Red-shift Of Emission Wavelength Research Articles

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.

Wavelength Red-Shift of Long Wavelength InGaN/GaN Multi-Quantum Well by Using an InGaN Underlying Layer

Long-wavelength GaN based light-emitting diodes are of importance in full color displays, monolithic white light- emitting diodes and solid-state lighting, etc. However, their epitaxial growth faces great challenges because high indium (In) compositions of InGaN are difficult to grow. In order to enhance In incorporation and lengthen the emission wavelength of a InGaN/GaN multi-quantum well (MQW), we insert an InGaN underlying layer underneath the MQW. InGaN/GaN MQWs with various InGaN underlying layers, such as graded InyGa1−yN material with linearly increasing In content, or InyGa1−yN with fixed In content but different thicknesses, are grown by metal-organic chemical vapor deposition. Experimental results demonstrate the enhancement of In incorporation and the emission wavelength redshift by the insertion of an InGaN underlying layer.

Preparation and luminescence properties of Eu2+-doped CaSi2O2-dN2+2/3d phosphors

Eu2+-doped CaSi2O2-dN2+2/3d phosphors for white LED lamps were prepared by solid-state reaction, and the effects of heat-treatment conditions and the overall composition of host lattice on the optical properties have been discussed. Eu2+-doped CaSi2O2-dN2+2/3d displayed a single broad emission band peak at 540 nm, which could be assigned to the allowed transition of Eu2+ from the lowest crystal field component of 4f65d to 4f7 ground-state level. The excitation band of samples, extending from UV to blue, is extremely wide, so the phosphors are suitable for white LED lamps in combination with a UV or blue LED dies. The highest PL intensity is found for the sample sintered at 1400 °C. Moreover, the emission intensity decreases when N partially replaces O. A red shift of emission wavelength did not occur with increasing of the N content.

Design and epitaxy of structural III-nitrides

We designed Mg-doped InGaN/GaN quantum well (QW), strained ultrathin InN/GaN QWs, and Mg and Si δ-doped AlGaN/GaN superlattices (SLs) by using first-principles simulations. The designed structures were grown by metal–organic vapor phase epitaxy (MOVPE) on high-quality thick GaN using an interruption technique. The injection-current-dependent electroluminescence characteristics of Mg-doped QW exhibit smaller injection-current-induced blueshift and energy separation, which indicate that modification of quantized levels has occurred in Mg-doped QW due to the reduction of polarization field. The cathodoluminescence intensity of the strained ultrathin InN/GaN QWs is enhanced with increasing barrier thickness due to the suppression of interwell coupling, while the emission wavelength redshifts significantly as the well width increases due to reduction of strain in the QW. Higher hole concentration and mobility are achieved in Mg and Si δ-doped p-type SLs compared to those in modulation-doped SL. These results are in excellent agreement with those of theoretical designs.

Controllable Fabrication and Cathodoluminescence Performance of High-index Facets GeO2 Micro- and Nanocubes and Spindles upon Electrical-field-assisted Laser Ablation in Liquid

We have developed a unique technique, the facile electrical-field-assisted laser ablation in liquid (EFLAL) without any catalyst or organic additives, to controllably fabricate the mass production of GeO2 micro- and nanoparticles with various shapes. By adjusting the applied electrical field, we synthesized the high-index facets GeO2 micro- and nanocubes and spindles, and propose the growth mechanisms of nanostructures upon EFLAL. On the basis of the cathodoluminescence measurements of dispersive GeO2 nanoparticles, we observed a shape-dependent red-shift of emission wavelength when the shape of GeO2 nanostructures transforms into spindle from cube, and then we established the physical model to address the anomalous red-shift of emission wavelength. Accordingly, we expecte EFLAL to be a general route to synthesize the micro- and nanostructures with metastable structures or metastable shapes.

Synthesis of Star Polymer Architectures with Site-Isolated Chromophore Cores

We report the synthesis of star polymers with site-isolated chromophores obtained by nitroxide-mediated polymerization and reversible addition−fragmentation chain transfer techniques through the “arm first” method. Linear polymer precursors such as α-alkoxyamine-terminated polystyrene (PS) and thioester-terminated poly(acrylic acid) (PAA) were prepared followed by the addition of fluorescent divinyl cross-linkers derived from the fluorene and thiophene families. We first synthesized organic soluble star polymers containing hexyl-functionalized fluorene and thiophene cross-linkers with PS. Second, ethylene oxide (EO)-functionalized fluorene cross-linkers were incorporated into PAA linear precursors to give water-soluble star polymers with site-isolated chromophore core units. The photoluminescence increased significantly while the emitting wavelength corresponded with highly dilute, nonconjugated chromophores in solution. This effect is indicative for a highly localized concentration of chomophores in the star polymer cores that are covalently connected but do not show the typical effects of concentrated monomers in solution such as aggregation, fluorescence quenching, and a red shift of emission wavelength. The site isolation of chomophores in star polymers leads to nanostructures with highly photoluminescent core units while the emanating linear polymers are available for further functionalizations.

