Brunetto Cortigiani

Magnetic Bistability in a Submonolayer of Sublimated Fe4 Single-Molecule Magnets

We demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations.

Epitaxy and alloying at the CoPt(111) interface: a study by X-ray photoelectron diffraction

Abstract Cobalt was deposited on the clean Pt(111) surface at room temperature by thermal evaporation. The overlayer growth mechanism and structure was examined by X-ray photoelectron diffraction, combined with X-ray photoelectron spectroscopy and low energy He+ scattering. During the initial stages of growth cobalt forms a uniform layer of a single atomic layer thickness. Further growth results in the formation of an epitaxial fcc cobalt multilayer. Annealing at moderate temperature causes the formation of a fcc CoPt alloy of variable thickness.

Enhanced Vapor-Phase Processing in Fluorinated Fe4 Single-Molecule Magnets

A new tetrairon(III) single-molecule magnet with enhanced volatility and processability was obtained by partial fluorination of the ancillary β-diketonato ligands. Fluorinated proligand Hpta = pivaloyltrifluoroacetone was used to assemble the bis(alkoxido)-bridged dimer [Fe2(OEt)2(pta)4] (1) in crystalline form, from which the new tetranuclear complex [Fe4(L)2(pta)6] (2) was synthesized in a one-pot reaction with H3L = 2-hydroxymethyl-2-phenylpropane-1,3-diol, NaOEt, and FeCl3 in a Et2O:EtOH solvent mixture. The structure of compound 2 was inferred from (1)H NMR, mass spectrometry, magnetic measurements, and DFT calculations. Direct current magnetic data are consistent with the expected metal-centered triangular topology for the iron(III) ions, with an antiferromagnetic coupling constant J = 16.20(6) cm(-1) between the central iron and the peripheral ones and consequent stabilization of an S = 5 spin ground state. Alternating current (ac) susceptibility measurements in 0 and 1 kOe static applied fields show the presence of a thermally activated process for magnetic relaxation, with τ0 = 2.3(1) 10(-7) s and U(eff)/kB = 9.9(1) K at zero static field and τ0 = 2.0(2) 10(-7) s and U(eff)/kB = 13.0(2) K at 1 kOe. At a pressure of 10(-7) mbar, compound 2 sublimates at (440 ± 5) K vs (500 ± 10) K for the nonfluorinated variant [Fe4(L)2(dpm)6] (Hdpm = dipivaloylmethane). According to XPS, ToF-SIMS, and ac susceptibility studies, the chemical composition, fragmentation pattern, and slow magnetic relaxation of the pristine material are retained in sublimated samples, suggesting that the molecular structure remains totally unaffected upon vapor-phase processing.

Lepidocrocite-like structure of the TiO2 monolayer grown on Ag(100)

Titanium oxide ultrathin films were grown on Ag(100) by evaporation of titanium in an O(2) atmosphere. The growth of the oxide films was monitored by means of XPS. The Ti2p XPS spectra indicate the formation of films with a TiO(2) stoichiometry in the whole range of coverages studied here. The STM results show that titania films appear to grow as islands of uniform thickness up to the completion of the first layer. Up to the formation of one complete monolayer, a (5 × 1) LEED pattern is observed. This pattern can be interpreted as a coincidence mesh and the lattice parameters of the oxide layer are very close to those of TiO(2) films with a lepidocrocite-like structure. However, the STM images show a long-range coincidence between the periodicity of the oxide film and that of the substrate along the short side of the oxide unit cell revealing that this lattice parameter is not exactly equal to that of the substrate. Above the monolayer coverage, additional spots become visible in the LEED pattern which can be interpreted in terms of the unit cell of rutile (110). The structural determination was carried out for the monolayer of titania by means of XPD and LEED intensity analysis. The results of these investigations demonstrate that the titania monolayer has a lepidocrocite-like structure. The DFT calculations carried out for the titania monolayer show the higher stability of the lepidocrocite structure with respect to other structures derived from crystallographic planes of bulk TiO(2) phases. Moreover, these calculations allow us to determine the oxide-substrate interface energy as well as to clarify the effect of the strain on the stability of the oxide layer.

Valence electronic structure of sublimated Fe4 single-molecule magnets: an experimental and theoretical characterization

The valence electronic structures of two single-molecule magnets (SMMs), [Fe4(L)2(dpm)6] and [Fe4(L)2(pta)6], (Hdpm = dipivaloylmethane, Hpta = pivaloyltrifluoroacetone, L3− = Ph–C(CH2O)33−), are investigated by means of ultraviolet photoemission spectroscopy (UPS) and ab initio calculations. The experimental UPS spectra of both compounds are analysed and compared with the total density of states (TDOS) computed with the hybrid functional PBE0. The substitution of half of the methyl groups in [Fe4(L)2(dpm)6] with fluorine atoms in [Fe4(L)2(pta)6] unexpectedly affects the spectrum shape in the Fermi region, thus becoming a useful fingerprint of the two SMMs. Moreover, a computational protocol at DFT + U level of theory is assessed on both compounds, which is in good agreement with the experimental spectroscopic and magnetic data. The basis for the future modelling of the adsorption of Fe4 clusters on surfaces is established.

