Red-shift Of Emission Wavelength Research Articles

Ba3Si6O12N2:Eu2+ green-emitting phosphor for white light emitting diodes: Luminescent properties optimization and crystal structure analysis

The Eu2+-doped Ba3Si6O12N2 green phosphor was synthesized by high temperature solid state method (twice sintered). It could be efficiently excited by UV–blue light (280–500nm) and showed a single intense broadband emission (470–625nm). The influence of sintering times on emission spectra of prepared Ba3Si6O12N2:0.2Eu2+ was discussed. It was a useful way to improve the luminescent property of phosphors materials by twice sintered. In addition, extending soaking time was also an effective approach to improve the luminescent property of phosphors materials in conventional solid-state reaction method. The effect of the crystal structure of Ba3Si6O12N2 and the concentration of the doped-Eu2+ in Ba3Si6O12N2:Eu2+ on the photoluminescence (PL) were also investigated. The phosphors have concentration quenching effect at x=0.15 and a systematic red-shift in emission wavelength with the increase of Eu2+ concentration. The result reveals that Ba3Si6O12N2:Eu2+ is a potential green-emitting phosphor for white LEDs.

Hybrid film of silver nanowires and carbon nanotubes as a transparent conductive layer in light-emitting diodes

A hybrid film of carbon nanotubes (CNTs) and silver nanowires (AgNWs) that could be regarded as a parallel circuit of CNTs and AgNWs was developed, which exhibited a low sheet resistance of 23 Ω/sq and transmittance at 550 nm of 93%. The relatively high, intertube contact resistance of CNTs was reduced by the metallic AgNWs, which acted as bridges to aid carrier transport between CNTs. A hybrid film of CNTs and AgNWs was used as a transparent conductive layer in an AlGaInP light-emitting diode (LED). Including the hybrid film in the LED increased the optical output power by about 1.6 times and decreased the red shift of emission wavelength from 13.11 to 9.7.

A double substitution of Mg2+-Si4+/Ge4+ for Al(1)(3+)-Al(2)3+ in Ce3+-doped garnet phosphor for white LEDs.

The influence of Mg(2+)-Si(4+)/Ge(4+) incorporation into Ce(3+)-doped Y3Al5O12 garnet phosphors on the crystal structure and luminescence properties is described in this work. X-ray diffraction with Rietveld refinements, photoluminescence spectra, absolute quantum yield, thermal quenching behavior, and lifetimes were utilized to characterize samples. The introduction of Mg(2+)-Si(4+)/Ge(4+) leads to an obvious red shift of emission wavelength under the excitation of blue light, especially for the series of Mg(2+)-Si(4+) substitutions, which is suited for white light-emitting diodes (LEDs) with low color temperatures and good color rendering using only a single phosphor. More interestingly, an additional emission band locating at high-energy was observed with ultraviolet excitation, which is different than previous literature. Under the excitation of ultraviolet, the emission color for the Mg(2+)-Si(4+) substitutions can be tuned from yellow-green to blue, which is expected to obtain single-phased phosphors with white emission excited with UV-LED chip. The usual Ce(3+) emission band at low energy has stronger quenching at high temperatures. The mechanisms for the observed phenomena are discussed.

The relationship between occupied sites and Mn2+ quantum efficiency in dual-emission Mg3Ca3(PO4)4: Eu2+, Mn2+ phosphors

Abstract Dual-emitting Eu2+ and Mn2+ codoped Mg3Ca3(PO4)4 phosphors have been synthesized, and energy transfer process from Eu2+ to Mn2+ was studied. The relationship between Eu2+ → Mn2+ energy transfer efficiency (ηT) and Mn2+ quantum efficiency (QE) with different Mn2+ occupied sites were compared and discussed. The ion-size compensation effect when Eu2+/Mn2+ occupying Ca2+ sites leads to the formation of Eu2+–Mn2+ cluster, which benefits the effective Eu2+ → Mn2+ energy transfer and efficient Mn2+ sensitized emission. The emission color of the phosphor can be tuned from blue to near-white and eventually to pink by adjusting the contents of Eu2+ and Mn2+. The Mn2+ emission wavelength red-shifts with increasing Mn2+ content, which is attributed to photon reabsorption process. The results indicated that Mg3Ca3(PO4)4: Eu2+, Mn2+ may serve as a potential color-tunable phosphor for NUV LED.

