Variation Of Photoluminescence Intensity Research Articles

Twin domains modulate light-matter interactions in metal halide perovskites

Despite the extensive insights gained in how the microstructure impacts the device performance of metal halide perovskites (MHPs), little is known about the effect of the ferroelastic twin domains on the optoelectronic properties of MHPs. In this work, the effect of the ferroelastic twin domains on the photoluminescence (PL) behavior of CH3NH3PbI3 is investigated by correlating measurements from multiple microscopies. PL spectra and the confocal PL lifetime maps reveal no difference in wavelength of emitted light and decay dynamics between the neighboring domains, whereas PL intensity is different. We propose that the PL intensity variation is induced by the difference in light-matter interactions between neighboring domains. These results suggest that the effect of ferroelastic twin domains on the intrinsic PL behavior is negligible. We expect that this work will stimulate researchers to further explore the impact of twin domains on the photophysical properties of MHPs.

Synthesis and photoluminescence of silver and europium codoped germinate tellurite oxyfluoride glasses

Germinate tellurite oxyfluoride glasses of composition TeO2-GeO2-WO3-Li2O3-EuF3-AgCl (TGEUAC) were synthesized by melt quenching and heat-treated for the growth of silver nanoparticles (AgNPs). The presence of surface Plasmon resonance (SPR) absorption band of silver species with Eu3+ ions absorptions peaks for differently heat-treated glass samples were measured by the absorption spectrum. The green, red and deep- red photoluminescence intensity variation of Eu3+ ions under the influence of AgNPs has been observed. Maximum three-fold enhancement of TGEUAC-6 sample for green, red and mainly deep- red emission of Eu3+ caused by SPR of silver nanoparticles has also been observed. On the other hand, emission quenching of after long heat-treated of other samples was attributed to back energy transfer from Eu3+ ions to silver species or local field saturation. The emission intensities characterized through the CIE diagram and energy transfer mechanism of the Ag NPs to Eu3+ ions in glasses are discussed for potential application of optical devices.

Visualizing the Impact of Light Soaking on Morphological Domains in an Operational Cesium Lead Halide Perovskite Solar Cell.

The dynamics of photogenerated carriers and mobile ions in operational cesium lead halide (CsPbI3) perovskite solar cells (PSCs) under working conditions are studied using nanoscale-resolved fluorescence lifetime imaging microscopy (FLIM). The temporally and spatially resolved photoluminescence (PL) changes in the perovskite film during and after bias light soaking are dynamically monitored. Through the analysis of the dynamic variations of PL intensity and PL lifetime of an open-circuit PSC, the impacts of light soaking are revealed by a dynamic model of photogenerated charge carrier and mobile ions. We confirmed the different behaviors between morphological domain interiors and domain boundaries during light soaking, which shed light on the engineering of the domain interiors in addition to the commonly considered domain boundary strategies. This work provides a full picture of the photogenerated process in an operational PSC and therefore guides the design and operation of perovskite-based optoelectronic devices.

Effect of Sr and Ca substitution of Ba on the photoluminescence properties of the Eu2+ activated Ba2MgSi2O7 phosphor

The effects of Sr and Ca substitution of Ba on the Ba1.98-xSrx(Cax)MgSi2O7:Eu2+ photoluminescence properties have been investigated. The physical mechanisms for the photoluminescence variations are discussed. With Rietveld refinement method, the crystal structure of Ba1.98MgSi2O7:0.02Eu2+ and the lattice parameters of Sr and Ca substituted phosphors were refined. The emission band shift, the photoluminescence intensity variation, the phosphor chromaticity evolution, the Eu2+ lifetime distribution and the thermal stability elevation were investigated. With Sr and Ca substitution, the cell is shrinks. The cell shrinkage is resulting in the increase of the Eu2+ 5d electron crystal field splitting intensity, which is the reason for the emission band shift towards the long wavelength band. The photoluminescence intensity is increased firstly and then decreased. The intensity variation is the competitive result between the increase of the crystal structure rigidity and the rise of the lattice defect. The correlated color temperature can be cut down and the color purity can be adjusted. The photoluminescence life time of Eu2+ is raised firstly and then decreased. For Sr and Ca substitution, the thermal stability can be elevated. With the forbidden band gap calculation, the reason for the thermal stability elevation was investigated that for the substituted phosphors the forbidden band gap is enlarged and then limits the Eu2+ 5d self-ionization from the splitting levels to the conduction band. This work reveals that the Sr and Ca substitution of Ba can elevate the Ba1.98-xSrx(Cax)MgSi2O7:Eu2+ photoluminescence properties and improve the applications for the White Light Emitting Diode.

