Time-resolved Photoluminescence Studies Research Articles

Emergence of Multicolor Photoluminescence in La0.67Sr0.33MnO3 Nanoparticles

Herein, we report the emergence of multicolor photoluminescence in a mixed-valence manganite nanoparticle La0.67Sr0.33MnO3 (LSMO NP) achieved through electronic structural modification of the nanoparticles upon functionalization with a biocompatible organic ligand, sodium tartrate. From UV–vis absorption, X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence study, and Raman spectroscopic measurements, it is revealed that ligand-to-metal charge transfer transitions from highest occupied molecular orbital (HOMO, centered in tartrate ligand) to lowest unoccupied molecular orbital (LUMO, centered in Mn3+/4+ of the NPs), and d–d transitions involving Jahn–Teller sensitive Mn3+ ions in the NPs plays the central role behind the origin of multiple photoluminescence from the ligand functionalized LSMO NPs.

Electronic, optical, and structural properties of (In,Ga)As/GaP quantum dots

We study in detail self-assembled (In,Ga)As quantum dots grown on GaP substrate from the structural, theoretical, and optical points of view. Single quantum dot morphology is first determined at the atomic level using plane-view scanning tunneling microscopy. Unusual ${C}_{2}$ symmetry properties with high-index ${136}$ facets are demonstrated for small quantum dots, whereas the apparent shape of the quantum dots approximately exhibits ${C}_{2v}$ symmetry, with the appearance of low-index ${111}$ facets when the quantum dot ripens. This is interpreted as a consequence of the competition between strain and surface energy during quantum dot formation. Electronic properties are then simulated using both k\ifmmode\cdot\else\textperiodcentered\fi{}p and tight-binding models. The indium content and geometry of the quantum dots are found to have a strong influence on the transition type (direct-indirect). Finally, temperature-dependent optical properties of quantum dots are analyzed between 10 and 375 K. Photoluminescence and time-resolved photoluminescence studies show a clear proximity of two different types of optical transitions. Supported by the theoretical calculations, these transitions are interpreted as a competition between conduction band states in the $X$ and \ensuremath{\Gamma} valleys.

Characterization of Cu(In,Ga)Se2Solar Cell Fabrication Process by Photoluminescence

Photoluminescence (PL), PL intensity mapping and time-resolved PL (TR-PL) studies have been applied to the Cu(In,Ga)Se2 (CIGS) solar cell fabrication process. Measurements have been done just after the respective cell process for the preparation of the Al/ZnO:Al/ZnO/CdS/CIGS structure, in which CdS has been formed by the chemical-bath deposition (CBD) while undoped and Al-doped ZnO layers were deposited by RF magnetron sputtering. PL intensity does not change by depositions of CdS and undoped ZnO buffer layers. PL intensity decreases by the deposition of the ZnO:Al film due to the cell shunt at the edge. The electrical cell isolation by the mechanical scribing leads to the increase in PL intensity because of the formation of the hetero-junction under the open circuit condition. The decay curves of the as deposited CIGS film, CdS/CIGS and ZnO/CdS/CIGS are non-exponential and composed of dominant fast decay and weak slow decay components. After the ZnO:Al deposition, PL decay is represented by the single exponential curve with long decay time. They are discussed in terms of the junction formation. PL intensity mapping after cell processes has been correlated with the solar cell performance.

Characterization of Cu(In,Ga)Se2 Solar Cell Fabrication Process by Photoluminescence

Photoluminescence (PL), PL intensity mapping and time-resolved PL (TR-PL) studies have been applied to the Cu(In,Ga)Se2 (CIGS) solar cell fabrication process. Measurements have been done just after the respective cell process for the preparation of the Al/ZnO:Al/ZnO/CdS/CIGS structure, in which CdS has been formed by the chemical-bath deposition (CBD) while undoped and Al-doped ZnO layers were deposited by RF magnetron sputtering. PL intensity does not change by depositions of CdS and undoped ZnO buffer layers. PL intensity decreases by the deposition of the ZnO:Al film due to the cell shunt at the edge. The electrical cell isolation by the mechanical scribing leads to the increase in PL intensity because of the formation of the hetero-junction under the open circuit condition. The decay curves of the as deposited CIGS film, CdS/CIGS and ZnO/CdS/CIGS are non-exponential and composed of dominant fast decay and weak slow decay components. After the ZnO:Al deposition, PL decay is represented by the single exponential curve with long decay time. They are discussed in terms of the junction formation. PL intensity mapping after cell processes has been correlated with the solar cell performance.

