Strong Cross-relaxation Research Articles

Influence of Eu3+ ions on 1.5 µm luminescence of Er3+ in TeO2–ZnO–Bi2O3 glasses

TeO2–ZnO–Bi2O3 glasses doped with Eu3+ and Er3+ ions were prepared and investigated for their physical, optical, and luminescent properties. The optical band gap is calculated for the indirect transition and is within the 3.04–3.10 eV range. The Urbach energy varies from 0.135 to 0.098 eV with the increase in Eu3+ compound content. Bi2Te2O8 and ZnTeO3 complex oxide phases are generated in the mixture as intermediate compounds in the course of charge thermal processing. The additional doping of TeO2–ZnO–Bi2O3:Er3+ glass samples with Eu3+ ions is shown to lead to the appearance of cross-relaxation channels of the Er3+ ion states and decreases the population of the 4I13/2 state of the Er3+ ions. Simultaneously, the excited 4I11/2 state of the Er3+ ions is also subjected to strong cross-relaxation decay. The Eu3+ ions are shown to decrease the 4I13/2 level population of Er3+ ions, which may facilitate the attainment of population inversion if the 4I13/2 level is at the lower level of the laser transition.

Bright Tm3+-based downshifting luminescence nanoprobe operating around 1800 nm for NIR-IIb and c bioimaging

Fluorescence bioimaging based on rare-earth-doped nanocrystals (RENCs) in the shortwave infrared (SWIR, 1000–3000 nm) region has aroused intense interest due to deeper penetration depth and clarity. However, their downshifting emission rarely shows sufficient brightness beyond 1600 nm, especially in NIR-IIc. Here, we present a class of thulium (Tm) self-sensitized RENC fluorescence probes that exhibit bright downshifting luminescence at 1600–2100 nm (NIR-IIb/c) for in vivo bioimaging. An inert shell coating minimizes surface quenching and combines strong cross-relaxation, allowing LiTmF4@LiYF4 NPs to emit these intense downshifting emissions by absorbing NIR photons at 800 nm (large Stokes shift ~1000 nm with a absolute quantum yield of ~14.16%) or 1208 nm (NIR-IIin and NIR-IIout). Furthermore, doping with Er3+ for energy trapping achieves four-wavelength NIR irradiation and bright NIR-IIb/c emission. Our results show that Tm-based NPs, as NIR-IIb/c nanoprobes with high signal-to-background ratio and clarity, open new opportunities for future applications and translation into diverse fields.

Reproducible Single-Droplet multiplexed detection through Excitation-Encoded Tri-mode upconversion solid sensors

The article reports a reproducible and resuable sensor for upconversion fluorescence multiplexed detection constructed by excitation-encoded core-multishell upconversion nanoparticles with a thin superhydrophobic SiO 2 layer, which outperforms the sensors with co-doped upconversion nanoparticles and multi-channel upconversion nanoparticle mixtures. Such solid-multi-mode sensor can achieve visual colorimetric and reproducible multiplexed simultaneous detection of multi-analytes. Upconversion multiplexed optical sensors are promising in clinical diagnostics, molecular mapping and environmental monitoring. However, traditional co-doped emitter sensors and mixture sensors show poor sensitivity and reproducibility due to the strong cross-relaxation quenching and instability. Herein, upconversion multi-shell nanoparticles are, for the first time, applied as excitation-controlled nanosensors for reproducible and channel-independent multiplexed detection. The multi-shell structure favors the excitation-encoded reproducible blue/green/red fluorescence signals from different upconversion shells in a single upconversion nanosensor, avoiding using instable upconversion nanoparticle mixtures for multiplexed detection. A reproducible and self-cleaning solid tri-mode sensor is constructed by the upconversion multi-shell nanoparticles coated with a thin superhydrophobic SiO 2 layer. The solid sensor shows superior single-droplet (≈10 µL) multiplexed detection capability with a low detection limit for dye molecules (∼0.1 μg/mL), high stability and excellent recyclability (more than 100 cycles). This work provides an intriguing strategy to achieve channel-independent, reproducible UC multiplexed sensors for simultaneous detection of multi-analytes.

