Upconversion Phosphors Research Articles

Simultaneous influence of Mg2+ and Sc3+ co-doping on upconversion luminescence and optical thermometry in β-NaYF4: Yb3+/Ho3+ microphosphor

A series of xMg2+/ySc3+ co-doped β-NaYF4:0.2Yb3+/0.02Ho3+ (x = y = 0 −0.2) upconversion (UC) green phosphors are synthesized using EDTA-assisted hydrothermal method. The structure, morphology and elemental compositions are investigated in detail using different experimental techniques. The lengthwise suppression of crystal due to co-doping is associated to the kinetics involved in the crystal growth in presence of Mg2+/Sc3+. The effect of Mg2+/Sc3+ co-doping on the UC green emission of β-NaYF4:0.2Yb3+/0.02Ho3+ microphosphors is studied and about 31-fold enhancement in the green emission (5F4,5S2→5I8) is observed for the 0.08Mg2+/0.12Sc3+ co-doped phosphor. The downconversion properties of the phosphors is also discussed to showcase the dual mode luminescence. The temperature sensing performance is investigated using luminescence intensity ratio (LIR) technique for non-thermally coupled levels (5F5/5F4,5S2) of Ho3+ ions and maximum relative sensitivity of 0.258% K−1 at 374 K is obtained for the optimized phosphor. The microphosphor can be sustained at high temperature as the UC emission intensity retains about 65% and 36% at 423 K and 574 K. From chromaticity diagram, the maximum color purity of 91.37% is obtained for 0.08Mg2+/0.12Sc3+ doping concentration. The estimated thermal parameters and color purity of Mg2+/Sc3+ co-doped β-NaYF4:0.2Yb3+/0.02Ho3+ UC phosphors suggest their applicability in fabricating temperature sensors and LEDs.

Ultraviolet and visible upconversion in Yb/Er-CaSiO3 β-wollastonite phosphors

Wollastonite (CaSiO3) is a well-known rock-forming mineral and an important constituent in ceramics and cement industries due to its outstanding mechanical, chemical, and thermal stabilities. Despite technological importance, functional properties such as photon upconversion in CaSiO3 wollastonite ceramics have not been studied. In this contribution, Yb- and Er-doped CaSiO3 (Yb/Er–CaSiO3) wollastonite ceramics were synthesized via microwave hydrothermal technique followed up by heat-treatment in an air environment. X-ray diffraction and transmission electron microscopy studies confirmed the β-wollastonite (2M) phase in the synthesized samples heat-treated at 1050 °C. X-ray photoelectron spectroscopy analysis has shown that the binding energy of Ca 2p orbitals decreases after doping, indicating a change in the crystal environment of Ca in the CaSiO3 and hence a successful incorporation of Yb3+ and Er3+ ions in the lattice. The 980 nm excitation resulted in ultraviolet, violet and strong green and red upconversion emissions as well as downshifting infrared emissions due to the energy transfer between Yb3+ and Er3+ ions. An absolute upconversion quantum yield in the 400–800 nm range is 0.04%. The most intense phonon band was observed at 969 cm−1 in the Yb/Er–CaSiO3 system. This study demonstrates that the β-wollastonite can be developed as a new kind of efficient upconversion phosphor material.

Synthesis and Upconversion Luminescence Properties of BaBiO2Cl:Yb3+,Er3+ Phosphor

The interaction of near-infrared (NIR) light with matter that produces high-energy visible light emissions is known as photon upconversion, which has shown promising applications in different fields, including optoelectronics, biomedicine and photovoltaics. In this paper, a novel BaBiO2Cl:Yb3+,Er3+ upconversion phosphor was successfully synthesized through a simple high-temperature solid-state reaction route. The crystal structure, phase purity, microstructure and upconversion luminescence properties of the as-prepared phosphor were characterized comprehensively. The XRD and SEM results clearly demonstrate the successful synthesis of the target phosphors with high purity. When excited by a 980 nm NIR laser, the as-prepared BaBiO2Cl:Yb3+,Er3+ phosphor exhibited intense red upconversion luminescence due to the Er3+ 4F9/2→4I15/2 transition, which enabled this phosphor to have high promise for important applications, such as anti-counterfeiting and advanced photonics.

