Microwave Synthesis of Blue Emitting Yb,Tm:NaYF4 Nanocrystals: Study of Reaction Parameters

Abstract

Up-converting (UC) materials are those materials that can absorb two or more low energy photons (from near infrared radiation) and convert them into an emission of higher frequency photons (visible light). These systems are very useful in a plethora of applications such as bioimaging, theragnostic, solar cells, sensors or anti-counterfeiting technologies. Among them, blue-emitting UC materials have received more and more attention, since this emission can trigger complex photochemical processes with special interest in biomedical fields and photocatalysis. This work presents the synthesis and hydrophilic surface functionalization of blueemitting Tm3+,Yb3+-doped NaYF4 nanocrystals, as one of the most efficient upconverting material, via an easy and ultra-fast microwave-assisted approach. The effect of several reaction parameters (i.e., temperature, reaction time and concentration) on the morphology and crystalline phase (cubic or hexagonal) of the materials has been studied. In this work, we obtained small ( < 100 nm) and rounded cubic-phase nanocrystals (kinetic product), and also larger hexagonal crystals (thermodynamic product) upon different reaction conditions. However, it was found that the large hexagonal crystals could revert into the cubic phase and reduce their size through a dissolution-nucleation mechanism because of a controlled microwave treatment. The optical characterization of the materials was also conducted and the emission of blue light upon NIR radiation detected. Finally, the application of these up-converting materials as additives to obtain functional light-emitting plastic films was assessed.