Photoluminescence Decay Dynamics Research Articles

Insights into the energy transfer mechanism inCe3+−Yb3+codoped YAG phosphors

Two distinct energy transfer (ET) mechanisms have been proposed for the conversion of blue to near-infrared (NIR) photons in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$. The first mechanism involves downconversion by cooperative energy transfer, which would yield two NIR photons for each blue photon excitation. The second mechanism of single-step energy transfer yields only a single NIR photon for each blue photon excitation and has been argued to proceed via a ${\mathrm{Ce}}^{4+}\ensuremath{-}{\mathrm{Yb}}^{2+}$ charge transfer state (CTS). If the first mechanism were operative in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$, this material would have the potential to greatly increase the response of crystalline Si solar cells to the blue/UV part of the solar spectrum. In this work, however, we demonstrate that blue-to-NIR conversion in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$ goes via the single-step mechanism of ET via a ${\mathrm{Ce}}^{4+}\ensuremath{-}{\mathrm{Yb}}^{2+}$ CTS. The photoluminescence decay dynamics of the ${\mathrm{Ce}}^{3+}$ excited state are inconsistent with Monte Carlo simulations of the cooperative (one-to-two photon) energy transfer, while they are well reproduced by simulations of single-step (one-to-one photon) energy transfer via a charge transfer state. Based on temperature dependent measurements of energy transfer and luminescence quenching we construct a configuration coordinate model for the ${\mathrm{Ce}}^{3+}$-to-${\mathrm{Yb}}^{3+}$ energy transfer, which includes the ${\mathrm{Ce}}^{4+}\ensuremath{-}{\mathrm{Yb}}^{2+}$ charge transfer state.

New electroluminescent carbazole-containing conjugated polymer: Synthesis, photophysics, and electroluminescence

A new electroluminescent polymer, poly{9-(2-ethylhexyl)carbazole-2,7-diethynylene-alt-tris[2,5-bis(2-ethylhexyloxy)-1,4-phenylenevinylene]} (PCzE-PPV), is synthesized, and its photophysical and electrochemical properties and electroluminescence (EL) are studied. In solution, an intense photoluminescence (PL) emission with a maximum at about 520 nm is observed. PL decay dynamics in solution are best described by a monoexponential function with a lifetime of 0.76 ns. Thin films exhibit an intense PL emission with a slightly red-shifted maximum at 532 nm compared to that in the solution spectra. The polymer oxidizes and reduces quasi-reversibly. The ionization potential (HOMO level) of 5.3 eV and the electron affinity (LUMO level) of 2.80 eV are evaluated from cyclovoltammetric measurements. The electrochemical bandgap value (2.45 eV) is in good agreement with the optical bandgap value. Using new polymer, light-emitting devices (LEDs) with a luminance higher than 3000 cd m−2 and low onset voltages at about 3 V are fabricated. The shape of EL spectra of the LEDs is similar to that of PL spectra of the thin films.

Photoluminescence profile imaging of Eu(III), Tb(III) and Eu(III)/Tb(III)-doped yttrium oxide nanosheets and nanorods

Eu(III), Tb(III) and Eu(III)/Tb(III)-doped Y2O3 nanosheets and nanorods were prepared by hydrothermal synthesis and post-thermal treatment. Their morphology, crystallinity, photoluminescence and UV–visible absorption profiles were fully examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) crystallography, photoluminescence imaging and UV–vis absorption spectroscopy. The as-prepared samples commonly showed the crystal structure of Y4O(OH)9NO3, which crystallized to cubic phase Y2O3 after 550°C-thermal annealing. The sharp emissions of Eu(III) between 580 and 720nm were attributed to 5D0→7FJ (J=0,1,2,3,4) transitions, and those of Tb(III) between 450 and 720nm were attributed to 5D4→7FJ (J=6,5,4,3) transitions. Energy transfer from Tb(III) to Eu(III) was confirmed by the photoluminescence decay dynamics. In the cubic Y2O3 structure, Eu(III) was embedded at the C2 symmetry site and dominated by the 5D0→7F2 transition. Conversely, Tb(III) was oxidized to non-luminescent Tb(IV) in the cubic structure.

Photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals

We have studied the photocarrier localization and recombination dynamics in Cu2ZnSnS4 single crystals at room temperature. The band-gap energy and tail states below the band edge were evaluated by a combination of photoluminescence (PL), PL excitation, photocurrent, and femtosecond transient reflectivity spectroscopy. The photocarriers are rapidly localized to shallow tail states within a typical time constant of several picoseconds to a few tens of picoseconds. The sub-nanosecond PL decay dynamics indicate the importance of multiple carrier trapping processes in the shallow tail states. Therefore, it is concluded that the tail states dominate the optical responses of Cu2ZnSnS4 single crystals.