Optical absorption, photoluminescence and structural analysis of CdS quantum dots in weak confinement

The diffusion-controlled growth of CdS quantum dots (QDs) dispersed in a silicate glass matrix was investigated. It was found that the size of CdS QDs can be controlled by either heat treatment at various temperatures for a fixed duration or varying times at a constant temperature. Pastel yellow colored glass samples were obtained due to the presence of CdS petite crystals. X-ray diffraction (XRD) was used for determining the average dot size which varied from 3.8 to 30 nm. The typical quantum confinement effect was clearly observed from the blue shift measured in the optical absorption edge with decreasing dot size in the absorption spectroscopy. The band gap of CdS QDs ranges from 2.41 to 2.82 eV. Measured photoluminescence (PL) at an excitation wavelength of 350 nm showed the red shift of emission wavelength with increasing thermal treatment time and temperature in agreement with the increasing dot sizes. The half-width of PL spectra seems to indicate qualitatively the size distribution of dots and is consistent with the treatment parameters.

Bovine serum albumin‐confined silver nanoclusters as fluorometric probe for detection of biothiols

Fluorescent bovine serum albumin-confined silver nanoclusters (BSA-AgNCs) were demonstrated to be a novel and environmentally friendly probe for the rapid detection of biothiols such as cysteine (Cys), homocysteine (Hcy) and glutathione (GSH). The sensing was ascribed to the strong affinity between the mercapto group of the biothiols and the silver nanoclusters. The fluorescence intensity of BSA-AgNCs was quenched efficiently on increasing the concentration of biothiol, corresponding with a red-shift in emission wavelength. However, the fluorescence of the silver nanoclusters was almost unchanged in the presence of other α-amino acids at 10-fold higher concentrations. By virtue of this specific response, a new, simple and rapid fluorescent method for detecting biothiols has been developed. The linear ranges for Cys, Hcy and GSH were 2.0 × 10(-6) to 9.0 × 10(-5) M (R(2) = 0.994), 2.0 × 10(-6) to 1.2 × 10(-4) M (R(2) = 0.996) and 1.0 × 10(-5) to 8.0 × 10(-5) M (R(2) = 0.980), respectively. The detection limits were 8.1 × 10(-7) M for Cys, 1.0 × 10(-6) M for Hcy and 1.1 × 10(-6) M for GSH. Our proposed method was successfully applied to the determination of thiols in human plasma and the recovery was 94.83-105.24%. It is potentially applicable to protein-stabilized silver nanoclusters in a chemical or biochemical sensing system.

Green synthesis of nitrogen-doped carbon dots from konjac flour with "off-on" fluorescence by Fe3+ and l-lysine for bioimaging.

Highly nitrogen-doped carbon dots (N-CDs) are prepared by the pyrolysis of konjac flour under mild conditions followed with a simple extraction by ethanol and water. The N-CDs exhibit excellent pH-switched photoluminescence (PL), and their PL intensity can be facilitated by either mixing with NaOH and basic amino acids or by surface passivation with non-amine-terminated polyethylene glycols of different molecular weights. Further, the fluorescence of N-CDs can be quenched with Fe3+ and recovered with l-lysine, accompanied with a red-shift of emission wavelength. In addition, the low toxicity and strongly fluorescent N-CDs are applied for cell imaging, and the quenched fluorescence by Fe3+ can be recovered inside the living cells.

