Time-resolved Photoluminescence Properties Research Articles

Photophysical Properties of the PVK-MEH-PPV/PCBM Composite for Organic Solar Cells Application: Synthesis, Characterization and Computational Study.

The physical and chemical properties of a new organic composite including PVK-MEH-PPV bi-block copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were recorded. The functionalization and the charge transfer that occurs between donor and acceptor were examined and computed. In fact, the stationary and time-resolved photoluminescence properties were used to examine the effect of the PCBM on the optical properties of the PVK-MEH-PPV matrix. The photoluminescence quenching accompanied by faster PL decay confirmed the charge transfer and interaction process. The electrical and optoelectronic properties and the charge carriers’ injection in the resulting composite were examined. The experimental conclusion was corroborated and confirmed by a calculation based on density functional theory (DFT). Hence, the combination of experimental and theoretical results indicated that the result composite can be applied as an active layer for organic solar cells.

Photoluminescence and time resolved photoluminescence properties in as grown ZnO thin films prepared by DC reactive sputtering for optoelectronic devices

Results of steady-state and time-resolved Photoluminescence (PL) measurements performed on ZnO thin films, deposited on (001) p-doped silicon substrate, by DC reactive sputtering technique for different growth times are presented. In all cases, two emission bands are observed. One is an exciton emission band and the second an intense and broad visible emission band.As the growth time increases, the intensity of the exciton emission decreases while that of the visible emission increases. Time-resolved PL (TRPL) measurements at the UV emission peak show a bi-exponential decay behavior. As the growth time increases, the fast and slow decay lifetimes increase together with a decrease of the amplitude ratio (AR) of the slow decay component to the rapid one due to the reduction of the non-radiative pathways when the oxygen deposition time is short. Moreover, slower rise time of the TRPL signal is observed when the growth time increases, proving the increase of the trap states density in the ZnO band gap. These results suggest that the amount of oxygen deposited as reactive gas in DC reactive sputtering technique must be optimized to improve the performance of UV light-emitting applications.

Dark-level trapping, lateral confinement, and built-in electric field contributions to the carrier dynamics in c-plane GaN/AlN quantum dots emitting in the UV range

c-plane GaN/AlN quantum dots (QDs) are promising zero-dimensional quantum nanostructures that exhibit single photon emission properties up to room temperature and even above. In this context, it is of prime interest to gain a deeper insight into the recombination dynamics of photogenerated electron–hole pairs captured by such dots. Hence, in this work, we study the time-resolved photoluminescence (PL) properties in the low injection regime and at cryogenic temperatures of c-plane GaN/AlN QD ensembles emitting above the bulk GaN bandgap in order to properly understand the nature of the recombination channels behind the observed non-exponential decay time profiles. Such decays reveal the existence of a relaxation channel competing with the radiative recombination one. It is thus observed that for the former process the dynamics is independent of the dot height, which is attributed to a reversible nonradiative transfer that could be mediated by a spin-flip process to a dark-level state. The radiative recombination process is recognizable thanks to the characteristic dependence of its lifetime with the emission energy, which is well accounted for by the built-in electric field inherent to quantum nanostructures grown along the c axis and the variations in the lateral confinement at play in such QDs. Those conclusions are drawn from the analysis of the time evolution of the PL spectra by means of a simple analytical model that enables to exclude any screening of the built-in electric field.

Influence of interstitial UO22+ doping on the valence control of Eu and energy transfer to substitutional Eu3+ and Sm3+ in SrBPO5.

Controlling the valence mixing of Eu3+/Eu2+ and energy transfer between activator ions in solid solution is an important process to improve the efficiency and specificity of phosphors. In this work, the structural and optical properties of stillwellite type SrBPO5 doped with uranium/europium/samarium, as prepared by conventional solid-state reaction synthesis, were investigated. PXRD studies and Rietveld analysis were carried out to determine the structure, phase purity, and coordination environment of the dopants in the host matrix. Samarium existed only as a trivalent cation in SrBPO5 synthesised in an air atmosphere, whereas europium exhibited abnormal reduction and Eu2+ co-existed with Eu3+. Unlike Eu and Sm, which replaced Sr at its sites, uranium gets stabilised as UO22+ in the interstitial vacant space in the SrBPO5 lattice. Uranium co-doping strongly influenced the Eu valence distribution by favouring the re-oxidation of Eu2+ to Eu3+. Possible mechanisms of Eu abnormal reduction in SrBPO5 and Eu valence control based on the electron transfer from substitutional Eu2+ to interstitial UO22+ are discussed. The photoluminescence and time-resolved photoluminescence properties of Eu/Sm/U doped SrBPO5 were investigated systematically. Uranium co-doping significantly enhanced the emission intensities of trivalent Eu and Sm through the exchange-type energy transfer. Besides, the Eu3+ luminescence intensity was further amplified in the presence of uranium due to the partial oxidation of Eu2+ to Eu3+. By controlling the uranium to Eu concentration ratio, the Eu-SrBPO5 phosphor could be tuned to different CIE coordinates.

