Günther Schwabegger

Color tuning of nanofibers by periodic organic-organic hetero-epitaxy.

We report on the epitaxial growth of periodic para-hexaphenyl (p-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

Natural resin shellac as a substrate and a dielectric layer for organic field-effect transistors

Biocompatible and sustainable electronic-grade materials are integral for the development of electronics for biointegration and ‘use-and-throw’ applications. Herein we report the use of the natural resin shellac in organic field-effect transistors. Shellac was employed to cast robust and smooth substrates suitable for durable transistor devices. In addition shellac displays excellent insulating properties enabling its use as a high-quality dielectric layer for organic field-effect transistor (OFET) devices. We demonstrate that two common organic semiconductors, pentacene and C60, show hysteresis-free operation in OFETs that employ shellac both as the substrate and as dielectric material. Shellac is a fully biocompatible (even edible) material that offers many advantages for OFET fabrication, including high dielectric breakdown fields, simple solution processing from ethanol solutions, and low temperature crosslinking at 50–70 °C. This work shows that shellac as a biomaterial can enable OFET applications where biocompatibility is necessary.

UV-induced modulation of the conductivity of polyaniline: towards a photo-patternable charge injection layer for structured organic light emitting diodes

In macromolecular electronics the organic material should offer—besides the desired electronic properties—the possibility of lateral patterning. Here, we report on a novel polyaniline derivative bearing photosensitive N-formamide groups. UV illumination of this polymeric material leads to a decarbonylation reaction resulting in polyaniline which can be subsequently protonated to yield the conductive emeraldine salt. Due to the fact that the conductivity depends on the conversion of the photoreaction, a selective adjustment of the conductivity by means of UV light is feasible. These photo-induced conductivity changes were corroborated in thin films by conductive AFM measurements. Further, we demonstrate the versatility of this polymer with respect to a patterned modification of the conductivity as well as its application as a photo-patternable charge injection layer for structured OLEDs. This makes the new polymer an interesting candidate for electrodes and interconnects in various organic electronic devices.

Morphological and Structural Investigation of Sexithiophene Growth on KCl (100)

The morphology and structure of sexithiophene deposited on KCl (100) substrates was investigated by scanning force microscopy and specular X-ray diffraction measurements. Two different needle-like structures with {010} and {4̅11} contact planes have been observed as well as islands of almost upright standing sexithiophene molecules with a {100} contact plane. Furthermore an azimuthal alignment of all three crystal orientations was observed by X-ray diffraction pole figure measurements, and the growth directions reflect the 4-fold rotational symmetry of the substrate surface. In addition the analysis of crystals with {4̅11} and {100} contact planes unveiled that they share a common crystallographic direction which is explained by ledge directed epitaxy.

Alternately deposited heterostructures of α-sexithiophene–para-hexaphenyl on muscovite mica(001) surfaces: crystallographic structure and morphology

Multi-component systems of para-hexaphenyl (p-6P) and α-sexithiophene (α-6T) molecules show great promise for tuning the fluorescence colour of optically active films. As the opto-electronic properties of rod-like molecules in thin films strongly rely on their anisotropic orientation, a technique for preparation of well-defined, anisotropic multicomponent systems is required. We demonstrate that a p-6P film of less than two nanometer thickness grown on muscovite mica(001) substrates acts as an efficient alignment layer for epitaxial growth of α-6T crystallites. On top of such a p-6P alignment layer, multilayer heterostructures of alternately deposited p-6P and α-6T molecules were grown. Combined X-ray diffraction and transmission electron microscopy studies show that molecules forming α-6T crystallites align parallel to those in the p-6P crystallites leading to the perfect adoption of their herring-bone structures. This alignment is desirable for optical applications and we show that it is preserved for heterostructures composed of up to 120 alternately deposited p-6P (0.8 nm) and α-6T (3.4 nm) nominal layers (120 cycles). Although for co-evaporated α-6T–p-6P molecules formation of a mixed crystal polymorph is reported, we show that in periodically deposited α-6T–p-6P heterostructures phase separation occurs and both molecules crystallize in their well-known equilibrium structures.

