1,721,320 research outputs found
Chiral control of spin-crossover dynamics in Fe(II) complexes
Full text linkIron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in solution, associated for stereocontrol with the enantiopure Delta- or Lambda-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-(KO1)-O-2,O-2)phosphorus(V) (P(O2C6Cl4)(3)(-) or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics
Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles
No full textWe present temperature-dependent (from room temperature to 80 degrees C) absorption spectra of Au/SiO2 core-shell nanoparticles (NPs) (core diameter: similar to 25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (epsilon(1)) and imaginary (epsilon(2)) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.LS
Non-equilibrium Inter-site Dynamics of Photoexcited Kitaev α-Li<inf>2</inf>IrO<inf>3</inf>
No full textThe inter-site transition dynamics in a 2D honeycomb lattice of the Mott-insulator Li2irO3 was investigated via broadband time-resolved spectroscopy. Through two excitation regime corresponding to charger transfer (3.1eV) and across Mott-gap excitation (0.8eV), distinct inter-site transition dynamics were revealed and attributed to the distinct photoexcited quasiparticles, i.e., doublon and holon, in the 2D honeycomb lattice.LUMESPH-S
Spectroscopy and kinetic studies of electron-hole recombination in CdSe nanoparticles : effect of size, shape, and lattice structure
Full text linkThis thesis presents an experimental study of the energy and time-resolved optical response of chemically prepared CdSe nanoparticles with different sizes, shapes (dots, rods, and tetrapods), and lattice structures (wurtzite and zinc blende). The first part of the thesis concerns a model system: spherical CdSe quantum dots with a wurtzite lattice structure. We have investigated their fluorescence spectra and decay kinetics as a function of size, laser excitation power, detection energy, and temperature. The experiments were performed using a nanosecond time-correlated single photon counting technique and have revealed kinetics from different short and long-lived states. In contrast to comparable literature studies, we covered a wide temporal window (up to 1 µs). The study shows at least four order of magnitude of signal to background ratio. Generally, at low excitation power, we always find four independent decay components covering a range of lifetimes between one nanosecond up to hundreds of nanoseconds. The first component is < 2 ns and probably corresponds to fast relaxation and trapping processes. A second component of 3-8 ns can be attributed to the lifetime of charged excitons, while the third one is in the range of 20-30 ns is due to the radiative electron-hole recombination. Finally, a longer decay time of 80-100 ns with low amplitude (< 10 %) appears for all sizes and experimental condition. This component is related to trap states. At high power (corresponding to more than one exciton per particle), an additional fast component, in the same range as the first one (< 2 ns), appears and is due to the multiexciton effect. The second part of the thesis concerns CdSe semiconductor nanocrystals from a slightly different point of view focusing on the crystal structure, by investigating quantum dots with a cubic lattice structure, synthesized in our group. We performed low- and high-resolution luminescence and excitation spectroscopy, and time-resolved spectroscopy as a function of size and temperature from room temperature down to 4K. These measurements reveal the optical properties and relaxation processes, and finally infer the amount of the structure-dependent field effect on the band edge exciton structure. Overall, we observe no difference in all these properties compared to wurtzite dots. We conclude that the crystal field is much less important than shape asymmetries and exchange interaction. The appearance of a permanent dipole moment, in a cubic lattice structure of spherical quantum dots, can explain the similar response of the two different lattice structure dots in our experiments. The third part of this work concerns the investigation of the shape effect on the spectroscopic and kinetic properties of CdSe nanocrystals. New fabrication methods have enabled the synthesis of high-quality CdSe nanorods and tetrapods. We investigated the temperature dependence of the absorption and fluorescence spectra of the different shapes of CdSe nanocrystals in the range of 4 to 300 K. We find that, while the shift of the fluorescence maximum indicates a little dependence on the shape, the broadening of the emission spectrum behaves very differently for dots and rods, indicating major differences in the broadening mechanisms for different shapes. Tetrapods behave more similar to dots, which suggest that the lowest exciton state is centered at the core. This is confirmed by the decay kinetics, which is again identical between dot and tetrapod nanocrystals, while an opposite temperature dependence decay was recorded for the kinetics of nanorods. We attribute this behaviour in tetrapods to the dot like centre. Nanorods show very different kinetics, because the lowest exciton state becomes allowed.LS
