New silicone variant acts as semiconductor

“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 are traditionally insulating materials, which means they resist the flow of electricity or heat. Their water-resistant properties make them especially useful in biomedical devices, sealants, electronic coatings, and more.

Conventional semiconductors are rigid. Semiconducting silicone offers the potential to enable flexible electronics and colourful silicone.

On a molecular level, silicones are made up of a backbone of alternating silicon and oxygen atoms with organic groups attached to the silicon. 3D formations of polymer chains emerge as they connect to one another, AKA cross-linking, which alter the material’s physical properties including strength or solubility.

While studying the cross-linking structures in silicone, the researchers came across the potential for electrical conductivity in a copolymer, 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 the 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 explained. “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 polymers also allow its spectrum of colours. Electrons jump between the ground and excited states by absorbing and emitting particles of light. The light emission depends on the length of the copolymer chain, which the team can manipulate.

Longer chain lengths mean smaller jumpers and lower energy photons, resulting in a red tint. Shorter chains require bigger jumps and emit higher energy light towards the blue end of the spectrum.

To showcase 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 colours based on copolymer chain length is especially 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 as much light.

“This allows an unexpected interaction between electrons across multiple bonds including Si—O—Si bonds in these copolymers,” said Laine. “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.”