Controllable growth of Ga2O3 hexagonal prism with pyramid end and monitoring size-dependent electron transfer process in perylene/Ga2O3 conjugate system via FLIM

Hexagonal prisms with hexagonal pyramid end of Ga2O3 microstructures were successfully prepared on gallium arsenide substrate by double cell electrochemical etching method under various growth conditions. The field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and fluorescence lifetime imaging microscopy (FLIM) were used to characterize the hexagonal prisms. The EDS analysis confirmed that the prepared prisms were composed dominantly of gallium (Ga) and oxygen (O) with atomic ratio of ~ 2:3. The binding energy of the Ga 2p and Ga 3d XPS peaks revealed that the formation of Ga–O bonding with the highest oxidation state of Ga (Ga3+) and the formed hexagonal microstructures were Ga2O3. The PL spectra at room temperature under excitation at 290 nm exhibit a strong blue emission peak located at about 470 nm. The origin of this PL peak can be attributed to exciton recombination at different intrinsic defect sites such as O vacancies and Ga vacancies. Moreover, FLIM technique has been employed to probe time-resolved photoluminescence properties of individual Ga2O3 prisms with hexagonal tip. FLIM images provide two-dimensional spatial maps of the carrier recombination lifetime which considered the most critical parameter to improve the performance of optoelectronic devices. It has been concluded that the carrier recombination lifetime was strongly sensitive to the size of the hexagonal prism-shaped structures due to surface-related defects. Additionally, a quantitative analysis of a conjugate system prepared by functionalizing hexagonal Ga2O3 prisms with perylene organic dye molecules was reported in this work. The size-dependent electronic coupling between the conduction band level of gallium oxide and excited states of perylene molecules was investigated. We observed that highly efficient electron transfer process occurs for aggregated molecules due to excimer trap states. The grown prism structures seem to be interesting for applications in optoelectronic devices as well as to be used as an effective host medium for organic dye molecules in solar cell applications.

Improved optical properties of ion beam irradiated (K,Na)NbO3 thin films

In the present study, we have demonstrated the effect of swift heavy ion (SHI) irradiation on photoluminescence (PL) and time-resolved photoluminescence (TRPL) properties of potassium sodium niobate (KNN) thin films deposited on Si and quartz substrates using RF magnetron sputtering. Ion beam irradiation of crystalline KNN films was carried out at room temperature using 100 MeV Ni ions with different fluences such as 1 × 1012, 5 × 1012 and 1 × 1013 ions/cm2. Various modes obtained in Raman spectra of films are related to NbO6 octahedron which confirms the crystalline phase of KNN. Moreover, the decrease in peak intensities with ion fluence is attributed to defects produced after SHI irradiation. X-ray photoelectron spectroscopy results show the increase in oxygen vacancies after irradiation. The optical properties of pristine and irradiated films were evaluated using UV–Vis–NIR spectroscopy and a significant improvement in optical transmittance upon irradiation was observed. The optical band gap of films is decreased to 3.14 eV upon irradiation at 1 × 1012 ions/cm2. PL spectra of films were obtained with excitation wavelength of 274 nm and, the results depict the emission wavelengths (band-to-band or near band edge) of KNN thin films in regime of 313–367 nm (3.96–3.38 eV). The origin of blue and green emission is due to defects created as a result of ion irradiation. The green luminescence evolved at higher fluence (i.e. 1 × 1013 ions/cm2) may be attributed to intrinsic defects such as oxygen vacancies. The obtained Commission Internationale de I’Elcairage (CIE) color coordinates are shifted towards white point in chromaticity diagram as a function of ion fluence. TRPL result reveals the decay lifetime of KNN films in nanosecond regime and, is varied with ion fluence. At fluence of 1 × 1013 ions/cm2, film exhibited the minimum average decay lifetime (1.75 ns) which suggests that KNN can be a potential candidate for optical switching, optical display and sensors, and opto-electronic device applications.