The Epitaxial Growth of Self-Assembled Ternaphthalene Fibers on Muscovite Mica.

The morphology and structure of 2,2′:6′,2″-ternaphthalene (NNN) deposited on muscovite mica(001) substrates was investigated by scanning force microscopy (SFM) and specular X-ray diffraction measurements. Consistently, both methods reveal the coexistence of needle-like structures with a {111} contact plane and {001} orientated island-like crystallites, which are built up by almost upright standing NNN molecules. Both orientations are characterized by a well-defined azimuthal alignment relative to the substrate surface, which is analyzed by X-ray diffraction pole figure (XRD-PF) measurements. Based on XRD-PF and SFM analysis, the azimuthal alignment of {001} orientated crystallites is explained by ledge-directed epitaxy along the fibers’ sidewalls. These fibers are found to orient along two dominant directions, which is verified and explained by a doubling of the energetically preferred molecular adsorption site by mirror symmetry of the substrate surface. The experimental findings are confirmed by force-field simulations and are discussed based on a recently reported growth model.

Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices.

Geometrical structure and interface dependence of bias stress induced threshold voltage shift in C60-based OFETs.

The influence of the nature of interface between organic semiconductor and gate dielectric on bias stress electrical stability of n-type C60-based organic field effect transistors (OFETs) was studied. The bias stress induced threshold voltage (Vth) shift was found to depend critically on the OFET device structure: the direction of Vth shift in top-gate OFETs was opposite to that in bottom-gate OFETs, while the use of the dual-gate OFET structure resulted in just very small variations in Vth. The opposite direction of Vth shift is attributed to the different nature of interfaces between C60 semiconductor and Parylene dielectric in these devices. The Vth shift to more positive voltages upon bias stress in bottom-gate C60-OFET was similar to that observed for other n-type semiconductors and rationalized by electron trapping in the dielectric or at the gate dielectric/C60 interface. The opposite direction of Vth shift in top-gate C60-OFETs is attributed to free radical species created in the course of Parylene deposition on the surface of C60 during device fabrication, which produce plenty of hole traps. It was also realized that the dual-gate OFETs gives stable characteristics, which are immune to bias stress effects.

Interface Properties of Organic para-Hexaphenyl/α-Sexithiophene Heterostructures Deposited on Highly Oriented Pyrolytic Graphite

It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules. We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

Photo-Fries-based photosensitive polymeric interlayers for patterned organic devices

This work reports on the investigation of the photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB), which undergoes the photo-Fries rearrangement upon illumination with UV-light, used as interfacial layers in organic electronic devices. Two cases were investigated: the use of a blend of PPNB with poly-vinylcarbazole (PVK) as an interlayer in para-sexiphenyl (PSP) based organic light emitting diodes (OLEDs) and the use of PPNB as gate dielectric layer in organic field effect transistors (OFETs). The photo-Fries rearrangement reaction causes a change of the polymer chemical structure resulting in a change of its physical and chemical properties. The electroluminescence spectra and emission of the PSP OLEDs are not affected when fabricated with a non-UV-illuminated PPNB:PVK blend. However, the electroluminescence is totally quenched in those OLEDs fabricated with UV-illuminated PPNB:PVK blend. Although the dielectric constant of PPNB increases upon UV-treatment, it is demonstrated that those OFETs built with UV-treated PPNB as gate dielectric have lower performance than those OFETs built with non-UV-treated PPNB. Furthermore, the effect of the UV-illumination of PPNB and PPNB:PVK blend on the growth of the small molecules C60 and PSP has been studied by atomic force microscopy. Using photolithography, this kind of photochemistry can be performed to spatially control and tune the optical and electrical performance of organic electronic devices.