Decrease In Luminescence Intensity Research Articles

Blinking suppression of CdTe quantum dots on epitaxial graphene and the analysis with Marcus electron transfer

We have prepared epitaxial graphene by a Si sublimation method from 4H-SiC. Single-particle spectroscopy of CdTe quantum dots (QDs) on epitaxial graphene covered with polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG) showed the suppression of luminescence blinking and ∼10 times decreased luminescence intensity as compared with those on a glass. The electronic coupling constant, H01, between CdTe QDs and graphene was calculated to be (3.3 ± 0.4) × 102 cm−1 in PVP and (3.7 ± 0.8) × 102 cm−1 in PEG based on Marcus theory of electron transfer and Tang-Marcus model of blinking with statistical distribution.

The luminescence of BaF2 nanoparticles upon high-energy excitation

The dependence of X-ray excited luminescence intensity on BaF2 nanoparticle size was studied. A sharp decrease of self-trapped exciton luminescence intensity was observed when the nanoparticle size is less than 80 nm. The main mechanism of the luminescence quenching is caused by the escape of electrons from the nanoparticles. Escape of electrons from nanoparticles is confirmed by the considerable increase of luminescence intensity of the polystyrene scintillator with embedded BaF2 nanoparticles comparing with pure polystyrene scintillator.

Effect of Capping Agent and Medium on Light-Induced Variation of the Luminescence Properties of CdTe Quantum Dots: A Study Based on Fluorescence Correlation Spectroscopy, Steady State and Time-Resolved Fluorescence Techniques

The influence of ligand and solvent on light-induced modulation of the emission behavior of the quantum dots (QDs) has been studied for CdTe QDs capped with hexadecylamine (HDA), mercaptopropionic acid (MPA), and 1-(1-undecanethiol)-3-methyl imidazolium bromide (SMIM) in CHCl3, H2O, and [bmim][PF6] ionic liquid, respectively, using steady state and time-resolved fluorescence and fluorescence correlation spectroscopy techniques. While an aqueous solution of CdTe/MPA QDs exhibits fluorescence enhancement and a small blue shift of the emission peak (λmaxem) in the early stages of the light irradiation, such enhancement could not be observed in the case of CHCl3 solution of CdTe/HDA and [bmim][PF6] solution of CdTe/SMIM. Instead, exposure to light leads to a rapid reduction in luminescence intensity and large blue shift of λmaxem in the case of CHCl3 solution of CdTe/HDA and a very slow decrease of luminescence intensity with negligible shift of λmaxem in the case of an ionic liquid solution of CdTe/SMIM. The time-resolved fluorescence behavior of the QDs is found to be consistent with the steady state results. Fluorescence correlation spectroscopy measurements on the other hand reveal a large decrease of the amplitude of correlation at time zero [G(0)] for the aqueous solution of CdTe/MPA and negligible change in the G(0) value for the ionic liquid solution of CdTe/SMIM with increasing excitation power. The mechanism of these light-induced changes of the luminescence behavior of the QDs is investigated.

Evolution of the structure of near-surface ultrahigh molecular weight polyethylene nanolayers during orientational drawing

A comparative investigation of the surface structure of ultrahigh molecular weight polyethylene film filaments obtained with different draw ratios from xerogels prepared from 1.5 wt % polymer solutions in decaline and mineral oil has been performed using a SUPRA-55V scanning electron microscope and a nanoluminograph for recording thermoluminescence of ultrathin near-surface layers of solids. It has been found that, with an increase in the draw ratio, the luminescence intensity decreases, and the peaks responsible for the segmental mobility are shifted toward higher temperatures. It has been assumed that this is associated with the improvement of the structure of near-surface layers of the polymer (with a decrease in the number of microcavities and segments of molecules with a high degree of coiling). It has also been revealed that the peaks observed in glow curves of the oriented gel samples from polymer solutions in decaline are shifted more significantly than those of the gel samples from polymer solutions in mineral oil, and the extremely oriented films are characterized by a large discretization of kinetic units of motion.

