Diego Gianolio

Low-dimensional systems investigated by x-ray absorption spectroscopy: a selection of 2D, 1D and 0D cases

Over the last three decades low-dimensional systems have attracted increasing interest both from the fundamental and technological points of view due to their unique physical and chemical properties. X-ray absorption spectroscopy (XAS) is a powerful tool for the characterization of such kinds of systems, owing to its chemical selectivity and high sensitivity in interatomic distance determination. Moreover, XAS does not require long-range ordering, that is usually absent in low-dimensional systems. Finally, this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (diffraction anomalous fine structure, extended energy-loss fine structure, pair distribution function, x-ray emission spectroscopy, high-energy resolution fluorescence detected XAS and x-ray Raman scattering). Subsequently, a selection of significant applications of XAS to two-, one- and zero-dimensional systems will be presented. The selected low-dimensional systems include IV and III–V semiconductor films, quantum wells, quantum wires and quantum dots; carbon-based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.

Influence of additives in defining the active phase of the ethylene oxychlorination catalyst

The understanding, at the atomic level, of the role played by additives (dopants or promoters) in the chemistry of an industrial catalyst is a very complex and difficult task. We succeeded in this goal for the ethylene oxychlorination catalyst (CuCl(2)/gamma-Al(2)O(3)), used to produce dichloroethane, a key intermediate of the polyvinyl chloride chemistry (PVC). Among the most used additives for both fluid and fixed beds technologies (LiCl, KCl, CsCl, MgCl(2), LaCl(3), CeCl(4)) we have been able to highlight that KCl, and CsCl, forming in reaction conditions a mixed phase with CuCl(2), strongly modify the catalyst behaviour. In particular, these additives are able to displace the rate determining step from the CuCl oxidation (undoped catalyst) to the CuCl(2) reduction. This results from the decrease of the rate of the latter reaction, thus the overall activity of the system. For all remaining additives the rate determining step remains the CuCl oxidation, as for the undoped catalyst. These results have been obtained coupling the catalyst activity monitored with a pulse reactor working in both non-depletive and depletive modes with time resolved XANES spectroscopy performed under in operando conditions (i.e. coupled with mass spectrometry). Formation of CuK(x)Cl(2+x) and CuCs(x)Cl(2+x) mixed phases has been proved monitoring the Cu(II) d-d transitions with UV-Vis spectrometer and the CO stretching frequency of carbon monoxide adsorbed on reduced catalyst by in situ IR spectroscopy. Finally, of high relevance is the observation that the fully oxidized catalyst is inactive. This unexpected evidence highlight the role of coordinatively unsaturated Cu(I) species in adsorbing ethylene on the catalyst surface indicating that copper, in the working catalyst, exhibits a (I)/(II) mixed valence state.

Preference towards Five-Coordination in Ti Silicalite-1 upon Molecular Adsorption

. B becomes less pronounced and shifts to higher en-ergies, while C varies depending on the kind of ligand: whenwater (ammonia) is adsorbed it becomes less (more) pro-nounced.Insights into the experimental vtc-XES data can be gainedfrom symmetry arguments because the intensities of the vtc-XES spectral features are related to the matrix elements

SrFe0.5Ru0.5O2: Square-Planar Ru2+ in an Extended Oxide

Low-temperature topochemical reduction of the cation disordered perovskite phase SrFe(0.5)Ru(0.5)O(3) with CaH(2) yields the infinite layer phase SrFe(0.5)Ru(0.5)O(2). Thermogravimetric and X-ray absorption data confirm the transition metal oxidation states as SrFe(0.5)(2+)Ru(0.5)(2+)O(2); thus, the title phase is the first reported observation of Ru(2+) centers in an extended oxide phase. DFT calculations reveal that, while the square-planar Fe(2+) centers adopt a high-spin S = 2 electronic configuration, the square-planar Ru(2+) cations have an intermediate S = 1 configuration. This combination of S = 2, Fe(2+) and S = 1, Ru(2+) is consistent with the observed spin-glass magnetic behavior of SrFe(0.5)Ru(0.5)O(2).

Photo-induced pyridine substitution in cis-[Ru(bpy)(2)(py)(2)]Cl(2): a snapshot by time-resolved X-ray solution scattering.

Determination of transient structures in light-induced processes is a challenging goal for time-resolved techniques. Such techniques are becoming successful in detecting ultrafast structural changes in molecules and do not require the presence of probe-like groups. Here, we demonstrate that TR-WAXS (Time-Resolved Wide Angle X-ray Scattering) can be successfully employed to study the photochemistry of cis-[Ru(bpy)(2)(py)(2)]Cl(2), a mononuclear ruthenium complex of interest in the field of photoactivatable anticancer agents. TR-WAXS is able to detect the release of a pyridine ligand and the coordination of a solvent molecule on a faster timescale than 800 ns of laser excitation. The direct measurement of the photodissociation of pyridine is a major advance in the field of time-resolved techniques allowing detection, for the first time, of the release of a multiatomic ligand formed by low Z atoms. These data demonstrate that TR-WAXS is a powerful technique for studying rapid ligand substitution processes involving photoactive metal complexes of biological interest.

X-ray transient absorption structural characterization of the 3MLCT triplet excited state of cis-[Ru(bpy)2(py)2]2+

The excited state dynamics and structure of the photochemically active complex cis-[Ru(bpy)2(py)2](2+) have been investigated using optical transient absorption (OTA) and X-ray transient absorption (XTA) spectroscopy, and density functional theory (DFT). Upon light-excitation in aqueous solution cis-[Ru(bpy)2(py)2](2+) undergoes ultrafast dissociation of one pyridine ligand to form cis-[Ru(bpy)2(py)(H2O)](2+). OTA measurements highlighted the presence of two major time components of 1700 ps and 130 ps through which the system decays to the ground-state and evolves towards the photoproduct. XTA data were acquired after 150 ps, 500 ps, and 3000 ps from laser excitation (λexc = 351 nm) and provided the transient structure of the (3)MLCT state corresponding to the longer time component in the OTA experiment. In excellent agreement with DFT, XTA shows that the (3)MLCT geometry is characterized by an elongation of the dissociating Ru-N(py) bond and a shortening of the trans Ru-N(bpy) bond with respect to the ground state. Conversely, calculations show that the (3)MC state has a highly distorted structure with Ru-N(py) bonds between 2.77-3.05 Å.

Synthesis, characterization and CO2 uptake of a chiral Co(II) metal–organic framework containing a thiazolidine-based spacer

The polytopic ligand thiazolidine-2,4-dicarboxylic acid (H2L) has been synthesised on a large scale starting from the naturally occurring amino acid L-cysteine. The (R,R)/(S,R)diastereomeric mixture has been separated into its constituents through selective precipitation of the pure (R,R) isomer from concentrated H2O/MeOH solutions. The enantiomerically pure ligand (H2L-RR) has been reacted with CoCl2·6H2O under hydrothermal conditions, with the final product being [Co(L-RR)(H2O)·H2O]∞ (1). The obtained coordination polymer is optically pure, and it maintains the chiral information that is present in its building block. Two different kinds of channels are present in the 3D structure of 1: one hydrophobic (with the sulfur atoms of the thiazolidine rings exposed) and the other hydrophilic [with the aquo ligand on Co(II) exposed, and hosting the crystallization water solvent]. 1 has been characterized through a combination of X-ray diffraction (single-crystal and powder) and spectroscopic (CD, IR, UV-Vis, XANES, EXAFS) techniques. Finally, CO2 adsorption tests conducted at 273 K and (pCO2)max = 920 torr have shown a good carbon dioxide uptake, equal to 4.7 wt%.

High energy resolution core-level X-ray spectroscopy for electronic and structural characterization of osmium compounds

A comprehensive study of the bulk solid OsCl3 and the molecular ion [Os(bpy)2(CO)Cl](+) is presented illustrating the application of RIXS and HERFD XANES spectroscopies to the investigation of both bulk materials and molecular complexes. In order to analyze the experimental results, DFT simulations were performed taking into account spin-orbit interaction. Calculations for both compounds resulted in good agreement with the experimental RIXS and HERFD XANES data, shedding light on the details of their local atomic and electronic structure. In particular, the spatial distribution of molecular orbitals was obtained, which allowed the determination of the origin of the absorption peaks. It was shown that for materials containing heavy atoms, only the application of advanced RIXS and HERFD XANES spectroscopies makes it possible to extract the information on local atomic and electronic structure details from XANES data.

Structural Characterization of Multi-Quantum Wells in Electroabsorption-Modulated Lasers by using Synchrotron Radiation Micrometer-Beams

Advanced optoelectronic devices, the backbone of modern communication technology, require the monolithic integration of different functions at chip level. An example of devices fulfilling this requirement are multi-quantum well (MQW) electroabsorption-modulated lasers (EMLs) employed in long-distance, high-frequency optical fiber communications technology. Such devices are realized by using the selective area growth (SAG) technique. Optimization of the growth parameters is carried out by empirical approaches as direct structural characterization of the MQW is not possible with laboratory X-ray sources, owing to the micrometer-variation of composition and thickness inherent to the SAG technique. Micrometer-resolved X-ray (m-X-ray) beam available at third generation synchrotron radiation sources, such as the ID22 at the European Synchrotron Radiation Facility (ESRF), allowed us to directly measure the determinant structural parameters of MQW EML structures such as well and barrier widths and mismatches with a 2 mm spatial resolution. In more detail, optoelectronic devices, widely used in the generation and detection of optical signal for telecom and datacom applications, consist of different III-V semiconductors alloys deposited on suitable substrates. The deposition can be performed by different techniques that are based on the crystal rebuilding of a semiconductor used as substrate, such as metal

Performance of B18, the Core EXAFS Bending Magnet beamline at Diamond

B18 has been operational since April 2010 and has hosted a total of 63 EXAFS user experiments up to Jan 2012. B18 is contributing to research programs across a wide range of scientific disciplines, e.g. solid state physics and materials, catalysis, chemistry, soft matter, surfaces and biomaterials. We present a review of its present performance and capabilities.