffracion Patterns with and without the de‐scanning below the sample, as well as Quasi‐Parallell STEM HAADF image with and without precession).

Using this electron microscope configuration, we are able to obtain images of organic materials without an excessive degradation compared to the static NBD‐TEM mode of the microscope. Moreover, the Precession‐assisted Quasi‐Parallel illumination STEM mode is suitable for electron diffraction tomography of both inorganic and organic structures, since sample drift and eucentricity at each tilting step may be controlled without changing the selected operative values in the projector system, usually corresponding to 12 cm for a 200KV high voltage. It also reduces the total time to obtain the whole data for the structure determination. Finally, the use of a precessed beam avoids the main dynamical effects on the diffraction patterns being able to solve structures with kinematical approximations.