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Horacio D. Espinosa

Northwestern researchers build world’s smallest
universal material testing system

The design, development, and manufacturing of revolutionary products such as the automobile, the airplane, and the computer owe a great deal of their success to the large-scale material testing systems that have provided engineers and designers with a fundamental understanding of the mechanical behavior of various materials and structures.

In the world of nanotechnology, however, where the mechanical characterization of materials and structures takes place on the scale of atoms and molecules, existing material testing systems are useless. The development of a universal nanoscale material testing system (n-MTS), which could fit in existing electron microscopes (instruments that can magnify images approximately 1 million times) and possess the resolution and accuracy needed to mechanically test nanoscale objects, has been a major challenge within the scientific community.

Now researchers at Northwestern have designed and built the first complete micromachine that makes possible the investigation of nanomechanics phenomena in real time. The findings were published last fall in PNAS (Proceedings of the National Academy of Sciences of the United States of America). The machine, which can fit in tiny spaces as required by in situ transmission electron microscopy, successfully characterized the mechanical properties of nanowires and carbon nanotubes.

The n-MTS developed by Horacio D. Espinosa, professor of mechanical engineering, and his colleagues consists of an actuator and a load sensor fabricated by means of microtechnology, a derivative of the computer industry. The load sensor is based on differential capacitive sensing, which provides a load resolution of about 10 nanonewtons. This is the first n-MTS that provides continuous observation of specimen deformation and failure with subnanometer resolution while simultaneously measuring electronically the applied forces with nanonewton resolution. The integration of electro- and thermomechanical components at the microscale made the achievement possible.

One of the challenges overcome by the researchers was the integration of micro-electro-mechanical systems (MEMS) and circuits for measurement of electronic signals. They solved this problem by using a double-chip architecture consisting of a MEMS chip and a microelectronic sensing chip. Another challenge was the mounting of individual nanostructures on the testing device. Using a nanomanipulator inside a dual-beam scanning electron microscope and focused ion beam apparatus (a new tool available to nanoscientists), the researchers picked up nanostructures, cut them to the desired length, and nanowelded the structures onto the n-MTS using electron-beam–induced deposition of platinum.

As reported in the PNAS paper, the system capabilities were demonstrated by in situ electron microscopy testing of free-standing polysilicon films, metallic nanowires, and carbon nanotubes. Espinosa’s team achieved the first real-time instrumented in situ transmission electron microscopy observation of carbon nanotube failure under tensile loading.

In 1959 Nobel laureate Richard Feynman delivered a talk at the California Institute of Technology titled “There is Plenty of Room at the Bottom,” in which he envisioned the possibility of making very small machines. “Our MEMS-based nanoscale material testing system represents another milestone along the path of miniaturization anticipated by Feynman,” said Espinosa. “We expect it will have a similar impact and produce the same level of opportunities as the development of the universal testing machine had in the last century.”

The n-MTS can potentially be applied to characterize the mechanical, thermal, and electromechanical properties not only of nanowires and nanotubes but also of a large number of organic materials, including DNA, proteins, and nanofibers.

Espinosa’s coauthor on the PNAS paper was graduate student Yong Zhu. Their research was supported by the National Science Foundation.

—Megan Fellman