Transient Luminescence Research Articles

Transient and stable color centers in neutron irradiated MgO

The transient absorption and luminescence induced by the pulsed electron beam have been investigated in the MgO single crystal containing transition metal ion (Cr, Mn, Fe) impurities and preliminary irradiated by the fast neutrons. It is supposed that the different behavior of the absorption spectra of the MgO samples preliminary irradiated by the different fast neutron fluence is connected with the destruction of the hole centers and with the creation of interstitial protons and the formation of the microphase Mg(OH)2. We assume that the luminescence band at ∼3.2eV is connected with F+ color centers.

Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-μm thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3 ns and diffusion coefficient of 1 cm 2/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.

Study of transient luminescence of three kinds of Ru complexes bound to DNA

The transient luminescence of three kinds of ruthenium complexes [Ru(bpy)2(7-CH3-dppz)]2+, [Ru(bpy)2(7-F-dppz)]2+ and [Ru(phen)2(7-F-dppz)]2+ bound to calf thymus DNA (ctDNA) has been studied by using the time-resolved spectroscopy. The results show that the luminescence is due to the radiative decay from the charge-transfer states to the ground state. By the interaction with DNA, the radiativeless rate of the photoexcited Ru complex molecules decreases, which results in the increase of luminescence lifetime and efficiency. The structure of the Ru complex has an important impact on the interaction with DNA. The [Ru(bpy)2(7-CH3-dppz)]2+ shows the longest luminescence lifetime (about 382 ns), while the [Ru(bpy)2(7-F-dppz)]2+ shows the shortest lifetime (about 65 ns). The possible origin of the luminescence dynamics is discussed.

Effect of exciton rotational relaxation on luminescence decay of low-doped porphyrin side-chain polymers

Effect of rotational motion of porphyrin molecules on the luminescence dynamic of low-doped porphyrin side-chain polymers has been studied by using the theory of exciton rotational diffusion. It is shown that the rotational motion of porphyrin molecules is an important way that induces the nonradiative relaxation of excitons. The more easily porphyrins rotate， the more rapidly energy decays， therefore， the faster the luminescence decay process. In the case of low doping concentration of porphyrins and long interchain distance， the rotational motion of porphyrin molecules is the main reason of fast luminescence decay and low luminescence efficiency. The transient luminescence relaxation processes of two kinds of polymers were fitted with the theoretical expression， the theoretical analysis are in good agreement with the experimental results.

带-NH2的嘧啶环对钌配合物与DNA相互作用的瞬态发光特性的影响

带-NH2的嘧啶环对钌配合物与DNA相互作用的瞬态发光特性的影响

Trapping mechanism in the afterglow process of the rare-earth activated Y 2O 2S phosphors

Phosphorescence properties are investigated in Y 2O 2S phosphors doped with rare-earth (lanthanoid, Ln) ions. Luminescence afterglow with a decay time of several ten milliseconds is observed at room temperature in the phosphors activated by Nd, Sm, Eu, Dy, Ho, Tm, Er, and Yb. The depths (thermal activation energies) of the traps causing the afterglow are measured with the transient luminescence method. It is concluded that the excited electron and the hole in the conduction and valence bands are trapped separately in the states (impurity levels) located in the vicinity of the Ln 3+ ion. The trapping depths of the level range from 0.3 to 1.1 eV and are dependent on the electron affinity of the Ln 3+ ion estimated from the energy difference between the 4f n+1 and the 4f n configurations in the 4f shell of the ion.

Effect of intrachain rotation of porphyrin molecules on luminescence efficiency in two kinds of low-doped porphyrin side-chain polymer films

Transient luminescent dynamics in two kinds of low-doped porphyrin side-chain polymer films, poly[porphyrin acrylate-acrylonitrile] (p[(por)A-AN]) and poly[porphyrin acrylate-styrene] (p[(por)A-S]), were measured using time-resolved photoluminescence spectroscopy. The luminescence is shown to decay more rapidly with the decrease of the intrachain porphyrin comonomer concentration. We calculated the interaction potential energy between the intrachain porphyrin molecules by using the method of ${\mathrm{MM}}^{+}$ force field. The results show that the intrachain aggregation of the porphyrin molecules can induce larger rotational barrier, slowing the nonradiative decay of the emissive exciton and leading to higher luminescence efficiency. The transient luminescence relaxation processes in p[(por)A-S] and p[(por)A-AN] films were numerically fitted with the theory of the exciton rotation diffusion. The experimental results are in good agreement with the theoretical analysis.

Carrier recombination and diffusion in GaN revealed by transient luminescence under one-photon and two-photon excitations

Carrier recombination and diffusion dynamics in a 100-μm-thick GaN grown by hydride vapor phase epitaxy on semi-insulating GaN:Mg substrate have been studied by means of transient photoluminescence under one-photon (1P) and two-photon (2P) excitations. For 2P bulk excitation the luminescence transients featured an exponential decay with the time constant of 1100ps, which was mainly due to carrier capture to nonradiative deep traps. Meanwhile for 1P surface excitation, the luminescence transients showed a nonexponential decay with the mean time constant of 440ps, which was shown to be due to both carrier in-depth diffusion and recombination.