Pure and Efficient Single-Photon Sources by Shortening and Functionalizing Air-Suspended Carbon Nanotubes

A single-photon source (SPS) based on a single-walled carbon nanotube (SWCNT) is a promising candidate for uncooled on-chip quantum information optoelectronics because a single photon can be generated at both room temperature and telecommunication wavelengths on silicon chips. However, for the applications of quantum information, such as quantum computing and quantum cryptography, higher performance SPSs that exhibit both high purity and high efficiency of single-photon generation are required. Here, we theoretically propose high-performance SPSs that simultaneously achieve high-purity and high-efficiency single-photon generation by using short and functionalized air-suspended SWCNTs. The simulated exciton dynamics, time-resolved photoluminescence, and photon correlation properties indicate that exciton–exciton annihilation, end quenching, and trapping in the defect introduced by functionalization such as oxygen or aryl doping play important roles in determining the emission and single-photon properties, which strongly depend on SWCNT length and excitation intensity. We found that high performance SPSs that exhibit simultaneously high single-photon purity of 99.87% and high single-photon generation efficiency of 99.84% can be realized by using air-suspended functionalized SWCNTs with a length of approximately 100 nm under high excitation conditions. This ideal SPS can enable high rate and long-distance quantum key distributions at room temperature.

Time-resolved photoluminescence and optical properties of a specific organic azo dye

The aim of this work is to measure the photoluminescence of the Disperse Blue 79 (DB 79) deposited as thin films and study its morphological properties by the mean of the Atomic Force Microscopy technique in order to apply it as an Organic Emitting Light Diode (OLED). The films were prepared by the mean of the home-made Physical Vapor Deposition apparatus in a high vacuum on BK7 glass substrates. The linear optical properties were studied by photoluminescence and decay time spectra. The photoluminescence was measured in a broad ranged from 25 to 300 K in a high vacuum, while the decay time technique was investigated to measure the studied sample’s life-time in real time. Obtained results confirm homogeneous morphology and good optical quality of investigated organic azobenzene dye. These results allow the DB 79 dye to be used for the first time in production of the Organic Light Emitting Diodes (OLEDs).

Stress-induced optical waveguides written by an ultrafast laser in Nd3+, Y3+ co-doped SrF2 crystals.

In this paper, we report on the ultrafast laser-induced birefringence, refractive index changes, and enhanced photoluminescence properties in the volume of neodymium (Nd), yttrium (Y) co-doped strontium fluoride (SrF2) and Nd, Y co-doped calcium fluoride (CaF2) crystals. The optical waveguides written with 500kHz repetition rate provided lowest propagation loss of 1.63±0.21 dB cm-1 for transverse magnetic (TM) polarization at 632.8nm in Nd,Y:SrF2 crystal. The measured retardance can be interpreted by stress-induced birefringence related to the permanent volume expansion, photo induced by a non-spherical irradiated zone. The absorption, steady-state, and time-resolved photoluminescence properties are also carried out in and out of the laser irradiated zone, enabling the local changes of the Nd and Y network in Nd,Y:SrF2, as well as well-preserved Nd fluorescence in the written optical waveguides.

Plasmon Dephasing in Gold Nanorods Studied Using Single-Nanoparticle Interferometric Nonlinear Optical Microscopy

We report the polarization-dependent and time-resolved photoluminescence (PL) properties of gold nanorods (AuNRs). AuNRs corresponding to three different length-to-diameter aspect ratios (AR)—1.86, 2.91, and 3.90—were examined using single-nanorod spectroscopy and imaging; the nanorod volume was approximately constant over the three sample types. For each AuNR, an aspect ratio-independent transverse plasmon resonance (TSPR) was detected at 2.41 eV. Aspect-ratio-dependent longitudinal surface plasmon resonances (LSPRs) were observed at 2.08 ± 0.19 eV, 1.76 ± 0.12 eV, and 1.53 ± 0.15 eV for the 1.86-AR, 2.91-AR, and 3.90-AR samples, respectively. On the basis of both excitation and emission polarization-resolved two-photon photoluminescence (TPPL) measurements, AuNR PL emission proceeded by plasmon-mediated radiative electron–hole recombination. The resonant LSPR mode frequencies of the nanorods were determined from interferometrically detected TPPL signals. For these measurements, the interpulse time delay...

Temperature-Dependent Exciton and Trap-Related Photoluminescence of CdTe Quantum Dots Embedded in a NaCl Matrix: Implication in Thermometry.