Tm3+ heavily doped NIR-III bioprobe with 1 µm Stokes shift towards deep-tissue applications

Recently developed rare earth ions doped NIR-NIR bioprobes, with excitation and emission both falling in the NIR region, have attracted wide attention due to their deep-tissue penetration, high signal-to-noise ratio, and high imaging resolution. The current NIR-NIR bioprobes focused mainly on the 1.5 µm emission of Er3+. Here, we developed a novel bioprobe utilizing the 1.8 µm emission of Tm3+ upon 0.8 µm excitation. After an inert shell effectively suppressing the surface quenching effect, the strong cross relaxation 3H4 + 3H6 → 3F4 + 3F4 between heavily doped Tm3+ greatly improves the luminescence intensity at 1.8 µm. As a result, the formed Tm3+ based NIR-III bioprobe exhibits better penetration ability of the state-of-the-art Er3+ based NIR-III bioprobe, and holding an even larger Stokes shift beneficial for the multiplexed bioimaging and labeling applications.

Multiple channels to enhance near-infrared emission from SiO2-SnO2:Er3+ films by Ba2+ ion doping.

Ba2+ ions co-doped SiO2-SnO2:Er3+ thin films are prepared using a sol-gel method combined with a spin-coating technique and post-annealing treatment. The influence of Ba2+ ion doping on the photoluminescence properties of thin films is carefully investigated. The enhancement of near-infrared (NIR) emission of Er3+ ions by as much as 12 times is obtained via co-doping with Ba2+ ions. To illustrate the relevant mechanisms, X-ray diffraction, X-ray photoelectron spectroscopy and comprehensive spectroscopic measurements are carried out. The enhanced NIR emission induced by Ba2+ co-doping can be explained by more oxygen vacancies, improved crystallinity and strong cross-relaxation processes between Er3+ ions. The incorporation of Ba2+ ions into SiO2-SnO2:Er3+ thin films results in a considerable enhancement in the NIR emission, making the thin films more suitable for Si-based optical lasers and amplifiers.

Na+-Driven Nucleation of NaYF4:Yb,Er Nanocrystals and Effect of Temperature on Their Structural Transformations and Luminescent Properties

Ultrasmall pure hexagonal phase NaYF4:Yb,Er is successfully prepared via the solvothermal method. The upconversion (UC) luminescence of hexagonal phase NaYF4:Yb,Er nanocrystals is ten times stronger than that of cubic phase nanocrystals with the same size of 6 nm. XRD results reveal that heating above 673 K leads to conversion of the hexagonal phase to the high-temperature cubic phase, whereas the cubic nanocrystals undergo phase transformation from normal cubic at room temperature to hexagonal at 673 K and further to high-temperature cubic above 773 K. The hexagonal nanocrystals exhibit emission enhancement after heat treatment up to 573 K. Further heating above 773 K induces a decreasing trend in emission due to the phase transition to the high-temperature cubic phase. The cubic phase exhibits decreasing luminescence with temperature due to strong cross relaxation and then increasing luminescence above the temperature of 673 K due to the hexagonal phase transformation. The luminescent properties of both...

Energy Transfer Highway in Nd3+-Sensitized Nanoparticles for Efficient near-Infrared Bioimaging.

Despite the large absorption cross-section of Nd3+ dopant as a sensitizer in lanthanide doped luminescence system, the strong cross-relaxation effect of it impedes the promotion of doping concentration and thus reduces the utilization of excitation light. In this work, we introduce a highly efficient acceptor, Yb3+ ion, which can quickly receive energy from Nd3+ ions, to construct an energy transfer highway for the enhancement of near-infrared emission. By using the energy transfer highway, the doping amount of Nd3+ ions in our NaYF4:Yb,Nd@CaF2 core/shell nanoparticles (CSNPs) can be markedly elevated to 60%. The quantum yield of CSNPs was determined to be 20.7%, which provides strong near-infrared luminescence for further bioimaging application. Remarkably, deep tissue penetration depth (∼10 mm) in in vitro imaging and high spatial resolution of blood vessel (∼0.19 mm) in in vivo imaging were detected clearly with weak autofluorescence, demonstrating that probes can be used as excellent NIR biosensors.

