IMR: Development of a TEM Testing Stage with Atomic Position Resolution for Student Training, Education, and Research

理想的、无缺陷的结晶固体的最终内聚强度长期以来一直是一个有趣的理论抽象：“真正的”材料由于永远存在的缺陷而永远不会表现出理想的强度。然而，纳米结构的机械失效在定性和定量上都不同于块状材料的失效行为，因此提供了将“真实”与“理想”联系起来的巨大机会。纳米结构的合成、表征和建模的最新发展保证了一种仪器的开发，这种仪器将与高分辨率透射电子显微镜结合使用，并将能够对断裂进行全面的研究，例如：半导体和绝缘体纳米线，以及单壁碳纳米管。提出了一种基于TEM mems的亚埃级分辨率机械加载平台。该仪器的硬件由五个部分组成：一个柔性传动机构、一个运动致动器、一个MEMS试样夹（coupon）、一个TEM夹和一个位置控制系统。该仪器将允许研究由于其新颖结构而具有基础兴趣的材料系统，以及由于其电学，热学和机械性能而具有实际重要性的材料系统。我们建议(i)开发和制造这种新的仪器；（ii）开发在测试平台上配置纳米线和纳米管的可靠方法；（iii）对纳米结构的力学进行初步实验测量以验证系统性能；（iv）与多尺度理论和建模专家进行互动，结合电子结构、分子力学和连续介质力学计算。对所提议的仪器及其所能执行的测量有强烈兴趣的人。该团队具有设计、制造、测试和使用基于TEM mems的测试阶段所需的技能，用于研究材料在机械载荷下的响应，以及具有零到几个原子尺度缺陷的纳米结构的断裂和疲劳。对于硅、锗、掺杂硅和锗纳米线，以及调制成分的纳米线（如“条纹”和核壳结构、TiO2纳米线和单壁碳纳米管）的广泛潜在应用，强调了对纳米线断裂和疲劳的基本理解的重要性。为了解决缺陷、界面、化学环境、循环机械载荷（疲劳）、应变速率和电流的存在对纳米线断裂力学的影响，将产生一种新的仪器和重要的新方法。据设想，纳米线（直径比头发丝小100倍）将用于许多重要的应用，如纳米电子学（作为逻辑、存储器和互连元件），由于其高表面体积比和对表面相互作用的优异灵敏度，作为化学传感元件，在纳米机电系统(NEMS；作为机械部件、机电部件、执行器、应变片、流量传感器等)，在结构复合材料中，单晶纳米线的完美结晶有望赋予卓越的刚度、强度和韧性，并可能用于能量转换装置（如热电元件）。正是由于这些原因，除其他外，了解单晶纳米线的详细力学及其失效行为至关重要。了解纳米线断裂（材料如何断裂）和疲劳（例如，材料如何反复加载，最终会失效）将为其随后在各种机械应力存在的应用中提供重要的知识基础。这项工作将对新型仪器产生强烈影响，这些仪器将在美国得到进一步开发和销售。基于TEM mems的测试阶段（这是一个很小的测试阶段，可以放入透射电子显微镜中，并具有允许其运行的微机电系统组件），以及对单个纳米线的机械响应的研究，将激发世界各地科学家和工程师的想象力，从而点燃纳米结构力学的国际努力。科学家、工程师和普通大众的这种“想象力的捕捉”将意味着，这里概述的工作将为使用聪明的工程技术开发能够在如此精细的控制水平上可控地变形纳米结构的仪器，以及由这种方法产生的机械响应、断裂和疲劳的基础研究提供教科书式的例子。这项仪器开发工作包括一个重要的教育推广计划，包括研究生和博士后研究计划，本科生（包括少数民族）学生和高中教师的暑期研究培训，化学和工程课程中提供的课程材料的补充，与美国国家科学基金会西北大学纳米科学与工程学习与教学中心合作，开发7-12年级的课程。还有一个重要的第二阶段的计划，即向感兴趣的公司进行技术转换，从而确保美国和世界各地的研究人员可以使用这些仪器，以迅速加速它们的使用。这将提高建立纳米线重要机械和机电特性数据库的速度，这也将加速纳米线在重要应用中的使用。

The ultimate cohesive strength of an ideal, defect-free crystalline solid has long been an interesting theoretical abstraction: 'real' materials never exhibit their ideal strengths due to ever-present defects. However, mechanical failure in nanostructures is qualitatively and quantitatively distinct from the failure behavior of bulk materials, thus providing a tremendous opportunity to connect the 'real' to the 'ideal'Recent developments in the synthesis, characterization, and modeling of nanostructures warrant the development of an instrument that will operate in conjunction with high-resolution transmission electron microscopy, and will enable a comprehensive study of fracture in, e.g., semiconductor and insulator nanowires, and single walled carbon nanotubes. A novel TEM MEMS-based mechanical loading stage with sub-Angstrom-level resolution is proposed. The hardware of the proposed instrument consists of five components: a compliant transmission mechanism, a motion actuator, a MEMS specimen holder (coupon), a TEM holder, and a position control system. This instrument will allow study of material systems that are of fundamental interest due to their novel structure, and of practical importance due to their electrical, thermal, and mechanical properties. We propose (i) to develop and fabricate this new instrumentation (ii) to develop robust methods for configuring nanowires and nanotubes onto testing platforms (iii) to perform preliminary experimental measurements of the mechanics of nanostructures to verify system performance, and (iv) to interact with experts in multiscale theory and modeling that combines electronic structure, molecular mechanics and continuum mechanics calculations, who have an intense interest in the proposed instrument and the measurements it can perform. The proposed team has the skills necessary to design, fabricate, test, and use the TEM MEMS-based testing stage, for the study of materials response under mechanical load, and of fracture and fatigue of nanostructures having zero to a few atomic-scale