Optical Properties and Concentration Quenching Mechanism of Er3+ Heavy Doped Gd2(MoO4)3 Phosphor for Green Light-Emitting Diode.

Upconversion materials capable of converting low-energy excitation photons into high-energy emission photons have attracted considerable interest in recent years. However, the low upconversion luminescence seriously hinders the application of upconversion phosphors. Heavy lanthanide doping without concentration quenching represents a direct and effective method to enhance the emission intensity. In this study, Er3+ heavy doped Gd2(MoO4)3 phosphor with a monoclinic phase was prepared by a sol–gel process. Under excitation at 976 nm, Gd2(MoO4)3:Er3+ phosphor emitted remarkably intense green emission, and Er3+ concentration up to 20 mol% did not cause concentration quenching. Here, we discuss the upconversion mechanism and concentration quenching. When the Er3+ concentration was in the range of 30–60 mol%, the concentration quenching was governed by the electric dipole–dipole interaction, and when the concentration was greater than 60 mol%, the concentration quenching was controlled by the exchange interactions. The result provides a schematic basis for identifying a phosphor host with heavy lanthanide doping.

Structure and Upconversion Luminescence Properties of Pr3+-Doped Y2SiO5 Phosphor

Ultraviolet-C light has significant application prospects in the fields of disinfection, air purification, etc. Herein, an effective UVC upconversion phosphor Y2SiO5:Pr3+ was successfully prepared, as evidenced by the XRD results. The diffuse reflection spectra of Y2SiO5: Pr3+ phosphors presented two distinct absorption bands corresponding to the electron transitions of 3H4 → 3P2 and 3H4 → 1D2, and the Eg was determined to be 4.22 eV for the Y2SiO5 host, which is very close to the Eg (4.172 eV) calculated by DFT. Under the excitation of 460 nm laser, two emission peaks centered at ~247 nm and ~ 258 nm were found in the range of 230-280 nm, which are attributed to the transitions of 4f5d → 3H4 and 4f5d → 3H5 of Pr3+ ions. With the help of temperature-dependent emission spectra, the phosphor demonstrated impressive thermal stability up to 150 °C. These findings indicate that the Y2SiO5: Pr3+ phosphor has the potential application in disinfection.

A building-block strategy for dynamic anti-counterfeiting by using (Ba,Sr)Ga2O4:Sm3+ new red persistent luminescent phosphor as an important component

Long persistent luminescence materials developed to commercial standards are primarily concentrated in the blue and green regions, with only a few in the red region. Red, as one of the three basic colors, can be mixed in various proportions with blue and green to yield various colors. The development of red persistent phosphors has a broader application potential but remains a challenge. A solid-state reaction method was used to synthesize new red persistent luminescent materials of Ba1-xSrxGa2O4:Sm3+ (x = 0–0.09). In BaGa2O4, both Sr2+ and Sm3+ preferentially occupy the Ba2+ site rather than the Ga3+ site. When exposed to UV light at 254 nm, the phosphors emit the characteristic red emission of Sm3+ at wavelengths ranging from 500 nm to 750 nm. After removing the UV light source, an intense red afterglow that lasted more than 1400 s was observed. The red afterglow signal reappears after a heating process. Doping Sr2+ reduces the trap depth and improves the red persistent luminescence significantly. Because the escaped electrons from traps compensate for the emission loss of Sm3+ during the heating process, the red phosphors have unimaginably luminescent thermal stability. Thus, the emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors show multimode luminescence, with the output signal being time and temperature sensitive, indicating that they are potential luminescent materials for anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications of dynamic display information is proposed, with red persistent phosphors serving as an important component combined with upconversion phosphors of NaYF4:Yb3+, Tm3+, and green persistent phosphors of SrAl2O4:Eu2+, Dy3+.

