Effect Of Surface Elasticity On The Mechanical And Electrical Properties Of Nanostructures

Nanostructures, such as nanowires, nanoflims or nanoplates, and nanotubes, hold a promise for a wide diversity of applications as, for instance, sensors, actuators, transistors, probes, resonators and etc. in nanoelectromechanical systems (NEMS). Determination of the mechanical and electrical properties of nanostructures becomes a critical issue in the design process of these nanodevices. Although many efforts have been devoted to study the properties of nanoelements, the conflicts and a consideration gap between the experiments and theories still exist. Therefore, to investigate the physical properties of nanostructures is still an open issue for further study. Especially, on the other hand, surface effects become hot topics for examining the physical properties of nanostructures. Surface elasticity and surface stress have been recognized as important factors that may explain the experimentally measured results. However, the existing surface models can not provide better theoretical agreements with experimental results especially when the dimensions of nanostructures drop down to several tens of nanometers. This academic dissertation just focuses on the problems of surface effects to investigate the physical properties of nanostructures. Specifically, we propose an improved surface elasticity model called modified core-shell model, based on which we systematically investigate the mechanical, electrical properties of nanostructures, and give more reasonable and valid theoretical results which agree well with experimental data at the smaller size.At first, on the basis of the existing surface elasticity models, we develop a modified core-shell (MC-S) model incorporating into the surface elasticity effect. By the modified core-shell model, we investigate the effect of surface elasticity on the elastic property of circular nanowires under bending and tension loading modes, and the bending of rectangular nano beams or plates. From the comparisons to relevant experiments and computational results, e.g., molecular statistical thermodynamics (MST), molecular dynamics (MD) simulation, and the existing theoretical models, e.g., core-surface model and core-shell model, it is demonstrated that our surface elasticity model is more reasonable especially when the dimensions of nanostructures reduce to few tens of nanometers; and can also well predict the prominent size effects of nanostructures at small size scale. This study might be helpful for the interpreting various phenomena associated with the size effects of elastic properties of nanostructures.And secondly, using the core-surface model, modified core-shell model, and stress consistency model, respectively, we mainly investigate the influence of surface elasticity on the piezoelectric potential distribution of a deformed ZnO nanowire. From the theoretical analysis, the results show that the values of piezoelectric potential generated in ZnO nanowires are decreased due to the surface stiffening. Furthermore, the effect of surface elasticity has a significant impact on the piezoelectric potential of a bent ZnO nanowire, actually it reduces the gap between theoretical estimation and experiment measurements, and provide a more closed theoretical result to the experimental data. Therefore, it accounts for that the models considering the surface effects are more reasonable especially for the dimensions of nanostructures below100run. This study may provide a valuable investigation on the recognizing the surface effects on the piezoelectric properties of nanostructures.Finally, based on the modified core-shell model, combining the buckling theory of Euler-Bernoulli beam, we investigate the surface effects of, surface elasticity and surface stress (i.e., residual surface tension), on the buckling behaviors of nanowires under uniaxial compression load. Two boundary conditions of nanowires, a fixed-fixed nanowire and a fixed-free nanowire, are mainly examined. The theoretical solutions indicate that the surface elasticity affects the buckling behaviors of the nanowires significantly, especially as the dimension of the nanowire decreases. In addition, the comparison of the effects between the surface elasticity and residual surface tension on the buckling of the nanowire is also discussed. Since the residual surface tension can induce a transverse load along the nanowire, the impact of residual surface tension on the buckling of the nanowire is much stronger than that of surface elasticity. This study might be in favor of understanding the buckling behaviors of nanobeams in nanodevices.