Development of a Method to Observe Local Atomic Vibrations by STEM Using a Segmented Detector

Aberration-corrected scanning transmission electron microscopy (STEM) is a powerful technique for directly observing atomic structures and chemistry in local regions of materials. Annular dark-field (ADF) method, in which electrons scattered at high angles are detected by an annular detector, is widely used as a useful tool for direct observation of atomic positions. It is known that the electrons scattered at high angles are dominated by thermal diffuse scattering (TDS) [1], which depends on the atomic number and atomic displacement parameter (ADP) of the atoms. It is considered that anisotropic atomic vibrations cause anisotropic TDS, but conventional annular detectors have not been able to measure local atomic vibrations quantitatively, including anisotropy of vibration. On the other hand, segmented detectors [2] can acquire images from multiple detection areas divided in the azimuthal direction, enabling the acquisition of signals that depend on the direction of electron scattering. In this work, we investigate the possibility of direct observation of anisotropic atomic vibrations by atomic-resolution STEM with a segmented detector. First, we performed image simulations based on the quantum excitation of phonons (QEP) multislice approach [3] and extracted the electrons which have scattered by anisotropic atomic vibrations. STEM observation conditions, such as convergence angle, defocus, accelerating voltage, and detector acquisition angle, were optimized to increase the signal due to anisotropic atomic vibrations. We performed experimental STEM observations for a model sample under thus obtained optimal conditions and quantitatively compared the experimental results with the theoretical simulations. Details will be reported in the presentation.<br/><br/>[1] S. J. Pennycook et al. Ultramicroscopy 37, 14 (1991).<br/>[2] N. Shibata et al., Microscopy 59, 473 (2010).<br/>[3] B. D. Forbes et al., Phys. Rev. B 82, 104103 (2010).