Shift In Photoluminescence Spectra Research Articles

Novel isocyanobenzene carbazole-based thermally activated delayed fluorophors towards solution-processed blue emitting OLED devices with high efficiency

Achieving efficient blue emission is an exigent challenge for the application of thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Carbazole benzonitrile-based TADF molecules exhibit promising performance in achieving the goal. Here, by replacing the cyano group in the sensitizer with the isocyano group, a 9 nm hypochromatic shift in photoluminescence spectra of the sensitizer and an enhanced efficiency of the Förster energy transfer (FET) in TADF-sensitized fluorescence (TSF) is found. By using the isocyano-based compounds as sensitizers, a maximum external quantum efficiency of 24.5% is obtained in solution-processed OLEDs, surpassing the cyano-based counterparts with similar structures. Moreover, the full width at half maximum (FWHM) of the emitter in the electroluminescence spectrum is also decreased from 47 nm to 38 nm, indicating a more complete energy transfer.

Radiation-induced scintillation properties of Ce-doped lutetium aluminosilicate glasses doped with cerium prepared by using a xenon image furnace

Ce-doped Lu2O3–Al2O3–SiO2 glasses were prepared using a xenon image furnace and their physical, optical, and scintillation properties were investigated. The photoluminescence (PL) properties were discussed based on the variations in the coordination environment of the different glass compositions. The characteristic shifts of PL spectra were explained in terms of (i) effects of crystal field due to the presence of different coordination environments which Ce3+ can take and (ii) effects of self-absorption due to the increase in Ce4+ based on the results of Raman spectra, transmission spectra, and PL lifetimes. The PL quantum yield (QY) tends to be higher for specimens with the emission peak located on the relatively shorter wavelength side, i.e., those with significant Ce3+ in the 6-coordinated environment and low Ce4+ content. This tendency also affects scintillation properties, with both scintillation spectral intensity and light yield under γ-ray irradiation showing characteristics that correlate positively with PL QY.

Optical and morphological properties of ZnO and Alq3 incorporated polymeric thin layers fabricated by the dip-coating method

In this report, we present the influence of polymer matrix on morphological and optical properties of thin films containing zinc oxide (ZnO), tris(8-hydroxyquinolinato)aluminium (Alq3) and ZnO:Alq3. Polyvinylpyrrolidone (PVP) and poly(methyl methacrylate) (PMMA) dissolved in isopropanol and tetrahydrofuran, respectively, were used as polymeric matrices of fabricated composites. The analysed thin layers were deposited on Si substrates using a dip-coating method and characterized by Fourier-Transform Infrared Spectroscopy (FTIR), Energy-Dispersive X-ray spectroscopy (EDX), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and photoluminescence (PL). It was found that adding the polymer to Alq3 causes a blueshift in absorption compared to pure Alq3 layers. We also observed photoluminescence in the region of 2.2–2.8 eV for ZnO:Alq3:PMMA and ZnO:Alq3:PVP, as well as for Alq3:PMMA at room temperature. PL measurements showed that adding ZnO to Alq3:polymer matrix did not result in any shift in PL spectra compared to the results of Alq3:polymer layer. AFM and SEM measurements show that relatively smooth films were obtained in the case of composites based on PVP and PMMA. Moreover, a change in the size of ZnO agglomerates depending on polymer used is observed for the three-component layers. We also noticed that the values of the refractive index are higher for the samples in the PVP matrix. However, the opposite behaviour was observed in the case of the extinction coefficient.

The influence of deposition time on the photoluminescent properties of SiOxCy thin films obtained by Cat-CVD from monomethyl silane precursor

In recent years, silicon-related photoluminescent materials have attracted great attention due to their optical properties. This work aims to study the deposited silicon oxycarbide (SiOxCy) thin films by using the Organic Catalytic Chemical Vapor Deposition (O-Cat CVD) technique. Monomethyl Silane gas [MMS; (CH3-SiH3)] was used as an organometallic silicon precursor, and tantalum (Ta) filament was used as a catalyst. The influence of deposition time on the photoluminescent properties, resulting from the Si/C ratio in thin layers, was assessed. The nature of shifted silicon-oxygen stretching bonds from their stoichiometry value was discussed by Fourier Transform Infrared (FTIR) analysis. The chemical composition of the SiOxCy matrix was confirmed by X-ray Photoelectron Spectroscopy (XPS) analysis. Wide and intense photoluminescence (PL) emissions were observed for as-deposited films at substrate temperatures as low as 200 °C and the origin of emissions was explained by different defect mechanisms. Moreover, the shift of PL spectra from the red to the blue band was also explained. This investigation provides further understanding of the SiOxCy thin film properties to improve its optical properties and produce it more safely.

The red light emission in 2D (C4SH3CH2NH3)2SnI4 and (C4OH7CH2NH3)2SnI4 perovskites.

Two-dimensional (2D) perovskites have a large exciton binding energy due to the structure of the quantum confinement, which produces a faster radiative recombination, and so are promising potential materials for light-emitting diodes. However, most of the highly efficient hybrid halide perovskites are based on the toxic Pb-based materials, so the replacement of Pb with less toxic and suitable substitute elements has been investigated for environmental efficient materials. Herein, we report the Sn-based 2D perovskites, which include (TPM)2SnI4 (TPM = C4SH3CH2NH3) and (TFF)2SnI4 (TFF = C4OH7CH2NH3), as red emission materials. Structural characterization by single crystal X-ray diffraction reveals that (TPM)2SnI4 undergoes a structural evolution from the orthorhombic space group Cmc21 (100 K) to Pbca (298 K), while the (TFF)2SnI4 perovskite exhibits the monoclinic space group P21/c at 100 K and 298 K. The inorganic framework of (TFF)2SnI4 was separated by the bilayer TFF chains with an empty space, which is an effective structure to increase the quantum confinement effect. The band gaps of the (TPM)2SnI4 (1.80 eV) and (TFF)2SnI4 (1.73 eV) compounds indicate the direct band gap semiconductor materials. From the time-resolved photoluminescence results, it can be seen that (TPM)2SnI4 produces uniform short emission (0.73 ns) throughout the entire powder crystals, whereas (TFF)2SnI4 has a uniform and long emission life time (47 ns). Temperature-dependent photoluminescence (PL) studies indicate that the (TPM)2SnI4 and (TFF)2SnI4 perovskites have a strong split red emission at low temperature due to the vibration of the inorganic framework. As the temperature increases, the PL spectra shift to the high energy region and the emission intensity decreases. The PL spectra of (TPM)2SnI4 and (TFF)2SnI4 perovskites have maximum peak wavelengths at 622 nm and 640 nm, and show the photoluminescence quantum yields of 0.30% and 1.71%, respectively.

Strong electron-ion coupling in gradient halide perovskite heterojunction

The electron-ion coupling in iontronics has great significance and potential for energy conversion and storage devices. However, it is a substantial challenge to integrate iontronics into all-solid-state semiconductor circuits and explore the electron-ion coupling in semiconductor devices. Here, by utilizing the organic-inorganic halide perovskite, we fabricate a planar heterojunction with a gradient distribution of halide anions. The diode-like halide migration was investigated by bias voltage induced asymmetric blue shift in photoluminescence spectrum. This pseudo-ionic diode behaviour was found to result from asymmetric charge injection characterized by I–V curves and gradient-halides-induced vacancy hopping indicated by energy dispersive spectroscopy (EDS). The planar gradient perovskite heterojunction provides a viable route for extending iontronic devices into the regime of all-solid-state semiconductors.

Formation of highly luminescent strained silicon nanowires due to surface effects and possible Plasmon role

ABSTRACT Metal Assisted Chemical Etching (MACE) is applied for generation of SiNWs (SiNWs). For the first time, the curved SiNWs are directly generated using MACE approach without any surface post-treatment after formation of SiNWs. This leaves the surface of the SiNWs intact without changing the surface composition. For comparison, straight SiNWs are generated under the same etchant and catalyst conditions. The generated curved nanostructures show a much higher Photo Luminescence (PL) compared to the straight SiNWs. The PLs in both cases of straight and curved nanowires are believed to be due to quantum confinement effects on the surface of the SiNWs. Blue shift of PL spectrum alongside its much higher intensity for the curved, is believed to be due to surface effects and possible coupling of the Plasmons with the silicon’s quantum confined bands, highly enhancing light extraction out of SiNWs.

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.

Optical properties of ultrathin ZnO films fabricated by atomic layer deposition

Ultrathin zinc oxide (ZnO) films with thickness down to several nanometers have been fabricated by atomic layer deposition. The chemical composition, surface morphology, crystalline structure, optical properties and photoluminescence characteristics of the ZnO films with various thicknesses were investigated. The X-ray photoelectric spectroscopy illustrates that the deposited films are uniformly distributed with zinc and oxygen atoms. Spectroscopic ellipsometric measurements reveal that both the refractive index and extinction coefficient decrease monotonically as the thickness of the film decreases. The thickness of the ZnO films obtained via ellipsometry is consistent with X-ray reflectivity measurements. The excitonic emission peak of the photoluminescence spectrum shifts to higher energies when the film thickness decreases to a few nanometers. This blueshift phenomenon is consistent with the expansion of the fundamental optical bandgap determined from ellipsometry, which could be well described by the effective mass model using physically meaningful parameters. The results obtained in this study could provide guidance to the design and optimization of optoelectronic devices based on ultrathin ZnO films.

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.

Structural, morphological and photoluminescence characteristics of Cd0.9-xZn0.1S quantum dots: Effect of Fe2+ ion

The Fe2+ incorporated Cd0.9-XZn0.1S QDs were synthesized via co-precipitation route. XRD phase analysis showed that the formation of the cubic zinc blende structure of all Cd0.9-XZn0.1FeXS QDs with varying the Fe2+ concentration (where x = 0, 0.01, 0.03 and 0.05). The average crystallite size was calculated and it found to be within the range of 2−3 nm. SEM pictures showed the spherical shaped QDs that are arranged in cloud form. The absorption intensity received from the UV–vis study was enhanced for the increase of Fe2+ composition and showed a blue shift from that of Cd0.9-XZn0.1S QDs. A wide band gap was exhibited by the addition of Fe2+ and it reaches the maximum as 4 eV. The Cd0.9-XZn0.1FeXS QDs showed a blue shift in Photoluminescence spectra (PL) to be due to the replacement of Cd2+ with smaller Fe2+. PL spectra were recorded for an excitation wavelength of 380 nm (NBE) and 650 nm (DLE). A wide range of band gap tailoring through tuning of crystallite size opens an opportunity to use in the optoelectronic device, labeling and imaging applications.

Emission and HR-XRD varying in GaAs/AlGaInAs heterostructures with InAs quantum dots at annealing

GaAs/Al0.30Ga0.70As/AlGaInAs/ heterostructures grown by molecular beam epitaxy with embedded InAs quantum dots (QDs) have been investigated before and after thermal annealing at 640 °C for 2 h. Two types of QD structures with the different compositions of capping layers: (Al0.30Ga0.70As (#1) and Al0.10Ga0.75In0.15As (#2)), are studied using the photoluminescence (PL), X-ray diffraction (XRD) and high-resolution XRD (HR-XRD) techniques. The high PL intensity, smaller half width of PL bands and lower energy of the ground state (GS) emission are detected in the structure with the Al0.10Ga0.75In0.15As capping layer. The blue shift of PL spectra is detected after annealing and this shift is more significant in the structure with Al0.10Ga0.75In0.15As capping as well. The last effect has been explained by the efficient Ga/In inter-diffusion at the AlGaInAs/InAs QD interface in #2 owing to the smaller In-As binding energy in comparison with Al-As and Ga-As ones in the studied alloy. The composition variation of the QDs and quantum wells (QWs) due to Ga/In intermixing at annealing has been modeled on the base of the numerical simulation of HR-XRD scans with the help of X′ Pert Epitaxy software.

Jet nebuliser spray pyrolysed indium oxide and nickel doped indium oxide thin films for photodiode application

Indium oxide thin films at different substrate temperatures (250−450 °C) and nickel doped indium oxide films with various doping concentration of 10 and 15 wt% were coated (at 400 °C) using jet nebulizer spray pyrolysis technique confirmed the cubic phase of the films from X-ray diffraction analysis. The micro-strain effects were also verified by Williamson-Hall method. Field emission scanning electron microscopy (FE-SEM) confirmed temperature dependence in the formation of octahedron layered geometry. The influence of temperature on surface irregularity and its effect on band gap shift confirmed using optical characterization was attributed to Burstein-Moss shift. The influence on doping was reinforced with the emission peak shift of photoluminescence spectra in ultra violet, violet and blue regions (394, 421 and 467 nm). Based on confirmations from these characterizations, n-In2O3/p-Si junction diodes were fabricated for as prepared In2O3 and 10 and 15 wt% concentrations nickel doped In2O3 at 400 °C. P-N junction diodes fabricated and analyzed showed high sensitivity for 15 wt% nickel doped indium oxide under illumination condition. On account of this 15 wt% nickel doped junction diode is a good candidate for optoelectronic devices.

Recycling and utilization of agro-food waste ashes: syntheses of the glasses for wide-band gap semiconductor applications

Present glasses are synthesized by the melt-quench method. The ashes of rice husk and sugarcane leaves are taken to synthesize the present samples. The eggshell powder is used to modify the glass-network and reduced the melting temperature of rice husk ash and sugarcane leaves ash. The as-quenched samples are characterized using different experimental techniques to study their structural and optical properties. The optical band gaps are found in the insulating range, i. e. 3.34–3.72 eV. The as-prepared glasses showed good luminescent properties due to some trace transition elements oxides present in the agro-food wastes. The agro-food wastes could be used as a resource and valuable materials to synthesize value-added engineering materials like glasses. The addition of eggshell powder into the main content showed the blue shift in photoluminescence spectra with enhanced intensity. This property may be exploited to develop UV–visible light-emitting diodes and some other non-linear optical devices.

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.

Luminescent properties of MgAl2O4 ceramics doped with rare earth ions fabricated by spark plasma sintering technique

MgAl2O4 ceramics doped with rare earth ions (Eu2+ and Ce3+ ions) were fabricated by spark plasma sintering technique. A complex characterization of the crystalline and defect structure of the ceramic by XRD was carried out. Absorption, excitation, photo- and cathodoluminescence spectra were studied. The photoluminescence spectrum shifts to the blue region with a maximum at λem = 475 nm for the MAS:0.1Ce ceramics. The nature of this luminescence can be caused by the radiative transitions in the cerium ion 5d–4f. The emission spectrum of MAS:0.1Eu has a “green” band emission in range of 400–700 nm centered around 500 nm, which can be ascribed to the allowed 4f65d1→4f7 (5d–4f) transition of Eu2+. In the millisecond time range, simultaneously with the emission of the complex host centers, the impurity luminescence bands of the chromium ion are recorded. It was shown that cathodoluminescence spectra in nanosecond time range can be decomposed into several emission bands at 2.72, 3.01, 3.37, 3.63–3.82 eV caused by F-type centers. It was demonstrated that the Eu2+ and Ce3+ ions lead to change the intensity ratio of the luminescence bands. The luminescence decay kinetics of synthesized spinel ceramics in nano- and millisecond time range were investigated in detail.

Synthesis of Zinc Oxide Nanorods via the Formation of Sea Urchin Structures and Their Photoluminescence after Heat Treatment.

A protocol for the aqueous synthesis of ca. 1-μm-long zinc oxide (ZnO) nanorods and their growth at intermediate reaction progression is presented, together with photoluminescence (PL) characteristics after heat treatment at temperatures of up to 1000 °C. The existence of solitary rods after the complete reaction (60 min) was traced back to the development of sea urchin structures during the first 5 s of the precipitation. The rods primarily formed in later stages during the reaction due to fracture, which was supported by the frequently observed broken rod ends with sharp edges in the final material, in addition to tapered uniform rod ends consistent with their natural growth direction. The more dominant rod growth in the c direction (extending the length of the rods), together with the appearance of faceted surfaces on the sides of the rods, occurred at longer reaction times (>5 min) and generated zinc-terminated particles that were more resistant to alkaline dissolution. A heat treatment for 1 h at 600 or 800 °C resulted in a smoothing of the rod surfaces, and PL measurements displayed a decreased defect emission at ca. 600 nm, which was related to the disappearance of lattice imperfections formed during the synthesis. A heat treatment at 1000 °C resulted in significant crystal growth reflected as an increase in luminescence at shorter wavelengths (ca. 510 nm). Electron microscopy revealed that the faceted rod structure was lost for ZnO rods exposed to temperatures above 600 °C, whereas even higher temperatures resulted in particle sintering and/or mass redistribution along the initially long and slender ZnO rods. The synthesized ZnO rods were a more stable Wurtzite crystal structure than previously reported ball-shaped ZnO consisting of merging sheets, which was supported by the shifts in PL spectra occurring at ca. 200 °C higher annealing temperature, in combination with a smaller thermogravimetric mass loss occurring upon heating the rods to 800 °C.

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.

Investigating surface effects of GaN nanowires using confocal microscopy at below-band gap excitation

Abstract