Combined Temperature and Pressure Sensing Using Luminescent NaBiF₄:Yb,Er Nanoparticles

The room temperature synthesis and photoluminescence properties of hexagonal phase NaBiF₄ nanocrystals doped with lanthanide ions have been presented. An efficient infrared Stokes emission and upconverted photoluminescence from the rare-earth ions (Yb³⁺, Er³⁺) codoped into bismuth–fluoride nanomatrix allows for dual modality of temperature and pressure sensing in VIS and NIR spectral regions. Upconversion emission shows nearly quadratic dependence of the photoluminescence intensity on the excitation light power, confirming a two-photon induced process and allowing two-photon sensing by nanocrystals with low-power CW laser diodes. The determined temperature sensitivity in the visible range varied from 1.07%/K at 303 K down to the minimum value of 0.27%/K at 523 K. Both Stokes and anti-Stokes emission bands are affected by matrix compression, with the determined emission wavelength pressure coefficients for the NIR range lines exceeding that of ruby. These novel nanocrystals combine the simplified green chemistry synthesis approach and features of lanthanide ions, providing a unique platform for many nanophotonics and sensing applications.