Accurate and reliable temperature measurement of many special inaccessible objects is a challenging task. Optical temperature sensing is a promising method to achieve it. The current status of optical thermometry of rare-earth ion doped phosphors is reviewed in detail. Based on the mechanisms of optical temperature sensing of different phosphors, temperature dependent luminescence spectra, the fluorescence intensity ratio technique in the data fitting process, and errors of the energy difference between thermally coupled levels, we describe the recent developments in the use of optical thermometry materials. The most important results obtained in each case are summarized, and the main challenges that we need to overcome are discussed. Research in the field of phosphor sensors has shown that they have significant advantages compared to conventional sensors in terms of their properties like greater sensitivity, freedom from electromagnetic interference, long path monitoring, and independence of compatibility with electronic devices.

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Optical temperature sensing of rare-earth ion doped phosphors

Luminescent ratiometric thermometers combining high spatial and temporal resolution at the micro- and nanoscale, where the conventional methods are ineffective, have emerged over the last decade as an effervescent field of research, essentially motivated by their potential applications in nanotechnology, photonics, and biomedicine. Among the distinct luminescent thermal probes, lanthanide-based materials play a central role in the field due to their unique thermometric response and intriguing emission features (eg, high quantum yield, narrow bandwidth, long-lived emission, large Stokes shifts, and ligand-dependent luminescence sensitization). This chapter offers a general overview of recent examples of single- and dual-center Ln3 +-based thermometers, emphasizing those working at nanometric scale, being focused on how to quantify their performance accordingly to the relevant parameters: relative sensitivity, temperature uncertainty, spatial and temporal resolution, repeatability (or test–retest reliability), and reproducibility. The emission mechanisms supporting single- and dual-center emissions are reviewed, together with the advantages and limitations of each approach. Illustrative examples of the rich variety of systems designed and developed to sense temperature are provided and explored. Finally, we discuss the challenges and opportunities in the development of highly sensitive luminescent ratiometric thermometers that are currently facing the scientists in this exciting research field.

Lanthanides in Luminescent Thermometry

Temperature is important because it has an effect on even the tiniest elements of daily life and is involved in a broad spectrum of human activities. That is why it is the most commonly measured physical quantity. Traditional temperature measurements encounter difficulties when used in some emerging technologies and environments, such as nanotechnology and biomedicine. The problem may be alleviated using optical techniques, one of which is luminescence thermometry. This paper reviews the state of luminescence thermometry and presents different temperature read-out schemes with an emphasis on those utilizing the downshifting emission of lanthanide-doped metal oxides and salts. The read-out schemes for temperature include those based on measurements of spectral characteristics of luminescence (band positions and shapes, emission intensity and ratio of emission intensities), and those based on measurements of the temporal behavior of luminescence (lifetimes and rise times). This review (with 140 references) gives the basics of the fundamental principles and theory that underlie the methods presented, and describes the methodology for the estimation of their performance. The major part of the text is devoted to those lanthanide-doped metal oxides and salts that are used as temperature probes, and to the comparison of their performance and characteristics.

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Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review.

Yb3+/Er3+ codoped Ba5Gd8Zn4O21 up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol–gel method. UC spectra are composed of green emission from 2H11/2/4S3/2 → 4I15/2 transitions and red emission from 4F9/2 → 4I15/2 transition of Er3+ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level (4S3/2) lifetimes under excitation of 490 nm light, and the latter method was explained by the non-steady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er3+ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels (2H11/2/4S3/2) and red emitting stark levels (4F9/2...

Broad-Scope Thermometry Based on Dual-Color Modulation up-Conversion Phosphor Ba5Gd8Zn4O21:Er3+/Yb3+

Detailed spectroscopic analysis of the electronic configuration of Cr3+ in Bi2Ga4O9 is reported. The material exhibits unique luminescent properties arising from the crystal field experienced by Cr3+, with simultaneous strong sharp and broadband near-infrared emissions from the 2E and 4T2 excited states, in a wide range of temperature. The system displays dual near-infrared emission characterized by a remarkable thermal sensitivity over the whole explored range of temperatures, reaching a value of 0.7%·K–1 in the physiological range. Moreover, the possibility to absorb and emit in the first biological window, allows one to consider the system as a new promising candidate for ratiometric fluorescent thermal sensing in biotechnological applications.

Ratiometric Optical Thermometer Based on Dual Near-Infrared Emission in Cr3+-Doped Bismuth-Based Gallate Host

Metamaterials offer new unusual electromagnetic properties, which have already been demonstrated, and many postulated new functionalities are yet to be realized. Currently, however, metamaterials are mostly limited by narrow band behavior, high losses, and limitation in making genuinely 3D materials. In order to overcome these problems an overlap between metamaterial concepts and materials science is necessary. Engineered self-organization is presented as a future approach to metamaterial manufacturing. Using directional solidification of eutectics, the first experimental realization of self-organized particles with a split-ring resonator-like cross section is demonstrated. This unusual morphology/microstructure of the eutectic composite has a fractal character. With the use of TEM and XRD the clear influence of the atomic crystal arrangement on the microstructure geometry is presented. The materials obtained present very high anisotropy and can be obtained in large pieces. Metallodielectric structures can be created by etching and filling the space with metal. The next steps in the development of self-organized materials exhibiting unusual properties are discussed.

How Far Are We from Making Metamaterials by Self-Organization? The Microstructure of Highly Anisotropic Particles with an SRR-Like Geometry

Temperature dependent emission spectra and decay times of trivalent dysprosium (Dy3) activated Y4Al2O9 (YAM) crystals have been studied for the first time (to our knowledge) The ratio of emission lines intensity can be used in temperature measurements, as it is not dependent on the variability of absolute intensity The Boltzmann model was applied for modeling the temperature variation of the 4I15/2 and 4F9/2 states emissions relative intensities 455 and 481 nm, respectively The calculated approximation gives highest sensor sensitivity of about 3×10(-3)°C-1 for the 600°C-800°C range, which allows for an expectation of usefulness of Dy3+:YAM in high-temperature luminescence thermometry Also, the measured decay times are suitable for temperature sensing

Blue emissions in Dy3+ doped Y4Al2O9 crystals for temperature sensing.

The self-organized rodlike microstructure of terbium-scandium-aluminum garnet−terbium-scandium perovskite, Tb3Sc2Al3O12−TbScO3, eutectic crystals has been studied. The growth of the eutectic by the...

S. Turczyński

Papers

How Far Are We from Making Metamaterials by Self-Organization? The Microstructure of Highly Anisotropic Particles with an SRR-Like Geometry

Blue emissions in Dy3+ doped Y4Al2O9 crystals for temperature sensing.

Self-organized, rodlike, micrometer-scale microstructure of Tb3Sc2Al3O12-TbScO3:Pr eutectic

Temperature and concentration quenching of Tb3+ emissions in Y4Al2O9 crystals

Optical transitions intensities of Dy3+:Y4Al2O9 crystals