Temperature-dependent Upconversion Luminescence Research Articles

Defect level induced negative thermal quenching of β-Ba3LuAl2O7.5: Yb3+, Er3+ phosphor

Temperature-dependent behavior of phosphors has recently attracted significant research attention. This is due to the widespread applications of phosphors. Unfortunately, the thermal quenching effect in many phosphors greatly limits their performance at high temperatures. Herein, negative thermal quenching phosphor β-Ba3LuAl2O7.5: 0.3Yb3+, 0.03Er3+ is synthesized using the high-temperature solid-state method. X-ray Rietveld refinement and EPR results indicate the defect state of oxygen vacancy. Temperature-dependent upconversion luminescence spectra demonstrate that the green and red emission intensities increase with temperature to reach maximums at 398 K and 348 K respectively. Diffuse reflectance spectrum and downshifting luminescence spectrum prove that the defect state is near the 4F7/2 state of Er3+ ions. The defect state traps electrons at room temperature and releases them to populate the 4F7/2 state of Er3+ ion at elevated temperatures to cause the observed thermal enhancement of upconversion luminescence. This material's negative thermal quenching effect makes it a suitable candidate for applications in displays and anti-counterfeiting.

Multi-mode up-conversion emission of Tm@Yb@Er phosphors for color modulation and thermometry

Rare earth doped up-conversion materials play an important role in information security, colorful display and temperature sensing. In this work, the up-conversion luminescence of NaYF4:0.2Tm/20Yb@NaYF4:20Yb@NaYF4:2Er/20Yb phosphors, synthesized by typical co-precipitation method, are investigated under different excitation condition. The emission color of sample can be tuned from yellow to blue-green through changing pump laser from 1550 to 980 nm because the Tm3+ ions in the core cannot be excited by 1550 nm laser. More interesting, the value of red-to-green (R/G) and blue-to-green (B/G) can be finely manipulated by tuning the excitation pulse width and frequency. This phenomenon can be interpreted by different rising processes of the green, red and blue up-conversion emission. In addition, the core-shell structured sample displays a temperature-dependent color change, ranging from blue-green to purple. The green emissions, radiated from thermal coupled levels (2H11/24S3/2), are used in thermometry and the max relatively sensing sensitivity reaches 0.01 K−1 at 303 K. The findings indicate that the core-shell Tm@Yb@Er phosphors, with excitation-dependent and temperature-dependent up-conversion luminescence, have potential applications in information security and temperature sensing.

Optical temperature sensing of Er3+/Yb3+-doped PSN–PMN–PT crystal based on temperature-dependent upconversion luminescence

The optical temperature sensing of Er3+/Yb3+-doped Pb(Sc1/2Nb1/2)O3 (PSN)–Pb(Mg1/3Nb2/3)O3 (PMN)–PbTiO3 (PT) (6PSN–61PMN–33PT with 2- wt% Er2O3 and 2- wt% Yb2O3) crystal was investigated based on temperature-dependent upconversion luminescence. Apparent difference in variation of luminescence intensity with temperature was observed for different emission bands. Moreover, excellent relative sensitivity and comparable absolute sensitivity were demonstrated based on the thermally coupled levels (2H11/2 and 4S3/2) and nonthermally coupled levels (4S3/2 and 4F9/2), respectively. These findings hold substantial implications for promoting the development of ferroelectric crystals towards electric-optical integration.

Near-zero thermal quenching of upconversion green emission implemented with the polymorphic phase transition

Thermally stable upconversion luminescence is highly expected for applications in high-temperature environments. Herein, the mechanism of the phase transition inducing near-zero thermal quenching in upconversion green light is reported. The phosphor Ba3Gd0.78Yb0.2Er0·02Al2O7.5 is synthesized by the conventional high-temperature solid-state method and quenched in air during cooling. In-situ XRD and DSC curves demonstrate the polymorphic phase transition in the subsequent heating. The temperature-dependent upconversion luminescence exhibits stable green emission instead of thermal quenching when the material is excited by a 980 nm near-infrared laser. The more intense green emission resulting from the low symmetry of the low-temperature phase and its increase in the phase content with temperature are responsible for the thermal stability in the luminescence of the material. This research provides an approach to utilizing phase transformation to obtain stable emissions in upconversion phosphors.