Temperature-dependent optical studies of semiconductor quantum dots (QDs) are fundamentally important for a variety of sensing and imaging applications. The steady-state and time-resolved photoluminescence properties of CdTe QDs in the size range from 2.3 to 3.1 nm embedded into a protective matrix of NaCl are studied as a function of temperature from 80 to 360 K. The temperature coefficient is found to be strongly dependent on QD size, with the highest sensitivity obtained for the smallest size of QDs. The emission from solid-state CdTe QD-based powders is maintained with high color purity over a wide range of temperatures. Photoluminescence lifetime data suggest that temperature dependence of the intrinsic radiative lifetime in CdTe QDs is rather weak, and it is mostly the temperature-dependent nonradiative decay of CdTe QDs which is responsible for the thermal quenching of photoluminescence intensity. By virtue of the temperature-dependent photoluminescence behavior, high color purity, photostability, and high photoluminescence quantum yield (26%-37% in the solid state), CdTe QDs embedded in NaCl matrices are useful solid-state probes for thermal imaging and sensing over a wide range of temperatures within a number of detection schemes and outstanding sensitivity, such as luminescence thermochromic imaging, ratiometric luminescence, and luminescence lifetime thermal sensing.

Time-Resolved Photoluminescence Properties of ZnO Thin Films Evaporated by Atomic Layer Deposition

We have studied time-resolved photoluminescence (PL) properties of ZnO thin films which were grown by layer-by-layer manner using atomic layer deposition (ALD) method. The deposited film thickness was estimated by X-ray reflectance spectroscopy, which proved that the ALD system deposited -1.57 Å/cycle in thickness. As-grown films showed typical band-edge and surface defect induced emissions. The evaluated PL lifetimes showed interesting trend as the film thickness approaching to exciton Bohr diameter. Moreover, PL lifetime imaging technique clearly showed the characteristic lifetime modulation with changing film thickness in both band-edge and surface defect emissions.

Stacking fault related luminescence in GaN nanorods

Optical and structural properties are presented for GaN nanorods (NRs) grown in the [0001] direction on Si(111) substrates by direct-current reactive magnetron sputter epitaxy. Transmission electron microscopy (TEM) reveals clusters of dense stacking faults (SFs) regularly distributed along the c-axis. A strong emission line at ∼3.42 eV associated with the basal-plane SFs has been observed in luminescence spectra. The optical signature of SFs is stable up to room temperatures with the activation energy of ∼20 meV. Temperature-dependent time-resolved photoluminescence properties suggest that the recombination mechanism of the 3.42 eV emission can be understood in terms of multiple quantum wells self-organized along the growth axis of NRs.

Enhanced blue light emission in transparent ZnO:PVA nanocomposite free standing polymer films

ZnO:PVA nanocomposite films were prepared and their fluorescence and time resolved photoluminescence properties were discussed. X-ray diffraction and infrared spectroscopy results confirmed the ZnO:PVA interaction. Optical absorption spectra showed two bands at 280 and 367nm which were ascribed to PVA and excitonic absorption band, respectively. Fluorescence spectra showed that the blue emission of ZnO was enhanced about tenfold through chemical interface electron transfer. The electron transfer from ZnO to PVA and its decay dynamics were experimentally analyzed through time resolved fluorescence measurements. The study revealed that the excited electrons found pathway through PVA to ground state which was slower than the pure ZnO nanoparticles.

Steady state and time resolved photoluminescence properties of CuInS2/ZnS quantum dots in solutions and in solid films

The photoluminescence properties of CuInS2/ZnS quantum dots (QDs) dispersed in solutions of different concentrations and solvent polarity and deposited as solid films on quartz substrates by drop-casting and spin-coating are studied. Both steady state and time-resolved photoluminescence spectroscopy have been used. The CuInS2/ZnS QDs in solutions exhibit a red-shift of their absorption and photoluminescence spectra by increasing concentration and solvent polarity. In addition, they exhibit a three-exponential decay with time constants 1–3, 20–40 and 200–300ns, depending on solvent, concentration and detection wavelength. In films, a red-shifted photoluminescence spectrum is observed for films made by drop-casting compared to those prepared by spin-coating. The time-resolved photoluminescence decays in films, apart from the three mechanisms observed in solutions, also exhibit a fast decay component of <1ns, which is more pronounced in the spin coated films and especially at long emission wavelengths. The time resolved photoluminescence spectra in the drop-casted films experience a larger transient red-shift than in the spin-coated ones, indicative of a possible energy transfer among adjacent QDs. In general, it is shown that the chemical environment and the presence of defects play a central role in the recombination processes.

