| 1. |
- Biasin, Elisa, et al.
(author)
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Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated[Co(terpy)2]2$
- 2016
-
In: Physical Review Letters. - : American Physical Society (APS). - 1079-7114 .- 0031-9007. ; 117:1
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Journal article (peer-reviewed)abstract
- We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.
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| 2. |
- Canton, S. E., et al.
(author)
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Watching the dynamics of electrons and atoms at work in solar energy conversion
- 2015
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In: Faraday discussions. - : Royal Society of Chemistry. - 1359-6640 .- 1364-5498. ; 185, s. 51-68
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Journal article (peer-reviewed)abstract
- The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray
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| 3. |
- Canton, Sophie, et al.
(author)
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Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses.
- 2015
-
In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6
-
Journal article (peer-reviewed)abstract
- Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.
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| 4. |
- Chábera, Pavel, et al.
(author)
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A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence
- 2017
-
In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 543:7647, s. 695-699
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Journal article (peer-reviewed)abstract
- Transition-metal complexes are used as photosensitizers1, in light-emitting diodes, for biosensing and in photocatalysis2. A key feature in these applications is excitation from the ground state to a charge-transfer state3,4; the long charge-transfer-state lifetimes typical for complexes of ruthenium5 and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron6 and copper7 being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs6,8,9,10, it remains a formidable scientific challenge11 to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered12 photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers13,14,15. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes4,16,17. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.
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| 5. |
- Chábera, Pavel, et al.
(author)
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Band-selective dynamics in charge-transfer excited iron carbene complexes
- 2019
-
In: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 216:2019, s. 191-210
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Journal article (peer-reviewed)abstract
- Ultrafast dynamics of photoinduced charge transfer processes in light-harvesting systems based on Earth-abundant transition metal complexes are of current interest for the development of molecular devices for solar energy conversion applications. A combination of ultrafast spectroscopy and first principles quantum chemical calculations of a recently synthesized iron carbene complex is used to elucidate the ultrafast excited state evolution processes in these systems with particular emphasis on investigating the underlying reasons why these complexes show promise in terms of significantly extended lifetimes of charge transfer excited states. Together, our results challenge the traditional excited state landscape for iron-based light harvesting transition metal complexes through radically different ground and excited state properties in alternative oxidation states. This includes intriguing indications of rich band-selective excited state dynamics on ultrafast timescales that are interpreted in terms of excitation energy dependence for excitations into a manifold of charge-transfer states. Some implications of the observed excited state properties and photoinduced dynamics for the utilization of iron carbene complexes for solar energy conversion applications are finally discussed.
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| 6. |
- Chábera, Pavel, et al.
(author)
-
FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-To-Ligand Charge-Transfer Excited-State Lifetime
- 2018
-
In: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:3, s. 459-463
-
Journal article (peer-reviewed)abstract
- The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3′-dimethyl-1,1′-bis(p-Tolyl)-4,4′-bis(1,2,3-Triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-To-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.
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| 7. |
- Corani, Alice, et al.
(author)
-
Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode.
- 2016
-
In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7, s. 1096-1101
-
Journal article (peer-reviewed)abstract
- There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.
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| 8. |
- Ericson, Fredric, et al.
(author)
-
Electronic structure and excited state properties of iron carbene photosensitizers - A combined X-ray absorption and quantum chemical investigation
- 2017
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 683, s. 559-566
-
Journal article (peer-reviewed)abstract
- The electronic structure and excited state properties of a series of iron carbene photosensitizers are elucidated through a combination of X-ray absorption measurements and density functional theory calculations. The X-ray absorption spectra are discussed with regard to the unusual bonding environment in these carbene complexes, highlighting the difference between ferrous and ferric carbene complexes. The valence electronic structure of the core excited FeIII-3d5 complex is predicted by calculating the properties of a CoIII-3d6 carbene complex using the Z+1 approximation. Insight is gained into the potential of sigma-donating ligands as strategy to tune properties for light harvesting applications.
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| 9. |
- Fredin, Lisa A., et al.
