Long-wavelength Peak Research Articles

The investigation on strain relaxation and double peaks in photoluminescence of InGaN/GaN MQW layers

Two emission peaks were observed in the low temperature photoluminescence (LTPL) spectra of an InGaN/GaN multiple quantum well (MQW) structure before and after nanopillar fabrication. After nanopillar fabrication it is found that among the two peaks the longer wavelength peak exhibits a clear blue shift and has a much stronger enhancement in LTPL intensity than the shorter one. Combined with x-ray diffraction and spatially resolved cathodoluminescence analyses, the difference induced by nanopillar fabrication is ascribed to different strain relaxation states in the lower and upper quantum well layers. It is found that the lower QW layers of the as-grown MQW which causes the longer wavelength PL peak are more strained, while the upper ones are almost fully strain-relaxed. Therefore, the nanopillar fabrication induces much less strain relaxation in the upper part of the MQW than in the lower one.

FIRST SPECTROSCOPIC IDENTIFICATION OF MASSIVE YOUNG STELLAR OBJECTS IN THE GALACTIC CENTER

We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas-phase absorption from CO2 (15.0um), C2H2 (13.7um) and HCN (14.0um). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15um CO2 ice absorption features, with a remarkable double-peaked structure. The prominent long-wavelength peak is due to CH3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center.

Synthesis, structures, and photochromic properties of 2-methylthieno[3,2-b][1]benzothiophen-3-ylfulgide

The novel heterocyclic fulgide, 3-(1-methylethylidene)-4-[1-(2-methylthieno[3,2-b]-[1]benzothiophen-3-yl)ethylidene]dihydrofuran-2,5-dione, was obtained and isolated as a Z-isomer. Its structure was confirmed by electronic absorption spectroscopy, 1H NMR spectroscopy, and X-ray diffraction analysis. The novel thienothiophene fulgide is photochromic in both solutions and the crystalline state. Its colored cyclic form resists photodegradation and is thermally stable. When benzothiophene is annulated with the thienyl fragment, the long-wavelength absorption peak of the cyclic isomer of the novel fulgide experiences a bathochromic shift compared to the earlier described 3-thienylfulgide. The TD B3LYP/6-311G**-calculated spectroscopic characteristics of the fulgide isomers suggest the formation of their photostationary mixture under irradiation with λ = 365 nm.

Rational routes to formyl-substituted chlorins.

Two distinct approaches have been developed for the synthesis of chlorins bearing formyl groups: (1) reaction of an acetal-substituted 1-acyldipyrromethane with 2,3,5,6-tetrahydro-1,3,3-trimethyldipyrrin to give upon hydrolysis a 5-formylchlorin and (2) Pd-mediated coupling of a bromochlorin with a one-carbon synthon (hydroxymethyl tributyltin or CO) to give a 13-, 15-, or 3,13-formylchlorin. The zinc chlorins exhibit long-wavelength peak absorption maxima ranging from 626 to 667 nm, indicating the wavelength tunability afforded by formyl substitution.

Ellipsoidal Core–Shell Dielectric-Gold Nanostructure: Theoretical Study of the Tunable Surface Plasmon Resonance

The dependence of surface plasmon resonance (SPR) characters of core-shell structure ellipsoidal nanorods was investigated as functions of the gold shell width and dielectric constant. A dipole-limit calculation indicated that, there are four SPR absorption peaks occur. The two shorter wavelength peaks are attributed to the transverse resonances, whereas the two longer wavelength peaks are attributed to the longitudinal resonances. Both transverse and longitudinal resonances are split into inner surface and outer surface modes, respectively. When the core size is a fixed value, the two transverse surface plasmon peaks red shift as the gold shell width is increased. On the other hand, both the inner and outer surface modes attributed to the longitudinal surface plasmon peaks blue shifts distinctly and nonlinearly. Further more, both the inner and outer surface plasmon modes are also dependent on the embedding medium and core dielectric constant. The longitudinal SPR from inner surface is more sensitive to the core dielectric constant, whereas the longitudinal SPR from outer surface is more sensitive to the surrounding medium.

