Vacuum-assisted colossal enhancement of up-conversion luminescence of lanthanide-doped nanoparticles upon NIR laser irradiation - a new strategy for phosphor development

Materials science has experienced significant advances in the development of new techniques to synthesize materials with unique properties, which are essential for various applications. For instance, lanthanide-doped structures have been combined with other optically-active compounds to produce materials with unique properties for anti-counterfeiting, bio-applications, optical sensors, photovoltaics, and so on. There are many different chemical synthesis routes (bottom-up; wet chemistry methods) and physical approaches (top-down) to obtain materials, and each of them has its own advantages and disadvantages, depending on the specific requirements of the given material and its final application. This study opens up completely new routes for the precise and localized crystal growth of optical materials and huge enhancement of their up-conversion luminescence intensity (by 2-orders of magnitude). It was possible by irradiation of the initially prepared nanoparticles with the NIR (975 nm) laser under lowered pressure conditions, i.e., vacuum. As exemplary compounds, we used the lanthanide-doped YVO4 nanomaterials, containing either Yb3+-Tm3+ or Yb3+-Er3+ ions. It was shown that the loss of air (vacuum) significantly improves the heating rate of the samples and induces the growth of the crystals. In addition, confocal Raman spectroscopy was used to verify the evolution of the materials structure before and after laser treatment. Permanent enhancement (by 2 orders of magnitude) of up-conversion luminescence via localized crystal growth, induced by NIR laser irradiation under vacuum conditions. This effect is caused by vacuum-enhanced light-to-heat conversion efficiency.