Spin tuning of electron-doped metal-phthalocyanine layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state

Designing optoelectronic properties by on-surface synthesis: formation and electronic structure of an iron-terpyridine macromolecular complex

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms, and intra-ligand conformational changes, lead to Fe-tpy coordination and formation of these nanochains. Low-temperature Scanning Tunneling Microscopy and Density Functional Theory were used to elucidate the atomic-scale morphology of the system, providing evidence of a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning Tunneling Spectroscopy reveals highest occupied orbitals with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photo-induced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthesis methods, and is mediated by the bottom-up on-surface approach used here

Self-Assembled Nanometer-Scale Magnetic Networks on Surfaces: Fundamental Interactions and Functional Properties

Nanomagnets of controlled size, organized into regular patterns open new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Self-assembling processes on various types of substrates allow designing fine-structured architectures and tuning of their magnetic properties. Here, starting from a description of fundamental magnetic interactions at the nanoscale, we review recent experimental approaches to fabricate zero-, one-, and two-dimensional magnetic particle arrays with dimensions reduced to the atomic limit and unprecedented areal density. We describe systems composed of individual magnetic atoms, metal-organic networks, metal wires, and bimetallic particles, as well as strategies to control their magnetic moment, anisotropy, and temperature-dependent magnetic behavior. The investigation of self-assembled subnanometer magnetic particles leads to significant progress in the design of fundamental and functional aspects, mutual interactions among the magnetic units, and their coupling with the environment

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc, and TbPc deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc and TbPc remain largely unaltered. TbPc deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc in molecular crystals, opposite to TbPc deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for TbPc, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc or MnPc

Effect of cholesterol on the dipole potential of lipid membranes

The membrane dipole potential, ψd, is an electrical potential difference with a value typically in the range 150 – 350 mV (positive in the membrane interior) which is located in the lipid headgroup region of the membrane, between the linkage of the hydrocarbon chains to the phospholipid glycerol backbone and the adjacent aqueous solution. At its physiological level in animal plasma membranes (up to 50 mol%), cholesterol makes a significant contribution to ψd of approximately 65 mV; the rest arising from other lipid components of the membrane, in particular phospholipids. Via its effect on ψd, cholesterol may modulate the activity of membrane proteins. This could occur through preferential stabilization of protein conformational states. Based on its effect on ψd, cholesterol would be expected to favour protein conformations associated with a small local hydrophobic membrane thickness. Via its membrane condensing effect, which also produces an increase in ψd, cholesterol could further modulate interactions of polybasic cytoplasmic extensions of membrane proteins, in particular P-type ATPases, with anionic lipid headgroups on the membrane surface, thus leading to enhanced conformational stabilization effects and changes to ion pumping activity.Australian Research Counci

Electronic structure of metal phthalocyanines on Ag (100)