Deep Spitzer Spectroscopy of the "Flying Saucer" Edge-on Disk: Large Grains beyond 50 AU

We present deep Spitzer IRS low-resolution (λ/Δλ ~ 100) 5-35 μm spectroscopy of the edge-on disk the Flying Saucer (2MASS J16281370-2431391) in the Ophiuchus molecular cloud. The SED exhibits the characteristic two-peak shape predicted for a circumstellar disk viewed very close to edge-on. The short-wavelength peak is entirely due to photons scattered off the surface of the disk, while the long-wavelength peak is due to thermal emission from the disk itself. The Spitzer spectrum represents the first spectroscopic detection of scattered light out to 15 μm from a bona fide, isolated edge-on disk around a T Tauri star. The depth and the wavelength of the mid-infrared valley of the SED give direct constraints on the size distribution of large grains in the disk. Using a 2D continuum radiative transfer model, we find that a significant amount of 5-10 μm-sized grains is required in the surface layers of the disk at radii of 50-300 AU. The detection of relatively large grains in the upper layers implies that vertical mixing is effective, since grain growth models predict that the grains would otherwise settle deep in the disk on short timescales. Additionally, we tentatively detect the 9.66 μm S(3) line of H_2 and the 11.2 μm emission feature due to PAHs.

MBE growth and optical properties of highly tensile-strained In 1−xGa xAs/In 0.52(Ga 0.4Al 0.6) 0.48As multi-quantum-wells using digital alloy

Highly tensile-strained (TS) InGaAs/lattice-matched (LM) InGaAlAs MQWs for 1.3 μm emission wavelength were grown by MBE and their properties were characterized by photoluminescence (PL) measurements and cross-sectional transmission electron microscopy (TEM). The energy band of the TS-InGaAs/LM-InGaAlAs MQW was theoretically calculated using a 6×6 Luttinger-Kohn Hamiltonian. The tensile strains for the wells were varied from 1.0% to 1.5%, while lattice match was used for barriers. The wells and barriers have well-defined interfaces for TS-InGaAs/LM-InGaAlAs MQWs with a tensile strain ( ε) of 1.0% and 1.25%, respectively. However, significant non-planarity between wells and barriers was observed for TS-InGaAs/LM-InGaAlAs MQWs with a tensile strain of 1.5%. For ε=1.0%, 1.25%, and 1.5%, two peaks were observed in each PL spectrum. The longer wavelength peak is attributed to an electron–light hole (E1–LH1) transition while the shorter wavelength peak to an electron–heavy hole (E1–HH1) transition. While the E1–HH1 transition is dominant at ε=1.0%, the E1–LH1 transition is dominant at ε=1.25% and 1.5%. The E1–LH1 transition was clearly observed with increasing well number. The total PL intensity increased as the well number increased from 1 to 4 QWs. However, the total PL intensity decreased with 5 QWs. Therefore, the maximum well number is limited to 4, constituting a compromise between well number and strain relaxation.

Influence of dielectric core and embedding medium on the local field enhancement for gold nanoshells

The dependence of the local field enhancement in gold nanoshells was investigated as a function of the core and embedding medium dielectric constant. Numerical calculations based on quasistatic theory indicated two local field factor peaks. If the core dielectric constant is a fixed value and the dielectric constant of surrounding medium is increased from 1.0 to 6.0, the shorter wavelength peak redshifts and decreases nonlinearly, whereas the longer wavelength peak redshifts, increases first and then decreases. On the contrary, if the surrounding dielectric constant is a fixed value and the core dielectric constant is increased from 1.0 to 6.0, the shorter wavelength peak redshifts and increases nonlinearly, whereas the longer wavelength peak redshifts and decreases nonlinearly.

