金属纳米线中的拍频现象及其对基于共振的弹性性质测量的意义
Beat Phenomena in Metal Nanowires, and their Implications for Resonance-Based Elastic Property Measurements
摘要：
一维纳米结构如纳米线的弹性性质通常通过在其共振频率下致动纳米线，然后使用基本梁理论将共振频率与弹性刚度相关联来实验测量。在本工作中，我们利用大规模分子动力学模拟报告了一种新的拍现象在[110]取向的银纳米线。拍现象是由[110]纳米线的正交基本方向上的晶格间距的不对称性引起的，即，[-110]和[001]方向，这导致两个不同的主惯性矩。因此，发现沿任何其他方向施加的致动基于相对于这两个基本方向的致动角分解成两个正交振动分量，该现象可推广到<110>不同材料的FCC纳米线（Cu、Au、Ni、Pd和Pt）。使用基于硬球假设的离散转动惯量模型来解释拍现象，该模型被用来显示，表面效应增强拍现象，而效果随着纳米线的横截面尺寸或纵横比的增加而减小。最重要的是，由于拍现象的存在，我们证明了在共振实验中，只有一个单一的频率分量预计将被观察到，此外，对于大范围的致动角，较低的频率比较高的频率更可能被检测到，这意味着从共振获得的杨氏模量的实验预测实际上可能低于预测。因此，本研究具有显着的影响，通过共振测试获得的杨氏模量的实验解释。
英文摘要：
The elastic properties of 1D nanostructures such as nanowires are often measured experimentally through actuation of the nanowire at its resonance frequency, and then relating the resonance frequency to the elastic stiffness using elementary beam theory. In the present work, we utilize large scale molecular dynamics simulations to report a novel beat phenomenon in [110] oriented Ag nanowires. The beat phenomenon is found to arise from the asymmetry of the lattice spacing in the orthogonal elementary directions of the [110] nanowire,i.e., [-110] and [001] directions, which results in two different principal moments of inertia. Because of this, actuations imposed along any other direction are found to decompose into two orthogonal vibrational components based on the actuation angle relative to these two elementary directions, with this phenomenon being generalizable to <110> FCC nanowires of different materials (Cu, Au, Ni, Pd and Pt).The beat phenomenon is explained using a discrete moment of inertia model based on the hard sphere assumption, the model is utilized to show that surface effects enhance the beat phenomenon, while the effect is reduced with increasing nanowires cross-sectional size or aspect ratio.Most importantly, due to the existence of the beat phenomena, we demonstrate that in resonance experiments only a single frequency component is expected to be observed, particularly when the damping ratio is relatively large or very small.Furthermore, for a large range of actuation angles, the lower frequency is more likely to be detected than the higher one,which implies the experimental predictions of Young's modulus obtained from resonance may in fact be under predictions. The present study therefore has significant implications for experimental interpretations of Young's modulus as obtained via resonance testing.
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