PL-decay Profile Research Articles

Multi-color emission composites of white carbon dots and dyes through inner filter effect

A series of multi-color carbon dot composites were obtained by transforming white carbon dots (WCDs) into three primary colors using the dyes' inner filter effect (IFE). WCDs were combined with three distinct dyes (methyl red, bromocresol green, and methylene blue), and three fluorescence colors (red, yellow, and blue) were obtained. The colors of the emitting light excited by the same UV light were similar to the colors of the respective dyes, and have a large Stokes shift. The presence of the IFE between WCDs and dyes was proved through a variety of methods, the analysis of the relationship between the UV of dyes and fluorescence spectra of WCDs, the excitation-emission matrix, as well as the comparison PL decay profiles of WCDs before and after dye addition. The Stern-Volmer quenching constant KSV exhibited no temperature dependence during the IFE process with increasing concentrations of dyes. The UV spectra of the mixture of WCDs and dyes were simply the sum of two individual components, which implies that there is no chemical interaction between WCDs and dyes. Since methyl red and bromocresol green are acid-base indicators, the fluorescence color of the carbon dot composite also changed continuously during the dilute NaOH solution titration process. In furtherment, distributing the binary system of “WCDs + dye” within poly (vinyl alcohol) (PVA), three colors fluorescent composite films of “WCDs / Dye / PVA” were produced successfully.

High-throughput transient photoluminescence spectrometer for deep learning of thermally activated delayed fluorescence materials

The fast acquisition of high dynamic range PL decay profile is realized and new PL decay parameters for deep learning of thermally activated delayed fluorescence materials are presented.

The growth and luminescence mechanism of novel near-infrared Cs4Eu(Br,I)6:Sm crystal

A novel NIR emitting scintillator with 825 nm was developed by doping Sm2+ in Cs4Eu(Br,I)6 crystal. The single crystals of Cs4Eu(Br,I)6:Sm were successfully grown by the Bridgman method. The crystal structure and valence state was approved by XRD and XPS methods. The luminescence properties and energy transfer were systematically investigated. The energy transfer mechanism is variable for different Sm2+ concentration. The PL decay profiles show that the Sm2+-related emission has fast decay of nanosecond. The proportion of fast decay can be tunable by adjusting Sm2+ doped concentration. The Sm2+ doped europium -host material will have a potential application for unclear detection as near-infrared (NIR) scintillators.

Decorating wide band gap CH3NH3PbBr3 perovskite with 4AMP for highly efficient and enhanced open circuit voltage perovskite solar cells

Halides based organic-inorganic solar cells have attracted huge attention for efficient energy harvesters owing to their low fabrication cost, large functioning area, long lifetime, and high conversion efficiency. Here we report the fabrication of pristine and aminomethyl piperidinium (4AMP) doped CH3NH3PbBr3 based thin film using spin coating technique to explore its potential for energy harvesting. The structural analysis confirmed the development of the rhombohedral structure of pristine and doped CH3NH3PbBr3. SEM and AFM images revealed that grains for the pure sample are highly compact linked with each other with sharp grain boundaries but their size increased when doped with 4AMP. A direct bandgap of 2.341 eV is noticed for pristine which is slightly decreased to 2.311 eV as a result of doping. The calculation of space charge limited current, PL decay profile analysis, charge transfer resistance, and Mott Schottky analysis provide quite convincing evidence regarding the high conversion efficiency of these films. A sufficiently high value of short circuit current density resulted in high efficiency of pristine MAPbBr3 i.e., 8.46% which increased to 10.21% due to improvement of solar cells parameters such as Voc of 1.46 V when doped with 4AMP. These values of efficiencies make the halide based organic compositions viable for solar cell applications.

An Optically Reconfigurable Förster Resonance Energy Transfer Process for Broadband Switchable Organic Single-Mode Microlasers

An Optically Reconfigurable Förster Resonance Energy Transfer Process for Broadband Switchable Organic Single-Mode Microlasers

Temperature dependence of photoluminescence properties of water-soluble CdS quantum dots

We investigated the size dependence of band-edge photoluminescence (PL) dynamics for CdS quantum dots (QDs). The temperature dependence of the PL-decay profiles of CdS QDs with an average diameter of 3.7–6.0 nm was measured. The PL-decay profiles became longer as the temperature increased. Further, it was found that the temperature dependence of the PL-decay profiles depends greatly on the QD size. These experimental results can be understood by considering that the magnitude of the splitting energy between the bright-and dark-exciton states depends on the QD size and becomes larger as the QD size becomes smaller.

