Mapping Elevated Temperatures with a Micrometer Resolution Using the Luminescence of Chemically Stable Upconversion Nanoparticles.

Sample Preparation

The synthesis procedure reported in the work of Geitenbeek et al. was used to obtain multiple batches of NaYF 4 :Er 3+ (2%),Yb 3+ (18%) nanoparticles dispersed in cyclohexane. 17 Deposition of the nanoparticles was achieved by drop casting on a microheater (Wildfire Nano-Chips, commercially available from DENSsolutions). The thickness of the dried nanoparticle layer (a few μm) was estimated using the concentration of the nanoparticle dispersion. Microcrystalline NaYF 4 :Er 3+ (2%),Yb 3+ (18%) samples were obtained following the procedure of Krämer et al. 22 Characterization The particle size and morphology were investigated using an FEI Tecnai 12 transmission electron microscope operating at 120 keV. Samples were prepared by drying the diluted nanoparticle dispersion onto carbon-coated copper grids. The luminescence of the dried nanoparticle layer was mapped using a Nikon TE2000-S inverted microscope fitted with a C1si confocal scanner. 23 A 980 nm laser diode was connected to the confocal scanner head using a single-mode fiber. The laser light was reflected using a 680 nm shortpass dichroic mirror and focused on the nanoparticle layer using a 10× CFI Plan Fluor (NA = 0.3) air objective or a 40× CFI S Plan Fluor ELWD (NA = 0.6) air objective. The luminescence was directed back to the confocal scanner head by the same objective, passed through the dichroic mirror, coupled into a 50 μm multimode fiber, passed through a 680 nm shortpass filter, dispersed by an equilateral SF10 glass prism (Linos), and finally detected using a back-illuminated CCD (Princeton Instruments, NTE/CCD-1340). All maps were obtained with a pixel dwell time of 100 ms. For the calibration of the luminescence, powders of dried nanoparticles were heated with a Linkam THMS600 microscope stage. Selective excitation of the microcrystalline NaYF 4 :Er 3+ (2%),Yb 3+ (18%) sample with 404 and 448 nm light was carried out using an Edinburgh Instruments FLS920 spectrofluorometer equipped with TMS300 monochromators, a R928 photomultiplier tube, and a Xe lamp (450 W). The reflection measurements were performed on a Nikon Ti-U inverted microscope. The microheater was illuminated in wide field with a broad-spectrum LED (Lumencor Sola) by a 40× Nikon CFI Plan Fluor (NA = 0.75) air objective. The reflected light was collected by the same objective and collimated outside of the microscope using a relay lens system. The light was focused on a mechanical slit at the entrance of an Andor Kymera 193i spectrometer and dispersed with a 150 lines/mm grating on an Andor iXon EMCCD.

Simulation

Finite element simulations were carried out using COMSOL Multiphysics. The electric currents and heat transfer modules were two-way coupled to include the temperature-dependent resistivity. The model assumes a vacuum environment. Convective heat transfer through air is not taken into account, but radiation is included. The validity of the model was shown in our previous work. 24

Supplementary Material an1c00657_si_001.pdf