Time-resolved photoluminescence properties of hybrids based on inorganic AlGaN/GaN quantum wells and colloidal ZnO nanocrystals

Dynamic properties are studied for the AlGaN/GaN quantum well (QW) structures with and without the coating of colloidal ZnO nanocrystals (NCs). The QW exciton recombination rate was reduced in such hybrids compared to the bare QW structure only in the sample with the thinnest cap layer of 3nm. Assuming that one of the recombination mechanisms in this hybrid is non-radiative resonant energy transfer (NRET) between the QW and the energy acceptor material i.e. ZnO NCs, the maximum pumping efficiency was estimated to be ∼42% at 60K. The NRET effect is, however, vanished after several months despite that the hybrid structures are composed of chemically stable components.

Time-resolved photoluminescence properties of ion-beam-synthesized β-FeSi2 and Si-implanted Si

Temporal decay characteristics of 1.54 µm photoluminescence (PL) were investigated in β-FeSi2 and Si-implanted Si samples grown by ion-beam-synthesis (IBS). In the samples, the band-edge PL of β-FeSi2 (A-band) and the dislocation-related PL (D1-band) of Si were both observed at ∼0.8 eV. Regarding the dependence of the PL decay curves on excitation power density (P), PL decay curves without extrinsic effects were obtained at a low P of P ≤ 4.3 mW/cm2. The PL decay times obtained at a low P showed clear differences between the A-band and the D1-line. The result showed that the band-edge PL of β-FeSi2 was distinguished from the dislocation-related PL of Si by the PL decay times. The intrinsic PL decay times of β-FeSi2 were determined to be τ1 = 70–100 ns and τ2 = 550–670 ns at 5 K.

Enhanced ultraviolet fluorescence in surface modified ZnO nanostructures: Effect of PANI

ZnO:polyaniline nanocomposite (ZnO:PANI) films were prepared and their steady state fluorescence and time resolved photoluminescence properties were discussed. X-ray diffraction and infrared spectroscopy analyses confirmed the interaction and formation of ZnO:PANI composite films. Optical absorption spectrum of pure PANI showed two bands at 325 and 625nm which were ascribed to π→π∗ transition in the benzoid and exciton formation in the quinoid rings, respectively. Pure ZnO nanoparticles exhibited a band at 369nm was due to their exciton absorption and the composite films showed a broad band in the visible region and small intensity band at the UV region. Fluorescence spectra showed that the ultra violet emission of ZnO was enhanced about tenfold due to the electron transfer from PANI to ZnO nanoparticles and the suppression of visible emission was attributed to the surface passivation effect. The transfer of electron from PANI to ZnO and its decay dynamics were experimentally analyzed through time resolved fluorescence measurements.

Optical spectroscopic investigation of m-plane GaN thin films

M-plane GaN thin films grown on γ-LiAlO2 substrate were investigated at different temperatures by photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. The origin of two distinct emissions, P1 and P2 observed in the PL spectra were established by analyzing their PL and TRPL properties at different temperatures. The P1 emission is attributed to the excitons bound to the stacking faults (SFs). The P2 shows an anomalous “S-shaped” emission shift with increasing temperature (T), and the associated mechanism is discussed. The radiative life time ‘τr’ for P2 emission exhibits the T3/2 dependence at higher temperatures and deviates at lower temperatures whereas the radiative life time ‘τr’ for P1 emission does not show the T3/2 dependence with temperature. The polarization-dependent PL study reveals that P2 emission involves free holes in the transition at room temperature.

High-Temperature Photoluminescence of CdSe/CdS Core/Shell Nanoheterostructures

The steady-state and time-resolved photoluminescence properties of CdSe/CdS heterostructures are studied as a function of temperature from 300 to 600 K. The emission properties of samples are found to behave similarly to bulk CdSe, with the samples maintaining high color purity and a slightly contracting band gap at elevated temperature. Photoluminescence from CdSe/CdS samples is maintained with high stability over prolonged illumination and multiple heating and cooling cycles. Structures synthesized with variation in the core and the shell dimensions show that the preservation of emission intensity at high temperature depends strongly on the microscopic structure of the samples. For samples synthesized by seeded growth, the size of the CdSe core is highly correlated with the fraction of preserved sample photoluminescence intensity at high temperature. Temperature-dependent lifetime data suggest that the core structure predicts the stability of photoluminescence at elevated temperatures by controlling the radiative rate. The rate of electron capture, for which the volume fraction of the core is a structural proxy, underpins the ability for radiative processes to compete with thermally induced nonradiative decay pathways. Heterostructures synthesized below 200 °C using highly reactive organometallic precursors show markedly lower thermal stability than samples prepared by seeded growth at 360 °C, suggesting that the temperature of nanocrystal synthesis has direct consequences for the thermal stability of photoluminescence.