The human eye can perceive over a million distinct colours, thanks to cone cells that send signals to the brain, allowing us to detect subtle differences in hue, intensity and saturation. Replicating this vast and nuanced colour palette in display technologies calls for a delicate balance of precision, stability and vibrancy.
Thin-film coatings, often just nanometres thick, are applied to surfaces like glass or metal to precisely manipulate light. They play a vital role in applications demanding long-lasting, high-quality colours, from vibrant displays and light-sensitive sensors to smart windows that regulate heat and light.
Jinghua Teng, a Senior Principal Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), explained how these coatings produce stable, vivid colours with exceptional purity and brightness, avoiding the iridescence common in other systems. “This scalable, cost-effective technology also improves energy efficiency, product durability and visual clarity in the devices we rely on every day,” Teng said.
However, the perfect thin-film coating remains elusive. “It’s difficult for any existing structural colouring technology to meet all these requirements,” Teng explained, pointing to issues such as colour mixing, poor stability and the need for complex materials that degrade over time.
In collaboration with Ranjan Singh at Nanyang Technological University, Singapore; Teng, Sreekanth K.V. and IMRE colleagues developed an aperiodic distributed Bragg reflector (A-DBR), a specialised thin-film structure made of alternating layers of silicon dioxide (SiO₂) and and antimony trisulfide (Sb₂S₃). The material is designed to overcome the limitations of existing technologies by enhancing colour purity and stability while reducing unwanted reflections.
By using materials with varying refractive indices, the structure maximises reflection. Sb₂S₃, with a higher refractive index than materials such as titanium dioxide (TiO₂), reduces the number of layers required, while its phase-change properties allow for tuneable colours, said Teng. The aperiodic design also enhances spectral control and reflectivity.
With their A-DBR, the team effectively suppressed unwanted reflections, achieving narrowband, vibrant colours like orange and yellow with 90 percent reflectivity. The A-DBR enabled multistate colour tuning by switching between amorphous and crystalline states, shifting wavelengths up to 110 nm while maintaining purity. Importantly, the colours remain stable and consistent, unaffected by viewing angle or light polarisation, even at a 60-degree incident angle.
“Our tuneable A-DBR outperforms existing structural colouring methods by providing high colour purity, wide colour gamut coverage, high brightness and angular control,” said Sreekanth, adding that it’s also cost-effective and easy to scale, making it ideal for next-generation display panels that don’t require backlighting, reducing energy use.
Speaking on the team’s plans, Teng said, “We’re now exploring various phase change materials for tuneable optics, including tuneable terahertz devices for 6G communications.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
