Thermally Coupled Energy Levels Research Articles

Color-Tunable Upconversion Luminescence and Multiple Temperature Sensing and Optical Heating Properties of Ba3Y4O9:Er3+/Yb3+ Phosphors

Upconversion (UC) luminescence materials doped with rare earth ions are extensively investigated as optical temperature probes by the fluorescence intensity ratio technique. However, most Er3+-doped materials are still suffering from low sensing sensitivity. In the present study, we attempt to develop high-sensing-sensitivity Er3+-doped materials based on the thermally coupled energy levels (TCLs) from Stark sublevels as well as the properties at subzero temperatures, for which there is continuous lack of research. Er3+/Yb3+ codoped Ba3Y4O9 (BYO) phosphors were produced via a solid-state reaction. Excited by 980 nm, various output colors, including bright green, yellow, and red, in BYO:Er3+/Yb3+ phosphors as well as the relative emission intensities could be regulated through altering Yb3+ concentrations. Subsequently, on the basis of all 12 pairs of TCLs especially from Stark sublevels of 2H11/2, 4S3/2, and 4F9/2 of Er3+ ions, multiple temperature-sensing performances are evaluated over a wide range of 73–573 K. The results show that the maximum sensitivity of the 2H11/2 and 4S3/2(1) levels is approximately 1-fold higher than that of traditional TCLs of 2H11/2/4S3/2 at elevated temperature and the maximum sensitivity based on the 2H11/2(1) and 2H11/2(2) levels is more than 12 times higher than that of the traditional TCLs of 2H11/2/4S3/2 at subzero temperature. Therefore, it is expected to realize high-sensitivity temperature detection from subzero to elevated temperatures by combining two pairs of different TCLs. In addition, the potential of Er3+/Yb3+ codoped BYO phosphors to be used as an optical heater is studied. The generated temperature can be accurately monitored by BYO:Er3+/Yb3+ phosphors and regulated by adjusting the excitation power, which indicate that BYO:Er3+/Yb3+ phosphors can be used as an optical heating device.

Improved upconversion efficiency and thermal stability of NaYF4@SiO2 photonic crystal film

Fluorescent photonic crystal films (PCFs), which consisted of monodisperse NaYF4:10Yb, 0.5Tm@SiO2 or NaYF4:10Yb, 0.5Er, 0.2Tm@SiO2 core-shell spheres (10%, 0.5% and 0.2% represent the reactant mole percentage), were successfully fabricated. The morphologies and structures of PCFs were measured, while the fluorescent properties of NaYF4 in PCFs with different photonic band gap (PBG) positions were investigated. Besides, the energy transfer process in thermal coupled energy levels of NaYF4:10Yb, 0.5Tm@SiO2 under different temperatures was discussed. The results indicated that the short wave emission of NaYF4 can be enhanced by the regulation of photon mode density (PMD) in photonic crystals. Particularly, when the infrared 800 nm emission peak was in the range of PBG position of the PCF sample, the energy transfer process of 3H4-3H6 was suppressed and the short wave emission was enhanced by 1.64 times. Due to the well thermal radiation ability of silica shell, the thermal coupled energy levels (3F3 and 3H4) of NaYF4:10Yb, 0.5Tm@SiO2 core-shell spheres exhibited more stable emission intensities than NaYF4:10Yb, 0.5Tm nanoparticles, when the experimental temperatures varied from 30 to 300 °C.

A novel upconversion luminescent material: Li+- or Mg2+-codoped Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors and their temperature sensing properties

A novel upconversion (UC) luminescent phosphor Bi3.84W0.16O6.24:Tm3+, Yb3+ was synthesized via co-precipitation method. Distinct visible blue, red, and near-infrared light and weak purple and ultraviolet light UC emissions were all detected under 980 nm excitation in Tm3+–Yb3+-codoped Bi3.84W0.16O6.24 phosphor. Addition of Li+ or Mg2+ ions in Tm3+-and Yb3+-codoped Bi3.84W0.16O6.24 phosphors further enhanced the blue and red emission intensities. The slope values of the log–log graph of pump power and emission intensity decreased to 1 for the blue emission because of the saturation effect, which was investigated in detail by theoretical and experiment analyses. Optical temperature sensing performances were evaluated in the NIR region from 313 K to 573 K based on the 3H4(1) and 3H4(2) of Tm3+ ions as thermal coupled energy levels. The corresponding optimum sensitivities were 0.000676, 0.00103, and 0.000696 K−1 at 323 K for free, Li+ ion-, and Mg2+ ion-codoped Bi3.84W0.16O6.24: Tm3+, Yb3+ phosphors, respectively. The introduction of alkali metals (Li) or alkaline earth metals (Mg) also enhanced the temperature sensitivity of the Bi3.84W0.16O6.24:Tm3+, Yb3+ phosphors in the NIR region.