El uso de moléculas orgánicas en dispositivos tecnológicos ofrece una serie de ventajas: su tamaño (~nm), su capacidad de auto ensamblan dando lugar a la formación de estructuras funcionales, y la posibilidad de adaptar sus propiedades electrónicas y magnéticas a través de los métodos de síntesis molecular. Sin embargo, la implementación de dispositivos orgánicos depende fundamentalmente de la comprensión entre la interacción de las moléculas y los electrodos de metal, así como las interacciones molécula-molécula. Esta tesis estudia las propiedades estructurales, electrónicas y magnéticas de las metal-ftalocianinas (MePc) adsorbidas sobre un sustrato metálico. MePcs son complejos metal-orgánicos con una química versátil y una estructura simple y robusta Mediante el uso de microscopía de efecto túnel (STM) y espectroscopia de efecto túnel de escaneo (STS) se ha estudiado la interacción molécula-sustrato y molécula-molécula de cuatro MePc (Me = Fe, Co, Ni y Cu) empezando desde moléculas individuales hasta multicapas de MePc en superficies de Ag (100). Se ha observado que la adsorción molecular de las MePc es paralela al plano del sustrato y es impulsada por la optimización del enlace entre el ligando con el sustrato de Ag, dando lugar a dos posibles orientaciones del eje molecular en la superficie. El desajuste de simetría entre la molécula y el sustrato provoca una hibridación asimétrica de diferentes orbitales moleculares. Como resultado, la molécula aquiral muestra un contraste quiral en la topografía de STM, quedando intacta su estructura química. Las fuerza laterales de van der Waals transfieren unívocamente la quiralidad electrónica de las moléculas individuales a niveles supramoleculares. La maduración de Ostwald provoca una ruptura espontanea de la simetría de la mezcla racémica inicial de los clústeres, obteniendo como resultado capas moleculares mesoscópicas homoquirales. Las propiedades electrónicas y magnéticas dependen de toda la molécula (ligando e ión central). Todas las moléculas reciben aproximadamente un electrón de la superficie, y en función del tipo de ión central y del carácter de sus d-orbitales frontera, el resultado es differente. Para los estados-d de baja energía o los que se encuentren en el plano molecular, como ocurre en NiPc / CuPc, son los orbitales de los ligandos orgánicos los que aceptan el electrón adicional conllevando a su hibridación. Esto hace que el electrón no apareado cree un momento magnético adicional en estas moléculas dejando intacto el estado magnético de los iones centrales. Para la CuPc, la interacción de canje intramolecular entre los electrones d y π conduce a la creación de un nuevo estado triplete (S = 1) en el estado fundamental. En cambio, en las moléculas de FePc y CoPc, la interacción a través de los estados-d fuera del plano molecular, provoca una reorganización compleja de carga dando lugar a la formación de un sistema de valencia mixta. En ambos casos esta interacción conduce a una reducción del momento magnético de los iones. Se ha estudiado la posibilidad de manipular el estado de espín de las MePc dopando átomo-por-átomo con donadores de electrones alcalinos. Se ha encontrado que en las moléculas de CuPc es posible dopar selectivamente tanto los estados del ligando como los del metal cambiando la colocación del alcalino. Esto permite controlar el estado de espín (0, 1/2, 1) y de carga (hasta 2 electrones adicionales) con un único dopante alcalino. El dopaje en monocapas de CuPc muestra que la interacción electrostática entre los átomos de Li de los complejos Li @CuPc vecinos conduce a un cruce de configuraciones estables en función de la dosis de Li, favoreciendo la transferencia a los estados de iones. Por último, se ha investigado el efecto de los dopantes magnéticos como los átomos de Fe en las moléculas de NiPc y CuPc. Se ha observado que no hay ninguna evidencia directa de un acoplamiento entre los momentos magnéticos de Fe y Cu. Sin embargo, la temperatura de Kondo de los átomos de Fe varía debido a cambios locales en su entorno modulados por la presencia del MePc. En general, estos resultados proporcionan una visión comprensiva de la interacción de las MePc con sustratos metálicos, así como de las propiedades magnéticas y de transporte en clústeres moleculares, monocapas y multicapas.The use of organic molecules in technological devices offers a number of advantages: their intrinsic small size (~nm), their ability to self assemble into functional structures, and the possibility to tailor their electronic and magnetic properties through the methods of molecular synthesis. However, the implementation depends crucially on the understanding of the interaction between molecules and metal electrodes as well as molecule-molecule interactions. This thesis deals with the structural electronic and magnetic properties of Metal phthalocyanines (MePc) adsorbed on a metallic substrate. MePc are well-known metal organic complexes, with a versatile chemistry combined with a relatively simple and robust structure. By using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS), we studied the molecule-substrate and molecule-molecule interaction for four MePc (Me = Fe, Co, Ni Cu) starting from single molecules up to multilayer films on a Ag(100) surface. We found that the flat adsorption of MePc molecules is driven by bond optimization between the ligand with the underlying Ag substrate, resulting in two possible orientations of the molecular axis on the surface. The symmetry mismatch between molecule and substrate leads to an asymmetric hybridization of different molecular orbitals. As a result the structurally unperturbed achiral molecules show chiral contrast in STM topography. Lateral vdW forces univocally transfer the electronic chirality of single molecules to the organizational level in clusters. Ostwald ripening of the originally racemic mixture of clusters leads to spontaneous symmetry breaking resulting in mesoscopic homochiral molecular layers. The electronic and magnetic properties depend on the whole molecule (ligand and central ion). While all of molecules receive approximately one electron from the surface, depending on type of central ion, and the character of its frontier d-orbitals, the consequences are different. For d states lying low in energy or within the molecular plane, as is the case for NiPc/CuPc, the main hybridization channel are the organic ligands orbitals, leading to the acceptance of an extra electron here. This unpaired electron causes of an additional magnetic moment in these molecules, whereas the magnetic state of the central ions remains unperturbed. For CuPc intramolecular exchange coupling between d and π electrons leads to the creation a triplet (S=1) ground state. In contrast for FePc and CoPc, the interaction through the d states, oriented out of the molecular plane, creates a complex charge reorganization and mixed valence regime. In both cases this interaction leads to a reduction of ion's magnetic moment. We studied the possibilities to manipulate the spin and change state of MePcs through atom-by-atom doping with Alkali electron donors. For CuPc we found that ligand and metal states can be selectively doped by changing the alkali-molecule bonding configuration. This permits tailoring of the spin (0, 1/2, 1) and charge state with up to 2 extra electrons, and 1 with a single alkali dopant. The doping of CuPc monolayer films shows that the electrostatic interaction between Li atoms of neighboring Li@CuPc complexes leads to a crossover of stable configurations depending on the Li dosage, favoring the transfer to the ion states. Finally, we investigated the effect of magnetic dopants such as Fe atoms on NiPc and CuPc molecules. No direct evidence of a coupling between the magnetic moments of Fe and Cu was observed. However the Kondo temperature of the Fe atoms varies due to changes in their local environment modulated by the presence of the MePc. Overall, these results provide a comprehensive view of the interaction of MePc with metal substrates as well as the magnetic and transport properties of molecular cluster, monolayer and multilayers

