Showing papers on "Photoexcitation published in 2009"
TL;DR: These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which are identified as a simple 1MLCT→3 MLCT→5T cascade from the initially excited state.
Abstract: X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer ( 1 MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1 MLCT→ 3 MLCT→ 5 T cascade from the initially excited state. The time scale of the 3 MLCT→ 5 T relaxation corresponds to the period of the iron-nitrogen stretch vibration.
489 citations
TL;DR: In this paper, the authors studied the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates.
Abstract: Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.
230 citations
TL;DR: In this paper, the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit were studied and shown to be dominated by excess hole carriers, with a 12-ps mono-exponential decay that refects the minority-carrier recombination time.
Abstract: We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma Non-equilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 12-ps mono-exponential decay that refects the minority-carrier recombination time
221 citations
TL;DR: In this paper, an exciton fission process that converts one singlet exciton into two triplet excitons was employed to increase the quantum efficiency of an organic multilayer photodetector beyond 100%.
Abstract: We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H=0.4 T, while the C60 photocurrent is relatively unchanged.
204 citations
TL;DR: Photocatalytic splitting of water into H( 2) and O(2) under visible light irradiation is achieved using a coumarin-dye-adsorbed lamellar niobium oxide for hydrogen evolution.
141 citations
TL;DR: In this article, a model of carrier trapping based on the electron transfer theory was proposed to accurately reproduce the photoexcitation dynamics of colloidal nanocrystals over a wide temperature range in five core−shell CdSe/CdZnS samples.
Abstract: Surface effects significantly affect the photoexcitation dynamics of colloidal nanocrystals, but their influence is hard to study because of sample complexity and the typically small extinction coefficient of trap states. Using temperature-dependent time-resolved photoluminescence (PL) measurements, we investigate the perturbations induced by surface-localized carrier traps on the exciton dynamics of the nanocrystals. We present a model of carrier trapping that is based on Marcus’ electron-transfer theory and use it to accurately reproduce PL dynamics over a wide temperature range in five core−shell CdSe/CdZnS nanocrystal samples. The resulting pictures of carrier dynamics are then used to identify features in the PL data that may be used in subsequent experiments to reveal information about the energy and distribution of surface-localized trap states. We find that in certain cases, the shape of the ensemble distribution of trap energies can be accurately determined from data recorded at a single temperat...
136 citations
TL;DR: In this article, the effect of electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electronelectron, electronhole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included.
Abstract: Electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electron-electron, electron-hole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included. The Elliott-Yafet mechanism is also fully incorporated. This approach offers a way toward thorough understanding of electron spin relaxation both near and far away from the equilibrium in the metallic regime. The dependences of the spin relaxation time on electron density, temperature, initial spin polarization, photo-excitation density, and hole density are studied thoroughly with the underlying physics analyzed. We find that these dependences are usually qualitatively different in the nondegenerate and degenerate regimes. In contrast to the previous investigations in the literature, we find that: (i) In $n$-type materials, the Elliott-Yafet mechanism is less important than the D'yakonov-Perel' mechanism, even for the narrow band-gap semiconductors such as InSb and InAs. (ii) The density dependence of the spin relaxation time is nonmonotonic and we predict a peak in the metallic regime in both $n$-type and intrinsic materials. (iii) In intrinsic materials, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel' mechanism. We also predict a peak in the temperature dependence of spin relaxation time which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scattering in intrinsic materials with small initial spin polarization. (iv) In $p$-type III-V semiconductors, the Bir-Aronov-Pikus mechanism dominates spin relaxation in the low-temperature regime only when the photoexcitation density is low. When the photoexcitation density is high, the Bir-Aronov-Pikus mechanism can be comparable with the D'yakonov-Perel' mechanism only in the moderate temperature regime roughly around the Fermi temperature of electrons, whereas for higher or lower temperature it is unimportant. The relative importance of the Bir-Aronov-Pikus mechanism decreases with the photoexcitation density and eventually becomes negligible at sufficiently high photoexcitation density. The effect of electric field on spin relaxation in $n$-type III-V semiconductors is also studied with behaviors very different from those in the two-dimensional case reported. Finally, we find good agreement of our calculation with the experimental results.
