Luminescence Kinetics Research Articles

A Study of Vanadate Group Substitution into Nanosized Hydroxyapatite Doped with Eu3+ Ions as a Potential Tissue Replacement Material.

In this study, nanosized vanadate-substituted hydroxyapatites doped with 1 mol% and 2 mol% Eu3+ ions were obtained via the precipitation method. To evaluate the structure and morphology of the obtained compounds, the XRPD (X-ray powder diffraction) technique, Rietveld refinement, SEM-EDS (scanning electron microscopy-energy-dispersive spectrometry) and TEM (transmission electron microscopy) techniques as well as FTIR (Fourier transform infrared) spectroscopy were performed. Moreover, the chemical formula was confirmed using the ICP-OES (Inductively coupled plasma optical emission spectroscopy spectroscopy). The calculated average grain size for powders was in the range of 25 to 90 nm. The luminescence properties of vanadium-substituted hydroxyapatite were evaluated by recording emission spectra and excitation spectra as well as luminescence kinetics. The crucial step of this research was the evaluation of the biocompatibility of the synthesized nanomaterials. Therefore, the obtained compounds were tested toward sheep red blood cells and normal human dermal fibroblast to confirm the nontoxicity and biocompatibility of new nanosized Eu3+ ion-doped vanadate-hydroxyapatite. Moreover, the final step of the research allowed us to determine the time dependent ion release to the simulated body fluid environment. The study confirmed cytocompatibility of vanadium hydroxyapatite doped with Eu3+ ions.

Radiative dynamics and delayed emission in pure and doped InP/ZnSe/ZnS quantum dots.

We have used time-correlated single photon counting to elucidate the radiative dynamics of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) that differ in the amount and distribution of excess indium. Stoichiometric QDs having an In:P atom ratio very near unity exhibit simple luminescence kinetics. The photoluminescence (PL) rises with the 40ps instrument response function and exhibits a decay that is close to a single exponential with a time constant that decreases from 32 to 28ns with increasing shell thickness. QDs having excess indium (In:P ratio of 1.15-1.63) show a significant component of a slower rise time assigned to transient population of indium-based hole traps in the ZnSe shell. They also have a slower PL decay, attributed to an equilibrium between these traps, which are optically dark, and the emissive valence-band state. This results in a radiative lifetime that increases from 32 to 48ns with increasing shell thickness. Different treatments of the InP cores prior to shell deposition result in different core/shell interfaces as indicated by resonance Raman spectroscopy, as well as differences in the amplitude and timescale of the slow PL rise and the PL decay time. These are interpreted in terms of different radial distributions of the indium-based hole traps, which can be related to differences in the interfacial lattice strain.

A novel single-phased white light emitting phosphor with single Eu2+ doped whitlockite structure

Exploring novel efficient single-phased full spectrum phosphor with individual activator doping for white light emitting diodes is highly significative and benefit for scientific activities. Herein, by designing and modifying the β-Ca3(PO4)2-type structure, the Ca19MgNa2(PO4)14: Eu2+ (CMNP) phosphor was proposed as a single-phased full spectrum white phosphor. The phase purity and clear crystal structure were then determined by employing the Rietveld refinement and energy dispersive X-ray spectroscopy. PLE and PL spectra indicated CMNP: xEu2+ (0.1% ≤ x ≤ 1.0%) phosphors could be well fitted with 365 nm LED chip and exhibited a full spectrum emission band covering all visible lights of 400–750 nm belonging to the 5d−4f transitions of Eu2+. Further, the selective occupation of Eu2+ ions in different cation sites and luminescence kinetics depending on the Eu2+ dopants were characterized by theoretical calculation, Rietveld refinement results and decay curves. Finally, the WLED device fabricated by (near)-UV WLED chip and CMNP: 0.75 %Eu2+ phosphor could present excellent color temperature (6643 K) and CIE coordinate (0.308, 0.346). The current work manifests that Ca19MgNa2(PO4)14: Eu2+ had excellent luminescent property, which could be used in optical applications.