New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2,7-Bis(diphenylphosphine oxide)-9,9-dimethylfluorene

We report the synthesis, crystal structure, and photophysical and electroluminescent properties of a new charge transporting host material for short wavelength phosphor-doped organic light emitting devices (OLEDs) based on 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene (PO6). The PO moiety is used as a point of saturation between the fluorene bridge and the outer phenyl groups so that the triplet exciton energy of PO6 is 2.72 eV, similar to that of a dibromo substituted fluorene, but it is more amenable to vacuum sublimation and has good film forming properties. Computational analysis (B3LYP/6-31G*) predicts the highest occupied molecular orbital and lowest unoccupied molecular orbital energies of PO6 to be lower by 1.5 and 0.59 eV, respectively, compared to a similar diphenylamino substituted derivative. In a simple bilayer OLED device, PO6 exhibits structured UV electroluminescence at a peak wavelength of 335 nm and structured lower energy emission with peaks at 380 and 397 nm, similar to the solid film and crystalline solid photoluminescence spectra. The longer wavelength peaks are attributed to aggregate formation via strong intermolecular interactions (PO···HC and edge-to-face CH···π contacts) and longer range electrostatic interactions between PO moieties leading to ordered regions in the film. Devices incorporating PO6 as the host material doped with iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) exhibited sky blue emission with peak external quantum efficiency (ηext,max) of 8.1% and luminous power efficiency (ηp,max) of 25.1 lm/W. At a brightness of 800 cd/m2, generally considered to be sufficient for lighting applications, the ηext and ηp are 6.7% and 11.8 lm/W and the operating voltage is 5.6 V, which is significantly lower than has been demonstrated previously using this dopant.

Single molecule sensitivity in SERS: importance of junction of adjacent Ag nanoparticles

An enormous SERS (Surface Enhanced Raman Scattering) signal from dye and adenine on hot or blinking silver nanoparticles is extinguished by duration of measurement possibly due to thermal diffusion or desorption of adsorbed molecules. Simultaneously, the elastic scattering peak at ca. 630 nm disappears. The Three-dimensional Finite Difference Time Domain (FDTD-3D) method manifests this scattering peak as originating from enhanced coupling of localized surface plasmon (LSP) on adjacent Ag particles through absorption of adspecies located at their junction. Distinct emission peaks were observed at 550-600 nm and 600-750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were attributed to excited electrons on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths, indicating it arises from inelastic scattering of excited electron by surface roughness or by adsorbed molecules.

Simulation of the surrounding medium controlled local field enhancement for silver nanorods

The dependence of the local field enhancement (LFE) in silver nanorods was investigated as a function of the surrounding medium dielectric constant. Numerical calculations based on the Drude model and quasi-static theory indicated that, because of the morphological anisotropy of the silver nanorods, there are two peaks of local field factor (LFF) take place at ultraviolet and visible regions respectively. With increasing the dielectric constant of surrounding medium from 1.0 to 4.0, the shorter wavelength peak red shift linearly and increase nonlinearly, whereas the longer wavelength peak red shift nonlinearly and decrease nonlinearly. Further more, with increasing the dielectric constant of surrounding medium, the full width at half maximum (FWHM) of longer wavelength LFF peak increase obviously, whereas the FWHM of shorter wavelength peak remains unchanged ultimately.

High-Performance Narrow-Bandwidth Multicolor InAs/AlGaAs/GaAs Quantum Dot Infrared Photodetector

By introducing a 2 nm Al0.3Ga0.7As cap layer on InAs/GaAs quantum dots, a high-performance narrow-bandwidth multicolor quantum dot infrared photodetector (QDIP) with peak responses at 5.4, 9.5, and 15.8 µm was fabricated successfully. The highest peak responsivities reached 0.79, 0.47, and 0.63 A/W at a bias of -1.2 V. The two shorter-wavelength responses originating from two different-sized InAs quantum dots were due to the transitions of InAs quantum dots from the ground state to the GaAs conduction band. The separation of the two peaks was amplified by the insertion of the 2 nm AlGaAs cap layer. The longer-wavelength peak at 15.8 µm was due to the transition from the ground state to the first excited state followed by the tunneling out of small quantum dots. The increase in photocarrier lifetime by the barrier is proposed to explain the very high responsivity.

Inelastic scattering and emission correlated with enormous SERS of dye adsorbed on Ag nanoparticles

With respect to the enormous enhancement in SERS of dye molecules adsorbed on hot Ag nanoparticles, two distinct inelastic and emission peaks were observed at 550–600 and 600–750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were originated from excited electron on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths, indicating it arises from inelastic scattering of excited electron by surface roughness or by adsorbed molecules.

