Time-delayed sensing strategy based on a fluorescence decay profile for achieving highly sensitive optical thermometry.

A time-delayed temperature sensing method based on a fluorescence decay profile was proposed for the purpose of boosting the temperature sensitivity of optical thermometry. The thermal quenching effect of the LiCa3ZnV3O12 sample was investigated to evaluate the viability of the method. Specifically, by applying the integral intensity obtained after a certain delay time in the normalized decay profile as a probing parameter, high relative sensitivity with a maximum of 13.5% K-1 was achieved. The high relative sensitivity along with the good reversibility verified by the temperature cycling test indicate that the time-delayed sensing strategy proposed here is promising for excellent optical thermometry.

Explaining the temperature-induced shift of the V-O charge transfer band experimentally and design of single-excitation ratiometric optical thermometry.

Temperature-induced redshift of the V-O charge transfer band (CTB) is promising for designing high performance optical thermometry. The shift mechanism is considered as the thermal populations of high vibrational energy levels of the VO4 3- ground state. Direct experimental evidence for this, however, is still lacking. In this work, Tm3+-doped YVO4 with various doping concentrations was studied to achieve strong 1D2 emission of Tm3+. The temperature dependent CTB was studied at low temperatures to give direct evidence experimentally for the shift mechanism of the CTB using YVO4:20% Tm3+. It was found that the V-O CTB does not shift when the temperature is lower than a certain temperature (60 K), verifying the proposed shift mechanism experimentally. In addition, based on the temperature quenching of 1D2 emission of Tm3+ and the redshift of the CTB, single-excitation ratiometric thermometry was carried out using YVO4:30% Tm3+,6% Sm3+. High relative sensitivity was achieved with a maximal value reaching up to 3.86% K-1 at approximately 355 K.

Design of opposite thermal behaviors in Tm3+/Eu3+ co-doped YVO4 for high-sensitivity optical thermometry.

Temperature-induced redshift of the V-O charge transfer band edge and the temperature quenching effect were combined for designing ratiometric optical thermometry. Following this approach, opposite thermal behaviors of Tm3+ and Eu3+ emissions were realized in the range of 300 to 380 K in Tm3+/Eu3+ co-doped YVO4. Applying the temperature dependent fluorescence intensity ratio of Eu3+ to Tm3+ as temperature readout, the maximal relative sensitivity reaches up to 4.6%K-1 around 330 K. This result makes our proposed strategy an excellent candidate for developing high-sensitivity optical thermometry.

Ratiometric optical thermometry based on temperature-induced shift of V-O charge transfer band edge.

Ratiometric optical thermometry was designed using temperature-induced shift of V-O charge transfer band (CTB) edge combined with temperature-induced variation of Tb3+ emission in YV1-xPxO4. P was introduced into YVO4 lattice to form YV1-xPxO4 solid solution successfully, with the purpose of enhancing Tb3+ emission. Under 352 nm excitation which locates in the tail of the V-O CTB, emission spectra of YV0.3P0.7O4:Tb3+, Eu3+/Sm3+ were recorded at a series of temperatures ranging from 300 to 440 K. It is demonstrated that Tb3+ and Eu3+/Sm3+ emissions exhibit opposite temperature dependences. The mechanisms for such opposite variations have been interpreted in detail. Based on the varied fluorescence intensity ratio of Eu3+/Sm3+ to Tb3+ with temperature, high relative sensitivity was obtained with a maximal value of 2.85% K-1 around 365 K. Our results imply that the proposed strategy is a promising candidate for high-sensitive optical temperature sensing.

Optical thermometry based on cooperation of temperature-induced shift of charge transfer band edge and thermal coupling.

A novel optical thermometry is put forward, based on the cooperation of temperature-induced red shift of the charge transfer band (CTB) edge of the vanadates and thermal population of the thermally coupled energy levels (TCELs). Particularly, temperature-dependent CTB of Sm3+ (Er3+) doped LuVO4 was investigated from 300 to 480 K. Then, under the excitation of 360 nm at which the excitation efficiency enhances with temperature due to the temperature induced red shift of the CTB edge, temperature-dependent emissions of the TCELs of Sm3+ and Er3+ were investigated. The results indicate that the emission from the upper-level in the TCELs exhibits a dramatic increase, along with the increase of temperature. High relative sensitivity of 4304/T2 was obtained, which is remarkably superior to the previous reported sensors, using the temperature-dependent fluorescence intensity ratio of TCELs. This suggests that the proposed strategy is a promising candidate for highly sensitive optical thermometry.

