Predicting the behaviour of lone-pair electrons

Abstract

Coherent Diffraction Imaging is a technique for the threedimensional reconstruction of single objects using coherent X-ray nano-beams. This technique, when used on Bragg reflections [1][2][3][4][5], can be used to recover both the shape and strain inside the objects.In semi-conductor nanowires (NWs), the knowledge of the exact strain state is essential for the tuning of the physical properties (conduction, photo-emission). While ensemble-averaged studies can provide quantitative information about the shape and strain state [6], this is only possible in the case of a very low dispersion of the structural properties. This presentation will be focused on the study of small (diameter<200nm) homogeneous and heterogeneous NWs and strained Silicon-On-Insulator lines, which have been studied using Coherent Bragg Imaging (CBI) [7,8].In the case of heterogeneous nanowires, we will show that it is possible to use the small size of existing nano-beams (e.g. 300x500 nm^2 using a Fresnel Zone Plate on beamline ID01 of the ESRF) to perform CBI measurements on different parts of heterogeneous nanowires, and therefore analyze the strain state near the interface.We will also discuss the current challenges presented by the method: (i) the requirement to determine the amplitude and phase of the incident focused X-ray beam, (ii) the difficulties of keeping a single object inside a nano-beam while rotating it over ~1° to record the 3D scattering pattern , and (iii) the effects of radiation damage due to the high flux (>10 4 ph/s/nm^2) on the sample. We present a new approach to probe bulk dislocations by using coherent x-ray diffraction. Coherent X rays are particularly suited for bulk dislocation studies because lattice phase shifts induce typical diffraction patterns which strongly depend on the fine structure of dislocation cores. The strength of the method is demonstrated by performing coherent diffraction of a single dislocation loop in silicon. The measured dissociation is proved to be unusually large compared to known dissociations. This work opens a route for the study of dislocation cores in a static and dynamical regime, and under various external constraints. Predicting the behaviour of lone-pair electrons David Brown, BIMR McMaster University, Hamilton (Canada). Email: idbrown@mcmaster.caIf one defines the bonding strength of an ion as the valence of a typical bond, the valence matching rule states that stable bonds will be formed only between ions having bonding strengths that do not differ by more than a factor of two. This rule limits the range of compounds that can form and indicates that weak cations bond only to weak anions and strong cations only to strong anions. This rule is obeyed in all compounds whose atoms can be assigned a formal cation or anion charge (valence) regardless of whether the bonding is ionic or covalent, with only two exceptions: the presence of steric stress or electronic anisotropies. Ions with lone pairs, namely cations in lower oxidation states and anions, also obey this rule in compounds ...