Annealing-Induced Changes in the Nature of Point Defects in Sublimation-Grown Cubic Silicon Carbide.

In recent years, cubic silicon carbide (3C-SiC) has gained increasing interest as semiconductor material for energy saving and optoelectronic applications, such as intermediate-band solar cells, photoelectrochemical water splitting, and quantum key distribution, just to name a few. All these applications critically depend on further understanding of defect behavior at the atomic level and the possibility to actively control distinct defects. In this work, dopants as well as intrinsic defects were introduced into the 3C-SiC material in situ during sublimation growth. A series of isochronal temperature treatments were performed in order to investigate the temperature-dependent annealing behavior of point defects. The material was analyzed by temperature-dependent photoluminescence (PL) measurements. In our study, we found a variation in the overall PL intensity which can be considered as an indication of annealing-induced changes in structure, composition or concentration of point defects. Moreover, a number of dopant-related as well as intrinsic defects were identified. Among these defects, there were strong indications for the presence of the negatively charged nitrogen vacancy complex (NC–VSi)−, which is considered a promising candidate for spin qubits.

Excitation dynamics of MAPb(I1-xBrx)3 during phase separation by photoirradiation: Evidence of sink, band filling, and Br-Rich phase coarsening

Here, we report the radiative recombination behavior of a binary perovskite, MAPb(I0.2Br0.8)3, during phase separation by investigating photoluminescence (PL) spectrum modulations under different laser irradiation conditions: power density, wavelength (379 and 470 nm), and irradiation time. During laser irradiation, the original PL spectrum was divided into three PL peaks assigned to the 1st I-rich, 2nd I-rich, and Br-rich phases. The observed modulations of the intensity and wavelength of the PL spectrum could be explained by the multiple effects of the charge carrier sink, band filling, and Br-rich phase coarsening. Under a weak laser power density, the observed PL predominantly showed the sink effects of the charge carriers through the excited state potential minimum. As the laser power density or irradiation time increased, new high-energy PL bands appeared owing to the band filling and Br-rich phase formation. With a further increase in the laser power or irradiation time, the PL from the Br-rich phase was intensified, but its PL peak position was red-shifted owing to the quantum size effect from the Br-rich phase coarsening. These variations in PL intensity and charge carrier behavior were carefully studied using time- and space-resolved fluorescence imaging microscopy.

Effects of Transition Energy on Intra-Band Photoluminescence of Zinc Oxide (ZnO) Semiconductor under Low injection Level

This paper presents the effects of the transition energies on photoluminescence intensities in Zinc Oxide compound semiconductor due to the intra-band transition of free carriers. The excitation of free carriers from the valence band to conduction band and from different localized state to the conduction band by the illumination of sufficient energy is considered. A theoretical model for minority carrier trapping is also investigated to explain the dependence of the photoluminescence on the trap energy. Variation of photoluminescence intensities along with localized state energy and transition energy is considered at different temperatures. As temperature increases the photoluminescence due to the transition of free electrons from the conduction band to the valence band, from the conduction band to the localized states and from the localized states to the valence band are increasing.

Correlation of Optical, Structural, and Compositional Properties with V-Pit Distribution in InGaN/GaN Multiquantum Wells.

InGaN/GaN double heterostructures and multiquantum wells (MQWs) have been successfully developed since more than 20 years for LED lightning applications. Recent developments show that state-of-the-art LEDs benefit from artificially generated V-pit defects. However, the control of structural and chemical properties plays a tremendous role. In this paper, we report on the lateral distribution of V-pit defects and photoluminescence of InGaN/GaN MQWs grown on thick GaN on patterned sapphire substrates. The synchrotron-based scanning X-ray diffraction microscopy technique K-map was employed to locally correlate these properties with the local tilt, strain, and composition of the InGaN/GaN MQW. Compositional fluctuation is the main factor for the variation of photoluminescence intensity and broadening. In turn, V-pit defects align along small-angle grain boundaries and their strain fields are identified as a reason for promoting the InGaN segregation process on a microscale.