Time-resolved photoluminescence studies of the optical quality of InGaN/GaN multi-quantum well grown by MOCVD—antimony surfactant effect

The influence of antimony on the optical quality of InGaN/GaN multi-quantum well (MQW) grown by metalorganic chemical vapor deposition has been investigated by means of photoluminescence and time-resolved photoluminescence for a set of samples obtained for the Sb/(In+Ga) flow ratio varying from 0% to 0.12%. It has been observed that by using proper Sb flow it is possible to improve the optical properties of InGaN/GaN MQWs; however, too large Sb flows cause their optical quality to deteriorate. The Sb-related improvement of optical properties has been observed as (i) ∼30% increase of PL intensity, (ii) reduction of temperature-induced photoluminescence quenching and (iii) elongation of PL decay time by 30%. The atomic force microscopy study and second ion mass spectrometry profiles show that optical quality improvement is connected with surfactant properties of antimony.

Progress on one-dimensional zinc oxide nanomaterials based photonic devices

Abstract One-dimensional nanostructures hold the most attractive and excellent physiochemical characteristics which exhibit the paramount influence on the fundamental and technological nanoelectronic as well as nanophotonic applications. In this review article, we present a detailed introduction to the diverse synthetic procedures which can be utilized for the fabrication of single-, planar- and three-dimensional ZnO nanostructures. More specifically, a thorough discussion regarding luminescence characteristics of the one-dimensional ZnO nanostructures is presented for ultraviolet and visible regions. We summarize the room temperature spontaneous emission and stimulated emission along with the interaction of the incident beam with material cavity to produce resonant optical modes and low-temperature time resolved photoluminescence studies. The most recent published results on the white light emitting diodes fabricated with the combination of ZnO nanotubes with p-GaN and ZnO nanorods with p-organic polymers on glass and disposable paper are discussed. Additionally, the significant results on optically and electrically pumped lasers are discussed; along with an overview on the future of ZnO nanostructures based photonic devices.

Probing Inter- and Intrachain Exciton Coupling in Isolated Poly(3-hexylthiophene) Nanofibers: Effect of Solvation and Regioregularity.

We report wavelength and time-resolved photoluminescence studies of isolated extended (1-10 μm length) poly(3-hexylthiophene) (P3HT) nanofibers (xNFs) cast on glass from suspension. The PL spectra of xNFs show multiple vibronic replicas that appear to be associated with the existence of both H- and J-type aggregates. The PL spectra of xNFs made from regioregular (rr)- (93%) and highly regioregular (hrr)-P3HT (98%) both show similarities in PL spectra suggestive of common chain packing features, as well as subtle differences that can be attributed to higher long-range order in the hrr-xNFs. Specifically, PL spectral measurements on isolated xNFs made from highly regioregular (>98%) P3HT showed a red-shifted electronic origin (≈30 meV) and increased 0-0/0-1 PL intensity ratio for the J-type species, suggestive of enhanced structural coherence length and intrachain order.

Self-assembled GaN quantum wires on GaN/AlN nanowire templates

We present a novel approach for self-assembled growth of GaN quantum wires (QWRs) exhibiting strong confinement in two spatial dimensions. The GaN QWRs are formed by selective nucleation on {112[combining macron]0} (a-plane) facets formed at the six intersections of {11[combining macron]00} (m-plane) sidewalls of AlN/GaN nanowires used as a template. Based on microscopy observations we have developed a 3D model explaining the growth mechanism of QWRs. We show that the QWR formation is governed by self-limited pseudomorphic growth on the side facets of the nanowires (NWs). Quantum confinement in the QWRs is confirmed by the observation of narrow photoluminescence lines originating from individual QWRs with emission energies up to 4.4 eV. Time-resolved photoluminescence studies reveal a short decay time (~120 ps) of the QWR emission. Capping of the QWRs with AlN allows enhancement of the photoluminescence, which is blue-shifted due to compressive strain. The emission energies from single QWRs are modelled assuming a triangular cross-section resulting from self-limited growth on a-plane facets. Comparison with the experimental results yields an average QWR diameter of about 2.7 nm in agreement with structural characterization. The presented results open a new route towards controlled realization of one-dimensional semiconductor quantum structures with a high potential both for fundamental studies and for applications in electronics and in UV light generation.

Low-temperature treatment of semiconducting interlayers for high-efficiency light-emitting diodes based on a green-emitting polyfluorene derivative

We investigate the scope for low-temperature treatment of exciton/electron blocking interlayers in light-emitting diodes based on poly(9,9′-dioctylfiuorene-alt-benzothiadiazole) (F8BT). We focus on poly(9,9′-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) (TFB) interlayers processed at temperatures up to 50 °C, i.e., far below the glass transition temperature of TFB (∼156 °C). Continuous-wave and time-resolved photoluminescence studies confirm the formation of both excitons and exciplex species, as a result of the F8BT/TFB intermixing. Interestingly, however, we can still increase the electroluminescence external quantum efficiency from 0.05% to 0.5% and 1% for progressively thicker TFB films. We propose that a degree of intermixing may become acceptable as a trade-off to achieve low-temperature processability.