Origin of the temperature-induced redshift of the charge transfer band of GdVO4.

The charge transfer band (CTB) of the VO43- groups in vanadates shifts to longer wavelengths with increasing temperature. The origin of this temperature-induced redshift was explored by studying the temperature-dependent excitation and emission spectra of GdVO4 ranging from 300 to 480 K. The influences of the thermal population and the decline of the charge transfer gap on the spectral shift were analyzed using the configurational coordinate diagram. We conclude that the thermal population of vibrational sublevels of the ground electronic energy level dominates the temperature-induced redshift of the CTB. Taking advantage of the redshift and the thermal quenching, a novel ratiometric temperature-sensing strategy was proposed. Drastic temperature dependence was achieved, indicating a promising candidate for an optical thermometer with high sensing performance.

Nonlinear Composition-Dependent Optical Spectroscopy of Ba2xSr2-2xV2O7.

In general, adjusting the composition of a fluorescent material is an effective way to tune its luminescent properties such as peak energy and bandwidth. In most solid-solutions, the emission peak shifts linearly with the materials' composition, which is referred to as Vegard's Law. However, we found extraordinary variations in our samples Ba2xSr2-2xV2O7, that is, both the excitation and emission peaks show nonlinear dependence on the composition x, and the same is true for the spectral bandwidths. The nonlinearities are not due to structural anomaly, as all the samples are confirmed to be solid-solutions by X-ray diffraction measurements. To explain these phenomena, we proposed a model by considering the disorder of Ba(2+) and Sr(2+) distributions in solid-solutions and the changes of configurations between the ground and excited electronic states. This novel phenomenon could be applied to further exploit new fluorescent materials.

Strategy for thermometry via Tm³âº-doped NaYF4 core-shell nanoparticles.

Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, ³F(2,3) and ¹G4, of Tm³âº in ß-NaYF4:20%Yb³âº, 0.5%Tm³âº/NaYF4:1%Pr³âº core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53% K⁻¹ at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry.

Pr(3+)-doped beta-NaYF4 for temperature sensing with fluorescence intensity ratio technique.

Pure beta-NaYF4:0.8%Pr3+ powder sample was synthesized by the hydrothermal method. The temperature dependence of the fluorescence intensity ratio (FIR) of emission bands corresponding to the 3P1 --> 3H5 and 3P0 --> 3H5 transitions was measured in the temperature range of 120 K to 300 K excited by a 473 nm continuous wave (CW) laser. The dependence of the FIR on temperature is well fitted with an exponential function and the effective energy difference obtained is 457 cm(-1), which gives further an absolute temperature sensitivity of 0.01352 K(-1) at 300 K. The monotonous increase of FIR with temperature and high absolute temperature sensitivity demonstrate that this material can be used as temperature sensor. In addition, mono-dispersed NaYF4:1%Pr3+ nanoparticles were also synthesized.

Local field effect on the photoluminescent spectra of Eu3+ ions in glass.

To investigate the effect of the dielectric medium on the spontaneous emission rate of an isolated emitter, two series of glass samples of various compositions lightly doped with the Eu3+ ion were prepared by melt-quenching method. According to the enhancement factors for emission rates due to the refractive index of the dielectric medium, we qualitatively analyzed the intensities of the electric dipole and magnetic dipole transitions by comparing the emission spectra of the samples with different compositions, viz. various refractive indices. This preliminary result indicates that the local-field effect on the spontaneous emission rates follows the virtual-cavity model, which is derived by assuming that single-ion emitters enter the medium without disturbing the medium, i.e., as interstitial ions or by replacing host ions of low polarizability.

A novel strategy for thermometry based on the temperature-induced red shift of the charge transfer band edge.

We report a novel strategy for optical temperature sensing using the temperature-induced red shift of the charge transfer band (CTB) edge of the VO43- groups in GdVO4:5% Sm3+. Excitation spectra were recorded at a series of temperatures ranging from 300 to 480 K. It is demonstrated that an excitation intensity of around 360 nm corresponding to the tail of the CTB and an excitation intensity of 407.6 nm corresponding to the 6H5/2 → 4F7/2 transition of Sm3+ exhibit opposite temperature dependence. Based on this, the relative sensitivity was obtained to be 3313/T2 in our investigated temperature range, which is remarkable progress compared with the optical temperature sensors reported previously. We believe that this work broadens the pathway for the design of highly sensitive temperature sensing materials.