Cross-relaxation Energy Transfer Process Research Articles

Enhancement of green emission from Ca14Al10Zn6O35: Tb3+ phosphors via cross-relaxation energy transfer by Li+ ions

Tb3+ doped Ca14Al10Zn6O35 phosphors were prepared by a traditional solid-state reaction approach. The Tb3+ doped phosphors exhibit blue and green emission under the 285 nm excitation. The green to blue emission ratio is gradually enhanced as the Tb3+ concentration increases, which is ascribed to the enhanced cross-relaxation energy transfer (CRET) processes between Tb3+ ions. Li+ ions were introduced to the composition as an attempt to address the charge mismatch induced by Tb3+ for Ca2+ ion substitution. An enhancement of green emission from Tb3+ ions is observed after co-doping with Li+ ions. The lifetime measurement confirms that the CRET efficiency and energy transfer rate are simultaneously increased by Li+ ions. The results suggest that the incorporation of Li+ ions is a feasible approach to overcome charge vacancy defects and enhance CRET processes.

Orange color emitting Sm3+ ions doped borosilicate glasses for optoelectronic device applications

An intense orange color emitting Sm3+ activated Li2O–PbO–ZnO–Al2O3–B2O3–SiO2 (LPZABS) glasses were propitiously fabricated by using sudden quenching method to study the luminescent potentiality using spectroscopic techniques such as XRD, FT-IR, optical absorption, photoluminescence (PL) and PL decay. XRD and FT-IR reveals the glassy nature and various functional groups present in LPZABS host glass respectively. Judd-Ofelt parameters derived from absorption spectra are used to estimate various radiative parameters for the excited states of Sm3+ ions in LPZABS glasses. Under 400 nm excitation, the luminescence spectra in the as prepared glasses exhibit three emission bands that corresponds to 4G5/2 → 6H5/2 (562 nm), 4G5/2 → 6H7/2 (598 nm) and 4G5/2 → 6H9/2 (645 nm) transitions of Sm3+ ions. Among these three, 4G5/2 → 6H9/2 transition observed in orange region (598 nm) is relatively more intense and prominent. The PL decay curves recorded for 4G5/2 fluorescent level reveal exponential behavior and single exponential fitting is applied to evaluate the experimental lifetimes (τexp). The τexp values are found to be decreasing with Sm3+ ion content due to cross-relaxation energy transfer process. The results reveal that the as prepared glasses can be effectively used in fabricating intense visible orange color emitting optoelectronic devices.

Multipath energy transfer and NIR luminescent properties of dual-lanthanide complexes

Near infrared (NIR)-emitting lanthanide ions, especially Sm3+, Nd3+, Yb3+, and Er3+, show special advantages in non-damage and deep-reaching biological imaging and telecommunication field. But most lanthanide ions have low luminescent efficiency in the NIR region, as a result of insufficient protection of the metal centers against non-radiative deactivation processes and/or inefficient inter- or intramolecular energy transfer. In recent years, organic ligands have been used as antennas to sensitize lanthanide ions to improve their luminescent efficiency. Because the chemical properties of lanthanide ions are very similar, mixed lanthanide complexes can be formed. At the same time, lanthanide ions have many energy levels, the energy transfer is easy to occur between different lanthanide ions. Thus, we can make use of these multi-path energy transfers to obtain efficient dual/multi-lanthanide multi-component NIR luminescent materials, which can be used for the ideal multi-channel near-infrared luminescence anti-counterfeiting bar code application. Herein, A series of NIR luminescent mono- or dual-lanthanide complexes were synthesized by TVPNTB ligand with lanthanide chloride salts. The monocrystalline structures of these rare earth complexes were obtained by X-ray single crystal diffraction. These lanthanide complexes are all hetero-isomorphic structures, crystallized in the monoclinic system and belong to P 21/ c space group. The phase purity was also proved by powder X-ray diffraction (PXRD). For the series of one component lanthanide complexes, the ligand TVPNTB can transfer energy to the excited state energy level of Sm3+, Nd3+, Er3+, Yb3+, Pr3+, Dy3+, and Ho3+, and produce NIR luminescence of the order of microsecond lifetime. Among them, the NIR luminescence studies of several lanthanide complexes such as Pr3+, Dy3+, Er3+, and Ho3+ are rarely reported. The results showed that the TVPNTB ligand has good near-infrared photosensitization effects on Sm3+, Nd3+, Yb3+, and Er3+. Moreover, by comparing the energy level difference of these ions, it is found that the energy level difference of Sm3+ from 4G2/5 to 6F5/2 is close to the energy level difference of Yb3+ from 2F5/2 to 2F7/2 in the ground state, and the energy level difference of Nd3+ from 4F3/2 to 4I11/2, which may lead to the cross relaxation energy transfer between rare earth ions and enhance the luminescence of Yb3+ and Nd3+. So we prepared a series of dual-lanthanide complexes, such as TVPNTB-Sm x Nd1 −x Cl3, TVPNTB-Sm x Yb1 −x Cl3 and TVPNTB-Sm x Er1 −x Cl3. Energy transfer from the antenna ligand can sensitize both NIR luminescence of the two lanthanide ions, which might serve as NIR polychromatic anti-counterfeiting luminescent barcode materials. We also investigated the energy transfer properties between the dual lanthanide ions, and confirmed that crossing relax energy transfer from Sm3+ to Nd3+, or from Sm3+ to Yb3+ were realized. The largest energy transfer efficiency ( η ) reaches 39% and 36%, leading to 4 or 2 times longer of the decay lifetimes of Nd3+ and Yb3+, respectively. Through the study of NIR luminescence performance of dual-lanthanide complexes with different proportions, it is clear that in addition to the antenna energy transfer from ligand to lanthanide ions, there is also a cross-relaxation energy transfer process from Sm3+ to Nd3+ and Sm3+ to Yb3+, thus achieving the effective energy transfer between dual-lanthanide complexes, thus further improving the NIR luminescence life of Nb3+ and Yb3+. The multi-path energy transfer strategy summarized in this study not only provides a basis for the preparation of novel high-performance NIR luminescent rare earth complexes, but also provides a reference for the design and synthesis of other multifunctional composite optical materials.

