Photoluminescence Decay Measurements Research Articles

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Simultaneous monitoring of competing photo-induced weathering processes in colloidal CdSe/ZnS semiconductor quantum dots using multivariate frequency-domain dynamic photoluminescence measurements

Coincident photo-induced weathering processes in oleic acid capped CdSe/ZnS semiconductor quantum dots (QDs), stimulated by exposure to artificial sunlight in samples with different headspace volumes, were monitored using multichannel frequency-domain photoluminescence (PL) decay measurements. The combination of process monitoring using matrix-formatted measurements and multivariate data mining permits resolution of the spectra, frequency-domain decays and kinetic profiles of the PL emitted by distinct QD subpopulations. The data indicate that three components contribute to the PL: photodarkened QDs that emit bluer, shorter-lived spectra reflecting photoionization-induced QD charging and ligand desorption; photobrightened QDs that emit redder, short-lived spectra indicative of photooxidative QD surface passivation; and photoripened QDs that emit very weak, red-shifted, large bandwidth, very long-lived spectra reflecting increasing nanoparticle growth and polydispersity. Analysis of the kinetic traces of the QD subpopulations during photoirradiation using systems of exponentials revealed the extent of coupling between the coincident photoinduced processes. The assignment of these components was supported by their emission maxima, spectral bandwidths, average and irradiation-time specific PL decay times, irradiation-dependent kinetic profiles, TEM images and literature precedents. As expected, the results show that QD photodarkening is faster and photobrightening is slower in the samples that have more exposure to water and oxygen. They also show that the dominant photodarkening and photobrightening pathways in those samples are not as strongly coupled, indicating the importance of contributions by additional processes in that case. Transmission electron microscopy (TEM) images were consistent with the PL results, but the QD samples proved too polydisperse to be characterized by single-beam dynamic light scattering measurements.

Ag-CsPbI3 nanocomposite structure in glasses boosts photoluminescence for efficient light emitting diodes

Nanocomposite structures based on perovskite nanocrystals (NCs) and metal nanostructures within transparent monolithic media hold promise for a wide range of applications in optoelectronics. However, the microstructure of such nanocomposites and the reason for enhanced emissions remain incompletely understood. In this work, we present the successful creation of Ag-CsPbI3 nanocomposite within a highly transparent borosilicate glass (Ag-CsPbI3 NCs@glass). We reveal that Ag and CsPbI3 NCs form after thermal treatment of the glass, either in the form of Ag-CsPbI3 heterostructures or of separate Ag and CsPbI3 NCs. Based on combined characterization of Mott-Schottky and steady-state photoluminescence (PL) of the products, we attribute the enhanced PL in the nanocomposite of Ag-CsPbI3 to the regulation of CsPbI3 lattice by Ag ions and/or the plasmonic effect of Ag NCs. Temperature-dependent time-resolved PL decay measurements suggest that Ag NCs could play an important role in the relaxation process of excited carriers by increasing the radiative recombination. Moreover, the Ag-CsPbI3 nanocomposite structure within the glass matrix curtails PL quenching caused by water molecules and high temperature. Using the Ag-CsPbI3 nanocomposite, we achieve high-performance white light-emitting diodes with a color rendering index of 83.9 and luminous efficiency of 58.07 lmW−1. This work elucidates the underlying mechanism on the enhanced PL in nanocomposites containing perovskite NCs and metal nanostructures, and aids in the rational design of glass-based perovskite nanocomposites for a diversity of optoelectronic applications.

Passivation of defective states in single crystal MAPbBr3 and their optoelectronic properties study

We fabricated high-quality Methylpropyl lead bromide (MAPbBr3) single crystals (SC) by introducing moderate amount electron-rich amino-terminated polyoxypropylene (PEA) additives via inverse temperature crystallization (ITC) method. The intramolecular vibrations and defective states of prepared MAPbBr3 single crystals were systemically studied by the in situ variable-temperature Fourier Transform Infrared (FTIR), X-ray photoelectron spectroscopy (XPS), the synchrotron-based X-ray absorption near edge structure (XANES) et al. We find the proper amount PEA additives would prevent the formation of intermediate states and crystal boundaries, passivating the density of defective states. The X-ray detection properties of single-crystal MAPbBr3 crystals were investigated, using both high dose and low dose X-ray photons at 40 keV in synchrotron beamlines. The photocurrent of optimized PEA-treated MAPbBr3 single crystals demonstrate better performance compared with the as-prepared single crystals. Meanwhile, the decay time of MAPbBr3 single crystal is decreased from 376 ns to 17.1 ns by Time-resolved photoluminescence (TRPL) decay measurement, owing to the defect passivation by 0.5 wt% PEA additives. The PEA-treated single crystals possess much faster decay time than the commercialized Cerium-doped Yttrium Aluminum Garnet (Ce:YAG) scintillator (∼100ns). These results help elucidate the passivation of defective states and contribute paths to manufacture high-quality single crystal perovskites for next-generation optoelectronic devices.