Opposite luminescence thermal behavior of upconversion core/shell nanocrystals for anticounterfeiting.

Inorganic luminescent materials generally suffer from thermal quenching due to accelerated nonradiative transitions at high temperature, whereas Yb3+ sensitized core nanocrystals with small sizes (<30 nm) exhibit a temperature-dependent upconversion luminescence (UCL) enhancement. The related mechanism of the anomalous UCL thermal behavior is still under debate. In this work, we find that the UCL of NaGdF4:Yb/Tm@NaGdF4 inert-shell nanocrystals declines at elevated temperature, while that of NaGdF4:Yb/Ho@NaGdF4:Yb active-shell ones is enhanced. The thermally-induced UCL enhancement of active-shell nanocrystals is attributed to a gradually attenuated surface quenching effect. The initiators of the surface quenching are H2O molecules, which mainly attenuate Yb3+ excited states through an overtone energy transfer. The energy transfer is a coupling effect between ion dipoles of Yb3+ and atomic dipoles of H2O. Utilizing the opposite UCL temperature-dependence of active- and inert-shell nanocrystals, we designed their hybrids, which exhibit temperature-responsive multicolor emissions. The color-tunable hybrids are demonstrated to be excellent candidates for producing anticounterfeiting patterns with high security but simple recognition methods.

Blue frequency upconversion emission and optical thermometry of Ca3(PO4)2: Tm3+/Yb3+

Rhombohedral phase Ca3(PO4)2: Tm3+/Yb3+ phosphor is synthesized via hydrothermal route, and then frequency upconversion emission is studied upon 980 nm excitation. The samples have resulted strong blue emission around 472 nm and two weak emission bands at 650 and 700 nm wavelengths. The blue emission was optimized by changing the Yb3+ ion concentration, and optimum emission is observed for the sample having 0.5 mol% Tm3+ and 5 mol% Yb3+. The temperature-dependent upconversion luminescence studies were done under 980 nm excitation. The Judd–Ofelt intensity parameters were calculated using the absorption spectra and found in Ω2 > Ω4 > Ω6 order. Further temperature sensitivity of sample is explored by considering 1G4 and 3F2,3 levels as non-thermally coupled. Maximum absolute sensor sensitivity is found to be 3.63 × 10−3 K−1 at 623 K. The results indicate that Ca3(PO4)2 host is suitable for achieving intense blue upconversion emission and may be helpful for biological imaging considering the biocompatibility nature of Ca3(PO4)2.

NaGd9(SiO4)6O2:Yb3+-Er3+/Tm3+: Optical thermometric materials of high-sensitivity by using different strategies

Herein, a series of Er3+/Yb3+ and Tm3+/Yb3+ co-doped NaGd9(SiO4)6O2 (abbreviated as NGS) phosphors were prepared by solid-state reaction method. The phase purity and optical properties were investigated systematically by XRD, luminescence spectra and decay curves. The temperature-dependent upconversion luminescence was studied by using the fluorescence intensity ratio (FIR) technique. For NGS:1%Er3+,20%Yb3+, the variation of FIR for 521 and 554 nm emissions with temperature was evaluated. For NGS:0.5%Tm3+,10%Yb3+, four emission peaks were utilized to characterize the optical temperature-sensing properties. High absolute and relative sensitivities were obtained. The above results indicated that the as-prepared phosphors could show potential applications in optical thermometer.

A novel upconversion optical thermometers derived from non-thermal coupling levels of CaZnOS:Tm/Yb phosphors

The growing interest in precise, rapid and contactless optical temperature sensor has led to the more demand for novel materials. In this work, Tm3+/Yb3+ co-doped CaZnOS phosphors were synthesized via a simple method of precipitation. The upconversion luminescence (UC) properties and optical temperature behavior were investigated. The strongest UC intensity was achieved with the doping concentration of 1:9 (Tm:Yb) and the emission bands located at 483 ​nm, 652 ​nm and 792 ​nm were ascribed to 1G4→3H6, 1G4→3F4 and 3H4→3H6 transitions, respectively. The optical thermometry using fluorescence intensity ratio (FIR) based on non-thermal coupling levels (NTCLs, 1G4 and 3H4) was employed to assess temperature sensing performance through analyzing temperature-dependent UC spectra of CaZnOS: Tm/Yb (1:9) sample. The maximum sensing sensitivity was 0.0262 ​K−1 ​at 393 ​K, as a comparation, which is more than 20 times higher than that of thermal coupling levels (TCLs, 1G4(a), and 1G4(b)). The result shows the validity of optical thermometry based on NTCLs, simultaneously suggesting that CaZnOS has great potential in temperature sensors.