Effect of well width on three-color quantum dots-in-a-well infrared detectors

Three-color InAs-InGaAs quantum dots-in-a-well (DWELL) detectors having different well sizes are presented. Three DWELL detectors (1388, 1373, and 1299) with different quantum well (QW) widths (120, 110, and 90/spl Aring/, respectively) have been characterized showing response peaks at three distinct wavelengths. The detector 1388 has peak wavelengths at /spl sim/6.25, /spl sim/10.5, and /spl sim/23.3 /spl mu/m. The two peaks at 6.25 and 10.5 /spl mu/m are believed to be due to bound-to-bound transitions from the ground state in the dot to a state in the well, whereas the longer wavelength peak (/spl sim/23.3 /spl mu/m) which has a detectivity of 7.9/spl times/10/sup 10/ cm/spl middot//spl radic/Hz/W at 4.6 K under -2.2-V bias is due to an intersubband transition between the dot levels. The operating wavelength of these detectors in the short wavelength region can be tailored by changing the width of the QW. Spectral responsivity curves of 1373 and 1299 show a shift of the short wavelength peaks toward decreasing wavelength while the long wavelength peak remains around /spl sim/23.3 /spl mu/m confirming that the particular transition is due to the quantum dot.

Resonance light scattering properties of Eu 3+ in gold colloid

Gold colloidal containing rare-earth ions Eu 3+ were prepared at room temperature. Fluorescence spectra and resonance light scattering (RLS) spectra of Eu 3+ ions and gold colloid containing Eu 3+ were measured. For solution containing Eu 3+, RLS features show two peaks at the edges of the visible light wavelength region. The short wavelength peak takes place at about 400 nm and the longer wavelength peak is the corresponding 1/2 fraction frequency RLS peak, which takes place at about 780 nm. When gold colloids were added to the solution containing Eu 3+, both these two RLS peaks were enhanced. We believe that the energies, which are absorbed by the surface plasmon resonance in the gold nanoparticles, are efficiently transferred into the Eu 3+ ions to cause the increased scattering.

Fluorescence excitation-emission matrix characterization of a commercial humic acid

Excitation-emission matrix fluorescence spectroscopy (EEM) has been widely used to elucidate the origin and structure of humic substances in natural environments. Due to its high sensitivity, good selectivity and non-destructive advantage, the EEM was applied to characterizing a commercial Fluka humic acid (FHA). The results showed that the EEMs of FHA has several Ex/Em peaks. Ionic strength (0-0.05mol/L KClO4) exerted little effect on the fluorescence properties of FHA, while the concentrations (5-100mg/L) of FHA and pH (2-12) had significant effects. A red shift in the longer wavelength peak region was observed when the concentrations or pH values increased. The fluorescence intensity increased with increasing pH, but slightly decreased in the case of pH=5.0. The protonation constants (lgK'(subscript HL)) of peak B were calculated to be 3.57 and 3.13, indicating that peak B was due to carboxyl groups. The r(superscript (A/B)) values range from 0.61 to 2.59. A strong linear relationship between r(superscript (A/B)) and pH was also observed. This indicates that the fluorescence peaks A and B posses similar inherent fluorescence characteristics.

Classification of edible oils using synchronous scanning fluorescence spectroscopy

Total luminescence and synchronous scanning fluorescence spectroscopy techniques were tested as regards their ability to characterize and differentiate edible oils, including soybean, sunflower, rapeseed, peanut, olive, grapeseed, linseed and corn oils. Total luminescence spectra of all oils studied as n-hexane solutions exhibit an intense peak, which appears at 290 nm in excitation and 320 nm in emission, attributed to tocopherols. Some of the oils exhibit a second long-wavelength peak, appearing at 405 nm in excitation and 670 nm in emission, belonging to pigments of the chlorophyll group. Additional bands were present in the intermediate range of excitation and emission wavelengths in some oils, arising from unidentified compounds. Similarly, bands attributed to tocopherols, chlorophylls and unidentified fluorescent components were detected in the synchronous-scanning fluorescence spectra. Classification of oils based on their synchronous fluorescence spectra was performed using a non-parametrical k nearest neighbours method and linear discriminant analysis. Both methods provided very good discrimination between the oil classes with low classification error. The results presented demonstrate the capability of the fluorescence techniques for characterizing and differentiating vegetable oils.