Extending Infrared Emission via Energy Transfer in a CsPbI3-Cyanine Dye Hybrid.

Directing energy flow in light harvesting assemblies of nanocrystal-chromophore hybrid systems requires a better understanding of factors that dictate excited-state processes. In this study, we explore excited-state interactions within the CsPbI3-cyanine dye (IR125) hybrid assembly through a comprehensive set of steady-state and time-resolved absorption and photoluminescence (PL) experiments. Our photoluminescence investigations reveal the quenching of CsPbI3 emission alongside the simultaneous enhancement of IR125 fluorescence, providing evidence for a singlet energy transfer. The evaluation of both photoluminescence (PL) quenching and PL decay measurements yield ∼94% energy transfer efficiency for the CsPbI3-IR125 hybrid assembly. Transient absorption spectroscopy further unveils that this singlet energy transfer process operates on an ultrafast time scale, occurring within 400 ps with a rate constant of energy transfer of 1.4 × 1010 s-1. Our findings highlight the potential of the CsPbI3-IR125 hybrid assembly to extend the emission of halide perovskites into the infrared region, paving the way for light energy harvesting and display applications.

(Invited) Coupled Quantum Emitters in Carbon Nanotubes

The development of covalent diazonium functionalization of carbon nanotubes brings unique opportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and super-localization measurements and their light emission properties using time-correlation techniques and photo-luminescence decay measurements. We show that two pairs show qualitatively different behaviors in terms of spectral diffusion and luminescence saturation, which we interpret using two models of electronically coupled or uncoupled two-level systems.

High-Pressure Near-Infrared Luminescence Studies of Fe3+-Activated LiGaO2.

A 0.25% iron (Fe3+)-doped LiGaO2 phosphor was synthesized by a high-temperature solid-state reaction method. The phosphor was characterized utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure photoluminescence, and photoluminescence decay measurement techniques using diamond anvil cells (DACs). The powder X-ray analysis shows that the phosphor is a β polymorph of LiGaO2 with an orthorhombic crystallographic structure at room temperature. The SEM result also confirms the presence of well-dispersed micro-rod-like structures throughout the sample. The photoluminescence studies in the near-infrared (NIR) range were performed at ambient, low-temperature, and high-pressure conditions. The synthesized phosphor exhibits a photoluminescence band around 746 nm related to the 4T1 → 6A1 transition with a 28% quantum efficiency at ambient conditions, which shifts toward longer wavelengths with the increase of pressure. The excitation spectra of Fe3+ are very well fitted with the Tanabe-Sugano crystal-field theory. The phosphor luminescence decays with a millisecond lifetime. The high-pressure application transforms the β polymorph of LiGaO2 into a trigonal α structure at the pressure of about 3 GPa. Further increase of pressure quenches the Fe3+ luminescence due to the amorphization process of the material. The prepared phosphor exhibits also mechanoluminescence properties in the NIR spectral region.

Temperature-dependent luminescence in pure and In+-Activated Cs3Cu2I5 single crystals

We report our investigation of the low-temperature optical properties of high-quality Cs2Cu3I5:In+ perovskite single crystals grown by the vertical Bridgman technique. Temperature-dependent radioluminescence in the range 15–300K reveals two main emissions for In+ doped samples at ∼450 nm and the other at ∼620 nm assigned to self-trapping exciton and In+ emissions respectively. Wavelength-resolved thermoluminescence reveals that In+ emission centers are the preferred recombination center for trapped charges. Further investigation of photoluminescence emission/excitation maps reveals a complex electronic structure that arises from spin-orbit coupling-related transitions in In+ and is only observable at low temperatures. Temperature-dependent photoluminescence decay measurements reveal some instances of charge transfer between the self-trapped excitons and In+ ions, as well as some thermally-assisted radiative recombination processes.

