Red Upconversion Luminescence Research Articles

Yb³⁺/Er³⁺-Codoped Bi₂O₃ Nanospheres: Probe for Upconversion Luminescence Imaging and Binary Contrast Agent for Computed Tomography Imaging.

In this work, water-soluble Yb(3+)/Er(3+) codoped Bi2O3 upconversion (UC) nanospheres with uniform morphology have been successfully synthesized via a solid-state-chemistry thermal decomposition process. With 980 nm near-infrared irradiation, the Bi2O3:Yb(3+)/Er(3+) nanospheres have bright UC luminescence (UCL). Moreover, multicolor UC emissions (from green to red) can be tuned by simply changing the Yb(3+) ions doping concentration. After citric acid molecules were grafted on the surface of Bi2O3:20% Yb(3+)/2% Er(3+) nanospheres, the MTT assay on HeLa cells and CCK-8 assay on osteoblasts show that the UC nanospheres exhibit excellent stability and biocompatibility. The possibility of using these nanoprobes with red UCL for optical imaging in vivo has been demonstrated. Furthermore, Bi(3+) and Yb(3+) containing nanospheres as binary contrast agent also exhibited significant enhancement of contrast efficacy than iodine-based contrast agent via X-ray computed tomography (CT) imaging at different voltage setting (80-140 kVp), indicating they have potential as CT imaging contrast agent. Thus, Yb(3+)/Er(3+) codoped Bi2O3 nanospheres could be used as dual modality probe for optical and CT imagings.

Green and Red Up-Conversion Luminescence of Er3+/Yb3+ Co-Doped 0.94Na0.5Bi0.5TiO3-0.06 BaTiO3 Ceramics

The Er3+/Yb3+ co-doped 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-BT:Er3+/Yb3+) cera-mics were synthesized by a traditional solid state reaction method. The XRD patterns show that all the ceramics are the single phase with perovskite structure. Under the excitation of 980 nm laser diode, the samples exhibit bright green up-conversion (UC) emission bands, which corresponds to the radiative transitions from (4S3/2,2H11/2) to 4I15/2, respectively. By tailoring the ratio of Er3+/Yb3+, the optimal UC emission properties were obtained in NBT-BT:0.01Er, 0.10Yb ceramics. The thermal behavior of UC emission in the ceramics was also investigated and the maximum sensitivity based on fluorescence intensity ratio (FIR) technology was approximately 0.0040K−1 at 502K. Moreover, ferroelectric and dielectric properties were enhanced with small amount of Er3+/Yb3+ co-doped. These ceramics, with excellent properties, may be a potential candidate in the application for electro-optical devices.

Optical properties of Er3 +-doped oxyfluoride glasses

Er3+-singly doped and Er3+/Yb3+-codoped 50SiO2–(50−x)BaF2–xZnF2(SBZx) oxyfluoride glasses are prepared and the optical properties of Er3+-singly doped glasses are investigated by using the Judd–Ofelt theory. Bright green and red upconversion luminescence of Er3+/Yb3+-codoped glasses is obtained under 980nm excitation. Furthermore, factors affecting this phenomenon such as glass composition, doping concentration of Er3+ and Yb3+ ions, and pump power are discussed in details.

Ce3+ dopants-induced spectral conversion from green to red in the Yb/Ho: NaLuF4 self-crystallized nano-glass-ceramics

Yb/Ho: NaLuF4 self-crystallized glasses were successfully synthesized. After further heat-treatment (forming nano-glass-ceramics), Ho3+ upconversion luminescence was enhanced by about 25 times, owing to the increased size and improved crystallinity of cubic NaLuF4 nanocrystals. Impressively, the introducing of Ce3+ dopants into glass ceramics resulted in an increase of red to green emission ratio of Ho3+ from ∼0.11 to ∼32 and ultimately induced an intense near-single-band red upconversion luminescence. Such color tunable luminescence was proved to be attributed to the competition of radiation transitions from Ho3+5S2,5F4 green-emitting and 5F5 red-emitting intermediate states.