Temperature-dependent upconversion luminescence multicolor tuning and temperature sensing of multifunctional β-NaYF4:Yb/Er@β-NaYF4:Yb/Tm microcrystals

Multifunctional β-NaYF4:Yb/Er@β-NaYF4:Yb/Tm microcrystals with simultaneous white-light emission, temperature-dependent upconversion luminescence multicolor tuning and good temperature sensitivity.

Gold nanorods conjugated upconversion nanoparticles nanocomposites for simultaneous bioimaging, local temperature sensing and photothermal therapy of OML-1 oral cancer cells

ABSTRACT The major challenge in photothermal therapy (PTT) is to develop nanocomposites that simultaneously exhibit bioimaging and PTT under a single near-infrared (NIR) irradiation with high therapeutic efficiency. Herein, we present a multifunctional nanocomposite synthesized by linking NaYF4:Yb3+,Er3+ upconversion nanoparticles (UCNPs) with gold nanorods (AuNR) to exhibit fluorescence labeling, local temperature sensing and photothermal functions simultaneously with a single NIR laser excitation. The AuNR-NaYF4:Yb3+,Er3+ nanocomposite particles displayed better photothermal properties compared with pure AuNRs or a blend of AuNRs and NaYF4:Yb3+,Er3+ UCNPs. The temperature-dependent upconversion luminescence (UCL) property was used to determine local temperature at the nanocomposite particles, which is useful for selecting appropriate irradiation dosage for PTT. The therapeutic performance of the nanocomposites in PTT for OML-1 oral cancer cells was determined. For cell labeling, we successfully labeled streptavidin-linked nanocomposite particles on the surface of OML-1 oral cancer using anti-human epidermal growth factor receptor 2 (anti-Her2) antibody. Finally, the nanocomposite particles caused exceptional destruction of cancer cells up to 70% dead cells under 976 nm laser irradiation for only one min at 0.3 W/cm2 which is below the maximal permissible exposure of human skin.

Upconversion process in BaY2 F8 :Yb3+ ,Ho3+ phosphor for optical thermometry.

A BaY2 F8 :Yb3+ ,Ho3+ phosphor was synthesized using a simple precipitation method. The temperature-sensing properties of BaY2 F8 :Yb3+ ,Ho3+ phosphor were investigated using the fluorescence intensity ratio (FIR) of the thermally coupled energy levels (5 F4 and 5 S2 ) of Ho3+ . Energy transitions 5 F4 →5 I8 and 5 S2 →5 I8 gave rise to emission peaks at 538 nm and 549 nm, respectively. The areas under these two emission peaks were used to calculate the FIR. Temperature-dependent upconversion luminescence was recorded in the temperature range 303-623 K using a 980 nm laser excitation source. The results suggested that the BaY2 F8 :Yb3+ , Ho3+ phosphor has the potential to be used as a non-contact optical temperature sensor.

Temperature-dependent upconversion luminescence in ferroelectric 1.5%Pr/3%Yb:0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 transparent ceramics for non-contact thermometry and optical heating

Non-contact optical thermometry based upon fluorescence intensity ratio (FIR) has attracted much attention because of its excellent accuracy and sensitivity. Recently we reported the upconversion luminescence in Pr3+/Yb3+ co-doped 0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 transparent ceramics [Z. Lv et al., Ceramics International 45 (2019) 10924–10929]. In this article we study the temperature dependence of upconversion emissions from the 1.5%Pr/3%Yb:0.75Pb(Mg1/3Nb2/3)O3–0.25PbTiO3 transparent ceramics. The FIR between the 1D2-3H4 and 3P0–3H4 emissions fits a thermally coupled-levels-like equation, and exhibits the maximum relative sensitivity of 1.03% K−1 at the temperature 320 K. Meanwhile, the FIRs of IRed/IGreen, IRed/IBlue and IRed/IBG show linear responses versus temperature and illustrate the high constant sensitivities of 1.09%–2.9% K−1, but in a narrow temperature range of 140–240 K. The multiple temperature-sensing performances offer us chances to select distinct response functions on the basis of the practical demands. In addition, laser induced heating was observed in the transparent ceramics. The generated temperature in the ceramics can be accurately monitored and regulated from 329 to 377 K by changing the excitation power from 1.58 to 8.61 W/cm2. The high heating efficiency of ~6.83 K cm2/W makes the transparent ceramics suitable for optical heater. The transparent ceramics exhibit excellent piezoelectric and ferroelectric performances, and thus they are promising candidates for multifunctional optical-electro devices besides non-contact thermometer and optical heater.

