World's Smallest Universal Material Testing System

In the world of nanotechnology, however, where the mechanicalcharacterization of materials and structures takes place on the scaleof atoms and molecules, the existing material testing systems areuseless. The development of a universal nanoscale material testingsystem (n-MTS), which could fit in existing electron microscopes(instruments that can magnify images approximately one million times)and possess the resolution and accuracy needed to mechanically testnanoscale objects, has been a major challenge within the scientificcommunity.

Now researchers at Northwestern University have designed andbuilt the first complete micromachine that makes possible theinvestigation of nanomechanics phenomena in real time. The findings arepublished online this week by the Proceedings of the National Academyof Sciences (PNAS). The machine, which can fit in tiny spaces asrequired by in situ transmission electron microscopy (TEM),successfully characterized the mechanical properties of nanowires andcarbon nanotubes.

The n-MTS developed by Horacio D. Espinosa, professor ofmechanical engineering, and his colleagues consists of an actuator anda load sensor fabricated by means of micro technology (a derivative ofthe computer industry). The load sensor is based on differentialcapacitive sensing, which provides a load resolution of about 10 nanoNewtons. This is the first nanoscale material testing system thatprovides continuous observation of specimen deformation and failurewith sub-nanometer resolution while simultaneously measuringelectronically the applied forces with nano-Newton resolution. Theintegration of electro-mechanical and thermo-mechanical components atthe micro scale made the achievement possible.

One of the challenges overcome by the University researcherswas the integration of micro-electro-mechanical systems (MEMS) andcircuits for measurement of electronic signals. They solved thisproblem by using a double-chip architecture consisting of a MEMS chipand a microelectronic sensing chip.

Another challenge overcome by the team was the mounting ofindividual nanostructures on the testing device. Using ananomanipulator inside a dual-beam scanning electron microscope andfocused ion beam apparatus (a new tool available to nanoscientists) theresearchers picked up nanostructures, cut them to the desired lengthand nanowelded the structures onto the n-MTS usingelectron-beam-induced deposition of platinum.

As reported in the PNAS paper, the system capabilities weredemonstrated by in situ electron microscopy testing of free-standingpolysilicon films, metallic nanowires and carbon nanotubes (CNTs).Espinosa's team achieved the first real-time instrumented in situtransmission electron microscopy observation of CNTs failure undertensile loading.

In 1959, Nobel Laureate Richard Feynman delivered a talk atthe California Institute of Technology entitled "There is Plenty ofRoom at the Bottom" in which he envisioned the possibility of makingvery small machines. "Our MEMS-based nanoscale material testing systemrepresents another milestone along the path of miniaturizationanticipated by Feynman," said Espinosa. "We expect it will have asimilar impact and produce the same level of opportunities as thedevelopment of the universal testing machine had in the last century."

The n-MTS can be potentially applied to characterize themechanical, thermal and electro-mechanical properties not only ofnanowires and nanotubes but also of a large number of organicmaterials, including DNA, proteins and nanofibers.

In addition to Espinosa,the other author on the PNAS paper is graduate student Yong Zhu. Theresearch was supported by the National Science Foundation under AwardsNo. DMR-0315561 and CMS 0120866. Nanomanipulation was carried out inthe Center for Microanalysis of Materials at the University ofIllinois, which is partially supported by the U.S. Department of Energyunder grant DEFG02-96-ER45439.