Cathodoluminescence of Diamond: Features of Visualization

Zonal and sectorial heterogeneities in natural diamonds provide information on the growth conditions and post-growth changes. Cathodoluminescence (CL) microscopy revealed these heterogeneities in a very detailed manner with high spatial resolution. In this study, factors affecting the CL images of two natural diamonds were analyzed and the results of cathodoluminescence studies in steady-state (SS-CL) and scanning modes were compared. SS-CL was observed using an optical microscope, and scanning mode was evaluated via SEM (SEM-CL). It was demonstrated that the relative brightness of the <111> and <100> growth sectors in diamond crystals depends on the nature of defects in them and on the method of image detection (steady-state/scanning versus color/panchromatic). The differences between SS-CL and SEM-CL images can be attributed to the kinetics of luminescence and spectral sensitivity of the detectors. It was established that the nature of lattice defects around small inclusions can be changed (e.g., the intensity of blue luminescence from nitrogen-vacancy defects (N3V) decreases due to their transformation into nitrogen–hydrogen defects (N3VH). The hydrogen disproportion between the sectors is caused by different growth mechanisms. Hydrogen atoms in the diamond matrix can affect the kinetics of transformation of the defects by transforming a part of N3V to N3VH.

Influence of synthesis conditions on structural and spectroscopic properties of the K2SrP2O7 pyrophosphate doped with the Eu3+ and Eu2+ ions

This study is focused on the synthesis condition influence related to structural and spectroscopic properties of the potassium strontium pyrophosphate (K2SrP2O7) co-doped with Eu3+ and Eu2+ ions. The experimental parameters of the urea assisted combustion synthesis were optimized by temperature and time of calcination as well as reduction atmosphere composition. The obtained materials were structurally characterized using the X-Ray Powder Diffraction (XRPD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Spectroscopic properties were determined by emission spectra, excitation spectra, luminescence kinetics and luminescence temperature quenching measurements. It was found that increasing concentration of Eu3+ ions caused an increase of spectroscopic factors, which correspond to the relative integrated luminescence intensities of 5D0–7FJ transitions as well as structural defects. Moreover, the successful Eu2+ ion incorporation was proven by optical spectroscopic measurements, which showed the allowed f-d transitions.

Förster resonance energy transfer in finite length systems and porous media

Förster Resonance Energy Transfer (FRET) is a mechanism widely used to describe the luminescent behavior of a great variety of systems. However, there is a strong need for a complete model of Förster theory to understand donor-acceptor interactions in different confinement geometries and their effects on the luminescent dynamics. Here, a methodology based on Förster theory that allows finding the probability distribution of donors and acceptors in systems of any geometry is presented. We have considered the particular cases of lines with finite length, finite cylinders, and bi-dimensional arrangements of lines and cylinders. Furthermore, using our results, we calculate the quantum yield as a function of the geometric parameters of the structures. Our numerical results show that this methodology is useful to calculate and predict the donor luminescent dynamics in structures of finite length. We find that our model explains the decrease of the transferred energy from donors to acceptors with the separation between lines or cylinders, this behavior is physically consistent. The model presented here extends the use of FRET to the study of structures with bi-dimensional arrays of lines or cylinders (as could be the case of porous media), and finite cylinder structures (as could be the case of nanotubes). In our knowledge, this work is the first theoretical approach to describe the donor-acceptor luminescent dynamics in this kind of systems.

Spectroscopic properties of Yb3+-doped fluorophosphate glasses with low phosphate content

A series of new fluorophosphate glasses of 5Ba(PO3)2–10BaF2–18.5CaF2–10MgF2–18.5SrF2–(38–x)AlF3–xYbF3 (x = 0.5, 1, 2, 5, 8, 10) composition was synthesized. Experimental characterization, including absorption, luminescence, and luminescence kinetics measurements, was performed. Based on experimental data, the optimal concentration of Yb3+ ions, providing the highest gain, was established. To characterize potential laser performance of the studied glasses several parameters, including pump saturation, minimal pump intensity, and minimal fraction of excited ions needed to balance gain and absorption, were calculated. Dependence of gain cross section on the excited state population was determined to characterize amplifying properties of the glasses.

Lanthanide-Doped Luminescent Nanophosphors via Ionic Liquids.