The effect of intermolecular interactions on photoluminescence of a porphyrin side-chain polymer

Photoluminescence properties and exciton decay dynamics in a porphyrin side-chain polymer, poly[porphyrin acrylate- acrylonitrile (abbreviated p[(por)A-AN]), have been investigated by femtosecond time-resolved photoluminescence spectroscopy. All the luminescences of p[(por)A-AN] films are due to the emissive decay of the photoexcited singlet excitons in the porphyrins. The luminescence efficiencies and lifetimes are increased for samples from pure films to dilute blend films. However, they are increased as the intrachain concentration of the porphyrin sidechain groups is decreased. The intrachain rotation motions of porphyrin sidechain groups result in the initial ultrafast luminescence decays, which are much faster than those due to the interchain interactions. All the samples show no significant red-shift and broadening of the transient luminescence spectra. The interchain and intrachain nonradiative exciton relaxation processes may play an important role in the luminescence dynamics in the p[(por)A-AN] films. The possible origin of different intrachain and interchain dynamic behaviours in p[(por)A-AN] films is discussed.

Optically stimulated luminescence of persistent luminescence materials

For around ten years, intense attention has been devoted to materials which exhibit strong persistent luminescence. The Eu 2+ doped SrAl 2O 4 is probably the best one. Co-doping with other trivalent rare earth ions, especially with Dy 3+, enhances the persistent luminescence. Co-doping with Sm 3+ leads to a drastic decrease in the efficiency of persistent luminescence, however. In this paper, the results of a careful analysis of the dynamic luminescence properties of SrAl 2O 4:Eu 2+ and co-doped either with Dy 3+ or Sm 3+ under different excitation schemes are presented. The temporal transients and time resolved luminescence spectra were analysed with pulsed UV excitation at 355 nm provided by the third harmonic of a Nd:YAG laser. Then continuous wave excitation at 360 nm was also applied simultaneously with the pulsed UV excitation. The profile of the transient luminescence was found to be strongly dependent on the excitation scheme, especially on the pumping power at 355 nm. In addition to the Eu 2+ decay, other population processes were found to occur. Two components were observed for all the materials studied. In order to carry out a complete analysis of the data, the positions of the 4f and 5d energy levels for the R 3+ and R 2+ ions in the energy gap of SrAl 2O 4 were determined.

Primary Photoexcitations and the Origin of the Photocurrent in Rubrene Single Crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.

H+-Coupled Sugar Transporter, an Initiator of Sugar-induced Ca2+-signaling in Plant Cells

Using Ca2+-dependent photoprotein aequorin-transformed tobacco BY-2 cell suspensions, the sugar-induced increase in cytosolic free Ca2+ concentration ([Ca2+]cyt) was investigated by measuring the luminescence intensity. When 0.5 M sucrose or some other sugars were fed to the cells, strong and transient luminescence was observed. Salts or sugar analogues didn't show this effect. In addition, the intensity of sucrose-induced aequorin luminescence was gradually enhanced when cells were exposed to sugar-starvation. This was observed with the concurrent expression of the sucrose/H+ co-transporter, NtSUT1A. The [Ca2+]cyt increase may initiate Ca2+-signaling leading to the expression of genes related to biosynthesis of storage carbohydrates in a sink organ. The sugar-signaling may play an important role in the conversion on nutritional stage of plant tissue, source organ to sink organ.

Efficient red electrophosphorescent top-emitting organic light-emitting devices

We have fabricated top-emitting red electrophosphorescent organic light-emitting devices with the 4,4′- N, N′-dicarbazole-biphenyl (CBP): bis[2-(2′-benzothienyl)-pyridinato- N, C 3′]iridium(acetylacetonate) (BtpIr(acac)) as a emitting layer. Emission spectra show peaks in the red region at 620 and 670 nm due to phosphorescent guest, accompanied by a very weak peak at 450 nm due to host CBP. Electrical characteristics indicate low turn-on voltage and good electrophosphorescence efficiency. The insertion of an Alq 3 electron injection layer between the emitting layer and cathode significantly improves the device properties. Transient luminescence excitation has been employed to study the emission mechanism in the host–guest system. Delay, rise and decay times of the devices decrease with the applied voltage amplitude. The efficiency of electron injection and the nature of the sites where triplet exciton form strongly influence the emission and electrical properties of top-emitting phosphorescent devices.