Continuous-Wave Operation of GaN Based Multi-Quantum-Well Laser Diode at Room Temperature

Room-temperature operation of cw GaN based multi-quantum-well laser diodes (LDs) is demonstrated. The LD structure is grown on a sapphire (0001) substrate by metalorganic chemical vapour deposition. A 2.5μm × 800μm ridge waveguide structure is fabricated. The electrical and optical characteristics of the laser diode under direct current injection at room temperature are investigated. The threshold current and voltage of the LD under cw operation are 110 mA and 10.5 V, respectively. Thermal induced series resistance decrease and emission wavelength red-shift are observed as the injection current is increased. The full width at half maximum for the parallel and perpendicular far field pattern (FFP) are 12° and 32°, respectively.

Effects of methyl substitution of metal (II) bis(2-(2-hydroxyphenyl) benzothiazolate) chelates on optical properties

The influence of methyl substitution on three-position of metal (II) bis(2-(hydroxyphenyl)benzothiazolate) on thermal stability, photophysical, electronic structure, and electrochemical properties was investigated. Experimental results show that methylated complexes were more thermally stable and had 10–20nm red shift in emission wavelength. Quantum chemistry calculation indicated that the methyl substitution affects mostly the highest occupied molecular orbitals charge distribution. In addition, the introduction of electro-donating methyl group can produce a decrease in the energy of the lowest unoccupied molecular orbitals.

Characteristics of dilute-nitride GaAsSbN∕InP strained multiple quantum wells

Ga As Sb N ∕ In P strained layer superlattice (SL) structures have been grown using low-temperature metal organic chemical vapor deposition with N composition varying from 0.6% to 1.6%. High-resolution x-ray diffraction measurements indicate that GaAsSbN∕InP SLs with defect-free layers and abrupt interfaces were achieved. Low-temperature photoluminescence measurements reveal that nitrogen incorporation into the GaAsSbN layers extends the emission wavelength, increases the conduction band offset, and dramatically changes the As∕Sb ratio. In parallel with the experimental efforts, simulation studies using a ten-band k∙p model are carried out to correlate the emission properties of these SL structures with experiment. Photoluminescence measurements indicate an emission wavelength redshift with respect to the simulated values.

Red-shift of emission wavelength caused by reabsorption mechanism of europium activated Ca- [formula omitted]-SiAlON ceramic phosphors

The optical properties of divalent europium activated Ca- α -SiAlON were investigated and the excitation and emission processes were discussed. The emission wavelength is influenced by the activator concentration of Eu more than the codopant concentration of Ca, in case of the same matrix composition of α -SiAlON. The red-shift of emission increases when the concentration of Eu increases. In addition, it was revealed that the red-shift of emission would be caused by the construction of the package of optical device. The correlated color temperature becomes low by the red-shift for the white light-emitting diode (LED) lamp using Ca- α -SiAlON:Eu phosphor when the phosphor powder concentration increases in the transparent resin which coats the primary light source of a blue LED die. The additional red-shift can be observed when the excessive amount of phosphor coating is intentionally poured to the white LED lamps. These red-shifts of emission wavelength could be explained by the classical configurational coordinate model.

Luminescence properties of (Ca,Y)-α-SiAlON:Eu phosphors

(Ca,Y)-α-SiAlON:Eu phosphors for white LED lamps were prepared and their luminescence properties were investigated. A red shift of emission wavelength occurred when Ca was partially replaced by Y. Raising the sintering temperature also caused the red shift. As a result, the peak emission wavelength varied from 585 nm to 608 nm, with increasing of the Y content and the sintering temperature, and therefore the chromaticity range of (Ca,Y)-α-SiAlON phosphors extended to longer wavelength by these results. The corresponding correlated color temperature (CCT) of white LED lamps using (Ca,Y)-α-SiAlON:Eu also extended to the temperature as low as 1700 K.

The application of an InGaAs∕GaAsN strain-compensated superlattice to InAs quantum dots

Application of InGaAs∕GaAsN strain-compensated superlattice (SCSL) to InAs quantum dots (QDs) has been studied with atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and temperature-dependent photoluminescence (PL) measurements. The insertion of a tensile-strained GaAsN layer between InGaAs layers with high In concentrations can compensate the compressive strain in the InGaAs layers and reduce the flattening of QDs during the growth of the successive InGaAs layers. Compared with QDs capped with a single InGaAs layer of a high In concentration, QDs capped with such SCSLs can achieve almost the same redshift of emission wavelength, while the optical property is highly improved. The mechanism responsible for this is discussed based on the AFM, RHEED, and PL measurements.

Low-potential electrochemical polymerization of fluorene and its alkyl-polymer precursor

High quality polymer films can be easily produced by low-potential anodic oxidation of fluorene and its alkyl-polymer precursor through 9,9 position (9,9-dibromohexylfluorene, 9-(6-bromohexyl)fluorene, poly(fluorene-9,9-diyl-alt-alkan-1,6-diyl); Scheme 1) in boron trifluoride diethyl etherate (BFEE) or mixed electrolytes of BFEE + CHCl 3. As-formed polymer films showed good redox behavior and were characterized by UV–vis, infrared, fluorescence spectroscopy and thermal analysis. Fluorescence spectral studies both in solution and in solid-state revealed that as-formed polymer films were good blue-green light emitters with strong emissions at about 420 and 560 nm. The substitutions of alkyl group and alkyl-polymer fluorene precursors were helpful for the red shift of emission wavelength of as-formed polyfluorenes.