Cost-efficient single photon-sensitive nanosecond gated spectrometer based on commercial grade image intensifier

A commercial grade low-cost image intensifier was modified with a fast transistor gating circuit for time-resolved photoluminescence spectra and decay measurements. Gating exposure of 20 ns and spectral sensitivity in the 300–1000 nm range were achieved. Operation frequency in 5 Hz to 0.3 MHz range was verified by a pulsed laser diode. Pulsed repetitive image detection was integrated on a cooled CMOS camera. High dynamic range and wide spectral resolution were achieved. The thermal noise, reduced by three orders of magnitude at -16°C cooling and a high gain of 104, allowed single photon detection. Application of the spectrometer for sensitive photoluminescence decay measurements at pulsed laser excitation was performed on MAPbBr3 perovskite and CsI(Tl) scintillator crystals. The decay lifetimes of 400–600 ns and 100–6000 ns were determined, respectively. CsI(Tl) scintillator showed much stronger x-ray luminescence intensity due to larger defect density.

Ce[formula omitted][formula omitted] Cr3＋ energy transfer in Y[formula omitted]Al[formula omitted]MgSiO12 garnet host and application in horticultural lighting

In this study, the photoluminescence properties of Y3Al3MgSiO12:Cr3＋ (YMASG:Cr3＋) and Y3Al3MgSiO12: 2%Ce3+, Cr3＋ (YMASG:2%Ce3+, Cr3＋) were investigated, and the energy transfer from Ce3+ to Cr3＋ were examined. All phosphor samples were prepared by the aqueous sol–gel method. The phase purity and surface morphology of the obtained sample was evaluated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). To determine the photoluminescence (PL) properties of YMASG:Cr3＋ and YMASG:2%Ce3+, Cr3＋, emission and excitation spectra along with reflection spectra were recorded. PL decay measurements were also performed, and PL lifetime values were determined. It was found that Ce3+ → Cr3＋ energy transfer efficiency increases with increasing Cr3＋ concentration. To investigate the temperature-dependent emission of YMASG:Cr3＋ and YMASG:2%Ce3+, Cr3＋, PL emission, and PL decay measurements were conducted in a temperature range from 77 to 500 K. Temperature-dependent PL decay measurements also revealed that the Cr3＋ ions mainly cause thermal quenching of YMASG:2%Cfe3+, 1%Cr3＋ emission. To check the suitability of synthesized materials to be applied for greenhouse illumination, two pcLEDs containing YMASG:Cr3＋ and YMASG:Ce3+, Cr3＋ phosphors were constructed. PcLED prototypes applicability for efficient horticulture lighting was also investigated within this study.

Time-Resolved Photoluminescence in GeSn Film by New Infrared Streak Camera Attachment Based on a Broadband Light Upconversion

GeSn coatings on commercial Si substrates have gained increased interest for application in infrared detectors and lasers. The characterization of these materials is crucial for their quality assessment and in describing device performance for commercialization. The time-resolved photoluminescence is an efficient method for contactless, time-resolved characterization of GeSn optoelectronic properties. For this purpose, in this work, we developed an infrared streak camera attachment based on the broadband upconversion of infrared photoluminescence by using powerful nanosecond 1064 nm pulses. The attachment achieved picosecond time resolution, being limited by the laser pulse duration, jitter, and temporal resolution of the streak camera. The spectral range for time-resolved photoluminescence in the 1100–2400 nm interval was obtained, outperforming the range of commercial infrared InGaAs streak cameras. The developed setup was applied for excitation-dependent time-resolved photoluminescence decay measurements in a GeSn and compared to the conventional upconversion technique with the optically delayed picosecond gate pulses. The new setup provided 2D spectro-temporal images for analysis. The photoluminescence decay times in the 30–80 ps range were obtained in the GeSn layer depending on the excitation pulse energy and spectral emission wavelength. Carrier thermalization was observed as a redshift of the photoluminescence spectra with time.

Barrier Molecules Based on Formamidinium for Low-Dimensional Perovskite Solar Cells

Low-dimensional perovskites are attractive materials for solar cells; this mainly includes barrier molecules based on ammonium or diammonium groups. In this work, we used barrier molecules based on a formamidinium (FA) functional group instead of the traditional ammonium functional group. Two FA-based barrier molecules were studied and compared: phenyl acetimidamide chloride (PhACCl) and benzamidine chloride (BzmCl). The main difference between the barrier molecules is the distance of the FA group from the aromatic ring, which results in the formation of two-dimensional (2D) perovskite in the case of PhACCl, whereas BzmCl formed a quasi-1D (one-dimensional) structure based on single-crystal X-ray diffraction (XRD). Absorbance, photoluminescence (PL), and XRD measurements show the existence of a 2D structure for PhACCl. The existence of a 1D phase in perovskite based on BzmCl as the barrier molecule inhibits charge transfer and forms pinholes in the film. Charge extraction and PL decay measurements show a high recombination rate with BzmCl, whereas with PhACCl there is better alignment of the energy levels to the selective contacts; this minimizes the hysteresis and enhances the cycle stability. The photovoltaic performance of low-dimensional n = 7 cells, based on PhACCl, shows 11.5% efficiency, with an open-circuit voltage of 1.08 V. This study investigates the use of FA-based barrier molecules and shows the importance of steric hindrance in the formation of a 2D perovskite structure.

