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LiYb 1-x Er x (MoO 4) 2 (x = 0.01–1.0) solid solutions with Er3+ as a dopant synthesized via conventional solid-state reaction. Phase formation of phosphors was confirmed by X-ray diffraction (XRD) analysis. The XRD results show that as the Er³⁺ concentration (x) increases, the LiYb₁₋ₓErₓ(MoO₄)₂ compounds adopt a tetragonal structure with a space group of I 4 ‾ (No. 82). This study reports on the optical properties, including both upconversion (UC) and downconversion (DC) luminescence. The emission and excitation spectra were employed to characterize the synthesized solid solution. The excitation spectra showed that LiYb 1-x Er x (MoO 4) 2 can be efficiently excited by the incident lights of 350–500 nm, which coincides well with the emission wavelength of the blue LED chips. The phosphors exhibit bright green emissions at two distinct wavelengths, where one is ranging from 500 to 540 nm and the other from 540 to 580 nm. Luminescence intensity and decay were examined as a function of Er³⁺ or Yb3+ concentration, to explore potential applications. The up-converted luminescence spectra of the samples were recorded under the excitation of a 975 nm laser. The UC process was studied through power and concentration dependence of up-conversion luminescence. The UC emission was confirmed to be generated by two-photon processes. [ABSTRACT FROM AUTHOR]

A novel class of Er3+/Tm3+/Yb3+ co-doped Sc 4 Zr 3 O 12 nanocrystals have been successfully synthesized via a hydrothermal method. The enhanced green and red emissions of Er3+are achieved through the mediation of Tm3+. Decay curves and DFT calculations indicate this enhancement is due to the cross relaxation processes and narrowed bandgap. Based on the temperature-dependent upconversion characteristics, a multi-mode optical thermometer is developed using the fluorescence intensity ratio (FIR) technique of thermally coupled levels (TCLs) and non-thermally coupled levels (NTCLs), as well as the valley to peak ratio (VPR) technique. The absolute sensitivity (S a) and relative sensitivity (S r) derived from 1G 4 (b)/2H 11/2 NTCLs reach as high as 11.84% K−1 and 1.89% K−1, respectively. Additionally, the optimal sensitivities occur at 303 K, perfectly aligning with the physiological temperature range. A comparison of different fitting models for NTCLs reveals that the exponential model offers the best performance for this system, with a correlation coefficient of 99.9%. This work provides new insights into upconversion enhancement of multi-emission centers and lays the theoretical and experimental foundation for developing high-sensitivity and high-precision optical nanothermometers. [Display omitted] [ABSTRACT FROM AUTHOR]

Recent advances in rare earth (RE) oxide application in energy conversion, particularly upconversion (UC) and downconversion (DC), have made remarkable progress. With rising energy demand and environmental concerns, the pursuit of clean energy sources such as solar, biomass, wind, hydropower, geothermal, and nuclear has gained momentum. Conventional photovoltaic systems convert sunlight into electricity but are limited by spectral sensitivity. Non-conventional solar cells, using RE ions, can harness lower-intensity infrared (IR) and ultraviolet (UV) light, converting it into visible wavelengths through UC and DC processes, aligning with green chemistry principles. However, the solar spectrum's broad range (280–2500 nm) poses challenges as most photoactive materials are sensitive to only visible light (400–800 nm), wasting over 50 % of energy. Photon conversion, including upconversion, addresses this issue by converting IR photons into the visible range. This work exposes current works based on RE doped materials as upconverters, with the pairs Yb3+/Er3+, Yb3+/Eu3+, Yb3+/Tm3+, Yb3+/Ho3+, and Yb3+/Pr3+. New downconversion materials, featuring both lanthanides and non-lanthanides, efficiently convert high-energy UV photons into lower-energy visible light, mitigating spectral mismatch losses that affect organic solar cells (OSC). Additionally, these materials protect OSCs from UV-induced degradation, offering dual benefits. Some recent work based on Pr3+/Yb3+, Dy3+, Sm3+, Tb3+, Ce3+, Er3+, and Eu3+ doped materials as downconverters are discussed in this review. Integration of upconversion and downconversion materials holds promise for enhancing traditional solar cell efficiency, for example : RE 2 O 3 doped with Yb3+/Er3+, Yb3+/Nd3+, Yb3+/Ho3+, Yb3+/Ce3+, Yb3+/Dy3+, Yb3+/Sm3+, Yb3+/Eu3+, and Yb3+/Tm3+. Besides applications, in this review we discuss the physical, chemical and spectroscopic properties of the RE elements, with main focus on their luminescent properties. Furthermore, the complexity of these luminescent mechanisms, involving different types of electronic transitions, quantum selection rules such as spin and Laporte rules, and how the presence of a ligand field can relax these rules, allowing transitions that generate their unique luminescent properties are discussed. This review explores these concepts and the application of RE materials in energy conversion, offering insights into advancements and prospects for enhancing efficiency and sustainability in renewable energy technologies. [ABSTRACT FROM AUTHOR]

