Ultraviolet Upconversion Research Articles

Citric acid-assisted phase controlled synthesis of NaYF4:Yb3+,Tm3+ crystals and their intense ultraviolet upconversion emissions

The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.

Seven-photon ultraviolet upconversion emission of Er3+ induced by 1,540-nm laser excitation

The multi-photon ultraviolet upconversion emission properties and synergistic effect are investigated in BaSr2Y6O12:Er3+ phosphor. The deep-ultraviolet emissions centered at 274, 297 and 324-nm are observed under the 1,540-nm excitation, which results from a seven-, six- and six-photon upconversion process, respectively. A synergistic effect is found, which shows that the red emission intensity under 351- and 1,540-nm dual excitation is 4.7 % time stronger than the sum of red emission intensities under the 351 and 1,540-nm single excitation. This phenomenon is attributed to the 4I13/2 and 4I11/2 levels of Er3+ from non-radiative transition process under the 351-nm excitation are excited again to 4F9/2 level by absorbing 1,540-nm photon in the 351- and 1,540-nm dual-excitation process.

Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm3+ doped NaYF4 phosphors by Yb3+ codoping

2 mol% Tm3+ doped NaYF4 phosphors with 0–98 mol% Yb3+ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm3+–Yb3+:NaYF4 phosphors was investigated with increasing of Yb3+ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm3+ were observed from the Tm3+–Yb3+:NaYF4 phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb3+ codoping. The violet and blue emissions for the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+ can be tuned by varying Yb3+ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the 3F2 → 3H6 and 3F3→3H6 transitions of Tm3+ was strongly related to the Yb3+ codoping concentration and temperature, implying a potential application of Tm3+–Yb3+:NaYF4 phosphors for optical temperature sensing.

Different dynamics of ultraviolet upconversion in Tm3+:ZBLAN glass under blue laser excitation

Ultraviolet upconversion luminescence around 292nm, 350nm and 363nm in Tm3+:ZBLAN glass was observed clearly by pulsed excitation at 464nm. Upconversion dynamics was discussed in detail by an analysis of the measured intensity dependence, decay curves and excitation spectra of upconversion luminescence, from which it was clarified that upconversion luminescence around 292nm and 350nm from 1I6 level was attributed to excited state absorption, while that around 363nm from 1D2 level was attributed to energy transfer process.

Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol–gel derived glass-ceramics

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650°C of the (Yb3+/Er3+) codoped CaF2–SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 16nm size. For pumping at 973nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at ∼408nm) and near-ultraviolet emission (at ∼380nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated.

Efficient near-infrared to visible and ultraviolet upconversion in polycrystalline BiOCl:Er3+/Yb3+ synthesized at low temperature

Er3+/Yb3+ co-doped BiOCl poly-crystals were synthesized by the conventional solid state method at 500 °C, which exhibited good crystalline and low phonon energy. Under 980 nm excitation, the samples showed intense red upconversion (UC) luminescence (Er3+: 4F9/2→4I15/2) as well as other four UC emission bands, including ultraviolet (UV) emission at 380 nm, violet emission at 411 nm, green UC emissions at 525 and 545 nm and near-infrared (NIR) emission between 800 and 850 nm, corresponding to the transitions of 4G11/2, 2H9/2, 2H11/2, 4S3/2 and 4I9/2→4I15/2 of Er3+, respectively. Interestingly, including the violet and green UC emissions, the red one originated a nearly three-photon process in this system, and a possible UC mechanism was proposed for the enhanced red emission.

Adjustable upconversion luminescence of Er3+-doped Lu2O3 upon Ar+ laser excitation

Lu2O3:Er3+ powders were synthesized by chemical coprecipitation. Ultraviolet, violet and blue upconversion properties of Lu2O3:Er3+ powders as a function of Er3+ concentration were investigated upon 514.5nm laser excitation. The cross relaxation of 2P3/2+4I15/2→2H9/2+4I13/2 was supposed to be responsible for the adjustable spectral distribution of upconversion emissions. Based on spectral, concentration and pump power dependence analyses, the upconversion mechanisms were proposed. Moreover, Stokes luminescent properties of Lu2O3:Er3+ powders were also observed upon 514.5nm laser excitation. These spectra showed the same concentration-dependent characteristics, and the dependence of emission spectra on Er3+ concentration could be well explained with the cross relaxation of 4F7/2+4I11/2→4F9/2+4F9/2.

Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–Gd3+ codoped NaLuF4 microcrystals

Optical temperature sensing characteristics based on the ultraviolet (UV) upconversion luminescence (UCL) of Gd3+ ions are reported here for the first time. Under 980 nm excitation, the temperature dependence five-photon UV UCL from the 6PJ and 6IJ levels of Gd3+ ions in NaLuF4:Yb3+, Tm3+, Gd3+ microcrystals were investigated systematically. The fluorescence intensity ratios (FIR) of two pairs of thermally coupled levels (6P5/2, 6P7/2 and 6I9/2, 6I7/2) were studied as a function of temperature in a range of 298–523 K. The maximum sensor sensitivities were found to be about 0.0004 K−1 (333 K) and 0.0029 K−1 (298 K) by exploiting the UC emissions from the 6PJ and 6IJ levels, respectively. This suggests that the Gd3+-based UCL materials are promising prototypes for application as multi-mode probes for use in bio-separation, MRI imaging, optical thermometers, etc.

Enhanced deep-ultraviolet upconversion emission of Gd3+ sensitized by Yb3+ and Ho3+ in β-NaLuF4 microcrystals under 980 nm excitation

The shortest ever recorded and enhanced 204.5 nm ultraviolet (UV) upconversion (UC) emission from 6G7/2 levels of Gd3+ ions was observed in NaLuF4:20%Yb3+,2%Ho3+,20%Gd3+ microcrystals (MCs) under 980 nm near infrared (NIR) excitation for the first time. The 6GJ levels of Gd3+ ions can be efficiently populated by the energy transfer (ET) processes of Yb → Ho → Gd and Yb → Gd. The effect of the pH value on the morphology of the MCs was explored and the UC mechanisms in the tri-doped NaLuF4 samples were analyzed in detail based on the emission spectra and the dependence of the luminescence intensity on the laser pumping power where a 6-photon UC process of the 204.5 nm emission is observed. The contrast spectra of UC luminescence suggest that β-NaLuF4 MCs are a better UC host material than the β-NaYF4 counterpart. Furthermore, this paper also provides an effective and facile approach to modulating the sizes of β-NaLuF4 MCs by manipulating the pH values of the initial reaction solutions and the doping of Gd3+ ions.

Femtosecond Fluorescence Dynamics of Trp-Trp Dipeptide in Water and its Interaction with DNA

We reported here the systematic measurements of fluorescence decay and quenching of tryptophan dipeptide Trp-Trp in water, and its interaction with DNA by using an ultraviolet upconversion fluorescence measurement system with time resolution better than 100 fs together with a time correlated single photon counting apparatus on the 100 ps to 30 ns time scale. We analyzed the effect of acidity and phosphate ion etc upon Trp-Trp fluorescence decay profiles. Fluorescence data reveal multiexponential decay of Trp-Trp dipeptide in water, and Trp-Trp can interact with DNA. It is shown that Trp-Trp is subject to ultrafast quenching with DNA and also senses nearby water dynamics. The time constant for the bulk water “solvent relaxation” remains near 1 ∼ 2 ps, but a new ultrafast decay, fit to an exponential time constant of ∼ 10 ps with associated amplitude positive or negative (>380 nm) depending on the emission wavelength, has been found. Candidate mechanisms (including ET to nearby acceptors and/or collisional quenching etc.) and the analysis of time resolved emission spectra will be discussed.

Up- and down-conversion luminescence in the oxyfluoride glass ceramics containing Ba2+1.5xYb1−xF7:Tb3+ nanocrystals

Transparent oxyfluoride borosilicate glass ceramics containing Ba2+1.5xYb1−xF7:Tb3+ nanocrystals were successfully prepared by a melt-quenching method with subsequent heat treatment. The precipitated crystalline phase in the glass matrix changed gradually from BaF2 to Ba2+1.5xYb1−xF7 with the increase of YbF3 content, which was confirmed by the results of XRD, HRTEM and EDX measurements. The ultraviolet and visible up-conversion and near-infrared quantum cutting down-conversion emissions were observed and interpreted. These materials could be used to modify the solar spectrum and enhance the silicon solar cell efficiency by the up-conversion and down-conversion luminescence of Tb3+–Yb3+ couples in the oxyfluoride borosilicate glass ceramics.

