Local strain measurements at dislocations, disclinations and domain boundaries
Résumé
We present the state of the art in strain mapping at the nanoscale using aberration - corrected high - resolution transmission electron microscopy, HRTEM and HAADF - STEM, and dark - field electron holography (DFEH) [1]. In particular, we will focus on the examination of localized strains around defects. A broader comparison of TEM strain mapping techniques can be found i n a recent review [2]. High - resolution images can be analysed by geometric phase analysis (GPA) [3] or applying peak - finding routines to determine the positions of individual atomic columns. GPA is best adapted to measuring the deformation of the crystall ine lattice, as the example concerning a five - fold twinned Pt nanoparticle will show (Figure 1). These star - shaped particles [4] have a disclination - like strain fields around their centres, as found for Au nanoparticles [5]. In multiferroic materials, peak - finding can be used to determine the relative displacements of atom columns within a unit cell [6]. The local polarization and strain can be mapped around dislocations and in the vicinity of domain walls (Figure 2). DFEH was developed to measure strain over wide fields of view to high precision and nanometre spatial resolution. We have nevertheless been applying the technique to the study of quantum dots to relatively high resolution [7]. As with any TEM technique the samples are necessarily thin which allows some of the strain to be relaxed: the well - known thin film effect. In addition, dynamical scattering effects the strain information [8]. These issues will be addressed using a combination of finite - element modelin g (FEM) and dynamical - scattering simulations. This overview will, in addition, show results from the recently installed I2TEM microscope (Hitachi), an instrument specifically designed for DFEH experiments and aberration - corrected HRTEM over wide fields of view and for in - situ experiments (Figure 3).