Time-resolved Photoluminescence Techniques Research Articles

Linear and nonlinear optical properties of ZnO thin films deposited by pulsed laser deposition

This paper presents the structural and optical properties of ZnO thin films deposited by using pulsed laser deposition technique on quartz substrates. The deposition process was carried out at various temperatures of the substrates from room temperature to 825K in order to investigate these properties of the films and their mutual influence. The structural and morphological properties of the films were investigated by X-Ray Diffraction and Atomic Force Microscopy measurements, respectively. The quality of the films was improved with an increase of the substrate temperature. The linear optical properties of the films were studied by classic and time-resolved photoluminescence spectra in the broad range of the temperature from 13K to 325K. Classic photoluminescence measurement allowed us to estimate band gap energy as a function of the temperature. An innovative time-resolved photoluminescence technique lets us precisely measure the decay time in the real time. Results of these measurements reveal a simple exponential decay behavior typical for well oriented crystalline thin films. Presented spectra confirm high structural and linear optical quality of investigated films. The nonlinear optical properties of the films were investigated by Third Harmonic Generation technique. Our results also indicated that the substrate temperature strongly affected nonlinear optical properties and the values of third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices.

Exciton Quenching Behavior of Thermally Activated Delayed Fluorescence Molecules by Charge Carriers

The quenching of singlet excitons by injected charge carriers in molecules that display thermally activated delayed fluorescence (TADF) was investigated using time-resolved transient photoluminescence (PL) techniques. Injected electrons did not affect the excitons; however, injected holes caused significant quenching. Using a rate-equation analysis, the hole-induced exciton quenching rate was determined to be between 10–11 and 10–12 cm3 s–1. Interestingly, the TADF emission component was enhanced in the presence of injected holes, plausibly due to a reduction of the singlet-exciton energy level.

Ratiometric iridium(III) complex-based phosphorescent chemodosimeter for Hg(2+) applicable in time-resolved luminescence assay and live cell imaging.

A novel iridium(III) complex-based probe Ir4-1 has been designed and synthesized conveniently by incorporating the chemodosimeter into phosphorescent luminophor, which displayed ratiometric luminescence change from yellowish-green to reddish-yellow only toward Hg(2+) ions in aqueous media via desulfurization and intramolecular cyclization with a broad pH range of 5-10. The phosphorescent chemodosimeter could eliminate effectively the signal interference from the short-lived fluorescent background, and the signal-to-noise ratio of the detection was improved distinctly by using time-resolved photoluminescence technique. Furthermore, the mechanism of phosphoresce change of the chemodosimeter was analyzed in detail by time-dependent density functional theory (TD-DFT) calculations, and the probe with long-wavelength emission could be applied to label cells and monitor intracellular Hg(2+) effectively by luminescence ratio imaging.

Exploitation and Application of a Highly Sensitive Ru(II) Complex-Based Phosphorescent Chemodosimeter for Hg2+ in Aqueous Solutions and Living Cells

A novel Ru(II) complex-based phosphorescent probe Rubpy-1 was designed and synthesized conveniently by incorporating of chemodosimeter into the luminophor, which exhibits good water solubility, longer excitation wavelength, and rapid turn-on phosphorescent response only toward Hg(2+) in aqueous system under physiological pH. The spectral response mechanism and Hg(2+)-promoted structure change of the chemodosimeter were analyzed in detail by theoretical calculations and electrospray ionization mass spectrometry. When time-resolved photoluminescence techniques were used, the Rubpy-1 could eliminate effectively the signal interference from the short-lived background fluorescence in complicated media, accompanied by the significant improvement of the signal-to-noise ratio and the accuracy of the detection. Furthermore, Rubpy-1 showed low cytotoxicity and excellent membrane permeability toward living cells, which was successfully applied to monitor intracellular Hg(2+) effectively by confocal luminescence imaging.

Effects of spin diffusion on electron spin relaxation time measured with a time-resolved microscopic photoluminescence technique

We performed measurements at room temperature for a GaAs/AlGaAs multiple quantum well grown on GaAs(110) using a time-resolved microscopic photoluminescence (micro-PL) technique to find what effects spin diffusion had on the measured electron spin relaxation time, τs, and developed a method of estimating the spin diffusion coefficient, Ds, using the measured data and the coupled drift-diffusion equations for spin polarized electrons. The spatial nonuniformities of τs and the initial degree of electron spin polarization caused by the pump intensity distribution inside the focal spot were taken into account to explain the dependence of τs on the measured spot size, i.e., a longer τs for a smaller spot size. We estimated Ds as ∼100 cm2/s, which is similar to a value reported in the literature. We also provided a qualitative understanding on how spin diffusion lengthens τs in micro-PL measurements.