Time-resolved fluorescence up-conversion study of radiative recombination dynamics in III-nitride light emitting diodes over a wide bias range

The technique of femtosecond fluorescence up-conversion was employed to explore the transient photoluminescence and carrier decay dynamics in c-plane (In,Ga)N multi-quantum-well light emitting diodes over a wide bias range. By investigating the bias dependence of initial transient photoluminescence intensity and the luminescence lifetime, the field and carrier density effects on the radiative recombination coefficient were revealed for both low and high current injection conditions, and in good agreement with the theory.

Observation of photo darkening in self assembled InGaAs/GaAs quantum dots

Photo darkening was observed in epitaxial InGaAs/GaAs quantum dots (QDs). The photoluminescence (PL) intensity of the QDs showed a non-reversible decrease under continuous laser irradiation. The time constants varied from tens of minutes to several hours, depending on the applied laser power. Based on the spectral evolution, it was concluded that the observed phenomenon should originate from laser induced structural damage and a sustained increase of non-radiative recombination rate in the wetting layer. Additionally, according to the PL decay dynamics at different laser powers, it is argued that there should exist other processes that hinder PL degradation at a high laser power.

Controlling exciton decay dynamics in semiconducting single-walled carbon nanotubes by surface acoustic waves

We show that the photoluminescence intensity and decay dynamics of semiconducting single-walled carbon nanotube films can be remotely controlled by surface acoustic waves (SAW) launched on the piezoelectric substrate LiNbO3. Time-resolved measurements in the picosecond regime reveal that photoluminescence quenching results from a decrease of the radiative recombination rate by up to 25% for the accessible SAW amplitudes. The SAW-induced piezoelectric field acts as a quasi-static perturbation that polarizes the luminescent exciton state reducing the oscillator strength of the radiative transition following a quadratic field dependence. Surface acoustic waves could be used for the remote and contact-free electrical control of high-speed electronic and optoelectronic nanotube-based devices.

Electrochemic and Photophysics Properties of Pyridine-Based Electron Transmission Material

A pyridine-based polybenzobisoxazole (PBOPy) was synthesized via solution condensation polymerization. The polymer was characterized by Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, thermogravimetric analysis, ultraviolet-visible (UV-Vis) absorption spectroscopy, photoluminescence (PL) spectroscopy, electrochemical cyclic voltammetry, and time-resolved PL decay dynamics. Results showed that the introduction of pyridine groups made the oxidation potential (1.82 eV) relatively larger than that of poly(p-phenylenebenzobisoxazole) (PBO) (1.65 eV), while the reduction potential (−1.18 eV) was smaller than that of PBO (−1.28 eV). It can be concluded, by comparing UV-Vis absorption spectroscopy and PL spectroscopy of PBOPy in film and in methanesulfonic acid (MSA), that PBOPy has a stronger protonation effect in the latter. The optical properties of PBOPy exhibited a PL emission band at 473 nm in thin films and a strong orange fluorescence (498 nm) after being protonated with MSA. The protonation effect resulted in a large Stokes shift, about 71 nm.

Photoexcitations From Intrachain and Interchain Excitons of Surface Plasmon Mediated Conjugated Polymers for PLED

Picosecond time-resolved spectroscopy studies are performed on surface plasmon (SP) mediated conjugated polymers. Huang-Rhys factor and photoluminescence (PL) decay time at low temperature were unchanged with the respect to thickness of Ag nanostructures due to a decrease in the surface plasmon coupling rates (Okamoto , Appl. Phys. Lett., vol. 87, 07112, 2005). However, Huang-Rhys factor and PL decay time at room temperature were increased with respect to the thickness of the Ag nanostructures. Relatively strong emission from the interchain excitons of surface plasmon (SP) mediated conjugated polymers in the film-like Ag nanostructure was verified by PL decay dynamics from time-resolved photoluminescence (TR-PL) measurement.

Size-Controlled Electron Transfer and Photocatalytic Activity of ZnO–Au Nanoparticle Composites

This Letter describes size-controlled photocatalytic activity of ZnO nanoparticles coated with glutathione-protected gold nanoparticles with diameters of 1.1, 1.6, and 2.8 nm. The photocatalytic activity of the ZnO–Au composites was found to increase with increasing gold size for both oxidative and reductive catalytic reactions. Photoluminescence decay dynamics of the composites showed that the electron-transfer rate from the photoexcited ZnO to gold nanoparticle also increased as the gold size increased. These results demonstrate that the photogenerated electron transfer and the resulting catalytic activity of the composites can be controlled by the size of the mediating gold capacitors.

