Mixed-Sandwich Titanium(III) Qubits on Au(111): Electron Delocalization Ruled by Molecular Packing.

Organometallic sandwich complexes are versatile molecular systems that have been recently employed for single-molecule manipulation and spin sensing experiments. Among related organometallic compounds, the mixed-sandwich S = 1/2 complex (η8-cyclooctatetraene)(η5-cyclopentadienyl)titanium, here [CpTi(cot)], has attracted interest as a spin qubit because of the long coherence time. Here the structural and chemical properties of [CpTi(cot)] on Au(111) are investigated at the monolayer level by experimental and computational methods. Scanning tunneling microscopy suggests that adsorption occurs in two molecular orientations, lying and standing, with a 3:1 ratio. XPS data evidence that a fraction of the molecules undergo partial electron transfer to gold, while our computational analysis suggests that only the standing molecules experience charge delocalization toward the surface. Such a phenomenon depends on intermolecular interactions that stabilize the molecular packing in the monolayer. This orientation-dependent molecule-surface hybridization opens exciting perspectives for selective control of the molecule-substrate spin delocalization in hybrid interfaces.

Quantum dynamics of a single molecule magnet on superconducting Pb(111).

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(III) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

Substrate mediated interaction of terbium(III) double-deckers with the TiO2(110) surface.

A terbium(iii)-bis(phthalocyaninato) neutral complex was deposited on the rutile TiO2(110) surface, and their interaction was studied by Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the TiO2 rutile surface favours the adsorption of isolated molecules adopting a lying down configuration with the phthalocyanine planes tilted by about 30° when they lie in the first layer. The electronic and chemical properties of the molecules on the surface were studied by XPS as a function of the TiO2(110) substrate preparation. This study evidences that strong molecule-substrate interactions are present and a charge transfer process occurs from the molecule to the surface.

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.

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.

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 ß-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.

Magnetic and spectroscopic investigation of thermally and optically driven valence tautomerism in thioether-bridged dinuclear cobalt-dioxolene complexes.

A series of dinuclear cobalt complexes of general formula [Co(Mentpa)(diox-S-diox)Co(Mentpa)](PF6)2·MeOH (n = 0, 2, 3) was prepared through the synthesis of the bis-bidentate ligand 6,6'-((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(3,5-di-tert-butyl-benzene-1,2-diol) (diox-S-diox). The ancillary ligands Mentpa are obtained by the tripodal tris(2-pyridylmethyl)amine (tpa) ligand through successive introduction of methyl groups into the 6 position of the pyridine moieties. As expected, the steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, determining the charge distribution of the metal-dioxolene adduct at room temperature. Magnetic measurements and X-ray photoelectron and X-ray absorption spectroscopies indicate that the charge distributions low-spin-Co(III)-catecholate and high-spin-Co(II)-semiquinonate characterize the complexes formed by the tpa and Me3tpa tetradentate ligands, respectively. The complex formed by the Me2tpa ligand undergoes a thermal- and light-induced interconversion of the two states, in agreement with the existence of a valence tautomeric equilibrium. All complexes were stable and behaved reproducibly under X-ray irradiation. This work points out a fast and simple chemical approach to structurally and electronically modify the catechol ring while leaving its coordination capabilities unaffected. These findings afford a robust chemical method to prepare sulfur-functionalized dioxolene ligands as new molecular bricks for chemical functionalization of noble metal surfaces with this class of molecular switches.

Magnetic molecules as local sensors of topological hysteresis of superconductors.

Superconductors and magnetic materials, including molecules, are key ingredients for quantum computing and spintronics. However, only a little is known about how these materials interact in multilayer nanostructures like the hybrid architectures nowadays under development for such advanced applications. Here, we show that a single layer of magnetic molecules, Terbium(III) bis-phthalocyaninato (TbPc2) complexes, deposited under controlled UHV conditions on a superconducting Pb(111) surface is sensitive to the topology of the intermediate state of the superconductor, namely to the presence and evolution of superconducting and normal domains due to screening and penetration of an external magnetic field. The topological hysteresis of the superconducting substrate imprints a local evolution of the magnetisation of the TbPc2 molecules in the monolayer. Element and surface selective detection is achieved by recording the X-ray magnetic circular dichroism of the Tb atoms. This study reveals the impressive potential of magnetic molecules for sensing local magnetic field variations in molecular/superconductor hybrid devices, including spin resonators or spin injecting and spin filtering components for spintronics applications.

Lepidocrocite-like structure of the TiO(2) 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.

Unraveling the mechanism of the one-pot synthesis of exchange coupled Co-based nano-heterostructures with a high energy product.

The development of reproducible protocols to synthesize hard/soft nano-heterostructures (NHSs) with tailored magnetic properties is a crucial step to define their potential application in a variety of technological areas. Thermal decomposition has proved to be an effective tool to prepare such systems, but it has been scarcely used so far for the synthesis of Co-based metal/ferrite NHSs, despite their intriguing physical properties. We found a new approach to prepare this kind of nanomaterial based on a simple one-pot thermal decomposition reaction of metal-oleate precursors in the high boiling solvent docosane. The obtained NHSs are characterized by the coexistence of Co metal and Co doped magnetite and are highly stable in an air atmosphere, thanks to the passivation of the metal with a very thin oxide layer. The investigation of the influence of the metal precursor composition (a mixed iron-cobalt oleate), of the ligands (oleic acid and sodium oleate) and of the reaction time on the chemical and structural characteristics of the final product, allowed us to rationalize the reaction pathway and to determine the role of each parameter. In particular, the use of sodium oleate is crucial to obtain a metal phase in the NHSs. In such a way, the one-pot approach proposed here allows the fine control of the synthesis, leading to the formation of stable, high performant, metal/ferrite NHSs with tailored magnetic properties. For instance, the room temperature maximum energy product was increased up to 19 kJ m-3 by tuning the Co content in the metal precursor.

