Michel P. de Jong

Influence of the electrode work function on the energy level alignment at organic-organic interfaces

The energy level alignment at interfaces, in stacks comprising of (4,4′-N,N′-dicarbazolyl-biphenyl) (CBP), (4,4,4″-tris[3-methyl-phenyl(phenyl)amino]-triphenylamine) (m-MTDATA), and a conductive substrate, has been studied. We show that the alignment of energy levels depends on the equilibration of the chemical potential throughout the layer stack, while any electronic coupling between the individual layers is of lesser importance. This behavior is expected to occur for a broad class of weakly interacting interfaces and can have profound consequences for the design of organic electronic devices.

Electronic and magnetic structure of C60/Fe3O4(001): a hybrid interface for organic spintronics

We report the electronic and magnetic characterization of the hybrid interface constituted by C60 molecules and an epitaxial Fe3O4(001) surface grown on GaAs(001). Using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we demonstrate that a stable C60 sub-monolayer (ML) can be retained on the Fe3O4(001) surface upon in-situ annealing at 250°C. A carbon K-edge dichroic signal of 1% with respect to the XAS C 1s → π* peak intensity has been observed, indicative of a weaker electronic interaction of C60 with Fe3O4(001) compared to the previously reported case of C60/Fe(001). Remarkably, the Fe L-edge XMCD spectrum of Fe3O4(001) reveals a reduced Fe3+/Fe2+ ratio upon C60 sub-ML adsorption. This observation has been ascribed to electron donation by the C60 molecules, as a consequence of the high work function of Fe3O4(001). Our present work underlines the significance of chemical interactions between inorganic magnetic surfaces and molecular adsorbates for tuning the electronic and magnetic properties of the interfaces, which have profound impact on spin-polarized charge transport in hybrid organic-inorganic spintronic devices.

Modeling charge transfer at organic donor-acceptor semiconductor interfaces

We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor material, as determined from metal-organic semiconductor contacts. These pinning levels can be obtained from simple density functional theory calculations.

Boosting the Boron Dopant Level in Monolayer Doping by Carboranes

Monolayer doping (MLD) presents an alternative method to achieve silicon doping without causing crystal damage, and it has the capability of ultrashallow doping and the doping of nonplanar surfaces. MLD utilizes dopant-containing alkene molecules that form a monolayer on the silicon surface using the well-established hydrosilylation process. Here, we demonstrate that MLD can be extended to high doping levels by designing alkenes with a high content of dopant atoms. Concretely, carborane derivatives, which have 10 B atoms per molecule, were functionalized with an alkene group. MLD using a monolayer of such a derivative yielded up to ten times higher doping levels, as measured by X-ray photoelectron spectroscopy and dynamic secondary mass spectroscopy, compared to an alkene with a single B atom. Sheet resistance measurements showed comparably increased conductivities of the Si substrates. Thermal budget analyses indicate that the doping level can be further optimized by changing the annealing conditions.

Magnetic Properties of bcc-Fe(001)/C-60 Interfaces for Organic Spintronics

The magnetic structure of the interfaces between organic semiconductors and ferromagnetic contacts plays a key role in the spin injection and extraction processes in organic spintronic devices. We present a combined computational (density functional theory) and experimental (X-ray magnetic circular dichroism) study on the magnetic properties of interfaces between bcc-Fe(001) and C(60) molecules. C(60) is an interesting candidate for application in organic spintronics due to the absence of hydrogen atoms and the associated hyperfine fields. Adsorption of C(60) on Fe(001) reduces the magnetic moments on the top Fe layers by ∼6%, while inducing an antiparrallel magnetic moment of ∼-0.2 μ(B) on C(60). Adsorption of C(60) on a model ferromagnetic substrate consisting of three Fe monolayers on W(001) leads to a different structure but to very similar interface magnetic properties.

Synthesis and characterisation of poly(distyrylbenzene-block-hexa(ethylene oxide)) and its fluorinated analogue—two new block copolymers and their application in electroluminescent devices

Two new soluble block copolymers are reported in which chromophores and hexa(ethylene oxide) units alternate along the polymer backbone. In polymer 1 the chromophore was the distyrylbenzene unit. T ...

Bottom-Up Single-Electron Transistors

As the downscaling of conventional semiconductor electronics becomes more and more challenging, the interest in alternative material systems and fabrication methods is growing. A novel bottom-up approach for the fabrication of high-quality single-electron transistors (SETs) that can easily be contacted electrically in a controllable manner is developed. This approach employs the self-assembly of Au nanoparticles forming the SETs, and Au nanorods forming the leads to macroscopic electrodes, thus bridging the gap between the nano- and microscale. Low-temperature electron-transport measurements reveal exemplary single-electron tunneling characteristics. SET behavior can be significantly changed, post-fabrication, using molecular exchange of the tunnel barriers, demonstrating the tunability of the assemblies. These results form a promising proof of principle for the versatility of bottom-up nanoelectronics, and toward controlled fabrication of nanoelectronic devices.

Microscopic origin of the reduced magnetocrystalline anisotropy with increasing oxide contenct in Co80Pt20:oxide thin films

Angle-dependent x-ray magnetic circular dichroism at the Co L2.3 edges has been utilized to systematically study Co80Pt20 : WO3 perpendicular magnetic recording thin films, in which the magnetocrystalline anisotropy significantly drops as the oxide volume fraction increases. The microscopic origin of this phenomenon in the studied films can be mainly attributed to an increase in orbital moment normal to the grain–oxide interface, with increasing oxide volume fraction, which arises from a more pronounced effect of symmetry breaking at the grain–oxide interface in smaller grains.