Flexible electronics, which can 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 clearly one of the most important building blocks of modern electronics, but even when formed into wires it is not very stretchy.
Recently, theoretical calculations have shown that it is possible to stretch silicon nanowire by as much as 23 percent, depending on the structure and direction of stretch. This begs an obvious question: why haven’t we been able to do this?
Recently, an international team of scientists and engineers directly investigated the elastic stress limit of single-crystalline Si nanowires. The team found that it is possible to stretch the Si nanowires almost to their theoretical limit.
Test Si nanowires
In their study, the researchers grew single-crystalline Si nanowires about 100 nm in diameter using a method called “vapour-liquid solid.” In this method, a liquid is placed on 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 particularly notable for the speed at which the crystals are formed.
The team used a device that could evaluate microscale mechanics and measured the 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 stretched repeatedly to more than 10 percent elastic load. A few samples came surprisingly close to the theoretical elastic limit, reaching 17-20 percent and showing about 16 percent tensile stress.
To better understand the strain limitations, the scientists performed charge-discharge tests at different strain rates. They found that the deformations were completely reversible and that the wires were unaffected by the order of loading and unloading (technically referred to as “hysteresis-free”). They also found that the nanowires showed brittle fractures rather than plastic deformation.
These findings suggest that the Si nanowires exhibited pure elastic deformation to the point of failure via a brittle fracture. Most materials show some plastic deformation prior to fracture, so this behavior is unusual.
The scientists think they were able to approach the theoretical limit of stretching because the monocrystalline structure had few defects and was atomically smooth.
How do these findings affect the future of silicon nanowires?
This team showed that it is possible to stretch a silicon crystal lattice and move the atoms slightly out of their typical equilibrium positions. This type of “elastic strain engineering” can result in dramatically different material properties. For example, the driving force to pull these atoms back to their equilibrium position may be high enough to alter chemical reactivity. Maybe one day we can use this to make silicon take on an unusual chemistry.
But the work also shows that, even under near-ideal conditions, silicon stretching will be limited and prone to failure. If we want flexible electronics, we may have to look elsewhere.
Scientific progress2016. DOI: 10.1126/sciadv.1501382 (About DOIs).