Preview

PDF, English Download (45MB) | Terms of use

Abstract

Single crystals of organic semiconductors are chemically pristine and exhibit nearly perfect long-range structural order. As such, they provide an ideal platform to investigate intrinsic properties. Vibrational spectroscopy techniques, such as Raman and Fourier-transform infrared spectroscopy (FT-IR), are widely employed techniques for the characterization of organic materials. They are versatile tools that can be used to study molecular packing and polymorphism in crystalline organic semiconductors, albeit with poor spatial resolution. Two fundamentally different scanning probe techniques with infrared spectroscopy and imaging capabilities offer a spatial resolution below 100 nm - atomic force microscopy-infrared spectroscopy (AFM-IR) and scattering-type infrared scanning near-field optical microscopy (IR-SNOM). This thesis compares the AFM-IR and the IR-SNOM with each other and to the conventional FT-IR spectroscopy with regard to their applicability to small-molecule organic semiconductors. To this end, single crystals of TIPS-pentacene, TIPS-tetraazapentacene, rubrene and per uorobutyldicyanoperylene carboxydiimide (PDIF-CN2) are used as the testbed. Significant differences are observed in the spectra of the crystals depending on the technique and polarization of incident light that are associated with the intrinsic molecular structure and packing as well as the different working principles of the applied methods. Furthermore, the imaging mode of the AFM-IR and the IR-SNOM is tested on solution-deposited microcrystals of PDIF-CN2. Micro- and nanostructures of layered organic materials can also be created by liquid-phase exfoliation (LPE), a popular technique used to produce two-dimensional nanosheets from layered inorganic crystals. The orthorhombic and the triclinic polymorphs of rubrene are dispersed in aqueous surfactant solution by ultrasonication. Distinct nanostructures of rubrene, referred to as nanorods and nanobelts, are formed that are isolated via liquid cascade centrifugation. Their crystalline nature is confirmed through electron diffraction measurements and Raman spectroscopy. Absorbance and photoluminescence (PL) of the dispersions are found to be similar to rubrene solutions due to random orientations of the nanostructures, however, their PL lifetimes are comparable to the macroscopic crystals. The likely arrangement of rubrene molecules within the nanorods and the nanobelts is deduced from AFM images, electron diffraction patterns, and IR-SNOM spectra.