Precise assessment of temperature is crucial in many physical, technological, and biological applications where optical thermometry has attracted considerable attention primarily due to fast response, contactless measurement route, and electromagnetic passivity. Rare‐earth‐doped thermographic phosphors that rely on ratiometric sensing are very efficient near and above room temperature. However, being dependent on the thermally‐assisted migration of carriers to higher excited states, they are largely limited by the quenching of the activation mechanism at low temperatures. In this paper, we demonstrate a strategy to pass through this bottleneck by designing a linear colorimetric thermometer by which we could estimate down to 4 K. The change in perceptual color fidelity metric provides an accurate measure for the sensitivity of the thermometer that attains a maximum value of 0.86 K−1. Thermally coupled states in Er3+ are also used as a ratiometric sensor from room temperature to ∼140 K. The results obtained in this work clearly show that Yb3+−Er3+ co‐doped NaGdF4 microcrystals are a promising system that enables reliable bimodal thermometry in a very wide temperature range from ultralow (4 K) to ambient (290 K) conditions.
Optical robustness, uniformity, ergodicity, statistical aging, etc. dictate
the applicability of nanocrystals.
Based on a series of multimodal statistical analyses such as the Kolmogorov–Smirnov
test, Lévy statistics, etc., we demonstrate that for CsPbBr3 perovskite nanocrystals (PNCs): (a) the extent of heterogeneity
in the quality and associated physical processes is minimal; (b) the
optical robustness is very high, and (c) indeed, a single PNC can
depict optical behavior of its ensemble. In addition, toward prospective
applications, an optically robust CsPbBr3 PNC exhibits
(i) near-ergodicity and (ii) minimal statistical
aging, which are extremely vital and complementary to its
high defect tolerance.
Upconversion luminescence bands from Yb 3+ / Er 3+ codoped into a matrix such as NaGdF 4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.
Lead- and tin-based chalcogenide semiconductors like PbTe or SnSe have long been known to exhibit an unusually low thermal conductivity that makes them very attractive thermoelectric materials. An apparently unrelated fact is that the excitonic bandgap in these materials increases with temperature, whereas for most semiconductors, one observes the opposite trend. These two anomalous features are also seen in a very different class of photovoltaic materials, namely, the halide perovskites such as CsPbBr3. It has been previously proposed that emphanisis, a local symmetry-breaking phenomenon, is the one common origin of these unusual features. Discovered a decade ago, emphanisis is the name given to the observed displacement of the lead or the tin ions from their cubic symmetry ground state to a locally distorted phase at high temperature. This phenomenon has been puzzling because it is unusual for the high-temperature state to be of a lower symmetry than the degenerate ground state. Motivated by the celebrated vibration-inversion resonance of the ammonia molecule, we propose a quantum tunneling-based model for emphanisis where decoherence is responsible for the local symmetry breaking with increasing temperature. From the analytic expression of the temperature dependence of the tunnel splitting (which serves as an order parameter), we provide three-parameter fitting formulas that capture the observed temperature dependence of the ionic displacements as well as the anomalous increase in the excitonic bandgap in all the relevant materials.
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