Lanthanide (Ln3+) ion(s)-doped or rare-earth ion(s)-doped nanomaterials have been considered a very important class of nanophosphors for various photonic and biophotonic applications. Unlike semiconductors and organic-based luminescent particles, the optical properties of Ln3+-doped nanophosphors are independent of the size of the nanoparticles. However, by varying the crystal phase, morphology, and lattice strain of the host materials along with making core-shell structure, the relaxation dynamics of dopant Ln3+ ions can be effectively tuned. Interestingly, a judicious choice of dopant ions leads to unparallel photophysical dynamics, such as quantum cutting, upconversion, and energy transfer. Recently, ionic liquids (ILs) have drawn tremendous attention in the field of nanomaterials synthesis due to their unique properties like negligible vapor pressure, nonflammability, and, most importantly, tunability; thus, they are often called “green” and “designer” solvents. This review article provides a critical overview of the latest developments in the ILs-assisted synthesis of rare-earth-doped nanomaterials and their subsequent photonic/biophotonic applications, such as energy-efficient lighting and solar cell applications, photodynamic therapy, and in vivo and in vitro bioimaging. This article will emphasize how luminescence dynamics of dopant rare-earth ions can be tuned by changing the basic properties of the host materials like crystal phase, morphology, and lattice strain, which can be eventually tuned by various properties of ILs such as cation/anion combination, alkyl chain length, and viscosity. Last but not least, different aspects of ILs like their ability to act as templating agents, solvents, and reaction partners and sometimes their “three-in-one” use in nanomaterials synthesis are highlighted along with various photoluminescence mechanisms of Ln3+ ion like up- and downconversion (UC and DC).

Synthesis, luminescent properties, and thermal stabilities of Gd2(1-x)Eu2x(MoyW1-yO4)3 (0≤x≤0.2, 0≤y≤1) solid solution phosphors

Through a solid-state reaction method, Gd2(1-x)Eu2x(MoyW1-yO4)3 (0 ≤ x ≤ 0.2, 0 ≤ y ≤ 1) solid solution phosphors were synthesized. The crystalline structures, the morphologies, and the compositions of the solid solution phosphors were characterized by X-ray diffraction (XRD) patterns, field emission scanning electron microscope (FE-SEM) images, energy-dispersive spectra (EDS), respectively. The diffuse reflection spectra (DRS), room temperature photoluminescence (PL) excitation and emission spectra, luminescence decay curves, and high temperature PL spectra were recorded to characterize the luminescent properties, the luminescent dynamics, and the thermal behaviors of the phosphors. The synthetic temperatures were explored to determine the optimal synthetic temperature. The Eu3+ concentration quenching effects were observed in the orthorhombic phase of Gd2(1-x)Eu2x(MoO4)3 phosphors and the monoclinic phase of Gd2(1-x)Eu2x(WO4)3 phosphors. By introducing Mo6+ to Gd2(1-x)Eu2x(MoyW1-yO4)3 phosphors, crystalline phase variations and red shift excitation bands were recorded. Enhancing Eu3+ excitation and emission intensities were recorded in the Gd2(1-x)Eu2x(MoyW1-yO4)3 solid solution phosphors with the concentration of W6+ increasing. The obtained Gd2(1-x)Eu2x(MoyW1-yO4)3 phosphors can be used in phosphor converted based ultraviolet (UV) LEDs.

From Structure to Luminescent Properties of B2O3-Bi2O3-SrF2 Glass and Glass-Ceramics Doped with Eu3+ Ions.

Glass-ceramics with the composition B2O3-Bi2O3-SrF2 were synthesized by the conventional melt-quenching technique and subsequent crystallization of the parental glasses. The temperature at which the ceramization was carried out was selected based on differential scanning calorimetry (DSC) analysis. The structure of the studied materials and the formation of SrF2 nanocrystals were confirmed by the Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. It was found that the amount of strontium fluoride introduced into the parental borate-bismuth glass has a significant impact on the growth of SrF2 nanocrystals. In particular, the influence of the crystalline SrF2 phase on luminescence intensity and kinetics was studied using Eu2O3-doped samples. An increase in luminescence intensity was observed in the samples in which SrF2 nanocrystals were formed. This is most likely related to the fact that some of the Eu3+ ions were (after annealing of the glass) located in the crystalline structure of strontium fluoride. This was confirmed both by the luminescence lifetime obtained based on the luminescence decay curves and the calculated Judd–Ofelt parameters, Ω2 and Ω4. The results achieved confirm that the glasses and glass-ceramics described in this work could be considered as a new phosphor for light-emitting diodes (LEDs).