115 citations
TL;DR: Time-resolved crystallography and density functional theory calculations are used to analyze the geometric and electronic changes that occur upon photoexcitation of crystalline Cu(I)(dmp)(dppe) in crystalline and show a large rearrangement of the electron density on the Cu atom upon excitation.
Abstract: Time-resolved crystallography and density functional theory calculations are used to analyze the geometric and electronic changes that occur upon photoexcitation of [Cu(I)(dmp)(dppe)]+ in crystalline [Cu(I)(dmp)(dppe)][PF6] [dmp = 2,9-dimethyl-1,10-phenanthroline; dppe = 1,2-bis(diphenylphosphino)ethane]. In the pump−probe experiment, laser and X-ray pulses are synchronized to capture an image of the instantaneous molecular distortions in the transient triplet state. Parallel theoretical calculations, with the phenyl groups replaced by methyl groups, yield information on the distortion of the isolated cation and the change in electron density upon excitation. The experimental distortions are significantly less than the calculated values and are different for the two independent molecules in the asymmetric unit; these findings are attributed to the constraining influence of the crystal matrix. The calculations indicate that the electron transfer upon excitation is mostly from the dmpe ligand to the dmp lig...
113 citations
TL;DR: In this article, the authors focused on the in-site observation of various reactive oxygen species (ROS), such as singlet oxygen (1O2) and the hydroxyl radical (•OH), generated by the photoexcitation of TiO2 nanomaterials using single-molecule fluorescence spectroscopy.
Abstract: Heterogeneous photocatalysts have both potential and demonstrated applications for use in the water-splitting reaction that produces hydrogen, the degradation of organic pollutants, the surface wettability conversion, etc. In this feature article, we have focused on the in-site observation of various reactive oxygen species (ROS), such as singlet oxygen (1O2) and the hydroxyl radical (•OH), generated by the photoexcitation of TiO2 nanomaterials using single-molecule fluorescence spectroscopy. The spatially resolved photoluminescence (PL) imaging techniques enable us to determine the location of the (photo)catalytically active sites that are related to the heterogeneously distributed defects on the surface. We also present the results that revealed the formation and reaction dynamics of the photogenerated charge carriers in individual TiO2 nanoparticles. Furthermore, we introduce the single-molecule single-mismatch detection of the nucleotide sequence upon the photoexcitation of a novel nanoconjugate consi...
91 citations
TL;DR: The analysis of the structural and energetic properties of the molecule during the jumps reveals the main role that the ethynylene triple bond plays in the unidirectional energy transfer process.
Abstract: The ultrafast dynamics of electronic and vibrational energy transfer between two- and three-ring linear poly(phenylene ethynylene) units linked by meta-substitution is studied by nonadiabatic molecular dynamics simulations. The molecular dynamics with quantum transitions(1, 2) method is used including an “on the fly” calculation of the potential energy surfaces and electronic couplings. The results show that during the first 40 fs after a vertical photoexcitation to the S2 state, the nonadiabatic coupling between S2 and S1 states causes a fast transfer of the electronic populations. A rapid decrease of the S1−S2 energy gap is observed, reaching a first conical intersection at ≈5 fs. Therefore, the first hopping events take place, and the S2 state starts to depopulate. The analysis of the structural and energetic properties of the molecule during the jumps reveals the main role that the ethynylene triple bond plays in the unidirectional energy transfer process.
75 citations
TL;DR: In this article, the authors investigated the morphology of the interfacial polymer layer and the photoexcitation dynamics in the picosecond regime of poly(3-hexylthiophene) (P3HT)/ZnO solar cells.
Abstract: To understand the critical factor(s) that influence short-circuit current in poly(3-hexylthiophene) (P3HT)/ZnO solar cells, we investigate the morphology of the interfacial polymer layer and the photoexcitation dynamics in the picosecond regime. Thin (∼6 nm) films of P3HT deposited on bare ZnO and ZnO modified with an alkanethiol monolayer are used as model systems for the heterojunction interface. Results are compared with thin P3HT films on glass for the behavior of the polymer alone. Synchrotron grazing incidence X-ray diffraction spectra of P3HT thin films deposited on glass and on an alkanethiol-modified ZnO surface identify a crystalline P3HT interfacial layer, while an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay dynamics of initial photoexcited states, the samples are interrogated by pump–probe spectroscopy with sub-picosecond time resolution. Compared to P3HT/ZnO composite films, the decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly slower with alkanethiol modification, indicating a reduction in early-stage charge recombination. These experiments demonstrate how the interfacial polymer morphology has a critical role in determining device performance.
TL;DR: The first experimental observation of electron dynamics at metal surfaces in the subfemtosecond range is provided using a full time-dependent approach within a one-dimensional model that includes the main ingredients of the short time physics involved in the experiment.
Abstract: The recent work of Cavalieri et al. [Nature (London) 449, 1029 (2007)] has provided the first experimental observation of electron dynamics at metal surfaces in the subfemtosecond range. We explain the experimental findings using a full time-dependent approach within a one-dimensional model that includes the main ingredients of the short time physics involved in the experiment.
TL;DR: In this paper, Nanosecond pulsed laser ablation of bulk silicon crystal upon the excitation of 532 nm was conducted in supercritical CO2 to generate silicon nanocrystals, whose properties were studied by seven experimental methods.
Abstract: Nanosecond pulsed laser ablation of bulk silicon crystal upon the excitation of 532 nm was conducted in supercritical CO2 to generate silicon nanocrystals, whose properties were studied by seven experimental methods. According to the photoluminescence spectra and fluorescence microscope images, emissions of near-ultraviolet, violet, blue, green, and red were observed in air, at room temperature, and without cooling in liquid nitrogen or a helium cryogenic system. A preferable emission channel of carriers, generated by photoexcitation of Si/SiO2 of core/shell structure, was responsible for interface states with defect sites. This luminescence process caused color changes and intensity increase, enhanced by a factor of 100, where thermal properties of supercritical CO2 were maximized, due to critical anomaly. It was found that colors and intensities of photoluminescence of silicon nanocrystals are controlled by a cooling rate during ablation, whose quantity is manipulated by the supercritical fluid pressure.
TL;DR: The remarkable elongation of the CS lifetime results from the strong binding of Mg(2+) to the PDI(*-) moiety in the CS state.
Abstract: Zinc phthalocyanine−perylenebisimide pentameric arrays, ZnPc(PDI)4 1 and 2, have been synthesized. ZnPc(PDI)4 1 has no substituents in the PDI bay positions, while ZnPc(PDI)4 2 presents four phenoxy groups in the bay positions of each perylene. In both cases, the PDI moieties are directly connected to the ZnPc. As a consequence of aggregation, photoexcitation of 1 affords the intermolecular exciplex rather than the charge-separated state. In contrast to 1, photoexcitation of 2, which contains sterically demanding substituents in the PDI moieties, affords the charge-separated (CS) state, which was clearly detected by its transient absorption spectrum in femtosecond laser flash photolysis measurements. The CS lifetime was determined to be 26 ps. The addition of Mg(ClO4)2 to a benzonitrile solution of 2 and the photoexcitation affords the long-lived CS state with the lifetime of 480 μs, whereas no such long-lived CS state was formed in the case of 1 under such conditions. The remarkable elongation of the CS ...
TL;DR: In this article, a model for photoexcitation dynamics and photoluminescence of strongly coupled $J$-aggregate microcavities was developed based on a description of the film as a disordered Frenkel exciton system, in which relaxation occurs due to the presence of a thermal bath of molecular vibrations.
Abstract: We have developed a model accounting for the photoexcitation dynamics and the photoluminescence of strongly coupled $J$-aggregate microcavities. Our model is based on a description of the $J$-aggregate film as a disordered Frenkel exciton system, in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton polaritons are formed, mixing super-radiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a cw nonresonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular, both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a particular set of discrete optical molecular vibrations active in relaxation processes are investigated.
TL;DR: A simple method to induce optical activity in the terahertz (THz) region using photoexcited carriers in a semiconductor substrate with metal two-dimensional chiral masks is proposed and demonstrated.
Abstract: We propose and demonstrate a simple method to induce optical activity in the terahertz (THz) region using photoexcited carriers in a semiconductor substrate with metal two-dimensional chiral masks, which does not show optical activity without photoexcitation. The three-dimensional chirality induced by the combination of photocarriers and metal mask gives rise to the optical activity. With this simple method, we can develop THz polarization modulation techniques applicable for biology, chemistry, and material sciences.
