Decrease In Luminescence Intensity Research Articles

The influence of synthetic conditions on optical properties of cadmium selenide quantum dots

In this work we consider the technique used to prepare CdSe quantum dots (QDs) both in a high-boiling solvent (octadecene) and in an aqueous solution. It has been shown that, if octadecene is used as a medium for synthesis, the increase in synthetic temperature and reaction time leads to a substantial decrease in the QDs trap luminescence intensity. In addition, an increase in the synthesis temperature reduces the scatter of particles in sizes. In addition to the hydrofobic QDs, CdSe QDs have also been synthesized in an aqueous solution using mercaptopropionic acid as a stabilizer. Using these QDs as an example, the possibility of increasing the stability of QDs covered by mercaptopropionic acid in the presence of sodium sulfite has been shown for the first time. The role of sodium sulfite consists of the chemical binding of dissolved oxygen.

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F 2, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16–150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16–200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F 2 laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼ t −1. For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups. Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects. The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.

IR luminescence of neodymium(III) and ytterbium(III) complexes with acylpyrazolones in solutions

Acylpyrazolones (L1–L5) have been synthesized and the conditions of their complexation with neodymium(III) and ytterbium(III) ions in aqueous solutions have been elucidated. The component ratio in the synthesized complexes is Nd(Yb): L = 1: 1. The conditions of excitation and luminescence of the ligands and complexes have been studied. The formation of mixed-ligand complexes upon the introduction of trioctyl- or triphenylphosphine oxide leads to a considerable rise of neodymium and ytterbium. In the presence of 1,10-phenanthroline and bathophenanthroline, competing complexation leads to a 20–70% decrease in luminescence intensity. The introduction of water-miscible organic solvents (30 vol %) decreases the Nd(III) and Yb(III) luminescence intensity by a factor of 9–20.

Photoluminescence and photoluminescence excitation studies in 80 MeV Ni ion irradiated MOCVD grown GaN

We report damage creation and annihilation under energetic ion bombardment at a fixed fluence. MOCVD grown GaN thin films were irradiated with 80 MeV Ni ions at a fluence of 1 × 10 13 ions/cm 2. Irradiated GaN thin films were subjected to rapid thermal annealing for 60 s in nitrogen atmosphere to anneal out the defects. The effects of defects on luminescence were explored with photoluminescence measurements. Room temperature photoluminescence spectra from pristine sample revealed presence of band to band transition besides unwanted yellow luminescence. Irradiated GaN does not show any band to band transition but there is a strong peak at 450 nm which is attributed to ion induced defect blue luminescence. However, irradiated and subsequently annealed samples show improved band to band transitions and a significant decrease in yellow luminescence intensity due to annihilation of defects which were created during irradiation. Irradiation induced effects on yellow and blue emissions are discussed.

Luminescence of localized states in silicon dioxide glass. A short review

The target is the description of the properties of localized states in silica glass, which relate to aspects of short-to-intermediate-range order. It has been observed that laser light interaction with localized states of silica glass leads to the creation of luminescence centers. Created luminescence centers, excited with laser light, provide intra-center luminescence of oxygen deficient centers (ODC) comprising a blue band at 2.7 eV and a UV band at 4.4 eV. Structurally, these ODCs are understood to comprise twofold-coordinated silicons that are commonly part of some larger local structure, and their luminescence bands can be suppressed by reaction with chlorine or hydrogen. Beside these processes laser light can give rise to charge separation. Recombination of well separated electrons and holes leads to recombination luminescence, which is similar to intra-center process luminescence but with greatly suppressed UV emission relative to the blue band and longer duration decay times. Evidence has been given that center of recombination at ODC sites involves an electron trap at the defect, whereas the hole is created as self-trapped holes (STHs) centers. Recombination results in the localized state recovering its initial state. Studies of temperature dependences of recombination luminescence intensity and decay show that intensive changes in these parameters take place in the known range of temperatures of STH liberation. Decrease of measured intra-center luminescence duration with increasing temperature is accompanied by less rapid decrease of recombination luminescence intensity. In the temperature range ~ 300 to 450 K, the blue luminescence intensity actually grows with heating, with little change in the decay time constant. As a rule, luminescence decay curves are non-exponential well described with stretched exponential function, showing first order fractal-kinetics. It is argued that discovered localized states of silica glass are connected with structure other than tetrahedrons. In dense silicon dioxide crystal with rutile structure (stishovite) luminescence similar to ODC luminescence of silica glass has been found.