Luminescence of highly excited nonpolar a ‐plane GaN epilayers

Luminescence transients have been studied in nonpolar a-plane fully coalesced epitaxial laterally overgrown (ELOG) GaN film grown over r-plane sapphire substrate under intense photoexcitation conditions. By increasing the excitation energy density (I) from 0.3 to 4.5 mJ/cm2, three regimes of carrier relaxation were revealed. These regimes are characterized by a specific time profile of the luminescence transient. For a low excitation density, I ≤ 0.35 mJ/cm2, the luminescence transient is nonexponential with an enhancement of the decay rate on the late stage of the transient due to carrier capture to nonsaturated deep traps. For an excitation density increased to about I ≅ 1 mJ/cm2, an establishment of the deep-trap saturation regime with a single luminescence decay time is observed. A further increase in the excitation density above 3 mJ/cm2 leads to an essentially nonexponential luminescence decay. Here, the initial stage of rapid recombination is attributed to laterally stimulated emission, while the late recombination stage is contributed by capture to the saturated deep traps and enhanced bimolecular emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

X‐ray and pulsed electron beam excited luminescence and optical absorption in KTaO 3 crystals

Pulsed electron beam excited time-resolved transient absorption and luminescence have been studied in nominally pure and Li-doped KTaO3 single crystals. It is found that under electron beam excitation in nominally pure KTaO3 the absorption bands peaks at ∼0.8 eV, ∼1.1 eV and 1.7 eV whereas in KTaO3:Li at ∼1.1 eV and ∼1.7 eV. The transient absorption decay varies over the spectrum thus revealing several short-lived centers. The main luminescence peak is centred at ∼2.7 eV in all KTaO3 specimens. Luminescence decay kinetics observed for the two bands slightly differ indicating different kinds of excited states are involved. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transient spectroscopy of InAs quantum dot molecules

Coupled pairs of InAs quantum dots are grown by molecular-beam epitaxy. Structural and optical characterization is done by means of transmission electron microscopy and photoluminescence, respectively. Photoluminescence spectra consist at least of three well-separated optical transitions that are assigned to molecular energy terms and a substantial exciton lifetime increase is observed. Detailed spectral analysis of the transient luminescence behavior indicates “intraterm” transitions that could be favorably used for the creation of midinfrared light sources.

Optical investigations of TlBr detector crystals

Shift of fundamental absorption edge, the position of main luminescence bands, the luminescence decay and transient absorption spectra in three TlBr crystals were studied. The γ-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties. The iodine impurity in TlBr was detected by optical methods. The role of impurities and crystal defects in γ-quanta detectors manufactured is discussed.

Transient processes and luminescence upconversion in zero-dimensional nanostructures

The experimental findings like the fast electronic energy relaxation in quantum dots, the fast decrease of photoluminescence from the ground state following the resonant excitation of quantum dot to the excitonic ground state, or the incomplete depopulation of excited state of electronic system of the quantum dot, or the effect of upconversion of population of electronic states and upconversion of photoluminescence (anti-Stokes luminescence) of quantum dots have attracted attention recently. These effects are sometimes interpreted in terms of carrier–carrier interaction in samples with quantum dots, intermediate states in quantum dot samples, thermal excitation of phonon subsystem and two-photon absorption. Basing on numerical calculations performed with relatively simple, but rather realistic, a model of two level quantum dot with a single electron coupled to phonons, we suggest to interpret the above phenomena in terms of the interaction of the electronic system of quantum dot with longitudinal optical phonons. In contrast to the lowest order effect of the optical line broadening and the continuous background in photoluminescence excitation experimental data observed in the presence of highly populated wetting layer, we show that the above listed effects are due to multiple scattering of electron in quantum dot, leading to multiphonon states of the optical phonon system. Using the technique of the non-equilibrium Green's functions we present the kinetic equations describing the relaxation process of the electronic system interacting with phonons. The numerical results are found in an interesting agreement with the available experimental data.

Luminescence transients in highly excited GaN grown by hydride vapor-phase epitaxy

Luminescence transients has been studied in GaN grown by hydride vapor-phase epitaxy (HVPE) under intense photoexcitation conditions. The HVPE grown GaN layer exhibited luminescence decay time of 205 ps, that implies the room-temperature free-carrier lifetime of 420 ps. The obtained carrier lifetime of HVPE-grown GaN is significantly higher that the typical values of carrier lifetime measured for GaN heterolayers grown by metalorganic chemical vapor deposition, what suggests high potential of HVPE growth technique for light-emitting diode and blue laser applications.

Exciton dynamics in supramolecular assemblies of p-phenylenevinylene oligomers

We have investigated the dynamics of photoexcitations in chiral assemblies of p-phenylenevinylene oligomers functionalized with hydrogen-bonding motifs. In the regime of low excitation densities, the luminescence transients are influenced by the migration of excitons to defect sites, indicative of fast diffusivity of excitons along the molecular assemblies. In addition, at high excitation densities, bimolecular exciton annihilation is shown to result in the fast depopulation of the stacks’ excitonic energy levels.