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.