The fate of charge carriers in solar materials investigated by time resolved X-ray and optical spectroscopy
Full text linkThis thesis investigates the photoinduced charge carrier dynamics of TiO2 nanoparticles and CsPbX3 (X = Cl, Br) nanocrystals, by means of ultrafast x-ray absorption spectroscopy (XAS) and transient absorption spectroscopy (TAS). TiO2 is among the most promising transition metal oxides for applications such as photocatalysis and photovoltaics. In the latter, TiO2 has been used over the years in dye-sensitized solar cells (DSSCs) as electron transport material. CsPbX3 belongs to the class of lead halide perovskites and is currently one of the most investigated materials for solar energy applications, due to the high conversion efficiencies and ease of preparation. These materials are commonly used in solar cells with an all-solid-state DSSC architecture as light absorbers. Solar energy conversion is governed by the generation of charge carriers, their subsequent evolution as excitons or free charge carriers, and eventually their localization. Ultrafast spectroscopy can gain insights into the evolution of charge carriers by following their dynamics in real time. For this reason, ultrafast XAS was the main technique used in this work, as it combines elemental and structural sensitivity to study the fate of charge carriers and their evolution under operating conditions. The early stages of electron localization in TiO2 anatase nanoparticles upon photoexcitation at 3.5 eV, are investigated by fs-XAS at the Ti K-edge using the synchrotron slicing technique. The results show that localization of electrons at Ti atoms occurs in < 300â fs, forming Ti3+ centres, in or near the unit cell where the electron is excited. Moreover, electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. Similar conclusions are drawn for another polymorph of TiO2, rutile, from ps-XAS at the Ti K-edge. Here electrons are trapped next to oxygen vacancies at 100 ps after photoexcitation. Electrons in rutile though, show a weaker tendency to localization than in anatase and this could explain the differences in photocatalytic performances between these two polymorphs. In the second part of this thesis, the ultrafast charge carrier dynamics of CsPb(ClBr)3 nanocrystals is investigated using fs-TAS in the visible region (~1.8-3.1 eV) upon photoexcitation at 3.1 eV. This material represents an ideal system to study the ultrafast physics of lead halide perovskite in general, because ultrafast TA studies suggest that the charge carrier dynamics are similar to the organic-inorganic materials but do not suffer from the same stability issues. Here, the ultrafast transition from free charge carriers to excitons is observed in a fluence dependent study, which sheds light on the interpretation of a long lived spectral feature rising from a transient electroabsorption effect. Finally, the charge carrier dynamics of CsPbX3 (X = Cl, Br) is investigated using ps-XAS at the Br K-edge, the Pb L3-edge and Cs L2-edge upon photoexcitation at 3.5 eV. The Br K-edge transients at 100 ps delay show evidence for a full electron charge being withdrawn from the Br atoms, i.e. the hole is localized due to formation of a small polaron. The transients at the Pb L3-edge point to the opposite, i.e. the electrons are fully delocalized as conduction band electrons and there is no hint of trapping. Lastly, the Cs L2-edge shows no transient signal, in agreement with predictions based on the partial density of states in the material.LS
Non-equilibrium dynamics of di-platinum complexes and molecular dyes in solution: Insights from transient absorption and two-dimensional Fourier transform spectroscopy
No full textUnraveling the interplay between electronic- and vibrational degrees of freedom on the earliest time scales of physical, chemical and biological processes is crucial to gaining insight into the mechanisms that govern the world around us, since it is during these primary steps that the fate of the excitation energy - be it solar, chemical or physical - is decided. A tiny structural change during the first 10s of femtoseconds can for example predetermine the dynamics of a system over microseconds, and newly developed techniques in time-domain spectroscopy have proved to successfully capture exactly these pivotal mechanisms.
In this thesis I use ultrafast spectroscopy to investigate the non-equilibrium dynamics of a prototypical di-platinum complex. Transient absorption spectroscopy is complemented with transient absorption anisotropy and fluorescence up-conversion to identify the spectral features of the intermediate excited states. The results show that the intersystem crossing from the singlet- to the triplet manifold of states happens on a 0.6 to 0.9 picosecond time scale, slower than previously assumed. As a consequence, the observable coherence is associated with a singlet excited state, rather than a triplet, and dephases prior to the intersystem crossing. The study further illustrates the importance of having a good understanding of the transient spectral signatures and yields insight into the structural dynamics of di-platinum complexes.
