arXiv subjects

A. Morgante

Publications and source records attributed to A. Morgante.

12 recordsLinked to original sources

arXiv2018

Ripple morphology of graphitic surfaces: a comparison between few-layer graphene and HOPG

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with diluted aqueous solutions of bio-organic molecules (dimethyl sulfoxide, DMSO, and L-Methionine). On pristine FLG we observe nicely ordered, three-fold oriented rippled domains, with a 4.7+/-0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction of the FLG surface with the molecular solution, the ripple periodicity relaxes to 6.2+/-0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off from FLG, as shown by quantitative analysis of XPS and Raman spectroscopy data which indicate a neat reduction of thickness. Upon sonication rippled domains are no longer observed. Regarding HOPG, we could not observe ripples on cleaved samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The complex of our comparative observations on FLG and HOPG can be hardly rationalized solely on the base of surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider the ripples as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as the interplane stacking, the interaction with molecules and/or with the AFM tip.

cond-mat.mtrl-sci↗

arXiv2010

Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant x-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance, and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) which present Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

cond-mat.mes-hall↗

arXiv2005

Electronic structure and molecular orientation of a Zn-tetra-phenyl porphyrin multilayer on Si(111)

The electronic properties and the molecular orientation of Zn-tetraphenyl-porphyrin films deposited on Si(111) have been investigated using synchrotron radiation. For the first time we have revealed and assigned the fine structures in the electronic spectra related to the HOMOs and LUMOs states. This is particularly important in order to understand the orbital interactions, the bond formation and the evolution of the electronic properties with oxidation or reduction of the porphyrins in supramolecular donor-acceptor complexes used in photovoltaic devices.

cond-mat.mtrl-sci↗

arXiv2004

The pseudomorphic to bulk fcc phase transition of thin Ni films on Pd(100)

We have measured the transformation of pseudomorphic Ni films on Pd(100) into their bulk fcc phase as a function of the film thickness. We made use of x-ray diffraction and x-ray induced photoemission to study the evolution of the Ni film and its interface with the substrate. The growth of a pseudomorphic film with tetragonally strained face centered symmetry (fct) has been observed by out-of-plane x-ray diffraction up to a maximum thickness of 10 Ni layers (two of them intermixed with the substrate), where a new fcc bulk-like phase is formed. After the formation of the bulk-like Ni domains, we observed the pseudomorphic fct domains to disappear preserving the number of layers and their spacing. The phase transition thus proceeds via lateral growth of the bulk-like phase within the pseudomorphic one, i.e. the bulk-like fcc domains penetrate down to the substrate when formed. This large depth of the walls separating the domains of different phases is also indicated by the strong increase of the intermixing at the substrate-film interface, which starts at the onset of the transition and continues at even larger thickness. The bulk-like fcc phase is also slightly strained; its relaxation towards the orthomorphic lattice structure proceeds slowly with the film thickness, being not yet completed at the maximum thickness presently studied of 30 Angstrom (i.e. about 17 layers).

cond-mat.mtrl-sci↗

arXiv2004

Surfactant-like Effect and Dissolution of Ultrathin Fe Films on Ag(001)

The phase immiscibility and the excellent matching between Ag(001) and Fe(001) unit cells (mismatch 0.8 %) make Fe/Ag growth attractive in the field of low dimensionality magnetic systems. Intermixing could be drastically limited at deposition temperatures as low as 140-150 K. The film structural evolution induced by post-growth annealing presents many interesting aspects involving activated atomic exchange processes and affecting magnetic properties. Previous experiments, of He and low energy ion scattering on films deposited at 150 K, indicated the formation of a segregated Ag layer upon annealing at 550 K. Higher temperatures led to the embedding of Fe into the Ag matrix. In those experiments, information on sub-surface layers was attained by techniques mainly sensitive to the topmost layer. Here, systematic PED measurements, providing chemical selectivity and structural information for a depth of several layers, have been accompanied with a few XRD rod scans, yielding a better sensitivity to the buried interface and to the film long range order. The results of this paper allow a comparison with recent models enlightening the dissolution paths of an ultra thin metal film into a different metal, when both subsurface migration of the deposit and phase separation between substrate and deposit are favoured. The occurrence of a surfactant-like stage, in which a single layer of Ag covers the Fe film is demonstrated for films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the formation of two Ag capping layers is also reported. As the annealing temperature was increased beyond 700 K, the surface layers closely resembled the structure of bare Ag(001) with the residual presence of subsurface Fe aggregates.

