Time-resolved Luminescence Measurements Research Articles

High-quality temperature imaging of Li2TiO3: Mn4+ thin film based on time-resolved luminescence measurement

With the rapid progress and mass production of integrated electronic devices in micro and nanometer scale, it is urgent to develop a technology that can monitor surface temperature distribution of integrated device for the purpose of optimizing the device design. Herein, a thermometry based on time-resolved measurements of luminescence from Li2TiO3: Mn4+ was presented, which uses the ratio of integrated fluorescence intensities in two different time windows. The scheme significantly improves the relative sensitivity of temperature sensing, with maximum value reaching 5.81 % K−1 at 342 K. With the help of a fluorescence microscope and an intensified charge coupled device (ICCD), the scheme based on the time-resolved measurement of Li2TiO3: Mn4+ was successfully applied to the temperature imaging of nickel circuit surface. By comparing the performance of temperature imaging via powder phosphor with that via thin film, it can be found that the thin film can better reflect the true temperature distribution on the surface of nickel circuit. What’s more, the Li2TiO3: Mn4+ thin film also improves the resolution of the temperature imaging (δT=0.61 K). Therefore, it is significant to explore the temperature measurement scheme based on the time-resolved measurement of Li2TiO3: Mn4+ thin film for realizing high-quality temperature imaging.

Photoluminescence properties of double perovskite Ca2LuTaO6:Bi3+/Sm3+ white light emitting phosphors

The high-temperature solid state reaction approach was used to successfully synthesize novel tunable-color emitting Ca2LuTaO6 phosphors with double perovskite structure that were un-doped, single-doped, and co-doped with Bi3+/Sm3+ ions. The crystal structure, microstructure, steady-state photoluminescence, time-resolved luminescence measurements, and thermal quenching features of the material were studied. The density functional theory method was used to compute the band gap of the Ca2LuTaO6, and Ca2LuTaO6: Bi3+, Sm3+ phosphors. The blue emission at 407 nm (with 313 nm excitation) is due to the host's luminescence, while the cyan emission at 480 nm (under 365 nm excitation) is related to the distinctive 3Pn − 1S0 transitions (n = 0, 1, 2) of Bi3+. Similarly, efficient red emission is observed in the case of Sm3+ single-doped Ca2LuTaO6 phosphors. In addition, the Ca2LuTaO6: 0.04Bi3+, 0.05 S m3+ phosphors exhibit energy transfer phenomena and have excellent thermal stability, the fluorescence intensity at 150 °C maintains 73% of that at 25 °C and has a small chromaticity shift. The intense blue emission at 407 nm was effectively converted to cyan, red, and white light through doping with Bi3+ and Sm3+, and the resulting energy transfer. The results obtained confirm that Ca2LuTaO6: Bi3+, Sm3+ phosphors as a single matrix have great potential for high-power WLED applications.

Luminescence Properties of Closely Packed Organic Color Centers Grafted on a Carbon Nanotube.

We report on the photoluminescence of pairs of organic color centers in single-wall carbon nanotubes grafted with 3,5-dichlorobenzene. Using various techniques such as intensity correlations, superlocalization microscopy, and luminescence excitation spectroscopy, we distinguish two pairs of color centers grafted on the same nanotube; the distance between the pairs is on the order of several hundreds of nanometers. In contrast, by studying the strong temporal correlations in the spectral diffusion in the framework of the photoinduced Stark effect, we can estimate the distance within each pair to be on the order of a few nanometers. Finally, the electronic population dynamics is investigated using time-resolved luminescence and saturation measurements, showing a biexponential decay with a fast overall recombination (compatible with a fast population transfer between the color centers within a pair) and a weak delayed repopulation of the traps, possibly due to the diffusion of excitons along the tube axis.