Electronic structure of metal phthalocyanines on Ag (100)

El uso de moléculas orgánicas en dispositivos tecnológicos ofrece una serie de ventajas: su tamaño (~nm), su capacidad de auto ensamblan dando lugar a la formación de estructuras funcionales, y la posibilidad de adaptar sus propiedades electrónicas y magnéticas a través de los métodos de síntesis molecular. Sin embargo, la implementación de dispositivos orgánicos depende fundamentalmente de la comprensión entre la interacción de las moléculas y los electrodos de metal, así como las interacciones molécula-molécula. Esta tesis estudia las propiedades estructurales, electrónicas y magnéticas de las metal-ftalocianinas (MePc) adsorbidas sobre un sustrato metálico. MePcs son complejos metal-orgánicos con una química versátil y una estructura simple y robusta Mediante el uso de microscopía de efecto túnel (STM) y espectroscopia de efecto túnel de escaneo (STS) se ha estudiado la interacción molécula-sustrato y molécula-molécula de cuatro MePc (Me = Fe, Co, Ni y Cu) empezando desde moléculas individuales hasta multicapas de MePc en superficies de Ag (100). Se ha observado que la adsorción molecular de las MePc es paralela al plano del sustrato y es impulsada por la optimización del enlace entre el ligando con el sustrato de Ag, dando lugar a dos posibles orientaciones del eje molecular en la superficie. El desajuste de simetría entre la molécula y el sustrato provoca una hibridación asimétrica de diferentes orbitales moleculares. Como resultado, la molécula aquiral muestra un contraste quiral en la topografía de STM, quedando intacta su estructura química. Las fuerza laterales de van der Waals transfieren unívocamente la quiralidad electrónica de las moléculas individuales a niveles supramoleculares. La maduración de Ostwald provoca una ruptura espontanea de la simetría de la mezcla racémica inicial de los clústeres, obteniendo como resultado capas moleculares mesoscópicas homoquirales. Las propiedades electrónicas y magnéticas dependen de toda la molécula (ligando e ión central). Todas las moléculas reciben aproximadamente un electrón de la superficie, y en función del tipo de ión central y del carácter de sus d-orbitales frontera, el resultado es differente. Para los estados-d de baja energía o los que se encuentren en el plano molecular, como ocurre en NiPc / CuPc, son los orbitales de los ligandos orgánicos los que aceptan el electrón adicional conllevando a su hibridación. Esto hace que el electrón no apareado cree un momento magnético adicional en estas moléculas dejando intacto el estado magnético de los iones centrales. Para la CuPc, la interacción de canje intramolecular entre los electrones d y π conduce a la creación de un nuevo estado triplete (S = 1) en el estado fundamental. En cambio, en las moléculas de FePc y CoPc, la interacción a través de los estados-d fuera del plano molecular, provoca una reorganización compleja de carga dando lugar a la formación de un sistema de valencia mixta. En ambos casos esta interacción conduce a una reducción del momento magnético de los iones. Se ha estudiado la posibilidad de manipular el estado de espín de las MePc dopando átomo-por-átomo con donadores de electrones alcalinos. Se ha encontrado que en las moléculas de CuPc es posible dopar selectivamente tanto los estados del ligando como los del metal cambiando la colocación del alcalino. Esto permite controlar el estado de espín (0, 1/2, 1) y de carga (hasta 2 electrones adicionales) con un único dopante alcalino. El dopaje en monocapas de CuPc muestra que la interacción electrostática entre los átomos de Li de los complejos Li @CuPc vecinos conduce a un cruce de configuraciones estables en función de la dosis de Li, favoreciendo la transferencia a los estados de iones. Por último, se ha investigado el efecto de los dopantes magnéticos como los átomos de Fe en las moléculas de NiPc y CuPc. Se ha observado que no hay ninguna evidencia directa de un acoplamiento entre los momentos magnéticos de Fe y Cu. Sin embargo, la temperatura de Kondo de los átomos de Fe varía debido a cambios locales en su entorno modulados por la presencia del MePc. En general, estos resultados proporcionan una visión comprensiva de la interacción de las MePc con sustratos metálicos, así como de las propiedades magnéticas y de transporte en clústeres moleculares, monocapas