Up-conversion luminescence of Mn4+ sensitized byYb3+, Er3+ in NaMgGdTeO6

A series of NaMgGdTeO6: Yb3+/Er3+/Mn4+ phosphors have been prepared by a high-temperature solid-state reaction method. The as-prepared products exhibit typical deep red 696 nm emission of Mn4+ under 980 nm laser excitation, which is performed through the Yb3+→Er3+→Mn4+ multi-step energy transfers. The Yb3+, Er3+ and Mn4+ concentrations dependent up-conversion emission spectra, the Mn4+ concentration dependent Er3+ 544 nm emission lifetimes, and the pumping power and temperature dependent up-conversion luminescence intensities have been investigated in detail. The preliminary study in this work extends the up-conversion spectral range of the traditional up-conversion materials and provides reference for exploring new transition metal based up-conversion phosphors.

Thermal decomposition synthesis of Er3+-activated NaYbF4 upconverting microparticles for optical temperature sensing

The Er3+-activated NaYbF4 microparticles were synthesized by a thermal decomposition method and their temperature-dependent upconversion luminescence dynamics were studied. The fluorescence intensity ratio change and optical temperature sensing have been evaluated by analyzing emission data obtained under 980 nm laser excitation. Maximum relative sensitivity of 3.46%·K−1 was achieved at the temperature of 175 K, providing encouraging results for temperature sensor application. It is necessary to mention that besides the promising temperature sensing capabilities, synthesized microparticles have potential for application as anti-counterfeiting pigments.

Upconversion luminescence of Ca2Gd8(SiO4)6O2:Yb3+-Tm3+-Tb3+/Eu3+ phosphors for optical temperature sensing

In order to develop new upconversion (UC) phosphors for optical temperature sensors, a series of Yb3+-Tm3+-Tb3+/Eu3+ doped Ca2Gd8(SiO4)6O2 (CGS) samples were designed by solid-state reaction method. The phase purity was examined by XRD patterns, which revealed that the obtained samples are all single-phase in spite of the rare earth (RE) ions doping. Upon 980 nm excitation, the emission peaks of all the doped luminescent ions (Tm3+, Tb3+ and Eu3+) have been observed, and the energy transfer processes among the RE ions in the UC luminescence were discussed with the help of the energy level diagram and decay curves. The competitive absorption between Tm3+ and Tb3+/Eu3+ from Yb3+ was proposed to interpret the sharp decrease of Tm3+ emission intensity. The temperature sensing behavior was evaluated by measuring the temperature-dependent UC luminescence of the typical CGS:10%Yb3+,0.2%Tm3+,10%Tb3+,1.25%Eu3+ phosphor. It has been found that the fluorescence intensity ratio (FIR) technique by employing ITb/IEu and ITb/ITm can generate high absolute sensitivity, but the relative sensitivity still needs further improvement compared with some other phosphors doped with Er3+/Ho3+/Tm3+.

Temperature-dependent upconversion luminescence and spectra characteristic of Er3+/Yb3+ co-doped fluorotellurite glasses

Er3+/Yb3+ co-doped fluorotellurite glasses were prepared. Intense green and red upconversion (UC) emissions corresponding to the transitions of 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 were observed under 980 nm laser excitation, benefiting from the advantages of low phonon energy and good stability of fluorotellurite glasses. Temperature-dependent UC luminescence was carried on glasses with different doping concentration of Er3+ in the range of 298–568 K. Fluorescence intensity ratios (FIR) and absolute sensitivities (Sa) were calculated, and the maximal sensitivity value of 54.09 × 10−4 K−1 was obtained at 531 K in the glass with the lowest Er3+ concentration at 0.1 mol%. This study indicates that Er3+/Yb3+ co-doped fluorotellurite glasses can be promising materials applied to non-touching temperature sensors.