Structural and spectroscopic properties of phosphate–tungsten glasses doped with Nd3+ and Tb3+

Glasses of composition 90NaPO3-10WO3-xNd2O3, (0 ≤ x ≤ 1.7 mol%) and 90NaPO3-10WO3-yTbF3, (0 ≤ y ≤ 5.0 mol%) have been prepared and characterized. 31P and 23Na magic-angle spinning (MAS) NMR and 23Na{31P} rotational echo double resonance results are consistent with previously published data, indicating the presence of Q(2) and Q(1)1 W units. In addition, linear dependences of 31P MAS-NMR linewidths and spin-lattice relaxation rates and the electron spin-spin relaxation behavior (in the case of Nd2O3 doping) are consistent with a random dopant distribution, For Nd2O3- doped glass samples, absorption spectra in the visible and infrared regions were collected and analysed, and photoluminescence excitation and emission spectra were obtained under 575 nm excitation. Using the Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and by using them, the radiative lifetime (τr) were calculated for the 4F3/2 excited state. The fluorescence quantum efficiency value for this level (QE = τf/τr) of 13% was determined for glass with x = 1.0. The photophysical properties of Tb- doped glasses were also studied and upon UV excitation (372 nm) the glasses show emissions in the green (540 nm, 5D4 → 7F5 transition) and blue (434 nm, 5D3 → 7F4). The relative luminescence intensity ratio 5D3 → 7F5/5D4 → 7F5 is higher for lower Tb3+ content (0.5 Tb glass sample), because for higher Tb3+ contents, a cross-relaxation energy transfer process takes place favoring the green emission.

Cross relaxation in CaTiO3 and LaAlO3 perovskite nanocrystals doped with Ho3+ ions

LaAlO3: Ho3+(0.5–10%) and CaTiO3: Ho3+ (0.1–2%) nanosized perovskites were prepared with the Pechini and sol–gel method, respectively. The spectroscopic properties of CaTiO3: Ho3+ were investigated for the first time. The cross relaxation energy transfer processes were determined as a function of Ho3+ concentration, by examination of the two strongest transitions observed in the visible region arising from 5S2 and 5F5 multiplets. The cross relaxation processes for the investigated Ho3+ levels were described. It was found that the clustering processes for CaTiO3 is so strong, that increase of dopant concentration weakly influence the cross relaxation processes, while in LaAlO3, where dopants are located randomly in the crystal lattice, clear monotonic dependence is observed.

Multicolor visible light upconversion emission in Tm^3+–Er^3+ codoped TeO_2–PbO glass under near-infrared laser radiation

The multicolor intense visible emission from Er3+ and Tm3+ ions codoped in TeO2–PbO glass upon excitation at ∼1064 nm radiation from a nanosecond pulsed Nd:YAG laser has been investigated. The optical parameters of erbium ions codoped in Tm3+–TeO2–PbO glass by using the Judd–Ofelt theory have been determined from the optical absorption spectra. No upconversion (UC) emission was seen in the singly doped Er3+-doped glass upon excitation at 1064 nm. Through the experiments, it has been found that, in the codoped glass, the UC luminescence from Er3+ (2.0 mol. %) in green and red regions corresponding to the H211/2→I415/2, S43/2→I415/2, and F49/2→I415/2 transitions is strengthened about ∼40, ∼60, and ∼130 fold, respectively, at 2.0 mol. % Tm3+ ion concentration in codoped glass. The observed UC emissions and the enhancement observed in their intensity have been explained on the basis of efficient energy transfer and cross-relaxation energy transfer processes.