Observation of Size-Dependent Thermalization in CdSe Nanocrystals Using Time-Resolved Photoluminescence Spectroscopy

We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ~25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions.

Size-dependent visible absorption and fast photoluminescence decay dynamics from freestanding strained silicon nanocrystals

In this article, we report on the visible absorption, photoluminescence (PL), and fast PL decay dynamics from freestanding Si nanocrystals (NCs) that are anisotropically strained. Direct evidence of strain-induced dislocations is shown from high-resolution transmission electron microscopy images. Si NCs with sizes in the range of approximately 5-40 nm show size-dependent visible absorption in the range of 575-722 nm, while NCs of average size <10 nm exhibit strong PL emission at 580-585 nm. The PL decay shows an exponential decay in the nanosecond time scale. The Raman scattering studies show non-monotonic shift of the TO phonon modes as a function of size because of competing effect of strain and phonon confinement. Our studies rule out the influence of defects in the PL emission, and we propose that owing to the combined effect of strain and quantum confinement, the strained Si NCs exhibit direct band gap-like behavior.

Poly[methyl(phenyl)silanediyl] modified with dansyl fluorophore: Synthesis and photophysics

AbstractThe syntheses and photophysical properties of novel luminescent polysilanes modified with dansyl fluorophores on various spacer (PMPSi‐n‐DNS) are reported. The modified polysilanes were prepared by condensation reaction of dansyl amines, such as 5‐(dimethylamino)naphthalene‐1‐sulfonohydrazide or N‐(ω‐aminoalkyl)‐5‐(dimethylamino)naphthalene‐1‐sulfonamides with formylated poly[methyl(phenyl)silanediyl], yielding Schiff bases. The aldehyde groups were incorporated into the parent polymer by a one‐step reaction with dichloromethyl methyl ether in the presence of Lewis acid. Influence of the alkyl chain length (n) on the photophysical properties was investigated using absorption, steady‐state, and time‐resolved photoluminescence (PL) spectroscopy. The excitation energy transfer from the polysilane backbone to dansyl fluorophore was proved. PL decay dynamics revealed the existence of more than one excited species. In solution, the decay curves of PMPSi‐DNS (no spacer) were double exponential, whereas for PMPSi‐n‐DNS (with spacer), and, in thin films, the decays were three‐exponential. Polymer light‐emitting devices (LEDs) were prepared from the blends of modified polysilane with electroluminescent polymer poly[2,5‐dimethoxy‐1,4‐phenylene‐1,2‐ethenylene‐2‐methoxy‐5‐(2‐ethylhexyloxy)‐(1,4‐phenylene‐1,2‐ethenylene)] (M3EH‐PPV). Compared with LED made of neat electroluminescent polymer, a significant performance improvement of blend LEDs was demonstrated. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Light emission from SrTiO3

AbstractWe summarized our recent optical studies on perovskite semiconductor SrTiO3 crystals. Photocarrier recombination dynamics of undoped, electron‐doped, and vacancy‐doped SrTiO3 crystals were studied by photoluminescence (PL) and transient absorption (TA) spectroscopy. By high‐density photoexcitation and electron doping, the blue PL is clearly observed in SrTiO3 crystals at room temperature. The PL spectrum and dynamics are sensitive to the photoexcited carrier density and the chemically doped electron density. The PL and TA decay dynamics under high‐density excitation are determined by nonradiative Auger recombination. It is demonstrated that the PL lifetime can be used as a probe for determining spatial profiles of carriers and defects in SrTiO3.

Blue photoluminescence of highly photoexcited rutileTiO2: Nearly degenerate conduction-band effects

We report the observation of blue photoluminescence (PL) from highly photoexcited rutile ${\text{TiO}}_{2}$ single crystals at room temperature. Under extremely high-density excitation, we found an anomalous dependence of the PL dynamics and intensity on the excitation density. By considering the nearly degenerate conduction bands of rutile, we revealed that the PL behavior is determined by the electron density in the lower-energy conduction band. The PL decay dynamics is well explained by the simple model involving two-body radiative recombination and three-body Auger recombination processes. We discuss the origin of the blue PL band.

Ultrafast decay of photoluminescence from high-density excitons inAlxGa1−xNmixed crystals: Diffusive propagation of exciton-polaritons

We report on the ultrafast decay dynamics of photoluminescence (PL) in highly excited ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals under exciton resonant excitation at low temperatures. When the excitation intensity is increased, the P-band emission appears due to exciton-exciton inelastic scattering processes. The PL intensity of the P band decays rapidly, with a much shorter decay time than the radiative recombination time of the excitons. We show that the ultrafast PL decay dynamics can be understood as due to the disorder-induced diffusive propagation of photonlike exciton-polaritons in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals.