cond-mat.mtrl-sci↗

arXiv2003

Quantum size effects in the low temperature layer-by-layer growth of Pb on Ge(001)

The electronic properties of thin metallic films deviate from the corresponding bulk ones when the film thickness is comparable with the wavelength of the electrons at the Fermi level due to quantum size effects (QSE). QSE are expected to affect the film morphology and structure leading to the low temperature (LT) ``electronic growth'' of metals on semiconductors. In particular, layer-by-layer growth of Pb(111) films has been reported for deposition on Ge(001) below 130 K. An extremely flat morphology is preserved throughout deposition from four up to a dozen of monolayers. These flat films are shown to be metastable and to reorganize into large clusters uncovering the first Pb layer, pseudomorphic to the substrate, already at room temperature. Indications of QSE induced structural variations of the growing films have been reported for Pb growth on Ge(001), where the apparent height of the Pb(111) monatomic step was shown to change in an oscillatory fashion by He atom scattering (HAS) during layer-by-layer growth. The extent of the structural QSE has been obtained by a comparison of the HAS data with X-ray diffraction (XRD) and reflectivity experiments. Whereas step height variations as large as 20 % have been measured by HAS reflectivity, the displacement of the atomic planes from their bulk position, as measured by XRD, has been found to mainly affect the topmost Pb layer, but with a lower extent, i.e. the QSE observed by HAS are mainly due to a perpendicular displacement of the topmost layer charge density. The effect of the variable surface relaxation on the surface vibration has been studied by inelastic HAS to measure the acoustic dispersion of the low energy phonons.

cond-mat.mtrl-sci↗

arXiv2002

From pseudomorphic to orthomorphic growth of Fe films on Cu3Au(001)

A few theoretical models predict Fe to display both ferromagnetic and antiferromagnetic phases, depending on the volume of the unit cell. A proper choice of the substrate allows the growth of thin Fe overlayers with structures different from the bcc bulk one. The Cu3Au(001) substrate is a suitable candidate for testing these magnetic properties since it has a lattice parameter (3.75 Angstroms) which lies closer to the ferromagnetic fcc Fe phase (3.66 Angstroms) than to the ferromagnetic bcc phase (4.07 Angstroms). We have investigated Fe films up to 40 Angstroms thickness by means of Grazing Incidence X-Ray Diffraction (GIXRD) and Photo/Auger--Electron Diffraction (ED). The combination of GIXRD and ED allows one to obtain quantitative information on the in--plane spacing "a" from the former technique, and the ratio between the vertical spacing "c" and "a", from the latter one. At low coverage the film grows pseudomorphic to the fcc substrate up to a limit thickness of 8 Angstroms. Above this limit the film is characterized by the coexistence of the pseudomorphic phase with another tetragonally strained phase. The latter phase is shown to fall on the epitaxial curve of tetragonally distorted fcc Fe phase. Finally, above 17 Angstroms the development of a body centered phase "alpha", whose unit cell is rotated of 45 deg. with respect to the substrate one, has been clearly observed. "alpha" is the dominating phase for film thickness higher than 25 Angstroms and its lattice constant evolves towards the orthomorphic phase in strict quantitative agreement with epitaxial curves calculated for the tetragonally distorted bcc iron phase.

cond-mat.mtrl-sci↗

arXiv2002

Study of the isotropic contribution to the analysis of photoelectron diffraction experiments at the ALOISA beamline

The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying ISO contribution originated by both intrumental details and physical properties of the samples. The origin of the various contributions to the ISO component has been identified since many years. Nonetheless in this work we present original developement of the ED analysis, which arises from the evolution of instrumental performance, in terms of analyzers positioning and angular resolution, as well as collimation and size of X-ray beams in third generation synchrotron sources. The analytical treatement of the instrumental factors is presented in detail for the end station of the ALOISA beamline (Trieste Synchrotron), where a wide variety of scattering geometries is available for ED experiments. We present here the basic formulae and their application to experimental data taken on the Fe/Cu3Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed as well as the overlayer thickness and surface roughness have been considered.