Preparation of long-afterglow colloidal solution of Sr 2MgSi 2O 7: Eu 2+, Dy 3+ by laser ablation in liquid

We have successfully prepared a novel nanoparticle solution of Sr 2MgSi 2O 7: Eu 2+, Dy 3+ with afterglow properties by means of laser ablation in liquid. This process also produced by-products of different kinds, depending on the liquid used. The amount of by-product and the size of the nanoparticles were controlled by the energy density of laser ablation. The amount of by-product was reduced by a decrease in the energy density, which also decreased the particle size of the nanoparticles. The PL spectrum of the nanoparticles was the same as that of the target materials used for laser ablation. The afterglow properties deteriorated with a decrease in particle size. We concluded that an increase in specific surface area caused by a decrease in particle size resulted in the decrease of luminescent intensity.

Generation of exciton in two semiconductors interface: SnO 2:Eu–Y 2O 3

Luminescence intensity corresponding to Eu 3+ for SnO 2:Eu 3+ nanoparticles dispersed in Y 2O 3 increases significantly with increasing heat-treatment temperature up to 500 °C after excitation through exciton of SnO 2. On further heat-treatment up to 900 °C, luminescence intensity decreases, whereas lifetime value for 5D 0 level of Eu 3+ increases. This can be explained on formation of phases Y 2− x Eu x Sn 2O 7 and Y 2− y Eu y O 3 during heat-treatment. The former does not contribute to luminescence intensity, whereas the latter contributes to luminescence intensity. Availability of optically active Eu 3+ ions decreases with heat-treatment and hence, luminescence intensity decreases. Enhancement of lifetime is related to homogeneous solid solution formation of Y 2− y Eu y O 3 during heat-treatment.

Modulation of 1.5 μm dislocation-related luminescence emitted from a direct silicon bonded interface by external bias

Interfacial dislocation networks were formed in silicon by direct bonding technology. Cathodoluminescence and deep level transient spectroscopy measurements verified that the D1 luminescence at 1.5 μm is associated with carrier transitions via a dislocation-related deep level at 0.35 eV. Both the experiment and theoretical calculations demonstrate that application of an external bias voltage on the bonding interface increases the majority carrier density at this deep level, thereby enhancing the local dislocation-related luminescence. However, beyond a critical voltage, corresponding to saturation of the majority carrier occupancy, the luminescence intensity decreases, due to the reduction in minority carrier density.

Radiation-induced luminescence of PET and PEN films under MeV ion and pulsed UV laser irradiation

Ion- and photo-induced luminescence of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films was investigated during irradiation by MeV H and He ions and an ultraviolet pulsed laser. At the beginning of ion irradiation, the PEN film emitted blue luminescence, whose intensity was an order of magnitude higher than that emitted by the PET film. Successive ion irradiation effectively reduced the luminescence centers, and the rate of decease in luminescence intensity depended on the energy deposited along the trajectory of the ions. Optical absorption measurements in the infrared region revealed an irradiation-sensitive feature of the PEN film. Moreover, a photo-induced band grew remarkably at 470 nm in the PET film under 266 nm pulsed laser irradiation, while the PEN film showed a moderate decrease in luminescence intensity at 440 nm.

Reduced bleaching in organic nanofibers by bilayer polymer/oxide coating

Para-hexaphenylene (p-6P) molecules exhibit a characteristic photoinduced reaction (bleaching) resulting in a decrease in luminescence intensity upon UV light exposure, which could render the technological use of the nanofibers problematic. In order to investigate the photoinduced reaction in nanofibers, optical bleaching experiments have been performed by irradiating both pristine and coated nanofibers with UV light. Oxide coating materials (SiOx and Al2O3) were applied onto p-6P nanofibers. These treatments caused a reduction in the bleaching reaction but in addition, the nanofiber luminescence spectrum was significantly altered. It was observed that some polymer coatings [a statistical copolymer of tetrafluoroethylene and 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole, P(TFE-PDD), and poly(methyl methacrylate), PMMA] do not interfere with the luminescence spectrum from the p-6P but are not effective in stopping the bleaching. Bilayer coatings with first a polymer material, which should work as a protection layer to avoid modifications of the p-6P luminescence spectrum, and second an oxide layer used as oxygen blocker were tested and it was found that a particular bilayer polymer/oxide combination results in a significant reduction in bleaching without affecting significantly the emission spectrum from the nanofibers.

Preparation and Characterization of Chromium(III)-Activated Yttrium Aluminum Borate: A New Thermographic Phosphor for Optical Sensing and Imaging at Ambient Temperatures.