Modifier’s influence on spectral properties of dysprosium ions doped lead boro-telluro-phosphate glasses for white light applications

Trivalent dysprosium doped Lead boro-telluro-phosphate glasses were synthesized with varying modifiers such as Li+, Zn2+, Ca2+, Sr2+, and Ba2+ using traditional route of glass preparation (melt quenching). The basic network formation and spectroscopical aspects of the as piped outed glasses was analyzed using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), UV–vis-NIR, photoluminescence (PL) and emission decay measurements. The existence of elementary functional groups pertaining to the present glass network was explored through FTIR spectral analysis. Bonding parameters (δ) were found to analyze the bonding nature of the rare earth ions and the surrounding metal ligand site. The optical bandgap, band tailing parameter and Urbach energy was found using Tauc’s method in order to identify the transparent region of the prepared glass samples. Furthermore, the symmetry of ligands around the Dy3+ ions has been explored with absorption spectral data by inspecting the Judd-Ofelt (JO) parameters and oscillator strengths of the as-synthesized samples. The radiative properties for instance branching ratios (βR), stimulated emission cross-section (σP) and lifetime (τR) of the meta stable states are estimated to realize the novel applications. The CIE colour coordinates and Y/B ratio values were estimated employing the photoluminescence spectra, whereas the colour purity and it’s colour correlated temperature (CCT) were determined following the (x,y) values thus confirms the generation of neutral white light emitting ability of the prepped samples. The decay or lifetime analysis shows the well fitted non-exponential behavior of the present Dy3+ doped Lead boro-telluro-phosphate glasses.

(Invited) Cavity Coupled Multi-Emitters in Carbon Nanotubes

The development of covalent diazonium functionalization of carbon nanotubes brings unique oportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and superlocalization measurements and their light emission properties using time-correlation techniques, photo-luminescence decay measurements.

Photoelectric Properties of Planar and Mesoporous Structured Perovskite Solar Cells.

The high efficiency of perovskite solar cells strongly depends on the quality of perovskite films and carrier extraction layers. Here, we present the results of an investigation of the photoelectric properties of solar cells based on perovskite films grown on compact and mesoporous titanium dioxide layers. Kinetics of charge carrier transport and their extraction in triple-cation perovskite solar cells were studied by using transient photovoltage and time-resolved photoluminescence decay measurements. X-ray diffraction analysis revealed that the crystallinity of the perovskite films grown on mesoporous titanium dioxide is better compared to the films grown on compact TiO2. Mesoporous structured perovskite solar cells are found to have higher power conversion efficiency mainly due to enlarged perovskite/mesoporous -TiO2 interfacial area and better crystallinity of their perovskite films.

Understanding the Interaction between Inorganic and Organic Excitonic Components of an Inorganic‐Organic Nanohybrid Associate

AbstractThe present study deals with the design, formation, and photophysical studies on light‐harvesting inorganic‐organic nanohybrid associates comprising a less‐toxic, inorganic core‐shell QD (Cu−In−S/ZnS) and an organic S0041 dye aggregate. One of the main objectives of this study is to understand the interaction between the inorganic and organic excitonic components of the hybrid system and its implication on the photophysical response of the system. The nanohybrid associate have been obtained through the self‐assembly of S0041 dye molecules (organic exciton) onto Cu−In−S/ZnS QD (inorganic exciton). The characterization of as‐synthesized QD have been done by conventional microscopic and spectroscopic measurements. The exciton domain length for the organic dye aggregates is estimated to be 24 nm. Steady‐state absorption, emission, and time‐resolved photoluminescence (PL) intensity decay measurements have been carried out to understand the interaction mechanism between inorganic and organic excitonic components and the overall photophysical behaviour of the hybrid system. The studies have revealed that the formation of hybrid associate is electrostatically driven and thermodynamically feasible. Interestingly, steady‐state and time‐resolved PL decay measurements and their analysis have revealed that the interaction between inorganic QD and organic aggregate is mediated through dipole‐dipole Förster energy transfer (ET) mechanism and the ET efficiency is found to be as high as 95%. The present system is expected to be used in fabrication of less‐toxic nanoscale light‐harvesting system.

