Demonstration of Temperature Dependent Energy Migration in Dual-Mode YVO: Ho/Yb Nanocrystals for Low Temperature Thermometry.

A dual mode rare-earth based vanadate material (YVO: Ho/Yb), prepared through ethylene glycol assisted hydrothermal method, demonstrating both downconversion and upconversion, along with systematic investigation of the luminescence spectroscopy within 12-300 K is presented herein. The energy transfer processes have been explored via steady-state and time-resolved spectroscopic measurements and explained in terms of rate equation description and temporal evolution below room temperature. The maximum time for energy migration from host to rare earth (Ho) increases (0.157 μs to 0.514 μs) with the material's temperature decreasing from 300 K to 12 K. The mechanism responsible for variation of the transients' character is discussed through thermalization and non-radiative transitions in the system. More significantly, the temperature of the nanocrystals was determined using not only the thermally equilibrated radiative intra-4f transitions of Ho but also the decay time and rise time of vanadate and Ho energy levels. Our studies show that the material is highly suitable for temperature sensing below room temperature. The maximum relative sensor sensitivity using the rise time of Ho energy level (F/S) is 1.35% K, which is the highest among the known sensitivities for luminescence based thermal probes.