(author)
-
Molecular and Interfacial Calculations of Iron(II) Light Harvesters
- 2016
-
In: ChemSusChem. - : Wiley. - 1864-5631. ; 9:7, s. 667-675
-
Journal article (peer-reviewed)abstract
- Iron-carbene complexes show considerable promise as earth-abundant light-harvesters, and adsorption onto nanostructured TiO2 is a crucial step for developing solar energy applications. Intrinsic electron injection capabilities of such promising FeII N-heterocyclic complexes (Fe-NHC) to TiO2 are calculated here, and found to correlate well with recent experimental findings of highly efficient interfacial injection. First, we examine the special bonding characteristics of Fe-NHC light harvesters. The excited-state surfaces are examined using density functional theory (DFT) and time-dependent DFT (TD-DFT) to explore relaxed excited-state properties. Finally, by relaxing an Fe-NHC adsorbed on a TiO2 nanocluster, we show favorable injection properties in terms of interfacial energy level alignment and electronic coupling suitable for efficient electron injection of excited electrons from the Fe complex into the TiO2 conduction band on ∼100 fs time scales.
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| 10. |
- Fullagar, Wilfred K., et al.
(author)
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Beating Darwin-Bragg losses in lab-based ultrafast X-ray experiments
- 2017
-
In: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 4:4
-
Journal article (peer-reviewed)abstract
- The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.
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| 11. |
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| 12. |
- Hammarström, Leif, et al.
(author)
-
Time-Resolved Laser Spectroscopy in Molecular Devices for Solar Energy Conversion
- 2017
-
In: Molecular Devices for Solar Energy Conversion and Storage. - Singapore : Springer Singapore. - 2196-6990 .- 2196-6982. - 9789811059230 - 9789811059247 ; , s. 385-432
-
Book chapter (other academic/artistic)abstract
- A complete characterization of solar energy conversion devices and the processes underlying their function is a challenge, and require a multitude of different experimental methods. This chapter discusses investigations of molecular solar cells and solar fuels devices by time-resolved laser spectroscopic methods. These methods have established important concepts we now use for understanding the function of devices for solar energy conversion into primary products. We give examples of scientific insight provided by ultrafast methods using detection in the regions from X-ray to THz radiation, and particularly highlight the case where the use of different methods has provided complementary information. Charge collection and solar fuel catalysis on the other hand occur on longer time scales, which opens for the use of time-resolved magnetic resonance and microwave conductivity methods. We also point out that, with suitable precautions, time-resolved laser spectroscopy is able to give information relevant for in operando device conditions
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| 13. |
- Harlang, Tobias, et al.
(author)
-
Iron sensitizer converts light to electrons with 92% yield.
- 2015
-
In: Nature Chemistry. - : Springer Science and Business Media LLC. - 1755-4330 .- 1755-4349. ; 7:11, s. 883-889
-
Journal article (peer-reviewed)abstract
- Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.
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| 14. |
- Hayashi, Hironobu, et al.
(author)
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Effects of Immersion Solvent on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells.
- 2015
-
In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 7:33, s. 18689-18696
-
Journal article (peer-reviewed)abstract
- Memory effects in self-assembled monolayers (SAMs) of zinc porphyrin carboxylic acid on TiO2 electrodes have been demonstrated for the first time by evaluating the photovoltaic and electron transfer properties of porphyrin-sensitized solar cells prepared by using different immersion solvents sequentially. The structure of the SAM of the porphyrin on the TiO2 was maintained even after treating the porphyrin monolayer with different neat immersion solvents (memory effect), whereas it was altered by treatment with solutions containing different porphyrins (inverse memory effect). Infrared spectroscopy shows that the porphyrins in the SAM on the TiO2 could be exchanged with the same or analogous porphyrin, leading to a change in the structure of the porphyrin SAM. The memory and inverse memory effects are well correlated with a change in porphyrin geometry, mainly the tilt angle of the porphyrin along the long molecular axis from the surface normal on the TiO2, as well as with kinetics of electron transfer between the porphyrin and TiO2. Such a new structure-function relationship for DSSCs will be very useful for the rational design and optimization of photoelectrochemical and photovoltaic properties of molecular assemblies on semiconductor surfaces.
