"Direct atomic imaging was instrumental in assessing the effectiveness of the test," Espinosa says.
The experimental findings revealed that the elastic stiffness of ZnO nanowires monotonically increases as their diameter decreases. Atomic level computational studies were also conducted to identify the reasons for the observed size effect.
"Our experimental method is the most direct and simplest in terms of data interpretation," says Bei Peng, a McCormick graduate student and co-author of the paper. "We feel quite certain on all the quantities we have measured. Moreover, the fact that the experimental trends and atomistic predictions agree is quite rewarding."
In this research article, the reason for the observed size dependence was also reported.
"Atoms on the surface of the wires are rearranged because they have fewer neighboring atoms as compared to atoms in the core of the nanowire," says Ravi Agrawal, a McCormick graduate student and co-author of the paper. "The resulting surface reconstruction leads to wire material properties very different to that encountered in bulk."
This phenomenon has been observed previously for various metallic nanowires with large surface-to-volume ratios, but the surface effect was confined to wires with diameters smaller than approximately 10 nm.
"Due to the ionic character of ZnO, the atoms interact via electrostatic forces, which are long-range in nature. Therefore, the size effect is found to be significant up to nanowires with diameters of about 80 nm," says Eleftherios Gdoutos, an undergraduate student and co-author of the paper.
"Our research approach based on a combined experimental-computational investigation of the mechanics of nanowires is very promising," Espinosa says. "We are currently employing MEMS devices that allow piezo-electric and piezo-resisti
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