Electrically Conductive Photoluminescent Porphyrin Phosphonate Metal–Organic Frameworks

AbstractHerein, the design and synthesis of a highly photoluminescent and electrically conductive metal–organic framework [Zn{Cu‐p‐H6TPPA}]⋅2 [(CH3)2NH] (designated as GTUB3), which is constructed using the 5,10,15,20‐tetrakis [p‐phenylphosphonic acid] porphyrin (p‐H8TPPA) organic linker, is reported. The bandgap of GTUB3 is measured to be 1.45 and 1.48 eV using diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy, respectively. The PL decay measurement yields a charge carrier lifetime of 40.6 ns. Impedance and DC measurements yield average electrical conductivities of 0.03 and 4 S m−1, respectively, making GTUB3 a rare example of an electrically conductive 3D metal–organic framework. Thermogravimetric analysis reveals that the organic components of GTUB3 are stable up to 400 °C. Finally, its specific surface area and pore volume are calculated to be 622 m2 g−1 and 0.43 cm3 g−1, respectively, using grand canonical Monte Carlo. Owing to its porosity and high electrical conductivity, GTUB3 may be used as a low‐cost electrode material in next generation of supercapacitors, while its low bandgap and high photoluminescence make it a promising material for optoelectronic applications.

Versatile compressive microscope for hyperspectral transmission and fluorescence lifetime imaging.

Increasing demand for multimodal characterization and imaging of new materials entails the combination of various methods in a single microscopic setup. Hyperspectral imaging of transmission spectra or photoluminescence (PL) decay imaging count among the most used methods. Nevertheless, these methods require very different working conditions and instrumentation. Therefore, combining the methods into a single microscopic system is seldom implemented. Here we demonstrate a novel versatile microscope based on single-pixel imaging, where we use a simple optical configuration to measure the hyperspectral information, as well as fluorescence lifetime imaging (FLIM). The maps are inherently spatially matched and can be taken with spectral resolution limited by the resolution of the used spectrometer (3 nm) or temporal resolution set by PL decay measurement (120 ps). We verify the system's performance by its comparison to the standard FLIM and non-imaging transmission spectroscopy. Our approach enabled us to switch between a broad field-of-view and micrometer resolution without changing the optical configuration. At the same time, the used design opens the possibility to add a variety of other characterization methods. This article demonstrates a simple, affordable way of complex material studies with huge versatility for the imaging parameters.

Noise effect on 2D photoluminescence decay analysis using the RATS method in a single-pixel camera configuration.

Using a random temporal signal for sample excitation (RATS method) is a new, capable approach to measuring photoluminescence (PL) dynamics. The method can be used in single-point measurement (0D), but also it can be converted to PL decay imaging (2D) using a single-pixel camera configuration. In both cases, the reconstruction of the PL decay and PL snapshot is affected by ubiquitous noise. This article provides a detailed analysis of the noise effect on the RATS method and possible strategies for its suppression. We carried out an extensive set of simulations focusing on the effect of noise introduced through the random excitation signal and the corresponding PL waveform. We show that the PL signal noise level is critical for the method. Furthermore, we analyze the role of acquisition time, where we demonstrate the need for a non-periodic excitation signal. We show that it is beneficial to increase the acquisition time and that increasing the number of measurements in the single-pixel camera configuration has a minimal effect above a certain threshold. Finally, we study the effect of a regularization parameter used in the deconvolution step, and we observe that there is an optimum value set by the noise present in the PL dataset. Our results provide a guideline for optimization of the RATS measurement, but we also study effects generally occurring in PL decay measurements methods relying on the deconvolution step.

Color tunable photoluminescence in KZABS: Tm3+ glasses under different sources of excitation for photonic applications

Potassium Zinc Alumino Borosilicate (KZABS) glasses doped with Tm3+ ions have been prepared by using a melt quenching process and further investigated through various spectroscopic tools such as optical absorption, photoluminescence (PL) excitation, PL emission, and PL decay measurements. The non-crystalline nature of the as-prepared glasses was confirmed by XRD measurements. PL spectral properties show blue and red colors under different excitation wavelengths. The absorption spectrum recorded in the UV-Vis-NIR range reveals the high bandgap energy. The Inokuti-Hirayama (I-H) model well fitted to non-exponential decay curves confirms the energy transfer between Tm3+-Tm3+ ions as dipole-dipole in nature. Under 359 and 466 nm excitation wavelength the titled glasses are showing CIE coordinates falling in deep blue and deep red color, respectively. A good color tunability can be seen from the blue to the red region under different excitation wavelengths. All the studies finally reveal the superiority of Tm3+ doped KZABS glasses for their usage in blue/red-emitting photonic devices needed to fabricate photonic devices such as w-LEDs.