Red Upconversion Emission Research Articles

Enhanced Red Upconversion Emission of NaYF4:Yb3+, Er3+, Mn2+ Nanoparticles for Near-infrared-induced Photodynamic Therapy and Fluorescence Imaging

Polyethyleneimine (PEI)-capped NaYF4 nanoparticles (NPs) codoped with Yb3+, Er3+, and Mn2+ ions (MnUNPs) were synthesized via a solvothermal method. The MnUNPs showed tissue-penetrable over-1000 nm (OTN-) near-infrared (NIR) and strong red upconversion (UC) emission under NIR excitation. Furthermore, the photosensitizers chlorin e6 (Ce6) and biocompatible poly(ethylene glycol) (PEG) were immobilized on the surface of the MnUNPs (PEG/Ce6-MnUNPs). The prepared PEG/Ce6-MnUNPs generated singlet oxygen (1O2) and displayed effective anticancer activity under NIR excitation.

Enhanced red upconversion emission of Er3+-doped ZnO by post-annealing

In this paper, we synthesized Er3+-doped Zinc oxide nanocrystals by conventional sol-gel method under 800°C, and further mesoporous-like structure was obtained through post-annealing procedure at 1300°C. Upconversion intensity was significantly enhanced by post-annealing, and the emission color was turned into relatively high purity red color (ratio of red to green emission is over 10). In addition, the mechanisms of post-annealing and upconversion luminescence were discussed in detail on the basis of the structural and spectral measurements.

An further enhancement in red upconversion emission in single LiYbF4: Ho3+ microparticle through codoping Ce3+ ions

In this work, tetragonal-phase LiYF4:Yb3+/Ho3+ and LiYbF4:Ho3+ microparticles were successfully synthesized and excited by near-infrared laser at 980nm through a confocal microscopy system. Upconversion (UC) emission tuning from green to red in single LiYF4:Yb3+/Ho3+ and LiYbF4:Ho3+ microparticles was achieved by increasing the Yb3+ and Ce3+ ion concentrations, respectively. A high intensity ratio of red to green emission was obtained in single LiYbF4:Ho3+ microparticle codoped with 12% Ce3+, which was boosted by approximately 19-fold in single LiYF4:Yb3+/Ho3+ microparticle. The cross-relaxation processes between adjacent Ho3+ ions, Yb3+ and Ho3+ ions, and Ho3+ and Ce3+ ions have been employed to enhance red UC emission and suppress green UC emission. The regulation mechanisms of UC emission of Ho3+ ions are discussed based on the emission spectra. The marked pure single-band red UC emission of single LiYbF4: Ho3+ microparticle can be applied in multicolor display technology and micro-optoelectronic devices.

Tunability of green–red up-conversion emission of co-doped Ca3WO6:Yb3+/Er3+ powders

Color tuning of Ca3WO6:Er3+/Yb3+ phosphors were prepared via high temperature solid method in this paper. The structure was investigated with X-ray diffraction (XRD) and the crystal morphology was observed through scanning electron microscopy (SEM). Visible up-conversion (UC) emissions were observed in green and red regions under the excitation of 980 nm diode laser. The luminescence study reveals strong 4S3/2 to 4I15/2 transition at 550 nm and strong 4F9/2 to 4I15/2 transition at 661 nm, respectively. In addition, it also can be observed that the green band is quenched and the red emission enhanced as the Yb3+ concentration increases. In consequence, the CIE coordinates changed from (0.30, 0.65) in the green region to (0.61, 0.38) in the red region at different Er3+/Yb3+ concentration. Thus, the tuning properties of Ca3WO6:Er3+/Yb3+ phosphors suggest that they could be good candidates for applications in lighting.

Synthesis of monodispersed hierarchical Yb(OH)xF3−x microcrystals and their conversion to Yb6O5F8

The fabrication of rare earth luminescent materials with monodispersed and hierarchical morphology is of great importance due to their various applications. In this report, monodispersed hierarchical Yb(OH)xF3−x microcrystals assembled by nanorod arrays were synthesized by a mild hydrothermal route. Calcination of Yb(OH)xF3−x resulted in the formation of Yb6O5F8 with the retention of morphology. The crystallographic structure relationship between Yb(OH)xF3−x and Yb6O5F8 and the transformation mechanism from Yb(OH)xF3−x to Yb6O5F8 were disclosed. The upconversion emission properties of the Er3+ doped samples were investigated. It can be found that Yb6O5F8: 5%Er3+ exhibited an intense red light upconversion emission with the strongest peaks around 658nm while Yb(OH)xF3−x: 5%Er3+ shows no upconversion emission.