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| 15. |
- Infahsaeng, Yingyot, et al.
(author)
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Transient photocurrent of bulk heterojunction solar cell characterized by ns-laser and sub-ms LED
- 2015
-
In: Proceedings of SPIE. - : SPIE. - 0277-786X .- 1996-756X. ; 9659
-
Conference paper (peer-reviewed)abstract
- We measure the transient photocurrent of APFO3:PCBM bulk heterojunction solar cells illuminated with ns-laser and sub-ms LED light sources. The ratio of the number of collective charges to the number of excited photon (external quantum efficiency, EQE) and the transient photocurrent fall times have been carried out with difference pulse durations and fluences. The EQEs characterized by ns-laser source are shown to obey the bimolecular recombination at high excitation fluences. The increasing of transient photocurrent fall times suggests that the fall times of free charge carriers are effected by deep trap density of state (DoS) and thus the free charge carriers have a sufficient time for bimolecular recombination at short circuit condition. At the same fluences, however, the EQEs characterized by sub-ms LED sources exhibit an excitation fluences independence of EQE. The transient photocurrent fall times with sub-ms LED sources are rather constant when the excitation fluences increases indicating that the deep trap DoS has less effect at short circuit condition for longer pulse duration.
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| 16. |
- Inganäs, Olle, et al.
(author)
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Solar energy for electricity and fuels.
- 2016
-
In: Ambio: a Journal of Human Environment. - : Springer Science and Business Media LLC. - 0044-7447 .- 1654-7209. ; 45:Suppl 1, s. 15-23
-
Journal article (peer-reviewed)abstract
- Solar energy conversion into electricity by photovoltaic modules is now a mature technology. We discuss the need for materials and device developments using conventional silicon and other materials, pointing to the need to use scalable materials and to reduce the energy payback time. Storage of solar energy can be achieved using the energy of light to produce a fuel. We discuss how this can be achieved in a direct process mimicking the photosynthetic processes, using synthetic organic, inorganic, or hybrid materials for light collection and catalysis. We also briefly discuss challenges and needs for large-scale implementation of direct solar fuel technologies.
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| 17. |
- Kalem, S., et al.
(author)
-
Si measurements : SiOx on Si
- 2017
-
In: Joint International EUROSOl Workshop and International Conference on Ultimate Integration on Silicon-ULIS, EUROSOI-ULIS 2017 - Proceedings. - 2472-9132. - 9781509053131 ; , s. 235-238
-
Conference paper (peer-reviewed)abstract
- We review the results of silicon measurements, which we have performed on suboxide SiOx formed on n and p type Si wafers with different surface textures. Localized vibrational modes through Raman and FTIR, light emission properties by photoluminescence (PL), energy critical points for optical transitions, excited state dynamics and non-linear electrical properties can be used as effective methods in investigating thin oxide layers on Si. Infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicating that disorder induced LO-TO optical mode coupling can be an effective tool in assessing the structural quality of the SiOx. Excited carrier dynamics and switching mechanisms can provide critical information about electronic quality of sub oxides for applications in CMOS circuits.
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| 18. |
- Kjær, Kasper S., et al.
(author)
-
Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]
- 2017
-
In: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 4:4
-
Journal article (peer-reviewed)abstract
- We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,20-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,20-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metalcentered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6-2N]2N-4, whereN=1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.
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| 19. |
- Kjær, Kasper Skov, et al.
(author)
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Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime
- 2019
-
In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 363:6424, s. 249-253
-
Journal article (peer-reviewed)abstract
- Iron’s abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer (CT) states of most Fe complexes are limited by picosecond or sub-picosecond deactivation through low-lying metal centered (MC) states, resulting in inefficient electron transfer reactivity and complete lack of photoluminescence. Here we show that octahedral coordination of Fe(III) by two mono-anionic facial tris-carbene ligands can suppress such deactivation dramatically. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is [phenyl(tris(3-methylimidazol-1-ylidene))borate]-, exhibits strong, visible, room temperature photoluminescence with a 2.0 ns lifetime and 2% quantum yield via spin-allowed transition from a ligand-to-metal charge-transfer (2 LMCT) state to the ground state (2 GS). Reductive and oxidative electron transfer reactions were observed for the2 LMCT state of [Fe(phtmeimb)2]+ in bimolecular quenching studies with methylviologen and diphenylamine.
