This schematic illustration shows the co-polymerization of vaporized sulfur for synthesizing the high refractive index thin film. Image: KAIST.
This schematic illustration shows the co-polymerization of vaporized sulfur for synthesizing the high refractive index thin film. Image: KAIST.

Researchers have developed a novel technology for enhancing the high transparency of refractive polymer film via a one-step vapor deposition process. The sulfur-containing polymer (SCP) film synthesized by Sung Gap Im's research team at the Korea Advanced Institute of Science and Technology (KAIST) boasts a high refractive index exceeding 1.9 while being fully transparent over the entire visible range. As the team reports in a paper in Science Advances, the SCP film also exhibits excellent environmental stability and chemical resistance, making it highly desirable for use in optoelectronic devices.

The refractive index is a ratio of the speed of light in a vacuum to the phase velocity of light in a material, and is used as a measure of how much the path of light is bent when passing through a material. With the miniaturization of various optical components used in optoelectronic devices such as cell phones, demand has been growing for high refractive index transparent materials that induce more light refraction with a thin film.

As polymers have outstanding physical properties and can be easily processed in various forms, they are widely used in a variety of applications, such as plastic eyeglass lenses. However, very few polymers developed so far have a refractive index exceeding 1.75, and existing high refractive index polymers require costly materials and complicated manufacturing processes.

Up to now, the core technologies for producing such materials have been dominated by Japanese companies, causing long-standing challenges for Korean manufacturers. Securing a stable supply of high performance, high refractive index materials is crucial for the production of optoelectronic devices that are lighter and more affordable, and which can be freely manipulated.

Using a one-step chemical reaction, the KAIST research team has now successfully manufactured a whole new polymer thin film with a refractive index exceeding 1.9 and excellent transparency. The SCP film showed outstanding optical transparency across the entire visible light region, presumably due to its uniformly dispersed, short-segment polysulfide chains.

Taking advantage of the fact that elemental sulfur is easily sublimated, the team was able to produce a high refractive index polymer by polymerizing vaporized sulfur with a variety of substances. This method suppresses the formation of overly long sulfur-sulfur chains while achieving outstanding thermal stability in high sulfur concentrations, and generating transparent non-crystalline polymers across the entire visible spectrum.

Due to the characteristics of the vapor phase process, the high refractive index polymer thin film can be coated not only onto silicon wafers or glass substrates, but onto a wide range of textured surfaces as well.

"This high-performance polymer film can be created in a simple one-step manner, which is highly advantageous in the synthesis of SCPs with a high refractive index," said Im. "This will serve as a platform material for future high-end optical device applications."

This story is adapted from material from KAIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.