Cross-relaxation Phenomena Research Articles

Achieving high-sensitivity dual-mode optical thermometry via phonon-assisted cross-relaxation in a double-perovskite structured up-conversion phosphor

By adjusting the phonon energy of the host crystal (Ca2+/Sr2+/Ba2+) LaLiTeO6:Yb3+, Tm3+ phosphor, combined with the cross-relaxation phenomenon, a high-sensitivity thermometer for dual-mode temperature measurement was constructed.

Constructing Efficient and Thermostable Red‐NIR Emitter via Cross Relaxation and Crystal‐Field Engineering of Holmium‐Based Perovskite‐Type Half Metal

AbstractLanthanide ions, as dopants, have evoked widespread research interest owing to the rich optical, magnetic, and electrical properties, but their luminescent intensity is always limited by typical concentration quenching. Herein, a holmium‐based double perovskite of Cs2NaHoCl6 is reported, which surprisingly breaks through this barrier and achieves efficient red‐NIR emission by virtue of the large unit cell, low phonon energy, high content of activators, and cross relaxation phenomenon between Ho3+. The heavy Ho3+ also endows the intriguing half‐metallic nature with a down‐spin conducting band and an up‐spin insulating band. After performing ion doping on crystallographic sites of Na+ and Ho3+, the photoluminescence quantum yield of such red‐NIR emitter under 450 nm excitation is dramatically promoted to 82.3%, benefiting from the improved crystal field environment that alleviates the parity forbidden rule and suppresses non‐radiative recombination loss. Furthermore, the heat‐favorable phonon‐assisted population processes enable the robust photostability against thermal quenching. By combining a 450 nm chip, the red‐NIR light‐emitting diodes are fabricated, in which the wide‐coverage NIR emissions are ideally suited for medical light source, night vision, nondestructive examination, and transmission imaging. It is believed that this work will open an avenue for enhancing the fluorescence of lanthanide ions and developing advanced spintronic materials.

High-Efficiency Red-Emitting Phosphor of Li0.5La0.5SrMgWO6: Mn4+, Pr3+

The 400–500 nm blue light and 630–750 nm red light play crucial roles in plant cultivation. In this work, a high efficiency red-emitting phosphor, Li0.5La0.5SrMgWO6:Mn4+, Pr3+, was synthesized through the solid-state reaction technique. The absorption and excitation spectra were measured, indicating that 467 nm was an suitable excitation wavelength for this material. Upon excitation with a 467 nm LED, the fluorescence characteristics were evaluated, revealing dual-band red emission peaks located at 621–654 nm and 695–710 nm. The quantum efficiency and pumping slope efficiency were measured to be 63% and 35%, respectively. Based on energy level theory and Tanabe-Sugano theory, the dual-band fluorescence mechanism was analyzed, revealing that cross relaxation phenomenon contributed to the high quantum efficiency. The temperature sensing property was also demonstrated. The relative sensitivity (Sr) exhibited a significant change from 0.05% K−1 to 0.85% K−1 with different temperatures. The phosphor’s high efficiency and exceptional temperature property confer it with extensive potential applications in the fields of plant cultivation and display technology.

Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120K when subjected to a sub-bandgap (405nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

Experimental and Theoretical Study of an Actively Q-Switched Tm:YLF Laser with an Acousto-Optic Modulator.

We report the characteristics of a diode-end-pumped, high-repetition-rate, acoustic-optic (AO) Q-switched Tm:YLF laser operating from 5 kHz to 10 kHz. In the continuous-wave (CW) regime, a maximum average output power of 8.5 W was obtained with a slope efficiency of 30.7%. Under the AO Q-switching regime, a maximum output power of 7.32 W was obtained at a repetition frequency of 5 kHz with a pulse width of 68 ns and a pulse energy of 1.4 mJ, corresponding to a peak power of 21.5 kW. A time-dependent rate equation model is introduced to theoretically analyze the results obtained in the experiment, in which the cross-relaxation phenomenon, upconversion losses and ground-state depletion are taken into account. Additionally, the evolution processes of population inversion density and intracavity photon number density with time are also presented. The theoretical results well predict the dependence of laser output characteristics of Tm:YLF crystal on the incident pump powers.

Spectroscopic investigation of upconversion and downshifting properties LaF3:Tb3+,Yb3+: A dual mode green emitter nanophosphor

The optimization of dopant concentration in upconverting nanophosphor is a real problem because of distinct limits defined by surface quenching, energy transfer and cross relaxation phenomenon. In this work, LaF3 based nanophosphors with different doping concentration of Yb3+ and Tb3+ ions were prepared by coprecipitation route. Particles are formed in spherical shape with narrow size distribution i.e. in ∼50–80 nm range. The volume of LaF3 unit cell shrinks with the doping of Tb3+ and Yb3+, whereas no variation in its phase were observed. The nanophosphors exhibit characteristics of dual mode spectral emitter. Thus, irrespective of excitation source (i.e. either UV or NIR), the intense green emission at 541 nm was observed. The material LaF3 with optimized molar doping concentration of 5%Tb3+ and 10%Yb3+ shows intense green emission on excitation with either UV or IR light. The time resolve luminescence emission were studied to explore the probability of involved energy transfer process between two dopant ions.