A Davies/Hahn multi-sequence for studies of spin relaxation in pulsed ENDOR

We extend earlier studies of the effects of relaxation on the intensities of pulsed ENDOR signals by introducing a Davies/Hahn (D/H) pulsed ENDOR multi-sequence that corresponds to a series of Davies sequences with the preparation pulse ‘turned off’. In this pulse train, the Hahn [π/2, π] detection pulse pair of sequence n−1 both generates the echo detected for that sequence and acts as the preparation portion of sequence n, in effect replacing the π preparation pulse of the Davies sequence. We show both theoretically, through a master-equation approach, and with both 1H ( I = 1/2) and 14N ( I = 1) ENDOR experiments on the non-heme Fe enzymes, superoxide reductase (SOR) ( S = 1/2) and AntDO ( S = 3/2), that under conditions of high electron–spin polarization (high microwave frequency/low temperature) the D/H multi-sequence allows simplification of ENDOR spectra by suppression of nuclear transitions associated with the m S = +1/2 ( α) manifold. As such suppression depends on the sign of A, it allows determination of this sign. The suppression as a function of the time between individual sequences is found to exhibit behaviors that can be classified into three regimes of the ratio of cross-relaxation to spin–lattice relaxation rates: strong cross-relaxation (X-case); comparable rates (XL); negligible cross relaxation (L). Interestingly, the ENDOR behavior of the S = 1/2 SOR center indicates it is an L case, while the S = 3/2 AntDO is an L case. Overall, the D/H protocol appears to be a robust and general tool for using relaxation effects to manipulate ENDOR spectra.

Silica gel glasses with a high efficiency of luminescence sensitization in the Ce3+-Tb3+ system

Terbium-and (Ce, Tb)-containing glasses prepared using the direct sol-gel-glass transition are studied. It is shown that glasses doped with one activator contain two main types of optical centers, namely, isolated and complex centers, which are characterized by weak and strong cross-relaxation quenching of luminescence from the 5D3 state of Tb3+ ions, respectively. The Ce4+-Tb3+ (Tb4+) complex centers are formed during sintering of coactivated xerogels in oxygen and can be transformed into Ce3+-Tb3+ centers through saturation of the samples with hydrogen. The Ce3+-Tb3+ centers exhibit efficient luminescence from the 5D4 state upon excitation into the absorption bands of Ce3+ ions.

Blue up-conversion emission in Yb- and Tm-codoped potassium yttrium tungstate

The spectroscopic characteristics of potassium yttrium tungstate crystal (KYW), codoped with ytterbium and thulium ions, have been investigated from the point of view of up-conversion blue lasing from G41 multiplet. An overall H63 multiplet splitting of ∼530 cm−1 for the trivalent thulium ion in the KYW host was obtained. A decay time of ∼112 μs for the G41→H63 blue luminescence in the material doped by 1 at. % of thulium was measured. The value of peak emission cross section for this transition is estimated to be (1.8±0.3)×10−20 cm2. Strong cross-relaxation quenching was found to be one of the main channels of nonradiative losses from the G41 and H43 multiplets. The cross-relaxation rate was significantly higher at higher concentration of thulium.

Near infrared to visible photon upconversion in Re4+ doped Cs2ZrBr6

Two visible luminescence bands are observed in Re4+:Cs2ZrBr6 under NIR excitation at temperatures between 10 and 300 K. We perform absorption, luminescence, excitation, and time resolved spectroscopic studies at various temperatures to characterize this upconversion luminescence and its driving mechanism. From the time dependent measurements we assign a ground state absorption/energy transfer upconversion type mechanism to the upconversion process. The visible to near-infrared photon ratio is 0.008 at T=50 K with a laser power density of 415 W/cm2. Most of the visible luminescence is quenched by a strong cross-relaxation process out of the highest emitting level as well as intense radiative interexcited state transitions. The ReBr62− unit is one of very few transition metal complexes with three emitting electronic levels in the near-infrared and visible range of the electromagnetic spectrum.

Effect of Tb3+ co-doping on the electron population densities of Tm3+ in Ge–As–Ga–S glasses

The 1.48-μm emission properties and electron population densities of the excited levels were investigated for Ge–As–Ga–S glasses doped with Tm3+ or Tm3+/Tb3+. Electron population inversion between the H43 and F43 levels in Tm3+ cannot be achieved without Tb3+ addition due to the strong cross-relaxation in Tm3+. Co-doping of Tb3+ resulted in the large decrease in the population density of the Tm3+:F43 level via the energy transfer of Tm3+:3F4→Tb3+:7F0,1,2. Calculated population densities of the two levels in Tm3+ using the rate equations showed that the population inversion became possible through the addition of Tb3+.

The strong cross relaxation of Er3+ and yb3+ ions in the oxyfluoride glass ceramics

Oxyfluoride glass ceramic is a new up-conversion fluorescent material with the low phonon energy just like fluoride and the good mechanical stability like oxide. In the oxyfluoride glass ceramics co-doped with Er3+ and Yb3+ ions, the red radiation is dominant while it is excited by the violet light. But in the general fluorescent material, the green radiation is dominant because the multi-phonon relaxation occurs step by step. The strong cross relaxation process of Er3+ and Yb3+ ions is mainly responsible for enhancement of the red radiation intensity in the oxyfluoride glass ceramics. The increase of the red radiation intensity of Er3+ ions by the violet light excitation shows that the rare earth ions are doped into fluoride micro-crystals which are planted into oxide host in the oxyfluoride glass ceramics.