Temperature sensing based on the 4F7/2/4S3/2−4I15/2 upconversion luminescence intensity ratio in NaYF4:Er3+/Yb3+ nanocrystals

Different from the popular 2H11/2/4S3/2–4I15/2 emission bands of Er3+, here we demonstrate for the first time the temperature detecting ability of the 4F7/2/4S3/2–−4I15/2 transitions of Er3+ embedded in NaYF4 nanocrystals via luminescent ratiometric method. Following the NIR irradiation at 980 nm, the characteristic 2H11/2/4S3/2–−4I15/2 transitions of Er3+, and the 4F7/2–4I15/2 emission line that is rarely studied are successfully detected. The 4F7/2/2H11/2/4S3/2 excited states are found to own the same lifetime. By studying on the temperature dependent upconversion luminescence, the 4F7/2/4S3/2 states, and the 2H11/2/4S3/2 ones are both confirmed to be thermally coupled. The luminescence intensity ratio between each pair of states thus could be used for thermal sensing. Using the 4F7/2/4S3/2–4I15/2 emission bands could achieve a larger relative sensitivity than the traditional 2H11/2/4S3/2–4I15/2 ones in the range of 332–483 K. Therefore, our work proposes a promising candidate for temperature measurement, which may find its application in the future.

Preparation and Properties of Rare-earth-doped Na2Ca3Si2O8 and BaZrSi3O9 Matrix Luminescent Materials

To achieve higher quality temperature measurement, this paper studies the rare-earth-doped Na2Ca3Si2O8 and BaZrSi3O9 matrix luminescent materials. The paper mainly adopts sol-gel method to prepare luminescent material in the form of thin film and powder. By comparing the thermodynamic reaction of the two materials and their respective properties, the differences between the two are further distinguished. The results illustrate the temperature-dependent up-conversion luminescence mechanism of thin film and explain the relationship between the up-conversion luminescence intensity and temperature. The results show that more accurate temperature measurements can be achieved.

Fabrication of highly porous Y2O3:Ho,Yb ceramic and its thermometric applications

Highly porous Y2O3:Ho,Yb ceramic with relative high compressive strength was successfully fabricated by using NaF as pore-forming agent for the first time. The porosity, pore size, compressive strength and temperature-dependent upconversion luminescence of the as-fabricated porous Y2O3:Ho,Yb ceramic were investigated in detail. It was found that the open porosity and compressive strength of the as-fabricated porous Y2O3:Ho,Yb ceramic were 43.12% and 28.53 MPa, respectively, and the pore size was 4–8 μm. Under 980 nm excitation, the upconversion luminescence intensity ratio (I550/I665) of the 550 nm and 665 nm emissions varied from 8.10 to 4.63 with the temperature increasing from 293 K to 873 K. Porous Y2O3:Ho,Yb ceramic with such properties could be applied to ultra-high temperature ceramic structures for space application, luminescent probes and temperature sensors.

Morphology controllable synthesis and upconversion luminescence of Gd4O3F6: Ho3+, Yb3+ for temperature sensing

A series of Yb3+, Ho3+ co-doped Gd4O3F6 with different morphologies (nanoparticles, oval and capsule-like submicrocrystals) and sizes are prepared by a facile hydrothermal route. The results of X-ray diffraction and scanning electron microscopy show that all pH values, organic additives and Li+ concentration play a significant effect on the crystallinity and morphology of the samples. And then, these conditions influence the upconversion (UC) luminescence intensity excited by 980 nm laser without changing the ratio of green and red emitting light obviously. Moreover, after introducing Tm3+, Gd4O3F6: Tm3+, Ho3+, Yb3+ exhibits excellent multicolor emission in the visible region. The corresponding UC mechanisms are elucidated through the laser power dependence of UC emissions. Furthermore, the temperature-dependent UC luminescence of Gd4O3F6: Ho3+, Yb3+ and Gd4O3F6: Tm3+, Ho3+, Yb3+ are investigated, and the results show that the ratio of green to red emission in Gd4O3F6: Ho3+, Yb3+ and Gd4O3F6: Tm3+, Ho3+, Yb3+ both respond single exponentially to temperature, which suggests their application for sense temperature.