Determination of silver ion based on the redshift of emission wavelength of quantum dots functionalized with rhodanine

A simple and selective method for the determination of silver ions was developed by utilizing the red-shift in emission wavelength of the core–shell CdSe/CdS quantum dots (QDs) functionalized with rhodanine upon the addition of Ag+. A linear relationship was observed between the shift and the increase in concentration of Ag+ in the range of 0.0125–12.5 μmol/L. The mechanism of the red-shift was investigated and suggested that the coordination between Ag+ and rhodanine on the QDs surface caused an increase of particle size, which resulted in the red-shift of the QDs’ emission wavelength. A detection limit of 2 nmol/L was achieved. The developed method showed superior selectivity and was successfully applied to the determination of silver in environmental samples.

Cathodoluminescence Studies of InGaN/GaN Multiple Quantum Well Structure Grown by Metal Organic Chemical Vapor Deposition

Scanning Electron Microscope (SEM) - Cathodoluminescence (CL) system is applied to blue emission InGaN/GaN Multiple Quantum Well (MQW) structure grown by Metal Organic Chemical Vapor Deposition (MOCVD) to study the luminescence properties of v-pit features and surface compositional variations. Emission from MQW layer and bulk n-GaN layer beneath are captured at various electron beam voltages and show that the luminescence efficiency of latter grown MQW layer may be related to that of the former grown n-GaN layer. Emission intensity variations are observed at v-pit free areas in the CL images, and it is observed that areas with higher emission intensity always show a red shift in emission wavelength. It may therefore indicate that high resolution CL system is able to image the indium inhomogeneities formed during the epitaxial growth process in nanoscale.

New Water‐Soluble Cationic Poly(p‐phenylenevinylene) Derivative: the Interaction with DNA and Selective Fluorescence Enhancement Induced by ssDNA

A new cationic cyano-substituted poly(p-phenylenevinylene) (N-CNPPV) is synthesized by Knoevenagel condensation. The water-soluble polymer shows different emission spectra in different solvents and displays unique fluorescent behaviors in the mixed solvents of water and THF. The new polymer can form a complex with ssDNA by adopting a more planar conformation, exhibiting red shift of emission wavelength and enhancement of fluorescence intensity. By investigating the fluorescent response of N-CNPPV to various surfactants, we demonstrate that the hydrophobic interaction and electrostatic interaction result in the selective response of N-CNPPV to ssDNA. This is the first report on selective fluorescence enhancement of conjugated polyelectrolyte induced by ssDNA.

Comparative Study of 1.3 and 1.5 m Light-Emitting Diodes GaAs-Based InAs/InGaAs and InAs/(Ga, In) (N, As) Self-Assembled Quantum Dots

This study presents self-assembled quantum dots structures made on GaAs substrate. The samples were grown by Molecular Beam Epitaxy (MBE) in the Stranski-Krastanow (SK) growth mode. Two types of quantum dots structures are performed under different growth conditions and are compared: the first type is composed of two structures with InAs quantum dots encapsulated with (Ga, In) (N, As) and the second one with boxes wrapped with InGaAs. The influence of encapsulation of quantum dots is highlighted and as already shown, the redshift of emission wavelength depends on nitrogen doping. Further investigations are done through the incorporation of indium and nitrogen in the quantum dots structures in order to understand their optimal doping level on electrical and optical properties. The effects of temperature, an important growth parameter, are examined through images of nanostructures obtained by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM) techniques.

Polyethyleneimine modified fluorescent carbon dots and their application in cell labeling

Fluorescent carbon dots (CDs) were solvothermaly synthesized in water–glycol medium by using glucose as carbon source and then modified with polyethyleneimine (PEI) for the first time to improve fluorescence quality. The as-prepared CDs were monodispersed sphere particles with a diameter of about 7.5nm, emitting strong fluorescence which is excitation wavelength-dependent, with a quantum yield of 3.5%. After PEI modification, there was a 300-fold enhancement in fluorescence intensity and also a red shift in emission wavelength. The as-prepared CDs were then conjugated with CEA8 antibody to label and image HeLa cells in vitro. We also tested the cytotoxicity of these CDs using HeLa cells. No apparent cytotoxicity was observed, demonstrating much better biosafety property compared with CdTe quantum dots.

