Hans Kraus

Abstract:

Realtime in situ temperature monitoring in difficult experimental conditions or inaccessible environments is critical for many applications. Non-contact luminescence decay time thermometry is often the method of choice for such applications due to a favorable combination of sensitivity, accuracy and robustness. In this work, we demonstrate the feasibility of an ultrafast PbI2 scintillator for temperature determination, using the time structure of X-ray radiation, produced by a synchrotron. The decay kinetics of the scintillations was measured over the 8–107 K temperature range using monochromatic pulsed X-ray excitation. It is found that lead iodide exhibits a very fast and intense scintillation response due to excitons and donor-acceptor pairs, with the fast decay component varying between 0.08 and 0.5 ns – a feature that can be readily exploited for temperature monitoring. The observed temperature dependence of the decay time is discussed in terms of two possible mechanisms of thermal quenching – transition over activation barrier and phonon-assisted escape. It is concluded that the latter provides a better fit to the experimental results and is consistent with the model of luminescence processes in PbI2. We evaluated the sensitivity and estimated the accuracy of the temperature determination as ca. ±6 K at 107 K, improving to ±1.4 K at 8 K. The results of this study prove the feasibility of temperature monitoring, using ultrafast scintillation of PbI2 excited by X-ray pulses from a synchrotron, thus enabling non-contact in-situ cryothermometry with megahertz sampling rate.

Abstract:

© 2018 Acta Materialia Inc. Luminescence lifetime thermometry for remote temperature monitoring of cryogenic objects requires materials that exhibit a suitably large change of the luminescence kinetics at low temperatures. Results of systematic studies of the temperature-induced changes in the luminescence of tungstates and molybdates with the general formula ABO4 (A = Ca, Sr, Zn, Cd; B = W, Mo) over the 4.5–300 K temperature range are summarized. It is shown through analysing changes of the emission and excitation spectra, as well as the decay kinetics, that in these materials the luminescence is due to the emission of self-trapped excitons, a process that exhibits strong temperature dependence. The main emphasis of the study is on establishing the factors that determine the character of the temperature dependence of the luminescence decay time constant. We discuss our findings in terms of a model that analyses the dynamics of radiative and non-radiative transitions between the excited and ground states of the emission center. Two thermally activated processes drive the observed changes. The first is the non-radiative decay of excited states, resulting in a decrease of the luminescence decay time constant at high temperatures. Additionally it is demonstrated that in molybdates and tungstates the fine splitting of the excited state facilitates a second mechanism for thermally activated exchange of charged carriers between the two split levels. This has a noticeable effect on the dynamics of the radiative decay at low temperatures. We established that the sensitivity of the luminescence lifetime to temperature changes can be estimated by using information on the energy structure of materials. It is concluded that within tungstates and molybdates under study SrWO4 is the most promising material for application in luminescence lifetime cryothermometry.

Abstract:

Intrinsic and extrinsic optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI3) thin films, whose background hole doping density was varied through SnF2 addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 1020 cm-3, a strong Burstein-Moss effect increases absorption onset energies by ~300meV beyond the band gap energy of undoped FASnI3 (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (1020 cm-3), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 1019 cm-3 and below, the charge-carrier mobility increases with decreasing temperature according to ~T-1.2, suggesting it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2´1018 cm^-3, charge-carrier mobilities reach a value of 67 cm2V-1s-1at room temperature and 470 cm2V-1s-1 at 50 K. Intra-excitonic transitions observed in the THz-frequency photoconductivity spectra at 5K reveal an exciton binding energy of only 3.1 meV for FASnI3, in agreement with the low bandgap energy exhibited by this perovskite.