A new thermographic phosphor based on chromium(III)-doped yttrium aluminum borate (YAB) is obtained as single crystals by high temperature flux growth and as a microcrystalline powder via solution combustion synthesis. The phosphor is excitable both in the blue (λmax 422 nm) and in the red part of the spectrum (λmax 600 nm) and shows bright NIR emission. The brightness of the phosphor is comparable to that of a well-known lamp phosphor Mn(IV)-doped magnesium fluorogermanate. At ambient temperatures, the Cr(III)-doped YAB shows high temperature dependence of the luminescence decay time, which approaches 1% per deg. The material shows no decrease in luminescence intensity at higher temperatures. The new phosphor is particularly promising for applications in temperature-compensated optical chemosensors (including those based on NIR-emitting indicators) and in pressure-sensitive paints.

Urea type of fluorescent organic nanoparticles with high specificity for HCO3− anions

A novel fluorescent HCO3− probe based on urea–thiadiazole–benzene (UTB) fluorescent organic nanoparticles (FONs) were designed and synthesized. UTB FONs exhibit excellent water solubility, high selectivity for sensing upon binding HCO3− in aqueous media. Under the optimum conditions, the relative fluorescent intensity is decreased linearly with increasing bicarbonate concentration in the range of 0–1000μM with a detection limit of 5.57×10−7M. It is found that the decreased luminescence intensity of the UTB FONs in a concentration of HCO3− ions dependence is best described by a Stern–Volmer equation. The possible mechanism is discussed.

Luminescence Behaviors of Ce3+ Ions in Chalcohalide Glasses

Luminescence behaviors of Ce3+ ions in GeS2–Ga2S3–CsCl chalcohalide glasses are reported for the first time. The dependence of Ce3+ concentration and host composition on absorption and photoluminescence spectra are investigated. Results show that these glasses exhibit broadband emission from 470 to 600 nm under a blue excitation. Luminescence intensity is enhanced with the increasing of Ce3+ ions and reaches the maximum at 0.08 mol%. The decrease of CsCl content results in the red shift of absorption cut‐off edge and emission band, and decreased luminescence intensity. The mechanism for these phenomena is discussed with the theory of optical basicity.

Nanophosphor aluminum oxide: Luminescence response of a potential dosimetric material

This work reports on the investigation of the radiation dosimetry properties of Al2O3 nanopowders. Samples were produced by solution combustion synthesis using three different organic fuels to check for the effect of synthesis conditions on the properties of interest. Luminescence characteristics were studied by thermoluminescence and optically stimulated luminescence (OSL) techniques. We found that samples produced using urea have characteristics similar to bulk Al2O3:C and may be suitable for personal dosimetry, while samples produced using glycine and hexamethylenetetramine (HMT) may be more suitable for applications where fast OSL decay is advantageous. While these results are promising and warrant further investigation, much has to be done to overcome the greatly decreased luminescence intensity of the nanomaterials as compared to bulk Al2O3:C.

Effects of gamma radiation on the photoluminescence properties of polycarbonate matrices doped with terbium complex

Luminescent films containing terbium complex [Tb(acac) 3(H 2O) 3] (acac=acetylacetonate) doped into a polycarbonate (PC) matrix were prepared and irradiated at low-dose gamma radiation with ratio of 5 and 10 kGy. The PC polymer was doped with 5% (w/w) of the Tb 3+ complex. The thermal behavior was investigated by utilization of differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). Changes in thermal stability due to the addition of doping agent into the polycarbonate matrix. Based on the emission spectra of PC:5% Tb(acac) 3 film were observed the characteristic bands arising from the 5D 4→ 7F J transitions of Tb 3+ ion ( J=0–6), indicating the ability to obtain the luminescent films. Doped samples irradiated at low dose of gamma irradiation showed a decrease in luminescence intensity with increasing of the dose.

The special features of the luminescence and light resistance of europium complexes and their mixtures with lanthanum complexes in polymethyl methacrylate

The kinetics of luminescence and transformation of short-lived products of the photolysis of europium and lanthanum complexes with thenoyltrifluoroacetone and 1,10-phenanthroline and their mixtures in polymethyl methacrylate films was studied by the nanosecond laser photolysis method with recording both light emission and absorption. Fast (535 and 585 nm, 5 D 1 → 7 F 0, 7 F 3, decay time 0.7 μs) and slow (613 nm, 5 D 0 → 7 F 2, luminescence rise and decay times 0.7 μs and 0.5 ms, respectively) luminescence was studied. Induced absorption with a maximum at 600 nm and decay time ∼3 ms was observed; this absorption was assigned to triplet states of the deprotonated form of thenoyltrifluoroacetone. The dependences of luminescence intensity on the concentration of the components in a mixture of complexes were analyzed, and synergistic effects of luminescence strengthening were estimated. The kinetics of a decrease in luminescence intensity during photolysis was studied. Possible mechanisms of a decrease in the relative initial process rate and an increase in the quasi-stationary value of relative luminescence intensity as the concentration of complexes in the polymer increased were discussed.

