Ethanol-processable CaF₂ nanocrystals for high-performance and low-index antireflective coatings

Reflection at glass/air interfaces remains a major source of optical loss in transparent optoelectronic devices, which can be effectively mitigated by introducing antireflective coatings based on low-refractive-index materials. Calcium fluoride (CaF₂) is an excellent candidate due to its intrinsically low refractive index, broad spectral transparency and outstanding stability; however, its solution processing using environmentally benign solvents has remained challenging. Here, we report a room-temperature synthetic route to ethanol-dispersible CaF₂ nanocrystals with an average size of 4.2 nm. These nanocrystals can be readily spin-coated onto glass substrates to form continuous films with a low refractive index of 1.41, which significantly increase the average transmittance of the coated glass from 93.3% to 95.9% over the 400-1000 nm wavelength range, corresponding to an overall enhancement of 2.6%. This work establishes a simple and effective surface-ligand engineering strategy that bridges room-temperature colloidal synthesis with ethanol-based processing, offering a low-cost and scalable route to high-performance antireflective coatings for applications in light-emitting diodes, thin film solar cells and photodetectors.