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。技术综述：发展了新的实验技术来研究一维金属纳米结构的尺寸塑性。此外，还提出了实验的原子模型，以追求变形场和断裂的一对一比较。本研究采用高分辨率透射电镜（TEM）对一维fcc晶体纳米结构进行原位拉伸实验，对其原子结构进行表征。测试样品的原子尺度模型将用于验证现有的EAM电位。在给定初始原子结构的单轴载荷下，本文将建立一个计算框架来确定金属fcc纳米结构的屈服和破坏准则。通过采用先前开发的基于微机电系统传感（MEMS）和驱动的纳米材料测试系统，使用样品制造和操作协议，可以克服测试合适尺寸和原子结构的样品的局限性。位错成核和/或活化将被识别和原子模拟以供比较。为了解决时间和空间分辨率问题，将进行高速瞬变电磁法成像。同样，为了捕获材料的不稳定性和故障，MEMS技术将扩展到实现带有反馈控制的负载敏感，这将确保在位移控制下进行测试。原子模型的边界条件将以实验中从TEM图像中获得的位移场的形式施加。预测的原子重排将直接与不同应变水平下的透射电镜观察结果进行比较。然后将模拟纳米线的变形，包括实验观察到的表面和内部缺陷。将进行系统的研究，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，但如果这项研究成功，将构成在原子水平上连接实验和模拟的重要一步。非技术概述：一维纳米结构被设想为下一代电子和传感器的关键部件，无论是纳米机电系统还是互连。根据该项目制定的协议将允许识别这些系统设计中必不可少的机械和电气特性，并且还将影响其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-计算结合方法可用于半导体纳米线、场效应晶体管和其他纳米系统的机电耦合研究。电学性质的应变依赖性、表面重建和分子间的相互作用，以及纳米级传感器和器件设计中必不可少的所有重要现象，都可以按照同样的思路进行研究。该项目的教育和推广部分将侧重于通过西北大学NSF-NSEC的现有项目，为本科生和少数民族学生提供参加为期9周的暑期实习的机会。同样，PI将在他在西北大学教授的微/纳米科学与工程双级课程实验中增加一个原位SEM纳米操作和纳米线测试实验室。PI还将为他的研究生课程“纳米工程专题”开发使用EAM型电位和反作用力场的原子/量子建模的课程材料。

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.