In addition, I have built and commissioned a two-dimensional photon echo experiment that is capable of acquiring broadband excitation- and detection frequency resolved transient spectra in the visible with a temporal resolution of approx. 10 femtoseconds. All relevant steps of the development are outlined in this thesis and 2D spectra of four different dye solutions highlight the capabilities of the experiment. The temporal evolution of the 2D spectra of nile blue is analyzed as an example and shows the information content that is contained within the data.LS
Ultrafast X-ray Spectroscopy of Heme Proteins
No full textWith the advent of X-ray free-electron lasers (XFELs) time-resolved X-ray spectroscopic techniques have advanced to the femtosecond regime. These are element selective techniques which offer unique insight into the electronic and chemical environment and dynamics of a sample. Specifically, X-ray emission spectroscopy probes the occupied density of states and is sensitive to the spin and local structure of the element of interest, whereas X-ray absorption spectroscopy is a tool for probing the unoccupied density of electronic states which makes it sensitive to the oxidation state, ligation and local structure around a specific atom. The tunable photon energy of the X-ray from XFELs allows to selectively probe the element of interest in the sample and additionally, the high intensity (1011 to 1012 photons per pulse) makes it possible to study dilute biological samples in physiological conditions. The sample studied in this work is myoglobin with nitric oxide as ligand, which has long been used as a model system to gain deeper understanding of the class of heme proteins. These proteins all have an iron porphyrine (heme) as an active center and play a crucial role in oxygen storage and transport in all mammals for example, amongst many other functions.
In heme proteins, the change of the low-spin (LS) hexacoordinated heme (ground state) to the high spin (HS) pentacoordinated domed form (excited state) is promoted by a reversible light induced ligand detachment, representing the â transition stateâ that ultimately drives the respiratory function. Here we investigate Myoglobin-NO (MbNO) by employing femtosecond Fe Kα and Kβ non-resonant X-ray emission spectroscopy (XES) at an XFEL upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) heme occurs in ~800 fs, and it proceeds via an intermediate (S = 1) spin state. The XES results also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ~30 ps. Thus, the en-tire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process. Femtosecond X-ray absorption near edge spectroscopy (XANES) experiments also performed at an XFEL show that NO dissociates in <75 fs and the intermediate (S = 1) spin state which has antibonding character is populated in ~110 fs. The XANES spectrum at short time delays (t=1 ps) shows a similarity to the steady state difference spectrum (deoxyMb minus MbNO) suggesting that at 1 ps the present species is very similar to deoxyMb in terms of electronic and local geometric structure. XAS time-traces at the pre- and rising-edge (7112, 7122.5 and 7127 eV) reveal the shortest pathway of geminate recombination which takes ~30 ps.LS
Picosecond X-ray Absorption Studies of Electron Dynamics in Titania Nanoparticles and Ruthenium Dye Sensitizer Complexes
No full textLS
Time-resolved X-ray and Optical Studies of Metalloproteins and Iridium Complexes
Full text linkA wide variety of physicochemical processes at a molecular level, in particular charge or energy transfer, electronic and vibrational relaxation, are at the origin of biological functionality of proteins and organometallic compounds. The work reported in this thesis is devoted to the study of these molecular dynamics in selected organometallic systems. In particular, it focuses on the role of these dynamics in the ultrafast photophysics and photochemistry of Myoglobin and iridium complexes having a transition metal active center of d6 configuration. The nature of ligand recombination to the active centre has a strong impact on the reactivity and activation of heme proteins. Using picosecond (ps) Fe K-edge X-ray absorption spectroscopy (XAS) we probed the NO-heme recombination with direct sensitivity to the Fe-NO binding in physiological solution. The transient XAS at 70 ps and 300 ps are identical, but they deviate from the difference between the static spectra of deoxyMb and nitrosyl myoglobin demonstrating the formation of an intermediate species, supposedly the 6-coordinated domed form that is populated on a time scale of ~200 ps and relaxes in ~30 ps. A broadband femtosecond (fs) luminescence study of Ir(ppy)3 (Ir1), Ir(ppy)2(pic) (Ir2), [Ir(ppy)2(bpy)]+ (Ir3) and Ir(ppz)3 (Ir4) provides the first extensive picture