Time-resolved Photoluminescence Studies Research Articles

Time-resolved photoluminescence study of tris(8-hydroxyquinolinato) aluminium with surface plasmon resonance of Ag nanoparticles

Surface plasmon resonance (SPR) of Ag nanoparticles (NPs), and its emission enhancement properties are investigated by time-resolved photoluminescence spectroscopy using tris(8-hydroxyquinolinato)aluminium (Alq3), which is generally applied in organic light-emitting diode, as the emitter. 5-nm-thick Ag island-films were fabricated, and annealed at various temperatures to vary the size of the Ag NPs. The peak position of the SPR band in the absorption spectra of Ag NPs blue-shifts with increasing annealing temperature, that is, the SPR frequency is controlled by the NP size. Comparison of the photoluminescence (PL) spectra of Alq3 with and without Ag NPs annealed at 150 °C indicates 1.2-fold enhancement of PL for Alq3 with Ag NPs compared to the reference Alq3. From the temperature dependence of the PL intensity and lifetime of Alq3 with Ag NPs, we evaluated the degree of enhancement of the radiative recombination rate in the presence of the localized surface plasmon of Ag NPs, in comparison with the competitive non-radiative quenching process.

Time-resolved photoluminescence studies on localization effects in orthorhombic phase of CH3NH3PbI3 perovskite thin film

We present the localization effects in orthorhombic phase of CH3NH3PbI3 perovskite materials by temperature-dependent photoluminescence and time-resolved photoluminescence. Our investigations indicate that the tetragonal inclusions in the orthorhombic phase of MAPbI3 at low temperature are the main reason for the formation of localization states with a depth of about 22.3 meV. The coexistence of the tetragonal and orthorhombic phases also results in an abnormal behavior of luminescence intensity from 12 to 80 K. The detailed understanding of luminescence characteristics upon the orthorhombic phase in MAPbI3 at low temperature can expand ideally the application fields of perovskite materials.

On the Origin of the Large Stokes-Shift of the Emission of CdS Nanoparticles Embedded in a Phosphate Glass Matrix

XRD and TEM characterisation evidenced the formation of well-dispersed CdS nanoparticles inside a phosphate glass matrix. Optical absorption and time-resolved photoluminescence study were carried out on the prepared glass samples. Optical absorption revealed the fast character of the growth of CdS nanoparticles in this medium. Photoluminescence spectra showed only one large band with a maximum at almost 740 nm, which was associated to transitions between energy levels within the bandgap of the CdS nanoparticles. From the steady state and time-resolved measurements, it was suggested that the emission comes mainly from sulfur vacancies inside the nanocrystals and on its surface, which act as deep traps for the photogenerated electrons. The creation of such vacancies was attributed to the loss of sulfur during the glass preparation as evidenced from a chemical analysis using energy dispersive X-ray spectrometry. These traps may be also induced by the fast growth of CdS nanocrystals in this matrix or laser exposure during PL measurements. These CdS-doped glasses with an intense absorption in the UV–Vis region and a large emission band with long lifetime and a large Stokes-shift are adequate for luminescent solar concentrators, photocatalytic applications and solid-state lasers.

Influence of Rutile-TiO2 nanorod arrays on Pb-free (CH3NH3)3Bi2I9-based hybrid perovskite solar cells fabricated through two-step sequential solution process

The organic–inorganic perovskite compound (CH3NH3)3Bi2I9 (MBI) has lower toxicity and higher stability than lead-based perovskite. In this work, we fabricated MBI perovskite-based solar cells (PSCs) through a two-step sequential-solution process by using rutile TiO2 nanorod array (NRA) films as the electron transporting layer (ETL) for the first time. The thickness, roughness, and crystal structure of the rutile TiO2 NRAs prepared by hydrothermal method with different reaction times were thoroughly characterized and correlated with the photovoltaic performance of PSCs. Results of morphological and structural analyses, as well as time-resolved photoluminescence studies, indicated the effect of TiO2 NRA film thickness on the carrier recombination and photovoltaic performance of MBI-based PSCs. The MBI-based device was also stable in an environment with >50% humidity for more than 67 days.

Amine-functionalized diatom frustules: a platform for specific and sensitive detection of nitroaromatic explosive derivative.