The design of color-tunable upconversion luminescence for expand the application of optical fields has been a hot research topic. Herein, Yb3+, Ho3+ co-doped YNb 1-x V x O 4 (x = 0, 0.25, 0.5, 0.75, 1) are synthesized by solid state method. The color tunability of UC luminescence from green to red is achieved by varying the component ratio of Nb5+ and V5+ in the mixed compositions. The continuous introduction of V5+ component strengthens the vibrational band of the mixed host, thus promoting the non-radiative relaxation of 5I 6 →5I 7 in activator Ho3+, which becomes the key to obtain the color-tunable UC luminescence. Besides, the Ho3+ activated system exhibits strong temperature dependent variation in UC luminescence intensity of 5F 4 /5S 2 →5I 8 and 5F 5 →5I 8. The optimal sensitivity parameters can be obtained as 35.64 × 10−3 K−1 for S A-max and 3.78 % for S R-max in YVO 4 :Yb3+,Ho3+ thermometer. Benefit by the luminescence color tunability and excellent thermometry performance, phosphors with host dependent upconversion of Ho3+ activated system are expected to achieve optical applications in various fields of anti-counterfeiting, display, and temperature sensing. [ABSTRACT FROM AUTHOR]

Luminescent materials usually suffer from thermal quenching at high temperature due to the aggravation of nonradiation relaxations. Herein, a giant enhancement of up conversion (UC) luminescence is observed in negative thermal expansion (NTE) Y 2 W 3 O 12 :Nd phosphor upon increasing the temperature from 310 to 790 K. In addition, the temperature-dependent UC emission is reversible. To penetrate into the mechanisms, the temperature-dependent luminescence intensity and lifetime are investigated for Nd doped negative and positive thermal expansion (PTE) phosphors. Based these investigation, we infer that the underlying mechanism of the giant enhancement for Y 2 W 3 O 12 :Nd phosphor are due to combined effects of phonon assisted absorptions and contraction of NTE crystals. Moreover, the observation of temperature-dependent near-infrared spectra (NIR) can support the mechanism from the other side. The present findings pave the way to overcome thermal quenching of Nd3+ ions and explore new thermal enhancement materials sensitized by Nd3+ ions. [ABSTRACT FROM AUTHOR]

In this work, SrLaAlO 4 :Er3+ (2 mol%),Yb3+ (4 mol%) upconversion phosphor was synthesized by using a combustion method. Later, CsPbBr 3-x I x (x = 0, 1.5 and 0.75) perovskite quantum dots (QDs) were synthesized by using hot injection method. According the analysis by X-ray diffraction, the SrLaAlO 4 :Er,Yb (SL:Er,Yb) phosphors and QDs had tetragonal and cubic phases, respectively. Moreover, the analysis by microscopy revealed that the SL:Er,Yb phosphors are composed of micro-grains with irregular morphology, while the CsPbBr 3-x I x QDs had a morphology of cubes. The surface of the SL:Er,Yb phosphors was decorated with CsPbBr 3-x I x QDs and obtained three composite powders: CsPbBr 3 +SL:Er,Yb, CsPbBr 1.5 I 1.5 +SL:Er,Yb and CsPbBr 2.25 I 0.75 +SL:Er,Yb. Those composites were firstly excited with UV light (380 nm) and produced emissions in the green and orange-red regions by downconversion. Interestingly, the emission intensity of the composite powders was 45–75 % higher than that for the individual CsPbBr 3-x I x QDs or SL:Er,Yb phosphors. Later, the same composite powders were excited with NIR light (980 nm), in consequence, intense green and yellow emissions were produced by upconversion. In particular, the CsPbBr 1.5 I 1.5 + SL:Er,Yb composite produced strong red emission by upconversion because the presence of the QDs on the SL:Er,Yb surface promoted the following cross relaxation process: 4I 11/2 (Er3+)+ 4F 7/2 (Er3+)→4F 9/2 (Er3+)+ 4F 9/2 (Er3+). In general, the quantum dots deposited on the SL:Er,Yb surface not only enhanced the red upconversion emission, but also provoked a color tuning when they are excited with NIR or UV light. Those last effects have not been reported in the literature previously. Thus, the results of this investigation demonstrate that combining perovskite quantum dots and the SL:Er,Yb forms a luminescent material able to tune its emission by upconversion or downconversion. This last property can be utilized for the design of new LED based lamps, which are employed for general illumination in houses and buildings. [ABSTRACT FROM AUTHOR]