Interactions of mixed ligand ruthenium(II) complexes containing an amino acid and 1,10-phenanthroline with DNA

Mixed ligand ruthenium(II) complexes containing an amino acid (AA) and 1,10-phenanthroline (phen), i.e. [Ru(AA)(phen) 2] n+ ( n=1,2, AA=glycine (gly), l-alanine ( l-ala), l-arginine ( l-arg)) have been synthesized. The interactions of these complexes and [Ru(phen) 3] 2+ with DNA have been examined by absorption, luminescence, and circular dichroism spectroscopic methods. Absorption spectral properties revealed that [Ru(AA)(phen) 2] + (AA=gly, l-ala) interacted with CT-DNA by the electrostatic binding mode. [Ru( l-arg)(phen) 2] 2+ exhibited the greatest hypochromicity, red shift, and binding constant, indicating that this complex may partially intercalate into the base-pairs of DNA. These results were also suggested by luminescence spectroscopy. CD spectral properties have been examined to understand the detailed interactions of the ruthenium(II) complexes with artificial DNA. In the case of Δ-[Ru( l-arg)(phen) 2] 2+, the solution on adding [poly(dG-dC)] 2 exhibited two well-defined positive peaks, which the shorter and longer wavelength peaks were assigned as originating from the major and the minor groove binding modes, respectively. Then, the solution on adding [poly(dA-dT)] 2 exhibited only one positive peak, which was assigned as a peak corresponding to the minor groove binding mode.

Characterization of Edible Oils Using Total Luminescence Spectroscopy

Total luminescence spectroscopy was used to characterise and differentiate edible oils and additionally, to control one of the major problems in the oil quality--the effect of thermal and photo-oxidation. We studied several vegetable oils available on the Polish market, including soybean, rapeseed, corn, sunflower, linseed and olive oils. Total luminescence spectroscopy measurements were performed using two different sample geometries: front-face for pure oil samples and right-angle for transparent samples, diluted in n-hexane. All the samples studied as n-hexane solutions exhibit an intense peak, which appears at 320 nm in emission and 290 nm in excitation, attributed to tocopherols. Some of the oils exhibit a second long-wavelength peak, appearing at 670 nm in emission and 405 nm in excitation, belonging to pigments of the chlorophyll group. Additional bands were present in the intermediate range of excitation and emission wavelengths; however, the compounds responsible for this emission were not identified. The front-face spectra for pure oils included chlorophyll peaks for most samples, and some additional peaks in the intermediate range, while the tocopherol peaks were comparatively less intense. The results presented demonstrate the capability of the total luminescence techniques to characterise and differentiate vegetable oil products, and additionally, to characterize the effect of thermal and photo-oxidation on such products. In the photo-oxidation experiments, special attention was paid to possible involvement of singlet oxygen. Experiments were done to monitor the highly specific O2(1delta(g)) --> O2(3sigma(g)-) singlet oxygen emission at 1270 nm. Thus, total luminescence spectroscopy presents an interesting alternative to time-consuming and expensive techniques such as gas or liquid chromatography, mass spectrometry and other methods requiring wet chemistry steps.

Model for bichromatic laser emission from a laser dye with nanoparticle scatterers

In this work, we present model for bichromatic laser emission from laser dye solutions containing randomly distributed scattering particles. We suggest that bichromatic emission is produced by two fluorescent aggregates: monomers and dimers. The shorter-wavelength laser peak was attributed to the monomer emission and the secondary longer-wavelength peak was attributed to the dimer emission. From neat dye solutions, using absorption and emission spectroscopies, the monomer and dimer absorption and emission cross sections were obtained. The partial overlap between the dimer absorption cross section and the monomer emission cross section indicates a unidirectional radiative and nonradiative energy transfers from excited monomers to fundamental dimers. The dynamics of the laser emission was modeled by a set of rate equations, whose solutions agree with our experimental data. It is also shown that nonradiative energy transfer plays a very important role in the dimer laser process, and helps to explain the relatively strong dimer laser peak intensities.