In the present study, an attempt was made to develop a proof of concept for the detection of nitroaromatic explosive derivatives through the photoluminescence (PL) quenching process using functionalized diatom frustules as a sensing platform. The diatom frustules are composed of nanostructured, highly porous biogenic silica material and emit strong, visible blue PL upon UV excitation. PL-active biosilica was isolated from the marine diatom Nitzschia sp. and was amine-functionalized to develop a sensing platform. Functionalized diatom frustules were further characterized using field emission scanning electron microscope and a series of spectroscopic methods. When nitroaromatic compounds were bound to the functionalized diatom frustules biosilica, the PL intensity from the functionalized biosilica was partially quenched due to the electrophilic nature of the nitro (-NO) groups. The quenching process confirmed the Meisenheimer complex formation and was investigated by using Fourier transform infrared spectroscopy and time-resolved photoluminescence studies. The developed platform was further evaluated for its sensitivity and specificity, and the limit of detection (LOD) of the assay was determined as 1μM for a series of nitroaromatic explosive compounds. In conclusion, the developed sensing platform will have great utility in the development of on-site detection platforms for sensitive detection of warfare explosive nitroaromatic compounds from the environment.

Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis

The present work focuses on understanding the heterojunction formation of graphene quantum dots (GQDs) and oxygen deficient TiO2 nanoparticle hybrid system and its enhanced photocatalytic activity under visible light illumination. We explain the formation of TiO2-GQD heterojunction through the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional (epoxy) groups in GQDs possibly via COTi bonds. Our FTIR, XPS and Raman results lend support to the proposed mechanism of heterojunction formation. In the TiO2/GQD hybrid, the Raman Eg(1) peak of anatase TiO2 is blue shifted indicating the strong interaction between the GQD and TiO2. The heterojunction formation was simulated through the density functional theory (DFT) calculation to obtain the optical spectrum on the hybrid between oxygen deficient TiO2 and oxygen functionalized GQDs. Interestingly, the calculated results for the hybrid structure show strong optical absorption in the visible to near infrared region, which is in close agreement with the experimental results. The TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation, as revealed from the steady state and time resolved photoluminescence studies. Interestingly, the photoluminescence intensity of the TiO2-GQD heterojunction was partially quenched indicating the electron transfer from GQDs to TiO2. The degradation rate constant (first order) for TiO2-GQD hybrid is 5.2 times higher than that of the TiO2. Free radical scavenger test revealed that OH radical played a major role in MB degradation as compared to O2°− radical. These results are significant for the development of metal free catalysts based on carbon nano-materials for ensuing optoelectronic, energy and environmental applications.

Synthesis, structural characterization, photo-physical and magnetic properties of cobalt salphen pseudo halide complexes showing meta-magnetic ordering

The solvo-thermal syntheses of [(CoSalphen)2Co (SCN)2]n (1), CoSalphen(NH3)(N3)(2), Na[CoIIIsalphen(N3)2](3), Na[CoIIIsalen(N3)2](4) and CoIIIsalen(NH3)(N3) (5) {salphen = N,N’-o-phenylene-bis(salicylideneimine)} are reported. The structural studies using X-ray diffraction measurements revealed that 1 crystalizes in a monoclinic C2/c space group. Two cobalt (II) metal centers in penta-coordinated and octahedral local coordination environments are bridged via alternating O and μ1,3 SCN bridges resulting in a novel 2D layered coordination polymer. Compound 2 is a trivalent mononuclear cobalt azido complex with an octahedral coordination environment. The magnetic investigations of 1 revealed ferromagnetic coupling (J = +49.1 cm−1) and meta-magnetic ordering. Time resolved photoluminescence studies of the complexes showed excited state lifetimes of (τ1 = 0.4675 ns, τ2 = 5.23 ns) for 1 and (τ1 = 0.5078 ns, τ2 = 6.79 ns) for 2.

Suppression of slow decaying emission in II-VI quantum dots with Förster resonance energy transfer

We report on time-resolved photoluminescence studies of Förster resonance energy transfer (FRET) in structures with two arrays of epitaxial Cd(Zn)Se quantum dots (QDs) of different sizes separated by the ZnSe barrier of a variable width. The acceleration of recombination rate of both fast and slowly decaying components of emission from the energy-donating small QDs with the decrease of the barrier width is well consistent with the FRET mechanism. The found Förster radii turn out to be different for the fast and slow components. The rate acceleration is accompanied by the strong suppression of the slow emission component related, presumably, to the dark excitons. These findings open a way to control the characteristic of QD-based devices.