Comparison of luminescence properties of bilayer and multilayer InAs/GaAs quantum dots

Coupled InAs/GaAs quantum dots have generated an interest for their longer emission wavelength and narrower line-width. However, a consensus has not been reached on the parameters of growth required to achieve a desired effect from coupling due to contradictory reports of shorter emission wavelengths. In this paper, we seek to compare the luminescence properties of bilayer quantum dots (BQDs) with those of multilayer quantum dots (MQDs), grown at a very low deposition rate, keeping all parameters constant. The BQD and MQD samples were grown by solid source MBE at a slow growth rate of 0.03 ML/s. A blueshift in the PL spectra for 11 layer coupled InAs/GaAs MQD heterostructure is observed compared to the BQDs for temperatures less than 180 K. This undesired blueshift is attributed to strain in the structure which overshadowed the usual redshift in emission wavelength in such structures due to electronic coupling. The variation in PL line-width with temperature in the MQD structure is found to be much lower than in the BQD. However the PL intensity of the MQDs fall at a faster rate with temperature compared to the BQD sample, due to strain generated non-radiative centers in the islands which favors in thermalization of carriers.

Effects of an InGaAs Cap Layer on the Optical Properties of InAs Quantum Dot Molecules

Self-assembled InAs quantum dot molecules are grown on GaAs substrates without following any special protocols by using metal-organic chemical vapor deposition. The effects of indium composition and the thickness of the InGaAs cap layer on the optical properties of InAs quantum dot molecules are investigated by photoluminescence. With increasing indium composition and thickness of the InGaAs cap layer, the ground-state wavelength of the emission spectrum redshifts and the peak intensity decreases. In addition, the structural and optical properties of quantum dots and quantum dot molecules are comparatively studied, and the results show that when quantum dots turn into quantum dot molecules, the emission wavelength red shifts.

Green Eu 2+-doped Ba 3Si 6O 12N 2 phosphor for white light-emitting diodes: Synthesis, characterization and theoretical simulation

The Eu 2+-doped Ba 3Si 6O 12N 2 green phosphor (Eu x Ba 3− x Si 6O 12N 2) was synthesized by a conventional solid state reaction method. It could be efficiently excited by UV-blue light (250–470 nm) and shows a single intense broadband emission (480–580 nm). The phosphor has a concentration quenching effect at x=0.20 and a systematic red-shift in emission wavelength with increasing Eu 2+ concentration. High quantum efficiency and suitable excitation range make it match well with the emission of near-UV LEDs or blue LEDs. First-principles calculations indicate that Ba 3Si 6O 12N 2:Eu 2+ phosphor exhibits a direct band gap, and low band energy dispersion, leading to a high luminescence intensity. The origin of the experimental absorption peaks is clearly identified based on the analysis of the density of states (DOS) and absorption spectra. The photoluminescence properties are related to the transition between 4f levels of Eu and 5d levels of both Eu and Ba atoms. The 5d energy level of Ba plays an important role in the photoluminescence of Ba 3Si 6O 12N 2:Eu 2+ phosphor. The high quantum efficiency and long-wavelength excitation are mainly attributed to the existence of Ba atoms. Our results give a new explanation of photoluminescence properties and could direct future designation of novel phosphors for white light LED.

