Electron diffraction and crystal orientation phase mapping under scanning transmission electron microscopy

Electron diffraction pattern acquisition in scanning transmission electron microscopy (STEM) mode is a very attractive technique for the study of the crystallographic characteristics of nanostructured materials. One of the most important aspects of this technique is to ensure an illumination on the sample as parallel as possible, which translates into reducing the convergence angle of the electron beam as much as possible. Different parameters of electron microscopes have a direct impact on the convergence angle of the electron beam; once these parameters are identified, and their effect on the convergence angle is studied, optimum conditions for the acquisition of electron diffraction patterns while in STEM mode (D-STEM) can be identified. In the present study, several of these parameters were identified and assessed; among these parameters we can mention the condenser aperture 2 size, the excitation of the condenser minilens, and the spot size used, among others. The results obtained allowed to identify the optimum conditions to produce a convergence angle smaller than 1 mrad, with an electron probe size smaller than 3 nm. When combined with precession electron diffraction (PED), this D-STEM technique allows obtaining crystal orientation phase maps with a spatial resolution determined mainly by the electron probe size. Several examples of these combined techniques applied to different nanostructured systems, like lead chalcogenide nanoparticles, Au clusters, GaN nanofilms, Co nanowires, and Au decahedral nanoparticles, are presented.