Tuning Excitonic Properties of Monolayer MoS2 with Microsphere Cavity by High-Throughput Chemical Vapor Deposition Method.

Tuning the optical properties of 2D direct bandgap semiconductors is crucial for applications in photonic light source, optical communication, and sensing. In this work, the excitonic properties of molybdenum disulphide (MoS2 ) are successfully tuned by directly depositing it onto silica microsphere resonators using chemical vapor deposition method. Multiple whispering gallery mode (WGM) peaks in the emission wavelength range of ≈650-750 nm are observed under continuous wave excitation at room temperature. Time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy are conducted to study light-matter interaction dynamics of the MoS2 microcavities. TRPL study suggests radiative recombination rate of carrier-phonon scattering and interband transition processes in MoS2 is enhanced by a factor of ≈1.65 due to Purcell effect in microcavities. TA spectroscopy study shows modulation of the interband transition process mainly occurs at PB-A band with an estimated F ≈ 1.60. Furthermore, refractive index sensing utilizing WGM peaks of MoS2 is established with sensitivity up to ≈150 nm per refractive index unit. The present work provides a large-scale and straightforward method for coupling atomically thin 2D gain media with cavities for high-performance optoelectronic devices and sensors.

In situ decoration of plasmonic Au nanoparticles on graphene quantum dots-graphitic carbon nitride hybrid and evaluation of its visible light photocatalytic performance

This work spotlights the development of a plasmonic photocatalyst showing surface plasmon induced enhanced visible light photocatalytic (PC) performance. Plasmonic Au nanoparticles (NPs) are decorated over the hybrid nanosystem of graphitic carbon nitride (GCN) and graphene quantum dots (GQD) by citrate reduction method. Surface plasmon resonance (SPR) induced enhancement of Raman G and 2D band intensity is encountered on excitation of the Plasmonic hybrid at 514.5 nm, which is near to the 532 nm absorption band of Au NPs. Time-resolved photoluminescence and XPS studies show charge transfer interaction between GQD-GCN and Au NPs. Plasmonic hybrid exhibits an enhanced PC activity over the other catalysts in the photodegradation of methylene blue (MB) under visible light illumination. Plasmonic photocatalyst displays more than 6 fold enhancement in the photodecomposition rate of MB over GQD and nearly 2 fold improvement over GCN and GQD-GCN. GQD-GCN absorbs mostly in the near visible region and can be photoexcited by visible light of wavelength () < 460 nm. Plasmon activation in Au NPs decorated GQD-GCN could exploit the entire UV–visible light for photocatalysis. Furthermore, plasmonic Au act as antennas for accumulation and enhancement of localized electromagnetic field at the interface with GQD-GCN, and thereby promotes photogeneration of large numbers of carriers on GQD-GCN. The carriers are separated by charge transfer migration from hybrid to Au NPs. Finally, the carriers on the plasmonic Au nanostructures initiate MB degradation under visible light. Our results have shown that plasmon decoration is a suitable strategy to design a carbon based hybrid photocatalyst for solar energy conversion.

Inside Perovskites: Quantum Luminescence from Bulk Cs4PbBr6 Single Crystals

Zero-dimensional perovskite-related structures (0D-PRS) are a new frontier of perovskite-based materials. 0D-PRS, commonly synthesized in powder form, manifest distinctive optical properties such as strong photoluminescence (PL), narrow emission line width, and high exciton binding energy. These properties make 0D-PRS compelling for several types of optoelectronic applications, including phosphor screens and electroluminescent devices. However, it would not be possible to rationally design the chemistry and structure of these materials, without revealing the origins of their optical behavior, which is contradictory to the well-studied APbX3 perovskites. In this work, we synthesize single crystals of Cs4PbBr6 0D-PRS, and investigated the origins of their unique optical and electronic properties. The crystals exhibit a PL quantum yield higher than 40%, the highest reported for perovskite-based single crystals. Time-resolved and temperature dependent PL studies, supported by DFT calculations, and structural ...

Spectroscopic Study of Thiophene-Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes.