Sunlight assisted photocatalytic mineralization of nitrophenol isomers over W6+ impregnated ZnO

The synthesis, characterization and sunlight photocatalytic activity of W6+ impregnated ZnO for the degradation of nitrophenol isomers (2-nitrophenol, 3-nitrophenol, and 4-nitrophenol) is reported. Compared to pure ZnO, the impregnated catalysts exhibited a relatively improved spectral response in the visible region. The significant decrease in luminescence intensity for W6+ impregnated catalysts, relative to pure ZnO, established the effective trapping of charge carriers by surface bonded W6+ states. The characterization of the impregnated catalysts by FESEM, XRD and XPS revealed the presence of impregnated W6+ entities at the surface without affecting the morphology of ZnO. The synthesized catalysts exhibited superior activity for the mineralization of three nitrophenol isomers under sunlight exposure. Based on HPLC analysis, ∼99% of 2- and 4-nitrophenol was degraded in a short span of 120min while a degradation of >80% was observed in 3-nitrophenol in the same period. Time scale TOC measurements ratified the mineralization capability of impregnated catalysts. The release of NO2−, NO3− ions in the solution, pH measurements and non-existence of hydroxylated aromatic intermediates revealed that superoxide anion radicals (O2−) are the major contributors in the degradation process. Besides the traces of coupling products, major intermediates identified by GC–MS analysis were either aliphatic dihydroxy alcohols, carboxylic acids or other oxygen containing species depicting that the degradation process proceeds through ring opening mechanism. The W6+ impregnated catalyst also furnished high activity in the visible portion of sunlight. Suitable kinetic models were applied to estimate the rate constants (k) of various processes involved in the degradation process. The stability of the photocatalyst against photocorrosion and the possible decrease in the activity of the catalyst in the successive use was also monitored.

2-Nitroimidazole-ruthenium polypyridyl complex as a new conjugate for cancer treatment and visualization

A novel long-lifetime highly luminescent ruthenium polypyridyl complex containing 2-nitroimidazole moiety [Ru(dip)2(bpy-2-nitroIm)]Cl2 (dip=4,7-diphenyl-1,10-phenanthroline, bpy-2-nitroIm=4-[3-(2-nitro-1H-imidazol-1-yl)propyl]-2,2′-bipyridine) has been designed cancer treatment and imaging. The luminescence properties of the synthesized compound strongly depend on the oxygen concentration. Under oxygen-free conditions quantum yield of luminescence and the average lifetime of emission were found to be 0.034 and 1.9μs, respectively, which is ca. three times higher in comparison to values obtained in air-equilibrated solution. The binding properties of the investigated ruthenium complex to human serum albumin have been studied and the apparent binding constant for the formation of the protein-ruthenium adduct was determined to be 1.1×105M−1. The quantum yield and the average lifetime of emission are greatly enhanced upon binding of ruthenium compound to the protein. The DNA binding studies revealed two distinguished binding modes which lead to a decrease in luminescence intensity of ruthenium complex up to 60% for [DNA]/[Ru]<2, and enhancement of emission for [DNA]/[Ru]>80. Preliminary biological studies confirmed fast and efficient accumulation of the ruthenium complex inside cells. Furthermore, the ruthenium complex was found to be relatively cytotoxic with LD50 of 12 and 13μM for A549 and CT26 cell lines, respectively, under normoxic conditions. The retention and cellular uptake of ruthenium complex is enhanced under hypoxic conditions and its LD50 decreases to 8μM for A549 cell line.

Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique

Blue emission due to self trapped exciton recombination in CaWO4 is believed to be very sensitive to the nature of the WO4 structural units. The present manuscript deals with the probing of structural differences existing in WO4 tetrahedra of CaWO4 particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L1 edge XANES and W L3 edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W6+ ions and disorder in WO4 tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO4 compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.

Boron nitride: A new photonic material

Rhombohedral boron nitride (r-BN) layers were grown on sapphire substrate in a hot-wall chemical vapor deposition reactor. Characterization of these layers is reported in details. X-ray diffraction (XRD) is used as a routine characterization tool to investigate the crystalline quality of the films and the identification of the phases is revealed using detailed pole figure measurements. Transmission electron microscopy reveals stacking of more than 40 atomic layers. Results from Fourier Transform InfraRed (FTIR) spectroscopy measurements are compared with XRD data showing that FTIR is not phase sensitive when various phases of sp2-BN are investigated. XRD measurements show a significant improvement of the crystalline quality when adding silicon to the gas mixture during the growth; this is further confirmed by cathodoluminescence which shows a decrease of the defects related luminescence intensity.