Surface effects on a photochromic spin-crossover iron(ii) molecular switch adsorbed on highly oriented pyrolytic graphite.

Thin films of an iron(ii) complex with a photochromic diarylethene-based ligand and featuring a spin-crossover behaviour have been grown by sublimation in ultra-high vacuum on highly oriented pyrolytic graphite and spectroscopically characterized through high-resolution X-ray and ultraviolet photoemission, as well as via X-ray absorption. Temperature-dependent studies demonstrated that the thermally induced spin-crossover is preserved at a sub-monolayer (0.7 ML) coverage. Although the photochromic ligand ad hoc integrated into the complex allows the photo-switching of the spin state of the complex at room temperature both in bulk and for a thick film on highly oriented pyrolytic graphite, this photomagnetic effect is not observed in sub-monolayer deposits. Ab initio calculations justify this behaviour as the result of specific adsorbate-substrate interactions leading to the stabilization of the photoinactive form of the diarylethene ligand over photoactive one on the surface.

Enhanced hydrogen photogeneration by bulk g-C3N4 through a simple and efficient oxidation route.

The search for new environment-friendly visible-light absorbing catalysts is an urgent task. g-C3N4 has excellent photocatalytic properties and the possibility of developing cost-effective routes to make this material a viable alternative to the currently used catalysts is required. In this work, we show that a simple chemical oxidation process of g-C3N4 with nitric acid allowed significantly enhancing the hydrogen photogeneration from aqueous triethanolamine, under simulated solar light. An 8-fold improvement of the H2 production, with respect to the pristine sample, was achieved by properly controlling the physical-chemical parameters of the oxidation process, reaching a value of about 4000 µmol g-1 h-1, which is one of the highest hydrogen production rates for bulk g-C3N4. Such high levels of photocatalytic activity result from the combination of improved surface area and changes in the electronic structure induced by the oxidation process.

Room temperature amine sensors enabled by sidewall functionalization of single-walled carbon nanotubes.

A new series of sidewall modified single-walled carbon nanotubes (SWCNTs) with perfluorophenyl molecules bearing carboxylic acid or methyl ester moieties are herein reported. Pristine and functionalized SWCNTs (p-SWCNTs and f-SWCNTs, respectively) were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM). The nitrene-based functionalization provided intact SWCNTs with methyl 4-azido-2,3,5,6-tetrafluorobenzoate (SWCNT-N-C6F4CO2CH3) and 4-azido-2,3,5,6-tetrafluorobenzoic acid (SWCNT-N-C6F4CO2H) attached every 213 and 109 carbon atoms, respectively. Notably, SWCNT-N-C6F4CO2H was sensitive in terms of the percentage of conductance variation from 5 to 40 ppm of ammonia (NH3) and trimethylamine (TMA) with a two-fold higher variation of conductance compared to p-SWCNTs at 40 ppm. The sensors are highly sensitive to NH3 and TMA as they showed very low responses (0.1%) toward 200 ppm of volatile organic compounds (VOCs) containing various functional groups representative of different classes of analytes such as benzene, tetrahydrofurane (THF), hexane, ethyl acetate (AcOEt), ethanol, acetonitrile (CH3CN), acetone and chloroform (CHCl3). Our system is a promising candidate for the realization of single-use chemiresistive sensors for the detection of threshold crossing by low concentrations of gaseous NH3 and TMA at room temperature.

Thermal and optical control of electronic states in a single layer of switchable paramagnetic molecules.

The control of the charge distribution in a monolayer of switchable cobalt-dioxolene complexes undergoing Valence Tautomerism (VT) has been achieved by means of thermal and optical stimuli. Thiol-derivatised VT molecules have been grafted on polycrystalline gold surface as monolayers from solution. X-ray photoelectron spectroscopy and time-of-flight secondary ions mass spectrometry evidenced the formation of a covalent bond between intact VT molecules and the surface and excluded the presence of physisorbed molecules. X-ray absorption spectroscopy revealed that the temperature- and light-induced conversion profiles of the monolayer closely reproduce the ones found for the crystalline phase. This study demonstrates that a wet chemistry based approach allows to transfer switchable paramagnetic molecules at the nanoscale, widening the playground to develop new hybrid molecular based architectures for novel technologies.

UHV deposition and characterization of a mononuclear iron(III) ß-diketonate complex on Au(111).

The adsorption of the sterically hindered ß-diketonate complex Fe(dpm)3, where Hdpm = dipivaloylmethane, on Au(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunnelling microscopy (STM). The high volatility of the molecule limited the growth of the film to a few monolayers. While UPS evidenced the presence of the ß-diketonate ligands on the surface, the integrity of the molecule on the surface could not be assessed. The low temperature STM images were more informative and at submonolayer coverage they showed the presence of regular domains characterized by a flat morphology and height of ≈0.3 nm. Along with these domains, tetra-lobed features adsorbed on the kinks of the herringbone were also observed. DFT-simulated images of the pristine molecule and its possible decomposition products allowed to assess the partial fragmentation of Fe(dpm)3 upon adsorption on the Au(111) surface. Structural features with intact molecules were only observed for the saturation coverage. An ex situ prepared thick film of the complex was also investigated by X-ray magnetic circular dichroism (XMCD) and features typical of high-spin iron(III) in octahedral environment were observed.

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.