y multicapas.The use of organic molecules in technological devices offers a number of advantages: their intrinsic small size (~nm), their ability to self assemble into functional structures, and the possibility to tailor their electronic and magnetic properties through the methods of molecular synthesis. However, the implementation depends crucially on the understanding of the interaction between molecules and metal electrodes as well as molecule-molecule interactions. This thesis deals with the structural electronic and magnetic properties of Metal phthalocyanines (MePc) adsorbed on a metallic substrate. MePc are well-known metal organic complexes, with a versatile chemistry combined with a relatively simple and robust structure. By using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS), we studied the molecule-substrate and molecule-molecule interaction for four MePc (Me = Fe, Co, Ni Cu) starting from single molecules up to multilayer films on a Ag(100) surface. We found that the flat adsorption of MePc molecules is driven by bond optimization between the ligand with the underlying Ag substrate, resulting in two possible orientations of the molecular axis on the surface. The symmetry mismatch between molecule and substrate leads to an asymmetric hybridization of different molecular orbitals. As a result the structurally unperturbed achiral molecules show chiral contrast in STM topography. Lateral vdW forces univocally transfer the electronic chirality of single molecules to the organizational level in clusters. Ostwald ripening of the originally racemic mixture of clusters leads to spontaneous symmetry breaking resulting in mesoscopic homochiral molecular layers. The electronic and magnetic properties depend on the whole molecule (ligand and central ion). While all of molecules receive approximately one electron from the surface, depending on type of central ion, and the character of its frontier d-orbitals, the consequences are different. For d states lying low in energy or within the molecular plane, as is the case for NiPc/CuPc, the main hybridization channel are the organic ligands orbitals, leading to the acceptance of an extra electron here. This unpaired electron causes of an additional magnetic moment in these molecules, whereas the magnetic state of the central ions remains unperturbed. For CuPc intramolecular exchange coupling between d and π electrons leads to the creation a triplet (S=1) ground state. In contrast for FePc and CoPc, the interaction through the d states, oriented out of the molecular plane, creates a complex charge reorganization and mixed valence regime. In both cases this interaction leads to a reduction of ion's magnetic moment. We studied the possibilities to manipulate the spin and change state of MePcs through atom-by-atom doping with Alkali electron donors. For CuPc we found that ligand and metal states can be selectively doped by changing the alkali-molecule bonding configuration. This permits tailoring of the spin (0, 1/2, 1) and charge state with up to 2 extra electrons, and 1 with a single alkali dopant. The doping of CuPc monolayer films shows that the electrostatic interaction between Li atoms of neighboring Li@CuPc complexes leads to a crossover of stable configurations depending on the Li dosage, favoring the transfer to the ion states. Finally, we investigated the effect of magnetic dopants such as Fe atoms on NiPc and CuPc molecules. No direct evidence of a coupling between the magnetic moments of Fe and Cu was observed. However the Kondo temperature of the Fe atoms varies due to changes in their local environment modulated by the presence of the MePc. Overall, these results provide a comprehensive view of the interaction of MePc with metal substrates as well as the magnetic and transport properties of molecular cluster, monolayer and multilayers

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Polynuclear metal-organic coordination complexes are often inaccessible by traditional synthetic chemistry methods. Here, the authors use on-surface supramolecular chemistry to form a planar trinuclear Fe complex, in which an accumulation of electrons around the positive mixed-valence polynuclear centre suggests a catalytically active core

Spin tuning of electron-doped metal-phthalocyanine layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state