Enhancement of the red upconversion luminescence in NaYF4:Yb3+, Er3+ nanoparticles by the transition metal ions doping

In recent years, the application of upconversion nanoparticles, especially in vivo, has so far largely been hampered due to the strong tissue absorption of short-wavelength light below 600nm. Therefore, it is an urgent need to carry out some new selective approaches to increase red emission efficiency. This work reports an effective strategy to enhance red upconversion emission of NaYF4:Yb3+, Er3+ nanocrystals by doping with the different concentrations of transition metal ions (Mn2+ or Fe3+). The upconversion emission spectra of the samples demonstrated that the red emissions were significantly enhanced and that different transition metal ions had a distinguishable effect on the red upconversion luminescence. Compared with the pure NaYF4:Yb3+, Er3+ nanoparticles, the red upconversion emission intensity of the nanoparticles co-doped with 10mol% Mn2+ was increased by nearly as high as 14 times. In addition, the red emission intensity was increased by about 24 times, which was observed in the sample co-doped with 30mol% Fe3+. The mechanism of the enhancement was discussed detailedly. Both the improvement of the nanoparticles׳ crystallinity and the redistribution of electron cloud around the Er3+ ions after introducing Mn2+ or Fe3+ were confirmed to be the origin of the improvement. The enhancement in red upconversion luminescence intensity makes the upconversion nanoparticles versatile imaging tools for diagnosis.

Up-conversion luminescence of Er3 +/Yb3 + co-doped LiYF4 nanocrystals in sol–gel derived oxyfluoride glass-ceramics

Up-conversion properties of Yb3+/Er3+ co-doped LiYF4 nanocrystals embedded in a sol–gel derived oxyfluoride glass-ceramic have been studied by comparison to the corresponding polycrystalline sample. Under 980nm laser light pumping, both Yb3+/Er3+ co-doped LiYF4 samples show green ((2H11/2, 4S3/2)→4I15/2) and red (4F9/2→4I15/2) up-conversion luminescences explained by a two-photon processes. The tendency for the saturation of the red up-conversion luminescence band observed in the glass ceramic was ascribed to a back energy transfer process Er3+–Yb3+ and cross-relaxation processes caused by the high Yb3+ and Er3+ ions concentrations in the nanocrystals.

Nd3+-Sensitized Ho3+ Single-Band Red Upconversion Luminescence in Core–Shell Nanoarchitecture

A strategy to achieve 808 nm excited single-band red upconversion luminescence of Ho(3+) via the core-shell nanoarchitecture design was provided. Specifically, the synthesized Yb/Ho/Ce: NaGdF4@Yb/Nd: NaYF4 active-core@active-shell nanoparticles were evidenced to enable high-content doping of Nd(3+) (∼10 mol %) in the shell layer and, thus, markedly enhance red upconversion emission from Ho(3+) activators in the core with the assistance of spatially confined doping of Nd(3+) ions and efficient energy transfer of Nd(3+) → Yb(3+)(shell) → Yb(3+)(core) → Ho(3+). Importantly, introducing Ce(3+) into the core was beneficial to the competition of radiation transitions from the two intermediate excited states of Ho(3+): (5)S2,(5)F4 (green-emitting) and Ho(3+): (5)F5 (red-emitting), which induced great enhancement in the red to green intensity ratio and ultimately intense single-band red upconversion emission. We believe that this preliminary study will provide an important advance in developing luminescent markers suitable for biolabeling applications.

Enhancement of Luminescence of YPO<sub>4</sub>:Eu<sup>3+</sup>,Yb<sup>3+</sup> Nanophosphor Synthesized by Hydrothermal Method

YPO4 nanowires (NWs) and nanoparticles (NPs) have been synthesized by hydrothermal methods respectively in the absence and presence of EDTA. The Eu3+ and Yb3+ codoped YPO4 NWs and NPs prepared directly from hydrothermal systems show efficient red down-conversion luminescence (DL) and upconversion luminescence (UL) at room temperature without further annealing process. EDTA plays a critical role in controlling the morphologies of YPO4 nanocrystals in hydrothermal systems and enhances both the DL and UL intensities of Eu3+ by the factor of 100%. Monodispersed Eu3+ and Yb3+ codoped YPO4 NPs will be a new candidate for application in infrared displays and biological labels.