In this study, we report a detailed spectroscopic study concerning the energy levels and vibrational structure of thiophene-pyrrole-containing S,N-heteroacenes. The aim of the study is first, to understand the differences in the photoluminescence (PL) efficiencies in this structurally similar series and second, to compare the electronic structure of S,N-heteroacenes to that of linear acenes and phenacenes, with a view to derive guidelines for the design of singlet fission materials. For S,N-heteroacenes comprising seven fused heterocyclic rings, we observe a higher PL quantum yield for derivatives with terminal thienothiophene units than for thienopyrrole-capped ones. This is assigned to a stronger tendency of the thienopyrrole-capped derivatives to form nonemissive associates in dilute solution, producing emissive excimers at higher concentration. By conducting time-resolved PL studies at 77 K, we further determine the lowest singlet and triplet energies for the S,N-heteroacenes with three, five, and seven fused rings. We show that their energies evolve with oligomer length analogously to those of phenacenes, yet in a fundamentally different way from that of linear acenes. This difference in evolution is attributed to the increasingly biradical character in acenes with increasing chain length in contrast to the S,N-heteroacenes and phenacenes.

Optical limiting properties and time-resolved photoluminescence study on a series compounds based on 1,3,5-triazine: From linear to tri-branched and six-branched structural configurations

Z-scan and time-resolved photoluminescence experiments were performed to study the optical limiting properties and the excited state dynamics of a series compounds based on 1,3,5-triazine. The optical limiting properties were greatly enhanced when multi-branched structural configurations were built. The more branches the better optical limiting properties. The fluorescence quantum yields of multi-branched compounds were decreased with the number of the branches. Obvious different excited states responses were determined by time-resolved photoluminescence. The fluorescence lifetimes were determined to be ∼2.9ns, ∼2.3ns, and ∼0.5ns, for compounds with linear, tri-branched, and six-branched structural configurations, respectively. The obtained ultrafast dynamics properties are well accorded with the fluorescence quantum yield results.

Ag doping of Zn-In-S quantum dots for photocatalytic hydrogen evolution: Simultaneous bandgap narrowing and carrier lifetime elongation

Visible-light-driven photocatalytic hydrogen production has been an ongoing hot topic for clean and renewable energy, for which I–III–VI multinary metal sulfides play an important role owning to their unique ability of bandgap manipulation by composition. In this work, we present the controlled synthesis and hydrogen evolution property of a series of Ag:Zn-In-S quantum dots (QDs) in an effort to a more comprehensive understanding of Ag doping. With increasing ratio of Ag, the UV–vis absorption and photoluminescence wavelength was observed to be tuned in a wide range from light green to dark red. With increased Ag doping, dramatic increase (over 70 times) of the photocatalytic activity was observed (maximized with Ag:In:Zn ratio of 1.5:10:5). More importantly, time-resolved photoluminescence study reveals that this synergistic enhancement effect of the photocatalytic activity by controllable Ag doping was correlated with the simultaneous bandgap narrowing and carrier lifetime elongation, which contribute to the enhanced visible light absorption and charge separation, repectively. The enhanced charge separation upon Ag doping were further proved by photocurrent and electrochemical impedance spectra measurements. A mechanism is proposed for this synergistic effect of photocatalysis by suitable Ag doping, where the long-lived deep donor-acceptor pair states play an important role. Our results provide an interesting view and useful guideline for photocatalyst design using the narrow bandgap multinary sulfides.

Trap-Free Hot Carrier Relaxation in Lead–Halide Perovskite Films

Photovoltaic devices that employ lead–halide perovskites as photoactive materials exhibit power conversion efficiencies of 22%. One of the potential routes to go beyond the current efficiencies is to extract charge carriers that carry excess energy, that is, nonrelaxed or “hot” carriers, before relaxation to the band minima is completed. Lead–halide perovskites have been demonstrated to exhibit hot-carrier relaxation times exceeding 100 ps for both single- and polycrystalline samples. Here, we demonstrate, using a combined time-resolved photoluminescence and transient absorption study supported by basic modeling of the dynamics, that the decay of the high-energy part of the photoluminescence occurs on a time scale (∼100 ps) very similar to the repopulation of the band minima when excited with a photon energy larger than 2.6 eV. The similarity between the two time scales indicates that the depopulation of hot states occurs without transient trapping of electrons or holes.