Frequency-domain time-resolved luminescence for In-operando study of efficiency roll-off in organic light-emitting diodes

Time-resolved luminescence measurements are invaluable for understanding light-emitting materials but are difficult to perform on operating organic light-emitting diodes (OLEDs) because the light generated by the OLED can disturb common time-resolved techniques. We show that frequency-domain luminescence can be used to overcome this problem, and apply it to study efficiency roll-off in OLEDs. We measure the exciton lifetime of a phosphorescent OLED based on the emitter bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate) iridium(III) (Ir(MDQ)2(acac)) whilst it is operating. By driving the OLED continuously at current densities of up to 128 mA/cm2, we observe that the exciton lifetime decreases with increasing current density as a result of triplet-polaron annihilation (TPA). The measurement presented can guide the development of new materials and device structures with improved efficiency at high brightness.

How tobacco (Nicotiana tabacum) BY-2 cells cope with Eu(III) - a microspectroscopic study.

The extensive use of lanthanides in science, industry and high-technology products is accompanied by an anthropogenic input of rare earth elements into the environment. Knowledge of a metal's environmental fate is essential for reasonable risk assessment and remediation approaches. In the present study, Eu(III) was representatively used as a luminescent probe to study the chemical environment and to elucidate the molecular interactions of lanthanides with a suspension cell culture of Nicotiana tabacum BY-2. Biochemical methods were combined with luminescence spectroscopy, two-dimensional microspectroscopic mappings, and data deconvolution methods to resolve the bioassociation behavior and spatial distribution of Eu(III) in plant cells. BY-2 cells were found to gradually take up the metal after exposure to 100 μM Eu(III) without significant loss of viability. Time-resolved luminescence measurements were used to specify the occurrence of Eu(III) species as a function of time, revealing the transformation of an initial Eu(III) species into another after 24 h exposure. Chemical microscopy and subsequent iterative factor analysis reveal the presence of four distinct Eu(III) species located at different cellular compartments, e.g., the cell nucleus, nucleolus and cell walls, which could be assigned to intracellular binding motifs. In addition, a special type of bioaccumulation occurs through the formation of a Eu(III)-containing oxalate biomineral, which is already formed within the first 24 hours after metal exposure. Oxalate crystals were also obtained in analogous experiments with Gd and Sm. These results indicate that tobacco BY-2 cells induce the precipitation of metal oxalate biominerals for detoxification of lanthanides, although they also bind to other cellular ligands at the same time.

Photophysical Properties of Eu3+ β-Diketonates with Extended π-Conjugation in the Aromatic Moiety

The influence of the degree of π-conjugation in biaroylmethane ligands upon Eu3+ luminescence efficiency in corresponding neutral tris-complexes was investigated in depth. The data obtained by both steady-state and time-resolved luminescence measurements gave an inside into electronic energy transfer mechanisms in the abovementioned complexes. It was shown that extension of the π-system in the naphthalene moiety in comparison to the phenyl one lead to a substantial decrease of both the S1 and T1 energy of the corresponding symmetrical β-diketones, which, in turn, led to a decrease of the total quantum yield of respective Eu3+ complexes. The obtained results are of interest for the rational design of highly luminescent complexes with NIR-emitting lanthanides, as the resonant levels energies are low and can hardly be sensitized by common ligands.

Review of Mn-Doped Semiconductor Nanocrystals for Time-Resolved Luminescence Biosensing/Imaging.

Colloidal semiconductor nanocrystals (NCs) have been developed for decades and are widely applied in biosensing/imaging. However, their biosensing/imaging applications are mainly based on luminescence-intensity measurement, which suffers from autofluorescence in complex biological samples and thus limits the biosensing/imaging sensitivities. It is expected for these NCs to be further developed to gain luminescence features that can overcome sample autofluorescence. On the other hand, time-resolved luminescence measurement utilizing long-lived-luminescence probes is an efficient technique to eliminate short-lived autofluorescence of samples while recording time-resolved luminescence of the probes for signal measurement after pulsed excitation from a light source. Despite time-resolved measurement being very sensitive, the optical limitations of many of the current long-lived-luminescence probes cause time-resolved measurement to be generally performed in laboratories with bulky and costly instruments. In order to apply highly sensitive time-resolved measurement for in-field or point-of-care (POC) testing, it is essential to develop probes possessing high brightness, low-energy (visible-light) excitation, and long lifetimes of up to milliseconds. Such desired optical features can significantly simplify the design criteria of time-resolved measurement instruments and facilitate the development of low-cost, compact, sensitive instruments for in-field or POC testing. Mn-doped NCs have recently been in rapid development and provide a strategy to solve the challenges faced by both colloidal semiconductor NCs and time-resolved luminescence measurement. In this review, we outline the major achievements in the development of Mn-doped binary and multinary NCs, with emphasis on their synthesis approaches and luminescence mechanisms. Specifically, we demonstrate how researchers approached these obstacles to achieve the aforementioned desired optical properties on the basis of the progressive understanding of Mn emission mechanisms. Afterward, we review representative applications of Mn-doped NCs in time-resolved luminescence biosensing/imaging and present the potential of Mn-doped NCs in advancing time-resolved luminescence biosensing/imaging for in-field or POC testing.

