Electronic structure, morphology and chemical reactivity of nanoclusters and low-dimensional systems: fast photoemission spectroscopy studies

2008/2009L'obiettivo di questa tesi è l'applicazione della spettroscopia di fotoemissione allo studio di nanoparticelle supportate e di sistemi a bassa dimensionalità. Ad una primo periodo dedicato allo sviluppo del rivelatore e del software per un nuovo analizzatore d'energia per elettroni installato presso la linea di luce SuperESCA ad Elettra, è seguita una fase durante la quale ho eseguito una serie di esperimenti mirati ad esplorare le potenzialità del nuovo apparato sperimentale. Il primo risultato ottenuto riguarda la comprensione della relazione che intercorre tra le variazioni della reattività chimica del sistema Pd/Ru(0001) e il numero degli strati di Pd cresciuti in modo pseudomorfico sul substrato di rutenio. La risoluzione temporale raggiunta con la nuova strumentazione ci ha permesso di studiare processi dinamici su una scala temporale fino ad ora inaccessibile per la spettroscopia di fotoemissione dai livelli di core: in particolare abbiamo studiato la crescita del grafene ad alta temperatura sulla superficie (111) dell'iridio e la reattività chimica di nanocluster di Pt supportati su MgO. Nel primo caso abbiamo messo in evidenza come la formazione del grafene proceda attraverso la nucleazione di nano-isole di carbonio che assumono una peculiare forma di cupola. Nel secondo caso siamo riusciti a seguire sia la dinamica del processo di adsorbimento di CO, sia la reazione CO + 1/2 O2 -> CO2 sulle nanoparticelle di Pt depositate su un film ultra-sottile di ossido di magnesio. Infine, abbiamo caratterizzato la morfologia di nanoparticelle di Pd, Pt, Rh e Au cresciute su diversi substrati a base di carbonio, in particolare grafite, nanotubi a parete singola e grafene. Tra i vari risultati abbiamo compreso come l'interazione metallo-substrato dipenda dalla dimensione delle nano-particelle e abbiamo evidenziato il ruolo centrale dei difetti del substrato nei processi di nucleazione e intercalazione.The objective of this thesis is the application of photoelectron spectroscopy for the investigation of supported nanoclusters and low-dimensional systems. After a first stage devoted to the development of the detector and the software for the electron energy analyser installed on the SuperESCA beamline at Elettra, during the PhD project I've performed a series of experiments aimed to explore the capabilities of the new experimental apparatus. One of the first results concerns the understanding of the relation between the modifications in the chemical reactivity of the Pd/Ru(0001) system and the thickness of the pseudomorphically grown Pd overlayer. The temporal resolution achieved with the new experimental set-up allowed us to study dynamical processes on a new time scale, in particular the graphene growth process at high temperature on the Ir(111) surface and the chemical reactivity of Pt nanoclusters supported on MgO. In the former case, we discovered that graphene formation proceeds via preliminary nucleation of dome-shaped C nano-islands. In the second case, we succeded in following both the dynamics of CO adsorption process and the CO + 1/2 O2 -> CO2 reaction on Pt nanoclusters grown on a ultra-thin film of magnesium oxide. Finally, the morphology of Pd, Pt, Rh and Au nanoclusers grown on different carbon-based substrates (namely graphite, single-walled carbon nanotubes and graphene) has been characterized. Among the results we report the understanding of the dependence of the metal-substrate interaction on the cluster size and the role of defects in the nucleation and intercalation processes.XXII Ciclo197

Translucency of Graphene to van der Waals Forces Applies to Atoms/Molecules with Different Polar Character

Graphene has been proposed to be either fully transparent to van der Waals interactions to the extent of allowing switching between hydrophobic and hydrophilic behavior, or partially transparent (translucent), yet there has been considerable debate on this topic, which is still ongoing. In a combined experimental and theoretical study we investigate the effects of different metal substrates on the adsorption energy of atomic (argon) and molecular (carbon monoxide) adsorbates on high-quality epitaxial graphene. We demonstrate that while the adsorption energy is certainly affected by the chemical composition of the supporting substrate and by the corrugation of the carbon lattice, the van der Waals interactions between adsorbates and the metal surfaces are partially screened by graphene. Our results indicate that the concept of graphene translucency, already introduced in the case of water droplets, is found to hold more generally also in the case of single polar molecules and atoms, which are apolar

Structural investigation of the Rh(110)-c(2x2)-CN phase

The Rh(110)-c(2x2)-CN phase has been examined by means of scanning tunneling microscopy (STM) and full dynamical low-energy electron diffraction (LEED). From STM large c(2x2) domains are observed. The detailed LEED-IV structural analysis indicates that CN is located in the grooves of the (110) surface, approximately atop second layer rhodium atoms. The CN molecules lie almost flat with their bond axes oriented perpendicular to the rhodium troughs. An outward relaxation of the first substrate interlayer distance and a strong buckling of the second Rh layer are induced by CN adsorption. Calculated and experimental intensity curves are in good agreement. An exhaustive set of other possible adsorption sites and configurations was tested and excluded on the basis of reliability-factor analysis

