Long considered as simple thermal and electric insulation, silicones today reveal an unsuspected electronic potential. A team of researchers from the University of Michigan shows that by modifying the angle of Si-Si bonds, these polymers can be transformed into colorful and flexible semiconductors. A discovery that opens the way to a new class of materials for flexible electronics and intelligent devices.
Silicones – or polysiloxans – are one of the most used materials in the world. Thanks to their unique combination of flexibility, water resistance and thermal stability, they serve in applications as varied as seals, medical implants, cosmetics or protective coatings. Always classified among insulation, these materials have so far been not associated with electronic properties. A team of researchers from the University of Michigan, coordinated by Zijing Zhang and Richard Laine, has just overthrew this dogma: silicones can, in certain configurations, become functional semiconductors.
Silicones: insulating materials… become drivers
From a molecular point of view, silicones are made up of a Si-Si chain, on which organic groups (methyl, vinyl, etc.) are grafted. These chains are flexible, insulating and inert. The key discovery concerns the angle formed by Si-Si connections, deemed fixed around 110 °, which prevents the circulation of electrons and gives silicones their insulating property. But the researchers show that by synthesizing copolymers of silicone containing “cage” units and linear units, this angle can be increased beyond 140 °, thus opening the way to a partial relocation of the electrons through the chain of silicon and oxygen.
This geometry facilitates the displacement of electrons, transforming the initial insulation into material with semiconductive properties. Completely unexpected behavior in usual silicones.
Semiconductor behavior observable by the spectrum of colors
The acquisition of this semiconductor property is also observable by the spectrum of colors of these copolymers under UV light. The electrons jump between the fundamental state and the excited state by absorbing and emitting photons. However, the light emission depends on the length of the copolymer chain, which can be controlled. The researchers have thus been able to observe that the long chains (which allow small jumps of energy) give the silicone a reddish shade, while the short chains (which involve higher energy jumps) give blue colors.
During tests, the researchers thus obtained a rainbow of colorful vials, visually illustrating the relationship between chain length and electronic transition. This optical behavior reinforces the thesis of semiconductor behavior, with a prohibited band (band gapthe interval between the valence band and the conduction strip) adjustable according to the structure and length of the copolymer.
Many potential applications
This discovery opens the way to a new class of functional silicones with particularly interesting properties. Unlike rigid conventional semiconductors, these semiconductor silicones are indeed flexible, and can be used in the field of printed electronics. Richard Laine evokes flexible screens, flexible solar cells, or even colorful interactive textiles.
This work breaks with the classic paradigm claiming that “Si-Si are insulating”. But despite its potential, this technology is only in its infancy. Several points remain to be deepened, such as the reliability of copolymers in terms of thermal resistance, photo-stability and aging, as well as their capacity for technical integration (compatibility with printing processes, membership, biocompatibility, etc.).
The transformation of a classic insulation into a semiconductor via a scientist structural control is a major breakthrough. By opening the door to electronic, colorful, flexible silicones, this research promises to redefine the role of these materials – so far passive – in electronics and intelligent materials. Beyond silicones, this concept of a geometric modification of atomic bonds can inspire new studies in other polymers classes, reformulating the way in which materials and circuits are designed. This study, published in the journal Macromolecular Rapid Communications, shows that it is sometimes enough to modify an angle, here that of the Si-Si, to unlock properties hitherto unsuspected.
