Stefan Kirstein

Electroluminescence of different colors from polycation/CdTe nanocrystal self-assembled films

Water soluble thiol capped CdTe nanocrystals are assembled into ultrathin films in combination with poly(diallyldimethylammonium chloride) (PDDA) by the self-assembly method of layer-by-layer adsorption of oppositely charged polyelectrolytes. Electroluminescent devices, which produce different color emissions, are fabricated by sandwiching CdTe/PDDA films between indium–tin–oxide (ITO) and aluminum electrodes using CdTe nanocrystals of different sizes. It is shown that the electroluminescence (EL) spectra of the CdTe/polymer films are nearly identical to the photoluminescence spectra of the corresponding CdTe nanocrystals in aqueous solutions. The devices produce room-light visible light output with an external quantum efficiency up to 0.1%. Light emission is observed at current densities of 10 mA/cm2 and at low onset voltages of 2.5–3.5 V, which depends on the thickness of the film indicating field-dependent current injection. A variation of the EL efficiency with the size of the CdTe particles is observ...

Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates

Self-assembled quasi one-dimensional nanostructures of pi-conjugated molecules may find a use in devices owing to their intriguing optoelectronic properties, which include sharp exciton transitions, strong circular dichroism, high exciton mobilities and photoconductivity. However, many applications require immobilization of these nanostructures on a solid substrate, which is a challenge to achieve without destroying their delicate supramolecular structure. Here, we use a drop-flow technique to immobilize double-walled tubular J-aggregates of amphiphilic cyanine dyes without affecting their morphological or optical properties. High-resolution images of the topography and exciton fluorescence of individual J-aggregates are obtained simultaneously with polarization-resolved near-field scanning optical microscopy. These images show remarkably uniform supramolecular structure, both along individual nanotubes and between nanotubes in an ensemble, demonstrating their potential for light harvesting and energy transport.

The influence of a viscous fluid on the vibration dynamics of scanning near-field optical microscopy fiber probes and atomic force microscopy cantilevers

The influence of a viscous fluid on the dynamic behavior of a vibrating scanning near-field optical microscopy fiber tip is investigated both theoretically and experimentally. A continuum mechanical description of a cylindric cantilever is used to calculate the resonance frequencies and the widths of the resonance bands. The linearized Naviers–Stokes equations are analytically solved and describe the interaction of the beam with the viscous fluid. The contribution of the liquid to the shift and the broadening of the resonance lines is summarized by two constants that can be derived from a master function and the kinetic Reynolds number. The theoretical values are compared with experimental data collected from an optical fiber which is used as a probe in a scanning near-field microscope. Agreement, with a relative error of less than 1%, is achieved. The theory is further developed for the application to atomic force microscopy cantilevers with a rectangular cross section. Experimental data taken from literature are in good agreement with the theory.

J-aggregates of amphiphilic cyanine dyes: Self-organization of artificial light harvesting complexes

The simultaneous chemical linkage of cyanine dye chromophores with both hydrophobic and hydrophilic substituents leads to a new type of amphiphilic molecules with the ability of spontaneous self-organization into highly ordered aggregates of various structures and morphologies. These aggregates carry the outstanding optical properties of J-aggregates, namely, efficient exciton coupling and fast exciton energy migration, which are essential for the build up of artificial light harvesting systems. The morphology of the aggregates depends sensitively on the molecular structure of the chemical substituents of the dye chromophore. Accordingly, lamellar ribbon-like structures, vesicles , tubes, and bundles of tubes are found depending on the dyes and the structure can further be altered by addition of surfactants, alcohols, or other additives. Altogether the tubular structure is the most noticeable structural motif of these types of J-aggregates. The optical spectra are characterized in general by a complex exciton spectrum which is composed of several electronic transitions. The spectrum is red-shifted as a total with respect to the monomer absorption and exhibits resonance fluorescence from the lowest energy transition. For the tubular structures, the optical spectra can be related to a structural model. Although the molecules itself are strictly achiral, a pronounced circular dichroism (CD) is observed for the tubular aggregates and explained by unequal distribution of left- and right-handed helicity of the tubes. Photo-induced electron transfer (PET) reactions from the dye aggregates to electron acceptor molecules lead to superquenching which proves the delocalization of the excitation. This property is used to synthesize metal nanoparticles on the aggregate surface by photo-induced reduction of metal ions.

Photoinitiated Growth of Sub-7 nm Silver Nanowires within a Chemically Active Organic Nanotubular Template

Self-assembled supramolecular nanotubes of J-aggregated amphiphilic cyanine dye in aqueous solution are employed as chemically active templates for the photoinitiated formation of silver nanowires with a very small and homogeneous diameter of (6.4 +/- 0.5) nm. Key features of the template are (1) its small and well-defined diameter; (2) its photochemical activity, which allows photoinitiation of the structure formation; and (3) the processability in aqueous solution. The latter includes the potential to remove the template after the reaction, or to functionalize it further, e.g. with optoelectronically active polycations, providing access to quasi one-dimensional hybrid structures with well-defined metallic nanowires as a core.

NEUTRON AND X-RAY REFLECTIVITY STUDIES OF SELF-ASSEMBLED HETEROSTRUCTURES BASED ON CONJUGATED POLYMERS

Neutron and x-ray reflectivity studies of modulated heterostructures consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterostructures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fitting procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/spacer interface. This roughness parame...

Chiral J‐Aggregates Formed by Achiral Cyanine Dyes

The self-assembly of helical supramolecular structures from chiral building units is a basic principle of biological materials. The mesoscopic structure of a chiral molecular aggregate, which formed spontaneously from a nonchiral J-aggregating cyanine dye in aqueous solution, is presented. In single crystals (as shown in the picture) a coexistance of planar molecules with left- and right-handed twisted conformers of the same dye are found; the latter may act as templates to build up the helical superstructures.

Crystalline two-dimensional domains of cyanine dyes at interfaces

Abstract The formation of two-dimensional crystalline J-aggregates of a negatively charged water-soluble cyanine dye at a monolayer of a dimethylammonium amphiphile is observed by fluorescence microscopy and spectroscopy. It is demonstrated that domain size and morphology and spectroscopic properties can be controlled via the surface layer. The formation of rod-shaped crystalline aggregates tightly bound to the surface layer is established. These domains are of uniform size and shape, and order due to long-rang electrostatic repulsion.

Structure and optical properties of a monolayer single crystal of a cyanine dye

Abstract Molecularly thin single crystals of a cyanine dye prepared by adsorption at a charged surfactant monolayer are studied by polarized optical spectroscopy and electron diffraction. Both techniques independently show a herringbone arrangement of the dye with the long axes of translationally inequivalent molecules forming an angle of 80°. The dye arranges in a primitive rectangular unit cell (a=21.2 A, b=6.14 A) adjacent to an epitaxially related hexagonal cell of the aliphatic tails of the surfactant.

Highly luminescent thin films from alternating deposition of CdTe nanoparticles and polycations.

Abstract Thin polymeric films with high concentration of CdTe-nanocrystals are obtained by the self-assembly process of layer-by-layer deposition. Water soluble CdTe particles stabilized by a shell of thioglycolic acid are combined with three different polyelectrolytes: poly(ethylenimine) (PEI), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethylammonium chloride) (PDDA). The film build-up is monitored by means of absorption and fluorescence spectroscopy. The electrostatic adsorption of the particle at the polymer surface is strongly determined by the pH value of the particle solution and the chemical structure of the polymer. Polycations with partially unsaturated nitrogen groups exert strong quenching on the particle fluorescence.