(Ti 1-x Si x)O 2 :Er/Yb (0 ≤ x ≤ 0.3) phosphor was synthesized using spray pyrolysis, and the upconversion (UC) properties were optimized by varying the calcination temperature and Si content (x). By introducing Si into the TiO 2 matrix, the thermal stability of titania was effectively improved, allowing the anatase phase to be maintained even when the (Ti,Si)O 2 :Er/Yb phosphor was calcined at temperatures above 800 °C. From investigating the effect of crystal size on the UC emission of (Ti,Si)O 2 :Er/Yb, it was found the optimal crystal size of Antase to achieve the highest UC emission intensity is approximately 5.5 nm, beyond which the UC intensity decreases rapidly. With increasing Si content, the crystalltie size decreased linearly and more amorphous SiO 2 domains were formed in the anatase TiO 2 matrix, which could consume Er/Yb ions to form SiO 2 :Er/Yb with low UC emission. Resultently, the UC emission intensity was significantly affected by Si content. The optimal calcination temperature and Si content to obtain the highest UC luminescence were determined as 700 °C and 10 %, respectively. The optimized (Ti,Si)O 2 :Er/Yb showed approximately 2.7 times improved UC luminescence than TiO 2 :Er/Yb. [ABSTRACT FROM AUTHOR]

Upconversion luminescence presents obvious advantages in optical temperature sensing, nevertheless the application in complex scenarios is limited by luminescence thermal quenching. Herein, NaYS 2 :Yb3+,Nd3+ phosphors are synthesized by the solid-gas reaction method and used for temperature sensing. A series of emissions for Nd3+ from visible to near-infrared region are achieved based on energy transfer from Yb3+ to Nd3+ under 980 nm excitation. With elevating the temperature, significant thermal enhancement effect of nearly three orders of magnitude is detected in near-infrared emission of 4F 7/2 → 4I 9/2 , which is attributed to phonon-assisted energy transfer of Yb3+→Nd3+ and the corresponding thermal population effect. The thermal behaviors of thermally coupled energy levels for 4G 7/2 /2G 9/2 and 4F 7/2 /4F 5/2 in NaYS 2 :Yb3+,Nd3+ are evaluated by luminescence intensity ratio technique. Remarkably, the optical thermometer based on the enhanced emission of 4F 7/2 /4F 5/2 shows excellent temperature measurement performance, which is expected to be applied in wide-temperature-range and highly-sensitive temperature sensing. These results not only provide a pathway to realize high performance temperature sensing, but also heighten the understanding of UC emission thermal enhancement. [ABSTRACT FROM AUTHOR]

Extremely intense blue upconversion luminescence (UCL) is got in Tm3+/Yb3+ co-doped BiTa 7 O 19 (BTO) phosphors, which are synthesized by solid phase sintering. The doping concentration of Tm3+ is fixed at 7 mol%, and the concentration of Yb3+ is adjusted. When the concentration of Yb3+ is 80 mol%, the strongest blue UCL can be obtained under 980 nm laser excitation. The blue UCL integral intensity under 808 + 980 nm laser co-excitation is 1.54 times of the sum of intensity under 808 and 980 nm single excitation, respectively. Blue UCL thermal enhancement behavior was found for all the samples under 808 or 785 nm laser excitation, and blue UCL thermal quenching happened under 660, 660 + 808, and 359 nm excitations. It is demonstrated that UCL thermal enhancement behavior is caused by oxygen defects. The possible electron transition pathways are proposed under excitation with different laser wavelengths. Using the luminescence intensity branching ratio (LIR) technique, the maximum relative temperature sensitivity range is calculated to be from 0.00610 to 0.00732 K−1 at 303 K. The experimental results show that BTO:Tm3+/Yb3+ has excellent blue UCL with thermal enhancement, which can have some applications in the field of UCL display and temperature sensing in high temperature environment. [ABSTRACT FROM AUTHOR]

