Flexible electronics, which could be used to control flexible robots, depend on the ability to produce electrical circuits that can be repeatedly stretched and bent while remaining operational. Silicon is obviously one of the most important building blocks of modern electronics, but even when it's shaped into wires, it isn't very stretchy.

Recently, theoretical calculations have indicated that it may be possible to stretch silicon nanowire by as much as 23 percent, depending on its structure and the stretch direction. This raises an obvious question: why haven't we been able to do so?

Recently, an international team of scientists and engineers has directly probed the elastic strain limit of single-crystalline Si nanowires. The team found that stretching the Si nanowires almost to their theoretical limit is possible.

Testing Si nanowires

In their investigation, the researchers grew single-crystalline Si nanowires that were about 100nm in diameter using a method called "vapor-liquid-solid." In this method, a liquid is placed onto a solid surface and then exposed to a vapor that can dissolve in it. Once the material in the vapor reaches supersaturation, crystals can grow at the liquid-solid interface. This method is mostly notable for the speed with which the crystals are formed.

The team used a device able to evaluate mechanics at the micro-scale and measured results using a device called a nanoindenter. This tool allowed them to pull the wires in one direction and quantitatively evaluate the resulting deformation. At room temperature, the silicon nanowires could be repeatedly stretched above 10 percent elastic strain. A few samples came startlingly close to the theoretical elastic limit, reaching 17-20 percent and exhibiting roughly 16-percent tensile strain.

For further understanding of the stretching limitations, the scientists performed loading-unloading tests with varied strain rates. They found that the deformations were fully reversible and that the wires weren't affected by the order of loading and unloading stress (technically, this is called "hysteresis free"). They also found that the nanowires exhibited brittle fractures rather than plastic deformation.

These findings suggest that the Si nanowires exhibited pure elastic deformation right up to the point that they failed via a brittle fracture. Most materials exhibit some plastic deformation prior to fracture, so this behavior is unusual.

The scientists think they were able to approach the theoretical limit on stretching because the nanosized single-crystal structure had few defects and was atomically smooth.

How do these findings affect the future of silicon nanowires?

This team demonstrated that it's possible to stretch a silicon crystal lattice and shift its atoms slightly out of their typical equilibrium positions. This type of "elastic strain engineering" can result in dramatically different material properties. As an example, the driving force to pull these atoms back to their equilibrium position may be high enough to alter the chemical reactivity. Maybe some day we could use this to get silicon to engage in some unusual chemistry.

But the work also shows that, even under close-to-ideal conditions, stretching silicon will be limited and prone to failure. If we want bendy electronics, we might have to look elsewhere.

Science Advances, 2016. DOI: 10.1126/sciadv.1501382 (About DOIs).