Direct-indirect crossover in GaxIn1-xP alloys

The energy and composition of the direct to indirect bandgap crossover in GaxIn1-xP significantly influences its potential for optoelectronic devices, such as solar cells and light emitting diodes, however considerable discrepancies still remain in the literature with regard to the precise value of the crossover composition xc. We revisit this issue in GaxIn1-xP films epitaxially grown on GaAs substrates. Observation of concurrent yet distinct direct and indirect transitions in Ga0.719In0.281P at 2 K using time integrated and time resolved photoluminescence studies places the crossover very near the composition xC = 0.71.

The optical properties of high quality ZnO/ZnMgO quantum wells on Si(111) substrates

The ZnO/Zn 0.85Mg 0.15O multiple quantum wells (MQWs) were fabricated on Si(111) substrates by plasma-assisted molecular beam epitaxy (P-MBE) using ZnMgO as buffer layers. The RHEED images indicated that the MQWs were of high quality. The free exciton (FE) emission line originated from the well region and its phonon replicas (FE-1LO, FE-2LO and FE-3LO) were observed in the 86 K photoluminescence (PL) spectrum. Blueshift of the FE line in the MQW sample as compared to that in the ZnO bulk was found at temperatures as high as room temperature. Time-resolved PL study on the FE line showed a fast lifetime of 140 ps. The high quality of the MQW structure was revealed by the observation of the quantum-confinement-induced blueshift and shortened lifetime of the FE emission.

Evidence for Defect-Enhanced Photoluminescence Quenching of Fluorescein by Carbon Nanotubes

We report on the definite role of surface defects in carbon nanotubes in the efficient photoluminescence (PL) quenching of fluorescein by single-walled (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Steady-state PL measurements show that SWCNTs are much more efficient quenchers than MWCNTs. The Stern–Volmer plot is found to follow a faster than exponential growth behavior for SWCNTs. Time resolved PL studies confirm a negligible contribution of dynamic quenching to the observed effect. Vacuum annealed SWCNTs containing fewer defects show very low quenching efficiency, though it exhibited high surface adsorption. A contribution of surface defects was further confirmed from study of quenching effects from a different set of SWCNT with high structural perfection, evidenced from Raman and thermogravimetric analysis. The mechanism of large quenching in SWCNTs is elucidated through systematic studies of absorption and emission spectroscopy.

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

Photoluminescence blinking--random switching between states of high (ON) and low (OFF) emissivities--is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this 'charging' model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept 'hot' electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Synthesis, Optical and Structural Properties, and Charge Carrier Dynamics of Cu-Doped ZnSe Nanocrystals

Stable and luminescent Cu-doped ZnSe nanocrystals (NCs) were synthesized in organic solvents with octadecylamine (ODA) as the capping ligand and characterized using a combination of optical and structural characterization techniques. Successful doping was achieved by adding Cu during the growth phase of the NCs when their size was ∼4 nm. The appearance of red-shifted, intense photoluminescence (PL) peak with doping indicates the incorporation of Cu in the NCs, and stability of dopant emission infers the internal doping of NCs. Extended X-ray absorption fine structure (EXAFS) studies revealed that Cu is surrounded by four neighbors in the lattice but is very close to the NC surface and gets oxidized when NCs are precipitated from the solution. For the undoped sample, time-resolved PL studies using time-correlated single photon counting (TCSPC) reveal the luminescence decay lifetimes of about 1.1, 12, and 60 ns that we attribute to near-bandedge, shallow trap (ST) state, and deep trap (DT) state emissions, respectively. In addition to these decay components, the Cu-doped sample was found to have a long-lived component with a lifetime of 630 ns. Luminescence decay lifetimes of near-bandedge and ST state emissions were slightly shortened by doping (1 and 10 ns, respectively), suggesting that the Cu dopant competes with these states in trapping the charge carriers from the conduction band (CB) or near-bandedge states. However, the presence of Cu was found to increase the lifetime of DT states from 60 to 100 ns probably due to a decrease in coupling of electron and hole states involved in this emission upon Cu doping. Synthesis of such stable, doped samples along with a better understanding of charge carrier dynamics is significant for emerging optical applications of these NCs.

FREQUENCY AND TIME RESOLVED SPECTROSCOPIC STUDIES OF CADMIUM SULFIDE NANOPARTICLES CAPPED WITH POLYVINYL PYRROLIDONE

Frequency and time domain interrogation of polyvinyl pyrrolidone (PVP) capped CdS nanoparticles synthesized by wet chemistry route have been performed. The optical absorption and temperature dependent photoluminescence (PL) properties have been studied. Time resolved PL studies have been performed at different wavelengths of the emission spectrum. The blue-shift of absorption band-edge confirms the quantum confinement effect due to the reduced size of the nanoparticles. Two PL peaks at 465 nm and 550 nm are observed. The temperature dependent PL study of CdS –PVP nanoparticles shows that the 550 nm peak originates from defect states and its intensity decreases exponentially with increasing temperature. Wavelength dependence of the PL lifetime suggests that the observed PL spectrum is the superposition of emissions originating from different excited states.

