Yunxin Liu

Influence of urea on microstructure and optical properties of YPO4:Eu3+ phosphors

YPO4:Eu3+ phosphors were synthesized by solution coprecipitation method assisted by urea in the precursor reaction solution. X-ray diffraction spectral analysis showed that the samples synthesized with urea had smaller particle size and lower crystallinity than those samples synthesized without urea. Moreover, the calculated strain result indicated that the Eu3+ site in the former exhibited a lower crystal field symmetry than that in the latter. Hence, the influence of crystal field symmetry dominated luminescence efficiency rather than crystallinity because the luminescence intensity observed in Eu0.05Y0.95PO4 synthesized with 1.0 g urea was six-fold higher than that of the as-synthesized sample. With increased concentration of Eu3+ ion, the luminescence intensity initially increased, and then subsequently decreased as the concentration of Eu3+ ion exceeded 12 mol.% due to concentration quenching. The optimal condition for YPO4:Eu3+ phosphor was Eu0.12Y0.88PO4 with 1.0 g urea added in the precursor. The luminescence intensity of the optimal condition was again enhanced 1.6-fold relative to that of Eu0.05Y0.95PO4 synthesized with 1.0 g urea.

Remarkable red-shift of upconversion luminescence and anti-ferromagnetic coupling in NaLuF4:Yb3+/Tm3+/Gd3+/Sm3+ bifunctional microcrystals

Lanthanide doped bifunctional materials are potentially important for developing multifunctional devices. Here, NaLuF4:Yb3+/Tm3+/Gd3+/Sm3+ optical-magnetic bifunctional microcrystals were successfully synthesized by hydrothermal method, which could emit ∼480 nm blue light from the 1G4→3H6 electronic transition and ∼800 nm infrared light from the 3H4→3H6 electronic transition of Tm3+ ion, under the excitation of 980 nm infrared light. By doping Sm3+ ion into NaLuF4:Yb3+/Tm3+/Gd3+, the infrared emission peak centered at 800 nm would shift obviously to longer wavelength. This indicated that Sm3+ ion could efficiently tune the energy level gaps of Tm3+ ions in NaLuF4 host which was demonstrated based on the crystal field theory. In addition, these NaLuF4:Yb3+/Tm3+/Gd3+/Sm3+ microcrystals presented unique ferromagnetic property instead of usually reported paramagnetic property. Importantly, the ferromagnetic property decreased with increasing the concentration of Gd3+ ion. This was in good agreement with Swifts theoretical investigation that the coexistence of light rare earth (Gd3+) and heavy rare earth (Yb3+/Tm3+) would lead to the anti-ferromagnetic coupling in the sub-lattices.

Controllable plasmonic sensing based on Fano resonance in a cavity coupled defective MDM waveguide

We report a simple plasmonic sensor based on the Fano resonance in a cavity coupled defective metal–dielectric–metal (MDM) waveguide. A theoretical expression for characterizing the sensing property is first derived. We show the formation and evolution of Fano resonance in this cavity coupled waveguide system. Based on the derived theoretical expression and the numerical analysis, we find that the cavity length plays an important role for tuning and optimizing the sensing performance. A highest figure of merit (FOM) of ~6100 is obtained. This plasmonic sensor possesses the advantages of easy fabrication and compactness. The findings and the proposed structure may provide some guidance for the fundamental research of the integrated plasmonic nanosensor applications and designs.

Direct evidence of reversible energy transfer between Er3+ and Tm3+ ions in upconversion microcrystals

Abstract Lanthanide doped NaYF 4 microcrystals were synthesized via a facile hydrothermal method. Multicolor upconversion luminescence was observed in NaYF 4 microcrystals doped with Yb 3+ /Er 3+ , Yb 3+ /Tm 3+ , and Yb 3+ /Er 3+ /Tm 3+ under the excitation of 980 nm infrared light. Importantly, the excitation power density dependence of upconversion emission intensity indicated clearly the energy transfer from Tm 3+ to Er 3+ ions under the excitation of low power density (510 2 −9×10 2 W/cm 2 ). Meanwhile, the inverse energy transfer from Er 3+ to Tm 3+ ions under the excitation of relatively higher power density (4.1×10 4 −4.9×10 4 W/cm 2 ) was also revealed. This was a direct evidence for reversible energy transfer between Er 3+ and Tm 3+ ions. Under the excitation of high power density (4.1×10 4 −4.9×10 4 W/cm 2 ), dark sensitizers were also motivated so that the bottleneck effect of high concentration Yb 3+ ion doping was broken. This was the main reason for realizing high upconversion efficiency of the samples with heavy doping of Yb 3+ ion.

Enhancement of red to orange emission ratio of YPO4：Eu3＋,Ce3＋ and its dependence on Ce3＋ concentration

Abstract Eu 3+ and Ce 3+ co-doped YPO 4 microspheres were synthesized by hydrothermal method without template. The emission spectra showed that the red emission centered at 618 nm could be readily increased relatively to the orange emission centered at 590 nm by controlling the doping concentration of Ce 3+ ion. The investigation based on excitation spectra and decay curves demonstrated that the doped Ce 3+ ions took two efficient energy transfers to Eu 3+ ions and affected the lifetime of the emission states of Eu 3+ ions so that the emission spectra of Eu 3+ ion were accordingly tuned with the Ce 3+ content increasing. This controllable red ( 5 D 0 → 7 F 2 ) to orange ( 5 D 0 → 7 F 1 ) emission ratio of YPO 4 :Eu 3+ ,Ce 3+ made it very promising for encoded anti-fake labels and bio-labels.