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| 20. |
- Leshchev, Denis, et al.
(author)
-
Tracking the picosecond deactivation dynamics of a photoexcited iron carbene complex by time-resolved X-ray scattering
- 2018
-
In: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 9:2, s. 405-414
-
Journal article (peer-reviewed)abstract
- Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)2]2+ (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)2]2+ after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (5MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the 5MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states.
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| 21. |
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| 22. |
- Liu, Yizhu, et al.
(author)
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Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers
- 2016
-
In: Accounts of Chemical Research. - : American Chemical Society (ACS). - 0001-4842 .- 1520-4898. ; 49:8, s. 1477-1485
-
Journal article (peer-reviewed)abstract
- ConspectusThe photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate FeII complexes (FeIIL6) have been widely considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer (3MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the 3MLCT state by increasing the ligand field strength, with special focus on making eg σ∗ orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from RuIIL6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field FeIIL6 complexes with significantly extended 3MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the 3MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the 3MC instead of the 5MC state likely dictates the deactivation of the 3MLCT state, a behavior distinct from traditional FeIIL6 complexes but rather resembling Ru analogues. A heteroleptic FeII NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the 3MLCT lifetime relative to its parental [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) complex. Again, a Ru-like deactivation mechanism of the 3MLCT state was indicated by QC studies. With a COOH-functionalized homoleptic complex, a record 3MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the 3MLCT lifetime indeed benefits efficient light harvesting with FeII NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy 3MLCT state to a TiO2 nanofilm was achieved by a stable FeII complex. This is in complete contrast to conventional FeIIL6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d6 FeII complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element.
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| 23. |
- Merdasa, Aboma, et al.
(author)
-
"Supertrap" at Work: Extremely Efficient Nonradiative Recombination Channels in MAPbI3 Perovskites Revealed by Luminescence Super-Resolution Imaging and Spectroscopy
- 2017
-
In: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 11:6, s. 5391-5404
-
Journal article (peer-reviewed)abstract
- Organo-metal halide perovskites are some of the most promising materials for the new generation of low-cost photovoltaic and light-emitting devices. Their solution processability is a beneficial trait, although it leads to a spatial inhomogeneity of perovskite films with a variation of the trap state density at the nanoscale. Comprehending their properties using traditional spectroscopy therefore becomes difficult, calling for a combination with microscopy in order to see beyond the ensemble-averaged response. We studied photoluminescence (PL) blinking of micrometer-sized individual methylammonium lead iodide (MAPbI3) perovskite polycrystals, as well as monocrystalline microrods up to 10 μm long. We correlated their PL dynamics with structure employing scanning electron and optical super-resolution microscopy. Combining super-resolution localization imaging and super-resolution optical fluctuation imaging (SOFI), we could detect and quantify preferential emitting regions in polycrystals exhibiting different types of blinking. We propose that blinking in MAPbI3 occurs by the activation/passivation of a "supertrap" which presumably is a donor-acceptor pair able to trap both electrons and holes. As such, nonradiative recombination via supertraps, in spite being present at a rather low concentrations (1012-1015 cm-3), is much more efficient than via all other defect states present in the material at higher concentrations (1016-1018 cm-3). We speculate that activation/deactivation of a supertrap occurs by its temporary dissociation into free donor and acceptor impurities. We found that supertraps are most efficient in structurally homogeneous and large MAPbI3 crystals where carrier diffusion is efficient, which may therefore pose limitations on the efficiency of perovskite-based devices.
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| 24. |
- Nogueira, Juan J., et al.
(author)
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Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block
- 2017
-
In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:5, s. 1004-1008
-
Journal article (peer-reviewed)abstract
- Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.
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| 25. |
- Pfeffer, M. G., et al.
