In order to exploit graphene (Gr) as a transparent electrode in organic optoelectronics, a profound understanding of molecular wetting and diffusion processes on this material is essential. Properties of Gr like band structure, work function, or elasticity depend significantly on the layer number. Here, we report on Gr layer dependent differences in the growth morphologies of hot wall epitaxy deposited sub-monolayer thin films of the one-dimensional conjugated parahexaphenyl (6P) molecule on exfoliated Gr/SiO2. At deposition temperatures around 363 K, 6P forms straight needles arranged in regular networks on single layer graphene. Within the first four Gr layers, the average needle height increases and the average needle length decreases thereby reducing the specific 6P-Gr interface area. The variations in morphology and wetting behavior with the Gr layer number arise from changes in the growth kinetics induced by differences in adhesion and diffusion properties with Gr thickness.

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxyThe growth of small conjugated molecules on graphene is of increasing interest, since the latter bears the potential to serve as a transparent electrode for organic solar cells and light emitting diodes. Here, parahexaphenyl thin films have been grown by hot wall epitaxy on SiO 2 supported exfoliated graphene. The arising morphologies-studied by atomic force microscopy-exhibit a strong dependence on deposition temperature. At temperatures from 280 K-333 K, islands consisting of almost upright standing molecules and needles composed from lying molecules coexist on the graphene flake. Between 363 and 423 K solely needles-consisting of lying molecules-are present on the graphene. The needles form well-ordered networks with relative orientation angles of $30 , $60 , and $90 reflecting the symmetry of the graphene substrate.

No abstract

The dependence of decomposition routes on intrinsic microstructure and stress in nanocrystalline transition metal nitrides is not yet fully understood. In this contribution, three Al0.7Cr0.3N thin films with residual stress magnitudes of −3510, −4660 and −5930 MPa in the as-deposited state were in-situ characterized in the range of 25–1100 °C using in-situ synchrotron high-temperature high-energy grazing-incidence-transmission X-ray diffraction and temperature evolutions of phases, coefficients of thermal expansion, structural defects, texture as well as residual, thermal and intrinsic stresses were evaluated. The multi-parameter experimental data indicate a complex intrinsic stress and phase changes governed by a microstructure recovery and phase transformations taking place above the deposition temperature. Though the decomposition temperatures of metastable cubic Al0.7Cr0.3N phase in the range of 698–914 °C are inversely proportional to the magnitudes of deposition temperatures, the decomposition process itself starts at the same stress level of ~−4300 MPa in all three films. This phenomenon indicates that the particular compressive stress level functions as an energy threshold at which the diffusion driven formation of hexagonal Al(Cr)N phase is initiated, provided sufficient temperature is applied. In summary, the unique synchrotron experimental setup indicated that residual stresses play a decisive role in the decomposition routes of nanocrystalline transition metal nitrides.

No abstract