The perovskite based phosphor materials are widely used to increase the efficiency of solar cells. In this work, Er3+ doped and Er3+/Yb3+ co-doped CaTiO 3 perovskite phosphor samples have been synthesized by solid state reaction technique at 1473 K. The phosphor samples show orthorhombic phase with Pnma (62) space group. The average crystallites and particles size of CaTiO 3 are increased in presence of Er3+ and Yb3+ ions. Er3+ doped CaTiO 3 phosphor samples give downshifting emission under 379 nm excitation. Though upconversion emission is seen in Er3+ under 980 nm excitation without Yb3+ ions in this host. The emission intensity of Er3+ ion is enhanced by 46 and 16 times for green and red emissions, respectively in presence of Yb3+. An intense quantum cutting (QC) emission is observed at 980 nm in presence of Yb3+ in CaTiO 3 :0.5Er3+ phosphor under 379 nm excitation. The QC efficiency has been found to be 119 % for CaTiO 3 :0.5Er3+/5 Yb3+ phosphor. Further an interesting phenomenon of back energy transfer (BEnT) from Yb3+ to Er3+ giving an intense NIR emission from Er3+ at 1002 and 1550 nm have been observed. The phosphor sample also shows an intrinsic optical bistablity (IOB) by upconversion. Thus, the prepared phosphor samples may be useful to increase the efficiency of c-Si solar cell, NIR emission for communication, bistable material and green light emitting source. [Display omitted] [ABSTRACT FROM AUTHOR]

To investigate a near-infrared-to-blue luminescence upconversion curing method for polymerizing resin cements under zirconia discs. Lava zirconia discs of different thicknesses (0.5–2.0 mm) were manufactured. First, the transmittances of the NIR and two blue lights (BLs) (LED and halogen lights) through these discs were measured. Second, NaYF 4 :Yb3+/Tm3+ upconversion phosphor (UP) powder was milled into 0.5-μm particle sizes. A light-curable resin cement VariolinkII base was chosen as the control (UP0), and an experimental cement (UP5) was prepared by adding 5 % UPs. These two cements were examined using multiphoton excitation microscopy for particle distribution. UP5 and UP0 were polymerized with or without zirconia shielding then subjected to a microhardness test. A multifold analysis was performed to examine the effects of zirconia thickness, curing protocols (pure BL or combined BL and NIR curing), and cement type. The transmittance of NIR was superior to that of BL through zirconia discs of all thicknesses. UP particles were homogeneously distributed in UP5 and emitted blue luminescence under 980-nm NIR excitation. UP5 showed higher microhardness values than UP0 under any curing protocol or zirconia shielding condition. The combination of 20-s BL and 40-s NIR curing yielded the highest microhardness in uncovered UP5. However, combining 40-s BL and 20-s NIR curing surpassed the other groups when the zirconia discs were thicker than 0.5 mm. NIR exhibits higher transmission through zirconia than BL. UP particles work as strengthen fillers and photosensitizers in cements. NIR upconversion curing could be a new strategy for polymerizing resin cements under thick zirconia restorations. • An NIR activated upconversion reaction was used to induce blue light (BL) emission. • The transmittance of NIR through zirconia discs is higher than conventional BL. • Phosphor particles work as photosensitizers and strengthen fillers in cements. • Combining BL and NIR effectively enhanced polymerization of cements under zirconia. [ABSTRACT FROM AUTHOR]

Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr 5 (PO 4) 3 F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications. • The xEr:S-FAP (x=0–0.05) transparent nanostructured ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural characteristics, upconversion luminescence property, and mechanism of as-synthesized xEr:S-FAP transparent ceramics were investigated for the first time. • The wide visible light spectrum from red light to green light was achieved by changing the doping amount of Er3+ from 0.5 to 5 at%. [ABSTRACT FROM AUTHOR]

In this work, Yb3+ and Er3+-co-doped Sr 9 In(PO 4) 7 upconversion fluorescent materials with dual functionality were synthesized though simple high-temperature solid state method. Based on International Commission on Illumination (CIE) color coordinates, the calculation of color purity at different temperatures demonstrates the attainment of highly pure green luminescence (color purity >90 %) for the synthesized materials. The analysis of activation energy proves that synthesized materials are stable. Utilizing the fluorescence intensity ratio (FIR) of the thermally coupled energy levels (2H 11/2 , 4S 3/2) as a metric for optical sensing performance, absolute sensitivity and relative sensitivity are determined, and their maximum values are 0.44 % and 1.13 %, respectively. Inspired by the variation in CIE coordinates with temperature, thermally responsive two-dimensional barcode image was developed, demonstrating the practical application of Sr 9 In(PO 4) 7 :Yb/Er materials in the field of information security and anti-counterfeiting. [ABSTRACT FROM AUTHOR]