Magnetic tuning of upconversion luminescence in Au/NaGdF4:Yb3+/Er3+ nanocomposite

Lanthanide-doped upconversion nanoparticles (UCNPs) NaGdF4:Yb3+/Er3+ have received increasing attention due to their unique optical-magnetic bifunctional properties. Here, we show that the luminescent intensity from NaGdF4:Yb3+/Er3+ nanoparticles decreases monotonously with increasing the applied magnetic field from 0 to 37.1 T, while plasmon-enhanced upconversion luminescence in Au/NaGdF4:Yb3+/Er3+ nanocomposite is independent of a magnetic field lower than 6 T. The surface plasmon resonances could compensate for the energetic mismatching between the excitation light and the energy-level gaps induced by magnetic field and enhance the radiative efficiency, which is the main factor for achieving this stable upconversion emission in this nanocomposite under a magnetic field not higher than 6 T. These findings provide a novel route for exploring the magnetic control of upconversion luminescence in lanthanide-doped bifunctional nanoparticles.

Synergistic effect of crystal structure and concentration quenching on photoluminescence of Er3+ doped upconversion nanocrystals

Abstract YbF 2.357 , YbF 3 , Ba 2 YbF 7 , and BaF 2 upconversion nanocrystals doped with emitter Er 3+ ion were synthesized in the same solvent system just with changing the molar ratio of Ba 2+ to Yb 3+ in the precursor, which corresponed to the crystal phases of rhombohedral, orthorhombic, tetragonal, and cubic, respectively. All the samples emitted both 660 nm red light and 543/523 nm green light which originated from Er 3+ -4f n electronic transitions 4 F 9/2 - 4 I 15/2 and 4 S 3/2 / 2 H 11/2 - 4 I 15/2 , respectively. It was worth mentioning that YbF 3 :Er 3+ , Ba 2 YbF 7 :Er 3+ , and BaF 2 :Er 3+ could emit dazzlingly bright light even under the excitation of a 980 nm CW laser with output power of 0.1 W. Upconversion emission mechanism analysis indicated that the intensity ratio of red to green light highly depended on the synergistic effect of crystal structure, concentration quenching, and particle size, but were not sensitive to crystallinity as previously reported for NaLnF 4 (Ln=lanthanide).

Upconversion NaYF 4 nanoparticles for size dependent cell imaging and concentration dependent detection of Rhodamine B

Upconversion nanoparticles (UCNPs) based on NaYF4 nanocrystals with strong upconversion luminescence are synthesized by the solvothermal method. The emission color of these NaYF4 upconversion nanoparticles can be easily modulated by the doping. These NaYF4 upconversion nanocrystals can be employed as fluorescence donors to pump fluorescent organic molecules. For example, the efficient luminescence resonant energy transfer (LRET) can be achieved by controlling the distance between NaYF4:Yb3+/Er3+ UCNPs and Rhodamine B (RB). NaYF4:Yb3+/Er3+ UCNPs can emit green light at the wavelength of ∼540 nm while RB can efficiently absorb the green light of ∼540 nmto emit red light of 610 nm. The LRET efficiency is highly dependent on the concentration of NaYF4 upconversion fluorescent donors. For the fixed concentration of 3.2 µg/mL RB, the optimal concentration of NaYF4:Yb3+/Er3+ UCNPs is equal to 4mg/mL which generates the highest LRET signal ratio. In addition, it is addressed that the upconversion nanoparticles with diameter of 200 nm are suitable for imaging the cells larger than 10 µm with clear differentiation between cell walls and cytoplasm.

Core/shell upconversion nanoparticles with intense fluorescence for detecting doxorubicin in vivo

Doxorubicin (Dox) is a chemotherapy medication used to treat cancer. Herein, we report a rapid and efficient method for detecting Dox in vivo based on a NaGdF4:Yb3+,Er3+@NaYF4 core/shell upconversion nanoparticles (UCNPs) probe. We found that the intensity ratio of green to red emission (IGVRE) bands of the core/shell NaGdF4:Yb3+,Er3+@NaYF4 nanoparticles was sensitive to Dox in blood samples, and drops as the concentration of Dox increases. In addition, the proposed UCNPs probe possessed the advantage that no nanoparticles leaked into the living body, thus overcoming the intrinsic defect (difficulty in removing UCNPs from blood vessels) of the fluorescence resonance energy transfer (FRET) approach. This proposed UCNP probe design and results may provide some guidance for the real-time and efficient detection of Dox, and can be helpful in biomedical applications.

Upconversion Luminescence and Magnetic Turning of NaLuF4

Fluorescent and magnetic bifunctional NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals were synthesized by the solvothermal method and subsequent surface modification. By changing the doping concentration of Gd3+, the shape, size, luminescent properties, and magnetic properties of the nanoparticles can be modulated. These NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals present efficient blue upconversion fluorescence and excellent paramagnetic property at room temperature. Based on the luminescence resonance energy transfer LRET, upconversion nanoparticles UCNPs were confirmed to be an efficient fluorescent nanoprobe for detecting acriflavine. It is easy to derive the concentration of acriflavine from the Integral Intensity Ratio of Green emission from acriflavine to Blue emission from UCNPs fluorescent signals. Based on this upconversion fluorescent nanoprobe, the detection limit of acriflavine can reach up to 0.32 μg/mL.