Radio frequency magnetron sputtered highly textured Cu2ZnSnS4 thin films on sapphire (0 0 0 1) substrates

This paper describes the growth of highly textured Cu2ZnSnS4 (CZTS) thin films on sapphire (0001) substrate by radio frequency (RF) magnetron sputtering from a single CZTS target. Working pressure is one of the most important parameters for a one-step RF magnetron sputtering process. The influence of working pressure on the composition and the structural, morphological, and optical properties of CZTS thin films has been investigated is this paper. The properties of Zn-rich and Cu-poor and stoichiometric CZTS thin films are compared to study the effect of film composition. Using photoluminescence (PL) and time-resolved photoluminescence (TRPL) techniques, two PL bands are observed from the Zn-rich and Cu-poor highly textured CZTS. In addition to the band to band transition at 1.39eV, a PL band at 1.26eV is ascribed to the donor–acceptor recombination. The band edge transition exhibits a PL lifetime of 17.2ns. In contrast, the band of donor–acceptor recombination exhibits a much longer lifetime of 177.3ns. The understanding of the film formation mechanism from single target sputtering may improve the standard preparation method for high quality CZTS absorbers.

Effect of carrier recombination on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3

Transient reflectivity (TR) from thin films (6–40 nm thick) of the topological insulator Bi2Se3 revealed ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ∼20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques.

Turn-on phosphorescent chemodosimeter for Hg2+ based on a cyclometalated Ir(III) complex and its application in time-resolved luminescence assays and live cell imaging.

A novel "turn-on" phosphorescent chemodosimeter based on a cyclometalated Ir(III) complex has been designed and synthesized, which displays high selectivity and sensitivity toward Hg(2+) in aqueous media with a broad pH range of 4-10. Furthermore, by time-resolved photoluminescence techniques, some interferences from the short-lived background fluorescence can be eliminated effectively and the signal-to-noise ratio of the emission detection can be improved distinctly by using the chemodosimeter. Finally, the chemodosimeter can be used to monitor Hg(2+) effectively in living cells by confocal luminescence imaging.

Speciation of uranium in solids using time resolved photoluminescence technique

A comprehensive study regarding the speciation of uranium in five different solids, namely, YBO3, Sr2P2O7, SrB4O7, SrBPO5 and SrZrO3 is presented using time resolved photoluminescence spectroscopy. The ‘Sr’ based hosts are considered as potential phosphor materials where as the borate based matrices are known to have near tissue equivalent absorption coefficients making them potential candidates for dosimetric applications. It was observed that, in case of the pyrophosphate, borophosphate and yttrium borate matrices, uranium gets stabilized as uranyl ( $$ {\text{UO}}_{2}^{2 + } $$ ), whereas, in case of the tetraborate and zirconate matrices, it was the uranate species, $$ {\text{UO}}_{6}^{6 - } $$ that gets stabilized preferentially.

Transparent amorphous zinc oxide thin films for NLO applications

This review focuses on the growth and optical properties of amorphous zinc oxide (ZnO) thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz and glass substrates at temperature equal to 350K. The amorphous nature of the films was verified by X-ray diffraction. Atomic Force Microscopy was used to evaluate the surface morphology of the films. The optical characteristics of amorphous thin films have been investigated in the spectral range 190–1100nm. Measurement of the polarized optical properties was shows a high transmissivity (80–99%) and low absorptivity (<5%) in the visible and near infrared regions at different angles of incidence. Linear optical properties were investigated by classic and Time-Resolved Photoluminescence (TRPL) measurements. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. An innovative TRPL technique has enabled the measurement of the photoluminescence decay time as a function of temperature. TRPL measurements reveal a multiexponential decay behavior typical for amorphous thin films. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064nm in picosecond regime for investigations of the nonlinear optical properties. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices based on refractive index changes. Presented spectra confirm high structural and optical quality of the investigated zinc oxide thin films.

