Dislocation surface nucleation in surfactant-passivated metallic nanocubes
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Research Letter
Dislocation surface nucleation in surfactant-passivated metallic nanocubes Mehrdad T. Kiani *, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA Radhika P. Patil *, and X. Wendy Gu, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Address all correspondence to Wendy Gu at [email protected] (Received 1 May 2019; accepted 7 June 2019)
Abstract The strength of single-crystalline nanoscale metals is controlled by dislocation nucleation from free surfaces. Surface properties such as crystallographic orientation, surface stress, and surface diffusion have been proposed as key parameters that control dislocation surface nucleation, but have not been confirmed experimentally. To investigate the influence of surface parameters, in situ scanning electron microscope mechanical testing is used to compress defect-free Ag and Cu nanocubes that are passivated with organic surfactants in order to tune their surface properties. Comparison between passivated nanocubes indicates that yield strength may depend on surfactant binding energy, but is also dependent on intrinsic material properties.
Dislocations are crystalline defects that determine the strength and mechanical behavior of metals. Dislocation motion and the interactions between dislocations and other defects are fairly well understood, but the thermal processes that govern dislocation nucleation remain mysterious.[1,2] Deformation in bulk materials involves dislocation motion and interactions with other defects, which make it challenging to isolate the materials’ response due to dislocation nucleation. Deformation in defect-free nanostructures is dominated by dislocation nucleation rather than dislocation motion. Thus, nanostructures with low dislocation density are ideal for studying dislocation nucleation.[3,4] Experimental studies and atomistic simulations on defect-free nanowires[5–7] and single-crystalline nanoparticles[8,9] show that defect-free nanostructures have extremely high yield strengths that approach theoretical limits. Yet, the atomistic mechanisms and probabilistic nature of dislocation nucleation, which results in scattered yield strengths for nominally identical nanostructures, are not well understood. It is necessary to observe dislocation nucleation in nanostructures with precise geometries and surfaces under well-controlled environmental conditions in order to isolate structural and thermodynamic parameters that control nucleation. Surface stress, surface diffusion, and crystallographic orientation have been proposed as key parameters in dislocation surface nucleation. Surface energy and surface stress have been shown to affect the yield strength of Ag and Cu nanowires in molecular dynamics (MD) simulations.[10,11] The effect of surface stress on yield strength becomes increasingly important as
* These authors contributed equally to this work.
the nanowires get smaller. Atomistic simulations of Cu nanowires with {100} facets and characteristic size of less
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