Mathieu Gonidec

Surface Supramolecular Organization of a Terbium(III) Double-Decker Complex on Graphite and its Single Molecule Magnet Behavior

The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. Molecular dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experiment, showing that-after diffusion on the graphite surface-the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface.

A Liquid‐Crystalline Single‐Molecule Magnet with Variable Magnetic Properties

This work was supported by the Marie Curie EST FuMaSSEC, EU NoE MAGMANet (515767-2), EMOCIONA (CTQ2006-06333/BQU), Grup de Recerca de Catalunya (2009 SGR-516) and Molecular Nanoscience (CSD2007-00010) and Molchip (MAT2009-13977-C03) projects.

Probing the Magnetic Properties of Three Interconvertible Redox States of a Single-Molecule Magnet with Magnetic Circular Dichroism Spectroscopy

The hysteresis of magnetization of the anionic, neutral, and cationic forms of a bis(phthalocyaninato)terbium-based complex ([Pc(2)Tb](-/0/+)) have been determined using magnetic circular dichroism (MCD) spectroscopy in frozen dilute solutions at low temperatures (1.5 K) showing that the three oxidation states of the complex exhibit single-molecule magnetic behaviors.

Engineering shadows to fabricate optical metasurfaces.

Optical metasurfaces-patterned arrays of plasmonic nanoantennas that enable the precise manipulation of light-matter interactions-are emerging as critical components in many nanophotonic materials, including planar metamaterials, chemical and biological sensors, and photovoltaics. The development of these materials has been slowed by the difficulty of efficiently fabricating patterns with the required combinations of intricate nanoscale structure, high areal density, and/or heterogeneous composition. One convenient strategy that enables parallel fabrication of periodic nanopatterns uses self-assembled colloidal monolayers as shadow masks; this method has, however, not been extended beyond a small set of simple patterns and, thus, has remained incompatible with the broad design requirements of metasurfaces. This paper demonstrates a technique-shadow-sphere lithography (SSL)-that uses sequential deposition from multiple angles through plasma-etched microspheres to expand the variety and complexity of structures accessible by colloidal masks. SSL harnesses the entire, relatively unexplored, space of shadow-derived shapes and-with custom software to guide multiangled deposition-contains sufficient degrees of freedom to (i) design and fabricate a wide variety of metasurfaces that incorporate complex structures with small feature sizes and multiple materials and (ii) generate, in parallel, thousands of variations of structures for high-throughput screening of new patterns that may yield unexpected optical spectra. This generalized approach to engineering shadows of spheres provides a new strategy for efficient prototyping and discovery of periodic metasurfaces.

Highly Reduced Double-Decker Single-Molecule Magnets Exhibiting Slow Magnetic Relaxation

F64Pc2Ln (1Ln, Ln = Tb or Lu) represent the first halogenated phthalocyanine double-decker lanthanide complexes, and 1Tb exhibits single-molecule magnet properties as revealed by solid-state magnetometry. The fluorine substituents of the phthalocyanine rings have a dramatic effect on the redox properties of the F64Pc2Ln complexes, namely, a stabilization of their reduced states. Electrochemical and spectroelectrochemical measurements demonstrate that the 1Tb(-/2-) and 1Tb(2-/3-) couples exhibit redox reversibility and that the 1Tb(-), 1Tb(2-) and 1Tb(3-) species may be prepared by bulk electrolysis in acetone. Low-temperature MCD studies reveal for the first time magnetization hystereses for the super-reduced dianionic and trianionic states of Pc2Ln.