Influence of Li+ doping on up-conversion and structural properties of Yb3+/Tm3+-doped cubic NaYF4 nanocrystals

The impact of Li+ doping on luminescence and structural properties of cubic NaYF4:Yb3+/Tm3+ colloidal nanocrystals was studied. The Yb3+/Tm3+ co-doped samples demonstrated significant red to blue emission ratio changes and high concentration quenching. By substituting Na+ with smaller Li+ ions within NaYF4 lattice, we expected to induce distortions in order to prevent the concentration quenching process, which is responsible for decrease of the luminescence intensity. The results indicated that the substitution of sodium ions by Li+ did not enhance Tm3+ up-conversion emission significantly in α-NaYF4:Yb3+/Tm3+. However, the influence of lithium ions on luminescence was not entirely the same in the case of 2% Tm3+ and 0.1% Tm3+ doped samples. The slight up-conversion intensity enhancement in the case of samples doped with 2% Tm3+ could result from the increase in the crystallites size and changes in the surface to volume ratio. The observed increase in lattice parameter and thus the increased distance between thulium ions in the lattice might also result in breaking apart of pairs responsible for cross-relaxation processes. Interestingly, in the 0.1% Tm3+ co-doped samples, initial luminescence intensity decrease followed by increase correlated with Li+ substitution was observed. Due to the existence of two different sites for incorporation of lithium ions into the host lattice, the changes in lattice parameters and resulting optical properties are not straightforward functions of variable amount of Li+.

The effect of temperature on the luminescence of molecular clusters of silver in photothermorefractive glasses

This paper presents the experimental results of a study of how temperature affects the luminescence of molecular clusters of silver in photothermorefractive glasses. It is shown that, after the glasses are UV-irradiated in the absorption band of cerium ions, intense and broad-band luminescence appears in the visible region. The luminescence centers responsible for this luminescence are neutral silver atoms and neutral molecular clusters of silver. Heat treatment at 350°C causes a tenfold increase of the luminescence intensity. The luminescence intensity decreases by a factor of 10 as the sample’s temperature is increased from 25°C to 200°C. The temperature dependence of the normalized luminescence intensity is shown for a fiber with silver introduced by ion exchange.

Thulium concentration quenching in the up-converting α-Tm3+/Yb3+ NaYF4 colloidal nanocrystals

Luminescence spectra, power dependence and luminescence lifetimes were measured and studied in pure cubic phase NaYF4:20%Yb3+, x%Tm3+ (where x=0.1%, 0.2%, 0.5%, 1% and 2%) nanocolloidal phosphors synthesized by thermal decomposition. Increasing concentration of Tm3+ ions led to decrease of luminescence intensity, increase in the slopes of power dependence as well as to the decrease in the luminescence decay- and rise-times, helping to resolve the mechanisms of up-conversion and concentration quenching.

Structure and NIR-luminescence of ytterbium(iii) beta-diketonate complexes with 5-nitro-1,10-phenanthroline ancillary ligand: assessment of chain length and fluorination impact

Seven new tris(β-diketonear-nate)ytterbium(III) complexes with the general formula [Yb(β-diketonate)3(5NO2phen)] (where the β-diketone is either 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione, 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione, 1,1,1-trifluoro-2,4-pentanedione, 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, 1,1,1,5,5,6,6,7,7,7-decafluoro-2,4-heptanedione, 2,4-hexanedione or 2,6-dimethyl-3,5-heptanedione, and 5NO2phen = 5-nitro-1,10-phenanthroline) were synthesized and characterized by elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy and photoluminescence spectroscopy. Single crystal X-ray structures have been determined for three fluorinated complexes and ground state geometries of the other four complexes have been predicted using the Sparkle/PM6 model. These experimental structures and those designed by semi-empirical models reveal octacoordination around the Yb(3+) ion. Photoluminescence studies and lifetime measurements show that the increase in the fluorinated β-diketonate chain length is associated with a decrease in Yb(3+) luminescence intensity of the (2)F5/2→(2)F7/2 transition at around 980 nm and the (2)F5/2 excited state lifetime, while the ligand lifetime value remains almost unaffected. Finally, fluorination of the ligands is only advised when the complexes are to be used for co-doping with isostructural Er(3+) complexes for optical amplifiers, since it leads to a slight decrease in luminescence intensity for the same β-diketonate chain length.