Preparation and characterization of some electrospun polysulfone nanocomposites reinforced with Ni doped SnO2 nanoparticles

Nickel doped tin dioxide nanoparticles (NPs) have been synthesized by precipitation method. Polysulfone (PSU)/NPs nanocomposites were prepared by electrospinning procedure using two precursor solutions. The resultant nanostructures were investigated by X-ray diffraction, electron scanning microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX), which confirmed well oriented nanofibers and good dispersion of NPs over them. The nanocomposites exhibit a strong UV emission band positioned at about 374nm (λex=320nm) and the blue and green emissions are suppressed. The time-resolved photoluminescence studies have been performed and the lifetimes were evaluated. The fluorescence decay reveals a biexponential behavior. A higher thermal stability was observed after incorporation of nanoparticles in nanofiber composites. Gas response properties (methanol, ethanol, ammonia, formaldehyde and butane) at different temperatures from 50 to 180°C were discussed. Gas sensing experiments suggest that PSU/Ni doped tin oxide nanocomposites showed highly enhanced sensing performances to methanol and ammonia as compared to pure PSU/tin oxide nanofiber. The PSU/NPs nanocomposites could be used for methanol and ammonia detection at room temperature.

Photoluminescence and time-resolved photoluminescence studies of lateral carriers transfer among InAs/GaAs quantum dots

We report on the lateral transfer and thermal escape of carriers in InAs quantum dots (QDs) grown on a GaAs substrate by solid source molecular beam epitaxy by mean of photoluminescence (PL) and time-resolved PL measurements. The temperature-dependent PL spectra are discussed in terms of the inhomogeneous size distribution of the QDs and the carrier tunneling process from small to large QDs. The dependence of the photoluminescence decay time on the emission-wavelength is attributed to lateral carriers’ transfer within QDs with an interdot carrier tunneling time of 910 ps under low excitation conditions.

ZnSe quantum dots sensitized electrospun ZnO nanofibers as an efficient photoanode for improved performance of QDSSC

In the present study, zinc selenide (ZnSe) quantum dots (QDs) with an average size of ~2.6nm were prepared by hot injection method and used as a sensitizer onto the electrospun ZnO nanofibers using 3-mercaptapropionic acid as a linker agent. The optical absorption, photoluminescence and time-resolved photoluminescence (TRPL) studies for ZnSe sensitized ZnO NFs were performed to give insight about the improvement in optical properties. The performances of fabricated QDSSCs was examined in detail using cobalt sulfide (CoS) as a counter electrode and polysulfide redox couple (S2−/Sx2−) as an electrolyte. The ZnSe QDs sensitized ZnO nanofibers showed an appreciable improvement in short circuit current density (6.60mA/cm2) with a maximum power conversion efficiency of 1.24% under 1 sun illumination of 100mW/cm2. This enhancement is mainly due to better light harvesting ability of ZnSe QDs and ZnO NFs, and lower recombination of photoinjected electrons with the polysulfide electrolyte. The improvement in power conversion efficiency (PCE) and reduction in back electrons recombination are supported by photovoltaic and electrochemical impedance studies. Finally, stability test was carried out over a span of 30 days (720h) under one sun illumination to know about the practical applicability of the resultant QDSSC.

An Interactive Quantum Dot and Carbon Dot Conjugate for pH-Sensitive and Ratiometric Cu2+ Sensing.

Herein we report the photoinduced electron transfer from Mn2+ -doped ZnS quantum dots (Qdots) to carbon dots (Cdots) in an aqueous dispersion. We also report that the electron transfer was observed for low pH values, at which the oppositely charged nanoparticles (NPs) interacted with each other. Conversely, at higher pH values the NPs were both negatively charged and thus not in contact with each other, so the electron transfer was absent. Steady-state and time-resolved photoluminescence studies revealed that interacting particle conjugates were responsible for the electron transfer. The phenomenon could be used to detect the presence of Cu2+ ions, which preferentially, ratiometrically, and efficiently quenched the luminescence of the Qdots.

Boosting Visible-Light-Driven Photocatalytic Hydrogen Evolution with an Integrated Nickel Phosphide-Carbon Nitride System.

Solar light harvesting by photocatalytic H2 evolution from water could solve the problem of greenhouse gas emission from fossil fuels with alternative clean energy. However, the development of more efficient and robust catalytic systems remains a great challenge for the technological use on a large scale. Here we report the synthesis of a sol-gel prepared mesoporous graphitic carbon nitride (sg-CN) combined with nickel phosphide (Ni2 P) which acts as a superior co-catalyst for efficient photocatalytic H2 evolution by visible light. This integrated system shows a much higher catalytic activity than the physical mixture of Ni2 P and sg-CN or metallic nickel on sg-CN under similar conditions. Time-resolved photoluminescence and electron paramagnetic resonance (EPR) spectroscopic studies revealed that the enhanced carrier transfer at the Ni2 P-sg-CN heterojunction is the prime source for improved activity.