Red Shift In Photoluminescence Spectra Research Articles

Non-alternant Benzodifluoranthene Tetraimides from 7,8,9,10-Fluoranthene Diimides: Synthesis, Structure, and Optical-Limiting Properties.

A novel tetraimide-functionalized non-alternant π-conjugated system, namely, benzodifluoranthene tetraimides (BDFTI), has been designed and synthesized through highly efficient UV-photocyclization of a vinyl-bridged fluoranthene diimide dimer (i.e., FDI-V). The synthesis of FDI-V starts from a straightforward three-step route to produce novel 7,8,9,10-fluoranthene diimide (FDIs) building-blocks, followed by nearly complete bromination and then Stille-coupling reaction to give the desired dimer. The analysis by X-ray crystallography confirms a near-coplanar geometry for FDIs, while BDFTI shows a U-shaped and distorted backbone configuration proven by theoretical optimizations. The tetraimide BDFTI exhibits several advantages over the FDI cores, including an extended absorption band and a red-shift in photoluminescence spectra. This enhancement can be attributed to the presence of additional electron-deficient imide units, which promotes increased intramolecular charge transfer and improved electron affinity. All the imides show a local aromatic characteristic owing to the incorporation of pentagon rings in the π-frameworks. The fully fused BDFTI exhibits nonlinear optical properties as analyzed by the open-aperture Z-scan technique, demonstrating superior optical-limiting performance compared to vinyl-bridged FDI-V. The versatile UV-photocyclization chemistries provide a pathway for developing complex and unique multiimide-functionalized π-conjugated systems, paving the way for creating high-performance optical-limiting materials.

Distyrylbenzene-based semi-conducting molecular material for OLED applications: Synthesis, characterization and TD-DFT calculations

New semi-conducting molecular materiel (M-DSB) based on the distyrylbenzene π-conjugated system was synthesized via the Wittig reaction. The structural properties of M-DSB, displaying three different stereoisomers E-E, Z-Z and Z-E, were investigated through density functional theory (DFT) optimization. The detailed TD-DFT analysis provided insights into the structure-property relationships and revealed good agreement with experimental data. This photoactive material exhibits nearly the same UV–visible spectra in dilute solution and as solid thin film, with a maximum absorption at 363 nm and an optical gap of 2.99 eV. A broad and red-shifted photoluminescence spectrum was obtained in the solid state with blue emission compared to the solution which shows purplish blue emission with relatively narrow emission. The HOMO/LUMO energy levels were calculated by cyclic voltammetry measurements and indicate a p-type semiconductor material. The temperature and frequency dependence of the conductivity shows an activation energy value of about 0.89 eV for the M-DSB thin layer. Impedance spectroscopy of the ITO/M-DSB/Al device indicates a typical behavior of a single relaxation process and confirms that the electrical response becomes faster and the mobility of the charge carriers is enhanced by increasing the temperature.

Pressure Effect on Electronic and Excitonic Properties of Purely J-Aggregated Monolayer Organic Semiconductor

Different from monolayer inorganic semiconductors, such as transition metal dichalcogenides, monolayer organic semiconductors derived from perylene have attracted much attention because of their strong absorption and bright photoluminescence (PL). Pressure has been proved to be an effective tool in probing the exciton behavior in monolayer semiconductors. Here, by studying the high-pressure behavior of purely J-aggregated monolayer organic semiconductors experimentally and theoretically, we find a redshift of PL spectra due to a decrease of band gap, which is consistent with fluorescent images taken under pressure. The PL center dominates the perylene group and the band edges are flat, indicating Frenkel exciton in the monolayer organic semiconductor under ambient conditions. With increasing pressure, the band edges become more dispersive, suggesting the exciton transform to Wannier-Mott exciton, which is commonly observed in inorganic semiconductors.