defects. The importance of a fundamental understanding of fracture and fatigue in nanowires is underscored by the broad range of potential applications envisioned for Si, Ge, doped Si and Ge nanowires, as well as nanowires of modulated composition such as 'striped' and core-shell structures, TiO2 nanowires, and single walled carbon nanotubes. A new instrument and important new methods will result, to address the influence of defects, interfaces, chemical environment, cyclic mechanical loading (fatigue), strain rate, and the presence of an electric current, on the fracture mechanics of nanowires. It is envisioned that nanowires (100 times smaller in diameter than a hair) will be used in a host of important applications, such as in nanoelectronics (as logic and memory and interconnect elements), as chemical sensing elements due to their high surface to volume ratio and exceptional sensitivity to surface interactions, in nanoelectromechanical systems (NEMS; as mechanical components, electromechanical components, actuators, strain gauges, flow sensors, others), in structural composites where the crystalline perfection of single crystal nanowires is expected to confer exceptional stiffness, strength, and toughness, and potentially in energy conversion devices (as thermoelectric elements). It is for these reasons, among others, that it is critically important to understand the detailed mechanics of single crystal nanowires and their failure behavior. An understanding of nanowire fracture (how a material breaks) and fatigue (how a material that is repeatedly loaded, for example, eventually will fail) will provide an important base of knowledge for their subsequent use in diverse applications where mechanical stress will be present.This work will have a strong impact on novel instrumentation, which will be further developed and sold in the United States. The TEM MEMS-based testing stage (this is a tiny testing stage that can fit into a transmission electron microscope and has microelectromechanical systems components that allow it to function), and studies of the mechanical response of individual nanowires, will capture the imagination of scientists and engineers around the World, so that an international effort on mechanics of nanostructures will be ignited. This same "capturing of the imagination" of scientists and engineers and the general public will mean that the work outlined here will provide textbook examples of the use of clever engineering to develop instruments that can controllably deform nanostructures at such fine levels of control, and of fundamental studies of mechanical response, fracture, and fatigue that result from such approaches. This instrument development effort includes a significant program in education outreach, including research programs for graduate students and postdoctoral fellows, summer research training for undergraduate (including minority) students and high school teachers, additions to course materials being offered both in chemistry and engineering courses, and curriculum development for grades 7-12 in coordination with the NSF Center for Learning and Teaching in Nanoscale Science and Engineering centered at Northwestern University. There is also a plan for the important second phase of technology transition to interested companies, and thus of follow through to ensure that such instrumentation will be available to researchers in the USA and around the World, for rapid acceleration of their use. This will increase the rate of creation of databases of important mechanical and electromechanical properties of nanowires, which will also accelerate their use in important applications.