Near-infrared to short-wavelength upconversion temperature sensing in transparent bulk glass ceramics containing hexagonal NaGdF_4: Yb^3+/Ho^3+ nanocrystals

Transparent glass ceramics containing hexagonal NaGdF4: Yb3+, Ho3+ nanocrystals were successfully fabricated via self-crystallization, which was further confirmed by XRD, TEM, HRTEM, and STEM-HADDF, as well as upconversion (UC) emission spectra. Impressively, GC750 exhibited fascinating upconversion luminescence, and the corresponding 5F1/5G6 and 5F2,3/3K8 states of Ho3+ were proven to be thermally coupled energy levels (TCELs), resulting in high temperature-sensitive behaviors based on fluorescence intensity ratio (FIR) for optical thermometry. As a consequence, a high relative sensitivity of 1.43%·K−1 at 390 K was achieved, offering a great potentially application in optical thermometry.

Optical thermometry based on up-conversion luminescence of Tm3+ doped transparent Sr2YF7 glass ceramics

Tm3+/Yb3+ co-doped transparent glass ceramics (GC) containing Sr2YF7 nanocrystals were successfully manufactured by traditional melt-quenching technique. Their structural and up-conversion luminescent properties were systemically investigated through X-ray diffraction (XRD), transmission electron microscope (TEM) and a series of spectroscopy methods. Tremendously enhanced up-conversion emissions with obvious Stark splitting and prolonged luminescence lifetime were observed after crystallization. These optical spectroscopy results manifest that Tm3+ ions have incorporated into the Sr2YF7 crystalline lattice with low phonon energy (~ 400cm−1) after heat-treatment. The 3F2,3 and 3H4 levels of Tm3+ can be explored as thermally coupled energy levels (TCEL) for temperature sensing because of their befitting energy gap (~ 2000cm−1). At the same time, the population of 1G4 state of Tm3+ is indirectly originated from 3H4 one. Therefore the temperature sensing performances of Tm3+ doped Sr2YF7 GC were investigated by fluorescence intensity ratio (FIR) between 3F2,3 → 3H6 and 1G4 → 3F4 up-conversion emissions of Tm3+. Results show that the theoretical maximum value of relative sensitivity SR-max is 1.16%K−1 at 428 K, and absolute sensitivity SA keeps increasing with temperature. Such Tm3+/Yb3+ co-doped Sr2YF7 GC may be excellent candidate for optical temperature sensors.

Self-crystallized novel transparent Na5Yb9F32: Er3+ glass-ceramics for optical thermometry and spectral conversion

Novel self-crystallized transparent Na5Yb9F32: Er3+ glass-ceramics (GCs) were successfully elaborated through high temperature melt-quenching route for the first time. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and photoluminescence emission spectra confirmed that the Na5Yb9F32 nanocrystals (NCs) were well-formed without any additional phase. In addition, X-ray photoelectron spectroscopy (XPS) was studied for investigating valence states of elements in the Na5Yb9F32: Er3+ GCs and it was suggesting that Er3+ ions was doped into the Na5Yb9F32: Er3+ GCs. Interestingly, the XRD patterns and TEM images demonstrate that the Na5Yb9F32 NCs have already formed during melt-quenching process without any further heat treatment. Due to improved crystallinity, reduced surface-to-volume ratio and more Er3+ incorporation into NCs after further heat treatment, both up-conversion (UC) and down-conversion (DC) show enhanced luminescence intensity after heat treatment. The emission color of Na5Yb9F32 GCs changed under different ultraviolet excitation wavelength was addressed and interpreted by photoluminescence. Furthermore, via the fluorescence intensity ratio (FIR), the temperature-dependent green UC emissions of Na5Yb9F32: Er3+ GCs were studied under 980 nm laser excitation. Provided with broad operating temperature range (300–773 K), large energy gap of thermal coupled energy levels (835 cm−1) and high sensitivity (1.33% K−1 at 300 K), the Na5Yb9F32: Er3+ GCs may have potential application in temperature senor.

