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The Sensitivity of Nanocrystalline Luminescent Thermometer in High and Low Excitation Density Regimes Lukasz Marciniak, Klaudia Waszniewska, Artur Bednarkiewicz, Dariusz Hreniak, and Wieslaw Strek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01636 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016
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The Journal of Physical Chemistry
The Sensitivity of Nanocrystalline Luminescent Thermometer in High and Low Excitation Density Regimes Lukasz Marciniak†, Klaudia Waszniewska†, Artur Bednarkiewicz†, Dariusz Hreniak†, Wieslaw Strek† . † Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland AUTHOR INFORMATION Corresponding Author *Lukasz Marciniak +48 71 395 4188
[email protected] Okolna st. 2 50422Wroclaw, Poland
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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ABSTRACT
Two figures of merit are typically taken into account studying rare earth doped materials and processes suitable for optical nanothermometry, namely temperature sensing range and the sensitivity. To optimize the composition of such phosphors and make quantitative comparison between different materials, other factors, such as excitation density and pulse duration, have to been included. Owing to metastable character of lanthanide excited states, the excitation intensity has a critical importance but has been disregarded so far. Our studies show, based on new nanocrystalline Er:LiYbP4O12 luminescent thermometer, the influence of the excitation power density on its suitability for temperature sensing. The highest sensitivity was reached for LiYbP4O12: 0.1% Er nanocrystals upon pulsed excitation – 2.88%/K at average power below 25 mW/cm2, while the same material displayed lower ~0.5 %/K sensitivity at higher 50-300 mW/cm2 excitation intensities. The mechanism responsible for the observed sensitivity changes was discussed in terms of competition between thermalization and nonradiative depopulation processes.
INTRODUCTION Optical remote thermometers based on luminescent nanoparticles doped with rare earth ions have been studied extensively owing to their potential applications in many fields of science, biomedicine and technology1-14. Although sufficient sensitivity (up to 1.5 % K-1), satisfying sensing range (10-500 K) 4, 15-17 and simple readout in Vis (540nm) or NIR (900-1100nm) spectral regions have been demonstrated, the impact of experimental conditions, in particular excitation intensity, on these key features has been disregarded so far. The studies carried within the current framework, show the critical importance of excitation scheme on both sensitivity and sensing range of luminescent thermometers based on Yb and Er up-converting phosphor. 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
Independently from the temperature sensing scheme, i.e. Boltzmann populations within same ion or temperature sensitive energy transfer between neighbor ions, rare-earth ions exhibit Laporte forbidden transitions, which makes the electronic levels metastable and thus susceptible to saturation under high excitation intensities. A good experimental evidence of similar phenomenon is the fact, the up-conversion rate is proportional to the square power of excitation intensity only for low excitation rate, while for higher excitation intensities, the upconvertion rate approaches linear dependence versus pumping rate 18. Even more evidences are provided by the quantum yield measurements - following the saturation model developed by Liu et al.19, the quantum yield, defined as a ratio of photons emitted to the absorbed ones, tends to saturate not only because saturating intermediate upconverting levels, but also owing to reduced absorption due to ground state depletion. Apparently, the saturation should also diminish the extent of luminescence changes in response to temperature variation. These effects have not been considered yet in literature, however, based on our current studies, play extremely important role for optical nanothermometry. These facts have severe implications for further studies, since, not only the so far obtained figures of merit (such as sensitivity or sensing range) are improperly compared between different studies (because excitation intensities are rarely provided in experimental conditions) but also the composition of luminescent phosphors are not really optimal. By simple decreasing the excitation intensity from 50-300 mW/cm2 to
