U. del Pennino

Grafting Derivatives of Mn6 Single-Molecule Magnets with High Anisotropy Energy Barrier on Au(111) Surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.

Oxidation behaviour of nickel silicides investigated by AES and XPS

The oxygen interaction with Ni silicide surfaces in a wide range of composition, from NiSi2 to Ni3Si, has been investigated at room temperature by AES and XPS techniques. It has been found that the Si oxidation is enhanced over that of pure Si in all the Ni-Si compounds, and that the strongest oxidation occurs in Ni richer silicides. In general, oxygen bonds with Si leaving the metal unoxidized, but in Ni3Si evidence for the occurrence of Ni oxidation has been found at high exposures.

Volta effect and liquidlike layer at the ice surface

The Volta potential difference between Ih ice single crystals and different metals has been measured as a function of the temperature by using the vibrating electrode technique. Reproducible results have been obtained with ice–gold and ice–copper contacts, indicating that in these cases the equilibrium has been attained. At temperatures below −30 °C the dependence is linear, and we think that this is the effect of the impurity content on the ice Fermi level. At higher temperatures a term which is logarithmic in (Tm−T), where Tm is the melting temperature of ice, is added. This additional pd has proved to be localized at the ice surface and ranges from 0 to 150 mV; it can be directly related to the appearance and thickening of the polarized, liquidlike layer predicted by N. H. Fletcher, if we assume that a constant electric field exists in the layer interior. The preferred dipole orientation turns out to be that with the oxygens outwards. We also obtained the approximate value of 4.3 eV for the ice work fu...

First spectroscopic investigation of the Yb/Si interface at room temperature

We report the results of a preliminary synchrotron radiation photoemission and Auger experiment on the Yb/Si interface, prepared in situ by depositing monolayers of Yb onto vacuum‐cleaved Si(111) surfaces at room temperature. Spectra were taken for the Si 2p and Yb 5p core levels at several different values of escape depth. The evolution of the valence band and the Si L2,3 VV Auger line shape with coverage also were depth examined. The first monolayer of Yb is adsorbed onto the Si surface without apparent mixing or reaction. For 1.5–2 monolayers, there is evidence of chemical interaction from core level shifts and changes in the Si LVV line shape. At higher coverages, the surface becomes essentially Yb metal; the region of intermixing appears to be limited to 2–3 atomic layers at room temperature.

Els investigation of the Si-Pd interface

Abstract Energy loss and Auger spectra of Pd deposited onto Si(111) at increasing coverages are presented, together with those of pure Pd and of bulk Pd 2 Si obtained in situ. The loss evolution with coverage is interpreted with the aid of a simple model calculation of the surface loss function of an ideal Si-Pd system. The results indicate that for coverages between 1 and 7 A an intermixed phase forms at the interface, while for higher coverages pure Pd grows on the surface. In the first stage some losses seem to shift in energy with coverage due to interface mode coupling.

Oxygen chemisorption and oxide formation on Ni silicide surfaces at room temperature

Abstract Auger spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used in a comparative study of the room temperature oxidation of Ni silicides of increasing silicon content, from Ni 3 Si to NiSi 2 . The results were compared with those for the oxidation of pure Si and Ni. All suicide surfaces in the exposure range between 0.2 and 10 4 L follow two-step oxidation kinetics: the first step is characterized by an oxygen uptake rate higher than in the second one. Attention was focused on the oxygen induced modifications of metal and silicon AES and XPS spectra in silicides, which are indicative of changes in the local electronic structure and in the chemical bonding. In general oxygen bonds with silicon leaving the metal unaffected; however, at high exposures, characteristic feature of the Ni-oxygen bonds appear in the Ni(MVV) Auger line of the Ni-rich silicides. The presence of Ni atoms enhances considerably the Si oxidation process in silicides with respect to pure Si, in terms both of a higher Si oxidation state and a higher oxygen uptake; this enhancement is stronger in Ni-rich silicides than in Si-rich silicides. The oxygen induced contributions in the Si(LVV) Auger line show structures at 76 and 83 eV, and those in the Si 2p photoemission spectra show binding energy shifts between −1 and −3.8 eV; we conclude that the oxidation products are mainly silicon suboxides, like Si 2 O 3 and SiO; only on Ni 3 Si at 10 4 L, a significant contribution of SiO 2 was found. The Ni catalytic effect on Si oxidation has been discussed in terms of the suicide heat of formation, of the breaking of the silicon sp3 configuration in silicides and of the metal atom dissociative effect on the O 2 molecule.

AES study of room temperature oxygen interaction with near noble metal-silicon compound surfaces

Abstract We have carried out a comparative study of room temperature (RT) oxidation of near noble metal silicides similar in stoichiometry (M2Si) and electronic structure. Core-valence-valence (CVV) Auger line measurements on Ni2Si, Pd2Si and Pt2Si surfaces before and after exposure to 104 L of oxygen were performed. We compare the results with those for the oxidation of pure Si. In general oxygen interacts with Si atoms only. In Ni2Si, however, features ascribed to the onset of oxidation of Ni atoms appear in the Ni(MVV) line. In the Ni2Si and Pd2Si case, the Si reaction rate is increased with respect to that of pure Si, the strongest oxidation enhancement being obtained in Ni2Si. However, Si cannot be oxidized to SiO2 in these conditions; a SiOx (x

Azimuthal dependence of reflection high resolution electron energy loss of Si(111)(2×1)

Abstract High Resolution Electron Energy Loss Spectroscopy has been used, with low energy of the primary beam and azimuthal resolution, to study the anisotropy of surface dielectric properties of Si(111)(2 × 1), in the range of the surface electronic excitations. By eliminating the effect of the kinematic prefactor, we are able to obtain from the data the surface Loss Function. Its dependence on q ∥ and ω is discussed in term of a model of surface dielectric function.

The Si(111)–Pd interface: Spectroscopic evidence of chemical processes at liquid nitrogen temperature

We present the results of angle integrated photoelectron spectroscopy (hν=21.2 eV) on Si(111)–Pd interfaces. We have found that some intermixing between Si and Pd takes place also when the interface is prepared at liquid nitrogen temperature (LNT). After preparation at LNT the intermixing increases at room temperature (RT). The results are discussed in connection with the interface formation mechanism.

Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate.

Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.