(author)
-
Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light
- 2015
-
In: Angewandte Chemie-International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 54:17, s. 5044-5048
-
Journal article (peer-reviewed)abstract
- To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N boolean AND N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.
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| 26. |
- Piatkowski, Piotr, et al.
(author)
-
Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film.
- 2016
-
In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:1, s. 204-210
-
Journal article (peer-reviewed)abstract
- We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 μm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.
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| 27. |
- Ponseca, Carlito, et al.
(author)
-
Excited state and charge-carrier dynamics in perovskite solar cell materials.
- 2016
-
In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:8
-
Research review (peer-reviewed)abstract
- Organo-metal halide perovskites (OMHPs) have attracted enormous interest in recent years as materials for application in optoelectronics and solar energy conversion. These hybrid semiconductors seem to have the potential to challenge traditional silicon technology. In this review we will give an account of the recent development in the understanding of the fundamental light-induced processes in OMHPs from charge-photo generation, migration of charge carries through the materials and finally their recombination. Our and other literature reports on time-resolved conductivity, transient absorption and photoluminescence properties are used to paint a picture of how we currently see the fundamental excited state and charge-carrier dynamics. We will also show that there is still no fully coherent picture of the processes in OMHPs and we will indicate the problems to be solved by future research.
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| 28. |
- Ponseca, Carlito, et al.
(author)
-
Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM, and Spiro-OMeTAD: Role of Dark Carriers.
- 2015
-
In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 137:51, s. 16043-16048
-
Journal article (peer-reviewed)abstract
- Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.
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| 29. |
- Ponseca, Carlito, et al.
(author)
-
Revealing the ultrafast charge carrier dynamics in organo metal halide perovskite solar cell materials using time resolved THz spectroscopy.
- 2016
-
In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 8:12, s. 6249-6257
-
Journal article (peer-reviewed)abstract
- Ultrafast charge carrier dynamics in organo metal halide perovskite has been probed using time resolved terahertz (THz) spectroscopy (TRTS). Current literature on its early time characteristics is unanimous: sub-ps charge carrier generation, highly mobile charges and very slow recombination rationalizing the exceptionally high power conversion efficiency for a solution processed solar cell material. Electron injection from MAPbI3 to nanoparticles (NP) of TiO2 is found to be sub-ps while Al2O3 NPs do not alter charge dynamics. Charge transfer to organic electrodes, Spiro-OMeTAD and PCBM, is sub-ps and few hundreds of ps respectively, which is influenced by the alignment of energy bands. It is surmised that minimizing defects/trap states is key in optimizing charge carrier extraction from these materials.
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| 30. |
- Ponseca, Carlito S., et al.
(author)
-
Ultrafast Electron Dynamics in Solar Energy Conversion
- 2017
-
In: Chemical Reviews. - : American Chemical Society (ACS). - 0009-2665 .- 1520-6890. ; 117:16, s. 10940-11024
-
Journal article (peer-reviewed)abstract
- Electrons are the workhorses of solar energy conversion. Conversion of the energy of light to electricity in photovoltaics, or to energy-rich molecules (solar fuel) through photocatalytic processes, invariably starts with photoinduced generation of energy-rich electrons. The harvesting of these electrons in practical devices rests on a series of electron transfer processes whose dynamics and efficiencies determine the function of materials and devices. To capture the energy of a photogenerated electron-hole pair in a solar cell material, charges of opposite sign have to be separated against electrostatic attractions, prevented from recombining and being transported through the active material to electrodes where they can be extracted. In photocatalytic solar fuel production, these electron processes are coupled to chemical reactions leading to storage of the energy of light in chemical bonds. With the focus on the ultrafast time scale, we here discuss the light-induced electron processes underlying the function of several molecular and hybrid materials currently under development for solar energy applications in dye or quantum dot-sensitized solar cells, polymer-fullerene polymer solar cells, organometal halide perovskite solar cells, and finally some photocatalytic systems.
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| 31. |
- Ponseca, Carlito S., et al.