Structure, morphology and upconversion luminescence properties of Yb3+/Er3+ Co-doped NaY(WO4)2 phosphors

NaY(WO4)2:Yb3+/Er3+ upconversion phosphors with different Yb3+/Er3+ doping concentrations were synthesized by high temperature solid-state reaction at 1200 °C for 4 h. The phase structure, surface morphology and luminescent properties were characterized by X-ray diffraction, scanning electron microscope and fluorescence spectroscopy. The sample composed of 7% Yb3+ and 2.5% Er3+ has the best luminescent properties. At 980 nm excitation, the emission peak of all the samples locates at the green area. The energy transfer of Yb3+→Er3+ is dominant in the energy transfer. The results demonstrate NaY(WO4)2:Yb3+/Er3+ can be used as the feasible materials for biomarkers and upconversion white light-emitting diodes.

Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors

Host material plays an important role in obtaining efficient photon upconversion and downshifting luminescence. Generally, fluoride and oxyfluoride glasses-based materials are used for high-efficiency photon upconversion. However, the poor thermal stability of fluoride glasses and the toxicity of fluorine ions limit their applications. In this report, Yb/Ho-doped CaSiO3 wollastonite phosphors have been demonstrated as efficient red-emitting upconversion phosphors. The phosphors have been synthesized by microwave hydrothermal process followed by heat treatment at 1050 ​°C in the air environment. 2M phase of the β-wollastonite has been confirmed by X-ray diffraction and Raman spectroscopy while the existence of the Yb and Ho dopants in the CaSiO3 lattice has been confirmed by X-ray photoelectron spectroscopy. The synthesized samples showed strong red upconversion emission centered at 662 ​nm and near-infrared downshifting emissions at 2016 ​nm upon 980 ​nm excitation. The emissions were found to depend significantly on the Ho concentration. Temporal evolution of the main emission bands was investigated to show that the energy transfer upconversion from Yb to Ho ions was responsible for the efficient upconversion and downshifting phenomenon.

Upconversion Emission Studies in Er3+/Yb3+ Doped/Co-Doped NaGdF4 Phosphor Particles for Intense Cathodoluminescence and Wide Temperature-Sensing Applications.

Er3+/Yb3+ doped/co-doped NaGdF4 upconversion phosphor nanoparticles were synthesized via the thermal decomposition route of synthesis. The α-phase crystal structure and nanostructure of these particles were confirmed using XRD and FE-SEM analysis. In the power-dependent upconversion analysis, different emission bands at 520 nm, 540 nm, and 655 nm were obtained. The sample was also examined for cathodoluminescence (CL) analysis at different filament currents of an electron beam. Through CL analysis, different emission bands of 526 nm, 550 nm, 664 nm, and 848 nm were obtained. The suitability of the present sample for temperature-sensing applications at a wide range of temperatures, from room temperature to 1173 K, was successfully demonstrated.

450 nm Photon‐Pumped Ultraviolet‐C Luminescence for Multifunctional Applications

AbstractWith the implementation of the Minamata Convention on Mercury, there is an urgent need to find a new way to generate ultraviolet‐C (UVC) light to replace mercury lamp. Upconversion phosphors have the potential to generate UVC light as they are able to convert low energy photons into high energy emissions. Here a new Sr2SiO4:Pr3+ phosphor is reported that could emit UVC light under excitation of a commonly used 450 nm laser diode, a cheap blue light‐emitting diode (LED) with central emitting wavelength at 450 nm, and even sunlight. Moreover, the potential applications of the UVC light of Sr2SiO4:Pr3+ phosphor upon excitation at 450 nm are demonstrated in several different situations, including static marking and dynamic tracking. Demonstration of the unique advantage of photon‐upconverted UVC emission of Sr2SiO4:Pr3+ in the application of sterilization is mainly focused, especially when the deep ultraviolet (DUV) LED cannot work adequately. This work opens a new route for generating UVC light and expands the applications of photon‐upconverted UVC emission, which is expected to be an indispensable supplement to the deep ultraviolet LED.