cond-mat.mtrl-sci↗

arXiv2001

The order-disorder character of the (3x3) to (sqrt3 x sqrt3)R30° phase transition of Sn on Ge(111)

Growing attention has been drawn in the past years to the α-phase (1/3 monolayer) of Sn on Ge(111), which undergoes a transition from the low temperature (3x3) phase to the room temperature (\sqrt3 x \sqrt3)R30° one. On the basis of scanning tunnelling microscopy experiments, this transition was claimed to be the manifestation of a surface charge density wave (SCDW), i.e. a periodic redistribution of charge, possibly accompanied by a periodic lattice distortion, which determines a change of the surface symmetry. Recent He diffraction studies of the (3x3) long range order have shown the transition to be of the order-disorder type with a critical temperature Tc=220 K and belonging to the 3-state Potts' universality class. These findings clearly exclude an SCDW driven mechanism at 220 K, but they cannot exclude the occurence of a displacive transition at higher temperature. Here we present photoelectron diffraction data taken at 300 K and photoemission data taken up to 500 K (which is the maximum temperature where the (\sqrt3 x \sqrt3)R30° is stable) . From our analysis it is shown that the atomic structure of the Sn overlayer does not change throughout the transition up to 500 K. As a consequence the displacive hypothesis must be discarded in favour of a genuine order-disorder model.

cond-mat.mtrl-sci↗

arXiv2001

From bi-layer to tri-layer Fe nanoislands on Cu3Au(001)

Self assembly on suitably chosen substrates is a well exploited root to control the structure and morphology, hence magnetization, of metal films. In particular, the Cu3Au(001) surface has been recently singled out as a good template to grow high spin Fe phases, due to the close matching between the Cu3Au lattice constant (3.75 Angstrom) and the equilibrium lattice constant for fcc ferromagnetic Fe (3.65 Angstrom). Growth proceeds almost layer by layer at room temperature, with a small amount of Au segregation in the early stage of deposition. Islands of 1-2 nm lateral size and double layer height are formed when 1 monolayer of Fe is deposited on Cu3Au(001) at low temperature. We used the PhotoElectron Diffraction technique to investigate the atomic structure and chemical composition of these nanoislands just after the deposition at 140 K and after annealing at 400 K. We show that only bi-layer islands are formed at low temperature, without any surface segregation. After annealing, the Fe atoms are re-aggregated to form mainly tri-layer islands. Surface segregation is shown to be inhibited also after the annealing process. The implications for the film magnetic properties and the growth model are discussed.

cond-mat.mtrl-sci↗

arXiv2001

The order-disorder transition of the (3x3)Sn/Ge(111) phase

Growing attention has been drawn in the past years to the α-phase (1/3 monolayer) of Sn on Ge(111), which undergoes a transition from the low temperature (3x3) phase to the room temperature (\sqrt3 x \sqrt3)R30 one. On the basis of scanning tunnelling microscopy experiments, this transition was claimed to be the manifestation of a surface charge density wave (SCDW), i.e. a periodic redistribution of charge, possibly accompanied by a periodic lattice distortion, which determines a change of the surface symmetry. As further experiments with different techniques were being performed, increasing doubts were cast about the SCDW model. We have measured by He scattering the long range order of the 1/3 monolayer phase of Sn on the Ge(111) surface throughout the phase transition. The transition has been found of the order-disorder type with a critical temperature Tc=220 K. The expected 3-State Potts critical exponents are shown to be consistent with the observed power law dependence of the (3x3) order parameter and its correlation length close to Tc, thus excluding a charge density wave driven phase transition.

cond-mat.stat-mech↗

arXiv2001

Determination of the (3x3)-Sn/Ge(111) structure by photoelectron diffraction

At a coverage of about 1/3 monolayer, Sn deposited on Ge(111) below 550 forms a metastable (sqrt3 x sqrt3)R30 phase. This phase continuously and reversibly transforms into a (3x3) one, upon cooling below 200 K. The photoemission spectra of the Sn 4d electrons from the (3x3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and viceversa. The bond angle between Sn and its nearest neighbour atoms in the first Ge layer (Sn-Ge1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy, while keeping the photon polarization and the collection direction parallel to the Sn-Ge1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. This vertical distance is found to be 0.3 Angstroms larger for one Sn atom out of the three contained in the lattice unit cell. The (3x3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbour Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

cond-mat.str-el↗