The temperature-sensitive luminescence of nanoparticles enables their application as remote thermometers. The size of these nanothermometers makes them ideal to map temperatures with a high spatial resolution. However, high spatial resolution mapping of temperatures >373 K has remained challenging. Here, we realize nanothermometry with high spatial resolutions at elevated temperatures using chemically stable upconversion nanoparticles and confocal microscopy. We test this method on a microelectromechanical heater and study the temperature homogeneity. Our experiments reveal distortions in the luminescence spectra that are intrinsic to high-resolution measurements of samples with nanoscale photonic inhomogeneities. In particular, the spectra are affected by the high-power excitation as well as by scattering and reflection of the emitted light. The latter effect has an increasing impact at elevated temperatures. We present a procedure to correct these distortions. As a result, we extend the range of high-resolution nanothermometry beyond 500 K with a precision of 1–4 K. This work will improve the accuracy of nanothermometry not only in micro- and nanoelectronics but also in other fields with photonically inhomogeneous substrates.

on luminescence thermometry offers an alternative that is capable of measuring heat generation and diffusion on the microscopic scale. [3] Among the various choices of luminescent systems, [4-9] crystals doped with lanthanide (Ln 3+) ions represent a particularly promising class of luminescent thermometers, because their dimensions can be tuned from a few nanometers to several micrometers and their photoluminescence spectrum is sensitive to temperature. A characteristic feature of Ln 3+ ions is their rich energy level structure, which results in emission spectra with well-separated lines. Typically, the luminescence intensity ratio (LIR) between two of these emission lines is used as a sensitive measure for the temperature of the Ln 3+-doped crystal. After insertion of these ratiometric thermometers into a system of interest, remote operation simply involves excitation by light and detection of the luminescence with standard spectroscopic equipment. The performance of a ratiometric thermometer is determined by how sensitively the LIR reacts to temperature. In general, the performance at a given temperature (T) is quantified in terms of the relative sensitivity [10]

Recent technological developments require knowledge of temperature down to the micro- or even nano-scale. Lanthanide-doped nanoparticles became a popular tool to achieve this. Their temperature sensitive luminescence enables their application...

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr 3 nanoplatelets, before and after chemical treatment with PbBr 2 that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr 2 . More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Materials with temperature-dependent luminescence can be used as local thermometers when incorporated in, for example, a biological environment or chemical reactor. Researchers have continuously developed new materials aiming for the highest sensitivity of luminescence to temperature. Although the comparison of luminescent materials based on their temperature sensitivity is convenient, this parameter gives an incomplete description of the potential performance of the materials in applications. Here, we demonstrate how the precision of a temperature measurement with luminescent nanocrystals depends not only on the temperature sensitivity of the nanocrystals but also on their luminescence strength compared to measurement noise and background signal. After first determining the noise characteristics of our instrumentation, we show how the uncertainty of a temperature measurement can be predicted quantitatively. Our predictions match the temperature uncertainties that we extract from repeated measurements, over a wide temperature range (303–473 K), for different CCD readout settings, and for different background levels. The work presented here is the first study that incorporates all of these practical issues to accurately calculate the uncertainty of luminescent nanothermometers. This method will be important for the optimization and development of luminescent nanothermometers.

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range from biothermal imaging, [5,6] temperature monitoring in catalysis, [7][8][9][10] microelectronics, [11] or molecular logics [12] to the investigation of fundamental thermodynamic phenomena [13] at the micro-and nanoscale. One of the conceptually simplest ways of optical temperature sensing, also in terms of the required setup, is the exploitation of the luminescence intensity ratio (LIR) of two emission bands due to radiative transitions from two thermally coupled excited levels of an ensemble of non-interacting ions. In case of efficient thermal coupling between these levels by (multi) phonon transitions, the LIR follows Boltzmann's law. [14] Trivalent lanthanoid ions with the rich energy level structure arising from their partially filled 4f n (n = 1-13) configuration are primary representatives for this type of luminescence thermometry, with Er 3+ and its green-emitting 2 H 11/2 and 4 S 3/2 levels being the most prominent examples. [7,15] Boltzmann-type equilibrium between the two excited levels critically depends on the interplay between the depopulating radiative decay and the nonradiative multiphonon thermalization pathways. [14,16] The rate-determining step is the Apart from the energy gap law, control parameters over nonradiative transitions are so far only scarcely regarded. In this work, the impact of both covalence of the lanthanoid-ligand bond and varying bond distance on the magnitude of the intrinsic nonradiative decay rate between the excited 6 P 5/2 and 6 P 7/2 spin-orbit levels of Gd 3+ is investigated in the chemically related compounds Y 2 [B 2 (SO 4 ) 6 ] and LaBO 3 . Analysis of the temperature-dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited 6 P J ( J = 5/2, 7/2) levels are of the order of only 10 ms −1 (Y 2 [B 2 (SO 4 ) 6 ]:Gd 3+ : 8.9 ms −1 ; LaBO 3 :Gd 3+ : 10.5 ms −1 ) and differ due to the different degree of covalence of the GdO bonds in the two compounds. Comparison to the established luminescent Boltzmann thermometer Er 3+ reveals, however, that the nonradiative transition rates between the excited levels of Gd 3+ are over three orders of magnitude slower despite a similar energy gap and the presence of a single resonant phonon mode. This hints to a fundamental magnetic dipolar character of the nonradiative coupling in Gd 3+ . These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers.

Ongoing developments in science and technology require temperature measurements at increasingly higher spatial resolutions. Nanocrystals with temperature-sensitive luminescence are a popular thermometer for these applications offering high precision and remote read-out. Here, we demonstrate that ratiometric luminescence thermometry experiments may suffer from systematic errors in nanostructured environments. We place lanthanide-based luminescent nanothermometers at controlled distances of up to 600 nm from a Au surface. Although this geometry supports no absorption or scattering resonances, distortion of the emission spectra of the thermometers due to the modified density of optical states results in temperature read-out errors of up to 250 K. Our simple analytical model explains the effects of thermometer emission frequencies, experimental equipment, and sample properties on the magnitude of the errors. We discuss the relevance of our findings in several experimental scenarios. Such errors do not always occur, but they are expected in measurements near reflecting interfaces or scattering objects.