(author)
-
Understanding charge carrier dynamics in solar cell materials using time resolved terahertz spectroscopy
- 2015
-
In: IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves. - 9781479982721
-
Conference paper (peer-reviewed)abstract
- The need for developing highly efficient solar cell devices have never been so pressing until recently when the urgency of using renewable energy sources becomes more evident. There are several promising technologies being explored by many groups with the sole purpose of optimizing harvesting sunlight and converting it to useful electricity. These include, but not limited to, dye- and quantum dot-sensitized, bulk heterojunction organic, inorganic nanowires, and very recently perovskite-based solar cells. In this talk, charge carrier dynamics of an assortment of solar cell technologies probed using time-resolved terahertz spectroscopy will be presented. Electron injection, mobility, charge carrier lifetime and recombination dynamics will be discussed.
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| 32. |
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| 33. |
- Smolentsev, G., et al.
(author)
-
Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis
- 2015
-
In: Coordination Chemistry Reviews. - : Elsevier BV. - 0010-8545. ; 304, s. 117-132
-
Research review (peer-reviewed)abstract
- Transition metal coordination compounds have a rich photochemistry and are interesting candidates as both light harvesters (photosensitizers) and catalysts in photocatalytic systems. Knowledge of electronic and molecular structure of excited states of photosensitizers and intermediates of catalysts is a key topic for rational design of systems for artificial photosynthesis. We describe recent advances in the field of time-resolved X-ray absorption spectroscopy that provide information on local structure around metal centers, their orbital structure and oxidation state, and thereby insights into the mechanisms of their photochemical reactions. Photosensitizers with metal centers, multicomponent molecular catalytic systems, and supramolecular model sensitizer-catalysts with two metal centers are used as examples to demonstrate the possibilities of the technique. We overview different experimental methods that can be used to investigate intermediates with lifetimes in the range from hundreds of picoseconds to hundreds of microseconds. Theoretical methods to extract the structural and electronic information from X-ray absorption near edge structure spectroscopy (XANES) are also discussed. (C) 2015 Elsevier B.V. All rights reserved.
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| 34. |
- Sundström, Villy
(author)
-
Electron dynamics in solar energy converting materials
- 2016
-
In: 41st International Conference on Infrared, Millimeter and Terahertz Waves, IRMMW-THz 2016. - 9781467384858
-
Conference paper (peer-reviewed)abstract
- Solar energy is the most abundant renewable energy source available. Conversion of light into electricity and chemical energy are the two major paths for solar energy conversion. Nanostructured organic and hybrid materials are being explored for applications in photovoltaic solar energy conversion, as well as photocatalysis for solar fuel generation. Light harvesting, energy transport, charge photogeneration and recombination, charge transport are the elementary processes accounting for the conversion of light energy into useful charge carriers. We show how a combination of time resolved spectroscopy, with a focus on TR-THz spectroscopy, covering the time scales from femtoseconds to milliseconds is a powerful tool to study the light induced processes and provide mechanistic information valuable for design of novel or optimized materials.
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| 35. |
- Sundström, Villy
(author)
-
My memories of ahmed zewail - from a snowy northern Sweden to the nobel prize
- 2017
-
In: Personal And Scientific Reminiscences : Tributes To Ahmed Zewail - Tributes To Ahmed Zewail. - : WORLD SCIENTIFIC (EUROPE). - 9781786344366 ; , s. 229-236
-
Book chapter (peer-reviewed)abstract
- In the summer of 1993, two Russian scientists from the Institute of Physics in Nizhny Novgorod drove the 1, 934 km to Umeå, where I was based at that time, in their red Lada car loaded with a home-built Ti:Sa laser. Within a few days, the laser was put in operation, pumped by a small argon-ion laser, and produced stable -100 fs pulses at 790 nm. These pulses were applied to our favorite topic at that time, energy transfer within light-harvesting complexes of photosynthetic purple bacteria. Energy transfer times between the pigment molecules of two different LH2 complexes were measured and the work was published in Chem. Phys. Lett. (CPM). In March 1994, Ahmed was touring Sweden, and having seen our recently published LH2 work in CPL, he took the detour to northern Umeå. On March 24, I picked up Ahmed outside the “Stora Hotellet” in Umeå, dressed in a thin summer jacket and shoes suitable for California climate, but not for a cold, snowy, and gloomy day in March in northern Sweden. This was how I met Ahmed for the first time. We quickly moved to the chemistry department, where we had a full day of scientific discussions. We had continued the work on energy transfer in photosynthetic light-harvesting complexes and had a bunch of exciting new results that we thought indicated vibrational coherence and interexciton state relaxation processes. Ahmed showed his enthusiasm and support to the results, which we submitted to CPM a month later. Through this work, and much work that followed as well as work by other groups, a deep understanding of the primary processes of photosynthesis was reached (see, e.g, Ref. 4). Nevertheless, using new sophisticated ultrafast methods such as 2D electronic spectroscopy, increasingly more mechanistic detail is revealed still today (see, e.g., Ref. 5). At the dinner we had later in the evening, Ahmed encountered a northern specialty, which I think became somewhat of a favorite dessert - warm cloudberries with vanilla ice cream….
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| 36. |
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| 37. |
- Tian, Yuxi, et al.
(author)
-
Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites
- 2015
-
In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 6:20, s. 4171-4177
-
Journal article (peer-reviewed)abstract
- Photoluminescence (PL) of organo-metal halide perovskite semiconductors can be enhanced by several orders of magnitude by exposure to visible light. We applied PL microscopy and super-resolution optical imaging to investigate this phenomenon with spatial resolution better than 10 nm using films of CH3NH3PbI3 prepared by the equimolar solution-deposition method, resulting in crystals of different sizes. We found that PL of similar to 100 nm crystals enhances much faster than that of larger, micrometer-sized ones. This crystal-size dependence of the photochemical light passivation of charge traps responsible for PL quenching allowed us to conclude that traps are present in the entire crystal volume rather than at the surface only. Because of this effect, "dark" micrometer-sized perovskite crystals can be converted into highly luminescent smaller ones just by mechanical grinding. Super-resolution optical imaging shows spatial inhomogeneity of the PL intensity within perovskite crystals and the existence of <100 nm-sized localized emitting sites. The possible origin of these sites is discussed.
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| 38. |
- Tian, Yuxi, et al.
(author)
-
Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence.
- 2015
-
In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 15:3, s. 1603-1608
-
Journal article (peer-reviewed)abstract
- Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.
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| 39. |
- Tian, Yuxi, et al.
(author)
-
Mechanistic insights into perovskite photoluminescence enhancement: light curing with oxygen can boost yield thousandfold
- 2015
-
In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 17:38, s. 24978-24987
-
Journal article (peer-reviewed)abstract
- A light-induced photoluminescence (PL) enhancement in surface-deposited methylammonium lead iodide (CH3NH3PbI3) perovskites was investigated in detail using time-resolved luminescence microscopy. We found the PL intensity to increase up to three orders of magnitude upon light illumination with an excitation power density of 0.01-1 W cm(-2). The PL enhancement is accompanied by an increase of the PL lifetime from several nanoseconds to several hundred nanoseconds and also by an increase of the initial amplitude of the PL decay. The latter suggests excited state quenching at the subpicosecond timescale. We propose a model where the trapping sites responsible for non-radiative charge recombination can be de-activated by a photochemical reaction involving oxygen. The reaction zone is spatially limited by the excitation light-penetration depth and diffusion length of the charge carriers. The latter increases in the course of the light-curing process making the reaction zone spreading from the surface towards the interior of the crystal. The PL enhancement can be reversed by switching on/off the excitation light or switching the atmosphere between oxygen and nitrogen. Slow diffusion of the reactants and products and equilibrium between the active and "cured" trapping sites are proposed to be the reasons for peculiar responses of PL to such varied experimental conditions.
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| 40. |
- Uhlig, Jens, et al.
(author)
-
High-resolution X-ray emission spectroscopy with transition-edge sensors: present performance and future potential.
- 2015
-
In: Journal of Synchrotron Radiation. - 1600-5775. ; 22:Pt 3, s. 766-775
-
Journal article (peer-reviewed)abstract
- X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies.
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| 41. |
- Ullom, J N
(creator_code:cre_t)
-
X-ray Spectrometer
- 2019
-
Patent (other academic/artistic)abstract
- An x-ray spectrometer includes: an x-ray plasma source that produces first x-rays; an x-ray optic in optical communication with the x-ray plasma source and that: receives the first x-rays from the x-ray plasma source; focuses the first x-rays to produce second x-rays; and communicates the second x-rays to a sample that produces product x-rays in response to receipt of the second x-rays and second light; and a microcalorimeter array detector in optical communication with the sample and that receives the product x-rays from the sample.
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| 42. |
- Valverde-Chavez, David A., et al.
(author)
-
Intrinsic femtosecond charge generation dynamics in single crystal CH3NH3PbI3
- 2015
-
In: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5706 .- 1754-5692. ; 8:12, s. 3700-3707
-
Journal article (peer-reviewed)abstract
- Hybrid metal-organic perovskite solar cells have astounded the solar cell community with their rapid rise in efficiency while maintaining low-cost fabrication. The intrinsic material photophysics related to the generation of free charges, their dynamics and efficiency, however, remains to be understood. As fabrication techniques improve, larger crystal grain sizes have been shown to be a critical factor for improving both the optical and transport properties of the hybrid metal halide perovskites. In this work, we use pulses of multi-THz frequency light in the ultra-broadband 1-30 THz (4-125 meV) range to observe the ac conductivity in large single crystal CH3NH3PbI3. Our spectra reveal the ultrafast dynamics and efficiencies of free charge creation and extremely high charge carrier mobility as high as 500800 cm(2) V-1 s(-1). While quasi-equilibrium analysis of efficiencies through the Saha equation suggests a binding energy on the order of 49 meV, an observed reflectance feature appearing at high pump fluence occurs at 12 meV and is consistent with an orbital transition of the exciton, indicating a much lower Rydberg energy of 17 meV at room temperature. The signature of the exciton is found to vanish on a 1 ps time scale commensurate with the appearance of mobile carriers, consistent with thermal dissociation of the exciton to the continuum in the room temperature tetragonal phase.
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| 43. |
- Van Driel, Tim B., et al.
(author)
-
Atomistic characterization of the active-site solvation dynamics of a model photocatalyst
- 2016
-
In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
-
Journal article (peer-reviewed)abstract
- The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir 2 (dimen) 4 ] 2+, where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute-solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis.
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| 44. |
- Vanko, Gyoergy, et al.
(author)
-
Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)(2)](2+)
- 2015
-
In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:11, s. 5888-5902
-
Journal article (peer-reviewed)abstract
- Theoretical predictions show that depending on the populations of the Fe 3d(xy), 3d(xz), and 3d(yz) orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)(2)](2+). The differences in the structure and molecular properties of these B-5(2) and E-5 quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)(2)](2+) 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)(2)](2+) molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe-ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)-high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.
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| 45. |
- Yamamoto, Masanori, et al.
(author)
-
Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst
- 2016
-
In: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 52:94, s. 13702-13705
-
Journal article (peer-reviewed)abstract
- A new subporphyrin was synthesized for use as a molecular sensitizer in electrochemical and dye-sensitized photoelectrochemical water oxidation. A photoelectrochemical cell with a TiO2 electrode modified with the sensitizer and a molecular water oxidation catalyst generated higher photocurrent than reference cells that have electrodes modified with either the photosensitizer or the catalyst under visible light (lambda > 500 nm) illumination. Oxygen evolution was confirmed after photolysis by GC and GC-MS analyses using isotope-labeling experiments. The large molar extinction coefficients of the ring-contracted porphyrin in the visible region enabled kinetic analysis by time-resolved transient absorption spectroscopy, which also supported the photocatalytic activity.
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| 46. |
- Zhang, Wenkai, et al.
(author)
-
Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution
- 2016
-
In: Chemical Science. - 2041-6520. ; 8:1, s. 515-523
-
Journal article (peer-reviewed)abstract
- Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover-the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-;) ligands and one 2,2′-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2′-bipyridine)3]2+ by more than two orders of magnitude.
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