Green Emitting Cerium Doped CaS Whiskers Grown by Solid State Diffusion Method.

Undoped and cerium doped Calcium sulfide (CaS) phosphors were synthesized using solid state diffusion method. The X-ray diffraction pattern revealed that both undoped and doped CaS crystallites have cubic structure with average crystallite size varying from 20 to 30nm. Scanning electron micrographs indicated that Ce doped CaS phosphors were composed of whiskers with different dimensions and orientations. The optical properties of undoped and Ce doped particles were characterized using Photoluminescence (PL) and UV-Vis absorption spectroscopy. The PL emission spectrum of cerium doped CaS phosphors for an excitation wavelength 465nm showed a main peak at 500nm and a shoulder peak at 556nm due to 5d → 4f transition in Ce3+ ions. The variation of PL intensity with cerium concentration was investigated and the maximum PL intensity was obtained for a doping concentration of 3wt.%. The optical band gap of the samples was estimated from the diffuse reflectance spectrum and was found to increase with increase in cerium concentration. The enhanced optical properties of these phosphors can be exploited in various optoelectronic devices including displays and bioimaging techniques.

Structural phase transition, electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 (x = 0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO2/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700 °C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr3+ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.

Using photoluminescence to monitor the optoelectronic properties of methylammonium lead halide perovskites in light and dark over periods of days

The degradation of methyl-ammonium lead iodide (MAPbI3) upon exposure to air is a potentially limiting effect for large scale MAPbI3 photovoltaic production. Here, we report a systematic study on effects of air-exposure on the structural and optical properties of MAPbI3 thin films. The X-ray diffraction studies indicate a shrinking volume of MAPbI3 upon air-exposure as the material decomposes back to its precursors, methyl amine (CH3NH2) and lead iodide (PbI2). However, the photoluminescence (PL) yield and carrier lifetime measured with time-resolved photoluminescence, show an increasing trend upon air-exposure. These phenomena can be explained by self-passivation of MAPbI3 grains by PbI2 layer that reduces the number of non-radiative recombination centres at the grain boundaries. However, this process is not self-limiting and it eventually leads to a film that has completely reverted back to its precursor state. It is shown that this conversion of MAPbI3 film back to its precursors is also accelerated by exposure to laser illumination. Furthermore, we report unusual variation of PL intensity on a shorter time scale of a few seconds in all the films used for the experiment. The variations are found to follow different trends in the encapsulated samples as compared to the un-encapsulated samples. We propose that the decomposition followed by the ionic diffusion through film is responsible for such unusual behaviours.

Effect of annealing in hydrogen atmosphere on the photoluminescence properties of phosphor-in- glass in tellurate glass

A series of phosphor-in-glasses (PIGs) was prepared by one-step melt-quenching method sintering the mixture of tellurate raw material powder and YAG:Ce3+ phosphors at 450–700°C, of which some was annealed at 250°C for 3h under the hydrogen atmosphere. The test results show that the relative photoluminescence (PL) intensity of phosphor-in-glass sintered at 450–700°C was significantly strengthened from 42.29% to 174.72% after the hydrogen annealing, and the relative photoluminescence excitation (PLE) intensity from 21.00% to 207.90%. However, the relative photoluminescence intensity of the PIG sintered at 700°C before and after hydrogen annealing was nearly zero because of the destruction of crystal structural surrounding Ce3+ ion. The concentration of Ce3+ and Ce4+ measured by X-ray photoelectron spectrum analyser (XPS) in PIGs before and after hydrogen annealing obviously happened to change, which was associated with the variation of PL and PL excitation intensity. Besides, the X-ray diffraction (XRD) pattern of YAG:Ce3+ phosphor and PIGs sintered at different temperatures further presented the destruction of YAG:Ce3+ lattice as the sintering temperature increased. Therefore, the appropriate sintering temperature and hydrogen annealing can increase the PL and PLE intensity and the Ce3+ concentration.

Investigation on the pH-independent photoluminescence emission from carbon dots impregnated on polymer matrix.