In/Er-codoped LiNbO3 crystals with enhanced 1.5 μm emission and suppressed upconversion emission

1.5 μ m emission and green upconversion emission properties in In/Er-codoped LiNbO3 crystals were investigated by using both steady-state and time-resolved emission spectra. The 1.5 mol % In2O3 codoping results in the lengthening of 1.5 μm emission lifetime by 13.9% and simultaneously the shortening of 550 nm green upconversion emission lifetime by 52.5%. The decay behavior of the green upconversion emission of the In/Er-codoped crystal shows obviously nonexponential. These spectroscopic characteristics are related with the lower concentration of the cluster sites of Er3+ ion and possible cross-relaxation energy transfer processes. The above emission results are further supported by the theoretical emission results based on Judd–Ofelt theory.

Thulium Doped Phosphors Under VUV Excitation

AbstractDevelopment of highly efficient mercury free fluorescent lamps and plasma display panels has been a challenging task due to the need for a combination of phosphor properties that are difficult to obtain in a single material (high efficiency, short emission lifetime and weak sensitivity to aging process under VUV excitation). Quantum cutting mechanism is a way to improve the fluorescence efficacy. Here we describe quantum cutting involving pairs of Tm3+ ions in KY3F10. Efficient excitation in the vacuum UV is initiated to the 5d state of Tm3+. This is followed by a cross relaxation energy transfer (CRET) involving the excited ion in the 5d state and nearby Tm3+ in the ground state, producing a pair of Tm3+ in excited states of the 4f13 configuration. Both ions can then emit photons. The excitation and reflection spectra are studied as a function of Tm3+ concentration and temperature. An unusual enhancement of the reflectivity at excitation wavelengths corresponding to the Tm3+ 5d absorption peaks is shown to arise from strong 5d→4f emission which is confirmed from the VUV emission spectra. The strong reduction of the integrated 5d emission intensity and shortening of its lifetime with Tm3+ concentration indicates the effective presence of the desired CRET process that is required for the first step of the quantum cutting. High Tm3+ concentrations are required for efficient quantum cutting. Whereas the CRET from the 5d state is estimated to be quite efficient, the 4f13 states of Tm3+ also undergo a strong CRET and therefore, emission from the 4f13 excited states that are created from the first step are strongly quenched at high Tm3+ concentrations. As a result, quantum yields greater than unity are not achieved.

Effect of Nd3+ concentration quenching in highly doped lead lanthanum zirconate titanate transparent ferroelectric ceramics

The concentration dependence of the fluorescence quantum efficiency in Nd3+ doped lead lanthanum zirconate titanate (PLZT), transparent ceramics, is presented. The total emission decay of the emitting level F3∕24 is close to exponential, even for high Nd3+ concentration Nt, due to the very low probability of the cross relaxation energy transfer processes among ions. Owing to this low probability, it was inferred that Nd:PLZT presents lower concentration quenching than other laser materials as Nd:YAG. The figure of merit ηNt, where η is the fluorescence quantum efficiency, presents a maximum around 6.0wt% Nd2O3, indicating the good prospects of concentrated samples for miniaturization of the laser medium (microchip laser).

Photon avalanche effect inLaCl3:Pr3+

The phenomenon of photon avalanche, which involves both an excited state absorption process from a metastable level and a cross relaxation energy transfer process responsible for the large increases in population of this metastable level, has been investigated in ${\mathrm{LaCl}}_{3}:{\mathrm{Pr}}^{3+}.$ In this paper, we report on the intensity dependence of both the steady-state fluorescence and the temporal evolution of the avalanche signal as a function of temperature (80 K to room temperature). Comparisons are made between experimental results and mathematical models based on a six-level system described by a series of coupled rate equations combined with the propagation of a Gaussian beam. We develop a model with only one adjustable parameter, the excited state absorption cross section, which provides a good description of all of our experimental data. We have estimated values of the excited state absorption cross section for the various ${}^{3}{\stackrel{\ensuremath{\rightarrow}}{{\mathrm{H}}_{5}}}^{3}{\mathrm{P}}_{0}$ pumping transitions at different temperatures.