Temperature dependence of photoluminescence properties of water-soluble ZnSe quantum dots

We have investigated the optical properties of ZnSe quantum dots (QDs) prepared by a hydrothermal method. The photoluminescence (PL)-decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behaviour due to thermal quenching. The temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground state and two excited states of the lower-lying bound-exciton and higher-lying dark-exciton states. The analysis of the temperature dependence of the decay time indicates that the dark-exciton state contributes to the PL-decay process in the ZnSe QDs.

Multiple-Heterojunction in Single Titanium Dioxide Nanoparticle for Novel Metal-Free Photocatalysis

Despite longstanding controversy, TiO2 polymorphs heterojunction composed of anatase and rutile outperform the individual ones because of the energetic type-II band alignment at the heterojunction interface. Apart from the conventional polymorphs interface, the surface disordered TiO2 has been drawing interest and considered as an unpredictable strategy while is regarded as a fact for dramatically enhancing photocatalytic activity. However, introduction and localization of the disordered layer within conventional TiO2 nanoparticles with the heterojunction has remained a big challenge. Here we report a selective positioning of a disordered layer with different thickness ranges in between the anatase and rutile phases by a conceptually different synthetic route for highly efficient novel metal-free photocatalytic H2 production (denoted as DE-P25). With two step disorder engineering: Phase selective disordering of rutile TiO2 and thermal annealing process at temperature lower than phase transformation, the multiple heterojunction in DE-P25 single nanoparticle with the disordered layer of 2~3 nm could be synthesized which is confirmed by high resolution-TEM, electron energy loss spectroscopy (EELS) and inline electron holography mapping for the first time. EELS Ti-L2,3 spectrum geometry of DE-P25 showed distinctive oxygen-deficient variation between the two crystals which corresponds with the drastic decrease of potential and negative charge density within the disordered layer due to localized unpaired electrons. PL decay profiles exhibiting ultra-fast charge migration of DE-P25 indicate that the rutile phase incorporated with the disordered layer in the TiO2 nanoparticle induces dominant direct excition formation and reduces the self-trap of charge carriers, thus suppressing the electron/hole recombination, which perfectly coincides with the holography mapping analysis. This multiple heterojunction in single TiO2 nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile demonstrate not only superior interfacial charge separation efficiency, but also novel-metal free surface reactivity, which synergistically yields ~140 times higher H2 production rate than conventional anatase and rutile single heterojunction system. Figure 1

Unusual Nonemissive Behavior of Rubrene J-Aggregates: A Rare Violation.

Structure-property correlations in rubrene (RB) colloidal J-aggregates were unravelled by steady state and time-resolved spectroscopy in conjunction with excited state density functional calculations. The RB J-aggregate with a slippage angle θ = 30.4°, estimated from the monomeric transition dipole moment directions, exhibited a broad fwhm of 1073 cm-1 and a 5 nm red-shifted absorption band carrying a transition dipole moment (M⃗λagg = 1.80 D) almost equivalent to the monomeric dye (M⃗λmon = 1.89 D). A significantly low magnitude of exciton coupling energy, ΔEexc = -358 cm-1 for the rhombic-RB colloidal J-aggregates resulted owing to the weaker electronic communication between the largely separated RB subunits (r = 7.2 Å) and a restricted exciton delocalization over the RB J-dimer (N = 2). The RB J-dimer exhibited a perfect balance between the computed singlet (2.53 eV) and the triplet (1.29 eV) exciton energies for singlet fission (SF). Supporting this, the PL decay profile of the J-aggregates revealed a delayed fluorescence, substantiating triplet pair formation via SF. The experimental evidence for the long-lived triplet formation was furthermore confirmed by its transient absorption (T1 → TN) at 530 nm. Consequently, a high probability for SF and a low probability for triplet-triplet recombination, leading to a dramatic lowering in photoluminescence quantum yield from 0.172 down to 0.035 was noted. The electronic structure calculations for the RB J-dimer followed TD-DFT-M062X/6-31G+(d,p) level of theory following integral equation formalism polarizable continuum model (IEFPCM) in water. S1 excited state for RB J-dimer was carefully analyzed using integral overlap of electron and hole density distribution (ϕ) and the defined t-indexes along all three spatial directions, and was found to be of locally excited in character.