An in-depth study on physical properties of facilely synthesized Dy@CdS NPs through microwave route for optoelectronic technology

Structural analysis of Dy@CdS quantum dots (QDs) were investigated by the XRD and TEM/HRTEM. The particle size of Dy@CdS QDs was varies from 7.3 ± 0.8 to 6.6 ± 0.08 nm for 0.3% and 1.2% of Dy contents respectively. The vibrational spectra of pristine and Dy@CdS QDs exhibit the longitudinal phonon modes at 293 and 587 cm−1. Optical band gap of 2.5 eV was optimized at 0.6% Dy doped CdS sample and found to be decreased on further increasing the Dy3+ content from 0.9 to 1.2%. The peaks at 414 and 466 nm are attributed to excitonic and trapped emission in the photoluminescence (PL) spectra of Dy@CdS samples respectively. The red shifts in PL spectra were observed for Dy3+ content more than 0.6% in Dy@CdS QDs. The frequency dependent characteristics of dielectric constant, dielectric loss, tangent and ac conductivity of Dy@CdS QDs were investigated at 300 K. It was founded that lower frequency range is dominated by the space charge polarization whereas orientational polarization is dominant at higher frequencies by increasing the dielectric constant. The variation of ac electrical conductivity was carried out linearly in the frequency range 15 kHz- 10 MHz. The easy way of repeatability, tenability and scalability of Dy@CdS QDs are attributed for optoelectronics application.

The Optical and Microstructural Characterization of the Polymeric Thin Films with Self-Assembly Nanoparticles Prepared by Spin-Coating Techniques

anhydride-poly(ethylene glycol) co-polymer (A-PEGCP) has been synthesized from maleic anhydride, poly(ethylene glycol) and bisphenol-A diglycidyl ether without using any organic solvent. The thin films produced from A-PEGCP solution were spin-coated on ITO-coated glass. The nanoparticles are observed in the thin films. It is proposed that the nanoparticle is built by a self-assembly process with bisphenol-A aggregates and poly (ethylene glycol) moieties. The effects of concentration, thermal annealing, excitation wavelength and moisture on the optical and nanostructured characterization of the thin films are investigated in this study. Photoluminescence (PL) spectrum of the thin film on ITO-coated glass has a peak of about 450 nm that extends from 360 to 550 nm under 325 nm excitation. The increase in PL intensity is accompanied by a red shift of PL spectrum as concentration increases. Moreover, the slightly red shift of PL spectrum is also observed as annealing temperature increases. Meanwhile, PL intensity negligibly decreases with annealing temperature. The degradation in PL intensity is apparent due to moisture. The excitation-wavelength dependent photoluminescence (EWDP) is observed in the thin film. UV-Vis absorption spectra of the thin films are red-shifted with concentration due to more molecular aggregation. The highest occupied molecular orbital (HOMO) energy is −9.52 eV. The optical band-gap energy is 4.09–4.44 eV.

Position-sensitive detectors based on redshifts in photoluminescence spectra

We have proposed and demonstrated position-sensitive detectors based on the spectral changes in fluorescent waveguides. The first prototype is a transparent heat-shrink tubing containing an organic luminescent dye at its core. With a laser beam incident on this linear fluorescent tubing, the redshift in the photoluminescence (PL) spectrum observed at its edge increases with the distance from the incident point. The range for position sensing is 2 cm. It is extended to 280 cm by adopting a scintillating fiber in our second experiment. Two-stage conversion enables two-dimensional position detection. We have attached two linear fluorescent tubing to a planar 50 mm × 50 mm × 8 mm fluorescent waveguide. When a laser beam excites the first luminescent material at a single spot in the planer waveguide, PL photons propagate to its edges and excite the second luminescent material in the two linear waveguides. Photon division between these linear waveguides gives the first coordinate. The second coordinate is given by the redshift in the linear waveguides. We have observed that the maximum error in position estimation is 1.5 mm. Unlike the conventional semiconductor technologies, no electronic components are required for the sensor head. This robust technology might be suited for deployment in large-scale harsh environments.

Controlling the Emergence and Shift Direction of Mechanochromic Luminescence Color of a Pyridine-Terminated Compound.

In this study, mechanochromic luminescence was induced in a complex of mechano-inactive compounds. Dye/acid complexes containing the same π-conjugated backbones were prepared. While the luminophore showed blue and red shifts in photoluminescence spectra when combined with different acids by grinding, it exhibited slight mechanoresponsiveness itself. Also, compounds with similar molecular backbones to the dye/acid complex were synthesized to clarify the color change mechanism. The compounds showed both blue and red shifts in photoluminescence and diffuse reflectance spectra upon grinding, indicating that mechanochromic luminescence in the hydrogen-bonded complex is like its monomeric analogue and that aggregation structure plays an important role in mechanoresponsive behavior rather than the π-conjugated structure. It was shown that a color change can be mechanically induced by imitating the solid-state aggregation structure of other mechanoresponsive compounds without synthetic modification.

Effect of preparation methods and doping on the structural and tunable emissions of CdS

Fe, Mn and Mg doped CdS samples were prepared by thermolysis method in air and under flow of nitrogen. Structural, compositional and optical properties of the prepared samples were investigated using x-ray powder diffraction (XRD), scanning electron microscope (SEM/EDS mapping), Fourier transform infrared red (FTIR), UV–vis absorption and photoluminescence (PL) spectroscopes. Rietveld refinement of x-ray data showed that all the undoped and doped CdS samples prepared in air and under flow of nitrogen have both cubic and hexagonal structures. The percentages of hexagonal and cubic phases for all prepared samples were determined. The crystallite size increased for CdS prepared under flow of N2 compared with the sample prepared in air. The energy gap of all the samples was calculated using UV data. The intensity of PL emission changed according to the method of preparation and the kind of doping elements. PL emission revealed a blue shift for CdS prepared in air compared with CdS prepared under flow of nitrogen; also all doped samples showed a red shift of PL spectra compared with undoped samples. Undoped and doped CdS with Fe and Mg samples emitted violet and blue sub-spectra. Mn doped CdS prepared in air revealed violet, blue and yellow sub-spectra, while the sample prepared under flow of N2 emitted violet, blue and green sub-spectra.

Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals.

Cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable two-photon-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared with one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Because of inherent nonlinearity, the size dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited, leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr3 NCs is likely a general feature of the perovskite NCs and can be tuned via NC size distribution to influence their performance within NC-based nonlinear optical materials and devices.

New anthracene derivatives including diazocine for blue emitting materials

ABSTRACTThe new anthracene derivatives including diazocine, (5S,11S)-2,8-bis(10-phenylanthracen-9-yl)-6,12-dihydro-5,11-ethanodibenzo[b,f][1,5]diazocine (AK-P) and (5S,11S)-4,4′-((6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diyl)bis(anthracene-10,9-diyl))-dibenzonitrile (AK-PCN) have been successfully synthesized and characterized. The ultraviolet-visible (UV-Vis) absorption of AK-P and AK-PCN showed in the range of 354 to 393 and exhibited maximum photoluminescence (PL) emissions at 458 and 463 nm, respectively in the film state. This slight red-shift in PL spectra of AK-PCN may be due to the polarity of the CN, electron withdrawing group. Using the AK-P as an emitting layer (EML), a non-doped organic light emitting diode (OLED) device resulted in blue emission with current efficiency of 1.37 cd/A and C.I.E. of (0.20, 0.25). AK-P exhibited a higher EL efficiency than that of AK-PCN.

Properties of silicon nanocrystals with boron and phosphorus doping fabricated via silicon rich oxide and silicon dioxide bilayers

Effects of boron and phosphorus doping on the structural, electrical, and optical properties of silicon nanocrystals in superlattice thin films were investigated. Silicon nanocrystals were fabricated via magnetron sputtering of stoichiometric silicon rich oxide and silicon dioxide bilayers followed by high temperature annealing at 1100 degrees Celsius. The characterization techniques used include: high-resolution transmission electron microscopy with energy filtering, grazing incidence x-ray diffraction, Raman, photoluminescence, and photothermal deflection spectroscopy, as well as electrical measurements. Results showed that phosphorus doping causes the loss of the bilayer structure and an increase in the average size of the silicon nanocrystals due to softening of the silicon dioxide matrix during post-sputter annealing. The result was a decrease in quantum confinement and a redshift in photoluminescence spectrum with an absorption profile similar to crystalline silicon. The undoped (intrinsic) sample maintained its bilayer structure and displayed stronger quantum confinement with higher photoluminescence peak energy and higher absorption coefficient. In-between, the boron doped sample was more similar structurally to the intrinsic sample, although merging between bilayers resulted in an extensive silicon nanocrystalline network. Optically, it displayed different effects due to photoluminescence quenching and free carrier absorption. Finally, both doped samples exhibited a decrease in electrical resistivity.

Simple and valid strategy for the enhancement of the solid-emissive property of boron dipyrromethenes

This manuscript presents the strategy for obtaining solid-emissive properties from the BODIPY dyes. It is demonstrated that the emission properties can be preserved from the aggregation-caused quenching (ACQ) by introducing the phenyl groups into their boron center. We established the synthetic procedures for the introduction of single or dual phenyl groups into the boron center in BODIPYs. The synthesized BODIPYs having single and dual-phenyl groups showed unimodal photoluminescence spectra both in the solution and solid states with sufficient quantum yields. The pristine and the single-phenyl substituted BODIPYs showed similar optical properties in the solution state. In contrast, the dual-phenyl substituted BODIPY provided broad and red-shifted photoluminescence spectrum. Interestingly, the emission property of the dual-substituted BODIPY was recovered not only in the solid state but also in the viscous solvent. It was revealed that the suppression of the molecular motion should play a crucial role in the enhancement of the emission properties of the dual-substituted BODIPY. Furthermore, it is also presented that the steric hindrance by the phenyl groups can inhibit ACQ in the condensed state.

Steady state and time resolved photoluminescence properties of CuInS2/ZnS quantum dots in solutions and in solid films

The photoluminescence properties of CuInS2/ZnS quantum dots (QDs) dispersed in solutions of different concentrations and solvent polarity and deposited as solid films on quartz substrates by drop-casting and spin-coating are studied. Both steady state and time-resolved photoluminescence spectroscopy have been used. The CuInS2/ZnS QDs in solutions exhibit a red-shift of their absorption and photoluminescence spectra by increasing concentration and solvent polarity. In addition, they exhibit a three-exponential decay with time constants 1–3, 20–40 and 200–300ns, depending on solvent, concentration and detection wavelength. In films, a red-shifted photoluminescence spectrum is observed for films made by drop-casting compared to those prepared by spin-coating. The time-resolved photoluminescence decays in films, apart from the three mechanisms observed in solutions, also exhibit a fast decay component of <1ns, which is more pronounced in the spin coated films and especially at long emission wavelengths. The time resolved photoluminescence spectra in the drop-casted films experience a larger transient red-shift than in the spin-coated ones, indicative of a possible energy transfer among adjacent QDs. In general, it is shown that the chemical environment and the presence of defects play a central role in the recombination processes.

Single-crystal indium phosphide nanowires grown on polycrystalline copper foils with an aluminum-doped zinc oxide template

The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.

Interband optical transitions of a strained InxGa1−xAs/GaAs quantum dot/wetting layer with various In mole fractions

In this paper, a numerical approach for calculating interband optical transitions of an InxGa1−xAs/GaAs conical shaped quantum dot is presented with different In mole fraction. The electronic structure was calculated by solving one-band effective-mass Schrödinger equation taking into account the strain effects. The simulations are performed by the employing finite element method in cylindrical co-ordinates. The wetting layer thickness and QD height effects on the S and P bands energies are also investigated to be a guidance of optimal structure design for optoelectronic devices. The obtained numerical results indicate a red shift in photoluminescence spectra with increasing wetting layer thickness which are in a good agreement with experimental data.

Enhancement and stability of photoluminescence from Si nanocrystals embedded in a SiO2 matrix by H2-passivation

Si nanocrystals embedded in SiO2 (Si-NCs/SiO2) with efficient light emission were prepared by N2-annealing of amorphous SiOx (a-SiOx) and subsequent H2-passivation, and the effects of passivation on the photoluminescence (PL) from Si-NCs/SiO2 were studied. The H2-passivation was performed in a mixed gas of 5% H2+95% N2 at temperatures ranging from 400 to 700°C for varied times, which is effective for passivating dangling bonds and enhancing luminescence. The PL intensity increases with passivation time, shortly followed by a saturation that depends on the passivation temperature. The H2-passivation also results in a red shift of PL spectra. The effects of H2-passivation show nearly complete reversibility as revealed by the emitted luminescence. Subsequent heating of the passivated samples in N2 has an effect of depassivation which regenerates dangling bonds and the regenerated dangling bonds can also be passivated. Si-NCs/SiO2 are found to exhibit stable behaviors in passivation and depassivation processes after three cycles of passivation and depassivation treatments.

Luminescence Mechanism of ZnWO&lt;sub&gt;4&lt;/sub&gt; Nanopowder Synthesized by Microwave-Assisted Heating

The luminescence investigations on the calcinated zinc tungstate nanopowder (ZnWO4 NP) synthesized by microwave-assisted synthesis are presented using photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses. The X-ray diffraction (XRD) patterns exhibit that the significant wolframite structure of ZnWO4 NP can be detected at calcination temperatures above 300 °C. The 12 K PL and TRPL results demonstrated that the deformation of WO6 octahedra is responsible for the low-energy side of PL spectra and dominate the red-shifted PL spectra with increasing calcination temperatures.

Water-soluble CdSe/CdS and CdSe/Cd x Zn1−x S quantum dots with tunable and narrow luminescent spectra and high photoluminescence efficiency

Nearly monodisperse CdSe/CdS and CdSe/Cd x Zn1−x S core–shell quantum dots (QDs) were synthesized by the epitaxial growth of CdS or Cd x Zn1–x S shells on pre-synthesized CdSe cores through a facile organic route. The thickness effect of CdS and Cd x Zn1−x S shells on the properties of resulting core–shell QDs, such as the photoluminescence (PL) efficiency, PL full width at half-maximum (fwhm), and PL decay lifetime, were investigated. The PL efficiency is greatly enhanced from 9 % of CdSe cores to 64 % of CdSe/Cd x Zn1−x S core–shell QDs. The PL spectra are narrowed after coating (fwhm ~28–32 nm). The alloy Cd x Zn1−x S shell is especially effective to enhance the PL efficiency, and it leads to a smaller red-shift in PL spectra compared with that of the CdS shell. The water-soluble CdSe/Cd x Zn1−x S core–shell QDs were obtained by the encapsulation of amphiphilic poly(styrene-co-maleic anhydride) (PSMA)–ethanolamine (EA) polymers, which was initially dispersed in oil phase. It was found that the CdSe/Cd x Zn1−x S QDs in water still retained a high PL efficiency as well as a small value in fwhm after encapsulating the CdSe/Cd x Zn1−x S QDs with PSMA–EA polymers, expanding the applications of these core–shell QDs in aqueous environment.

Preparation of Organic Zn-Phthalocyanine-Based Semiconducting Materials and Their Optical and Electrochemical Characterization

In order to increase the species of organic semiconductors, new Zn-phthalocyanines-based organic materials were synthesized and characterized. The new compounds have been characterized by 1H and 13C using NMR, FTIR, and UV-Vis. The absorption, fluorescence, and electrochemical properties were also studied. Green photoluminescence was observed in dilute solutions. In solid thin films, π-π* interactions influenced the optical properties, and redshifted photoluminescence spectra were obtained; red emissions for ZnPAL (647 nm) and ZnPTr (655 nm) were found. By cyclic voltammetry, the electrochemical band gap was estimated to be 1.94 and 1.17 eV for ZnPAl and ZnPTr, respectively. Single-layer diode devices of an indium tin oxide/Zn-phthalocyanine/aluminum configuration were fabricated and showed relatively low turn-on voltages (3.3 V for ZnPAl and 3 V for ZnPTr).

Highly Reliable White LEDs Using Nitride Phosphors

Nitride phosphors are now attracting significant attention of material scientists and phosphor engineers, because they promise red-shifted photoluminescence spectra, small Stokes shift, high quantum efficiency and low thermal degradation. A number of interesting nitride phosphors have been developed and practiced for producing white light with LED chips. In this paper, the applications of nitride phosphors in highly reliable wLEDs will be overviewed.