Impact of Noise and Background on Measurement Uncertainties in Luminescence Thermometry

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

Photonic Artifacts in Ratiometric Luminescence Nanothermometry

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

Increasing the Power: Absorption Bleach, Thermal Quenching, and Auger Quenching of the Red-Emitting Phosphor K2TiF6:Mn4+

Mn4+-doped fluorides are popular phosphors for warm-white lighting, converting blue light from light-emitting diodes (LEDs) into red light. However, they suffer from droop, that is, decreasing performance at increasing power, limiting their applicability for high-power applications. Previous studies highlight different causes of droop. Here, a unified picture of droop of Mn4+-doped K2TiF6, accounting for all previously proposed mechanisms, is provided. Combining continuous-wave and pulsed experiments on samples of different Mn4+ content with kinetic Monte Carlo modeling, the contributions of absorption bleach, thermal quenching, and Auger quenching at different excitation densities, are quantified. This work contributes to understanding the fundamental limitations of these materials and may inspire strategies to make Mn4+-doped fluorides more efficient in high-power applications

Beyond the energy gap law : the influence of selection rules and host compound effects on nonradiative transition rates in boltzmann thermometers

P.N. and M.H. contributed equally to this work. H.A.H., P.N., M.H., and E.T. thank the Deutsche Forschungsgemeinschaft (DFG) for generous support (Project HO 4503/5-1). Open access funding enabled and organized by Projekt DEAL.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 6P5/2 and 6P7/2 spin–orbit levels of Gd3+ is investigated in the chemically related compounds Y2[B2(SO4)6] and LaBO3. Analysis of the temperature-dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited 6PJ (  J = 5/2, 7/2) levels are of the order of only 10 ms−1 (Y2[B2(SO4)6]:Gd3+: 8.9 ms−1; LaBO3:Gd3+: 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 Er3+ reveals, however, that the nonradiative transition rates between the excited levels of Gd3+ 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 Gd3+. These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers.Publisher PDFPeer reviewe

Beyond the Energy Gap Law: The Influence of Selection Rules and Host Compound Effects on Nonradiative Transition Rates in Boltzmann Thermometers

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 6P5/2 and 6P7/2 spin–orbit levels of Gd3+ is investigated in the chemically related compounds Y2[B2(SO4)6] and LaBO3. Analysis of the temperature-dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited 6PJ (J = 5/2, 7/2) levels are of the order of only 10 ms−1 (Y2[B2(SO4)6]:Gd3+: 8.9 ms−1; LaBO3:Gd3+: 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 Er3+ reveals, however, that the nonradiative transition rates between the excited levels of Gd3+ 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 Gd3+. These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers

The Formation of NaYF4: Er3+, Yb3+ Nanocrystals Studied by In Situ X-ray Scattering: Phase Transition and Size Focusing

β-NaYF4 nanocrystals are a popular class of optical materials. They can be doped with optically active lanthanide ions and shaped into core-multi-shell geometries with controlled dopant distributions. Here, we follow the synthesis of β-NaYF4 nanocrystals from α-NaYF4 precursor particles using in situ small-angle and wide-angle X-ray scattering and ex situ electron microscopy. We observe an evolution from a unimodal particle size distribution to bimodal, and eventually back to unimodal. The final size distribution is narrower in absolute numbers than the initial distribution. These peculiar growth dynamics happen in large part before the α-to-β phase transformation. We propose that the splitting of the size distribution is caused by variations in the reactivity of α-NaYF4 precursor particles, potentially due to inter-particle differences in stoichiometry. Rate equation modeling confirms that a continuous distribution of reactivities can result in the observed particle growth dynamics

Probing nearby molecular vibrations with lanthanide-doped nanocrystals

The photoluminescence (PL) of lanthanide-doped nanocrystals can be quenched by energy transfer to vibrations of molecules located within a few nanometers from the dopants. Such short-range electronic-to-vibrational energy transfer (EVET) is often undesired as it reduces the photoluminescence efficiency. On the other hand, EVET may be exploited to extract information about molecular vibrations in the local environment of the nanocrystals. Here, we investigate the influence of solvent and gas environments on the PL properties of NaYF4:Er3+,Yb3+ upconversion nanocrystals. We relate changes in the PL spectrum and excited-state lifetimes in different solvents and their deuterated analogues to quenching of specific lanthanide levels by EVET to molecular vibrations. Similar but weaker changes are induced when we expose a film of nanocrystals to a gas environment with different amounts of H2O or D2O vapor. Quenching of green- and red-emitting levels of Er3+ can be explained in terms of EVET-mediated quenching that involves molecular vibrations with energies resonant with the gap between the energy levels of the lanthanide. Quenching of the near-infrared-emitting level is more complex and may involve EVET to combination-vibrations or defect-mediated quenching. EVET-mediated quenching holds promise as a mechanism to probe the local chemical environment—both for nanocrystals dispersed in a liquid and for nanocrystals exposed to gaseous molecules that adsorb onto the nanocrystal surface

Bifunctional Europium for Operando Catalyst Thermometry in an Exothermic Chemical Reaction

Often the reactor or the reaction medium temperature is reported in the field of heterogeneous catalysis, even though it could vary significantly from the reactive catalyst temperature. The influence of the catalyst temperature on the catalytic performance and vice versa is therefore not always accurately known. We here apply EuOCl as both solid catalyst and thermometer, allowing for operando temperature determination. The interplay between reaction conditions and the catalyst temperature dynamics is studied. A maximum temperature difference between the catalyst and oven of +16 °C was observed due to the exothermicity of the methane oxychlorination reaction. Heat dissipation by radiation appears dominating compared to convection in this set-up, explaining the observed uniform catalyst bed temperature. Application of operando catalyst thermometry could provide a deeper mechanistic understanding of catalyst performances and allow for safer process operation in chemical industries

Mapping Temperature Heterogeneities during Catalytic CO2 Methanation with Operando Luminescence Thermometry

Controlling and understanding reaction temperature variations in catalytic processes are crucial for assessing the performance of a catalyst material. Local temperature measurements are challenging, however. Luminescence thermometry is a promising remote-sensing tool, but it is cross-sensitive to the optical properties of a sample and other external parameters. In this work, we measure spatial variations in the local temperature on the micrometer length scale during carbon dioxide (CO 2) methanation over a TiO 2-supported Ni catalyst and link them to variations in catalytic performance. We extract local temperatures from the temperature-dependent emission of Y 2O 3:Nd 3+ particles, which are mixed with the CO 2 methanation catalyst. Scanning, where a near-infrared laser locally excites the emitting Nd 3+ ions, produces a temperature map with a micrometer pixel size. We first designed the Y 2O 3:Nd 3+ particles for optimal temperature precision and characterized cross-sensitivity of the measured signal to parameters other than temperature, such as light absorption by the blackened sample due to coke deposition at elevated temperatures. Introducing reaction gases causes a local temperature increase of the catalyst of on average 6-25 K, increasing with the reactor set temperature in the range of 550-640 K. Pixel-to-pixel variations in the temperature increase show a standard deviation of up to 1.5 K, which are attributed to local variations in the catalytic reaction rate. Mapping and understanding such temperature variations are crucial for the optimization of overall catalyst performance on the nano- and macroscopic scale