Intense up-conversion emissions of Yb3+/Dy3+ co-doped Al2O3 nanopowders prepared by non-aqueous sol—gel method

Yb3+/Dy3+ co-doped Al2O3 nanopowders have been prepared by the non-aqueous sol—gel method and their up-conversion photoluminescence spectra are measured under excitation by a 980-nm semiconductor laser. The results show that there are comparatively abundant spectra of up-conversion emissions centered at 378, 408, 527 and 543, and 663 nm, corresponding to 4G9/2 → 6H13/2, 4G9/2 → 6H11/2, 4I15/2 → 6H13/2, and 4F9/2 → 6H11/2 transitions of Dy3+, respectively. Two-photon and three-photon processes are involved in ultraviolet, violet, green, and red up-conversion emissions. The energy transition between Yb3+ and Dy3+ is discussed.

Synthesis and upconversion luminescence properties study of NaYbF4:Tm3+ crystals with different dopant concentration

Tm3+ ion doped α-NaYbF4 nanocrystals (NCs) and β-NaYbF4 microcrystals (MCs) were prepared by one-step hydrothermal method through tuning pH values. Through the comparative study on the crystal growth mechanism guided by two different organic additives, the effect of Na+ cation on morphologies was discussed. Under 980-nm excitation, intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from both of them. The UC luminescence properties of the as-prepared MCs were studied based on changing the Tm3+ ion concentrations. The fluorescent lifetimes of MCs doped with different amount of Tm3+ ions indicate that the obtained NaYbF4:Tm3+ MC was an excellent matrix for UV UC emissions.

Upconversion emission enhancement of Gd3+ ions induced by surface plasmon field in Au@NaYF4 nanostructures codoped with Gd3+–Yb3+–Tm3+ ions

Au nanoparticles (NPs) attached β-NaYF4 nanocrystals codoped with Gd3+–Yb3+–Tm3+ were synthesized by a facial solution method. The UV–vis-near-infrared absorption spectrum shows typical surface plasmon resonance band of Au NPs in addition to the characteristic absorption peaks of Yb3+ ion. X-ray diffraction and selected area electron diffraction results indicate the existence of Au NPs. The transmission electron microscopic image reveals the formation of Au@NaYF4 nanostructures. Enhanced ultraviolet (UV) upconversion luminescence (UCL) was observed in the nanostructures under the excitation of 980-nm infrared laser. The largest enhancement factor was obtained as 76 for the 6IJ→8S7/2 emission of Gd3+ ions, which was much larger than those emission enhancement factors of Tm3+. It is for the first time to our knowledge that the emission enhancement of Gd3+ ions was obtained. Local field enhancement induced by Au NPs was found to be responsible for the UCL enhancement, which is the further experimental evidence of local field enhancement theory. Magnetic measurements of the Au@NaYF4 nanostructure indicated it would have potential application in magnetic resonance imaging.

Size-dependent upconversion luminescence in CaF2:Yb3 +,Tm3 + nanocrystals

CaF2:Yb,Tm nano- and micro-crystals with different sizes were synthesized by a simple hydrothermal method. All the nanocrystals present intense ultraviolet (UV), visible, and near infrared (NIR) upconversion (UC) luminescence. The relationship between the UC luminescence intensities and the sample sizes from 50nm to 1200nm was investigated. The UV UC luminescence intensity of the sample with the particle size around 600nm was the highest among all the samples, but the NIR UC luminescence intensity of the sample was the lowest. Our experimental results indicated that there was a size between 50nm and microscale at which the UV emission intensity was the highest for CaF2 matrix. The size-dependent UC luminescence was proposed and analyzed based on experimental data.

