Sep 20, 2005
Nanostructure strength goes under the microscope
Researchers at Northwestern University, US, have made a microelectromechanical systems (MEMS) device that enables mechanical testing of nanostructures while they are under constant observation in a scanning or transmission electron microscope (SEM or TEM). By using the technique on multiwalled carbon nanotubes, they discovered that the crystalline nanotubes became amorphous on tensile failure.
"Since existing in situ scanning and transmission electron-microscopy testing methodologies lacked simultaneous and independent measurement of load and deformation with adequate resolution," Horacio Espinosa of Northwestern University told nanotechweb.org, "we decided to use MEMS technology to create the smallest mechanical testing machine in the word that could fit in situ TEM holders."
The MEMS device contained a thermal actuator to apply a displacement to the specimen, and a load sensor based on capacitance. For use in a TEM the team etched a window in the back of the device.
"The challenges were to design and engineer a tiny electro-thermo-mechanical system that could provide the same versatility as current large-scale testing technology but at the same time achieve several orders of magnitude better resolution in load and displacement measurements," said Espinosa. "That required management of electronic noise, new microfabrication strategies, and optimal microstructural design."
The researchers used the device in a SEM to test the mechanical properties of freestanding polysilicon films and palladium nanowires and in a TEM on multiwalled carbon nanotubes.
"The testing micro-machine offers the same degree of versatility as large-scale ones," said Espinosa. "For instance, tests can be performed under displacement control to examine the onset and evolution of material instabilities such as shear localization, phase transitions and damage."
Two samples of polysilicon film had a Young's modulus of 155±5 GPa and failure strengths of 0.7 and 1.42 GPa, respectively. The palladium nanowires, which were 200 nm in diameter and 20-30 μm long, had a Young's modulus of 99.4±6.6 GPa and a fracture stress of 1.5 GPa. The multiwalled nanotubes, meanwhile, which had an outer diameter of 130 nm and inner diameter of 99 nm, had a fracture strength of 15.84 GPa and a failure strain of 1.56%.
Observation of the nanotubes by TEM during their failure revealed that the graphite shell structure disappeared once the tubes had broken. The post-failure structure consisted of nanoparticles of platinum embedded in an amorphous matrix of carbon. The scientists believe that a thin layer of amorphous platinum coated the nanotubes and transformed to a crystalline phase during the straining of the nanotubes. The crystalline carbon making up the nanotubes, on the other hand, became amorphous.
"The technology we developed can be applied to study nanoscale phenomena in general, including mechanical strength, and electrical and thermal transport properties of materials," said Espinosa. "This is of relevance, for instance, in nanoscale sensor technology and in the design of the next generation of electronics materials and devices, including their thermal management."
Now the scientists plan to examine the electromechanical properties of nanowires made from various materials. "We also plan to extend the device to study thermal transport in one-dimensional nanostructures as well as the properties of biomolecules," said Espinosa. Such biomolecules could include DNA and proteins.
The researchers reported their work in PNAS.
About the author
Liz Kalaugher is editor of nanotechweb.org.