A rule-breaking, colorful silicone that could conduct electricity

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A newly discovered silicone variant is a semiconductor, University of Michigan researchers have discovered—upending assumptions that the material class is exclusively insulating. 

"The material opens up the opportunity for new types of flat panel displays, flexible photovoltaics, wearable sensors or even clothing that can display different patterns or images," said Richard Laine, U-M professor of materials science and engineering and macromolecular science and engineering and corresponding author of the study recently published in Macromolecular Rapid Communications.

Silicone oils and rubbers—polysiloxanes and silsesquioxanes—are traditionally insulating materials, meaning they resist the flow of electricity or heat. Their water-resistant properties make them useful in biomedical devices, sealants, electronic coatings and more.

Meanwhile, conventional semiconductors are typically rigid. Semiconducting silicone has the potential to enable the flexible electronics Laine described as well as silicone that comes in a variety of colors.

On a molecular level, silicones are made up of a backbone of alternating silicon and oxygen atoms (Si—O—Si) with organic (carbon-based) groups attached to the silicon. Various 3D formations of polymer chains arise as they connect to one another, known as cross-linking, which alter the material's physical properties like strength or solubility. 

While studying different cross-linking structures in silicone, the research team stumbled upon the potential for electrical conductivity in a copolymer, which is a polymer chain containing two different types of repeating units—cage-structured and then linear silicones in this case. 

The possibility for conductivity arises from the way electrons can move across Si—O—Si bonds with overlapping orbitals. Semiconductors have two main states: the ground state, which doesn't conduct electricity, and a conducting state, which does. The conducting state, also known as an excited state, occurs when some electrons jump up to the next electron orbital, which is connected across the material like a metal.

Typically, Si—O—Si bond angles don't allow for that connection. At 110°, they are a long way from a 180° straight line. But in the silicone copolymer the team discovered, these bonds started out at 140° in the ground state—and they stretch to 150° in the excited state. This was enough to create a highway for electrical charge to flow.

"This allows an unexpected interaction between electrons across multiple bonds including Si—O—Si bonds in these copolymers," Laine said. "The longer the chain length, the easier it is for electrons to travel longer distances, reducing the energy needed to absorb light and then emit it at lower energies."

The semiconducting properties of the silicone copolymers also enable its spectrum of colors. Electrons jump between the ground and excited states by absorbing and emitting photons, or particles of light. The light emission depends on the length of the copolymer chain, which Laine's team can control. Longer chain lengths mean smaller jumps and lower energy photons, giving the silicone a red tint. Shorter chains require bigger jumps from the electrons, so they emit higher energy light toward the blue end of the spectrum.

To demonstrate the connection between chain length and light absorption and emission, the researchers separated copolymers with different chain lengths and arranged them in test tubes from long to short. Shining a UV light on the tubes creates a full rainbow as each absorbs and emits the light at different energies. 

The colorful array based on copolymer chain length is particularly unique because up to this point, silicones have only been known to be transparent or white because their insulating properties make them unable to absorb much light.

"We're taking a material everyone thought was electrically inert and giving it a new life—one that could power the next generation of soft, flexible electronics," said Zijing (Jackie) Zhang, U-M doctoral student of materials science and engineering and lead author of the study. 

The research was funded by the U.S. National Science Foundation (2103628) and the Thailand National Science, Research and Innovation Fund (NSRF) via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation (B16F640099).

Study: σ–σ* conjugation Across Si─O─Si Bonds (DOI: 10.1002/marc.202

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