The excited state dynamics of protein-encapsulated Au nanoclusters

A combination of static and time-resolved spectroscopic techniques has been applied to study the low-lying excited electronic states of protein-encapsulated Au nanoclusters in terms of their energy levels and relaxation mechanisms. The energy levels were determined using photoluminescence (PL) spectroscopy. The excited state dynamics were probed using time-resolved PL techniques as well as femtosecond transient absorption (TA) spectroscopy. A simple model was proposed to account for the key spectral and dynamic features observed. This study has added new insight to the optical and dynamic properties of metal nanoclusters of interest in many applications including solar energy conversion and biomedical imaging.

Carrier Escape Time and Temperature-Dependent Carrier Collection Efficiency of Tunneling-Enhanced Multiple Quantum Well Solar Cells

Tunneling enhancement of cell performance in InGaAs/GaAsP multiple quantum well (MQW) solar cells has been studied to investigate the potential in overcoming the carrier collection problem, which hinders the maximum performance of quantum structure solar cells. To accurately investigate the effects of the tunneling effect, the study was carried out in samples with different GaAsP barrier thickness, controlled absorption edge, and constant built-in field. The tunneling effect has been confirmed by evaluating carrier escape times using the time-resolved photoluminescence technique and measuring carrier collection efficiency at various temperatures. The collection efficiencies at low temperature are found to be remarkably improved when barrier thickness was below 3 nm, which can be regarded as the critical thickness for efficiently facilitating tunneling enhancement. It can also be concluded that the carrier transport model based on thermal and tunneling processes is practical enough to describe most of the carrier sweep-out dynamics in MQW solar cells.

Energy transfer process between Eu3+ and wide-band-gap SnO2 nanocrystals in silica films studied by photoluminescence excitation and time-resolved photoluminescence techniques

Eu3+ ions embedded in silica thin films co-doped with SnO2 nanocrystals were fabricated by sol–gel and spin-coating methods. SnO2 nanocrystals with controllable sizes were synthesized through precisely controlling the Sn concentrations. The influences of doping and annealing conditions on the photoluminescence intensity from SnO2 nanocrystals are systematically investigated. The effective energy transfer from the defect states of SnO2 nanocrystals to nearby Eu3+ ions has revealed by the selective photoluminescence excitation spectra. The efficiency of the Forster resonance energy transfer is evaluated by the time-resolved photoluminescence measurements, which is about 29.1 % based on the lifetime tests of the SnO2 emission.

Impact of thermal annealing on bulk InGaAsSbN materials grown by metalorganic vapor phase epitaxy

Two different thermal annealing techniques (rapid thermal annealing (RTA) and in-situ post-growth annealing in the metalorganic vapor phase epitaxy (MOVPE) chamber) were employed to investigate their impact on the optical characteristics of double-heterostructures (DH) of InGaAsSbN/GaAs and on the performance of single-junction solar cell structures, all grown by MOVPE. We find that an optimized RTA procedure leads to a similar improvement in the photoluminescence (PL) intensity compared with material employing a multi-step optimized anneal within the MOVPE reactor. Time-resolved photoluminescence techniques at low temperature (LT) and room temperature (RT) were performed to characterize the carrier dynamics in bulk InGaAsSbN layers. Room temperature carrier lifetimes were found to be similar for both annealing methods, although the LT-PL (16 K) measurements of the MOVPE-annealed sample found longer lifetimes than the RTA-annealed sample (680 ps vs. 260 ps) for the PL measurement energy of 1.24 eV. InGaAsSbN-based single junction solar cells processed with the optimized RTA procedure exhibited an enhancement of the electrical performance, such as improvements in open circuit voltage, short circuit current, fill factor, and efficiency over solar cells subjected to the in-situ MOVPE annealing technique.

Visible-ultraviolet vibronic emission of silica nanoparticles.

We report the study of the visible-ultraviolet emission properties and the structural features of silica nanoparticles prepared through a laboratory sol-gel technique. Atomic force microscopy, Raman and Infrared investigations highlighted the 10 nm size, purity and porosity of the obtained nanoparticles. By using time resolved photoluminescence techniques in air and in a vacuum we were able to single out two contributions in the visible emission: the first, stable in both atmospheres, is a typical fast blue band centered around 2.8 eV; the second, only observed in a vacuum around the 3.0-3.5 eV range, is a vibrational progression with two phonon modes at 1370 cm(-1) and 360 cm(-1). By fully characterizing the spectroscopic features of this structured emission, we determine its vibronic properties and clarify the different origins with respect to the blue luminescent defect.

