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Next-generation electronics could self-heal

The crystalline structure: this video shows colloidal beads (bright dots) that have assembled themselves on a liquid droplet to form a 3D, curved crystalline structure. The positive electric charges cause the beads to repel each other, leading them to arrange themselves in a honeycomb pattern. Each particle is equally distant from six others. Image courtesy William Irvine/U. Chicago.

Researchers have modeled and experimentally created a crystal capable of healing itself. The research has the potential to significantly improve conductive electronics, such as graphene, a two-dimensional honeycomb structure that conducts electricity 100 times faster than silicon, the material of choice in modern computer chips today.

In their paper, published in the journal Nature Materials, the researchers who specialize in ‘soft matter’ science, which is the study of a wide range of semi-solid substances, such as gels, foams, and liquid crystals, inserted an extra particle – also known as an interstitial –  in a single-layer crystal, creating a defect. Then, they watched a video of the structure heal itself: the key to this organic behavior was that the entire structure was curved.

Mark Bowick, a physicist at Syracuse University, New York, US, and his colleagues used computer models to predict that an extra particle added halfway between two scars on the crystal would create a defect that splits into two. Then, the strain on the crystal caused by these two defects would flow away, like ripples on a pond, as the particles rearrange their distances. Finally, the defects would migrate to opposite scars and disappear, and the original defect would be gone. Residual stress in the regions in-between scars drives the self-healing.

Interactive Data Language was used to determine particle locations and custom Matlab computer code identified defects.

Healing a defect: This video shows the hexagonal crystal pattern that naturally occurs when the crystalline structure is formed. The regular six-sided pattern imperfectly fits around the spherical droplet, so defects appear. Inserting an interstitial particle (black) allows the defect to heal. 

The stresses caused by the extra particle can be seen in the creation of yellow and red shapes.
 Image courtesy William Irvine/U. Chicago.


To test this experimentally and watch what happened, the researchers built a specialized instrument - essentially a microscope on top of a microscope. The first microscope acted as optical tweezers, which use a highly focused laser beam to provide an attractive or repulsive force, to grab the particle and place it on the surface of the crystal. The second microscope created 3D images of the experiment and confirmed what the team's models had predicted.

The healing properties of this structure could have important implications for conductivity of microscopic and electrically conductive materials such as graphene, which is only one atom thick. Even though it’s a very good conductor and incredibly strong, any defects could reduce the conductivity of graphene, so it needs to be as pure as possible. With this knowledge the researchers say that a piece of graphene could be flexed to remove the defects and improve its conductivity.   

Bowick says as his group is working on colloidal systems, these next steps for self-healing graphene development would need to be done by other experts.

- Adrian Giordani

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