NaYF4:Er3+/Yb3+ nanocrystals for nanothermometry

Abstract

In this work the temperature dependent upconversion luminescence of NaYF4:Yb3+/Er3+ nanocrystals is characterized over a large temperature range, relevant for thermometry in chemical reactors. The temperature of the nanocrystals determines the intensity ratio between the two green emission lines of erbium from the 2H11/2 level centred at 520 nm and from the 4S3/2 level centred at 540 nm. To this end, the green luminescence of erbium is monitored for different temperatures. The green emission of erbium is visible upon infrared excitation of ytterbium at 980 nm and following energy transfer upconversion. To allow for practical application of NaYF4: Yb3+/Er3+ as nanothermometer, the scale of nanocrystal synthesis is increased by a factor of four compared to existing procedures. The resulting monodisperse NaYF4 nanocrystals have a size of 22 nm (± 1 nm) and show temperature dependent upconversion luminescence up to 600 K. Above this temperature the NaYF4 nanocrystals melt together and the luminescence drops. A silica shell of ca. 10 nm has been grown around the NaYF4 nanocrystals to overcome this problem. The resulting NaYF4@SiO¬2 nanocrystals show temperature dependent luminescence up to 900 K. The intensity ratio between the 2H11/2 level and the 4S3/2 level changes from ca. 0.4 at 300 K to ca. 3 at 900 K. The temperature variations of the intensity ratio are identical for five consecutive cycles, showing that the durability of the NaYF4@SiO¬2 nanocrystals is high. The accuracy of the measurements is high, with standard deviations of 1 K and 5 K below and above 750 K. Our results show that NaYF4:Yb3+/Er3+ nanocrystals can be used as a non-invasive thermometer up to 900 K after encapsulation in a protective silica shell. The broad temperature range, temperature resolution, and durability make that this system can be used to measure temperature variations inside a chemical reactor.