Deformation and Fracture of Metallic Nanostructures - In-situ TEM Experiments and Atomistic Models

该奖项是根据2009年美国复苏和再投资法案(公法111-5)资助的。技术摘要：开发了新的实验技术来研究一维金属纳米结构的尺寸尺度塑性。此外，还提出了实验的原子化模型，以追求变形场和断裂的一对一比较。这项研究包括在透射电子显微镜上对一维面心立方晶体纳米结构进行原位拉伸实验，用高分辨电子显微镜对其原子结构进行了表征。测试样品的原子尺度建模将用于验证现有的EAM势。将制定一个计算框架，以确定金属面心立方纳米结构在给定初始原子结构的情况下在单轴载荷下屈服和失效的标准。通过采用以前开发的基于微电子机械系统(MEMS)和驱动的纳米材料测试系统，使用样品制造和操作协议，将克服测试合适尺寸和原子结构的样品的限制。位错成核和/或激活将被识别并以原子模型进行比较。为了解决时间和空间分辨率问题，将进行高速瞬变电磁成像。同样，为了捕捉材料的不稳定性和失效，MEMS技术将被扩展到实现具有反馈控制的载荷传感，从而确保在位移控制下进行测试。原子模型的边界条件将以位移场的形式施加到原子模型上，这些位移场是从实验期间拍摄的电子显微镜图像中获得的。然后将预测的原子重排与不同应变水平下的透射电子显微镜观测结果直接进行比较。然后，将对纳米线的变形进行建模，包括实验观察到的表面和内部缺陷。将进行一项系统的研究，以开发一种能够预测纳米线强度和失效的标准，给定其尺寸和原子结构。虽然非常具有挑战性，但如果成功，这项研究将成为寻求在原子层面上连接实验和模拟的重要一步。非技术综述：一维纳米结构被设想为下一代电子和传感器的关键部件，要么是纳米机电系统，要么是互连。在该项目下制定的议定书将能够确定在这些系统的设计中必不可少的机械和电气特性，并将影响在设计中使用的计算工具的开发。从长远来看，本项目开发的实验-计算相结合的方法可以用于研究半导体纳米线、场效应晶体管和其他纳米系统中的机电耦合。电性能的应变依赖、表面重建和分子之间的相互作用，这些都是纳米级传感器和器件设计中必不可少的重要现象，然后可以按照同样的想法进行研究。该项目的教育和推广部分将侧重于通过西北大学NSF-NSEC的现有计划，为本科生和少数族裔学生提供参加为期9周的暑期实习的机会。同样，PI将在他在西北大学任教的微/纳米科学与工程双层课程实验中增加一个关于纳米线的原位扫描电子显微镜纳米操纵和测试的实验室。PI还将为他的研究生水平课程纳米工程特别主题开发使用EAM类势和反应力场的原子/量子建模课程材料。

This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Novel experimental techniques are developed to investigate size-scale plasticity in one dimensional metallic nanostructures. Additionally, atomistic models of the experiments are proposed to pursue one-to-one comparison of deformation fields and fracture. The proposed research involves in-situ tensile experiments in the transmission electron microscope (TEM) on one-dimensional fcc crystal nanostructures which are characterized in terms of their atomic structure by high-resolution TEM. Atomic scale modeling of tested specimens will be used to validate existing EAM potentials. A computational framework will be formulated to identify criteria for yielding and failure of metallic fcc nanostructures under uniaxial loading given their initial atomic structure. Limitations in testing specimens of suitable size and atomic structure will be overcome by employing a previously developed nanoscale-material testing system based on micro-electro-mechanical systems sensing (MEMS) and actuation, using protocols for sample fabrication and manipulation. Dislocation nucleation and/or activation will be identified and modeled atomistically for comparison. In order to address temporal and spatial resolution issues, high-speed TEM imaging will be carried out. Likewise, in order to capture material instabilities and failure, the MEMS technology will be extended to achieve load sensing with feedback control, which will ensure testing under displacement control. Boundary conditions on the atomistic models will be imposed in the form of displacement fields obtained from TEM images taken during the experiments. The atomic rearrangement predicted will then be directly compared with the TEM observations at different strain levels. Deformation of the nanowires will then be modeled including the surface and internal defects experimentally observed. A systematic study will be carried out to develop a criterion capable of predicting nanowire strength and failure, given its size and atomic structure. Although very challenging, if successful this research will constitute a major step in the quest for connecting experiments and simulations at the atomic level. NON-TECHNICAL SUMMARY:One dimensional nanostructures are envisioned as key components in the next generation of electronics and sensors as either nanoelectromechanical systems or interconnects. The protocols developed under this project will allow the identification of mechanical and electrical properties that are essential in the design of these systems and will also impact the development of computational tools used in their design. In the long term, the combined experimental-computational approach developed under this project could be employed in the study of electro-mechanical coupling in semiconducting nanowires, field effect transistors, and other nanosystems. The strain dependence of electrical properties, surface reconstruction and interaction between molecules, all important phenomena essential in the design of nanoscale sensors and devices, could then be investigated following the same ideas. The educational and outreach component of this project will focus on providing opportunities to undergraduate and minority students, through existing programs within the NSF-NSEC at Northwestern University, to participate in 9-week summer internships. Likewise, the PI will add a lab on in-situ SEM nanomanipulation and testing of nanowires in the dual level course Experiments in Micro/Nano Science and Engineering he teaches at Northwestern University. The PI will also develop course materials on atomistic/quantum modeling using EAM type potentials and reactive force fields for his graduate level course Special Topics in Nano Engineering.