STM study of the nanostructures prepared by deposition of NiO on Ag(001)

Submonolayers of nickel oxide were grown on the Ag(001) surface by evaporation of Ni in the presence of O2 at a pressure of 1 ? 10?6?mbar. In the early stages of deposition, two-dimensional oxide islands with a (2 ? 1) periodicity grow on Ag(001). After annealing at about 500?K, the (2 ? 1) single atomic layer transforms into NiO(001) double layer islands. The 1:1 stoichiometry of the film is not influenced by whether the annealing is carried out in vacuum or in the presence of oxygen. However, the oxide islands exhibit different morphologies depending on the O2 pressure during the annealing process.

Ultrathin nickel oxide films grown on Ag(0 0 1): a study by XPS,LEIS and LEED intensity analysis

The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was prepared by evaporating Ni in presence of O2 at a pressure in the 10 � 6 mbar range. The growth of the oxide film was followed by XPS,LEIS and LEED. In the early stages of deposition,the film shows a (2 � 1) superstructure in LEED. After deposition of 2 ML of NiO,a sharp (1 � 1) LEED pattern is observed. The intensity versus electron energy curves of the LEED spots were measured for this NiO(1 � 1) film and analysed by means of the tensor LEED method. A good level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was obtained. We found that oxygen atoms at the oxide–substrate interface are on-top silver atoms. The interlayer distance in the oxide does not differ significantly from that in bulk NiO(0 0 1),within the accuracy of the analysis. An outward displacement (0.05 � 0.05 � of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The interlayer distance at the silver–nickel oxide interface is 2.43 � 0.05 � 2003 Elsevier Science B.V. All rights reserved.

Epitaxial growth of TiO2 films with the rutile (110) structure on Ag(100)

Ultrathin films of TiO(2) were grown on Ag(100) by evaporation of titanium in the presence of O(2) at a pressure in the 10(-4) Pa range and annealing at 770 K. The composition of the deposited films was monitored by XPS and LEIS. The morphology at the nanometric scale of the TiO(2) films and their crystallographic structure were investigated by means of STM, LEED and XPD. Above the monolayer coverage (at which the oxide film has a lepidocrocite-like structure), STM images show the formation of multilayer islands with a distribution of heights. XPD results indicate that these oxide islands have the rutile (110) structure and are epitaxially oriented with the sides of the oxide unit cell parallel to those of the substrate unit cell. The results of the DFT calculations justify the 3D growth of rutile (110) on Ag(100). The calculated strain energy required to match the metal substrate can explain the incommensurate growth of the overlayer in the direction of the long side of the oxide unit cell. The results of the calculations indicate that a commensurate growth of rutile (110) may be possible along the short side of the oxide unit cell, taking into account the relatively small strain energy to fit the lattice parameter of the substrate. The DFT calculations predict a considerable increase of the work function upon deposition of titania films on Ag(100), which can be attributed to a charge transfer from the metal to the 3d Ti empty states.

Low-Temperature Magnetic Force Microscopy on Single Molecule Magnet-Based Microarrays

The magnetic properties of some single molecule magnets (SMM) on surfaces can be strongly modified by the molecular packing in nanometric films/aggregates or by interactions with the substrate, which affect the molecular orientation and geometry. Detailed investigations of the magnetism of thin SMM films and nanostructures are necessary for the development of spin-based molecular devices, however this task is challenged by the limited sensitivity of laboratory-based magnetometric techniques and often requires access to synchrotron light sources to perform surface sensitive X-ray magnetic circular dichroism (XMCD) investigations. Here we show that low-temperature magnetic force microscopy is an alternative powerful laboratory tool able to extract the field dependence of the magnetization and to identify areas of in-plane and perpendicular magnetic anisotropy in microarrays of the SMM terbium(III) bis-phthalocyaninato (TbPc2) neutral complex grown as nanosized films on SiO2 and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and this is in agreement with data extracted from nonlocal XMCD measurements performed on homogeneous TbPc2/PTCDA films.

Chloride formation and photoreduction on the Cu(100) surface. A study by X-ray photoelectron spectroscopy and low energy ion scattering

Abstract Multi-atomic chloride layers are prepared on the clean Cu(100) surface by exposure to Cl 2 at 160 K at pressures of the order of 10 −3 Pa. The layer stoichiometry determined by XPS measurements corresponds to CuCl. Bringing the sample to room temperature, the layer undergoes a structural transformation where thicker islands are formed, while a fraction of the substrate surface becomes free. Exposure to radiation in the soft X-rays domain causes the photoreduction of the chloride layer and the formation of metallic copper. This metallic copper is located near or over the surface of the chloride.