Energy transfer and laser spectroscopy of Eu 3+ co-doped CaF 2:Sm 3+

We report on two heterogeneous Sm 3+ dimer centers in CaF 2: 0.05% Sm 3+ crystals co-doped with Eu 3+ which are remarkable in showing very efficient and complete transfer of the optical excitation from Sm 3+ to Eu 3+ through multi-phonon assisted processes. In contrast, the homogeneous Sm 3+ dimer centers are non-radiative because of strong cross-relaxation between the Sm 3+ ions.

High-Pressure NMR Relaxation Study of the Solute and Solvent Dynamics of Undercooled Aqueous Tetraethylammonium Bromide Solutions

Multinuclear spin−lattice relaxation rate measurements are reported over wide ranges of temperature, pressure, and concentration in undercooled aqueous solutions of tetraethylammonium bromide (Et4NBr). These dissolved organic cations with apolar surface groups provide model systems to investigate their influence upon the dynamics of the water molecules in the hydration shell as well as the influence of the hydration shell upon the dynamics of the dissolved hydrophobic ions. The isobaric relaxation time curves of all nuclei investigated show pronounced minima, indicating a strong slowing down of the dynamics of the solvent molecules in the hydration shell as well as of the dissolved hydrophobic ions. A VTF-like temperature dependence and corresponding scaling behavior allow glass transition temperatures to be deduced which signify the onset of motional arrest in these metastable solutions. Segmental motions of the aliphatic chains of the organic cations are discussed in terms of correlation times, and corresponding order parameters are given characterizing their spatial restrictions. At low temperatures all proton relaxation rates become equal to those of the methyl groups due to strong cross-relaxation effects, indicating that methyl groups act as relaxation sinks in these systems.

Population mechanisms of the green Er3+:LiYF4 laser

In computer simulations the mechanisms that lead to room-temperature continuous-wave green upconversion lasing in Er3+:LiYF4 are investigated. The rate-equation system considers the full erbium level scheme up to 2H9/2, ground-state depletion, excited-state absorption on the pump and laser wavelengths, three interionic processes, stimulated emission, and the crystal and resonator data of the experiments. Experimental results performed at the University of Hamburg, Germany, are reproduced in the simulation. The influence of different parameters as pump wavelength, absorption cross sections, interionic parameters, dopant concentration, and temperature is investigated. An avalanche effect which exploits the strong cross relaxation from the upper laser level and the upconversion from 4I13/2 leads to an efficient population of the upper laser level. At higher dopant concentrations the cross relaxation becomes detrimental to stimulated emission due to the depletion of the upper laser level. This concentration dependence can be considered as a general behavior of rare-earth-doped avalanche lasers.

Nuclear magnetic resonance relaxation of glycogen H1 in solution.

The NMR relaxation properties of the H1 proton of oyster glycogen in D2O and H2O solutions have been studied using nonselective, semiselective, and selective inversion recovery and Hahn spin-echo pulse sequences. The data were analyzed in terms of an isotropic, rigid-rotor dipole-dipole model including cross-relaxation. At 8.4 T in D2O, p = 5.4 +/- 0.4 s-1 and sigma = -4.5 +/- 0.4 s-1. The large, negative sigma value is consistent with strong cross-relaxation and a long correlation time. The relaxation data can be explained by a single correlation time, tau c = 2.7 x 10(-9) s, indicating significant internal mobility. With this value of tau c, and assuming that the structure of the glucose moieties was the same as in alpha-D-glucose crystals, the dipole sum contributing to T1 relaxation was calculated. The intra-ring relaxation was dominated by dipole fields from the H2 proton, but these only accounted for approximately 18% of the total relaxation. Most of the relaxation comes from inter-glucose relaxation. From modeling, this is dominated by the H4' across the alpha-1,4-glycosidic bond. The H1 longitudinal relaxation rates were significantly enhanced in H2O compared with D2O. This enhancement is not due to direct dipolar interaction between H1 and bulk water. Transverse relaxation rates were not significantly enhanced in H2O.