Study of Photoluminescence Decay by Time-correlated Single Photon Counting for the Determination of the Minority-carrier Lifetime in Silicon

The minority-carrier lifetime is a crucial parameter for the improvement of electronic or optoelectronic materials and particularly solar cells. We have investigated the potential of the time-correlated single photon counting (TCSPC) for the measurement of time-resolved photoluminescence (TRPL) and the direct determination of the effective minority-carrier lifetime in silicon. The TRPL technique was found to be in very good agreement with quasi-steady state and transient photoconductance results obtained from Sinton Instruments and micro-wave photo- conductance from Semilab. We have demonstrated the ability to evaluate the effective carrier lifetime of silicon substrates over a broad range of values (between a few μs and less than 500μs).

Carrier Dynamics in MOVPE-Grown Bulk InGaAsNSb Materials and Epitaxial Lift-Off GaAs Double Heterostructures for Multi-junction Solar Cells

ABSTRACTHigh performance and cost effective multi-junction III-V solar cells are attractive for satellite applications. High performance multi-junction solar cells are based on a triple-junction design that employs an InGaP top-junction, a GaAs middle-junction, and a bottom-junction consisting of a 1.0 – 1.25 eV-material. The most attractive 1.0 – 1.25 eV-material is the lattice-matched dilute nitride such as InGaAsN(Sb). A record efficiency of 43.5% was achieved from multi-junction solar cells including dilute nitride materials [1]. In addition, cost effective manufacturing of III-V triple-junction solar cells can be achieved by employing full-wafer epitaxial lift-off (ELO) technology, which enables multiple substrate re-usages. We employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in both pre- and post-ELO processed GaAs double heterostructures (DHs) as well as in MOVPE-grown bulk dilute nitride layers lattice matched to GaAs substrates.

Laser performance parameters of Yb3+ doped UV-transparent phosphate glasses

We report on optical investigation of phosphate glasses doped with Yb2O3. UV–Vis and FT-NIR optical absorption, photoluminescence, time-resolved photoluminescence and refractometry techniques are used. Cluster formation of Yb3+ was not observed as well as, no evidence of cooperative luminescence among Yb3+. An exponential decay of the 2F5/2→2F7/2 lifetime of ytterbium was found when the interionic distance decreases. The quantum efficiency and laser performance were also investigated. Relatively high figure of merit was found, 0.33×10−20cm2ms. The values of the minimum fraction of Yb3+ for minimum laser gain, pump saturation intensity and minimum absorbed pumping intensity were estimated.

Rational Design of Phosphorescent Chemodosimeter for Reaction‐Based One‐ and Two‐Photon and Time‐Resolved Luminescent Imaging of Biothiols in Living Cells

A selective phosphorescent biothiols probe is synthesized based on Ir(III) complex 1, which has 2,2'-biquinoline as the N^N ligand for realizing the satisfied two-photon absorption cross-section and two-functionalized 2-phenylpyridine ligands with an α,β-unsaturated ketone moiety as the thiol reaction site. The one- and two-photon optical properties of 1 are investigated through UV-vis absorption spectrum and photoluminescence spectrum. This Ir(III) complex can act as an excellent one- and two-photon excited "OFF-ON" phosphorescent probe for biothiols based on the 1,4-addition of biothiol to α,β-unsaturated ketones. Moreover, one- and two-photon-induced luminescent imagings of biothiols in living cells are also realized. Furthermore, the experiments of time-resolved photoluminescence technique and fluorescence lifetime imaging microscopy demonstrate that 1 is able to detect biothiols in the presence of strong background fluorescence. In addition, probe 1 is adsorbed into the shell of mesoporous silica nanoparticles with core-shell structure to form a nanoprobe, which can realize the ratiometric detection of biothiols in absolute water solution and living cells based on two phosphorescent signals.

Optical properties of oxide glasses with semiconductor nanoparticles co-doped with rare earth ions

This letter investigates PZABP glasses with nominal composition, 60P2O5. 15ZnO. 5Al2O3. 10BaO. 10PbO, doped with tellurium and co-doped with different concentrations of rare earth ions, ytterbium and europium. AFM, optical absorption, photoluminescence (PL) and time-resolved photoluminescence (TRPL) techniques characterized the vitreous systems. The formation of semiconductor nanoclusters ZnTe bulk-like and ZnTe quantum dots were identified. PL and TRPL show the energy transfer from the ZnTe nanocrystals and Eu3+ ions to Yb3+ ions. The optical properties presented qualify the system as potentially useful for light emission devices in the infrared region from 920 to 1060nm.