Mixed-Metal and Mixed-Ligand Lanthanide Metal-Organic Frameworks Based on 2,6-Naphthalenedicarboxylate: Thermally Activated Sensitization and White-Light Emission.

Trivalent lanthanide ions (Ln3+) hold an exceptional position in the field of optoelectronic materials due to their atomic-like emission spectra and long luminescence lifetimes. Metal-organic frameworks (MOFs) and coordination polymers are particularly suited as luminescent materials due to their structural diversity and ease of functionalization both at bridging ligands and/or metal centers. In this contribution, we present a series of mixed-metal Ln3+/Eu3+ (Ln = La, Gd) and mixed-ligand (2,6-naphthalenedicarboxylate (ndc2-) and 4-aminonaphthalene-2,6-dicarboxylate (andc2-)) MOFs belonging to three different structural types, with emissions spanning most of the visible region, thereby constituting favorable materials for color tuning and white-light emission. We investigate the thermal stability and photophysical properties of the synthesized materials with regard to their metal and ligand doping levels and structural type, where we discuss excimer and monomer emission. The photophysical study, involving both steady-state and time-resolved luminescence measurements, allows us to discuss the possible energy migration and Eu3+ sensitization pathways that take place within these materials following ligand excitation. Low-temperature luminescence studies led us to determine the energies of the ligand-based excited states and investigate their participation in thermally activated energy transfer mechanisms within the studied lattices. We observe emission quantum yields of up to 87% for the Eu3+-doped materials, while their ligand- and metal-doped counterparts show decreased quantum yields of up to 17%. Finally, we attempt fine color tuning by carefully adjusting the doping levels to achieve yellow and white-light emission.

High-Density, Localized Quantum Emitters in Strained 2D Semiconductors.

Two-dimensional chalcogenide semiconductors have recently emerged as a host material for quantum emitters of single photons. While several reports on defect- and strain-induced single-photon emission from 2D chalcogenides exist, a bottom-up, lithography-free approach to producing a high density of emitters remains elusive. Further, the physical properties of quantum emission in the case of strained 2D semiconductors are far from being understood. Here, we demonstrate a bottom-up, scalable, and lithography-free approach for creating large areas of localized emitters with high density (∼150 emitters/um2) in a WSe2 monolayer. We induce strain inside the WSe2 monolayer with high spatial density by conformally placing the WSe2 monolayer over a uniform array of Pt nanoparticles with a size of 10 nm. Cryogenic, time-resolved, and gate-tunable luminescence measurements combined with near-field luminescence spectroscopy suggest the formation of localized states in strained regions that emit single photons with a high spatial density. Our approach of using a metal nanoparticle array to generate a high density of strained quantum emitters will be applied to scalable, tunable, and versatile quantum light sources.