Intense Upconversion Luminescence of CaSc2 O4 :Ho(3+) /Yb(3+) from Large Absorption Cross Section and Energy-Transfer Rate of Yb(3.).

Concentration-optimized CaSc2 O4 :0.2 % Ho(3+) /10 % Yb(3+) shows stronger upconversion luminescence (UCL) than a typical concentration-optimized upconverting phosphor Y2 O3 :0.2 % Ho(3+) /10 % b(3+) upon excitation with a 980 nm laser diode pump. The (5) F4 +(5) S2 →(5) I8 green UCL around 545 nm and (5) F5 →(5) I8 red UCL around 660 nm of Ho(3+) are enhanced by factors of 2.6 and 1.6, respectively. On analyzing the emission spectra and decay curves of Yb(3+) : (2) F5/2 →(2) F7/2 and Ho(3+) : (5) I6 →(5) I8 , respectively, in the two hosts, we reveal that Yb(3+) in CaSc2 O4 exhibits a larger absorption cross section at 980 nm and subsequent larger Yb(3+) : (2) F5/2 →Ho(3+) : (5) I6 energy-transfer coefficient (8.55×10(-17) cm(3) s(-1) ) compared to that (4.63×10(-17) cm(3) s(-1) ) in Y2 O3 , indicating that CaSc2 O4 :Ho(3+) /Yb(3+) is an excellent oxide upconverting material for achieving intense UCL.

Effect of Li+ ions doping on microstructure and upconversion luminescence of CeO2:Er3+ translucent ceramics

CeO2:1%Er, CeO2:1%Er, 5%Li and CeO2:1%Er, 10%Li translucent ceramics were fabricated for the first time. Under the 980nm diode laser excitation, the ceramics gave intense green (520–570nm) and red (640–680nm) upconversion luminescence, which were ascribed to the radiative transitions of 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 of Er3+ ions, respectively. It was worthy to point out that the above upconversion luminescence intensity of CeO2:Er, Li ceramics was greatly enhanced, and the preliminary observation suggests that the transmittance of CeO2:Er, Li ceramics in the visible spectral region was improved when Li+ ions were introduced as ceramics fluxing agent. The effect of Li+ ions doping on microstructure and upconversion emission of CeO2:Er translucent ceramics was investigated and the possible reasons for these results were presented, respectively.

IR to visible upconversion luminescence of transparent (Mg,Er)–α-Sialon ceramics

A detailed study of upconversion (UC) process was carried out in Er2O3 doped Mg–α-Sialon ceramics. IR to visible upconverting transparent Sialon ceramics has been fabricated by hot press sintering method. Different Sialon phases, grain morphology, and chemical compositions were analyzed by X-ray diffraction and Transmission Electron Microscopy. With increasing the amount of Er2O3 in Mg–Sialon system, the α-Sialon phase was also increased. The UC emissions were observed around 527, 547 and 660nm wavelength corresponding to the 2H11/2, 4S3/2 and 4F9/2 transitions, respectively under 980nm continuous wave (CW) laser diode excitation. UC process in Er3+ is governed by two photonic processes. Increasing the doping concentration (Er2O3), dipole–dipole energy transfer (ET) mechanisms between nearby Er3+ ions become more efficient than excited state absorption (ESA). The intense UC was observed in the green region due to low phonon energy of Sialon system as revealed by Raman spectroscopy. The red UC luminescence was also observed, but the emission intensity was less as compared to the green emission. Moreover, the down-conversion luminescence was observed around 1530nm corresponding to 4I13/2 transition.

Selectively enhanced red upconversion luminescence and phase/size manipulation via Fe(3+) doping in NaYF4:Yb,Er nanocrystals.

Red upconversion luminescence (UCL) is selectively enhanced by about 7 times via Fe(3+) codoping into a NaYF4:Yb,Er nanocrystalline lattice. The maximum red-to-green ratio (R/G) as well as the overall integrated UCL intensity features at an Fe(3+) content of 20 mol%. The size and phase of nanocrystals are simultaneously manipulated via Fe(3+) doping with various concentrations by a facile hydrothermal method. Contrary to the literature, the pure hexagonal phase appears when Fe(3+) concentrations are from 5 to 20 mol%, meanwhile, the size of NaYF4:Yb,Er nanocrystals reaches its maximum at 10 mol%. The intensified visible UCL especially the dominant red emission is mainly ascribed to the energy transfer (ET) from |(2)F7/2, (4)T1g > (Yb(3+)-Fe(3+) dimer) to (4)F9/2 (Er(3+)) states as well as the distortion of the crystalline field symmetry upon Fe(3+) codoping. Dynamic investigation of (4)S3/2 and (4)F9/2 states under the pulsed laser excitation of 980 nm along with the diffuse reflectance data further supports the proposed mechanism of UC processes. The results show the remarkable promise of Fe(3+)-codoped NaYF4:Yb,Er nanocrystals as upconverting nanoprobes with high sensitivity and penetrability in deeper tissue for multimodal biomedical imaging.

Tunable Upconversion Luminescence and Energy Transfer Process in BaLa2ZnO5:Er3+/Yb3+Phosphors

BaLa2ZnO5:Er3+/Yb3+has been synthesized via a high temperature solid-state method, and the tunable upconversion luminescence and energy transfer process between Yb3+and Er3+in this system have been demonstrated. Upon 980 nm laser excitation, the intense green and red emission around 527, 553, and 664 nm were observed for BaLa2ZnO5:Er3+/Yb3+, which can be assigned to the characteristic energy level transitions of2H11/2→4I15/2,4S3/2→4I15/2, and4F9/2→4I15/2of Er3+, respectively. The critical Er3+quenching concentration (QC) was determined to be about 5 mol%, and the power studies indicated that mixture of 2- and 3-photon process was responsible for the green and red upconversion luminescence.

Tuning into single-band red upconversion luminescence in Yb(3+)/Ho(3+) activated nano-glass-ceramics through Ce(3+) doping.

Yb(3+)/Ho(3+) activated glass ceramics containing β-YF3 nanocrystals were successfully fabricated. The green ((5)S2/(5)F4→(5)I8) upconversion emission is dominant in the glass ceramics and is about 160 times stronger than that of the precursor glass, resulting from the partition of lanthanide activators into a low-phonon-energy crystalline lattice and the subsequent low probability of multi-phonon nonradiative relaxation from the (5)S2/(5)F4 and (5)I6 states to the lower ones. Upon the introduction of Ce(3+) ions into nano-glass-ceramics, two efficient cross-relaxation processes between Ho(3+) and Ce(3+), i.e., Ho(3+):(5)S2/(5)F4 + Ce(3+):(2)F5/2→Ho(3+):(5)F5 + Ce(3+):(2)F7/2 and Ho(3+):(5)I6 + Ce(3+):(2)F5/2→Ho(3+):(5)I7 + Ce(3+):(2)F7/2, are demonstrated to greatly suppress the population of the green-emitting (5)S2/(5)F4 state and to enhance the population of the red-emitting (5)F5 one, leading to the intense single-band red UC radiation of Ho(3+).

Up-conversion and Photoluminescence in Er3+ Single Crystal MgAl-spinel

Traditional and up-conversion luminescence of MgAl2O4 single crystal doped with erbium ions obtained by the Verneuil method has been investigated. The time resolved spectral measurements of the green and red up-conversion luminescence bands show that a build-up part is present in the up-conversion luminescence kinetics. This means that energy transfer process is involved in the creation of the luminescence. Considering rather small concentration of Er3+ in the material (0.12 mass %), the expected up-conversion mechanism should be excited state absorption since the average distance between erbium ions is high. The above-mentioned considerations suggest that clustering of the activator ions is present in the material, which is supported by SEM analysis.

A long-wave optical pH sensor based on red upconversion luminescence of NaGdF4nanotubes

A long-wave optical pH sensor based on intense red upconversion (UC) luminescence of NaGdF4 nanotubes and bromothymol blue was explored. By detecting the UC spectra of NaGdF4 nanotubes in aqueous solution, the sensing system could work on the pH measurement. The intensities of the red emission were linear with pH value changing from 6 to 8.