Temperature dependence of photoluminescence dynamics of self-assembled monolayers of CdSe quantum dots: the influence of the bound-exciton state

We have investigated the temperature dependence of photoluminescence (PL) dynamics of self-assembled monolayers (SAMs) of CdSe quantum dots (QDs). The PL decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behavior due to thermal quenching. Such anomalous temperature dependence of the PL-decay profile is explained using a four-state model which introduces a bound-exciton state into a conventional three-state model consisting of a ground state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state. Furthermore, it is proposed that the radiative decay time of QDs is strongly influenced by the presence or absence of the bound-exciton state.

Photoluminescence dynamics of exciton-exciton scattering in a lightly alloyed InGaN thin film

We have investigated the photoluminescence (PL) dynamics of a lightly alloyed In0.02Ga0.98N thin film under intense excitation conditions at 10 K. In the In0.02Ga0.98N thin film, a PL band due to exciton-exciton scattering, the so-called P band, appears with a thresholdlike nature in the excitation-power region higher than ∼3 μJ/cm2. Under the condition that the exciton-exciton scattering occurs, the PL-decay profile consists of a fast decay component of the P band (of the order of 10 ps) and a slow decay component of a localized exciton band. The decay time of the P band gradually shortens with increasing excitation power. The change in the decay time can be explained qualitatively by the photon-like characteristics of the lower polariton branch that is the final state of the P emission. Furthermore, we have clearly observed the temporal change in the peak energy of the P band, which reflects an effective temperature of the excitonic system. The energy shift of the P band suggests that the P emission process reaches equilibrium with the lattice temperature after ∼100 ps.

Temperature Dependence of Photoluminescence Dynamics in Colloidal CdS Quantum Dots

We have investigated the temperature dependence of photoluminescence (PL) dynamics in CdS quantum dots (QDs) prepared by a colloidal method. A size-selective photoetching process and a surface modification technique with a Cd(OH)2 layer enabled us to prepare size-controlled CdS QDs with high PL efficiency. The PL decay profiles became slower with an increase in temperature, contrary to an ordinary behavior. We have revealed that such anomalous temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground-state and two excited states: a lower-lying bound-exciton state and a higher-lying free-exciton state (the “dark-exciton state”) having an optically inactive triplet nature.

Optical properties of GaInNAs quantum wells on misoriented substrates grown by MOVPE

We have investigated the optical properties of GaInNAs/GaAs single quantum wells grown by metalorganic vapor-phase epitaxy on misoriented GaAs substrates with the use of photoluminescence (PL) spectroscopy sensitive to localization of carriers and photoreflectance (PR) spectroscopy sensitive to the intrinsic band-edge transition. We have found from systematic results of PL spectra, PL-decay profiles and PR spectra that the band-edge energy of the quantum well is lowered with increasing offset angle from 2° to 15° toward (1 1 1)A and that the localization of carriers due to disorders of the band-edge state is enhanced.

Photoluminescence dynamics of energy transfer between CdS quantum dots prepared by a colloidal method

We have investigated the energy transfer process in CdS quantum dots (QDs) prepared by a colloidal method. By applying a surface-modification method, the band-edge photoluminescence (PL) is intensely observed as a main PL band. The surface-modified QDs were dispersed in polyvinyl alcohol films, where the QD solutions were variously concentrated by a factor from 1 to 250. The change of the degree of concentration corresponds to the change of a mean distance between QDs. In the 1-, 10-, and 75-times concentrated samples, the decay profiles of the band-edge PL do not depend on detection energies and exhibit a slow decay in the order of hundreds of nanosecond. On the other hand, the PL-decay profiles in the 250-times concentrated sample exhibit faster decays as compared with the dilute samples. The faster decay suggests a new decay channel due to an effective energy transfer process between CdS QDs.