In situ passivation of Ga x In(1−x)P nanowires using radial Al y In(1−y)P shells grown by MOVPE

Ga x In(1−x)P nanowires with suitable bandgap (1.35–2.26 eV) ranging from the visible to near-infrared wavelength have great potential in optoelectronic applications. Due to the large surface-to-volume ratio of nanowires, the surface states become a pronounced factor affecting device performance. In this work, we performed a systematic study of Ga x In(1−x)P nanowires’ surface passivation, utilizing Al y In(1−y)P shells grown in situ by using a metal-organic vapor phase epitaxy system. Time-resolved photoinduced luminescence and time-resolved THz spectroscopy measurements were performed to study the nanowires’ carrier recombination processes. Compared to the bare Ga0.41In0.59P nanowires without shells, the hole and electron lifetime of the nanowires with the Al0.36In0.64P shells are found to be larger by 40 and 1.1 times, respectively, demonstrating effective surface passivation of trap states. When shells with higher Al composition were grown, both lifetimes of free holes and electrons decreased prominently. We attribute the acceleration of PL decay to an increase in the trap states’ density due to the formation of defects, including the polycrystalline and oxidized amorphous areas in these samples. Furthermore, in a separate set of samples, we varied the shell thickness. We observed that a certain shell thickness of approximately ∼20 nm is needed for efficient passivation of Ga0.31In0.69P nanowires. The photoconductivity of the sample with a shell thickness of 23 nm decays 10 times slower compared with that of the bare core nanowires. We concluded that both the hole and electron trapping and the overall charge recombination in Ga x In(1−x)P nanowires can be substantially passivated through growing an Al y In(1−y)P shell with appropriate Al composition and thickness. Therefore, we have developed an effective in situ surface passivation of Ga x In(1−x)P nanowires by use of Al y In(1−y)P shells, paving the way to high-performance Ga x In(1−x)P nanowires optoelectronic devices.

Time-resolved and excitation-emission matrix luminescence behaviour of boro-silicate glasses doped with Eu3+ ions for red luminescent application

This paper reports the time-resolved luminescence and excitation-emission matrix (EEM) measurements of three different boro-silicate host glasses doped with Eu3+. The effect of zinc and tellurium oxide addition on the photoluminescence (PL) and time-resolved emission spectra (TRES) of the glasses was also studied. At 392 nm of excitation wavelength, a bright red electric dipole (ED) transition 5D0 → 7F2 (615 nm) and a weak orange magnetic dipole (MD) transition 5D0 → 7F1 (590 nm) were observed. The asymmetric ratio (ED/MD) values were increased in the order Eu < Eu-Zn < Eu-Te. The decay associated spectra (DAS) showed a bright red emission transition (5D0 → 7F2) at 612 nm for all glasses. The time-resolved analysis revealed that, because of the glass network randomness, the luminescence emissions did not present the same lifetime or wavelength for the same transition bands, attributed to different environments (or interactions) experienced by Eu3+ ions in the glass network.

Optical Properties of Chiral Azo-Schiff Base Mn(II) and Zn(II) Complexes with Silver Nanoparticles

Herein we have originally designed chiral azo-salen Mn(II) and Zn(II) complexes for interacting silver nanoparticles (AgNPs) exhibiting localized surface plasmon resonance (LSPR). Understanding excited state and reaction intermediate during light irradiation to return to ground state may be important for such composite systems. Therefore, we investigated such optical properties for systems using time-resolved luminescence and transient absorption measurements. DMSO solutions of the four newly prepared and characterized complexes (MMn, MZn, CMn, and CZn) and ethanol solutions of the composite materials of each complex with AgNPs were served for optical measurements. The time-correlated single photon counting (TCSPC), the streak camera which is much shorter period of time than TCSPC and transient absorption measurement, was performed for the eight samples. The fluorescence lifetime of the sole complexes and the composite materials with AgNPs was derived from curve-fitting analysis of luminescence decay curves of TCSPC. Lifetime of the composite systems with AgNPs was longer than that of the corresponding sole metal complexes for three cases. It was revealed that composite systems may go through three reaction intermediates during relaxation from excited state to ground state.

Spectroscopic, photophysical, solution thermodynamics and computational study of europium and terbium complexes with a flexible quinolinol-based symmetric tripodal chelator.