Luminescence properties of PrF3-doped Sb2O3–ZnO–GeO2 glass phosphors for near-infrared wideband light-source

We synthesized PrF3-doped glass phosphors using a melt-quenching method and investigated the luminescence characteristic aiming at a new near-infrared wideband light-source. Optical activation of Pr3+ ions in the samples was confirmed by absorption spectra measurements. Broad PL spectra from 700 to 1100 nm with four PL peaks 730, 870, 930, and 1040 nm were observed by blue LED excitation in all samples. The observed PL peaks were attributed to 3P0 → 3F4, 1D2 → 3F2, 3P0 → 1G4, and 1D2 → 3F4 transitions of Pr3+ ions. With the increase in the PrF3 concentration, the integrated PL intensity around 1040 nm (1D2 → 3F4) was sharply decreased compared to the other PL peaks. We investigated the contribution of self-absorption and cross-relaxation phenomena to the spectral change by measuring glass thickness dependence of the PL spectra and measuring the PrF3 concentration dependence of the fluorescence lifetimes. The fluorescence lifetime of the PL peak at 1040 nm decreased with increasing PrF3 concentration owing to a non-radiative relaxation pathway stemming from the cross-relaxation phenomena. We found that the cross-relaxation phenomenon is a dominant factor in the luminescence properties of PrF3-doped glass phosphors.

Synthesis and characterization of AgYP2O7 pyrophosphate activated with Tb3+, Sm3+ and Dy3+ ions

Silver yttrium phosphates AgYP2O7 doped with Tb3+, Sm3+ and Dy3+ (x = 0, 1, 3 5, 10, 15, 20 mol%) were successfully prepared via solid state reaction. The similarity between all diffractograms of prepared phosphors confirm that they are a single pure phase. The Fourier Transform InfraRed (FTIR) analysis are in good agreement with structure type, the characteristic bands of P2O74−groups attributed to the symmetric and asymmetric stretching of P-O-P bridge, respectively are identified. The investigation on luminescence of Tb3+, Sm3+ and Dy3+ ions in this phase is reported. The cross-relaxation phenomenon widely known in phosphors based on Tb3+ is pointed out. A prediction of the nature of the occupied site by the rare earth ion was possible using the predominant intensity in both Sm3+ and Dy3+ ions.

An Optical Quantum Thermometer with Submicron Resolution Based on the Cross-Relaxation Phenomenon of Spin Levels

An optical quantum thermometer with a submicron spatial resolution that is based on the physical phenomenon of optical response in the system of spin centers in silicon carbide under conditions of cross-relaxation between the optically active centers in quadruplet spin state and triplet centers, where there is anomalously strong dependence of the splitting of the fine structure on temperature, has been proposed.

Photoluminescence and energy transfer of YAG: Ce3+, Gd3+, Bi3+

In this study, Gd[Formula: see text] and Bi[Formula: see text] ions act to redshift the emission band to orange region, and to enhance significantly the maximum emission of YAG: Ce[Formula: see text]. On account that size mismatch between the host and the doped Gd[Formula: see text] ion, the crystal structure turns soft, and the emission spectra are not tuned from 540 to 570[Formula: see text]nm but decreased the emission intensity. Accordingly, an effective way to increase emission intensity is to introduce Bi[Formula: see text] ion into the YAG: Ce[Formula: see text], Gd[Formula: see text] phosphors. Experimental results show partial overlapping between the emission band of Bi[Formula: see text] ion and the excitation band of Ce[Formula: see text] ion, indicating that the energy transfer from Bi[Formula: see text] to Ce[Formula: see text] ions exists in the (Y[Formula: see text]Ce[Formula: see text]Gd)Al5O[Formula: see text]: Bi[Formula: see text] phosphor. Bi[Formula: see text] ion can serve as the activator to provide energy for Ce[Formula: see text] ion via cross relaxation phenomenon. Therefore, the (Y[Formula: see text]Ce[Formula: see text]Gd)Al5O[Formula: see text]: Bi[Formula: see text] phosphor could have potential applications in warm white LEDs.