Stretched exponential profiles of photoluminescence decays related to localized states in InGaAsN/GaAs single-quantum wells

We have investigated photoluminescence (PL) dynamics related to localized states in In x Ga 1− x As 1− y N y /GaAs single-quantum wells (SQWs) with the constant In content of x = 0.32 and various N contents of y = 0 , 0.004 , and 0.008 . In order to determine the intrinsic band-edge energy, we used photoreflectance (PR) spectroscopy that is sensitive to the optical transitions at critical points. From systematic measurements of the PL and PR spectra, it is demonstrated that the slight incorporation of nitrogen considerably disorders the band-edge states of the InGaAsN SQWs, resulting from formation of localized states, so-called band-tail states. We find that the PL-decay profile related to the localized states generally exhibits a stretched exponential behavior peculiar to a disordered system at low temperatures, which means that randomness of alloy potential fluctuations including nitrogen dominates the PL dynamics.

Strong enhancement of band-edge photoluminescence in CdS quantum dots prepared by a reverse-micelle method

We have investigated the effects of surface modification on photoluminescence (PL) properties of CdS quantum dots (QDs) prepared by a reverse-micelle method. It is demonstrated that the modification of QD surface with a Cd(OH)2 layer leads to a strong enhancement of the band-edge PL intensity of CdS QDs in reverse micelles. The PL-decay profile before the surface modification exhibits a fast decay component less than 50ps. After the modification, the fast-decay component disappears. The drastic change of the PL-decay profiles before and after the surface modification corresponds to the marked increase of the PL intensity. This suggests that the strong enhancement of the band-edge PL intensity originates from the remarkable reduction of nonradiative recombination processes, which usually result in a very short decay time, due to surface defects of QDs.

Luminescence Quenching of Dyes by Oxygen in Core−Shell Soft-Sphere Ionic Liquids

We examined the photoluminescence and susceptibility to quenching by atmospheric oxygen of a series of phosphorescent dyes dissolved in a core−shell “soft-sphere” ionic liquid matrix consisting of a 7 nm diameter silica core surrounded by a mobile phase consisting of end-grafted flexible chains. While this medium is macroscopically uniform in composition, it is locally heterogeneous on the nanometer length scale. Luminescent dyes provide an opportunity for assessing some of the properties of this local heterogeneity. The PL decay profiles of both platinum ocatethyl porphine (PtOEP) and [Ru(dpp)3]Cl2 could be fitted to a simple exponential form over a range of partial oxygen pressures. Taking the behavior of PtOEP as “normal,” we calculate an oxygen permeability in the liquid of PO2 = 4 × 10-12 mol cm-1 s-1 atm-1, comparable to that of poly(n-butyl thionylphosphazene) (C4PATP) but smaller than that (18 ± 2 × 10-12 mol cm-1 s-1 atm-1 for poly(dimethylsiloxane) (PDMS). The smaller slope of the Stern−Volmer quenching plot for [Ru(dpp)3]Cl2 could be rationalized in terms of two factors, the known smaller cross section for quenching by oxygen for this dye coupled with the likelihood that this dye is located in a more rigid ionic environment characterized by a somewhat smaller local diffusion coefficent for oxygen.

Photoluminescence properties of localized states caused by nitrogen alloying in a GaInNAs/GaAs single quantum well

We have investigated photoluminescence (PL) properties of localized states induced by nitrogen alloying in a Ga 1− x In x N y As 1− y /GaAs single quantum well (SQW) with x=0.33 and y=0.008, comparing with those in a Ga 1− x In x As/GaAs SQW without nitrogen. In order to determine the intrinsic band edge, we also used photoreflectance (PR) spectroscopy that is sensitive to the optical transitions at critical points. The PR line width of the band-edge transition in the GaInNAs/GaAs SQW is much broader than that in the GaInAs/GaAs SQW, which indicates that considerable disorders are introduced by nitrogen alloying. The PL-peak energy of the GaInNAs/GaAs SQW, which has a Stokes shift of ∼20 meV at 10 K, continuously shifts to the band edge with an increase in excitation power, reflecting the localization nature of carriers. The PL-decay profile related to the localized states clearly exhibits a stretched exponential feature peculiar to a disordered system. It is concluded that the PL properties at low temperature are dominated by random potentials caused by nitrogen alloying.