15 research outputs found
Role of cerium oxide in bioactive glasses during catalytic dissociation of hydrogen peroxide
The addition of cerium oxide to bioactive glasses, important materials for bone tissue regeneration, has been shown to induce multifunctionality, combining a significant bioactivity with antioxidant properties. We provide a real time investigation of the evolution of the electronic properties of highly diluted cerium ions in a liquid environment containing hydrogen peroxide - the most abundant reactive oxygen species in living cells. This challenging task is undertaken by means of high-energy resolution fluorescence detected by X-ray absorption near-edge spectroscopy at the Ce L-3 edge. We investigate samples with variable compositions and different morphologies. We relate the observed spectroscopic changes not only to variations in the concentration of the two Ce oxidation states in the samples, but also to changes in the local atomic environment of Ce ions, providing a clear picture of the role of cerium ions in the dissociation of hydrogen peroxide. The obtained results contribute to the understanding of the mechanisms that come into play in the process and provide a basis for the optimization of the functionalities of this class of materials
Contraction, cation oxidation state and size effects in cerium oxide nanoparticles
An accurate description of the structural and chemical modifications of cerium oxide nanoparticles (NPs) is mandatory for understanding their functionality in applications. In this work we investigate the relation between local atomic structure, oxidation state, defectivity and size in cerium oxide NPs with variable diameter below 10 nm, using x-ray absorption fine structure analysis in the near and extended energy range. The NPs are prepared by physical methods under controlled conditions and analyzed in morphology and crystalline quality by high resolution transmission electron microscopy. We resolve here an important question on the local structure of cerium oxide NPs: we demonstrate a progressive contraction in the Ce-O interatomic distance with decreasing NP diameter and we relate the observed effect to the reduced dimensionality. The contraction is not significantly modified by inducing a 4%-6% higher Ce3+ concentration through thermal annealing in high vacuum. The consequences of the observed average cation-anion distance contraction on the properties of the NPs are discussed
Ultrafast dynamics in (TaSe4)2I triggered by valence and core-level excitation
Dimensionality plays a key role in the emergence of ordered phases, such as charge density-waves (CDW), which can couple to, and modulate, the topological properties of matter. In this work, we study the out-of-equilibrium dynamics of the paradigmatic quasi-one-dimensional material (TaSe4)2I, which exhibits a transition into an incommensurate CDW phase when cooled to just below room temperature, namely at TCDW = 263 K. We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively. Excitation of the core-levels by ultrashort X-ray FEL pulses at 47 eV and 119 eV induces reflectivity transients resembling those recorded when only exciting the valence band of the compound - by near-infrared pulses at 1.55 eV - in the case of the insulating sub-system. Conversely, the metallic sub-system displays relaxation dynamics which depend on the energy of photo-excitation. Moreover, excitation of the CDW amplitude mode is recorded only for excitation at a low-photon-energy. This fact suggests that the coupling of light to ordered states of matter can predominantly be achieved when directly injecting delocalized carriers in the valence band, rather than localized excitations in the core levels. Complementing this, table-top experiments allow us to prove the quasi-unidirectional nature of the CDW phase in (TaSe4)2I, whose fingerprints are detected along its c-axis only. Our results provide new insights into the symmetry of the ordered phase of (TaSe4)2I perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials
FEL stochastic spectroscopy revealing silicon bond softening dynamics
Time-resolved X-ray Emission/Absorption Spectroscopy (Tr-XES/XAS) is an
informative experimental tool sensitive to electronic dynamics in materials,
widely exploited in diverse research fields. Typically, Tr-XES/XAS requires
X-ray pulses with both a narrow bandwidth and sub-picosecond pulse duration, a
combination that in principle finds its optimum with Fourier transform-limited
pulses. In this work, we explore an alternative xperimental approach, capable
of simultaneously retrieving information about unoccupied (XAS) and occupied
(XES) states from the stochastic fluctuations of broadband extreme ultraviolet
pulses of a free-electron laser. We used this method, in combination with
singular value decomposition and Tikhonov regularization procedures, to
determine the XAS/XES response from a crystalline silicon sample at the
L2,3-edge, with an energy resolution of a few tens of meV. Finally, we combined
this spectroscopic method with a pump-probe approach to measure structural and
electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after
photoexcitation with an optical laser pulse at 390 nm allowed us to observe
perturbations of the band structure, which are compatible with the formation of
the predicted precursor state of a non-thermal solid-liquid phase transition
associated with a bond softening phenomenon
Nanoscale transient polarization gratings
We present the generation of transient polarization gratings at the
nanoscale, achieved using a tailored accelerator configuration of the FERMI
free electron laser. We demonstrate the capabilities of such a transient
polarization grating by comparing its induced dynamics with the ones triggered
by a more conventional intensity grating on a thin film ferrimagnetic alloy.