Preparation of Gd2O2S: Yb3+, Er3+, Tm3+ sub-micro phosphors by sulfurization of the oxides derived from sol-gel method and the upconversion luminescence properties

Yb3+, Er3+ and Tm3+ co-doped Gd2O2S sub-micro phosphors were synthesized by solid-state sulfurization of the oxide powders derived from sol-gel method. The crystal structure, morphology and upconversion luminescence properties of the phosphors were characterized by x-ray diffraction, scanning electron microscopic and fluorescence spectrum analysis methods. The phosphors exhibited typical hexagonal Gd2O2S phase when sulfurized at 800 °C for 2 h. Under the excitation of 980 nm laser diode, the Gd2O2S: Yb3+, Er3+, Tm3+ phosphors displayed distinct blue, green and red upconversion emissions centered at 481, 546 and 669 nm, respectively. The Gd2O2S phosphors using acetic acid as a chelating agent in the sol-gel process had the optimal upconversion emission property. The upconversion mechanism analysis revealed that the two-photon absorption was mainly responsible for the green and red upconversion emission of Er3+ ions, and the three-photon absorption was responsible for the blue upconversion emission of Tm3+ ions in the Gd2O2S: Yb3+, Er3+, Tm3+ phosphors.

Excellent optical thermometry based on upconversion emission in SrMoO_4:Er^3+ phosphor

In this paper, SrMoO4:Er3+ phosphor excited by 976 nm diode laser shows remarkably intense green and relatively weak red upconversion emissions. Subsequently, within the temperature range from 310.8 K to 629.4 K, we tested a temperature-dependent fluorescence intensity ratio originating from the thermally coupled 2H11/2 and 4S3/2 states. It is revealed that SrMoO4:Er3+ phosphor exhibits a high sensitivity of 0.0326 K−1 at 554.0 K, indicating that the phosphor can be regarded as a promising building block for optical temperature sensor.

808 nm NIR light excited single-band red upconversion emission in lanthanide-doped KMnF3 nanocrystals

A new type of upconversion nanoparticles (UCNPs) whose excitation and emission peaks are both located in the biological window has been developed based on the combination of Nd3+ sensitization and efficient energy transfer between Mn2+ and Er3+ ions. In our design, Nd3+ ions act as sensitizers for efficient absorption of 808 nm NIR photons, while the usage of Yb3+ ions as energy migrators facilitates the efficient energy transfer from Nd3+ to Er3+ ions. More importantly, the introduction of Mn2+ ions plays a key role in the realization of single-band red UC emission of Er3+ ions via the efficient energy transfer process between Mn2+ and Er3+. Consequently, upon 808 nm laser excitation, single-band red UC emission can be obtained in the as-synthesized Nd3+/Yb3+/Er3+ tri-doped KMnF3 nanocrystals, which could be considered as attractive alternative to conventionally used UCNPs for deep-tissue imaging with mitigation of the attenuation effect associated with the emission peaks outside biological window and the overheating constraints imposed by the conventional 980 nm laser excitation.

Enhancement of upconversion emission in Er:LiNbO3 by codoping with HfO2 under 1550 nm excitation

An increased red upconversion emission produced under excitation at 1550 nm was observed in hafnium and erbium co-doped lithium niobate (Hf (8 mol%)/Er:LiNbO3), which can be used to increase the photovoltaic efficiency of solar cells. Three 1550 nm photons were required to arouse the green and red emissions in Hfx(x= 2, 4, 6 mol%)/Er:LiNbO3, while the red emitting 4F9/2 state was populated by a hybrid two- and three-photon process in Hf(8 mol%)/Er:LiNbO3. The formation of Er3+ cluster sites (ErLi2+-ErNb2−) lead to an efficient cross relaxation process 4I13/2 + 4I13/2 → 4I15/2 + 4I9/2 and enhanced red emission in Hf(8 mol%)/Er:LiNbO3.

Enhancement of up-conversion emission and emerging cool white light emission in co-doped Yb3+/ Er3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications

Abstract A series of co-doped (Yb 3+ /Er 3+ ): Li 2 O-LiF-B 2 O 3 -ZnO glasses were prepared by standard melt quenching technique. Structural and morphological studies were carried out by XRD and FESEM. Phonon energy dynamics have been clearly elucidated by Laser Raman analysis. The pertinent absorption bands were observed in optical absorption spectra of singly doped and co-doped Yb 3+ /Er 3+ : LBZ glasses. We have been observed a strong up-conversion red emission pertaining to Er 3+ ions at 1.0 mol% under the excitation of 980 nm. However, the up-conversion and down conversion (1.53 µm) emission intensities were remarkably enhanced with the addition of Yb 3+ ions to Er 3+ : LBZ glasses due to energy transfer from Yb 3+ to Er 3+ . Up-conversion emission spectra of co-doped (Yb 3+ /Er 3+ ): LBZ glasses exhibits three strong emissions at 480 nm, 541 nm and 610 nm which are assigned with corresponding electronic transitions of 2 H 9/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 respectively. Consequently, the green to red ratio values (G/R) also supports the strong up-conversion emission. The Commission International de E′clairage coordinates and correlated color temperatures (CCT) were calculated from their up-conversion emission spectra of co-doped (Yb 3+ /Er 3+ ): LBZ glasses. The obtained chromaticity coordinates for optimized glass (0.332, 0.337) with CCT value at 5520 K are very close to the standard white colorimetric point in cool white region. These results could be suggested that the obtained co-doped (Yb 3+ /Er 3+ ): LBZ glasses are promising candidates for w-LEDs applications.

Tunable color upconverison emissions in erbium(III)-doped BiOCl microplates for simultaneous thermometry and optical heating

The authors have synthesized Er(III)-doped BiOCl microplates by a solid-state reaction method. Under irradiation with 980 nm light, the microplates emit both green and red upconversion emissions originating from the transitions of Er(III) ions. By properly adjusting the concentration of the Er(III) ions, the colors can be tuned from pure green to yellowish green. The optimal doping concentration is 3 mol%. Furthermore, the ratio of the two green emission bands of the Er(III) ions is strongly temperature-dependent. The doped microplates can be used to sense temperature in the 298 to 778 K range, indicating a maximum sensitivity of 2.8 × 10−3 K−1. The sensitivity is independent of Er(III) ion concentration. The internal heating properties of the microplates induced by the laser excitation are also discussed. Theoretical analysis reveals that the temperature of the doped BiOCl microplates increases from 276 to 324 K with elevating the laser power from 246 to 445 mW.

White light emission from Er2O3 nano-powder excited by infrared radiation

Phosphors of Er2O3 nano-crystalline powders were synthesized by the thermal decomposition method. The structural properties of the nano-powders were investigated with XRD and HRTEM measurements. The cubic phase with a = 10.540 Å was the only phase observed. The average crystalline sizes and the widths of the grain size distribution curves were determined to be 27.2, 18.7 and 9.7 nm, respectively. The spectroscopic properties of the Er2O3 nano-powder were studied by measuring the luminescence, decay and rise patterns under 808 and 975 nm diode laser excitations. A peculiar effect of the pressure was observed since an optically active ion (Er) is part of the complex and not a dopant. A broad band of the white light emission combined with blue, green and red up-conversion emission bands of Er3+ ions were observed at 0.03 mbar pressure under both excitation wavelengths. Only, an intense broad band white light emission was observed from these nanocrystals at atmospheric pressure. Rising patterns show that the white light intensity reaches its maximum value more rapidly under 975 nm excitation although it decays slower than that of 808 nm excitation. The color quality parameters such as the color coordinate (CRI), correlated color temperature and the color rendering index were found to vary with both the excitation wavelength and the ambient pressure indicating that these nanocrystals could be considered good white light emitting source under the infrared excitations.

White-light manipulation in Ho3+/Yb3+/Tm3+-doped LiNbO3 single crystals through transition metal Mn2+ ion doping

A strategy for ideal upconversion white-light emission in LiNbO3:Ho3+/Yb3+/Tm3+ single crystals is demonstrated simply through transition metal Mn2+ doping. The addition of different Mn2+ concentrations favors the probability of upconversion red emission transition, and the ratio of red-to-green is enhanced resulting in a purer upconversion white-light emission. It is observed the cross-relaxation of energy between Ho3+, Tm3+ and Mn2+ ions plays an important role in improving the red emission. The obtained results greatly benefit in the further discovery to design upconversion process and fabricate other ideal upconversion white-light materials.

Enhanced single-band red upconversion emission in Yb/Er: Na3ZrF7@Yb: Na3ZrF7 active-core/active-shell nanocrystals

Yb/Er: Na3ZrF7 NCs with single-band upconversion emission are prepared by a facile co-precipitation method and their particle sizes are tuned by varying Yb3+ doping concentration. The UC emission intensity of 20Yb/2Er:Na3ZrF7@20Yb:Na3ZrF7 core/shell NCs increases ∼33 times in comparison with the 20Yb/2Er:Na3ZrF7 core NCs.

Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF4:Yb3+,Er3+

The mechanism for red upconversion luminescence of Yb–Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, our results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O.

Near infrared to blue upconverting Tm3+/Yb3+/Li+:Gd2(MoO4)3 phosphors for light emitting display devices

Tm3+-Yb3+/Tm3+-Yb3+_Li+ codoped/tridoped Gd2(MoO4)3 phosphors using chemical co-precipitation method have been synthesized and characterised by XRD, FTIR, diffuse reflectance spectra and FE-SEM analysis. The downconversion emission spectra recorded by using 390nm radiation shows blue colour emission band due to the 1G4→3H6 transition of Tm3+ ion. Frequency upconversion emission study show overall intense blue, red and NIR upconversion emission bands due to the 1G4→3H6, 1G4→3F4 and 3H4→3H6 transitions of Tm3+ ion under 980nm laser diode excitation. The effect of incorporation of Li+ ions in the codoped phosphors produces small shifting in the XRD peaks towards the higher diffraction angle side and also intensity of the blue upconversion emission bands is enhanced around ∼510 times in the Tm3+-Yb3+_Li+:Gd2(MoO4)3 phosphors as compared to the Tm3+ doped phosphors. The confirmation of overall blue colour emission has been identified by colour co-ordinate analysis. The result shows that the developed tri-doped phosphors show improved blue colour emission and hence could be a proper material for near infrared to blue upconverter and blue colour emitting device fabrications.

Upconversion photon quantification of holmium and erbium ions in waveguide-adaptive germanate glasses

Visible upconversion photons have been quantified precisely in Ho3+/Yb3+ and Er3+/Yb3+ doped waveguide-adaptive aluminum germanate (NMAG) glasses, and effective red and green upconversion emissions generated from Ho3+ and Er3+ were illustrated in contrast. The emission photon numbers are identified as a positive correlation with the laser power densities, and stronger dominance of red emission in the Ho3+/Yb3+ doped NMAG glasses and more effectiveness of photon generation in Er3+/Yb3+ doped case were proved. When the power density is 1227 W/cm2, the absolute quantum yields for red and green (660 and 548 nm) upconversion fluorescences are derived to be 2.41 × 10−5 and 0.17 × 10−5 in Ho3+/Yb3+ doped NMAG glasses, and the ones (665 and 548 nm) in Er3+/Yb3+ doped NMAG glasses are 4.26 × 10−5 and 1.44 × 10−5. The macroscopic quantization of red and green upconversion emissions in Ho3+/Yb3+ and Er3+/Yb3+ doped waveguide-adaptive NMAG glasses provides the original referenced data for developing upconversion waveguide-typed irradiation light sources.

A modified energy transfer model for determination of upconversion emission of β-NaYF4:Yb,Er: Role of self-quenching effect

A modified energy transfer model by incorporating self-quenching effect is introduced to determine upconversion emission of β-NaYF4:Yb,Er. The simulation results agree well with existing experimental results, demonstrating the critical role of self-quenching effect in upconversion emission. Our results confirm that a 4.4-fold increase of green upconversion and 86-fold increase in the intensity of red upconversion emission could be realized by suppressing self-quenching. In addition, the optimal doping concentrations for integral emission intensity are found to be independent of excitation power, while the green to red ratio is found to rely significantly on excitation power. Our model offers mechanistic insight into upconversion emission processes and provides inspirations in improving upconversion emission efficiency through optimization of energy transfer pathways in different types of matrix sub-lattice.

Pure red upconversion emission from CaF2:Yb3+/Eu3+

Single phase cubic CaF2 microcrystals co-doped with Eu3+ and Yb3+ ions were synthesized through a coprecipitation process followed by annealing at 1200°C. Under the excitation of 980nm laser, the intensive emission at ~614nm (5D0 → 7F2) in the red region was observed at room temperature. The color coordinate of CaF2:0.1%Yb3+/0.1%Eu3+ was calculated with the Commission Internationale de I’Eclairage (CIE) chromaticity coordinate of (x=0.572, y=0.265). The excitation of Eu3+ ions came from the energy transfer of Yb3+-dimers through a cooperation sensitization process. Furthermore, not only the emissions from the 5Di (i=0, 1, 2, 3)→7Fj (j=0, 1, 2, 3) transitions but also the rarely reported emission (~398nm) from the 5L6 →7F0 transition was recorded. The cooperation sensitization process was so effective in the Yb3+/Eu3+ co-doped system that all the cooperative luminescence (CL) from Yb3+-clusters were quenched and only the red upconversion luminescence of Eu3+ ions could be seen.

Enhanced red upconversion emission and its mechanism in Yb3+–Er3+ codoped α-NaLuF4 nanoparticles

Red upconversion luminescence is greatly enhanced through manipulation of the initial solution pH.