Intense red up-conversion luminescence and temperature sensing property of Yb3+/Er3+ co-doped BaGd2O4 phosphors

In this study, intense red and extremely weak green up-conversion (UC) luminescence was obtained in BaGd2O4: x mol% Yb3+/y mol% Er3+ phosphors under the excitations of 980 nm and 1550 nm. The corresponding maximum integrated intensity ratios of the red to green UC emissions are 50.3 and 158.7, respectively. The UC luminescence mechanisms upon different excitations were discussed. It was confirmed that two-photon and three-photon processes were responsible for both the red and green UC emissions excited at 980 nm and 1550 nm, respectively. The energy transfer efficiency from Er3+ to Yb3+ was calculated according to the fluorescence lifetime measurement under 1550 nm excitation. Temperature sensing based upon the thermally coupled energy levels 2H11/2/4S3/2 as well as thermally coupled Stark sublevels of 4F9/2 level of Er3+ was investigated under the excitation of 980 nm. The maximum absolute sensitivities were respectively obtained to be 0.42% K−1 at 573 K and 0.18% K−1 at 298 K. Our results indicated that BaGd2O4: Yb3+/Er3+ phosphors might be a kind of promising red UC phosphors with optical temperature measurement function.

Synthesis and upconversion properties of KAlF4:Yb3+/Er3+ phosphor for bioimaging application

This article reports a simple and green synthesis route for the preparation of Er3+/Yb3+−co doped KAlF4 upconversion phosphor. When annealed at 450 °C, the crystalline phase of KAlF4 host changes from monoclinic to tetragonal. Upon 980 nm light irradiation, KAlF4: Er3+phosphor shows two emission bands centred at 662 nm (4F9/2 → 4I15/2) and834 nm (4S3/2 → 4I13/2)respectively, in which the red emission band corresponds to the intra-4f transitions of Er3+ ions and hence the upconversion luminescence mechanism observed due to the two-photon process. With co-doping of Yb3+ ions, upconversion luminescence in KAlF4: Er3+ phosphor enhanced to few extend andexhibit two strong emission band appears invisible spectral range that consists ofstrong red emission bandand two high-moderate green emission band. As a result, Er3+/Yb3+ co-dopedKAlF4, laser-light excited upconversion phosphor may be an excellent candidate for bioimaging applications.

Enhanced red up-conversion of β-NaYF4:Er3+,Tm3+ microcrystals for bio-imaging applications

Pure red fluorescence generated by near infrared (NIR) excitation is desired for bio-imaging with high biological penetration, but without cell damage or multi-photon photo-toxicity. In this work, β-NaYF4:Er3+ and β-NaYF4:Er3+,Tm3+ microcrystals were synthesized using a novel hydrothermal method. β-NaYF4:Er3+ microcrystals exhibited intense green and red up-conversion emissions under 976 nm laser diode excitation. The introduction of Tm3+ ions into β-NaYF4:Er3+ resulted in strong inhibition of green up-conversion emission (Er3+:2H11/2 /4S3/2→4I15/2) and, consequently, red up-conversion emission (Er3+:4F9/2→4I15/2) was significantly promoted. To simultaneously restrain green up-conversion emissions and enhance red up-conversion emissions, the optimal concentrations of Er3+ and Tm3+ ions were determined to be 20 mol% and 2.0 mol%, respectively. The red/green up-conversion emission intensity ratio increased from ~1.80 (β-NaYF4:20Er3+, mol%) to ~78 (β-NaYF4:20Er3+,2Tm3+, mol%) after the addition of 2 mol% Tm3+ ions to β-NaYF4:20Er3+ (mol%). The red up-conversion emission intensity of β-NaYF4:20Er3+,2Tm3+ was stronger than that for commercial red up-conversion phosphors (NaYF4 and ZnF2) under the same experimental conditions. Based on the results of the emission spectra, luminescence rise, and decay profiles, the mechanism of enhanced red up-conversion emission by Tm3+ ion co-doping in β-NaYF4:20Er3+,2Tm3+ (mol%) was proposed. The bio-fluorescence experiment under NIR excitation proves that β-NaYF4:Er3+,Tm3+ microcrystals have potential applications in bio-imaging.