Citric-assisted sol-gel based Er3+/Yb3+-codoped Na0.5Gd0.5MoO4: A novel highly-efficient infrared-to-visible upconversion material for optical temperature sensors and optical heaters

The Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors were prepared via a facile sol-gel method. Under 980nm excitation, the characteristic emissions of Er3+ ions were observed in all the samples and the upconversion (UC) emission intensity increased gradually with the increment of Yb3+ ion concentration, achieving its optimum value when the doping concentration was 0.11mol. Meanwhile, the UC mechanism for both green and red emissions in Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors was a two-photon process. Furthermore, the fluorescence intensity ratio of two green UC emissions from the thermally-coupled energy levels (2H11/2, 4S3/2) of Er3+ ions was studied as a function of temperature in the range of 298–778K. The maximum sensitivity was determined to be around 0.00856K−1 at 590K, which was independent of the UC emission intensity. The heating effect which was induced by the laser excitation was also investigated. It was found that the temperature of sample varied from 265.143 to 328.094K with elevating the pump power from 334 to 1032mW. These results demonstrated that the Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors may have potential applications in optical temperature sensors and optical heaters.

Excitation powder dependent optical temperature behavior of Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics.

The knowledge of the pump power for which the population of thermally coupled energy levels (TCL) changes with power increase is of valuable importance for optical temperature sensors. In this paper, novel Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics was fabricated successfully, and its structure is studied by XRD, TEM and HRTEM analyses. The 2H11/2/4S3/2, 4F9/2(1)/4F9/2(2), and 4I9/2(1)/4I9/2(2) levels of Er3+ are proved as TCL by analyzing the temperature dependent fluorescence intensity ratios. The spectrum split, thermal quenching ratio, population stability, and temperature sensitivity from three TCL are observed to be dependent on the pump power. A new fitting method has been developed to establish the relation between fluorescence intensity ratios and temperature. It is found that the combined use of 2H11/2/4S3/2 and 4F9/2(1)/4F9/2(2) as thermally coupled energy levels will get a more precise temperature reading from 62.7 K to 800 K with the help of low excitation power at 66.8 mW/mm2.

Luminescent nano-thermometer based on up-conversion luminescence behavior of transparent Ba2ErF7 nano-glass ceramics

Transparent novel Ba2ErF7 nano-glass ceramics was fabricated by melt-quenching technique. X-ray diffraction and transmission electron microscopy analyses show that tetragonal Ba2ErF7 nano-crystals are homogeneously precipitated among the glass matrix. The 2H11/2/4S3/2 and 4F9/2(1)/4F9/2(2) levels of Er3+ are proved as thermally coupled energy levels (TCL) by analyzing fluorescence intensity ratios of 523 nm/548 nm and 654 nm/666 nm emissions of Ba2ErF7 glass ceramics. Temperature-dependent spectra show that Ba2ErF7 nano-glass ceramics present broad operating temperature range, large energy gap of TCL (844 cm−1), and high theoretical maximum value of relative sensitivity, which suggests that Ba2ErF7 nano-glass ceramics may be excellent candidates for optical temperature sensors.

Optical Thermometry Based on Up‐Conversion Luminescence Behavior of Er3+ ‐Doped Transparent Sr2YbF7 Glass‐Ceramics

Transparent novel glass‐ceramics containing Sr2YbF7:Er3+ nanocrystals were successfully fabricated by melt‐quenching technique. Their structural and up‐conversion luminescent properties were systemically investigated by XRD, HRTEM, and a series of spectroscopy methods. The temperature‐dependent up‐conversion spectra prove that 2H11/2 and 4S3/2 levels of Er3+ are thermally coupled energy levels (TCEL). Consequently, the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 emissions of Er3+ in Sr2YbF7:Er3+ glass‐ceramics can be used as optical thermometry based on fluorescence intensity ratio (FIR) technique. Combined with low phonon energy and high thermal stability, Er3+ ions in Sr2YbF7 glass‐ceramics present broad operating temperature range (300–500 K), large energy gap of TCEL (786 cm−1) and high theoretical maximum value of relative sensitivity (62.14 × 10−4 K−1 at 560 K), which suggests that Sr2YbF7:Er3+ glass‐ceramics may be excellent candidates for optical temperature sensors.