Friction characteristics of graphene deposited on different substrates have been studied by atomic force microscopy (AFM). In particular, we compared mechanically exfoliated graphene transferred over ...Si SiO2 with respect to monolayer (ML) graphene grown in our laboratory by low temperature chemical vapor deposition on Ni(111) single crystal. Friction force measurements by AFM have been carried out as function of load under different environment conditions, namely vacuum (10−5 Torr), nitrogen and air. The typical decrease of friction force with increasing number of layers has been observed on graphene over Si SiO2 in all environment including vacuum. Continuum mechanical approximation has been used to analyze the friction versus load curves of ML graphene on Ni(111). Analysis shows that Derjaguin-Mueller-Toporov model is in good agreement with our experimental data indicating that overall behavior of the interface graphene-Ni(111) is relatively rigid respect to out of plane deformations. This result is consistent with the structural characteristics of the interface since graphene grows in registry with Ni(111) surface with covalent bonding character. Finally, the shear strength and the work of adhesion of the two systems with respect to AFM tip in vacuum have been compared. The result of this procedure indicates that shear strength and work of adhesion measured on graphene-Si SiO2 interface are always greater than those on graphene-Ni(111) interface.
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.
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Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic ...properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour.
The ultrahigh vacuum adsorption of cysteine layers on the Au(111) surface has been studied by means of X-ray photoelectron (XPS) and high-resolution energy loss spectroscopies (HREELS). ...Room-temperature deposition determined the formation of a quite heterogeneous first layer, where both weakly and strongly bound molecules coexist. Deposition at a slightly higher temperature (330 K) led instead to the formation of a homogeneous, self-assembled monolayer made of molecules chemisorbed through a thiolate bond. In the latter case, HREELS measurements have been interpreted in terms of a well-organized H-bond network made of zwitterionic molecules. Two vibrational modes, denoted as N and H modes, respectively, have been identified as distinguishing features of the homogeneous monolayer obtained at 330 K. The N mode lies at 3350 cm−1 and is attributed to a stretching vibration of the N−H···O bond. The H mode, observed at 74 cm−1 for full monolayer coverage, is assigned to a collective vibration of the two-dimensional H-bond network. At half-monolayer coverage, the H mode has been observed at 55 cm−1. This red-shift indicates a coverage dependence of the H-mode frequency, which clearly supports its intermolecular origin. This finding is a nice example of the extreme sensitivity of low-frequency vibrational modes to the details of molecule−molecule interactions.
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Producing wine entails the consumption of large quantities of fresh water, and consequently the production of large quantities of wastewater the treatability of which becomes increasingly difficult ...as purification standards get stringent, year after year. The problem is in the presence of aldehydes, phenolic compounds and detergents which are not easily broken down by the microorganisms of ordinary biological treatment. Wastewaters from wineries command high levels of Chemical Oxygen Demand (COD). In recent years research has focused on using Advanced Oxidation Processes (AOPs) as a means to abate the concentration of organic compounds and polluting contaminants, to render the wastewater suitable for ensuing biological treatment. The object of this study was to investigate how effective a particular type of AOP, the Fenton's reaction, could be as a pre-treatment step aimed at destroying the toxic and biologically stable organic contaminants to reduce the organic load on the ensuing biological treatment of the wastewater. In particular, our experiments showed that if oxidation with Fenton's reagents was followed by neutralization with lime and precipitation with bentonite, the COD reduction could be as high as 54%. Experimental tests were designed according to a 23 full factorial plan. Elaboration of results led to a model of COD reduction as a function of two significant factors. Finally, the flowsheet and the mass balance of the proposed process were defined.
The electronic properties of a prototype system suitable for dye-sensitized solar cell applications are investigated both experimentally and theoretically by means of electron spectroscopies ...(high-resolution electron energy loss spectroscopy, HREELS, and ultraviolet and X-ray photoemission spectroscopies, UPS and XPS) and first-principles density functional theory (DFT)-based calculations. The comparison of HREELS and UPS data with the DFT results allows the microscopic description of electronic structure modifications upon interface formation, and provides a quantitative evaluation of the ionization energy and electron affinity changes induced by functionalization: these variations can be associated to the electric dipole of the functional species and, thus, to the formation of an interface dipole layer.
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We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn6 family (general formula MnIII 6O2(R-sao)6(O2C-th)2L4−6, 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 MnIII 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 Mn6 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.
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The preparation, magnetic characterization, and X-ray structures of two polycationic Mn12 single-molecule magnets Mn12O12(bet)16(EtOH)4(PF6)14·4CH3CN·H2O (1) and ...Mn12O12(bet)16(EtOH)3(H2O)(PF6)13(OH)·6CH3CN·EtOH·H2O (2) (bet = betaine = (CH3)3N+−CH2−CO2 −) are reported. 1 crystallizes in the centrosymmetric P2/c space group and presents a (0:2:0:2) arrangement of the EtOH molecules in its structure. 2 crystallizes in the noncentrosymmetric P4̅ space group with two distinct Mn12 polycations, Mn12O12(bet)16(EtOH)2(H2O)214+ (2A) and Mn12O12(bet)16(EtOH)414+ (2B) per unit cell. 2A and 2B show a (1:1:1:1) distribution of the coordinated solvent molecules. Interestingly, bond valence sum calculations extracted from X-ray diffraction data indicate the presence of two Mn2+ ions in the Mn12 core for both 1 and 2. This finding is confirmed by X-ray absorption spectroscopy (XAS) measurements. A complete magnetic characterization, including subkelvin micro-SQUID magnetometry and inelastic neutron scattering (INS) measurements, permits to extract the parameters of the giant spin Hamiltonian of these polycations. Compared with the archetypal Mn12 acetate, an increase in the value of the ground spin state from S = 10 to S = 11 together with a decrease in the effective energy barrier, is observed for 1 and 2. Such a result is consistent with the reduction of two Mn3+ to the less anisotropic Mn2+ ion in the structures.
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The (4
×
2) epitaxial InAs(0
0
1) surface grown by molecular beam epitaxy and subjected to different surface treatments, namely amorphous As-de-capping and ion-bombardment annealing (IBA), is ...investigated by high-resolution angular-resolved UV photoelectron spectroscopy. Both treatments produce a semiconducting surface, ruling out the presence of metallic In aggregates. Binding energy shifts of 0.2–0.3
eV are measured for the valence-band levels of the IBA surface with respect to the de-capped surface, implying an important influence of the surface treatment on the subsurface region. The line-shape of the In-4d core levels, which consists of two different In-related surface doublets, is discussed in view of the recently proposed structural models based on dimers formation.
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