Disentangling Vacancy Oxidation on Metallicity-Sorted Carbon Nanotubes

Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O$_2$ and N$_2$, which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O$_2$ doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all the SWCNT's vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first principles density functional theory calculated Kohn-Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C$_2$$>$O), carbonyl (C$_2$$>$C$=$O), and ketene (C$=$C$=$O), as ordered by abundance. Upon oxidation of nearly all the SWCNT's vacancies, the central peak's intensity for the metallic SWCNT sample is 60\% greater than for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O$_2$ does not contribute to the measured XPS O~1s spectra

Probing the Atomic Arrangement of Sub-Surface Dopants in a Silicon Quantum Device Platform

High-density structures of sub-surface phosphorus dopants in silicon continue to garner interest as a silicon-based quantum computer platform, however, a much-needed confirmation of their dopant arrangement has been lacking. In this work, we take advantage of the chemical specificity of X-ray photoelectron diffraction to obtain the precise structural configuration of P dopants in sub-surface Si:P $\delta$-layers. The growth of $\delta$-layer systems with different levels of doping is carefully studied and verified using X-ray photoelectron spectroscopy and low-energy electron diffraction. Subsequent XPD measurements reveal that in all cases, the dopants primarily substitute with Si atoms from the host material. Furthermore, no signs of free carrier-inhibiting P$-$P dimerization can be observed. Our observations not only settle a nearly decade-long debate about the dopant arrangement but also demonstrate that XPD is well suited to study sub-surface dopant structures. This work thus provides valuable input for an updated understanding of the behavior of Si:P $\delta$-layers and the modeling of their derived quantum devices

Band dispersion in the deep 1s core level of graphene

Chemical bonding in molecules and solids arises from the overlap of valence electron wave functions, forming extended molecular orbitals and dispersing Bloch states, respectively. Core electrons with high binding energies, on the other hand, are localized to their respective atoms and their wave functions do not overlap significantly. Here we report the observation of band formation and considerable dispersion (up to 60 meV) in the $1s$ core level of the carbon atoms forming graphene, despite the high C $1s$ binding energy of $\approx$ 284 eV. Due to a Young's double slit-like interference effect, a situation arises in which only the bonding or only the anti-bonding states is observed for a given photoemission geometry.Comment: 12 pages, 3 figures, including supplementary materia

The highest oxidation state observed in graphene-supported sub-nanometer iron oxide clusters

Iron oxide nanoclusters are of interest for a broad range of applications, but limited experimental information on their oxidation mechanism is available outside of the gas phase. Here, the oxidation of graphene-supported size-selected Fe-n clusters is studied using high-resolution X-ray Photoelectron Spectroscopy.Size-selected iron oxide nanoclusters are outstanding candidates for technological-oriented applications due to their high efficiency-to-cost ratio. However, despite many theoretical studies, experimental works on their oxidation mechanism are still limited to gas-phase clusters. Herein we investigate the oxidation of graphene-supported size-selected Fe-n clusters by means of high-resolution X-ray Photoelectron Spectroscopy. We show a dependency of the core electron Fe 2p(3/2) binding energy of metallic and oxidized clusters on the cluster size. Binding energies are also linked to chemical reactivity through the asymmetry parameter which is related to electron density of states at the Fermi energy. Upon oxidation, iron atoms in clusters reach the oxidation state Fe(II) and the absence of other oxidation states indicates a Fe-to-O ratio close to 1:1, in agreement with previous theoretical calculations and gas-phase experiments. Such knowledge can provide a basis for a better understanding of the behavior of iron oxide nanoclusters as supported catalysts

Mixed cation halide perovskite under environmental and physical stress

Despite the ideal performance demonstrated by mixed perovskite materials when used as active layers in photovoltaic devices, the factor which still hampers their use in real life remains the poor stability of their physico-chemical and functional properties when submitted to prolonged permanence in atmosphere, exposure to light and/or to moderately high temperature. We used high resolution photoelectron spectroscopy to compare the chemical state of triple cation, double halide Cs-x(FA(0.83)MA(0.17))(()Pb-1-(x))(I0.83Br0.17)(3) perovskite thin films being freshly deposited or kept for one month in the dark or in the light in environmental conditions. Important deviations from the nominal composition were found in the samples aged in the dark, which, however, did not show evident signs of oxidation and basically preserved their own electronic structures. Ageing in the light determined a dramatic material deterioration with heavily perturbed chemical composition also due to reactions of the perovskite components with surface contaminants, promoted by the exposure to visible radiation. We also investigated the implications that 2D MXene flakes, recently identified as effective perovskite additive to improve solar cell efficiency, might have on the labile resilience of the material to external agents. Our results exclude any deleterious MXene influence on the perovskite stability and, actually, might evidence a mild stabilizing effect for the fresh samples, which, if doped, exhibited a lower deviation from the expected stoichiometry with respect to the undoped sample. The evolution of the undoped perovskites under thermal stress was studied by heating the samples in UHV while monitoring in real time, simultaneously, the behaviour of four representative material elements. Moreover, we could reveal the occurrence of fast changes induced in the fresh material by the photon beam as well as the enhanced decomposition triggered by the concurrent X-ray irradiation and thermal heating