Structural and optical properties of calcium neodymium hexaaluminates single crystals, potential laser materials

The structural and optical properties of calcium-neodymium hexaaluminates crystals Ca 1− x Nd x Mg x Al 12− x O 19 (labeled Ca 1− x Nd x ) with a magnetoplumbite (MP) structure are investigated. The floating zone method is used to grow single crystals in the composition range 0.1 ≤ x ≤ 0.7, although for high calcium content, the melting of the compounds is no longer congruent. The X-ray structural determination, optical absorption at 4 K, and ESR investigation agree in the localization of Nd 3+ at the regular large cations site of the MP structure with axial ( D 3 h ) symmetry. A set of crystal field and free ion parameters which fits the absorption spectrum of Nd 3+ in this site is calculated. When x increases, Nd 3+ ions tend to occupy also a second site with lower symmetry. Moreover some anomalous behavior observed in the absorption and ESR spectra at high neodymium concentration may be related to Nd 3+-Nd 3+ ion pairing. Fluorescence intensity and lifetime measurements as a function of the x value are reported. There is evidence of strong cross-relaxation between neighboring neodymium ions for high x values. The results obtained for the Ca 1− x Nd x compounds can be extended to other series in which Nd 3+ is replaced by another lanthanide ion. Preliminary investigations have been performed with Pr 3+ and are also reported.

NMR dynamics in disordered magnets

The excitation dynamics of site diluted magnets can be described at low energies (long length scales) by magnons, and above a crossover frequency, ωc, (short length scales) by fractons. The density of fracton states is given by $$N(\omega )\alpha \omega ^{\bar d - 1} $$ , where $$\bar \bar d$$ is the fracton dimensionality. Dilution gives rise to a characteristic length ξ∝(p−p c)ν, wherep c is the critical concentration for (magnetic) percolation. The crossover frequency ωc is proportional to ξ-1[1+(θ/2)], where θ is the rate at which the diffusion constant decays with distance for diffusion on an equivalent network. A fractal dimensionD describes the density of magnetic sites on the infinite network, and $$\bar \bar d = 2D/(2 + \theta )$$ . For percolating networks, $$\bar \bar d \simeq {4 \mathord{\left/ {\vphantom {4 3}} \right. \kern-\nulldelimiterspace} 3}$$ for all dimensions ≥2. Neutron scattering structure factor measurements by Uemura and Birgeneau compare well with calculations using fracton concepts. Magnons are extended at low energies, while the fracton states are geometrically localized, with a wave function envelope proportional to exp $$\{ - [{r \mathord{\left/ {\vphantom {r {\Lambda (\omega )}}} \right. \kern-\nulldelimiterspace} {\Lambda (\omega )}}]^{d_\phi } \} $$ . Here, $$\Lambda (\omega ) \propto \omega ^{{{ - \bar d} \mathord{\left/ {\vphantom {{ - \bar d} D}} \right. \kern-\nulldelimiterspace} D}} $$ is the fracton length scale at frequency ω. The exponentd ϕ lies between 1 andd min, the chemical length index (of the order of 1.6 in three dimensions). The localization of the magnetic excitations causes a spread in the NMR relaxation rates. A given nuclear moment will experience only a limited set of fracton excitations, resulting in an overall non-exponential decay of the NMR relaxation signal. When strong cross-relaxation is present, the relaxation will be exponential, but the temperature dependence will be strongly altered from the concentrated result.

Nuclear Magnetic and Quadrupole Resonance Studies of Phase Transitions in Crystalline (NH4)2SbF5 and (ND4)2SbF5

(NH4)2SbF5 undergoes two successive phase transitions at 169 and 292 K, both of which are of typical lambda-type and without thermal hysteresis. Proton and fluorine spin-lattice relaxation time measurements showed that the isotropic reorientation of the NH4 + ions is highly excited below the lower transition point with an activation energy of 15.5 kJ mol-1 and that the uniaxial reorientation of the square pyramidal SbF5 2- anion is excited above about 200 K with an activation energy of 42.0 kJ mol-1. There is strong cross relaxation between the proton and the fluorine over the whole temperature range of the measurements. The 121Sb and 123Sb nuclear quadrupole resonance frequencies show an anomalous temperature dependence; each resonance frequency assumes a minimum at around 100 K and increases on heating, reaches a maximum at the lower transition point, decreases drastically in the middle temperature phase and then fades out just below the upper transition point. The anomalous increase in the resonance frequencies stems from the reorganization of the N - H ... F type hydrogen bonds with the progress of the lower phase transition. The lower and the upper transition temperatures shift to 173.0 and 290.1 K, respectively, on deuteration of the ammonium ion.