M. C. G. Passeggi

Periodically Rippled Graphene: Growth and Spatially Resolved Electronic Structure

We grow epitaxial graphene monolayers on Ru(0001) that cover uniformly the substrate over lateral distances larger than several microns. The weakly coupled graphene monolayer is periodically rippled and it shows charge inhomogeneities in the charge distribution. Real space measurements by scanning tunneling spectroscopy reveal the existence of electron pockets at the higher parts of the ripples, as predicted by a simple theoretical model. We also visualize the geometric and electronic structure of edges of graphene nanoislands.

Chemical reactions at surfaces: titanium oxidation

Auger electron spectroscopy has been used to study the first stages of titanium oxidation. Titanium at room and high temperature, as well as thin films deposited over a metal substrate, were oxidized at low pressure. The authors found, using factor analysis, two diVerent regimes depending on the sample temperature. For temperatures below 200°C the oxidation is characterized by the presence of only one oxide phase: TiO 2 . At higher temperatures, the presence of Ti 2 O 3 is detected from the beginning of the process. For thin films [less than one monolayer (ML)] only TiO 2 was found, but for thicker films a new oxide phase is detected (TiO x , x 7 ML, only TiO 2 is detected.

Temperature effects in the early stages of titanium oxidation

The effect of the temperature on the first stages of the titanium oxidation process has been followed by means of Auger electron spectroscopy. We found, using principal component analysis, two different regimes depending on the substrate temperature. For temperatures below 200°C the oxidation is characterized by the presence of only one oxide phase: TiO2. At higher temperatures, the presence of Ti2O3 is detected from the beginning of the process.

The role of passivation in titanium oxidation : Thin film and temperature effects

Abstract Through the combined application of Auger electron spectroscopy (AES) and factor analysis (FA), we have studied the oxidation process of titanium under different regimes: bulk and thin film at room temperature (RT) and high temperature (HT). While the saturation regime at RT is produced by the formation of a passivated TiO2 film at the surface of bulk and thick oxide films, the saturation is governed by the substrate/titanium film interactions in the case of thinner films. The stoichiometry of the obtained oxide films also depends on the oxidation process. We found only TiO2 for RT bulk oxidation, Ti2O3 and TiO2 for HT, and TiO and TiO2 for thin films. Heating the sample, once the RT saturation was achieved, produces additional oxidation (post-oxidation), but the stoichiometry of the oxide films remains unchanged. For the thinner films, the post-oxidation produces almost no effect.

Single Crystal EPR of the Mixed-Ligand Complex of Copper(II) with l-Glutamic Acid and 1,10-Phenanthroline: A Study on the Narrowing of the Hyperfine Structure by Exchange

We report an EPR study at X- and Q-bands of polycrystalline and single crystal samples of the mixed copper(II) complex with L-glutamic acid (glu) and 1,10-phenantroline (phen), [Cu(glu)(phen)(H(2)O)](+) NO(3)(-)·2(H(2)O). The polycrystalline sample spectrum at Q-band showed well resolved g(∥ )and g(⊥) features and partially solved hyperfine structure at g(∥), typical for weakly exchange coupled systems. This is confirmed from the angular variation of the EPR spectra which shows for certain magnetic field orientations a partially solved hyperfine structure characteristic of weak exchange, whereas a single Lorentzian line corresponding to strong exchange is observed for others. Analysis and simulation of the single crystal EPR spectra were performed using the random frequency modulation model of Anderson. Numerical simulations of the angular variation of the EPR spectra showed that the narrowing of the hyperfine structure is due to an exchange-mediated mechanism in which transitions between any pair of lines are equally likely. The exchange interaction responsible for this process is mediated by hydrophobic interactions between two phen molecules and a mixed chemical path of the type CuA-O(ap)H···O-C-O(eq)-CuB, for which we evaluated an average isotropic exchange parameter |J| ≈ 25 × 10(-4) cm(-1).