Synthesis and properties of Eu&lt;SUP align="right"&gt;3+&lt;/SUP&gt;-doped LaPO&lt;SUB align="right"&gt;4 powders

Eu3+-doped LaPO4 powders were produced from direct liquid-liquid reaction of lanthanum chloride (LaCl3) and ammonium phosphate ((NH4)2HPO4). The sample was characterised by XRD and FTIR, which reveal that there is a phase transformation after the heat-treatment at 700oC. The optical properties of Eu3+-doped LaPO4 powders were investigated. Meanwhile, the influence of the dosage concentration of Eu3+ on the luminescent properties was studied. When above 5 mol%, the luminescence intensity decreases.

Electrical and Optical Characterization of n-GaN by High Energy Electron Irradiation

The effect of high energy electron irradiation on the electrical and optical properties of n-GaN is studied. Hall and photoluminescence measurements are carried out on the samples irradiated with different doses. The results obtained from Hall measurements show that electron concentration and mobility are proportional to electron irradiation doses. The PL results show that the near-band-edge intensity and yellow luminescence intensity decrease continually with the increasing electron irradiation. However, it is found that the ratio of the yellow luminescence intensity to the near-band-edge intensity increases with the increasing of electron irradiation dose. To interpret this phenomenon, we propose two theoretical models based on the charge transport mechanism and rate equations, respectively, and they are in good agreement with the experimental observations.

Selenium cannot substitute for sulfur in cell density‐independent bioluminescence in Vibrio fischeri

It has been proposed that selenium, an element chemically similar to sulfur, can participate in some of the same biological pathways as sulfur, although only a few studies have been confirmed this. In this study, we investigated the relationship between selenium and sulfur-dependent luminescence in Vibrio fischeri. The luminescence of V. fischeri was induced by the addition of sulfur-containing compounds such as Na₂SO₄ and L-cystine, and their luminescence was suppressed, in a dose-dependent manner, by the addition of the selenium-containing compounds Na₂SeO₄ and L-selenocystine. Since the viability of V. fischeri was not affected by the addition of low concentration of selenium-containing compounds, the decrease in luminescence intensity cannot be explained by cell death. Kinetic analysis performed using Lineweaver-Burk plots demonstrate that Na₂SeO₄ and L-selenocystine act as competitive suppressors in inorganic sulfur (Na₂SeO₄)-dependent luminescence. In contrast, these selenium-containing compounds act as uncompetitive suppressors in organic sulfur (L-cystine)-dependent luminescence.

Radiation hardness of single‐crystalline zinc oxide films

AbstractZinc oxide (ZnO) is a potential semiconductor to exhibit high radiation hardness since large threshold displacement energy for damage can be expected due to the small unit‐cell volume and large bandgap energy. In order to study the radiation hardness, single‐crystalline c‐axis‐oriented ZnO films with and without two‐dimensional electron gas, and bulk crystals of ZnO and gallium nitride (GaN) for comparison, were irradiated with 8 MeV protons at the wide range of fluences from 2×1013 to 1×1017 p/cm2. For both ZnO films, decrease of luminescence intensity followed by increase of electrical resistance was observed at larger fluences than ∼5×1014 p/cm2. This threshold fluence was found to be much larger and larger than those of GaN and ZnO bulk crystals, respectively, indicating that ZnO thin films should be useful for such device applications as the electronics for space satellites and nuclear reactors. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Assembly, stabilities, and photophysical behaviors of highly efficient luminescent materials fabricated from a terbium complex doped silica/polymer hybrids

A series of hybrid polymeric materials Tb-L xerogel, Polymer-Tb-L xerogels (polymer=PVB, PVP, or PMMA) fabricated from a new highly luminescent Tb(III) complex TbL2(NO3)3 (Φ=42.65%) of β-diketone ligand [L=N-(6-(2-methylpyridinyl))trifluoroketoacetamide] were successfully assembled by sol–gel process. The Fourier transform infrared spectra, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetry (TG), UV–visible spectra and photophysical behaviors of Tb(III) complex and Tb(III) complex doped hybrids (Tb-L xerogel, Polymer-Tb-L xerogels) are investigated in detail. The hybrids display more efficient unit mass luminescence emission, enhanced thermal stability, and improved exposure durability in comparison with the pure complex, due to the steric restriction effect from the rigid SiO and SiC polymeric network. In addition, the hybrids Polymer-Tb-L xerogels represent longer lifetime and higher quantum efficiency than that of the hybrid Tb-L xerogel. The result may support the conclusion that the polymers could not only enwrap the lanthanide complexes to keep the donors and acceptors close, but also transfer energy to the central Tb(III) ions. Comparatively, PVP-Tb-L xerogel represents the longest lifetime (1047.07μs) and highest quantum yield (34.09%). At the same time, concentration effects on the luminescence intensity were investigated. The luminescence intensity decreases, however, with increasing complex concentration in the Polymer-Tb-L xerogels. The research of this work attempted to assemble the highly efficient luminescent materials containing the optimal combination of lanthanide complex and matrices.