Citric acid-assisted hydrothermal synthesis of α-NaYF4:Yb3+,Tm3+ nanocrystals and their enhanced ultraviolet upconversion emissions

Tm3+ and Yb3+ ions co-doped α-NaYF4 nanocrystals (NCs) were prepared by a one step hydrothermal method with citric acid as the organic additive. The morphology of the as-prepared materials maintains a spherical shape when the surfactant amount, hydrothermal time, and hydrothermal temperature were varied. Under 980 nm excitation, intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from the α-NaYF4 NCs and their intensities were much higher than those of the β-NaYF4 NCs modified by trisodium citrate. The intensities of the 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than those of the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. It is for the first time, to our knowledge, that such intense UV UC emissions were obtained with α-NaYF4 NCs. The intense UV UC emissions and the relatively long fluorescent lifetimes for the 1I6, 1D2, and 1G4 levels of Tm3+ ions in α-NaYF4 NCs indicate that they could be one of the promising candidates for potential application in short-wavelength solid-state lasers.

(Gd,Yb,Tb)PO4 up-conversion nanocrystals for bimodal luminescence–MR imaging

Up-conversion (Gd,Yb,Tb)PO(4) materials and their potential for bimodal imaging have received little attention in the literature. Herein, we report the first study on the up-conversion emission of (Gd,Yb,Tb)PO(4) nanocrystals synthesized via a hydrothermal method at 150 °C. These materials exhibit ultraviolet, blue and green up-conversion emissions upon excitation with a 980 nm continuous wave laser diode. The intensity of the blue-emission band at 479 nm, ascribed to the cooperative up-conversion emission of a pair of excited Yb(3+) ions, depends on the Yb(3+)/Tb(3+) concentration ratio, calcination temperature and particle size. Strong green up-conversion emission of Tb(3+) is observed at 543 nm for the (5)D(4)→(7)F(5) transition. Relaxometry measurements reveal that the nanocrystals are efficient T(2)-weighted (negative) contrast agents which, combined with visible-light emission generated by infrared excitation, affords them considerable potential for being used in bimodal, photoluminescence-magnetic resonance, imaging.

近红外光诱导下NaYbF4∶Tm3+,Eu3+体系中Eu3+的上转换发光

NaYbF4∶0.01%Tm3+,20%Eu3+ nanocrystals were synthesized by hydrothermal method.The crystalline phase,morphology,and optical properties were characterized by X-ray diffraction,scanning electron microscope and luminescent spectra.The samples are nano-rods with uniform size distribution.Their length is 500 nm and diameter is 200 nm on average.Under 980 nm near-infrared laser excitation,ultraviolet and visible upconversion luminescence of Eu3+ was obtained.In the codoped NaYbF4 nano-rods,the energy transfer from Tm3+ to Eu3+ and the bridging effect of Tm3+ ions play important roles in populating the high-energy excited states of Eu3+.Furthermore,the upconversion emissions from 3P0→7Fj(j=0,1,2) of Eu3+ were observed for the first time.

Enhancement of upconversion luminescence of YAlO3: Er3+ by Gd3+ doping

The upconversion luminescences of YAlO3:Er3+ phosphor co-doping with different Gd3+ concentrations are investigated under the excitation of 980- and 532-nm diode lasers. A near ultraviolet upconversion emission at 410 nm is observed in YAlO3 under 532-nm excitation. Moreover, the inactive Gd3+ ions can improve the upconversion intensity efficiently in a certain range of concentration. Under 980-nm excitation, the visible upconversion emissions at 546 and 646 nm are enhanced by about 10 and 8 times at the Gd3+ concentration of 40%, respectively. The upconversion emission at 410 nm under 532-nm excitation is also enhanced by 7 times. The substitution of Gd3+ ions for Y3+ sites changes the local symmetry of Er3+, leading to the improvement of upconversion efficiency.

具有强紫外上转换发射特性的小尺寸、水溶性NaYF4∶Yb,Tm纳米晶的合成与表征

Uniform NaYF4∶18%Yb,0.5%Tm nanocrystals were synthesized via thermal decomposition method following high temperature,and then a change of the nanocrystals was made from oil-soluble to water-soluble by ligand-exchanged method.X-ray diffraction(XRD) analyses showed that the samples were β-NaYF4 nanocrystals.The TEM images exhibited that the nanocrystals had an average size of 35 nm.Under 980 nm near-infrared laser excitation,the samples showed intense ultraviolet upconversion emission.5-photon upconversion fluorescence of the Tm3+ ions in the nanocrystals was stronger than 4-photon and 3-photon upconversion fluorescence.Due to special optical properties and small size,the water-soluble nanocrystals would show potential applications in the biomedical field.