of their ligand dependence. After excitation to a ligand-centered state (266 nm), we directly clock the relaxation cascade of Ir1 leading to the lowest metal-to-ligand charge transfer (3MLCT) state. The cascade proceeds within â € 10 fs, being faster than some of the high-frequency modes of the system. In Ir4 the 3MLCT state decays (530 fs) by non-radiative channels. For Ir1-3, the early emission (100s ps the emission evolves to the steady-state showing evidence of dual emission for Ir1,2 due to a double-well minimum of the lowest 3MLCT state. For Ir3, the final emission stems from a ppy to bpy inter-ligand CT state. The studies of the electronic and geometric structure of Ir1-4 by Ir L3 XAS in solution reveals overall identical above-edge multiple scattering resonances and EXAFS indicating similar bond lengths and angles. Simulation of the XAS using TDDFT and FEFF9 shows good overall agreement with the experiment. The near-edge region of the spectra shows distinct differences among the studied complexes. Ir4 has stronger WL intensity than Ir1, indicating weaker Ï -back donation. The XAS spectra of Ir2,3 indicate larger 5d-electron density on the iridium. The XAS spectra of Ir1 and Ir4 show weak pre-edge resonance due to metal (2p3/2)-to-ligand charge transfer excitations. The absence of the pre-edge in Ir2,3 is due to the reduced covalency of the bonds. However, the existence of such transitions remains open for debate since Ir+3 has only occupied t2g orbitals in the low spin configuration. Using ps XAS, we have successfully captured the transient spectrum at the Ir L3-edge of both Ir1 and Ir4 probed 150 ps after laser excitation in solution. Upon excitation of a MLCT band (355 nm), we observed oxidation state change of Ir from +3 to +4. In DCM, the transient spectra are almost identical for both Ir1,4. The only significant changes in the transient spectra are those observed around the WL. The calculations reproduce the blue shift of the absorption spectrum due to the reduced electron density on the Ir, consistent with the MLCT excitation.LS
Photo- and Thermally- induced Effects on the Absorption Spectroscopy of Au/SiO2 and Au/TiO2 Core-shell Nanoparticles
No full textElectrons generated by Landau damping of the plasmon excitation on gold nanoparticles that can be injected into an adjacent semiconductor e.g. anatase TiO2, enhancing the light harvesting capabilities of solar energy conversion devices. The understanding of the electron injection processes is crucial for the optimization of the devices. The first step is to have an understanding of the bare gold nanoparticle itself, we thus report on ultrafast transient absorption studies of surface plasmon-excited ~25 nm diameter gold nanoparticles in solution, monitoring the surface plasmon resonance response in the visible (1.7-3.0 eV) as well as the inter- and intra-band transitions in the near-to-deep-UV (3.4-4.5 eV). However, in order to distinguish the thermal effects in absorption spectrum on gold NPs, we present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core-shell nanoparticles (core diameter: ~25 nm) in the range from 1.5 eV to 4.5 eV, which encompasses the localized surface plasmon resonance and the interband transitions. The changes of absorption spectra are well reproduced by theoretical calculations based on the experimentally measured temperature-dependence of the real (e_1) and imaginary (e_2) part of the gold nanoparticle dielectric constant. We also compare the results to ultrafast photo-induced transient absorption spectra upon excitation of the plasmon band, and conclude that while the plasmon band can be used to monitor the heating of the nanoparticles, the interband region is little affected by thermal effects and is dominated by electronic ones. Our deep-UV probed transient results on gold nanoparticles show that after the initial ultrafast electron-electron and electron-phonon scattering processes, low-energy hot electrons above the Fermi level remain in equilibrium with the hot lattice for tens to hundreds of ps. Then we investigate the photo-induced charge carrier transfer and relaxation in Au/TiO2, our results show that the bleach at the exction resonance in Au/TiO2 has a significant slow decay speed upon plasmon excitation, and its transient absorption intensity at long term decay gives a quadratic dependence on excitation fluence, implying the appearance of two-photon absorption in anatase TiO2 that enhanced by the surface plasmon resonance of gold nanoparticles. Besides, in the Au/SiO2 and Au/TiO2 NPs, we both observe acoustic vibrations in the UV region for the first time but with significantly different oscillation frequencies which were attributed to the different elastic properties of the shell.LS
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