Federica Bondino

Dopamine Adsorption on Anatase TiO2(101): A Photoemission and NEXAFS Spectroscopy Study

The adsorption of dopamine onto an anatase TiO(2)(101) single crystal has been studied using photoemission and NEXAFS techniques. Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry, resulting in the removal of band gap states in the TiO(2) valence band. Using the searchlight effect, carbon K-edge NEXAFS spectra indicate that the phenyl rings in the dopamine molecules are orientated normal to the surface. A combination of experimental and computational results indicates the appearance of new unoccupied states arising following adsorption. The possible role of these states in the charge-transfer mechanism of the dopamine-TiO(2) system is discussed.

Formation of Efficient Catalytic Silver Nanoparticles on Carbon Nanotubes by Adenine Functionalization

Stuck together: adenine/carbon nanotube hybrids trigger the formation of controlled-size catalytic silver nanoparticles on the nanotube surface. The catalytic efficiency of the resulting species was assessed in the oxidation of 2-methylhydroquinone to its corresponding benzoquinone, with complete recovery and without loss of activity of the catalyst.

Room temperature metalation of 2H-TPP monolayer on iron and nickel surfaces by picking up substrate metal atoms.

Here, it is demonstrated, using high-resolution X-ray spectroscopy and density functional theory calculations, that 2H-tetraphenyl porphyrins metalate at room temperature by incorporating a surface metal atom when a (sub)monolayer is deposited on 3d magnetic substrates, such as Fe(110) and Ni(111). The calculations demonstrate that the redox metalation reaction would be exothermic when occurring on a Ni(111) substrate with an energy gain of 0.89 eV upon embedding a Ni adatom in the macrocycle. This is a novel way to form, via chemical modification and supramolecular engineering, 3d-metal-organic networks on magnetic substrates with an intimate bond between the macrocycle molecular metal ion and the substrate atoms. The achievement of a complete metalation by Fe and Ni can be regarded as a test case for successful preparation of spintronic devices by means of molecular-based magnets and inorganic magnetic substrates.

Supramolecular Engineering through Temperature-Induced Chemical Modification of 2H-Tetraphenylporphyrin on Ag(111): Flat Phenyl Conformation and Possible Dehydrogenation Reactions

Scratching the surface: Formation of a monolayer of 2H-tetraphenylporphyrins (2H-TPP) on Ag(111), either by sublimation of a multilayer in the range 525-600 K or by annealing (at the same temperature) a monolayer deposited at room temperature, induces a chemical modification of the molecules. Rotation of the phenyl rings into a flat conformation is observed and tentatively explained, by using DFT calculations, as a peculiar reaction due to molecular dehydrogenation.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

Enhancement of room temperature ferromagnetism in N-doped TiO2−x rutile: Correlation with the local electronic properties

The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic ordering.

Structural determination of molecules adsorbed in different sites by means of chemical shift photoelectron diffraction: c(4×2)-CO on Pt(111)

Abstract In this letter we present a chemical shift photoelectron diffraction (CS-PED) study of the c(4×2) phase of CO on Pt(111), in which two molecular species are present. The angular dependence of both chemically shifted C1s core levels was independently determined in a high resolution photoemission experiment. By means of a full multiple scattering analysis, the local structure around each of the two CO species has been quantitatively evaluated. A configuration with half of the CO molecules on top and half in the bridge position gives good agreement with the experimental data, in accord with previous structural studies of this system. Our result highlights the capabilities of CS-PED for the study of complex adsorption systems where the same molecule occupies different adsorption sites.

Electronic structure of Mn-doped ZnO by?x-ray emission and absorption spectroscopy

We report an investigation of Mn-doped ZnO pellets with diluted Mn concentration by soft-x-ray emission and absorption spectroscopy. We have compared the electronic structure of two samples with different Mn concentration and different magnetic properties at room temperature: ferromagnetism in one case (Zn(0.98)Mn(0.02)O) and no magnetic order in the other (Zn(0.96)Mn(0.04)O). The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the nonmagnetic sample, a larger contribution of higher oxidation Mn states is present, which can be correlated to the suppressed ferromagnetism. The presence of oxygen atoms bonded to Mn ions and hybridized Mn 3d-O 2p states has been detected in both compounds. The partial density of states in the valence band has been measured with x-ray emission spectroscopy and the Mn 3d states have been found inside the bandgap of ZnO.

Surface Properties of Nanocrystalline PbS Films Deposited at the Water-Oil Interface: A Study of Atmospheric Aging

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

Spectroscopic evidence of in-gap states at the SrTiO3/LaAlO3 ultrathin interfaces

Experimental evidence of differences in the electronic properties of an insulating and a conducting SrTiO3/LaAlO3 interface is provided by soft x-ray spectroscopies. While core level photoemission measurements show that only at the conducting interface Ti ions with 3+ ionization state are present, by using resonant photoemission and x-ray absorption spectroscopies, it is shown that in both samples in-gap states with a Ti 3d character are present, but their density is higher at the conducting interface.