Elektronische Eigenschaften übergangsmetalldotierter Siliziumcluster

In dieser Arbeit wird die Wechselwirkung zwischen einem Übergangsmetallatom und einem Siliziumcluster untersucht. Die Untersuchungsmethoden beinhalten VUV Spektroskopie und elementspezifische Röntgenspektroskopie. Durch die Analyse der partiellen Ionenausbeute können die resonante Anregung und die direkte Photoionisation eines Clusters getrennt betrachtet werden. Erstmalig werden experimentelle Indizien für die vorhergesagte hohe Symmetrie von VSi16+ [1, 2] gezeigt. Untersuchungen an diesem Siliziumkäfig mit Titan und Chrom als Dotieratom zeigen, dass die Dotierung einen größeren Einfluss auf die Struktur des Käfigs hat als auf die lokale elektronische Struktur des Übergangsmetallatoms. Des Weiteren konnte die größenabhängige Änderung der Lokalisierung von Valenzelektronen näher bestimmt werden. Der in einem erstmalig eingesetzten Verfahren bestimmte Abstand von dem höchsten besetzten und dem niedrigsten unbesetzten Molekülorbital (HOMO-LUMO gap) in Abhängigkeit der Clustergröße zeigt eine gute Übereinstimmung mit theoretischen Berechnungen desselben. Eine besondere Stellung hat hierbei das hochsymmetrische VSi16+ mit einem gemessenen HOMO-LUMO gap von (1.9 ± 0.2) eV. [1] M. B. Torres, E. M. Fernandez, and L. C. Balbas. Physical Review B, 75(20):205425, 2007. [2] V. Kumar and Y. Kawazoe. Physical Rewiew Letters, 87(4):045503, 2001. [3] K. Jackson, E. Kaxiras, and M. R. Pederson. The Journal of Physical Chemistry, 98(32):7805, 1994.The interaction of a transition metal atom with a silicon cluster is investigated to understand the stabilizing effect responsible for silicon cage formation in transition metal doped silicon clusters. The analysis was done using VUV spectroscopy and element specific x-ray spectroscopy. Partial ion yield analysis allows observing resonant excitation and direct photoionization channels separately. First experimental indications for the predicted high symmetry [1,2] of VSi16+ are found. In addition, studies on different transition metal dopant atoms in this specific silicon cage show that deviation from electronic shell closure seems to affect the silicon cage more strongly than the local electronic structure at the transition metal dopant. Furthermore, the size of transition metal doped silicon clusters shows a strong influence on the localization of electrons and position of valence levels. The latter is studied using a novel analysis method based on the combination of VUV and x-ray spectroscopy. Determination of the HOMO-LUMO gap for a wide range of cluster sizes show good agreement with theoretical predictions. An enhanced HOMO-LUMO gap of (1.9±0.2) eV is observed in case of the highly symmetric VSi16+, which can be understood in terms of a spherical potential model [1, 3]. [1] M. B. Torres, E. M. Fernandez, and L. C. Balbas. Physical Review B, 75(20):205425, 2007. [2] V. Kumar and Y. Kawazoe. Physical Rewiew Letters, 87(4):045503, 2001. [3] K. Jackson, E. Kaxiras, and M. R. Pederson. The Journal of Physical Chemistry, 98(32):7805, 1994

Mapping of the Photoinduced Electron Traps in TiO2 by Picosecond X-ray Absorption Spectroscopy

Titanium dioxide (TiO2) is the most popular material for applications in solar-energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X-ray absorption spectroscopy at the Ti K-edge and the Ru L-3-edge to address this issue for photoexcited bare and N719-dye-sensitized anatase and amorphous TiO2 nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect-rich surface shell in the case of anatase TiO2, whereas they are inside the bulk in the case of amorphous TiO2. In the case of dye-sensitized anatase or amorphous TiO2, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds

Time-resolved Element-selective Probing of Charge Carriers in Solar Materials

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL)

Time-resolved Element-selective Probing of Charge Carriers in Solar Materials

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL)

NO binding kinetics in myoglobin investigated by picosecond Fe K-edge absorption spectroscopy

Diatomic ligands in hemoproteins and the way they bind to the active center are central to the protein's function. Using picosecond Fe K-edge X-ray absorption spectroscopy, we probe the NO-heme recombination kinetics with direct sensitivity to the Fe-NO binding after 532-nm photoexcitation of nitrosylmyoglobin ( MbNO) in physiological solutions. The transients at 70 and 300 ps are identical, but they deviate from the difference between the static spectra of deoxymyoglobin and MbNO, showing the formation of an intermediate species. We propose the latter to be a six-coordinated domed species that is populated on a timescale of similar to 200 ps by recombination with NO ligands. This work shows the feasibility of ultrafast pump-probe X-ray spectroscopic studies of proteins in physiological media, delivering insight into the electronic and geometric structure of the active center

Solvent Induced Luminescence Quenching: Static and Time-Resolved X-Ray Absorption Spectroscopy of a Copper(I) Phenanthroline Complex

We present a static and picosecond X-ray absorption study at the Cu K-edge of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) ([Cu(dmp)(2)](+); dmp = 2,9-dimethyl-1,10-phenanthroline) dissolved in acetonitrile and dichloromethane. The steady-state photoluminescence spectra in dichloromethane and acetonitrile are also presented and show a shift to longer wavelengths for the latter, which points to a stronger stabilization of the excited complex. The fine structure features of the static and transient X-ray spectra allow an unambiguous assignment of the electronic and geometric structure of the molecule in both its ground and excited (MLCT)-M-3 states. Importantly, the transient spectra are remarkably similar for both solvents, and the spectral changes can be rationalized using the optimized ground- and excited-state structures of the complex. The proposed assignment of the lifetime shortening of the excited state in donor solvents (acetonitrile) to a metal-centered exciplex is not corroborated here. Molecular dynamics simulations confirm the lack of complexation; however, in both solvents the molecules come close to the metal but undergo rapid exchange with the bulk. The shortening of the lifetime of the title complex and nine additional related complexes can be rationalized by the decrease in the (MLCT)-M-3 energy. Deviations from this trend may be explained by means of the effects of the dihedral angle between the ligand planes, the solvent, and the (MLCT)-M-3-(MLCT)-M-1 energy gap

Probing the dynamics of plasmon-excited hexanethiol-capped gold nanoparticles by picosecond X-ray absorption spectroscopy

Picosecond X-ray absorption spectroscopy (XAS) is used to investigate the electronic and structural dynamics initiated by plasmon excitation of 1.8 nm diameter Au nanoparticles (NPs) functionalised with 1-hexanethiol. We show that 100 ps after photoexcitation the transient XAS spectrum is consistent with an 8% expansion of the Au-Au bond length and a large increase in disorder associated with melting of the NPs. Recovery of the ground state occurs with a time constant of similar to 1.8 ns, arising from thermalisation with the environment. Simulations reveal that the transient spectrum exhibits no signature of charge separation at 100 ps and allows us to estimate an upper limit for the quantum yield (QY) of this process to be <0.1

Experimental and theoretical 2p core-level spectra of size-selected gas-phase aluminum and silicon cluster cations: chemical shifts, geometric structure, and coordination-dependent screening

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al 12 + , Si 15 + , Si 16 + , and Si 19 + are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed