Mead James L, Wang Shiliang, Zimmermann Sören, Huang Han
School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia.
Nanoscale. 2020 Apr 21;12(15):8237-8247. doi: 10.1039/d0nr01261k. Epub 2020 Apr 1.
It is imperative to understand the interfacial adhesive behaviour of nanowires (NW) integrated into a nanoelectromechanical system in order to design commercialisable nanogenerators as well as ultrasensitive sensors. Currently available interfacial adhesion characterisation techniques that utilise in situ electron microscopy subject nanoscale systems to a high-vacuum, electron-irradiated environment, potentially altering their interfacial interactions. Alternatively, force-sensing techniques conducted in air do not provide visual feedback of the interface, and therefore can only indirectly deduce adhesive properties. Here, we present an interface characterisation technique that enforces ZnO NWs to remain partially delaminated on a Si substrate, and permits optical observation of their deformed condition in air. NWs are draped over a wedge and are allowed to conform to their minimum energy state. We evaluate the strain energy stored in the suspended segment of each NW by determining their deflected shape from interferometry. We show that utilising a tailored Euler-Bernoulli beam model which accounts for the tapering and irregularity of a NW is crucial for accurately evaluating their interfacial adhesion energy. A nominal energy per unit interface area value of [capital Gamma, Greek, macron] = 51.1 ± 31.9 mJ m is obtained for the ZnO NW-Si substrate interface; a magnitude lower than that found using electron microscopy, and higher than the upper-bound of the theoretically predicted van der Waals interaction energy of γ = 7.2 mJ m. This apparent discrepancy has significant implications for any nanotribological study conducted inside an electron microscope. The results also implicate electrostatic and capillary interactions as significant contributors towards a NW's adhesive behaviour during device operation.
为了设计可商业化的纳米发电机以及超灵敏传感器，了解集成到纳米机电系统中的纳米线（NW）的界面粘附行为至关重要。目前可用的界面粘附表征技术利用原位电子显微镜，使纳米级系统处于高真空、电子辐照环境中，这可能会改变它们的界面相互作用。另外，在空气中进行的力传感技术无法提供界面的视觉反馈，因此只能间接推断粘附特性。在此，我们提出一种界面表征技术，该技术使ZnO纳米线在硅衬底上保持部分分层，并允许在空气中对其变形状态进行光学观察。纳米线被搭在一个楔子上，并使其符合其最低能量状态。我们通过干涉测量确定每个纳米线的偏转形状，来评估存储在其悬浮段中的应变能。我们表明，使用考虑纳米线的锥形和不规则性的定制欧拉 - 伯努利梁模型对于准确评估其界面粘附能至关重要。对于ZnO纳米线 - 硅衬底界面，获得的单位界面面积的标称能量值为[大写伽马，希腊字母，长音符号] = 51.1 ± 31.9 mJ/m²；该值低于使用电子显微镜测得的值，且高于理论预测的范德华相互作用能γ = 7.2 mJ/m²的上限。这种明显的差异对在电子显微镜内进行的任何纳米摩擦学研究都具有重要意义。结果还表明，静电和毛细管相互作用是器件运行期间纳米线粘附行为的重要贡献因素。
