Near-Infrared Upconversion Transparent Inorganic Nanofilm: Confined-Space Directed Oriented Crystal Growth and Distinctive Ultraviolet Emission

A well-designed, efficient, one-step assembly strategy is implemented in this work by constructing a confined nanospace to manufacture an approximately 120 nm thick inorganic upconversion (UC) nanofilm with highly (101) oriented and morphology-controllable crystal grains, as well as transparent and robust characteristics. The morphology and distribution density of crystal grains of the film can be tuned by varying space heights and precursor concentrations. The confined space incubates a stable growing environment for crystal grains to decrease crystal defects and grow bigger. Therefore, there are high populations of doped Tm ions and high efficiencies of radiation transitions to realize multiphotons ultraviolet (UV) (monitoring range: 300-400 nm) emissions under laser excitation with a wide power range. Quantum yields of the film in the UV region are 4.7 and 16.1 times higher than those of UC nanoparticles synthesized by the typical thermal decomposition method and hydrothermal method, respectively. The UV-enhanced UC film is demonstrated to have the ability to serve as a medium to realize near-infrared induced undersurface photochemical reactions, which may inspire broad applications, such as UC three-dimensional printing.