Highly luminescent, polymer nanocomposite films based on poly(vinyl alcohol) (PVA), and monodispersed carbon dots (C-dots) derived from multiwalled carbon nanotubes (MWCNTs), as coatings on substrates as well as free standing ones are obtained via solution-based techniques. The synthesized films exhibit pH-independent photoluminescence (PL) emission, which is an advantageous property compared with the pH-dependent photoluminescence intensity variations, generally observed for the C-dots dispersed in aqueous solution. The synthesized C-dots and the nanocomposite films are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), ultraviolet (UV)-visible spectroscopy and photoluminescence spectroscopy (PL) techniques. The TEM image provides clear evidence for the formation of C-dots of almost uniform shape and average size of about 8nm, homogeneously dispersed in aqueous medium. The strong anchoring of C-dots within the polymer matrix can be confirmed from the XRD results. The FTIR spectral studies conclusively establish the presence of oxygen functional groups on the surfaces of the C-dots. The photoluminescence (PL) emission spectra of the nanocomposite films are broad, covering most part of the visible region. The PL spectra do not show any luminescence intensity variations, when the pH of the medium is changed from 1 to 11. The pH-independent luminescence, shown by these films offers ample scope for using them as coatings for designing diagnostic and imaging tools in bio medical applications. The non-toxic nature of these nanocomposite films has been established on the basis of cytotoxicity studies.

Structural and emission properties of Tb3+-doped nitrogen-rich silicon oxynitride films

Terbium doped silicon oxynitride host matrix is suitable for various applications such as light emitters compatible with CMOS technology or frequency converter systems for photovoltaic cells. In this study, amorphous Tb3+ ion doped nitrogen-rich silicon oxynitride (NRSON) thin films were fabricated using a reactive magnetron co-sputtering method, with various N2 flows and annealing conditions, in order to study their structural and emission properties. Rutherford backscattering (RBS) measurements and refractive index values confirmed the silicon oxynitride nature of the films. An electron microscopy analysis conducted for different annealing temperatures (TA) was also performed up to 1200 °C. Transmission electron microscopy (TEM) images revealed two different sublayers. The top layer showed porosities coming from a degassing of oxygen during deposition and annealing, while in the region close to the substrate, a multilayer-like structure of SiO2 and Si3N4 phases appeared, involving a spinodal decomposition. Upon a 1200 °C annealing treatment, a significant density of Tb clusters was detected, indicating a higher thermal threshold of rare earth (RE) clusterization in comparison to the silicon oxide matrix. With an opposite variation of the N2 flow during the deposition, the nitrogen excess parameter (Nex) estimated by RBS measurements was introduced to investigate the Fourier transform infrared (FTIR) spectrum behavior and emission properties. Different vibration modes of the Si–N and Si–O bonds have been carefully identified from the FTIR spectra characterizing such host matrices, especially the ‘out-of-phase’ stretching vibration mode of the Si–O bond. The highest Tb3+ photoluminescence (PL) intensity was obtained by optimizing the N incorporation and the annealing conditions. In addition, according to these conditions, the integrated PL intensity variation confirmed that the silicon nitride-based host matrix had a higher thermal threshold of rare earth clusterization than its silicon oxide counterpart. Analysis of time-resolved PL intensity versus TA showed the impact of Tb clustering on decay times, in agreement with the TEM observations. Finally, PL and PL excitation (PLE) experiments and comparison of the related spectra between undoped and Tb-doped samples were carried out to investigate the impact of the band tails on the excitation mechanism of Tb3+ ions.

Computational Investigation to Improve the Efficiency of Multi-junction SiGe/Si Solar cell

The Intrinsic SiGe layer is implemented in the designing of thin-film SiGe solar cell in order to optimize and improve the electrical and optical behavior of the commercially available SiGe solar cells. The proposed device is simulated with Silvaco ATLAS TCAD by placing the intrinsic SiGe layer on a silicon base. Various parameters like molefraction of germanium in Si1-XGeX, thickness of newly introduced intrinsic region etc. were optimized to enhance the efficiency of thin-film SiGe solar cell. At a molefraction of x=0.48 of germanium and at a approximate thickness of 1μm of the intrinsic region, the efficiency of the designed cell is significantly improved and is found to be around 23.2% and the maximum value of cathode current density is 162mA/cm2, achieved at an open circuit voltage of 1.32V. In this regard the variation of photo-luminescence Intensity with wavelength for different mole fraction of germanium has been observed.