While the signal of the intensity grating is dominated by the thermoelastic
response of the system, such a contribution is suppressed in the case of the
polarization grating. This exposes helicity-dependent magnetization dynamics
that have so-far remained hidden under the large thermally driven response. We
anticipate nanoscale transient polarization gratings to become useful for the
study of any physical, chemical and biological systems possessing chiral
symmetry
Studio della struttura locale di droganti in quantum dots mediante spettroscopia di assorbimento di raggi X
La capacità della spettroscopia di assorbimento di riuscire a determinare la struttura locale di campioni di ogni tipo e concentrazione, dagli elementi puri ai più moderni materiali nanostrutturati, rende lo studio dei meccanismi di incorporazione di droganti in matrici di semiconduttori il campo che meglio ne esprime tutto il potenziale. Inoltre la possibilità di ottenere informazioni sulla struttura locale di un particolare elemento in traccia posto in sistemi senza ordine a lungo raggio risulta, ovviamente, nello studio dei semiconduttori di grandissimo interesse. Tuttavia, la complessità di determinate strutture, generate dalla incorporazione di elementi eterovalenti che ne modificano la simmetria, può far si che all’analisi sperimentale si debbano affiancare dei metodi avanzati ab-initio. Questi approcci garantiscono, attraverso la simulazione o di strutture atomiche o dello stesso spettro XAS, di ottenere una più completa e precisa interpretazione dei dati sperimentali. Nella fase preliminare di questo elaborato si illustrerà la fenomenologia della spettroscopia di assorbimento e i fondamenti teorici che stanno alla base dell’analisi della struttura fine di soglia. Si introdurranno contemporaneamente le tecniche sperimentali con cui si realizzano le misure di spettri di assorbimento su una beamline che sfrutta sorgente di radiazione di sincrotrone facendo riferimento agli strumenti montati sulla linea LISA (o BM08) presso l’European Synchrotron Radiation Facility di Grenoble su cui si sono realizzati gli esperimenti di questo lavoro. Successivamente si realizzerà una rassegna di alcuni esperimenti simbolo della analisi della struttura locale di droganti in semiconduttori mediante XAFS, andando ad approfondire i metodi sperimentali associati. Nella parte principale della tesi verranno descritti alcuni tipi di analisi avanzate effettuate su Colloidal Quantum Dots a base di solfuro di piombo drogati con antimonio. Tali sistemi, particolarmente interessanti per potenziali applicazioni in campo optoelettrico, sono stati analizzati mediante misure di fluorescenza ottenute sulla beamline LISA. La fase di analisi ha visto la progettazione di una suite di programmi in C++ per realizzare simulazioni di uno spettro XAS teorico completo basato su strutture ottenute (anche esse) da metodi ab-initio
Experimental data for "Superradiant Thomson scattering from graphite in the extreme ultraviolet"
<p>Pretreated experimental data of an extreme ultraviolet (EUV) Thomson scattering experiment on highly oriented pyrolytic graphite (HOPG), described in the PNAS paper "Superradiant Thomson scattering from graphite in the extreme ultraviolet" by the same authors. Details on the experimental geometry and setup are available in the paper. </p>
<p>The data contain information on the distribution of the Thomson scattering intensity, at varying the incident intensity. Given the observation that the ratio of the scattered to incident intensity is not constant throughout the experiment, but depends on the incident intensity, the pulse-by-pulse measured intensities are treated as follows. </p>
<p>Each independent dataset (at given incident wavelength and polarization) of our experiment was treated by averaging the ratio of I_scat to the incoming I_0, R_scat (I_0) = I_scat (I_0)/I_0, over several different sample points. The average was taken in small enough intervals of I_0. Statistically meaningful data were obtained rejecting I_0 intervals with too few (<5) data within them and (rare) fluctuations larger than twice the standard deviations. These data are reported in the available files. </p>
<p>The files are named after the value of the wavelength and polarization of the EUV free electron laser beam, exciting the HOPG sample.</p>
<p>"lambda0-4nm" -> incident wavelength 4.08 nm<br>"lambda0-5nm" -> incident wavelength 4.74 nm<br>"pol-s" -> polarization orthogonal to the scattering plane<br>"pol-p" -> polarization parallel to the scattering plane</p>
<p>The file column are as follows: <br>col. 1: incident EUV pulse intensity I_0 (central values for of the sampled intervals, see above)<br>col. 2: ratio between scattered and incident intensity R_scat = I_scat/I_0<br>col. 3: experimental error associated to col. 2, estimated as the standard deviation in the data relative to a given I_0 interval<br>col. 4: scattering angle.</p>
<p>Col. 1,2,3 are in arbitrary units, as discussed in detail in the paper. </p>
<p>Please note that in the case of lambda0-4nm, pol-s, two datasets are available, relative to two different graphite samples: these differ for a scaling value of I_0 (about the 20%), possibly associated to a slightly different surface quality and/or focusing conditions. </p>
<p> </p>
Contraction, cation oxidation state and size effects in cerium oxide nanoparticles
An accurate description of the structural and chemical modifications of cerium oxide nanoparticles (NPs) is mandatory for understanding their functionality in applications. In this work we investigate the relation between local atomic structure, oxidation state, defectivity and size in cerium oxide NPs with variable diameter below 10 nm, using x-ray absorption fine structure analysis in the near and extended energy range. The NPs are prepared by physical methods under controlled conditions and analyzed in morphology and crystalline quality by high resolution transmission electron microscopy. We resolve here an important question on the local structure of cerium oxide NPs: we demonstrate a progressive contraction in the Ce-O interatomic distance with decreasing NP diameter and we relate the observed effect to the reduced dimensionality. The contraction is not significantly modified by inducing a 4%-6% higher Ce3+ concentration through thermal annealing in high vacuum. The consequences of the observed average cation-anion distance contraction on the properties of the NPs are discussed
Lifetime of Photogenerated Positive Charges in Hybrid Cerium Oxide-Based Materials from Space and Mirror Charge Effects in Time-Resolved Photoemission Spectroscopy
Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes