Hetero-epitaxy of organic-organic nanofibers

During the last years organic materials became of increasing interest in many fields of research. On the one hand a nearly unlimited variety of molecules is available by chemical synthesis leading to a broad spectrum of applications. On the other hand the possibility to manufacture organic electronic based products at very low costs and the availability of flexible substrates opens niches not occupied by silicon-based electronics. During the last years the supramolecular self-assembly processes of several conjugated organic molecules have been studied, providing access to defined shape-persistent objects under thermodynamic control. Above all, it has been demonstrated that phenylenes, in particular para-hexaphenyl (p-6P), and functionalized quarter-phenylenes in combination with muscovite mica provide unique structural properties, ending up in parallel nano-needle formation being the key for photonic device applications e.g. waveguiding, polarized light emission/absorption and lasing. It turned out that material combinations, providing microscopic and macroscopic highly ordered nano-needles, are rare and consequently the possible spectrum of applications is constricted. In particular optical emission of such nano-structures is limited to the blue spectral range. Consequently novel approaches have to be found in order to enlarge the scope of organic nano-needles for future optical applications. Consequently it should be demonstrated within this project that organic-organic hetero-epitaxy represents a powerful technique being able to positively influence structural and morphological properties of organic nano-needles. In particular it should be demonstrated that the suggested approach leads to microscopic and macroscopic highly ordered structures due to parallel alignment of thiophenes and phenylenehiophene co- oligomers forced by p-6P nano-needle templates. A successful demonstration would indeed enlarge the possible spectrum of materials which can be used for the formation of self-assembled organic nano-needles and in a direct consequence optical emission and absorption of such highly interesting structures can be tuned in a broad spectral range. Additionally the ability of periodically grown organic-organic heterostructures in respect to a well controlled and stable doping procedure should be investigated. In particular it is expected that optical properties of nano- needles can be controlled and tuned by such a method in a precise and accurate way and in the optimum case a repetition of the preferential orientation of the subsequent layers could lead to an organic superlattice being the key for future photonic applications.

During the last years organic small molecules contributed to many applications of opto-electronics due to their abbility to show strong fluorescence paired with high quantum yield. In order to fabricate organic lasers, a detailed knowledge on the nucleation and crystallization processes of organic molecules is essential. Consequently, the main goal of this project was to fabricate and characterize organic-organic multilayer structures using organic self-assembled nano-fibres as template layers.The fundament was the fabrication of nano-fibres by depositing rod-like para-hexaphenyl (p-6P) molecules on muscovite mica substrates. The chosen molecules tend to self-assembly as parallel oriented crystals (fibres) which are characterized by highly polarised blue fluorescence (due a parallel molecular alignment) and their ability to act as optical waveguides. In further consequence these waveguides act as optical resonators being the fundament for random lasing emission in the blue spectral range.In order to tune the emission colour of such nano-lasers, the main goal of the project was to embed other molecular species. Exemplary, sexi-thiophene (6T) molecules were selected due to their fluorescence in the orange/red spectral region. Based on these two organic compounds it has been demonstrated that organic-organic hetero-structures can provide a proper starting point to reach the project goal namely tuning the lasing emission wavelength from the blue (p-6P) to the red (6T) spectral region. In particular it could be demonstrated that structural properties of the chosen p-6P template crystals can be adopted by 6T molecules deposited on top.Finally, it has been demonstrated that the fabrication of organic-organic multi-layer structures can be used to precisely tune the emission colour of nano-fibers from the blue via white to the green spectral range without changing the highly anisotropic molecular order within the fabricated crystals. Moreover, it was shown that the high anisotropy on the molecular level leads to polarised emission and that the obtained crystal quality is the key to reduce optical losses within the waveguides.All these results were the fundament for the fabrication of organic-organic multi-layer fibres which showed lasing in the green and orange spectral range.