Radiative defects, emission and structure of ZnO nanocrystals obtained by electrochemical method

Radiative defects, emission and structure of ZnO nanocrystals (NCs) have been studied by means of the scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. ZnO NCs were prepared by etching Zn sheets and annealing at 200–400°C in ambient air. The NC size and volume of ZnO crystalline phase increase with annealing temperature rising from 200°C up to 400°C. Simultaneously, XRD parameters and inter-planar distances in ZnO NCs vary no monotonically: decreasing upon annealing at 200–360°C and increasing at 400°C annealing. The PL intensity variation of orange, yellow, green and blue PL bands correlates with the change of XRD parameters. The study of PL band thermal decays, within the range 10–300K, permits to estimate decay activation energies and to analyze the nature of donors and acceptors responsible for the orange, yellow and green PL bands in ZnO NCs.The electrochemical method allows varying the PL intensity of different PL bands by a controllable way that is interesting for ZnO NC film application in “white” light emitting device structures.

Synthesis, Surface Modification and Optical Properties of Thioglycolic Acid-Capped ZnS Quantum Dots for Starch Recognition at Ultralow Concentration

In this research, water-soluble thioglycolic acid-capped ZnS quantum dots (QDs) are synthesized by the chemical precipitation method. The prepared QDs are characterized using x-ray diffraction and transmission electron microscopy. Results revealed that ZnS QDs have a 2.73 nm crystallite size, cubic zinc blende structure, and spherical morphology with a diameter less than 10 nm. Photoluminescence (PL) spectroscopy is performed to determine the presence of low concentrations of starch. Four emission peaks are observed at 348 nm, 387 nm, 422 nm, and 486 nm and their intensities are quenched by increasing concentration of starch. PL intensity variations in the studied concentrations range (0–100 ppm) are best described by a Michaelis–Menten model. The Michaelis constant (K m) for immobilized α-amylase in this system is about 101.07 ppm. This implies a great tendency for the enzyme to hydrolyze the starch as substrate. Finally, the limit of detection is found to be about 6.64 ppm.

Influences of Si-doped graded short-period superlattice on green InGaN/GaN light-emitting diodes.

We report significant improvement in optical and electrical properties of green InGaN/GaN light-emitting diodes (LEDs) by using Si-doped graded short-period InGaN/GaN superlattice (SiGSL) formed by so called indium-conversion technique. For comparison, a conventional LED without the superlattice (C-LED) and a LED with undoped graded superlattice (unGSL-LED) were prepared, respectively. The photoluminescence (PL) intensity of the SiGSL-LED was increased more than 3 times at room temperature (RT) as compared to C-LED. The PL intensity ratios of RT to 10K for the C-LED, unGSL-LED, and SiGSL-LED were measured to be 25, 40.9, and 47.5%, respectively. The difference in carrier lifetimes between 10K and RT for the SiGSL-LED is relatively small compared to that of the C-LED, which is consistent with the variation in PL intensity. The output power of a transistor-outline type SiGSL-LED was increased more than 2 times higher than that of the C-LED.

Towards understanding the unusual photoluminescence intensity variation of ultrasmall colloidal PbS quantum dots with the formation of a thin CdS shell.

In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell via a microwave-assisted cation exchange approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns-μs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed.

Optical Characterization of Si-Based Ge1−x Sn x Alloys with Sn Compositions up to 12%

Optical properties of germanium tin (Ge1−x Sn x ) alloys have been comprehensively studied with Sn compositions from 0 (Ge) to 12%. Raman spectra of the GeSn samples with various Sn compositions were measured. The room temperature photoluminescence (PL) spectra show a gradual shift of emission peaks towards longer wavelength as Sn composition increases. Temperature dependent PL shows the PL intensity variation along with the temperature change, which reveals the indirectness or directness of the bandgap of the material. As temperature decreases, the PL intensity decreases with Sn composition less than 8%, indicating the indirect bandgap Ge1−x Sn x ; while the PL intensity increases with Sn composition higher than 10%, implying the direct bandgap Ge1−x Sn x . Moreover, the PL study of n-doped samples shows bandgap narrowing compared to the unintentionally (Boron) doped thin film with similar Sn compositions due to the doping.