Synthesis, Characterization and Properties of 1,4-Bis(Naphthalen-1-Ylethynyl)Benzene and Its Derivatives: Monomers of Oligomers or Polymers Based on Linear 1,4-Phenylethynyl or 1,5-Naphthylethynyl Subunits

A series of novel monomers of oligomers or polymers based on linear 1,4-phenylethynyl or 1,5-naphthylethynyl subunits, 1,4-bis(naphthalen-1-ylethynyl)benzene (8a) and its derivatives (8b–8i), has been synthesized and characterized. The UV absorption and fluorescence emission properties of these compounds were analyzed and discussed through the energy gap ( E ) between HOMO and LUMO orbitals, the orbital diagrams and the charge densities of related carbon atoms. It is evident from these results that the introduction of the electron-donating substituent (–OR) to the central phenylene ring of 8a and the introduction of electron-withdrawing substituent (–NO2) to the end of the conjugated system can efficiently improve its solubility, give rise to red-shift of maximum absorption and emission wavelength. But the introduction of electron-withdrawing substituent (–NO2) will lead dramatically decrease in fluorescence efficiency. The novel synthesized compounds 8b–8e have potential prospects as the monomers of oligomers or polymers based on linear 1,4-phenylethynyl or 1,5-naphthylethynyl subunits for their excellent solubility and optical properties.

Conformational Changes Facilitate FXI Autoactivation to FXIa

AbstractAbstract 19Coagulation factor XI (FXI) is a uniquely dimeric coagulation protein, which in its activated form (FXIa) activates FIX to FIXa. We have previously shown that the dimeric structure of FXI is essential for normal autoactivation and activation by thrombin and FXIIa, but not for the expression of enzymatic activity against FIX (Wu W, et al J. Biol. Chem. 283:18655-18664, 2008). A comparison of three separate structures of FXI/XIa from our laboratory (i.e., the crystal structure of the catalytic domain of FXIa in complex with the kunitz protease inhibitor domain of protease nexin-2; the crystal structure of full-length, dimeric FXI; and the NMR structure of the FXI A4 domain) predicts a major conformational change accompanying the conversion of FXI to FXIa. We now show that when FXI binds to the negatively charged polymer, dextran sulfate and is autoactivated to generate FXIa, changes of intrinsic fluorescence are observed, i.e, a decrease in fluorescence intensity and a red shift of emission wavelength, which also suggests that a conformational change accompanies FXI activation. To investigate the mechanism of FXI zymogen activation and the allosteric transition accompanying the conversion of FXI to FXIa, which exposes binding sites for FXIa ligands, we have carried out fluorescence resonance energy transfer (FRET) studies to characterize the conformational changes accompanying zymogen activation. Using a sensitive free thiol quantitation assay, we confirmed the presence of a single free cysteine residue (Cys11) per subunit of recombinant FXI, which was quantitatively labeled with the thiol reactive fluorescence dye IAEDANS (5-({2-[(iodoacetyl)amino]ethyl}amino)naphthalene-1-sulfonic acid). Fluorescence excitation of AEDANS-labeled FXI at 280 nm shows a prominent dansyl emission peak (∼450 nm) in addition to the Trp emission peak (∼325 nm) indicative of efficient FRET from Trp donors to the AEDANS acceptor. Controls using a C11S mutant of FXI showed ∼10-fold lower levels of AEDANS labeling, confirming that Cys11 is the predominant labeling site. Autoactivation of FXI in the presence of dextran sulfate results in a major decrease in donor emission, but has little effect on acceptor emission. This indicates that, for wild-type FXI, FRET is dominated by transfer within the A1 domain originating from Trp55, which is located at a distance of 18 Å from Cys11, far closer than any other tryptophan. The changes in Trp emission, which are similar in the presence and abence of AEDANS, allow us to follow the kinetics of zymogen activation. The S557A active-site mutant of FXI, which cannot undergo autoactivation, showed no fluorescence changes upon addition of dextran sulfate, confirming that the observed decrease in Trp fluorescence is due to formation of active FXIa enzyme. In an effort to observe specific inter-domain FRET, we prepared an AEDANS labeled W55H mutant of FXI, which eliminates the Trp donor in the A1 domain that dominates energy transfer in wild-type FXI. Our data show that autoactivation of the W55H mutant is accompanied by a significant increase in AEDANS emission that can be attributed to the movement of the labeled Cys11 (in A1) relative to Trp228 in the A3 domain of the opposite dimer subunit. In the crystal structure of FXI, the distance for this donor-acceptor pair is 29 Å (compared to a distance of 40 Å for the second closest Trp, Trp407 in the catalytic domain), making it a sensitive and specific FRET probe for monitoring changes in domain arrangement associated with enzyme activation and ligand interactions. A comparison of the FXI crystal structure with our model of FXIa showed that the distance between the active site serines (Ser557) of each catalytic triad is shortened from ∼118 Å in the zymogen to 40–75 Å in the enzyme. Since the distance between the two scissile bonds of each subunit of FXI is also ∼75 Å, we propose that during autoactivation, either the active site of each catalytic domain of FXIa is positioned to cleave the Arg369-Ile370 bond of the opposite subunit (intersubunit transactivation) or a FXIa dimer positions its two active sites adjacent to the two scissile bonds of a separate FXI dimer (intermolecular activation). These studies support a model in which the autoactivating transition from zymogen to enzyme is accompanied by the movement of each catalytic domain of the dimer to facilitate efficient autoactivation of FXI. Disclosures:No relevant conflicts of interest to declare.