Applied anti-counterfeiting with strontium molybdate up-conversion phosphor synthesized via co-precipitation at low sintering temperature

Applied anti-counterfeiting with strontium molybdate up-conversion phosphor synthesized via co-precipitation at low sintering temperature

Study on improving the performance of Gd2O2S: Er3+, Yb3+ up-conversion phosphors by Lu3+ introduction

Multi-component luminescence materials (Lu0.09Gd0.91)2O2S: Er3+, Yb3+ with response wavelengths of 980 nm, 1064 nm, and 1550 nm were synthesized by the coprecipitation-high temperature solid-state reaction method. The optimum formula was obtained by the orthogonal experiment method. The effects of Lu3+ on crystal structure, fluorescence intensity, and temperature stability were systematically studied. The phase composition of the phosphors was analyzed by X-ray diffraction. With the increase of Lu3+ concentration, the fluorescence intensity first decreases, then increases slowly to a maximum, and finally decreases sharply. The sample exhibits characteristic emissions of red and green under three wavelengths of excitation. In addition, the up-conversion luminescence mechanisms excited at different wavelengths were discussed by means of the Intensity-Power test. Through a series of temperature spectra, it is found that the introduction of Lu3+ contributes to excellent temperature stability. Subsequently, the decay curve shows that the fluorescence lifetime of 554 nm increases first and then decreases with the increase of Lu3+ concentration, resulting from competition between γ and knr. This provides a useful idea for developing infrared detection materials with excellent temperature stability and luminescence properties.

Impact of Sc3+-Modified Local Site Symmetries on Er3+ Ion Upconversion Luminescence in Y2O3 Nanoparticles.

Rare earth (RE) doped yttria sesquioxide has been widely used as host materials for upconversion (UC) phosphors due to their high refractive index, wide band gap, and high melting point. Meanwhile, while fluoride matrices with low phonon cutoff energies exhibit stronger UC emissions, RE-doped oxides exhibit better thermal stability and higher thermal sensitivity when applied as optical temperature sensors. In this work, Sc3+ is substituted in RE-doped Y2O3 lattices to generate smaller cation sites, enhancing the crystal field and modifying the allowed optical transitions. Er3+ is used as a photoluminescent probe to study the effect of site position and symmetry on the UC performance. In comparison with the traditional hydrothermal method, Sc3+ is successfully incorporated into the Y2O3 lattice via the co-precipitation/molten salt method without segregating observed. The Judd–Ofelt analysis was applied to determine the local symmetry and efficiency changes. Sc was found to be able to improve the luminescence performances of Er in Y2–xScxO3 (YScO) hosts by adjusting the local symmetry level around the luminescent sites. The local symmetry level was reduced with less than 30 mol % of Sc doping concentration based on the changes in Ω2 values. Meanwhile, the YScO oxide was found to significantly improve the luminescence intensity and red-to-green ratio at a lower Yb3+ concentration (5 mol %) instead of a higher concentration (20 mol %) commonly used. This was attributed to an increased energy transfer between the closer Yb3+–Er3+ pairs. Overall, this work allows the spatial occupancy of luminescence centers in the metal oxide host materials to optimize the UC luminescence performance and develop a high-efficiency oxide material for high-temperature applications such as optical thermometry.

Designing Optical Thermometers Using Down/Upconversion Ca14Al10Zn6O35: Ti4+, Eu3+/Yb3+, Er3+ Thermosensitive Phosphors.

Down/upconversion Ca14Al10Zn6O35 inorganic phosphors codoped with Ti4+/Eu3+ or Yb3+/Er3+ were prepared. The crystal structure and downconversion luminescence properties of Ca14Al10Zn6O35:Ti4+, Eu3+ phosphors were studied in detail. Ti4+ and Eu3+ occupied Al3+ and Ca2+ sites in the host lattice, respectively. Under the excitation of 273 nm, the emission peak in the 300-570 nm band originated from the 2T2 → O2- transition of Ti4+. The f-f transition of Eu3+ ions generated multiple peaks in the 570-800 nm range. The emission intensity of Ti4+ and Eu3+ ions can be used as a fluorescence intensity ratio (FIR) signal. Based on the FIR technology, the maximum relative sensitivity (Sr) and the minimum temperature uncertainty (δT) reached 1.41% K-1 and 0.07 K, respectively. Meanwhile, the temperature-sensing behaviors were explored by the temperature-dependent upconversion spectra of Er3+- and Yb3+-codoped Ca14Al10Zn6O35 phosphors. Based on the fluorescence intensity ratio of thermal coupling levels (Er3+:2H11/2/4S3/2), the maximum Sr and minimum δT of upconversion phosphors reached 1.28% K-1 and 0.08 K in the temperature range of 293-473 K, respectively. Ca14Al10Zn6O35:Ti4+/Eu3+ (Yb3+/Er3+) phosphors realize temperature sensors with higher relative sensitivity, and it is a good candidate material for optical temperature measurement.