Effect of Cross-Linking and Organic Groups on the Visible Photoluminescence Characteristics of n-Octadecylsiloxanes

The photoluminescence (PL) properties of properly annealed n-octadecylsiloxanes, which are newly recognized highly emissive hybrid materials reported recently by us (Nishimura, A.; Sagawa, N.; Uchino, T. J. Phys. Chem. C 2009, 113, 4260), with different cross-linking structures are investigated using steady-state and time-resolved PL spectroscopy. We find that the observed PL intensity and the PL decay dynamics are strongly dependent on the degree of cross-linking of the organic−inorganic hybrid materials. We attribute these changes to the initial concentration of the residual silanol groups, which can be converted into emission and electron-trapping centers through the condensation reaction during annealing. These PL features are then compared with those of purely inorganic silica gels to evaluate the effect of organic groups on the PL characteristics of the n-octadecylsiloxanes. It has been found that organic groups contribute to the PL decay component in the time scale of ∼70 to ∼100 ns. Also, organic ...

A spectroscopic and photochemical study of Ag +-, Cu 2+-, Hg 2+-, and Bi 3+-doped Cd xZn 1−xS nanoparticles

A combination of stationary and time-resolved absorption and photoluminescence spectroscopy with flash and steady-state photolysis of Ag +, Cu 2+, Hg 2+, or Bi 3+-doped Cd x Zn 1− x S nanoparticles was used to assess the nature of the doping influence upon the optical properties of Cd x Zn 1− x S nanoparticles. The relationships between the type and the concentration of a dopant and the dynamics of the photoinduced processes in the doped nanoparticles are derived and discussed. A correlation is found between the magnitude of doping-induced changes in the intensity and decay dynamics of the deep trap photoluminescence and an enhancement of the transient bleaching recovery and acceleration of the photocorrosive degradation of the doped Cd x Zn 1− x S NPs compared to the undoped ones. The impact of the dopant upon the intensity of the luminescence and microsecond transient bleaching bands was found to grow substantially from Ag + to Cu 2+, Hg 2+ and Bi 3+. The same trend was found to hold for the acceleration of the steady-state photochemical corrosion of doped Cd x Zn 1− x S nanoparticles. The differences among the effect of the dopant ions studied were interpreted in terms of the depth and charge of surface states created by Cd 2+ (Zn 2+) substitution by a dopant.

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics.

Charge carrier trapping is an important phenomenon in nanocrystal (NC) decay dynamics because it reduces photoluminescence (PL) quantum efficiencies and obscures efforts to understand the interaction of NC excitons with their surroundings. Particularly crucial to our understanding of excitation dynamics in, e.g., multiNC assemblies, would be a way of differentiating between processes involving trap states and those that do not. Direct optical measurement of NC trap state processes is not usually possible because they have negligible transition dipole moments; however, they are known to indirectly affect exciton photoluminescence. Here, we develop a framework, based on Marcus electron transfer theory, to determine NC trap state dynamics from time-resolved NC exciton PL measurements. Our results demonstrate the sensitivity of PL to interfacial dynamics, indicating that the technique can be used as an indirect but effective probe of trap distribution changes. We anticipate that this study represents a step toward understanding how excitons in nanocrystals interact with their surroundings: a quality that must be optimized for their efficient application in photovoltaics, photodetectors, or chemical sensors.

Effect of Microscopic Structure and Porosity on the Photoluminescence Properties of Silica Gels

We have studied the photoluminescence (PL) characteristics of silica gels with different microscopic morphology, which are prepared by acid- and base-catalyzed hydrolysis of tetraethylorthosilicate in ethanol solution. All the gels show visible PL emission peaking at ∼420 nm after annealing the samples at ∼200 °C in air; however, the PL intensity and decay kinetics are strongly dependent on the microstructure and porosity of the initial gels. The annealing-induced PL intensity increases with decreasing porosity of the initial gels, whereas the PL decay dynamics tends to deviate from a pure exponential trend as the porosity of the initial gels becomes higher. These results indicate that the formation, stabilization, and radiative recombination process of the emission centers that are created during annealing are influenced by the microscopic morphology of the gels. These experimental results are interpreted in terms of the formation mechanism of the visible emission center in silica-based nanostructure materials proposed recently (Uchino, T.; Kurumoto, N.; Sagawa, N. Phys. Rev. B 2006, 73, 233203).