Wolfgang S. Bacsa

Evidence of anisotropic metallic behaviour in the optical properties of carbon nanotubes

We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet, in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. The fair agreement between our experimental data and the phenomenological MG-approach suggests that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

Blue organic light emitting diodes based on bicarbazyle derivates: Device stability and multilayer configuration

The device stability of blue light emitting diodes based on N,N′-diethyl-3,3′-bicarbazyle, (EtCz)2, are substantially improved by coevaporation with N,N′-diethyl-3,3′-bicarbazyle-6,6′ dicarbaboxylic acid, (OcCzCOOH)2. We attribute this effect to the formation of a network of hydrogen bonds in the alloy related to the presence of carboxyl groups. A three layer device with transport layers for both electrons and holes improves the quantum efficiency: With an Al cathode we obtain a luminance of 100 cd/m2 at current density levels of 50 mA/cm2. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. The incorporation of an electron transport layer splits and shifts the electroluminescent band to lower energies and we observe a significant additional downshift after a period of three weeks which we attributed to molecular diffusion at the interface between the emitting and elec...

The optical response of carbon nanotubes

Abstract We have conducted reflectivity measurements on carbon nanotubes from the far-infrared up to the ultraviolet (from 20 cm −1 up to 3× 10 4 cm −1 ), in order to achieve the complete excitation spectrum. We have analysed our experimental findings in terms of the Maxwell-Garnett (MG) effective medium model, where small conducting particles are dispersed in a dielectric host-medium. We establish that carbon nanotubes have an intrinsic and anisotropic metallic behaviour.

Coherent photon imaging in near-field optics

The coherent superposition of optical waves or optical interference is an integral part of the image formation in near-field optics and can be used to increase the lateral resolution. Cavity effects increase the resolution furthermore and shadow formation avoids aperture limitations in reflection-collection nearfield optics. The application range of optical scanning probe microscopy is extended by taking advantage of optical interference and standing waves by preserving at the same time spectroscopic imaging ability, the main advantage of optical scanning probe techniques.

Organic light-emitting diodes based on bicarbazyles

Bicarbazyles are stable and small organic molecules with external quantum yields of 0.1 percent in single layer devices. Devices from (EtCz)2 emit in the violet-blue spectral region. Multilayer configuration with transport layers show a significant higher brightness and quantum efficiency and addition of (OcCz COOH)2, improves the device stability. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. Although the applicability of the polaron concept to small molecules is somewhat questionable, the excellent result obtained with the model reveal the importance of molecular relaxation processes during the jump of the carrier.

PURIFIED CARBON NANOPARTICLES INVESTIGATED WITH HREM AND RAMAN SPECTROSCOPY

Previous studies of fullerene-related materials (nanotubes, onions, etc.) have been almost exclusively performed with electron microscopy methods due to the heterogeneous character of the deposits produced in the electric arc. We report Raman spectra of carbon nanotubes which have been purified following the procedure of Ebbesen et al. The purity of the sample has been confirmed by high-resolution electron microscopy. The Raman spectra of carbon nanotubes with an average diameter of 10–12 nm show very little disorder-induced scattering and no curvature-induced phonon softening, in contrast to unpurified material which contains a large number of carbon nanoparticles. These experimental findings are confirmed by lattice-dynamical calculations. Preliminary low-loss EELS measurements on nanotubes reveal significant differences for the graphite interband transition and collective valence-electron plasmon excitation, which are attributed to the cylindrical structure of the nanotubes.

Carbon Nanotubes and Aligned Carbon Nanotube Films

Carbon nanotube are slowly coming to the forefront of fullerene materials especially since mass quantities can be produced rather easily. A new methods to make aligned nanotube films hold many promises, not only for basic research but also for eventual applications. With this method the tubes can be aligned either parallel or perpendicular to the surface. Here we give a short review of the field with several examples of the microscopic properties of single tubes and the bulk properties of the aligned films.