Andreas Frömsdorf

Micelle and Vesicle Formation of Amphiphilic Nanoparticles

Nanoparticle brushes: Complex nanostructures can be formed by self assembly of amphiphilic CdSe/CdS core-shell nanoparticles that bear a brushlike layer of poly(ethylene oxide) chains. This route is based on controlling the volume fractions of hydrophilic and hydrophobic moieties within the particles and allows the formation of micellar, cylindrical, or vesicular nanoobjects (see picture).

Synthesis of Monodisperse PbS Nanoparticles and Their Assembly into Highly Ordered 3D Colloidal Crystals

We describe the synthesis of highly monodisperse PbS nanocrystals using an organometalic approach, whereby oleic acid and tri-n-octylphosphine serve as stabilizing ligands. The optical bandgap of the particles is tunable as a function of crystal-size throughout the near-infrared region from 800 to 2200 nm. The particles were characterized using X-ray diffraction, optical spectroscopy and transmission electron microscopy. Applying the three layer super saturation technique we were able to grow colloidal crystals of various shapes as shown by scanning electron microscopy. Small-angle X-ray scattering experiments indicate fcc lattice structure.

On the electric conductivity of highly ordered monolayers of monodisperse metal nanoparticles.

Monolayers of colloidally synthesized cobalt-platinum nanoparticles of different diameters characterized by transmission electron microscopy were deposited on structured silicon oxide substrates and characterized by scanning electron microscopy, grazing incidence X-ray scattering, and electric transport measurements. The highly ordered nanoparticle films show a thermally activated electron hopping between spatially adjacent particles at room temperature and Coulomb blockade at low temperatures. We present a novel approach to experimentally determine the particles charging energies giving values of 6.7-25.4 meV dependent on the particles size and independent of the interparticle distance. These observations are supported by finite element method calculations showing the self-capacitance to be the determining value which only depends on the permittivity constant of the surrounding space and the particles radius.

In Situ Synthesis and Alignment of Au Nanoparticles within Hexagonally Packed Cylindrical Domains of Diblock Copolymers in Bulk

We present a simple method to prepare hexagonally packed metallic nanocylinders based on gold nanoparticles embedded in a copolymeric matrix. The gold nanoparticles are generated selectively within the P4VP-rich cylindrical domains of a polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP) diblock copolymer. In order to achieve this selectivity, a gold precursor (HAuCl4) is coupled to the pyridine blocks of a spherical PS327-b-P4VP27 block copolymer. In consequence, the hybrid block copolymer is able to self-assemble in a hexagonally packed cylinders morphology. The application of mechanical oscillatory shear improved markedly the alignment of these nanocylinders, while simultaneously the gold precursor was reduced in situ into gold nanoparticles. Following rheological characterization in the linear viscoelastic regime, a set of alignment parameters were comprehensively selected and checked with a series of transmission electron microscopy (TEM) micrographs. An optimal temperature of alignment was found after systematic evaluation of samples sheared at different temperatures. The block copolymer exhibited an increase in the domain period as a consequence of chain rearrangements around the newly formed gold nanoparticles. The hexagonally packed morphology was preserved, and under the optimal conditions single grain sizes showed significant improvement to macroscale order in comparison to nonaligned samples. In contrast to current multistep lithographic techniques, the present method constitutes a simple path to produce three-dimensional organic-inorganic conductive nanowires with periodicities at the macroscopic level.

Covalent Attachment of Polymersomes to Surfaces

We show that vesicles made of block copolymers with aldehyde end groups can be covalently attached to aminated and non-aminated, untreated glass surfaces. The attached vesicles were sufficiently stable to allow a detailed investigation of vesicle shapes by confocal laser scanning microscopy (CLSM) and AFM in aqueous solutions allowing reconstruction of 3D images of the vesicle structure. Covalently attached PCL-PEO, PLA-PEO, and PI-PEO block copolymer vesicles have different footprint areas and different shapes due to their differences in bilayer stiffness.

Shallow HEMTs For Lateral Magnetic Superlattices

We study two‐dimensional electron gases subjected to modulated potentials and magnetic fields. With mask techniques it is possible to prepare lateral magnetic superlattices with periods below 100 nm. For a sufficiently strong modulation of the magnetic field at the location of the 2DEG at such small periods, optimized heterostructures with extremely shallow 2DEGs are required. Here, we report on the development of shallow HEMTs containing 2DEGs less than 20 nm below the surface and compatible deposition techniques for the magnetic layers.