Crystal configuration, luminescence dynamics and facile combustion-fabrication of high-brightness YAG:Sm3+ nanomaterials towards competent illuminating appliances, especially WLEDs and solar-cells

New Sm3+-activated YAG nanomaterials have been fabricated via low-temperature solution-combustion method, to advance the quality of multi-phase wLEDs (white-light-emitting diodes). Cubic-phase has been configured via XRD-facilitated Rietveld technique. Fine orange-red luminescence with high color-purity and quantum-efficiency, has been attained via energization of 429 nm wavelength, such that x = 0.02 Sm3+ content has been spotted as the best possible doping-content. Obtained orange-red-coordinates (0.5868, 0.4088) are proximate to commercialized AMBER LED-NSPAR 70 BS. (NICHIA CORP.). All in all, the fabricated materials can certainly be implemented for solid-state illumination appliances, mainly warm-WLEDs, bio-imaging, security-inks, signage and solar cells.

Formation of Gd2O3:Nd3+ nanocrystals in silica microcapillary preforms and hollow-core anti-resonant optical fibers

A silica microcapillary preform and a hollow-core anti-resonant optical fiber doped with small-size (<50 nm) Gd2O3:Nd3+ nanocrystals are fabricated by the liquid polymer-salt method for the first time, to our knowledge. Aqueous solutions of gadolinium nitrate and neodymium chloride and a soluble organic polymer (polyvinylpyrrolidone) are used to form Gd2O3:Nd3+ nanocrystals inside the air channels of a preform followed by drawing of an optical fiber. Characterization of microcapillary preforms and optical fibers doped with Gd2O3:Nd3+ nanocrystals is performed with XRD, SEM, optical microscopy and photoluminescence analysis. Basic technological factors affecting structural and luminescent properties of the fabricated materials are considered. Particularly, it is found out that microcapillary preforms and hollow-core anti-resonant optical fibers doped with Gd2O3:Nd3+ nanocrystals demonstrate photoluminescence (emission spectra and luminescence lifetimes) characteristic to Nd3+ ions in metal sesquioxides. In addition, it is revealed that luminescence kinetics in optical fibers is described by two exponential dependences with decay times τ1 = 12 µs and τ2 = 233 µs that can be attributed to the feature of a cubic crystal structure of Gd2O3.

Gigahertz Frame Rate Imaging of Charge-Injection Dynamics in a Molecular Light Source.

Light sources on the scale of single molecules can be addressed and characterized at their proper sub-nanometer scale by scanning tunneling microscopy-induced luminescence (STML). Such a source can be driven by defined short charge pulses while the luminescence is detected with sub-nanosecond resolution. We introduce an approach to concurrently image the molecular emitter, which is based on an individual defect, with its local environment along with its luminescence dynamics at a resolution of a billion frames per second. The observed dynamics can be assigned to the single electron capture occurring in the low-nanosecond regime. While the emitter’s location on the surface remains fixed, the scanning of the tip modifies the energy landscape for charge injection into the defect. The principle of measurement is extendable to fundamental processes beyond charge transfer, like exciton diffusion.

Ultrafast excited state dynamics and light-switching of [Ru(phen)2(dppz)]2+ in G-quadruplex DNA

The triplet metal to ligand charge transfer (3MLCT) luminescence of ruthenium (II) polypyridyl complexes offers attractive imaging properties, specifically towards the development of sensitive and structure-specific DNA probes. However, rapidly-deactivating dark state formation may compete with 3MLCT luminescence depending on different DNA structures. In this work, by combining femtosecond and nanosecond pump-probe spectroscopy, the 3MLCT relaxation dynamics of [Ru(phen)2(dppz)]2+ (phen = 1,10-phenanthroline, dppz = dipyridophenazine) in two iconic G-quadruplexes has been scrutinized. The binding modes of stacking of dppz ligand on the terminal G-quartet fully and partially are clearly identified based on the biexponential decay dynamics of the 3MLCT luminescence at 620 nm. Interestingly, the inhibited dark state channel in ds-DNA is open in G-quadruplex, featuring an ultrafast picosecond depopulation process from 3MLCT to a dark state. The dark state formation rates are found to be sensitive to the content of water molecules in local G-quadruplex structures, indicating different patterns of bound water. The unique excited state dynamics of [Ru(phen)2(dppz)]2+ in G-quadruplex is deciphered, providing mechanistic basis for the rational design of photoactive ruthenium metal complexes in biological applications.

Non-radiative luminescence decay with self-trapped hole migration in strontium titanate: Interplay between optical and transport properties

A novel model for the non-radiative decay of self-trapped excitons (STEs) in the advanced functional oxide SrTiO3 is proposed and supported by experimental observations. The study is based on the initial ionoluminescence stage for STE emission at 2.5 eV, and its dependence on temperature and electronic excitation rate under energetic heavy-ion irradiation. For all temperatures, this initial stage reaches rapidly a quasi-steady level, and then decreases as induced-structural damage increases. The quasi-steady luminescence exhibits a linear dependence on the excitation rate, suggesting a constant efficiency for STEs formation. An activation energy of 55 meV, essentially independent of the incident ion mass and energy of projectile-ion, is deduced from an Arrhenius-type relationship with irradiation temperature. This energy is in good agreement with experimental values, measured for non-radiative STE decay under ns-laser pulse excitation, and reasonably consistent with density functional theory calculations for migration of self-trapped holes (STHs) described by a small-polaron adiabatic hopping model. A new mechanism dealing with a non-radiative contribution to the STE transition is discussed, consisting of STH migration through thermally-activated hopping and annihilation with the STE-electron. Luminescence kinetics from the chromium intrinsic impurity strongly supports this model, being consistent with the annihilation of Cr3+ centers through recombination with migrating STHs.

Decay time dynamics of red and blue luminescence of surface-functionalized silicon quantum dots

Controlling the emission wavelength by the quantum confinement or the size effect of silicon quantum dots (Si QDs) has a limitation in that it can be only approximately 600 nm or above. To overcome this limitation, in the recent decade, numerous studies have been conducted for reproducing the entire visible range from blue to red from Si QDs by their surface functionalization. In this study, we fabricated alkyl- and amine-passivated Si QDs and investigated their emission mechanism following surface functionalization. We interpreted the emission mechanism of the surface-functionalized Si QDs by combined experimental analysis of the decay time dynamics and density functional theory-based model calculation of the highest occupied molecular orbital-lowest unoccupied molecular orbital electronic structures. Our comprehensive studies could provide insight into the debated problems of the luminescence mechanism of surface-functionalized Si QDs.

Tunable luminescence and realization of warm white light via energy transfer from Dy3+/Er3+/Sm3+ triply doped glasses for photonic applications

The spectroscopic exploration of Dy 3+ /Er 3+ /Sm 3+ singly, doubly and triply doped BaO + ZnF 2 +K 2 O + SiO 2 +B 2 O 3 glasses have been carried out through photoluminescence excitation and emission, decay profiles, CIE color co-ordinates and correlated color temperature (CCT) values. Co-occurrence of Dy 3+ , Er 3+ and Sm 3+ luminescent bands in the fluorescence spectra of Dy 3+ /Er 3+ /Sm 3+ triply doped multicomponent borosilicate glasses with different excitation wavelengths (348, 374, 379 and 400 nm) emphasizes the existence of energy transfer processes and is studied in detail. The luminescence intensity of Er 3+ and Sm 3+ bands has shown a remarkable reduction with the increase in Dy 3+ content, whereas Dy 3+ bands seemed to be enhanced monotonously. Luminescent kinetics of as-prepared samples were investigated thoroughly by using recorded decay profiles and the observed decrement in lifetime values validated proposed Er 3+ (or Sm 3+ ) to Dy 3+ energy transfer routes. The energy transfer efficiency was considerably increased with the increase in concentration of Dy 3+ ions and is substantially interpreted using partial energy level diagrams. The estimated CCT values and CIE color co-ordinates of the prepared samples were significantly changed from reddish-orange to warm white light and it suggests the suitability of the material for lighting applications. • Multicomponent borosilicate glass is optically activated with Dy 3+ /Er 3+ /Sm 3+ ions. • The influence of Dy 3+ ions on the spectroscopic properties of triply doped glasses was examined. • Luminescent kinetics of as-prepared samples were investigated thoroughly by using photoluminescence decay profiles. • Energy transfer from Er 3+ (or Sm 3+ ) to Dy 3+ occurs. • Tunable luminescence was observed in the glassy system under different excitation wavelengths.

Solution combustion synthesis and luminescence dynamics of CaTiO3: Eu3+, Y3+ nanophosphors

Perovskite titanate nanophosphors, Ca1-x-yYxTiO3:yEu3+ have been synthesized through solution combustion synthesis using metal-citrate complexation method. The optimum substitution of Y3+ ions in Ca0·93TiO3:0.07Eu3+ phosphor enhances the intensity of luminescence 10 times along with the realization of the color purity of 91% and the luminescence life time value to 1.3 ms. X-ray Photoelectron Spectroscopy studies confirm the 3+ oxidation state of yttrium and europium ions and the occupancy of these ions at Ca2+ sites. The sole presence of Raman modes representing the orthorhombic structure and the TEM analysis reveal the high degree of crystallinity of phosphors. The substitution of rare earth ions causes the distortion of local crystal structure symmetry by the tilting of TiO6 octahedra and this results in the shift of Raman modes of CaTiO3. The single exponential nature of luminescence decay curves suggests the occupancy of substituent ions at the Ca2+ sites. Judd- Ofelt analysis gives the quantum efficiency as 81.6% for the phosphor with optimum dopant concentration.

Luminescent kinetics of Dy3+ doped CaZn2(PO4)2 phosphors for white light emitting applications

In the present work, we have synthesized a series of Ca (1-X) Zn2 (PO4)2: X Dy3+ (X = 0.02, 0.04, 0.06, 0.08, 0.10) white light emitting phosphors via solid state reaction method. We account the structural and photoluminescence properties of all the prepared samples. The formation of CaZn2(PO4)2 was confirmed through X-ray diffraction and Fourier transform infrared spectra analyses. The luminescence properties of the phosphor materials were investigated through optical absorption and photoluminescence studies. The details of radiative properties were obtained through Judd- Ofelt intensity parameters and they were found to follow the trend Ω2 > Ω4 > Ω6. The samples were efficiently excited under near ultraviolet radiations to get excellent white light emission in terms of color rendering and luminous efficacy. Photo luminescent spectra of all the samples exhibit two prominent peaks in blue (474 nm) and yellow (572 nm) regions corresponding to the transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 respectively. It is also revealed that the luminescence intensity is maximum for the sample Ca(0.96)Zn2(PO4)2:0.04 Dy3+. The phosphor was characterized by high values of branching ratio and stimulated emission cross section for the 4F9/2 → 6H13/2 transition. The emission spectra were also analyzed using Commission Internationale de l’eẤclairage 1931 standard to understand the quality of white light emission. The chromaticity color coordinates of all the samples fall within the white light domain. The photoluminescence life time values of the 4F9/2 excited level was measured by exciting Dy3+ ions at 349 nm and was found to be 0.884 ms for the sample with optimum concentration. The internal quantum efficiency was also calculated and observed to have a very good value of about 74.9%.The present study confirms the Dy3+ doped CaZn2(PO4)2 phosphors to be a promising material for white light emitting diodes.

Sensitized Luminescence Kinetics as a Tool for Identification of Nonradiative Energy Transfer

A simple equation is proposed for calculating the sensitized luminescence kinetics of acceptors caused by nonradiative excitation energy transfer (Forster Resonance Energy Transfer (FRET)) from donors. As parameters, the equation includes the lifetimes of the donor and acceptor excited states and the experimental luminescence decay kinetics of donors in the presence of FRET. The equation does not specify a certain FRET mechanism. It is proposed to use this equation to fit the experimental sensitized luminescence kinetics in order to quantitatively confirm the existence of FRET between donor and acceptors.