Cooperative Upconversion Emission Research Articles

Evaluation of the energy transfer mechanism leading to tunable green-to-red cooperative up-conversion emission in Eu3+-Yb3+ co-doped CaF2 powders

The luminescence from rare-earth doped alkali earth metal fluorides has been studied for applications in photonics such as lamp and display technologies. In this context, the study of energy transfer (ET) processes among rare-earth ions is of great importance. Here, we report the study of the ET mechanisms involving Eu3+ and Yb3+ in CaF2 powders prepared by combustion synthesis. Cooperative up-conversion (UC) luminescence could be tuned between green and red colors by adjusting the doping concentrations of the rare-earth ions. The energy transfer rate from pairs of Yb3+ to Eu3+ was estimated by use of a rate-equation set for the relevant 4f electronic populations engaged in the UC production.

Bright blue cooperative upconversion emission of Yb3+ from langbeinite K2Ti1.887 Yb0.113(PO4)3 single crystal

Ytterbium doped langbeinite K2Ti1.887Yb0.113(PO4)3 (KTYP) single crystals were grown using high temperature molten salt flux growth method. Inclusion-free KTYP crystals with sizes up to 3×3×3mm3 were attained using K6P4O13 molten flux. The complete melting point of solute and solvent is above at 1223K. Langbeinite phase KTYP crystals are growing in higher concentration of Yb2O3 in flux solution. KTYP emits a strong cooperative blue emission centered at 488nm owing to Yb-Yb pairs under 980nm laser excitation. To the best of our knowledge, phosphate based langbeinite crystal found exhibit cooperative upconversion emission at ~488nm.

Broadband-sensitive cooperative upconversion emission of La(Ga0.5Sc0.5)O3:Er,Ni,Nb

We systematically conducted time-resolved photoluminescence measurements on La(Ga0.5Sc0.5)O3:Er,Ni,Nb to elucidate the dominant mechanism of Ni2+-sensitized Er3+ upconversion emission at approximately 0.98 µm under 1.0–1.45-µm excitation, which could significantly improve the conversion efficiency of crystalline silicon solar cells. After detailed analysis using rate equations describing the energy transfer involved in the Ni2+ and Er3+ and upconversion, we concluded that the upconversion emission is dominated by the excitation of two Er3+ emitters due to the resonance energy transfer from two Ni2+ sensitizers, followed by further excitation to the initial state of the upconversion emission caused by another energy transfer between the two first-excited Er3+, i.e., cooperative upconversion.

Intense cooperative upconversion emission in Yb/Er: TeO2–Li2O–WO3 oxyfluoride glass ceramics

Novel oxyfluoride glass ceramics based on Yb3+-doped TeO2–Li2O–WO3–ErF3 (TWLE) were prepared using the two-stage heat-treatment method. The structural characterisation of these materials with X-ray diffraction indicated that crystalline phases of LiYbErF4 nanocrystals grew in the glass ceramics. Upon excitation with a 980nm laser diode the Yb3+ doped (2mol%) TWLE glass-ceramic produced intense blue emission at 487nm, weak green emission at 525nm and 550nm, and red emission at 650nm at room temperature. The mechanism of the blue emission, which is attributed to cooperative upconversion from pairs of excited Yb3+ ions to ground-state Er3+ ions, and the weak red and green emissions, which are attributed to energy transfer from Yb3+ to Er3+ ions, were investigated. The results of a pump-power dependence measurement suggested that the blue cooperative emission is a two-photon process. A rate equation was used to describe the cooperative energy-transfer process among the donor and acceptor ions. The observed results were compared to the behavior of another glass materials that exhibits blue cooperative emission, revealing the considerable advantage of TWLE:2Yb glass ceramics for optical devices.

(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.

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO 2 layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb 3+–Yb 3+ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state 2F 5/2 to the 2F 7/2 ground state in the Yb 3+ ions. The Er implanted SiO 2 exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the 2H 11/2 or 4S 3/2 energy levels and from the 2H 9/2 or 4F 5/2 energy levels to the 4I 15/2 ground state, respectively. We have found that the energy transfer from Yb 3+ to Er 3+ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er 3+–Yb 3+ system.

Intense White Luminescence from Combustion Synthesized Ca12Al14O33: Yb3+/Yb2+ Single Phase Phosphor

The Ca(12)Al(14)O(33):Yb(3+)/Yb(2+) single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb(3+) and Yb(2+) states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb(3+) state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb(2+) ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb(2+) state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb(2+) to Yb(3+) has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb(3+) and Yb(2+) states have been examined and discussed. Quantum yield of emission has also been determined for different samples.