TL;DR: An intensity-stabilized diode laser absorption spectrometer was developed and used to perform a highly accurate study of the line shape of CO(2) absorption lines, in the spectral region around 5000 cm(-1), belonging to the nu(1) + 2nu(2)(0) + nu(3) combination band.
Abstract: An intensity-stabilized diode laser absorption spectrometer was developed and used to perform a highly accurate study of the line shape of CO2 absorption lines, in the spectral region around 5000 cm−1, belonging to the ν1+2ν20+ν3 combination band, at a temperature of 296.00 K. Standard and complex semiclassical models, including Dicke narrowing and speed-dependent broadening effects, were applied, tested, and compared in the pressure range between 0.7 and 4 kPa, in order to single out the model best reproducing the absorption profile and, hence, the physical situation of self-colliding CO2 molecules. Line intensity factors and self-broadening coefficients were determined. The 1-σ overall accuracy of our determinations is at a level of 0.1%, which is, to our knowledge, the highest ever reached.
TL;DR: It is proved that the tube diameter initially increases in response to ultrafast photoexcitation, and from excitation profiles, it is demonstrated that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when theTube diameter undergoes coherent radial breathing mode oscillations.
Abstract: Using predesigned trains of femtosecond optical pulses, we have selectively excited coherent phonons of the radial breathing mode of specific-chirality single-walled carbon nanotubes within an ensemble sample. By analyzing the initial phase of the phonon oscillations, we prove that the tube diameter initially increases in response to ultrafast photoexcitation. Furthermore, from excitation profiles, we demonstrate that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when the tube diameter undergoes coherent radial breathing mode oscillations.
TL;DR: The results indicate that if the S(1) state of the amino compound is sufficiently stabilized, other rapid decay channels like internal conversion to the ground state will minimize the transfer of population to the triplet manifold.
Abstract: We present a study of the dynamics following photoexcitation in the first electronic band of NO2-para-substituted nitronaphthalenes. Our main goal was to determine the interplay between the nitro group, electron-donating substituents, and the solvent in defining the relative excited-state energies and their photoinduced pathways. We studied 4-nitro-1-naphthylamine and 1-methoxy-4-nitronaphthalene in solution samples through femtosecond fluorescence up-conversion and transient absorption techniques. In all solvents, both compounds have ultrafast fluorescence decays, showing that, similarly to the parent compound 1-nitronaphthalene, these molecules have highly efficient S1 decay channels. The evolution of the transient absorption signals in the visible region reveals that for the methoxy-substituted compound, independently of solvent polarity, the photophysical pathways are the same as in 1-nitronaphthalene, namely, ultrafast intersystem crossing to an upper triplet state (receiver Tn state) followed by rel...
TL;DR: The photoluminescence properties of different NiO specimens (bulk single crystal and ceramics) have been measured at different temperatures between 10 and 300 K, before and after heat-treatment in a vacuum as discussed by the authors.
Abstract: The photoluminescence properties of different NiO specimens (bulk single crystal and ceramics) have been measured at different temperatures between 10 and 300 K, before and after heat-treatment in a vacuum. Considerable heat-treatment effects on the photoluminescence properties have been observed. Under 3.81 eV photoexcitation, at least two luminescence bands are observed around 3.0 and 2.3 eV. Reversible ultraviolet-laser-light (photon energy=3.81 eV)-induced photoluminescence spectral change has been also observed at room temperature for the NiO specimens, for the first time. The observed change is explained as a photo-induced associative detachment of O2 molecule on the specimen surface, while that observed change in O2 gas is explained as a photo-induced dissociative adsorption of O2 molecule on the specimen surface, accompanied by oxygen defects. The obtained results assort the observed luminescence in intrinsic and defect-related components.
TL;DR: The simulations provide direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron-phonon relaxation dynamics of semiconductor QDs.
