Thin film fabrication of upconversion lanthanide-doped NaYF₄ by a sol-gel method and soft lithographical nanopatterning

Lanthanide-doped NaYF₄ is one of the most efficient upconversion materials and fabrication of high-quality thin-films of this material would be useful in various applications. However, it has been a great challenge to fabricate thin films and/or nanopatterns of this material. In this work, we demonstrate the successful fabrication of the NaYF₄ thin films and nanopatterns via a sol-gel process and soft lithography. Upconversion of 980 nm near-infrared wavelength to various visible wavelengths (yellow, green, blue, etc.) has been realized using dopants and/or multilayered film structures. Nanopatterns enhance the light out-coupling efficiency and thus the upconversion intensity by a factor of ∼ 2–3. The fabrication of high-quality films and patterns is enabled by understanding the dependence of the structural, optical, chemical, and thermal properties of the precursor materials on the temperature. Our results will be an important basis for both scientific and technological advances in wavelength conversion in various areas such as displays, optoelectronics, photonics, and photovoltaics. - Highlights: • High quality NaYF₄ thin films are successfully fabricated via a sol-gel process. • Phase transition from cubic to hexagonal occurs at 340 °C. • The process is compatible with soft lithography by which nanopatterns are generated. • The nanopatterns enhance the light out-coupling efficiency. • Lanthanide dopants enable upconversion of infrared to various visible wavelengths.