Aimant monomoléculaire | EPFL Graph Search

Un aimant monomoléculaire ou nano-aimant moléculaire, appelé aussi SMM, de l'acronyme anglais Single Molecule Magnet, est une molécule faisant partie des composés de coordination qui a un comportement superparamagnétique : c'est un aimant uniquement en dessous d'une certaine température dite de blocage. Les aimants monomoléculaires sont des macromolécules, c'est-à-dire composés de 100 à atomes. Bien que découverts en 1993, nommés en 1996, l'idée du premier aimant monomoléculaire ( Mn_{12} ) fut décrite en 1980. Ils présentent quelques centres magnétiques couplés et isolés de l’environnement extérieur par des ligands volumineux (souvent des ligands organo-carboxylate). Le cœur magnétique est le plus souvent constitué de métaux de transition, les ponts permettant une interaction d'échange entre ces différents constituants sont souvent des composés oxygénés tels que O2−, OH−, OR− ou encore RCO2. Ils donneront naissances aux aimants mono-ioniques, ou Single-Ion Magnets (SIMs

Physics of optically driven nanojunctions

Philippe Andreas Rölli

Recent years have seen spectacular developments in the domain of nano-optics. Alongside the well-known techniques of super-resolution microscopy progress in nanofabrication has enabled important improvements in the fields of optical imaging and spectroscopy. Nowadays enhancements of specific near-field interactions by nano-antennae/cavities allows the optical readout of single molecule properties in a number of different systems. The observation of normal modes of vibration for example carries essential information on the orientation, link to the interface and environment in which the molecule is embedded. This thesis develops firstly a novel theoretical frame for these vibration-cavity interactions by following the optomechanical formalism, which has proven to be an accurate treatment of a wide variety of mechanical systems interacting with a laser field. By leveraging this framework, we investigate on the one hand several quantum limits. On the other hand, for the parameter space experimentally available and for well-designed cavities, novel regimes of interaction between the collective mode of vibration of an ensemble of molecules and an optical tone seem within reach. In parallel, this thesis experimentally explores one specific realization of such a system. In collaboration with the group of Christophe Galland we constructed the optical setups necessary to interrogate our nanostructures. The class of structures constructed, known as self-assembled nanojunctions or nanoparticles on mirror, reaches confinement of electric fields via plasmonic modes close to its fundamental limit and enables the study of many different mechanical systems whose thickness consist in a single molecule. Along this research we have developed techniques to characterize mechanical, electronic and thermal observables of nanojunctions interacting with an optical field. Our experimental study has enabled the discovery of a novel effect related to the electronic confinement of these quasi-0-dimensional structures. The characteristic photoluminescence of metallic nanostructures is found to consistently blink in nanojunctions akin to other quantum emitters. The mechanisms explaining in detail this blinking remain debatable but the numerous experimental data collected highlight the key role of the interface in the specificities of the blinking observed. This finding opens novel ways to characterize the important and to date ill-described question of the interfaces in nanojunctions. The observed modifications of the nanojunctions optical properties might constitute an interesting step towards the development of a novel class of functional materials ('gap-materials') with enhanced and tailored electro-optical properties. The last part of this thesis considers one technological application for this class of material by conceptually developing a new kind of detector based on the upconversion of an infrared photon to the visible; exploiting thus in a unique way the optical properties of the molecular system. The study of the quantum noises of these detectors highlights two exciting and technologically relevant directions for such devices : at room temperature these systems operate with levels of noise below state of the art devices, at lower temperatures these systems open an unexplored path towards single photon detection in a spectral range where this technology remains unavailable to date.

EPFL2020

Uniaxial 2D Superlattice of Fe-4 Molecular Magnets on Graphene

Harald Brune, Luca Gragnaniello, Fabian Paschke, Stefano Rusponi

We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assembled arrays of Fe4H single molecule magnets on graphene/Ir(111). The energetic positions of molecular states are probed by means of scanning tunneling spectroscopy, showing pronounced long- and short-ranged spatial modulations, indicating the presence of both locally varying intermolecular as well as adsorption-site dependent molecule substrate interactions. From the magnetic field dependence of the X-ray magnetic circular dichroism signal, we infer that the magnetic easy axis of each Fe4H molecule is oriented perpendicular to the sample surface and that after the deposition the value of the uniaxial anisotropy is identical to the one in bulk. Our findings therefore suggest that the observed interaction of the molecules with their surrounding does not modify the molecular magnetism, resulting in magnets that retain their bulk magnetic properties.

American Chemical Society2017

Magnetic Stability of Single Lanthanide Atoms on Graphene

Romana Baltic

This thesis presents a study of the magnetism of surface supported atoms performed principally with XMCD spectroscopy and multiplet calculations. The objective of the research was twofold: first, to study the underlying interactions and conditions governing the magnetization stability of surface supported atoms, with the aim of achieving long magnetic lifetimes, and second, to assemble single atom magnets in an ordered pattern. Through our study of

4f

lanthanide atoms on supporting substrates, we established that their magnetic stability is governed by their quantum level structure, in particular their ground

J_z

state and the height of the energy barrier for thermally assisted magnetization reversal. These features are ruled through the crystal field interaction with their supporting surface. The adsorption site of adatoms governs the symmetry of the crystal field and, consequently, the coupling between the different

J_z

levels. This in turn enables magnetization reversal either through quantum tunneling between

J_z

states or via scattering with the electrons and phonons of the substrate. To reduce the scattering events, it is necessary to decouple the adatoms from the metal substrate by using decoupling layers. Here we show that a single layer of graphene is sufficient to decouple Dy atoms from the underlying Ir(111) substrate, resulting in a magnetic lifetime of about 1000~s at 2.5~K. In addition, we show that the moiré pattern of the graphene/Ir(111) surface can be used as a template for the self-assembly of these single atom magnets into well ordered superlattices. Further, by studying multiple graphene/metal substrates we show that the interaction of graphene with the supporting substrate greatly influences the magnetization stability of adsorbed atoms. Finally, we show that replacing graphene with an insulating layer does not result in a stable magnetization of adsorbed atoms if its superior decoupling is not accompanied with an adequate crystal field symmetry.

EPFL2018