X-ray luminescence spectra of graded-gap AlxGa1−xAs structures irradiated by alpha particle

The influence of 241 Am alpha particle irradiation on X-ray luminescence spectra of the graded-gap Al x Ga 1− x As structures of different thicknesses is investigated. It is observed that the integral X-ray luminescence intensity of nonirradiated thin (15 μm) structure is 1.4 times less than that in the thick (32 μm) structure, and this difference increases to 3 times after 3×10 10 cm −2 dose of irradiation by alpha particle. The X-ray luminescence intensity of the energy hν <1.5 eV of thin nonirradiated structure is about 7 times less than that in thick one. The internal graded-gap electric field F gg is responsible of that large difference, because it shifts the X-ray generated carriers to the narrow-gap surface with great nonradiative surface recombination rate. The alpha particle irradiation increases nonradiative recombination rate and causes a decrease of the X-ray luminescence intensity of all spectra lines in the thin (15 μm) detector. The most significant drop in X-ray luminescence efficiency is observed from the region at narrow-gap surface after the initial stage (10 9 cm −2 dose) of alpha particle irradiation. In the 32 μm thick detector, the luminescence intensity of the energy hν =1.8 eV does not change up to 2×10 10 cm −2 of alpha particle irradiation dose. That means the high irradiation hardness of the thick graded-gap X-ray detector with optical response.

Interaction of ZnS nanoparticles with flavins and glucose oxidase: A fluorimetric investigation

Interactions of luminescence, water soluble ZnS nanoparticles (NPs) with flavins and glucose oxidase have been thoroughly investigated through optical spectroscopy. The photoluminescence of ZnS nanoparticles was quenched severely (∼60%) by riboflavin while other flavins such as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) show quenching to different extents under analogous conditions. However, interestingly no effect in luminescence intensity of ZnS NPs was observed with protein bound flavins such as in glucose oxidase. Fluorescence lifetime measurement confirmed the quenching to be static in nature. Scavenging of photo-generated electron of ZnS nanoparticles by the flavin molecules may be attributed to the decrease in luminescence intensity. Quenching of ZnS nanoparticles with flavins follows the linear Stern–Volmer plot. The Stern–Volmer constants decreased in the following order: K S−V (Riboflavin)> K S−V (FAD)> K S−V (FMN). This interaction study could generate useful protocol for the fluorimetric determination of riboflavin (vitamin B 2) content and also riboflavin status in biological systems.

Aqueous curium(III) phosphate species characterized by time-resolved laser-induced fluorescence spectroscopy

AbstractThe formation of aqueous Cm(III) phosphate complexes was studied at room temperature by time-resolved laser-induced fluorescence spectroscopy (TRLFS) in 0.1 M NaClO4solutions. The experiments were perfomed at a fixed total Cm(III) concentration of 3 × 107or 2 × 108 M by varying the phosphoric acid concentration (3 × 105–0.1 M) and the pH (1.4–6.0). The red shift of the excitation and emission spectra, as well as the increase of luminescence lifetimes clearly showed the influence of phosphate on the aqueous Cm(III) speciation. In acidic phosphate solutions ([H3PO4] ≤0.1 M, pH 1.4–2.6) an increase in luminescence intensity was detected due to complexation with H2PO4−. At [H3PO4] ≥4 × 104 M and between pH 4.0 and 6.0 in general a decrease in luminescence intensity affiliates the complexation with HPO42−. Two Cm(III)-phosphate complexes could be identified from the emission data, CmH2PO42+and CmHPO4+, having peak maxima at 599.6 and 600.8 nm, respectively. TRLFS in combination with ultra-filtration (1 kD) showed that the formation of CmHPO4+is accompanied by the generation of Cm(III)-phosphate colloids especially at [H3PO4] ≥0.002 M and pH ≥ 5. Cm(III)-phosphate colloids formed at pH 5 and 6 are characterized by an emission maximum at 603.1 nm. Based on the factor analysis of the emission data the stability constants of the two complexes were calculated to be logβ121= 20.23 ± 0.13 and logβ111= 16.54 ± 0.80 at an ionic strength of 0.1 M (NaClO4).