Abstract: State-of-the-art time domain density functional theory and non-adiabatic (NA) molecular dynamic simulations are used to study phonon-induced relaxation of photoexcited electrons and holes in Ge and Si quantum dots (QDs) The relaxation competes with productive processes and causes energy and voltage losses in QD solar cells The ab initio calculations show that quantum confinement makes the electron and hole density of states (DOS) more symmetric in Si and Ge QDs compared to bulk Surprisingly, in spite of the symmetric DOS, the electron and hole relaxations are quite asymmetric: the electrons decay faster than the holes The asymmetry arises due to stronger NA coupling in the conduction band (CB) than in the valence band (VB) The stronger NA coupling of the electrons compared to the holes is rationalized by the larger contribution of the high-frequency Ge–H and Si–H surface passivating bonds to the CB relative to the VB Linear relationships between the electron and hole relaxation rates and the CB and VB DOS are found in agreement with Fermi's golden rule The faster relaxation of the electrons compared to the holes in the Ge and Si QDs is unexpected and is in contrast with the corresponding dynamics in the majority of binary QDs, such as CdSe It suggests that Auger processes will transfer energy from holes to electrons rather than in the opposite direction as in CdSe, and that a larger fraction of the photoexcitation energy will be transferred to phonons coupled with electrons rather than holes The difference in the phonon-induced electron and hole decay rates is larger in Ge than Si, indicating that the Auger processes should be particularly important in Ge QDs The simulations provide direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron–phonon relaxation dynamics of semiconductor QDs
TL;DR: In this paper, the upconversion excitation efficiency of Tb3+−Yb3− codoped fluorophosphate glass was investigated in the 0.54 µm band.
Abstract: Tb3+–Yb3+ codoped fluorophosphate glasses were synthesized and properties of the visible emission at 0.54 μm were investigated. The upconversion excitation efficiency from Yb3+ to Tb3+ was studied by evaluation of the cooperative energy transfer efficiency from Yb3+ to Tb3+ (ηCET) and the back-transfer efficiency from Tb3+ to Yb3+(ηBT), which gave a positive and negative contribution to upconversion excitation, respectively. The ηCET was as high as 30% and the ηBT was less than 1% in the fluorophosphate glass. This indicates that Tb3+–Yb3+ codoped fluorophosphate glass is promising as laser and gain medium in the 0.54 μm band.
TL;DR: In this paper, the wavelength dependence of charge photogeneration in materials based on donor-acceptor charge-transfer complexes (CTCs) of the conjugated polymer MEH-PPV was investigated.
TL;DR: It is shown that the excess energy of the higher excitations partially survives both electron transfer steps and is seen by different distributions of unrelaxed ground states, which are generated by the back electron transfer and has unique UV-vis spectroscopic signatures.
Abstract: We have, for the first time for molecular systems in solution, shown a case where the variation of excitation energy influences the product distribution of a two-step electron transfer. The photoexcitation was to the porphyrin-localized S2 state or either of the S1(v = 1) or S1(v = 0) states of an aqueous Zn(II)-meso-tetrasulphonatophenyl-porphyrin−methylviologen (ZnTPPS4−/MV2+) complex. Both forward and back electron transfer occur on a subpicosecond time scale (τFET ≈ 0.2, τBET = 0.7 ps). The excess energy of the higher excitations partially survives both electron transfer steps. This is seen by different distributions of unrelaxed ground states, which are generated by the back electron transfer and has unique UV−vis spectroscopic signatures. State selective electron transfer opens interesting possibilities for reaction control, and the results represent initial steps in that direction
TL;DR: The results lead us to conclude that following Franck-Condon excitation, the time scale required to populate the lowest triplet state should be <10 ps.
Abstract: The excited-state dynamics of heteroleptic iridium(III) complexes have been investigated using femto-nanosecond time-resolved luminescence and transient absorption spectroscopy. Two prototypical examples illustrated here are iridium(III) bis[2-(2,4-difluorophenyl)pyridinato-N,C2′] quinaldinate (1) and iridium(III) bis[2-(2,4-difluorophenyl)pyridinato-N,C2′][2-(6-methylbenzoxazol-2-yl)phenolate] (2). Upon photoexcitation at, for example, 400 nm, the emission decay, monitored at the region of phosphorescence, for both 1 and 2 consists of a system response limited rise (<300 ps) and a single exponential decay. Further single wavelength as well as full spectrum of femto-picosecond transient absorption acquired in CH2Cl2 at room temperature reveals an ultrafast S1 → Tn intersystem crossing (<1 ps) and a rapid Tn → T1 internal conversion/vibrational relaxation within a time period of <10 ps. The results lead us to conclude that following Franck−Condon excitation, the time scale required to populate the lowest t...
TL;DR: In this paper, the authors studied photocarrier recombination processes in highly excited SrTiO3 crystals using pump-probe transient absorption (TA) and photoluminescence (PL) spectroscopy at room temperature.
Abstract: We studied photocarrier recombination processes in highly excited SrTiO3 crystals using pump-probe transient absorption (TA) and photoluminescence (PL) spectroscopy at room temperature. TA signals of nondoped SrTiO3 crystals clearly appear in the visible and infrared spectral region under intense interband photoexcitation, and TA spectra show Drude-like photon-energy dependence. Both TA and PL decay curves are well explained by the same simple rate equation including three-body Auger recombination and single-carrier trapping.
TL;DR: In this article, the authors observed ultrafast release and capture of charge carriers in InGaAs/GaAs quantum dots in a room-temperature optical pump-terahertz probe experiment sensitive to the population dynamics of conducting states.
Abstract: We observe ultrafast release and capture of charge carriers in InGaAs/GaAs quantum dots in a room-temperature optical pump-terahertz probe experiment sensitive to the population dynamics of conducting states. In case of resonant excitation of the quantum dot ground state, the maximum conductivity is achieved at approximately 35 ps after photoexcitation, which is assigned to release of carriers from the quantum dots. When exciting carriers into the conduction band of the barriers, depletion of the conductivity via carrier capture into the quantum dots with a few picosecond pump fluence-dependent time constant was observed.
TL;DR: The infrared measurements clearly indicate that trapping of injected electrons is the main mechanism responsible for the observed long-lived charge separation in TiO(2) mesoporous films.
Abstract: Close to the edge: Photoexcitation of alizarin coupled to the surface of mesoporous TiO2 films leads to ultrafast electron transfer to the TiO2 conduction band (see picture). Complex kinetics after photoexcitation depend on the excitation energy, and indicate a position of the alizarin excited state close to the TiO2 conduction band edge, where the density of acceptor states is reduced.
The photoinduced dynamics in Al2O3 and TiO2 mesoporous films sensitized by the strongly coupled alizarin dye is investigated by femtosecond transient absorption spectroscopy in the spectral range from UV to mid-IR. Alizarin/Al2O3 acts as a nonreactive reference system, in which no electron transfer is observed. For comparison, the photoexcitation of the alizarin dye coupled to the surface of TiO2 films leads to ultrafast electron transfer from the dye to the TiO2 conduction band on the sub-100-fs timescale. We observe a fast relaxation of the alizarin excited state as well as a fast recombination of injected electrons with the alizarin cation on the picosecond timescale, which gives rise to very complex kinetics at short delay times. The infrared measurements clearly indicate that trapping of injected electrons is the main mechanism responsible for the observed long-lived charge separation in TiO2 mesoporous films. The experimental findings can be explained by a position of the dye excited state close to the conduction band edge.
TL;DR: In this paper, a visible-light-driven layered photocatalyst of Bi-based PbBiO2Cl is prepared by solid-state reaction, and the optical band gap is determined to be 2.45 eV by UV-vis diffuse reflectance spectroscopy.
TL;DR: In this paper, the authors presented the first time-dependent density functional theory (TDDFT) calculation on a light-harvesting triad carotenoid-diaryl-porphyrin-C(60) system and provided an interpretation of the photoexcitation mechanism in terms of the properties of the component moieties.
Abstract: We present the first time-dependent density functional theory (TDDFT) calculation on a light-harvesting triad carotenoid-diaryl-porphyrin-C(60) Besides the numerical challenge that the ab initio study of the electronic structure of such a large system presents, we show that TDDFT is able to provide an accurate description of the excited-state properties of the system In particular, we calculate the photoabsorption spectrum of the supramolecular assembly, and we provide an interpretation of the photoexcitation mechanism in terms of the properties of the component moieties The spectrum is in good agreement with experimental data, and provides useful insight on the photoinduced charge-transfer mechanism which characterizes the system