Study of steady state and time resolved photoluminescence of thiol capped CdS nanocrystalline powders dispersed in N,N-dimethylformamide

The microwave (MW) assisted synthesis of thiol capped cadmium sulfide (CdS) nanocrystallites/quantum dots (QDs) was performed through the reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) by keeping the MW irradiation time fixed (40 s) in the presence of a thiol containing capping agent. Three capping agents, namely, benzyl mercaptan (BM), 1-butanethiol (BT) and 2-mercaptoethanol (ME) were used. The concentration of the precursors was varied to check the change in the average size of the thiol capped CdS nanocrystals formed. The nanocrystallites were characterized by usual procedure. The UV–vis absorption spectra and the photoluminescence (PL) spectra of the CdS nanocrystalline powders dispersed in DMF were studied. It was observed that with increase in concentration of the capping agent (BM), there is a shift in the nature of emission (PL) from trap associated PL to the band edge luminescence in the case of BM capped CdS nanocrystalline powders dispersed in DMF possibly due to better surface passivation. The relative PL quantum yield of the thiol capped CdS nanocrystalline powders dispersed in DMF was calculated under various experimental conditions. Time-correlated single-photon counting experiments were performed to study the time-resolved photoluminescence of the CdS nanocrystalline powders dispersed in DMF. The observed emission decay profiles have been simulated using the multiexponential model. The emission decay profiles for thiol capped CdS nanocrystalline powders dispersed in DMF depend on the nature of the capping agents (thiols) used to passivate the CdS nanocrystallites. The time resolved PL studies show that the average values of PL lifetime are related to the size and size distribution of the prepared thiol capped CdS nanocrystallites.

Stark spectroscopy and radiative lifetimes in single self-assembled CdTe quantum dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

RAPID THERMAL ANNEALING INDUCED ENHANCED BAND-EDGE EMISSION FROM ZnO NANOWIRES, NANORODS AND NANORIBBONS

In this letter, we report on the enhanced near band edge (NBE) photoluminescence (PL) emission by rapid thermal annealing (RTA) of ZnO nanowires, nanoribbons and nanorods synthesized by vapor transport method using a ZnO nanopowder source. As a result of RTA, the intensity ratio of NBE to green emission peak is nearly doubled for the nanowires, while this enhancement is one order of magnitude for the nanoribbons and nanorods. Time-resolved PL studies on the green emission band shows a single-exponential decay (time constant ~ns) after RTA. The Raman spectral shift of the RTA treated samples indicates reduced tensile strain in the annealed ZnO nanostructures. Our results demonstrate the effectiveness of RTA process for improving the crystallinity and optical properties of ZnO nanostructures.

Synthesis, characterization and optical studies on lanthanide-doped CdS quantum dots: new insights on CdS → lanthanide energy transfer mechanisms

Eu3+-Doped CdS hexagonal nanocrystals (NCs), with an average size of ∼4 nm, have been prepared through a simple methodology based on the direct precipitation of CdS NCs using 1-thioglycerol as a capping molecule. Steady-state and time-resolved photoluminescence studies (14–300 K) reveal an effective CdS NCs-to-Eu3+ energy transfer process involving surface localized states and mediated by a thermally activated exchange mechanism. Furthermore, the analysis of the optical data also points out the presence of two distinct Eu3+ local coordination sites with different relaxation kinetics and attributed to lattice-bound and surface-bound centres. The 5D0 lifetime values are longer than those reported previously for sol–gel derived Eu3+-doped CdS NCs.

Photoluminescence lifetime of Al-doped ZnO films in visible region

ZnO and Al-doped ZnO films have been deposited on quartz substrates by ultrasonically assisted chemical vapor deposition technique. Photoluminescence (PL) spectra of the films reveal that Al doping leads to suppression of defect related visible band. Time resolved photoluminescence studies have been carried out for the measurement of lifetime of deep level luminescence. The decay of PL intensity with time has been found to follow biexponential behavior. The relative contributions of fast decay component ( τ 1 ) and slow decay component ( τ 2 ) in total decay process are found to be ∼99% and ∼1% respectively. The values of τ 1 and τ 2 are found to decrease with Al doping in ZnO film. The decrease of both τ 1 and τ 2 is attributed to increase in non-radiative recombination due to reduction in grain sizes and the decrease in radiative recombination due to suppression of defects.