Up and down conversion fluorescence studies on combustion synthesized Yb 3+/Yb 2+: MO-Al 2O 3 (M=Ca, Sr and Ba) phosphors

The ytterbium ions doped MO-Al 2O 3 (M=Ca, Sr and Ba) phosphors have been synthesized through combustion technique and their up and down conversion fluorescence properties have been studied and compared. The samples were calcinated at different temperatures and their FTIR and XRD spectra have shown a close relationship. With 976 nm excitation all these phosphors show cooperative upconversion emission at 488 nm from the pairs of two Yb 3+ ions along with an unexpected broad upconversion band in the blue green region and has been assigned to arise from the defect centers. Contrary to this upconversion emission, calcium aluminate phosphor exhibits bright and very broad down-conversion fluorescence (FWHM≈160 nm) upon UV (266 nm) excitation due to Yb 2+ ions. The inter-conversion between the 3+ and 2+ valence states of Yb ion has been observed on calcinations of samples in open atmosphere and has been correlated to the emission properties. The Yb 2+ ions containing calcium aluminate phosphor has been found suitable for producing broad band light in the visible region (white light). Lifetime of the emitting states of Yb 3+ and Yb 2+ ions have also been measured and discussed.

Blue electroluminescence of ytterbium clusters in SiO2 by co-operative up-conversion

Metal-oxynitride-oxide-silicon (MONOS) structures containing a SiO2 layer implanted with different concentrations of ytterbium atoms were investigated. The electroluminescence (EL) measurements show that the MONOS:Yb devices can operate as blue or near infrared light emitters depending on the Yb concentration and annealing time. For an Yb concentration of up to 1.5% and annealing times of 30 min the near infrared EL with a peak at 975 nm corresponding to the 2F5/2→2F7/2 transition dominates. The short time annealed (6 s) sample containing 3% of Yb atoms exhibits mainly the blue EL due to co-operative up-conversion emission in the Yb3+–Yb3+ system. An enhancement of the red EL at 650 nm due to up-conversion energy transfer from the 2F5/2 excited state to the nonbridging oxygen hole center (NBOHC) is also presented.

Blue-green upconversion emission in ZrO2:Yb3+ nanocrystals

Strong blue-green cooperative upconversion emission was observed under infrared excitation in 70 nm average crystallite size ZrO2:Yb3+ nanocrystals prepared by sol-gel process. The structural characterization was performed by x-ray diffraction and high resolution transmission electron microscopy suggesting that crystalline phase of nanoparticles is controlled by active ion concentration. The cooperative absorption coefficient η∼2.5×10−22 is four orders of magnitude larger than the ones reported from bulk crystals. The highest emission intensity was obtained from the doped sample at 4 mol % and was the result of the simultaneous relaxation of two excited Yb ions. Decay time of the upconverted signal τCUC≈0.310 ms is half from the near infrared effective decay time confirming the cooperative process among Yb ions. Such strong cooperative effect is explained in terms of interaction enhancement due to the diminishing of Yb–Yb separation promoted partly by the surface recombination of nanocrystals.

Upconversion in Ho3+-doped YbF3 particle prepared by coprecipitation method

YbF3 particles doped with Ho3+ were synthesized by coprecipitation method, from which the ultraviolet and visible emission bands of the Ho3+ and the 480 nm cooperative upconversion emission of Yb3+–Yb3+ are observed under 980 nm excitation. Under the same excitation power, the emission intensity of Ho3+ in coprecipitation method is enhanced by about two times comparing to that in solid-state reaction method. The novel ultraviolet and violet emissions of the Ho3+ are firstly obtained which are centered at 360 (5G2→5I8),391 (3K7→5I8),412 (5G4→5I8), and 446 nm (5G5→5I8). The luminescence decay profiles of 545 and 652 nm visible emissions were obtained with a 980 nm pulsed laser. The excitation power dependence of the emission intensity was also measured and intensity saturation was observed. Based on the level structures of Ho3+, two- and three-photon processes are suggested to perform populations of 5S2 and 5G3 (Ho3+) levels, respectively. The dominant upconversion mechanism may be attributed to a cooperative sensitization process of two excited states of Yb3+ and energy transfers from Yb3+ to Ho3+.

Highly efficient cooperative up-conversion of Yb3+ in NaYF4

Strong blue cooperative up-conversion emission around 475 nm has been observed in Yb3+-doped hexagonal NaYF4. The influence of concentration of the Yb3+ ion on the luminescence intensity is investigated. It is found that the sample shows the strongest cooperative luminescence when the Yb3+ ion concentration is 75%. The investigation shows that hexagonal NaYbF4 is an efficient blue up-converted phosphor, which might be potentially applicable in three-dimensional solid-state fluorescence display.

Enhanced cooperative absorption and upconversion in Yb3+doped YAG nanophosphors

Abstract Enhanced cooperative absorption and upconversion emission of Yb 3+ pairs in nanocrystalline YAG is observed. The nanocrystallites synthesized by a modified sol–gel method have an average size of 96 nm. The estimated concentration normalized ratio of maximum visible absorption to maximum IR absorption emission of 7.55 × 10 −22 cm 3 is about four orders of magnitude higher than the highest reported value to date, and more than seven orders higher than for bulk monocrystalline YAG:Yb 3+ . Strong cooperative upconversion emission is observed under IR pump at 940 nm.