Coordination behaviour and photonic properties of a flexible tripodal multidentate ligand, 5,5',5''-(Cyclohexane-1,3,5-triyltris(oxy))tris(methylene)triquinolin-8-ol (CYTOM5OX), with Eu and Tb ions were explored through potentiometric, UV-visible and fluorescence spectrophotometric methods. The interaction of tripod towards the lanthanides has been examined for thermodynamic equilibrium, photophysical behavior, and time-resolved luminescence measurements. Besides, a theoretically empirical method followed by quantum mechanical treatment of these chelates has also been summarized. A noticeable change in the electronic spectra of these complexes disclose CYTOM5OX as a pH-sensitive optical sensor for lanthanides detection; also, a strong green fluorescence allows simultaneous sensing in the visible region in a competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. The thermodynamically stable Ln3+ complexes in aqueous solution show stability constants in the range log β110=33-35 and pLn3+ in the range of 21-22. The hydration number that was predicted by the coordination scan was substantiated by lifetime measurements reveal presence of coordinated water molecules in Eu(CYTOM5OX) and Tb(CYTOM5OX) complexes. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.

Auto-Phase-Locked Time-Resolved Luminescence Detection: Principles, Applications, and Prospects.

Time-resolved luminescence measurement is a useful technique which can eliminate the background signals from scattering and short-lived autofluorescence. However, the relative instruments always require pulsed excitation sources and high-speed detectors. Moreover, the excitation and detecting shutter should be precisely synchronized by electronic phase matching circuitry, leading to expensiveness and high-complexity. To make time-resolved luminescence instruments simple and cheap, the automatic synchronization method was developed by using a mechanical chopper acted as both of the pulse generator and detection shutter. Therefore, the excitation and detection can be synchronized and locked automatically as the optical paths fixed. In this paper, we first introduced the time-resolved luminescence measurements and review the progress and current state of this field. Then, we discussed low-cost time-resolved techniques, especially chopper-based time-resolved luminescence detections. After that, we focused on auto-phase-locked method and some of its meaningful applications, such as time-gated luminescence imaging, spectrometer, and luminescence lifetime detection. Finally, we concluded with a brief outlook for auto-phase-locked time-resolved luminescence detection systems.

Time-resolved and fluorescence excitation-emission matrix measurements of lanthanide (Gd3+, Tb3+ and Dy3+) doped silver-zinc borate glasses

This paper reports the time-resolved luminescence and excitation-emission matrix measurements of three different lanthanide ions Gadolinium, Gd3+ (GdG), Terbium, Tb3+ (TbG) and Dysprosium, Dy3+(DyG) doped silver-zinc borate glasses. The GdG and TbG glasses were excited at 267 nm and 371 nm, respectively, and time-resolved luminescence measurements used to evaluate how the randomness of the network can influence the lanthanide ion emission. The GdG glass exhibited ultraviolet radiation (UVR) at around 310 nm with a 6P7/2 → 8S7/2 transition. Similarly, a sharp band at 545 nm with a 5D4 → 7F5 transition provided a bright green coloured emission from the TbG glass. To further elucidate the Dy3+ emission, steady-state luminescence (SSL), excitation-emission matrix (EEM) studies and decay associated spectra (DAS) were obtained. These showed a bright yellow emission transition (4F7/2 → 6H9/2), 571 nm composed of species with decay times of 0.062, 0.148, 0.337 and 0.701 ms. Among them, an ideal decay time was observed at 0.701 ms along with a dependency of the DyG glass lifetimes on the emission intensity.

Ultrafast processes triggered by one- and two-photon excitation of a photochromic and luminescent hydrazone.

In this work we apply a combination of steady state and time resolved luminescence and absorption spectroscopies to investigate the excited-state dynamics of a recently developed molecular photoswitch, belonging to the hydrazone family. The outstanding properties of this molecule, involving fluorescence toggling, bistability, high isomerization quantum yield and non-negligible two-photon absorption cross section, make it very promising for numerous applications. Here we show that the light induced Z/E isomerization occurs on a fast <1 ps timescale in both toluene and acetonitrile, while the excited state lifetime of the Z-form depends on solvent polarity, suggesting a partial charge transfer nature of its low lying excited state. Time-resolved luminescence measurements evidence the presence of a main emission component in the 500–520 nm spectral range, attributed to the Z-isomer, and a very short living blue-shifted emission, attributed to the E-isomer. Finally, transient absorption measurements performed upon far-red excitation are employed as an alternative method to determine the two-photon absorption cross-section of the molecule.

