Next-Generation High Refractive Index Polymer Thin Films

Conformal optical coatings with high refractive indexes are crucial components in optoelectronic and energy applications. High refractive index polymers (HRIPs)are particularly compelling due to their desirable mechanical properties, such as flexibility and resiliency. However, it is challenging to prepare polymer thin films with high index (>1.8) while retaining optical transparency over visible wavelengths. This project builds on previous work where the refractive index of poly(4-vinylpyridine) (P4VP) was increased to over 2.0 through incorporation of halogen compounds. Here, we examine an alternative polymer derivatization approach and characterize the resulting refractive index, thermal stability, and optical transparency. Using initiated chemical vapor deposition (iCVD), 4-vinylpyridine is deposited onto Si wafer substrates and rapidly polymerized, producing P4VP thin-films with well-controlled thicknesses ranging from 400nm to 1000nm and RMS surface roughnesses less than 1 nm. P4VP films are then chemically derivatized with substituents bearing high molar refractivity in sealed jars at 100°C for 0.5 hours to 30 hours. Film thickness and optical properties are characterized using variable angle spectroscopic ellipsometry coupled with an in situ thermal cell. After a 6.5 hour treatment, the derivatized P4VP possesses a refractive index of 2.06, accompanied by a 10% increase in thickness and a thermal decomposition temperature well above 200°C. This is a significant improvement over previous results where charge transfer complexes degraded rapidly in ambient (room temperature) conditions. We further demonstrate the ability to selectively tune the refractive index of P4VP by changing reaction conditions during chemical derivatization, opening up optoelectronic applications for next-generation thermally robust HRIPs.