Synthesis of Barium Tungstate Up-Conversion Phosphor Applied in Temperature Sensing and Anti-Counterfeiting

Crystalline BaWO4 was synthesized by preparing a precursor by co-precipitation followed by heat treatment at 800 °C. For use as up-conversion phosphor materials, rare earth ions Er3+ and Yb3+ were co-doped. In the synthesized BaWO4:[Er3+]/[Yb3+], a strong peak from the (112) phase, the main peak, could be observed in the XRD analysis and had a tetragonal structure. The lattice constant was changed by the rare earth doping, and when the sample was excited at 980 nm, green light emission was confirmed. The luminescence intensity changed according to the change in the amount of Er3+, a rare earth ion, and the luminescence intensity increased strongly as the laser power of the excitation wavelength increased. This is a phenomenon caused by the two-photon effect, and it was possible to confirm the characteristics of the up-conversion phosphor. To investigate the use of the synthesized phosphor for temperature sensors and anti-counterfeiting, a flexible composite and ink were made, respectively. As the temperature increased, the luminous intensity of the flexible composite was weakened, and the produced ink was difficult to observe with the naked eye when applied to banknotes, but when illuminated with a 980 nm laser, the green label could be seen.

Comparative study of the luminescence of Bi doped LaOCl and LaOBr phosphor powders

In this work, Bi3+ doped lanthanum oxyhalides (LaOX, X = Cl or Br) were prepared via the solid-state reaction method in order to compare their photoluminescence (PL) properties and stability. Their structure and morphology were assessed using X-ray diffraction and scanning electron microscopy, while the annealing temperature and doping concentration were optimized for maximum PL emission intensity. The 3P1 → 1S0 emission transitions of Bi3+ ions in LaOCl:Bi and LaOBr:Bi were located in the ultraviolet at 344 nm and 358 nm, for excitation at 266 nm and 273 nm, respectively. The red-shift and lower intensity emission of LaOBr:Bi compared to LaOCl:Bi was attributed to the stronger nephelauxetic effect in the bromide host. Although LaOCl:Bi was not completely stable when stored exposed to the atmosphere for several months, LaOBr:Bi was significantly less stable and also showed accelerated degradation when exposed to a 254 nm UV lamp. Although previous reports for scintillators and upconversion phosphors based on these hosts have found LaOBr to be preferable, direct comparison of PL for these materials doped with Bi3+ ions showed that the LaOCl host was superior.

High crystalline quality of hydrophilic LiYF4: Tm3+/Yb3+ particles prepared by fluorination of layered yttrium hydroxides under assistance of DTPA

Up-conversion emissions can be achieved by doping Tm3+ in hydrophilic LiYF4 crystal particles, with Yb3+ as a sensitizer. They are considered to be excellent up-conversion materials used in bioimaging. However, the crystalline quality of hydrophilic LiYF4 particles prepared by traditional methods is not satisfied and lowers the fluorescent performance. Herein, some additives were employed in preparation of LiYF4 crystals via fluorination transformation from layered yttrium hydroxides (LYHs). The reaction mechanism of additives was discussed in detail. Especially, diethylenetriaminepentaacetic acid (DTPA) can effectively promote the crystallization and Oswald ripening of LiYF4 crystals. The as-prepared LiYF4: Tm3+/Yb3+ up-conversion phosphors have a marvelously intense emission at 793 nm and would be beneficial to the application in bioimaging.