Upconversion luminescence of nano-sized Yb and Tm codoped rare earth phosphates, that is, LaPO and YPO, has been investigated under high-pressure (HP, up to ∼25 GPa) and high-temperature (293-773 K) conditions. The pressure-dependent luminescence properties of the nanocrystals, that is, energy red shift of the band centroids, changes of the band ratios, shortening of upconversion lifetimes, and so forth, make the studied nanomaterials suitable for optical pressure sensing in nanomanometry. Furthermore, thanks to the large energy difference (∼1800 cm), the thermalized states of Tm ions are spectrally well-separated, providing high-temperature resolution, required in optical nanothermometry. The temperature of the system containing such active nanomaterials can be determined on the basis of the thermally induced changes of the Tm band ratio (F → H/H → H), observed in the emission spectra. The advantage of such upconverting optical sensors is the use of near-infrared light, which is highly penetrable for many materials. The investigated nanomanometers/nanothermometers have been successfully applied, as a proof-of-concept of a novel bimodal optical gauge, for the determination of the temperature of the heated system (473 K), which was simultaneously compressed under HP (1.5 and 5 GPa).

Anti-Stokes luminescence of up-converting nanocrystals SrF:Yb,Er can be used as a high pressure optical sensor alternative to the ruby fluorescence-scale. In nanocrystalline SrF:Yb,Er, high pressure reversibly shortens the emission lifetimes nearly linearly up to 5.29 GPa at least. Its advantage is the use of NIR (≈980 nm) radiation, highly penetrable for many materials. The shortening of up-conversion lifetimes has been attributed mainly to the changes in energy transfer rates, caused by decreased interatomic distances and increased overlap integrals between 4f electrons and the valence shells of ligand ions. The origin of high-pressure effects on the luminescence intensity, band ratio and their spectral position has been explained by the increased interactions and distortions of the crystal-field symmetry around the emitting ions in the compressed structure.

Upconverting core@shell type β-NaYF4:Yb3+–Er3+@SiO2 nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λex = 975 nm), i.e., upconversion luminescence of Er3+ and down-shifting emission of Yb3+, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er3+ and Yb3+ observed in the vis–near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er3+ levels (2H11/2 → 4I15/2/4S3/2 → 4I15/2) and the nonthermally coupled Yb3+/Er3+ levels (2F5/2 → 2F7/2/4I9/2 → 4I15/2 or 4F9/2 → 4I15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er3+ 525/545 nm (max S r = 1.31% K–1) and Yb3+/Er3+ 1010/810 nm (1.64% K–1) or 1010/660 nm (0.96% K–1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb3+/Er3+ bands, vs. the typically used Er3+ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis–NIR optical, contactless sensors of temperature.

the measured spectroscopic parameter is usually a pressure-induced line shift, i.e., spectral shift of the emission bands of Cr 3+ (ruby) or Sm 2+ . [14,15,20,22] Whereas, in the case of temperature the commonly measured parameter is the luminescence intensity ratio (LIR), i.e., band ratio of two thermally coupled levels (TCLs; separated by ≈200-2000 cm −1 ) of, e.g., Nd 3+ , Er 3+ , or Tm 3+ , which is directly related to the local temperature of the system (probe), and conforms Boltzmann distribution. [3,[23][24][25] A great number of optically active functional materials is based on the Ln 2+/3+ , because of their unique spectroscopic properties, such as multicolor photoluminescence induced by UV or near-infrared (NIR) (energy up-conversion) irradiation, narrow absorption/emission bands, large spectral shift of the emission bands in relation to the absorption ones, long emission lifetimes, etc. [26][27][28][29][30][31][32][33][34][35] Matrices hosting Ln 3+ ions are usually fluorides, oxides, vanadates, phosphates, and borates. [3,4,11,12,[19][20][21][22][23][24][25][26] This is mainly because of their resistance to photobleaching and high temperature treatment, as well as relatively low phonon energy in contrast to organic compounds. [3][4][5][19][20][21][22][23][24][25][26] Moreover, the Ln 3+ -doped inorganic materials may exhibit up-conversion (UC) phenomena, i.e., anti-Stokes emission of higher-energy photons, generated by the absorption of two or more lowerenergy photons. [32,[36][37][38][39] Thanks to the high absorption cross-section of Yb 3+ in the NIR range, and the presence of a ladder-like structure of Ln 3+ energy levels, the upconverting materials codoped with Yb 3+ / Ln 3+ (Ln 3+ = Ho 3+ , Er 3+ , Tm 3+ ) may work not only as temperature sensors, but also as optical "heaters," as during their irradiation with a high-power NIR lasers they locally heat up. [40][41][42][43][44][45][46][47][48] This is due to the occurrence of various nonradiative processes between the Ln 3+ ions, quenching luminescence of the material and leading to heat generation. [43][44][45][46][47][48] Thanks to the efficient light-to-heat conversion, the optical heating phenomenon can be utilized in photothermal therapies, thermophotovoltaics, formation of new materials under extreme conditions, etc. [43][44][45][46][47][48][49] Currently, temperature of the system can be optically monitored in a relatively broad range, starting from cryogenic up to around ≈10 3 K, whereas pressure could be monitored only in the "high-pressure" range (≈10 2 -10 6 bar). These limitations are associated with the fundamental concept of pressure sensing, i.e., measurements of physical parameters directly Currently the lowest optically determinable pressure values are around 10 2 bar, making the pressure below inaccessible for optical detection. This work shows for the first time how to overcome these limitations, and optically monitor the low pressure values in a vacuum region (from ≈10 −5 to 10 −2 bar), utilizing the light-induced and pressure-g...

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K −1 ) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb 3+ −Tm 3+ codoped YVO 4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm 3+ emissions associated with the 3 F 2,3 and 3 H 4 thermally coupled levels, that is, 3 F 2,3 → 3 H 6 / 3 H 4 → 3 H 6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Tm 3+ ( 3 H 4 → 3 H 6 ). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.