Oligomeric Fluorescent Labels for DNA

In an effort to find fluorescent labels that have large Stokes shifts and increased emission intensity, a strategy for fluorescence labeling of DNA was explored in which multiple individual fluorophores are incorporated at adjacent positions at the end of a DNA probe. To encourage close interactions, hydrocarbon and heterocycle fluorophores were substituted at C-1 of deoxyribose, replacing the DNA base. The C-glycosides studied contained the well-known fluorophores terphenyl, pyrene, and terthiophene. For comparison, a commercial fluorescein-dU nucleotide was examined. Oligomeric labels containing up to five fluorophores were tested. Interestingly, all four dyes behaved differently on multiple substitution. Fluorescein displayed strong self-quenching properties, with the quantum yield dropping severalfold with each additional substitution and with a constant, small Stokes shift. In contrast, pyrene showed increases in quantum yield on addition of more than one fluorophore and yielded efficient long-wavelength emission on multiple substitution, with Stokes shifts of >130 nm. Oligomeric terphenyl labels gave a small progressive red shift in absorption and a marked red shift in emission wavelength and showed a strong increase in brightness with more monomers. Finally, terthiophene oligomers showed self-quenching combined with increasing Stokes shifts. Overall, the results suggest that some oligomeric fluorescent labels exhibit properties not available in common fluorescein class (or other commercial) labels, such as large Stokes shifts and increasing brightness with increasing substitution.

Cu(I)(2,9-Bis(trifluoromethyl)-1,10-phenanthroline)2+ Complexes: Correlation between Solid-State Structure and Photoluminescent Properties

The 90K solid-state structures, room temperature absorption, and room temperature and 17 K emission spectra of seven different salts of [Cu(I)(bfp)(2)](+) (bfp = 2,9-bis(trifluoromethyl)-1,10-phenanthroline) have been determined. To quantify the distortion of the Cu coordination environment, a distortion parameter zeta is defined that is a combined measure of the flattening, rocking, and wagging distortions of the complex cations. In general, the distortion in the (bfp) cations is less than found previously for Cu(I)(dmp)(2) (dmp = 2,9-dimethyl-1,10-phenanthroline) salts, in particular the flattening is reduced because of the bulkier 2,9-substituents. The 17 K lifetimes range up to 1.8 mus in the series of solids examined and, with the marked exception of the BF(4)(-) salt, correlate linearly with the distortion parameter zeta. The emission wavelength red-shifts with decreasing lifetime, which implies that an increased ground-state distortion is associated with a smaller energy gap.

Thermal Crosstalk in Integrated Laser-Modulators

A monolithically integrated distributed feedback laser with an electroabsorption modulator has been investigated showing a redshift of emission wavelength with applied modulator bias voltage. This leads to an adiabatic chirp parameter /spl alpha//sub H/ of up to 0.13 that is usually zero for decoupled devices like hybrid laser modulator integrations. By measurement and corresponding simulation, the phenomenon was identified to be caused by thermal coupling between the sections.

Optical Properties of Eu2+ in α-SiAlON

New interesting α-SiAlON:Eu-based yellow oxynitride phosphors with the formula (CaxEuy)Si12-2x-3yAl2x+3yOyN16-y (x = 0.2−2.2, y = 0−0.25) have been prepared, and the effects of the activator concentration and the overall composition of the host lattice on optical properties have been investigated. It has been demonstrated that the Eu2+-doped α-SiAlON phosphors absorb strongly in the UV−visible part of the electromagnetic spectrum and exhibit an intense emission band with peaks at 583−605 nm. We have shown that the optical properties depend greatly on the Eu2+ concentration and the overall composition of the host lattice. The results indicate a concentration quenching effect at y ≥ 0.075 and a systematic red-shift in emission wavelength as the Ca2+ or the Eu2+ concentration increases.

Cathodoluminescence from Thin Film Zn2GeO4 : Mn Phosphor Grown by Pulsed Laser Deposition

Low voltage cathodoluminescent (CL) characteristics of thin-film phosphors grown by pulsed laser deposition were investigated. The effects of substrate heating (600-750°C) and substrate type (MgO, Si, and yttria-stabilized zirconia) on the cathodoluminescent properties were studied. A characteristic green emission peak at 540 nm was observed at substrate temperatures of 650, 700, or 750°C. However, the emission was red shifted to 650 nm for a substrate temperature of 600°C. The red shift in emission wavelengths from 540 to 650 nm was attributed to a change in ionic state of the activator from to While the spectral position was independent of substrate type, the relative intensities of the cathodoluminescent emission peak varied with both substrate type as well as substrate temperature. At a substrate temperature of 600°C, the crystal structure of the film was mixed polycrystalline and amorphous. However, at substrate temperatures ranging from 650-750°C, the films were only polycrystalline with varying degrees of crystallinity that correlated with the CL intensity. © 2004 The Electrochemical Society. All rights reserved.