Structural and optical properties of self-assembled InAs quantum dot molecules on GaAs substrates

Self-assembled laterally aligned InAs quantum dot molecules (QDMs) were grown on GaAs substrates by metal-organic chemical vapor deposition. The effects of growth temperature and InAs coverage on the structural and optical properties of QDMs are investigated by using atomic force microscopy and photoluminescence. It is found that through appropriately selecting growth parameters, QDMs composed of two closely spaced InAs quantum dots (QDs) are formed, and the distance between the two QDs can be controlled. Moreover, a redshift of emission wavelength attributed to the formation of QDMs is observed.

Phosphor-Free Apple-White LEDs with Embedded Indium-Rich Nanostructures Grown on Strain Relaxed Nano-epitaxy GaN

Phosphor-free apple-white light emitting diodes have been fabricated using a dual stacked InGaN/GaN multiple quantum wells comprising of a lower set of long wavelength emitting indium-rich nanostructures incorporated in multiple quantum wells with an upper set of cyan-green emitting multiple quantum wells. The light-emitting diodes were grown on nano-epitaxially lateral overgrown GaN template formed by regrowth of GaN over SiO2 film patterned with an anodic aluminum oxide mask with holes of 125 nm diameter and a period of 250 nm. The growth of InGaN/GaN multiple quantum wells on these stress relaxed low defect density templates improves the internal quantum efficiency by 15% for the cyan-green multiple quantum wells. Higher emission intensity with redshift in the PL peak emission wavelength is obtained for the indium-rich nanostructures incorporated in multiple quantum wells. The quantum wells grown on the nano-epitaxially lateral overgrown GaN has a weaker piezoelectric field and hence shows a minimal peak shift with application of higher injection current. An enhancement of external quantum efficiency is achieved for the apple-white light emitting diodes grown on the nano-epitaxially lateral overgrown GaN template based on the light -output power measurement. The improvement in light extraction efficiency, ηextraction, was found to be 34% for the cyan-green emission peak and 15% from the broad long wavelength emission with optimized lattice period.

A novel strategy for selective detection of Ag+ based on the red-shift of emission wavelength of quantum dots

Selective determination of Ag(I) ion was accomplished based on the red-shift of the emission band of quantum dots (QDs). Under optimal conditions, a linear relationship does exist between the red-shift of the emission and the concentration of Ag(I) in the range from 1.0 × 10−7 to 1.5 × 10−5 mol L−1, with a detection limit of 5.0 × 10−8 mol L−1. The method has been successfully applied to the determination of Ag(I) ion in water samples. The possible reaction mechanism was investigated by ultraviolet–visible absorption, fluorescence, Raman spectroscopies and by high resolution transmission electron microscopy. The results suggest that the red-shift in emission be attributed to the stabilization of a charge-transfer state, but not due to the aggregation induced by AgI(I) ion.