The upconversion fluorescence thermometry has been recognized as one of the most promising techniques for the temperature measurement and multi-mode fluorescence temperature sensing can improve the accuracy and widen the temperature range. Herein, a fluorescence thermometry was developed based on the MgAlON: Er3+ transparent ceramic. It was demonstrated that the enrichment of Er3+ in grain boundaries increased susceptibility to temperature variations of the cross-relaxation rate, which endowed the non-thermally coupled energy levels with temperature sensing properties and led to the multi-mode thermometric capability. A novel four-mode temperature sensor based on fluorescence intensity ratios (2H 11/2 /4S 3/2 and 4F 9/2 /4S 3/2) and fluorescent lifetimes (4F 9/2 and 4S 3/2) was realized in the temperature range of 180–500 K, which brought about the maximum absolute and relative sensitivities reached 0.0258 K−1 and 2.95% K−1, respectively. This work reveals that the MgAlON: Er3+ and the grain boundary functionalization of transparent ceramics could have potential applications for fluorescence thermometry. • A novel four-mode MgAlON: Er3+ fluorescent temperature sensor was prepared. • The maximum absolute and relative sensitivities of MgAlON: Er3+ reached 0.0258 K−1 and 2.95% K−1, respectively. • The grain boundary enrichment of Er3+ makes the cross-relaxation rate temperature-sensitive, forming a multi-mode fluorescence thermometry. [ABSTRACT FROM AUTHOR]

BiTa 7 O 19 (BTO) has two lattice structure, P–6C2 (188) ordered phase and P 6 3 / mcm (193) disordered phase. Ordered and disordered BTO:Er3+/Yb3+ upconversion phosphors (UCPs) were obtained by solid sintering with different temperatures. The lattice structure of all the samples was investigated by XRD, SEM and Raman spectroscopy, and upconversion luminescence (UCL) spectra were tested by spectrometers. The green integral UCL intensity of the disordered BTO:Er3+/Yb3+ UCP is 60.76 times than that of the ordered sample. By comparing Er3+/Yb3+ co-doped and Er3+ single-doped UCPs with different structures, it is found that the disordered structure enhances the energy transfer from Yb3+ to Er3+, which may be caused by the disordered structure reducing the distance between Yb3+ and Er3+. The absolute temperature sensitivity of the sample with the highest UCL intensity is 0.01005 K-1 at 352 K. All results suggest that the disordered structure of BTO host can greatly improve green UCL intensity and the disordered BTO:Er3+/Yb3+ UCPs are suitable for the application of optical temperature measurement. [ABSTRACT FROM AUTHOR]

Herein, Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Ho3+/Er3+/Tm3+ are synthesized through the solid-state reaction method. By utilizing Yb3+-mediated energy transfer in Nd3+-sensitized various activators (Ho3+/Er3+/Tm3+) system, 808 nm excited multi-color upconversion luminescence is realized. Yb3+ plays an important role in energy transfer bridging, and the introduction of Yb3+ in Nd3+-sensitized system can significantly improve emission of activators by nearly two orders of magnitude. The main luminescence mechanism of Nd3+→Yb3+→Activators (Ho3+/Er3+/Tm3+) is proved via measuring the emission characteristics and luminescence decay curves. With efficient upconversion luminescence, the optical thermometry behaviors for Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Ho3+/Er3+/Tm3+ are investigated. The maximum relative and absolute sensitivities are 1.08 % K−1 and 7.82 × 10−3 K−1 for Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Er3+; 1.01 % K−1 and 4.22 × 10−3 K−1 for Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Tm3+; and 1.08 % K−1 and 22 × 10−3 K−1 for Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Ho3+, respectively. Based on the multi-color upconversion luminescence and high sensitivity thermometry performance, the Na 3 Y(VO 4) 2 :Nd3+,Yb3+,Ho3+/Er3+/Tm3+ phosphors have potential applications in fields of solid-state display and safety sign of high-temperature. [ABSTRACT FROM AUTHOR]

