November 11, 2009 – Researchers at NC State U. say they have developed a way to measure properties of silicon nanowires using in-situ tensile testing, to quantify the material’s elastic and fracture properties.

Their work, published in the journal Nano Letters, attempts to do a better job of determining the properties of silicon nanowires (specifically those grown using the common vapor-liquid-solid synthesis process), which tantalizingly offer much higher surface-to-volume ratio vs. "bulk" silicon used ubiquitously in electronic devices. Specifically, they chose to "determine how much abuse these silicon nanowires can take" — how much they deform (warp/stretch until breakage), how much force they can absorb before cracking/fracturing, etc., according to project lead researcher Yong Zhu, assistant professor of mechanical engineering at NC State.

Silicon nanowires used in in-situ scanning electron microscopy mechanical testing. (Credit: North Carolina State University)

In-situ tensile testing inside a scanning electron microscope, utilizing a nanomanipulator as the actuator and a microcantilever used as the load sensor, enabled "real-time observation of nanowire deformation and fracture, while simultaneously providing quantitative stress and strain data," noted Qingquan Qin, paper co-author and NC State Ph.D student, adding that the "very efficient" process can test "a large number of specimens […] within a reasonable amount of time."

Their results: While "bulk" silicon is brittle and can’t be stretched or warped very much without breaking, silicon nanowires were able to show far more resilience and sustained "much larger" deformation. Also, as the nanowires get smaller they also show increased fracture strength and decreased elastic modulus. From the Nano Letters abstract:

The Young’s modulus decreased while the fracture strength increased up to over 12 GPa, as the nanowire diameter decreased. The fracture strength also increased with the decrease of the side surface area; the increase rate for the chemically synthesized silicon nanowires was found to be much higher than that for the microfabricated silicon thin films. Repeated loading and unloading during tensile tests demonstrated that the nanowires are linear elastic until fracture without appreciable plasticity.

Such properties "are essential to the design and reliability of novel silicon nanodevices," pointed out Zhu. The work provides a better understanding of size effects on mechanical properties of nanostructures, as well as giving nanodevice designers more options in what they can build, e.g. sensors, electronics, and solar cells.

SEM images of the Si nanowire with 60nm diameter before (a) and after (b) tension test, showing that the carbon deposition clamp was strong enough for testing Si nanowires with diameters up to 60nm without sliding. (Source: Nano Letters)