Hydrothermal changes in wheat starch monitored by two-dimensional NMR

The temperature-dependent changes in wheat starch powder and wheat starch-water mixtures were monitored in real-time throughout the heating/cooling program using a classical one-dimensional T1 method and a novel bi-dimensional approach to correlate spin-lattice and spin-spin relaxation times (T1-T2) including acquisition of the FID signal. The influence of two controlling factors (i.e. water content (11%, 35–50%, wet basis) and temperature (20–90°C and back to 20°C)) on water distribution and starch transformation was investigated. Quantitative analysis of 2D T1-T2 maps greatly facilitated the interpretation of T1 relaxation times, which have been interpreted rather narrowly in the literature when classically measured in one-dimension. Application of the new IR-FID-CPMG sequence allowed distinction between different proton pools with different T1 relaxation times, particularly when the starch gelatinization occurred. The quantification of each T1 component permitted to assign the short T1 to slow cross relaxation phenomena, highlighting proton chemical and/or diffusional exchanges between water and starch.

Two dimensional IR-FID-CPMG acquisition and adaptation of a maximum entropy reconstruction

By acquiring the FID signal in two-dimensional TD-NMR spectroscopy, it is possible to characterize mixtures or complex samples composed of solid and liquid phases. We have developed a new sequence for this purpose, called IR-FID-CPMG, making it possible to correlate spin–lattice T1 and spin–spin T2 relaxation times, including both liquid and solid phases in samples. We demonstrate here the potential of a new algorithm for the 2D inverse Laplace transformation of IR-FID-CPMG data based on an adapted reconstruction of the maximum entropy method, combining the standard decreasing exponential decay function with an additional term drawn from Abragam’s FID function. The results show that the proposed IR-FID-CPMG sequence and its related inversion model allow accurate characterization and quantification of both solid and liquid phases in multiphasic and compartmentalized systems. Moreover, it permits to distinguish between solid phases having different T1 relaxation times or to highlight cross-relaxation phenomena.

Dosimetric investigations of Tb3+-doped strontium silicate phosphor.

Tb(3+)-doped SrSiO(3) phosphor synthesised by co-precipitation technique exhibits intense green emission due to cross-relaxation phenomena between Tb(3+) ions. Dosimetric properties of this phosphor have been investigated using thermoluminescence (TL) technique. A dosimetrically useful glow peak observed was at 581 K along with a linear dose response over the wide dose range (100 mGy-4 Gy). TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined by different methods such as Chen's peak shape, initial rise, isothermal decay and variable heating rate methods. Results of these methods are compared and reported in this study.

Structural and luminescent properties of Eu3+-doped GdSrAl3O7 nanophosphor

Eu3+-doped GdSrAl3O7 nanophosphor with promising luminescent properties has been synthesized by low-temperature solution combustion synthesis. The structural properties examined by X-ray diffractometer, scanning electron microscopy, and transmission electron microscopy showed that pure tetragonal GdSrAl3O7: Eu3+ red nanophosphor having narrow size distribution in 50–55 nm range could be readily obtained at low temperature 550 °C. The photoluminescent excitation and emission spectra, life time, and concentration effect were studied in detail. Under excitation at 266 nm, Eu3+-doped GdSrAl3O7 nanophosphor revealed weak green emission and strong red emission attributed to 5D1 → 7F1–2 and 5D0 → 7F0–3 transitions of Eu3+ ion, respectively in the region of 525–700 nm. The red emission from 5D0 → 7F2 transition at 616 nm exhibits the highest intensity under the optimized concentration of 10 mol% after which the quenching mechanism became relevant. Quenching behavior of the europium in the GdSrAl3O7 host was explained by nonradiative cross-relaxation phenomenon. Moreover, Eu3+-doped GdSrAl3O7 nanophosphor can generate light from orange to deeper red by properly tuning the concentration of europium ions based on the energy transfer principle.

Heteronuclear Cross‐Relaxation Effects in the NMR Spectroscopy of Hyperpolarized Targets

Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as (15)N and (13)C. This report explores certain features arising in solution-state (1)H NMR upon polarizing low-γ nuclear species. Following solid-state hyperpolarization of both (13)C and (1)H, solution-phase (1)H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring (13)C nuclei appeared larger than the typically dominant (12)C-bonded (1)H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from (13)C→(1)H polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the (13)C-bound (1)H NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity (1)H NMR traces was explored.

Effects of ultraviolet excitation on the spectroscopic properties of Sm3+ and Tb3+ doped aluminophosphate glasses