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

This paper describes charge transport by tunneling across self-assembled monolayers (SAMs) of thiol-terminated derivatives of oligo(ethylene glycol) (HS(CH2CH2O)nCH3; HS(EG)nCH3); these SAMs are positioned between gold bottom electrodes and Ga2O3/EGaIn top electrodes. Comparison of the attenuation factor (β of the simplified Simmons equation) across these SAMs with the corresponding value obtained with length-matched SAMs of oligophenyls (HS(Ph)nH) and n-alkanethiols (HS(CH2)nH) demonstrates that SAMs of oligo(ethylene glycol) have values of β (β(EG)n = 0.29 ± 0.02 natom-1 and β = 0.24 ± 0.01 Å-1) indistinguishable from values for SAMs of oligophenyls (β(Ph)n = 0.28 ± 0.03 Å-1), and significantly lower than those of SAMs of n-alkanethiolates (β(CH2)n = 0.94 ± 0.02 natom-1 and 0.77 ± 0.03 Å-1). There are two possible origins for this low value of β. The more probable involves hole tunneling by superexchange, which rationalizes the weak dependence of the rate of charge transport on the length of the molecules of HS(EG)nCH3 using interactions among the high-energy, occupied orbitals associated with the lone-pair electrons on oxygen. Based on this mechanism, SAMs of oligo(ethylene glycol)s are good conductors (by hole tunneling) but good insulators (by electron and/or hole drift conduction). This observation suggests SAMs derived from these or electronically similar molecules are a new class of electronic materials. A second but less probable mechanism for this unexpectedly low value of β for SAMs of S(EG)nCH3 rests on the possibility of disorder in the SAM and a systematic discrepancy between different estimates of the thickness of these SAMs.

Looking Inside the Perchlorinated Trityl Radical/Metal Spinterface through Spectroscopy

We report on a spectroscopic multitechnique approach to study the metal/radical spinterface formed by a perchlorinated trityl radical derivative and either gold or silver. The spectroscopic fingerprint of their paramagnetic properties could be determined by comparison with their diamagnetic precursor and by DFT calculations. Thanks to the presented approach, we could gain unprecedented insight into the radical-metal interaction and how this latter perturbs the spin polarization and consequently the magnetoelectronic properties of the radical adlayer. Knowledge of the factors influencing the spinterface is an essential tool toward the tailoring of the properties of spin-based electronic devices.

Charge Tunneling along Short Oligoglycine Chains

This work examines charge transport (CT) through self-assembled monolayers (SAMs) of oligoglycines having an N-terminal cysteine group that anchors the molecule to a gold substrate, and demonstrate that CT is rapid (relative to SAMs of n-alkanethiolates). Comparisons of rates of charge transport-using junctions with the structure Au(TS)/SAM//Ga2O3/EGaIn (across these SAMs of oligoglycines, and across SAMs of a number of structurally and electronically related molecules) established that rates of charge tunneling along SAMs of oligoglycines are comparable to that along SAMs of oligophenyl groups (of comparable length). The mechanism of tunneling in oligoglycines is compatible with superexchange, and involves interactions among high-energy occupied orbitals in multiple, consecutive amide bonds, which may by separated by one to three methylene groups. This mechanistic conclusion is supported by density functional theory (DFT).

Combining Step Gradients and Linear Gradients in Density.

Combining aqueous multiphase systems (AMPS) and magnetic levitation (MagLev) provides a method to produce hybrid gradients in apparent density. AMPS—solutions of different polymers, salts, or surfactants that spontaneously separate into immiscible but predominantly aqueous phases—offer thermodynamically stable steps in density that can be tuned by the concentration of solutes. MagLev—the levitation of diamagnetic objects in a paramagnetic fluid within a magnetic field gradient—can be arranged to provide a near-linear gradient in effective density where the height of a levitating object above the surface of the magnet corresponds to its density; the strength of the gradient in effective density can be tuned by the choice of paramagnetic salt and its concentrations and by the strength and gradient in the magnetic field. Including paramagnetic salts (e.g., MnSO4 or MnCl2) in AMPS, and placing them in a magnetic field gradient, enables their use as media for MagLev. The potential to create large steps in density with AMPS allows separations of objects across a range of densities. The gradients produced by MagLev provide resolution over a continuous range of densities. By combining these approaches, mixtures of objects with large differences in density can be separated and analyzed simultaneously. Using MagLev to add an effective gradient in density also enables tuning the range of densities captured at an interface of an AMPS by simply changing the position of the container in the magnetic field. Further, by creating AMPS in which phases have different concentrations of paramagnetic ions, the phases can provide different resolutions in density. These results suggest that combining steps in density with gradients in density can enable new classes of separations based on density.