Synthesis and Spectral Studies of CdTe-Dendrimer Conjugates.

In order to couple high cellular uptake and target specificity of dendrimer molecule with excellent optical properties of semiconductor nanoparticles, the interaction of cysteine-capped CdTe quantum dots with dendrimer was investigated through spectroscopic techniques. NH2-terminated dendrimer molecule quenched the photoluminescence of CdTe quantum dots. The binding constants and binding capacity were calculated, and the nature of binding was found to be noncovalent. Significant decrease in luminescence intensity of CdTe quantum dots owing to noncovalent binding with dendrimer limits further utilization of these nanoassemblies. Hence, an attempt is made, for the first time, to synthesize stable, highly luminescent, covalently linked CdTe–Dendrimer conjugate in aqueous medium using glutaric dialdehyde (G) linker. Conjugate has been characterized through Fourier transform infrared spectroscopy and transmission electron microscopy. In this strategy, photoluminescence quantum efficiency of CdTe quantum dots with narrow emission bandwidths remained unaffected after formation of the conjugate.

Photoinduced phase transition in strongly correlated TTTA crystals probed with two-photon luminescence

Two-photon processes are used for studying photoinduced phase transition (PIPT) from diamagnetic low-temperature phase to paramagnetic high-temperature phase in strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystals. We have measured the Raman and luminescence spectra in the low-temperature phase after two-photon excitation with various excitation photon densities. After correction of re-absorption of the luminescence, the shape of the two-photon luminescence spectrum is in good agreement with that of the one-photon luminescence spectrum, which strongly suggests that the initial states of the two-photon luminescence are the same as those of the one-photon luminescence and are self-trapped excitons (STEs). As increasing the excitation photon density, PIPT to the high-temperature phase takes place with a threshold behavior as a sudden decrease of the two-photon luminescence intensity. This result suggests that STEs are the precursors of the PIPT and the two-photon luminescence can be a useful probe of whole PIPT dynamics from the microscopic lattice distortion to the macroscopic phase transition.

A STUDY ON THE INDIUM-INDUCED VARIATIONS IN PHOTOLUMINESCENCE PROPERTIES OF INDIUM-DOPED ZINCOXIDE NANORODS

In doped ZnO nanorods ( ZnO : In NRs) were grown on Si (111) substrates by using AuGe as a catalyst. In-concentration was controlled as high as 0.33 at.%. The length L to diameter D relationship of NRs revealed that the growth mechanism should be explained in terms of not a simple vapor–liquid–solid (VLS) mechanism but a diffusion-induced growth model. In-induced changes have been investigated from the viewpoint of photoluminescence (PL) property. At room temperature, an increase of extrinsic carrier concentration and interaction with excitons were estimated from the blueshift of peak position and decrease of luminescence intensity. Also, narrow linewidth of low-temperature PL spectra characterized the high structural quality of ZnO : In NRs. We have assigned several PL lines and confirmed the assignment by using excitation power and temperature dependence PL. Considerable evidence of crystal quality degradation was not observed from both measurements; however, we could observe that a defect and donor-bound-exciton complex dominate the PL spectrum. ZnO : In NRs shows good optical quality within our experimental range, and this result strongly supports the feasibility of In -doping to enhance the electrical property of ZnO -based nanostructure.

Photoluminescence Up-Conversion of Bioconjugated Hybrids on CdTe and Au Nanoparticles

Semiconductor nanomaterials have attracted considerable attention in the design of high efficiency PL up-conversion in heterojunctions or nanostructures at extremely low continuous wave (cw)-excitation intensity. In this study, bioconjugated hybrids were constructed using CdTe and Au nanoparticles (NPs), where two-fold PL enhancement was observed in the solution state. These results are in accordance with theoretical predictions of the local-field effects associated with the combined influence of strong localization of the collective plasmon modes in metallic-semiconducting hybrids and multi-photon absorption into its localized plasmon modes. The feasibility of the nanohybrids as sensors was demonstrated by breaking the bioconjugation through thermal stress, which induced a rapid decrease in luminescence intensity. It is believed that the phenomena is applicable to high-compacted optoelectronic devices and sensing systems that take advantage of both quantum confinement effects and nonlinear optical properties.