Silicon Nanowires Could Stretch Up to 23% Farther than Previously Thought

New advancements in finding the ideal elastic strain limit in silicon nanowires are showing that silicon can be stretched up 23% farther than previously thought possible.

Flexible electronics depend on stretchable electrical circuits that can bend while keeping their functionality. They are created by mounting electronic devices on flexible plastic substrates, such as silicon. Silicon is essential to any electronics you see today, but if you made silicon into wires, the elasticity of it isn't very high.

 

Purified silicon.

 

If we were able to transform a stiff circuit board that is loaded with chips and other electronic parts into something flexible enough to bend or fold, we could transform the way we create and use electronic devices.

Flexible electronic devices have a tremendous potential for use on as well as in the human body due to their stretchability and flexibility. Another application is photovoltaics; curved photovoltaic panels could provide to be much more useful than today's photovoltaic electricity generation. 

A research team has created silicon that has a very high elasticity, which is ideal for the types of electronic devices that were just described. 

 

Silicon nanowires as viewed through a scanning electron microscope. Image courtesy of nanotechweb.org.

 

Just recently, they've found that their theoretical calculations have pointed towards the ability to stretch silicon nanowire up to 23% farther than before. However, this is all dependent on the silicon material's structure and the direction in which stretching is desired. The team has discovered that it is possible to stretch the Si nanowire to its theoretical limit before breaking through the material directly.

Stretching the Possibility of Flexible Electronics

Hongti Zhang and his colleagues performed a series of experiments at room temperature (300 K) at the City University of Hong Kong and grew monocrystalline Si (single-crystal silicon) using the vapor-solid-liquid method. The vapor-liquid-solid method (PDF), also known as VLS, is used for growing one-dimensional structures, which silicon nanowire is.

Generally speaking, growing any type of crystal through direct adsorption of a gas phase onto a solid surface is very slow. The VLS method bypasses this by adding a catalytic liquid alloy phase which will absorb a vapor to a supersaturation level rapidly. After achieving a supersaturation level, the Si crystal can be formed from nucleated seeds at the liquid-solid interface. The size and physical properties that form are dependent upon the characteristics of the nanowires. Below is an illustration depicting the silicon nanowires synthesis using the VLS method.

 

The VLS method of silicon nanowire synthesis. Image courtesy of ResearchGate.

 

After achieving Si nanowire, the research team used a nanoindenter—a device used in nanoindentation that records small load and displacements with high accuracy and precision—to calculate results of their nanowire and a micro-scale. They used the nanoindenter to pull the nanowires in a single direction and then measure the wire's deformation.

They found that the nanowire was able to sustain 10% elastic strain multiple times while at room temperature. Also, they found that a few of their samples were able to approach the theoretical elastic limit of silicon, which is 17-20%!

The research team then went a step further with their research to better understand how the wire was deforming and why. They then applied transmission electron microscopy (TEM) analysis, which transmits a beam of electrons through the nanowire to test the deformations. After loading-unloading tests with TEM analysis, they were able to confirm that the deformation of the silicon nanowire was completely elastic.

The team also found that during the loading and unloading tests, the nanowire could still recover its original length instantly after unloading—without any signs of plastic deformations.

Below is a video of a nanowire in the process of being stretched.

What does this research point to?

It's astonishing how close the team was able to come to silicon's theoretical elastic strain limit. These results are going to be of high importance practically and for fundamental concepts for future developments. Not only will it be important for building new electronic devices, but also for applications such as nanomembranes and photovoltaics.

See more research leading up to nanowires on MIT's "Ultra-Strength Materials & Elastic Strain Engineering" page.