Crystal growth and fluorescence of downconversion films in confined space with ingoing channels

Abstract

Ultraviolet (UV) imaging technology plays an essential role in deep space exploration, fingerprint imaging, aerospace and other fields. To overcome the shortcomings of current detection technology arising from the limited detection band and low quantum efficiency, we propose to combine UV-responsive fluorescent film with visible light imaging technology to realize the UV imaging process through light conversion. The present work focuses on the preparation of UV-responsive light conversion thin films and their properties. A hydrothermally assisted in situ growth method is utilized to prepare downconversion (DC) NaYF4 thin films. The effects of reaction time on fluorescence and morphologies are investigated, followed by the creation of ingoing channels to achieve uniform light emission. The results show that the reflection fluorescence is enhanced by increasing reaction time from 8 to 24 h while fluorescence uniformity decreases, i.e., different locations on the DC films presented significant fluctuation in fluorescence intensity and morphologies. The reason is that sufficient time is allowed for crystal growth and Ostwald-ripening, which increases fluorescence intensity but resulting in various crystal structures. The use of 4 × 4 to 12 × 12 ingoing channels increases precursor concentrations in confined space, achieving uniform fluorescence. However, the crystal nucleation stage dominated by high precursor concentrations plays a crucial role in generation of small crystal grains, which reduces fluorescence intensity. Our work is beneficial for further optimization of reaction conditions for obtaining large-scale DC films with uniform and high fluorescence.