EPR spectroscopy and exchange interaction parameters in Cu(glycine)2·H2O

Abstract EPR measurements have been performed in single crystals of Cu(glycine) 2 ·H 2 O, at 9.7 and 33 GHz and at room temperature. this orthorhombic crystal has four chemically identical but magnetically non-equivalent copper ions per unit cell. The EPR data at both microwave frequencies show two partially resolved EPR lines for some of the field directions in the ab plane, while only a single line is observed along any direction in the other two crystallographic planes. This indicates that the four copper ions are arranged in two weakly coupled and symmetry-related subsets , each involving two non-equivalent and interacting copper sites. The decoupling of the spectra leads to g 1 = 2.256(2), g 2 = 2.071(2) and g 3 = 2.061(2) for the principal values of the molecular g -factors. The EPR data, used in conjunction with the classical Andersons model and the Kubo and Tomitas theory, allow to evaluate the exchange interaction parameters between the magnetically non-equivalent copper ions. The Andersons model and the observed EPR line positions data lead to | J ′/ k | = 19(3) mK for the intersubset exchange coupling. The analysis of the angular variation of the line width in terms o the Kubo and Tomitas theory, gives | J ′/ k | = 21(2) mK and | J / k | = 57(1) mK for the intersubset and intrasubset exchange couplings, respectively. These values are discussed in terms of the crystal structure of the compound and compared with the results obtained by other authors.

Auger electron spectroscopy analysis of the first stages of thermally stimulated oxidation of GaAs(100)

Abstract The first stages (exposures 4 L) of thermally stimulated oxidation of GaAs(100) have been studied using Auger electron spectroscopy and principal component analysis. We compare the GaAs oxidation processes taking place at high (700 K) and room temperatures, and during simultaneous electron bombardment and oxygen exposure. We found that while at room temperature, GaAs oxidizes via a one-phase process involving the simultaneous oxidation of Ga and As, the high temperature process is characterized by the presence of two different GaAs oxide phases. The first phase involves the simultaneous oxidation of Ga and As while in the second, only Ga oxides are formed. On the other hand, under simultaneous oxygen exposure and electron irradiation, two different oxide phases appear, both of them exhibiting the same features of the room temperature process, i.e., the simultaneous oxidation of Ga and As.

Oxide stoichiometry in the early stages of titanium oxidation at low pressure

Ti oxidation has been studied by using AES combined with PCA. The authors found three different compounds for an exposure of 60 L identified as Ti, TiO and TiO2. They also conclude that the formation of TiO2 starts at the expense of TiO.

Chemical changes of titanium and titanium dioxide under electron bombardment

The electron induced effect on the first stages of the titanium (Ti0) oxidation and titanium dioxide (Ti4+) chemical reduction processes has been studied by means of Auger electron spectroscopy. Using factor analysis we found that both processes are characterized by the appearance of an intermediate Ti oxidation state, Ti2O3 (Ti3+).

Magnetic Properties of Weakly Exchange-Coupled High Spin Co(II) Ions in Pseudooctahedral Coordination Evaluated by Single Crystal X-Band EPR Spectroscopy and Magnetic Measurements

We report single-crystal X-band EPR and magnetic measurements of the coordination polymer catena-(trans-(μ2-fumarato)tetraaquacobalt(II)), 1, and the Co(II)-doped Zn(II) analogue, 2, in different Zn:Co ratios. 1 presents two magnetically inequivalent high spin S = 3/2 Co(II) ions per unit cell, named A and B, in a distorted octahedral environment coordinated to four water oxygen atoms and trans coordinated to two carboxylic oxygen atoms from the fumarate anions, in which the Co(II) ions are linked by hydrogen bonds and fumarate molecules. Magnetic susceptibility and magnetization measurements of 1 indicate weak antiferromagnetic exchange interactions between the S = 3/2 spins of the Co(II) ions in the crystal lattice. Oriented single crystal EPR experiments of 1 and 2 were used to evaluate the molecular g-tensor and the different exchange coupling constants between the Co(II) ions, assuming an effective spin S′= 1/2. Unexpectedly, the eigenvectors of the molecular g-tensor were not lying along any preferential bond direction, indicating that, in high spin Co(II) ions in roughly octahedral geometry with approximately axial EPR signals, the presence of molecular pseudo axes in the metal site does not determine preferential directions for the molecular g-tensor. The EPR experiment and magnetic measurements, together with a theoretical analysis relating the coupling constants obtained from both techniques, allowed us to evaluate selectively the exchange coupling constant associated with hydrogen bonds that connect magnetically inequivalent Co(II) ions (|JAB(1/2)| = 0.055(2) cm(–1)) and the exchange coupling constant associated with a fumarate bridge connecting equivalent Co(II) ions (|JAA(1/2)| ≈ 0.25 (1) cm(–1)), in good agreement with the average J(3/2) value determined from magnetic measurements.