Structure of <i>p</i>‑Sexiphenyl Nanocrystallites in ZnO Revealed by High-Resolution Transmission Electron Microscopy

The structure of <i>para</i>-sexiphenyl (6P) nanocrystallites embedded in ZnO single crystals is resolved by cross-sectional high-resolution transmission electron microscopy (HRTEM) combined with image contrast simulations and X-ray diffraction measurements. The hybrid structures are prepared by subsequent physical vapor deposition of 6P on ZnO­(1010) templates followed by overgrowth with ZnO. Application of ultramicrotomy for HRTEM specimen preparation and imaging under different focus conditions provides direct access to the atomic and molecular structure of the hybrid interface and the organic inclusion. The hybrid stacks reveal a high structural perfection. The 6P nanocrystallites maintain a structure as in the bulk crystal. Individual 6P lattice planes can be traced up to the lateral and top interfaces with ZnO, indicating that all interfaces are defined on an atomic/molecular level. Further evaluation of the HRTEM images reveals peculiarities of 6P growth on ZnO­(1010). The common 6P β-phase coexists here with the rarely reported γ-phase. The ZnO surface structure induces two mirror-symmetric in-plane preferential orientations of the 6P nanocrystallites. The ZnO surface topography, on the other hand, is critical for the structural perfection of 6P. Although conformal growth is observed, ZnO step edges induce characteristic stacking faults in 6P nanocrystallites

Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift ΔE<i>_m</i> is mainly caused by dispersion effects and depends sensitively on the molecule’s specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide ΔE_<i>m</i>= −<i>QW</i>_<i>m</i> in two factors, where <i>Q</i> depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while <i>W</i>_m is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)

Fingerprint of Charge Redistribution in the Optical Spectra of Hybrid Inorganic/Organic Semiconductor Interfaces

Hybrid structures combining conjugated organic molecules and inorganic semiconductors hold the promise to merge the better of two worlds. To achieve optoelectronic functionality exceeding that of the individual constituent, both the electronic and the optical properties of the hybrid interface must meet certain requirements. Charge redistribution occurring upon deposition of conjugated organic molecules on semiconductor surfaces modulates the electrostatic potential at the hybrid interface. Here we show at the example of ZnO-based hybrid systems that even minuscule charge redistribution is accompanied by a profound modification of the excitonic absorption of the semiconductor. The changes in the optical spectra are detected in real time by differential reflectance spectroscopy (DRS) during the deposition of the molecules. Appropriate modeling of the spectra yields the magnitude of the change of the electrostatic potential. Our findings provide insight into the subtle interplay between optical and electronic properties at hybrid interfaces which is essential to design structures with truly superior function