Atomic Events at the Onset of Plasticity in the Au(001) Surface
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0976-EE06-08
Atomic Events at the Onset of Plasticity in the Au(001) Surface Esther Carrasco, Oscar Rodríguez de la Fuente, and Juan Manuel Rojo Física de Materiales, Universidad Complutense de Madrid, Av. Complutense s/n, Madrid, 28040, Spain
ABSTRACT Atomic defects, in the form of dislocation configurations, are identified following nanoindentation of Au(001) surfaces. Combined STM, dislocation theory and atomistic simulations fully characterise these defects and describe their motion. By comparing with AFM data the emission of those defects are correlated with the incipient stages of plasticity, the latter recognised from force vs penetration curves. The size distribution, the spatial pattern and the motion of these dislocations under stress have also been investigated and explained in terms of the theory of dislocations in a continuum. INTRODUCTION The ever increasing interest in nanostructures properties is fuelling the study of the local mechanical properties of solid surfaces. The elastic-plastic transition at the nanoscale and the dislocation and related defects involved in the first stages of plasticity are beginning to be understood by using nanoindentation techniques [1,2]. However, the atomic processes taking place at the initial stages of plasticity in solid surfaces are still full of uncertainties. In previous articles we have used scanning tunnelling microscopy (STM) to image the Au(001) surface after nanoindentation with the microscope tip. Local analysis around the nanoindentation point has showed that two special types of dislocation configurations are generated around the latter: ‘mesas’ and ‘screw-loops’ [3,4], both of which intersect the surface and give rise to permanent features that can be disclosed by STM. We have also revealed that a dislocation-mediated mechanism is responsible for a substantial fraction of the material pile-up around the nanoindentation. In addition, by using AFM we have shown that the discontinuities in the force vs penetration curves (sometimes called ‘pop-ins’) are related to well-characterised dislocation configurations and local modifications of the metal surface [5]. Here, we present new experimental and theoretical results which further shed light on the relationship between mechanical properties and defects parameters at the first stages of plasticity. In particular, we report on the comparison between experiment and atomistic simulations of dislocation emission associated to the pop-ins in force vs penetration curves. We further analyse the spatial distribution and motion of those dislocation configurations. EXPERIMENTAL AND SIMULATION METHODOLOGY A conventional ultrahigh-vacuum chamber equipped with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and a homemade scanning tunnelling microscope (STM) was used in the present investigation. Au(001) surfaces were prepared by the standard combination of Ar+ ion bombardment and high temperature annealing.
All the STM images shown in this paper were obtained in the constant-current mode with
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