A photoemission study of interfaces between organic semiconductors and Co as well as Al₂O₃/Co contacts

We have studied the energy-level alignment of ex situ, acetone cleaned Co and Al2O3/Co contacts to the organic semiconductors pentacene and rubrene by combined X-ray and ultraviolet photoemission spectroscopy. Our results demonstrate that the work function under these conditions is smaller than in the in situ cleaned, atomically clean case. Moreover, the studied interfaces are characterized by very small, short range interfaces dipoles and substantial injection barriers for holes. This represents essential information in view of their use in organic spintronic devices. Our core-level photoemission spectroscopy measurements rule out chemical reactions

Low-temperature solution synthesis of chemically functional ferromagnetic FePtAu nanoparticles

Magnetic nanoparticles are of great scientific and technological interest. The application of ferromagnetic nanoparticles for high-density data storage has great potential, but energy efficient synthesis of uniform, isolated, and patternable nanoparticles that remain ferromagnetic at room temperature is not trivial. Here, we present a low-temperature solution synthesis method for FePtAu nanoparticles that addresses all those issues and therefore can be regarded as an important step toward applications. We show that the onset of the chemically ordered face-centered tetragonal (L10) phase is obtained for thermal annealing temperatures as low as 150 C. Large uniaxial magnetic anisotropy (107 erg/cm3) and a high long-range order parameter have been obtained. Our low-temperature solution annealing leaves the organic ligands intact, so that the possibility for postanneal monolayer formation and chemically assisted patterning on a surface is maintained

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Electron transport and spin phenomena in hybrid organic/inorganic systems

This thesis describes several experiments in hybrid organic/inorganic systems, in which electron transport and/or spin behavior is studied. The basic concepts of organic electronics and spintronics are given, to understand the described spin-valve experiments. The problems and obstacles for injecting a spin-polarized current into organic materials and the potential of using organic single-crystals in spintronic devices are discussed. Different methods for fabricating organic single-crystal FET devices with FM electrodes are explained. First, the growth of organic single-crystals is discussed. Then, the fabrication of FM electrodes with shadow masks, photo- and e-beam lithography is explained. The interface of fabricated FM electrodes and organic materials is investigated. UV photoemission spectroscopy and X-ray photoelectron spectroscopy are performed to study the energy level alignment. The effect of the lithography processes on the spin injection properties of the FM electrode and a cleaning step applied to the interfaces are investigated. Measurements on rubrene single-crystal FETs with FM/tunnel barrier electrodes are discussed. The critical spin-valve properties of these devices are investigated and it is shown that this FET has the potential of being used as a spin valve. The current flowing through the device can be fitted to a back-to-back Schottky model, which show that electrons are injected via a well-defined tunnel barrier fabricated on top of the FM electrodes. Transferring Au electrodes to organic single-crystals by soft elastomeric stamps, with and without the facilitation of organic molecules, is investigated. Measurements in the space-charged-limited-current regime and on FET devices are presented and discussed. FM nanoparticles (NPs), capped with organic ligands, are discussed. A systematic study of the annealing of these NPs in solution at relative low temperatures is described. The annealing is needed to obtain high magnetocrystalline anisotropy. The low-temperature annealing leaves the organic ligands intact, which can be used for patterning. Experiments are performed on two-dimensional organic spin systems to study the interaction of the spins with their environment. The system consists of a thin Au film covered with a monolayer of molecules containing an unpaired spin. The signature of the Kondo effect, a local resistance minimum as a function of temperature, is observed under certain conditions. The behavior of this effect in the presence of a magnetic field is also investigated

Organic spintronics

We review the emerging field of organic spintronics, where organic materials are applied as a medium to transport and control spin-polarized signals. The contacts for injecting and detecting spins are formed by ferromagnetic metals, oxides, or inorganic semiconductors. First, the basic concepts of spintronics and organic electronics are addressed, and phenomena which are in particular relevant for organic spintronics are highlighted. Experiments using different organic materials, including carbon nanotubes, organic thin films, self-assembled monolayers and single molecules are then reviewed. Observed magnetoresistance points toward successful spin injection and detection, but spurious magnetoresistance effects can easily be confused with spin accumulation. A few studies report long spin relaxation times and lengths, which forms a promising basis for further research. We conclude with discussing outstanding questions and problems

Interaction of a 2-level system with 2D phonons

We report on the non-adiabatic interaction between 2D acoustic phonons and an artificial 2-level system in a GaAs/AlGaAs heterostructure. The 2-level system is formed by two discrete energy levels inside a double quantum dot, and monochromatic surface acoustic waves (~2 GHz) are generated by an on-chip interdigital transducer (IDT). An IDT for better performance is proposed

A photoemission study of interfaces between organic semiconductors and Co as well as Al2O3/Co contacts

We have studied the energy-level alignment of ex situ, acetone cleaned Co and Al2O3/Co contacts to the organic semiconductors pentacene and rubrene by combined X-ray and ultraviolet photoemission spectroscopy. Our results demonstrate that the work function under these conditions is smaller than in the in situ cleaned, atomically clean case. Moreover, the studied interfaces are characterized by very small, short range interfaces dipoles and substantial injection barriers for holes. This represents essential information in view of their use in organic spintronic devices. Our core-level photoemission spectroscopy measurements rule out chemical reactions

Serum alkaline phosphatase activity during zinc deficiency and long-term inflammatory stress.

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Magnetic Nanoparticle Assembly on Surfaces using Click-Chemistry

Controlled assembly of ferromagnetic nanoparticles on surfaces is of crucial importance for a range of spintronic and data storage applications. Here, we present a novel method for assembling monolayers of ferromagnetic FePt nanoparticles on silicon oxide substrates using “click chemistry‿. Reaction of alkyne-functionalized FePt nanoparticles with azide-terminated self-assembled monolayers (SAMs), on silicon oxide, leads to the irreversible attachment of magnetic nanoparticles to the surface via triazole linkers. Based on this covalent interaction, well-packed monolayers of FePt nanoparticles were prepared and nanoparticle patterns are generated on surfaces via microcontact printing (μCP)

Interaction of a 2-level system with 2D phonons

We report on the non-adiabatic interaction between 2D acoustic phonons and an artificial 2-level system in a GaAs/AlGaAs heterostructure. The 2-level system is formed by two discrete energy levels inside a double quantum dot, and monochromatic surface acoustic waves (~2 GHz) are generated by an on-chip interdigital transducer (IDT). An IDT for better performance is proposed