Internship 2015, Osmane CAMARA
Context. Plastic deformation of crystalline materials leads to surface atomic steps (slip traces) due to the movement of emerging dislocations. The height of these steps corresponds to the dislocation Burgers vector component normal to the surface and their orientation is linked to their gliding plane. These steps may give new insight on the plasticity mechanisms taking place in the bulk. They also interact with the native vicinal steps (atomic steps) present at the free surface and can be used to pattern the material in order to further confer some specific properties. In both cases, the stability of the surface configuration is of primary importance. To our knowledge, no detailed experimental study of the interaction between vicinal steps and slip traces has been reported.
Results. Au(111) single crystals were deformed at different temperatures. The emergence of each perfect dislocation shift the vicinal step by an atomic level along the out-of-plane direction. This leads to the atomic ‘chequered’ structure highlighted in the white frame in Fig. 1. The structure is thus characterized by three terraces of increasing atomic levels 0, 1 and 2 respectively). Two of them are opposite across an intersecting point labelled X. After an annealling at 300K, a destabilization of this structure is observed; the singularity (intersecting point X) has disappeared and is now replaced by two curved steps and appearance of a ‘bridge’ between the terraces of level 1. This strongly suggests a diffusion of gold atoms at the free surface along the steps. Atoms are going outside on the upper level 2, while they are going from the outside to the X point on the lower one 0. A modelling has been proposed in the framework of energetic considerations. It is shown that the destabilization of the initial ‘chequered’ structure is always energetically favorable whatever the intersecting angle between the slip trace and the vicinal step, in good agreement with the experimental STM investigations.
| Fig. 1. Nanostructure induced at the surface of Au(111) single crystals deformed at 180 K at a few % plastic strain. (a) Initial ‘chequered’ structure at 180K (b) Destabilization of the step structure after an annealing at 300K.