Li2O–BaO–Al2O3–La2O3–P2O5 glasses optically activated with rare earth ions with the 4f5, and 4f8 electronic configuration (Sm3+ and Tb3+, respectively) were analyzed by Raman spectroscopy, absorption, excitation photoluminescence, decay curves and temperature dependent photoluminescence. The spectroscopic characteristics of the as-prepared and heat treated samples at temperatures below and above Tg were studied as well as their room temperature photometric properties under ultraviolet excitation. All the doped glasses exhibit typical signatures of the lanthanides in their trivalent charge state. For the samarium doped glass heat treated at 250°C (<Tg) the Sm2+ luminescence was also observed. The analysis of the luminescence efficiency was performed in the interval range of 14K to room temperature, where the integrated intensity of the luminescence was found to decrease for the Sm3+ and Tb3+ ions in the studied temperature range. Luminescence decay curves were found to be non-exponential for the 4G5/2→6H7/2 and 5D3→7F4 transitions of the Sm3+ and Tb3+ ions, respectively. The results strongly suggest the occurrence of energy transfer processes through cross relaxation phenomena, mediated by dipole–dipole interaction in all the studied samples. The decay of the 5D4→7F5 emission of the Tb3+ ions was found to be single exponential with a time constant of ∼3.1ms. Based on the spectroscopic characteristics, models for recombination processes are proposed. The room temperature luminance photometric properties with ultraviolet excitation show that the samarium doped glasses have much lower luminance intensity (around 0.3Cd/m2) when compared with the 6–7Cd/m2 observed for the terbium doped ones.

Observing Cross-Relaxation with Depth-Resolved 8Li β-NMR

Abstract In the paper, we review some of the progress that has been made, at the ISAC β-NMR facility, towards observing 8 Li + high field cross-relaxation phenomena in copper. The clearest observation of these phenomena is via the appearance of peaks in the field dependence of the relaxation rates of the 8 Li + polarization.

Structural and Spectroscopic Studies of ${\rm Pr}^{3+}$-Doped ${\rm GdAlO}_{3}$

In this work, GdAlO3:Pr3+ was successfully prepared by the Pechini method at lower temperatures when compared to others methods such as solid-state synthesis and sol-gel process. In accordance to the XRD data, the fully crystalline single-phase GdAlO3 could be obtained at 900 °C. Luminescence measurements indicate Gd → Pr3+ energy transfer. In the emission spectra, the 3P0 →3 H4 (blue emission) and 1D2 →3 H4 (red emission) transitions of Pr3+ ions can be observed and the ratio between their intensities depends on the Pr3+ content due to the cross-relaxation phenomenon.

Solution State NMR Techniques Applied to Solid State Samples: Characterization of Benzoic Acid Confined in MCM-41

In this paper we present an NMR methodology to characterize small organic molecules confined in mesoporous materials. In particular, we demonstrate that NMR techniques issued from solution state NMR are well suited to characterize benzoic acid encapsulated in hexagonally ordered mesoporous silica MCM-41 possessing two different averaged pore sizes (30 and 100 Å). As evidenced by differential scanning calorimetry, entrapped benzoic acid molecules are highly mobile at room temperature due to confinement effect and possess a glass phase transition temperature around −55 °C. Thus, the 13C NMR characterization of encapsulated molecules has to be adapted to that particular behavior. In particular, the cross-polarization technique traditionally used in solid state NMR to record 13C magic angle spinning (MAS) spectra is of poor efficiency due to weak 1H−13C dipolar interaction. Nevertheless, the presence of 1H−13C cross-relaxation phenomenon (nuclear Overhauser effect, NOE) allows us to record 13C spectra through power-gated techniques, routinely used in solution state NMR, in order to enhance the 13C signal through NOE. Furthermore, the long T2′(1H) values (up to 22 ms) are compatible with the setup of J-coupling-based experiments such as MAS refocused {1H}−13C INEPT experiments allowing us to characterize the sample through chemical bonds. These results combined with those of MAS 1H NOESY experiments lead us to distinguish unambiguously different benzoic acid populations within the large pore sample. Finally, we show that cooling down the samples at −35 °C diminishes the mobility and allows the reintroduction of the 1H−13C dipolar interaction. Thus, 2D MAS {1H}−13C HETCOR experiments can be performed at low temperature to explore spatial proximities.

A Microscopic−Macroscopic Analysis for Mixed Energy Transfer Schemes in Doped Amorphous Solids

In this study we propose a methodology to treat mixed and more complex energy transfer schemes to reach the time-dependent intensities of luminescence in doped amorphous materials. We start outlining the differential equations for the time variation of microscopic probabilities of being in the relevant states and then transforming them into equations for the variation of the relevant macroscopic states populations and solve those equations. By this method, statistical approaches to the initially excited and up-converted states transient populations for up-conversion processes in the presence of cross relaxation in lanthanide-monodoped amorphous solid are calculated. The resultant formulations produce plots that show correct general tendencies and are coherent with what would be expected for systems exhibiting both mechanisms, hence they could be used in the fitting of experimental curves to calculate some important parameters. The new solution method is more convenient that the classical analysis because it permits the introduction of more realistic time dependent functions for the interacting optical centers and allows showing the time dependence of the macroscopic energy transfer rates in the equations for the dynamics of the involved populations. We apply our method to the particular case of mixing of up-conversion and cross relaxation phenomena; however, because of its general characteristics, we suggest it could be applied to other mixings or more complex schemes.