High-performance optical thermometer that can operate in the long-wavelength range of the second near infrared (1500 nm–1700 nm, NIR-IIb) biological window is in desperate need for bio-medical treatment. In this paper, a multipath temperature sensor Y 3 NbO 7 : Yb3+/Er3+ with pure cubic phase is synthesized by the high temperature solid state method. Under the excitation of 980 nm wavelength, efficient green upconversion emission attributed to thermally coupled Er3+: 2H 11/2 /4S 3/2 levels are found, through which an excellent temperature sensing properties are realized with the maximal absolute and relative sensitivity of approximately 0.54 % K −1 and 1.37 % K −1 as well as the minimal temperature resolution of about 0.024 K. More importantly, the sample exhibits remarkable temperature detection capacity in deep tissues, which is implemented by the thermally coupled Stark sublevels of Er3+: 4I 13/2 locating just in the NIR-IIb sub-window. Thanks to this appropriate response wavelength range, the maximal penetration depth of the NIR-IIb optical thermometer in the biological tissues can reach up to 8 mm. All the findings indicate that the present sample is a supernormal multipath thermometric probe with detectability in the deep biological tissues. [ABSTRACT FROM AUTHOR]

Series of Er3+/Yb3+-codoped Gd 2 Mo 3 O 12 (GMO:Er3+/2 x Yb3+) microparticles with dual-mode luminescence were fabricated with the assistance of sol-gel method. The phase structure, morphological information, upconversion (UC) and down-shifting luminescence properties of the final products were systematically investigated. At 980 nm light irradiation, strong UC emissions rise in all the products and their intensities depend on the dopant concentration, where the optimum status occurs at x = 0.25 and the two-photon capture procedure results in the UC emission mechanism. Besides, excited at 378 nm, the down-shifting photoluminescence is also gained in resulting samples. By means of the fluorescence intensity ratio technique, the thermometric properties of the studied samples were investigated through analyzing the diverse thermal quenching features of the green emissions from thermally coupled levels of Er3+. When the UC emission is selected, the maximum absolute and relative sensitivities of the GMO:Er3+/2 x Yb3+ microparticles are 0.0076 and 1.15% K−1, respectively, whereas they shift to 0.0036 and 0.89% K−1, respectively, when the down-shifting photoluminescence is adopted. Note that, the sensor sensitivities of developed microparticles are related to the luminescence mode, while they do not depend on Yb3+ content. These results suggest that the GMO:Er3+/2 x Yb3+ microparticles are promising thermosensitive luminescent materials for optical thermometry and their thermometric features can be tuned through changing luminescence mode. [ABSTRACT FROM AUTHOR]

In this study, nitrogen-doped carbon dots (NCDs) modified ultrathin carbon nitride (g-C 3 N 4) nanosheets were synthesized by a simple secondary calcination method, which realizes efficient photocatalytic hydrogen production. The secondary calcination process can not only thermally exfoliate g-C 3 N 4 into ultrathin nanosheets, but also uniformly anchor NCDs (∼3 nm) on the surface of g-C 3 N 4. The photocatalytic results manifested that a very small amount of NCDs (0.5 wt%) loading on g-C 3 N 4 could achieve a high photocatalytic hydrogen production rate (5.21 mmol g−1 h−1). The reasons for the enhanced photocatalytic activity were analyzed. The ultrathin g-C 3 N 4 nanosheets facilitated the migration of photogenerated carriers. Moreover, NCDs/g-C 3 N 4 exhibited enhanced visible light absorption owing to the up-conversion fluorescence effect of NCDs and the narrow band gap caused by the decoration of NCDs. Furthermore, NCDs as electron acceptors can promote carrier separation. This combination strategy provides a basis for the synthesis of inexpensive, metal-free, and efficient carbon-based photocatalysts. Nitrogen-doped carbon dots (NCDs) modified ultrathin carbon nitride (g-C 3 N 4) nanosheets were synthesized. The enhanced photocatalytic mechanism of NCDs/g-C 3 N 4 composites was analyzed. [Display omitted] • NCDs/g-C 3 N 4 nanosheets were synthesized by a simple secondary calcination method. • The ultrathin g-C 3 N 4 nanosheets (∼8 nm) facilitated the migration of carriers. • The modification of NCDs promotes light absorption and carrier separation abilities. • The H 2 production rate of g-C 3 N 4 loaded with 0.5 wt% NCDs reached 5.21 mmol g−1 h−1. [ABSTRACT FROM AUTHOR]