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UU2.03 - Bridging Surface Atomic Structure with In Situ Nanomechanics of ZnO Nanowires 
Date/Time:
April 22, 2014   2:15pm - 2:45pm
 
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Evaluating the mechanical properties of nanomaterials and understanding their size effect is basic for optimizing the performance of nano-electromechanical devices and systems. For example, the diameter dependence of elastic modulus in ZnO nanowires has been focused by extensive experimental and theoretical studies. However, the real mechanism of size effect has never been directly revealed in atomic-scale. Here we firstly show the diameter dependence of elastic modulus in [0001]-oriented ZnO nanowires measured in both bending and tensile modes by in situ scanning electron microscopy. As diameter is smaller than about 120 nm, moduli of nanowires deviate from the bulk value and increase dramatically with decreasing diameter, which can be well explained by a core-shell composite model in terms of stiffening effect correlated with a radial-distributed relaxation strain function. Then we show that by using aberration-corrected transmission electron microscopy combined with density functional theory calculations, the relaxation (atomic displacements) of ZnO{10-10} fresh surfaces can be directly quantified in picometer-level accuracies. The Zn-O bond in the outmost surface layer is shown to be contracted by 6~7 %, according well with theoretical predictions. Further, the under-surface distribution of Zn-O bond contractions are directly measured for the first time, which verify our model of relaxation strain function and thus validate the correlation between surface relaxation and elasticity size effect. In the end, we show a novel reversible reconstruction between wurtzite and body-centered-tetragonal lattice in ZnO{10-10} fresh surfaces, which has not been observed before and may open broad prospects for exploring the metastable surface structures and related applications of ZnO nanomaterials.
 


 
 
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