Enhanced upconversion luminescence and single-band red emission of NaErF4 nanocrystals via Mn2+ doping

In this paper, the monodispersed NaErF4:Yb, Mn nanoparticles (NPs) were successfully synthesized by a simple and mild solvothermal method. The as-prepared NPs were characterized by X-ray diffraction, transmission electron microscopy, and spectrophotometer. The crystal phase, size, morphology, and upconversion (UC) properties of these NPs can be readily tuned by doping Mn2+. With increasing Mn2+ contents, the crystal phase of NaErF4:Yb NPs was transferred from hexagonal to cubic, and then a new orthorhombic phase NaMn3F10 appeared. The morphology of NaErF4:Yb was tuned from nanodisks to nanocubes and then to large nanoflakes with increasing Mn2+ dopant contents. Moreover, the UC luminescence (UCL) intensity can be enhanced by increasing Mn2+ content. Interestingly, single-band red emission can be achieved when the Mn2+ content reaches 20%. This is mainly ascribed to the efficient energy transfer between Er3+ and Mn2+. The enhanced UCL intensity and single-band red UCL make these NaErF4:Yb/Mn NPs ideal probes for optical bioimaging owing to the low tissue absorption and deep tissue penetration of red light.

Up-conversion luminescence of Y2O3: Yb, Er under 1.55 μm excitation

This study explores a new method to achieve highly efficient red up-conversion luminescence by using Y2O3: Yb, Er pumping with a 1550nm laser diode (Y2O3: Yb, Er@1550nm). The strongest red emission, with a peak situated at 660nm, can be assigned to Er3+ ion 4I9/2→4I15/2 transition. The radiation brightness of Y2O3: Yb, Er@1550nm is comparable to that of Y2O3: Yb, Er under an equivalent 980nm pumping density (Y2O3: Yb, Er@980nm). Y2O3: Yb, Er@1550nm presents a very weak green emission as compared with that of Y2O3: Yb, Er@980nm. Consequently, Y2O3: Yb, Er@1550nm exhibits much better color purity and excellent color stability with increasing pumping power.

Effect of charge compensation on up-conversion and UV excited luminescence of Eu3+ in Yb3+–Eu3+ doped calcium aluminate CaAl4O7

Calcium aluminate powders co-doped with Yb3+ and Eu3+: Ca1−x−yYbxEuyAl4O7 (x = 0.02–0.06; y = 0.02), Ca1−x−yYbxEuyAl4O7 (x = 0.05, y = 0.01–0.07) were synthesized by a modified Pechini citrate process and their optical properties at 298 K and 77 K were investigated. Direct excitation of Yb3+ by means of 2F7/2 → 2F5/2 absorption at 980 nm leads to bluish-green luminescence centered around 488 nm due to the cooperative emission of Yb3+ and red emission of Eu3+ corresponding to 5D0 → 7FJ (J = 0–4) transitions. The near-infrared to red up-conversion luminescence of Eu3+ was assigned to a cooperative sensitization mechanism. The effect of activators concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied in detail. Na+ addition led to narrowing of luminescence bands, suggesting smaller disturbance/distribution of activator ions local symmetries. This was associated with significantly enhanced luminescence brightness of Eu3+ under UV excitation and, surprisingly, with noticeably reduced efficiency of Eu UC emission upon near-infrared excitation. It was also concluded that Yb3+ pairs transfer preferentially their excitation energy to Eu3+ ions with the local symmetry perturbed by certain defects present in the crystal lattice.

Enhanced broadband excited upconversion luminescence in Ho-doped glasses by codoping with bismuth.

We report enhanced green and red upconversion (UC) luminescence in Ho3+-doped oxyfluoride germanate glass by introducing bismuth near-infrared active centers as sensitizers. The UC excitation bands at 750 and 970 nm show a full width at half-maximum of 20 and 45 nm, respectively. Energy transfer from sensitizers, the excited-state absorption, and phonon-coupled absorption of Ho3+ jointly contribute to the enhanced UC luminescence. Our approach provides an efficient methodology to broaden the excitation bandwidth of UC luminescent materials, which may have the potential for promising application in solar cells.