  Currently available temperature measurements or imaging at nano-micro scale
are limited to fluorescent molecules and luminescent nanocrystals, whose
spectral properties respond to temperature variation. The principle of
operation of these conventional temperature probes is typically related to
temperature induced multiphonon quenching or temperature dependent energy
transfers, therefore, above 12%/K sensitivity and high thermal resolution
remain a serious challenge. Here we demonstrate a novel class of highly
sensitive thermographic phosphors operating in room temperature range with
milikelvin thermal resolution, whose temperature readings are reproducible,
luminescence is photostable and brightness is not compromised by thermal
quenching. Corroborated with phase transition structural characterization and
high spatio-temporal temperature imaging, we demonstrated that optically active
europium ions are highly and smoothly susceptible to monoclinic to tetragonal
phase transition in LiYO2 host, which is evidenced by changed number and the
splitting of Stark components as well as by smooth variation of contribution
between magnetic and electric dipole transitions. Further, reducing the size of
phosphor from bulk to nanocrystalline matrix, shifted the phase transition
temperature from 100oC down to room temperature. These findings provide
insights into the mechanism underlaying phase transition based luminescence
nanothermometry and motivate future research toward new, highly sensitive, high
temporal and spatial resolution nano-thermometers aiming at precise studying
heat generation or diffusion in numerous biological and technology
applications.