The Quest for Molecular Grippers: Photo-Electric Control of Molecular Gripping Machinery.

The quest for nanoscale molecular machines has inspired the search for their close relatives, molecular grippers. This path was paved by the development of resorcin[4]arene cavitands and their quinone-based redox-active congeners. In this Concept article, the efforts to design and establish the control of quinone-functionalized resorcin[4]arenes by electronic and electromagnetic stimuli is described. This was achieved by relying on paramagnetic semiquinone radical anions formed electrochemically or by photoredox catalysis. The gripper-like motion of such species could not be studied by conventional NMR spectroscopy. Instead, an entirely different approach had to be developed that included various electroanalytical and spectroelectrochemical methods, including UV/Vis/NIR spectroelectrochemistry, pulsed EPR and Davies 1 H ENDOR spectroscopy, transient absorption spectroscopy, and time-resolved luminescence measurements, besides density functional theory calculations and X-ray crystallography. The conceptual breakthroughs are reviewed as well as the current state and future perspectives of photoredox-switchable molecular grippers.

Niobium enhanced europium ion luminescence in hafnia nanocrystals

In this work we demonstrate a method where by adding Nb ions, Ln3+ ion luminescence intensity in HfO2 is increased for up to 15 times (in a sample containing 5 mol%Eu). The effect is described as niobium acting as a charge compensator and neutralizing the charge resulting from Ln3+ ion insertion in Hf4+ site and hence reducing the number of defects present. This is the second system where such an effect was observed, so it is expected that other metal oxides would show the same effect. The optical properties of HfO2: Eu3+ and HfO2: Eu3+, Nb5+, synthesized using the sol-gel method and annealed at various temperatures are studied. A conclusion that the structure of hafnia does not affect luminescence intensity directly and a larger role is played by factors such as defect presence and the size of the particles is drawn based on XRD and TSL measurements. Time-resolved luminescence measurements were also carried out and significant changes depending on dopant concentration and annealing temperatures were observed. Judd Ofelt theory was used to determine quantum efficiency and the local symmetry of Eu3+ ion sites.

Radiation-induced optical change of ion-irradiated CdSeS/ZnS core-shell quantum dots embedded in polyvinyl alcohol

Semiconductor quantum dots (QDs) are receiving increasing interest for the production of scintillator detectors and dosimeters. For this reason, the investigation of the effects of ionizing radiation on the optical properties of QDs has become of great importance. In this paper, we present the optical characterization of colloidal CdSeS/ZnS core-shell quantum dots (QDs) embedded in a polyvinyl alcohol (PVA) matrix and irradiated with 2 MeV H+ and He+ ions at different fluences. With steady-state luminescence measurements, we investigate the ion irradiation effects on the luminescence spectrum of the irradiated samples. With time-resolved luminescence measurements, acquired with a Time Correlated Single Photon Counting (TCSPC) system, we evaluate the radiation-induced change in the QD charge carrier dynamics. Luminescence spectra show the appearance of new features related to damage of the PVA host. Time-resolved measurements highlight a shortening of the QD luminescence time constants as the ion fluence increases.

Role of Surface States in Silver-Doped CdSe and CdSe/CdS Quantum Dots

The effects of Ag doping of CdSe and CdSe/CdS quantum dots (QDs) have been studied as a function of the surface composition. Throughout these studies, static and time-resolved luminescence and transient absorption measurements are used to determine the nature and rates of radiationless decay mechanisms and how these vary with the concentration of Ag dopants. In tributylphosphine-ligated CdSe, the photoluminescence quantum yield (PL QY) varies nonmonotonically with dopant concentration, increasing at low concentrations of Ag+ then decreasing with further addition. The initial increase is assigned to the passivation of preexisting surface hole traps by interaction of interstitial Ag+ with surface Se2– ions, whereas the subsequent decrease is due to the introduction of substitutional Ag+ dopants, which act as a new source of hole traps. Ligation of the particle surface with alkyl amines largely passivates the surface hole traps and thereby eliminates the initial increase in PL QY upon doping. CdSe/CdS QDs li...