No abstract

Complementary experimental and theoretical results on the coalescence of nanoparticles demonstrate the importance of the crystallographic orientation on the coalescence process. In situ hot-stage transmission electron microscopy studies on self-supporting films consisting of indium tin oxide nanoparticles clearly show rotations of neighboring particles preceding their coalescence. Both rotation and coalescence are observed well below half the melting temperature. The coalescence of two adjacent nanoparticles is simulated by means of a combination of the kinetic Monte Carlo method for atomic diffusion with an integration of the equations of motion for the rigid body degrees of freedom of the two particles. This allows analyzing the reorientation of the two crystal lattices prior to the merging process. Thus, nanoparticle coalescence has theoretically as well as experimentally been shown to be a two-step process: first a reorientation of adjacent nanoparticles, and second their complete or incomplete coalescence depending on the matching of the crystallographic orientations.

The organic molecule 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was studied by means of scanning tunneling microscopy (STM) on thin insulating NaCl films grown on a Cu(111) single crystal. The deposition of approximately two monolayers (ML) of sodium chloride onto a Cu(111) substrate at a sample temperature of about 350 K causes a rather rough growth of (100)-oriented NaCl islands up to a local height of 4 ML. For submonolayer coverages (0.1 and 0.4 ML) of PTCDA on a Cu(111) surface partly covered with NaCl, two different rod structures of PTCDA were found on the copper surface, which are in contrast to previously published data for PTCDA on Cu(111) showing a herringbone-like arrangement. These findings can be explained by the formation of a Na(x)-PTCDA complex. On NaCl covered areas, single PTCDA molecules adsorb at vacancies of [010] and [001] oriented steps of the NaCl(100) islands. In this case, the electrostatic forces between the polar step edges and the PTCDA molecules are dominant. The terraces of the alkali halide surface are free of PTCDA molecules.

If copper phthalocyanine ͑CuPc͒ molecules are deposited on a Au͑111͒ surface covered with a monolayer of C 60 , the molecules are found to adsorb individually onto the close-packed layer of C 60. As the adsorption site of the CuPc is not symmetric with respect to the underlying C 60 layer, the CuPc molecule has six equivalent orientations according to the hexagonal packing of the C 60. Scanning tunneling microscopy ͑STM͒ measurements reveal that at room temperature, the molecules may hop between these adsorption sites due to thermal activation and therefore appear as a ring with six maxima. This paper reviews the STM measurements carried out by Stöhr et al. in the light of molecular force field calculations. The potential energy for an individual CuPc molecule has been calculated as a function of the lateral position on the C 60 layer based on the van der Waals interactions between molecules. For each position, the minimal energy has been evaluated, taking the vertical and the rotational degrees of freedom into account. A map of the potential energy not only reveals the preferred adsorption sites, it also indicates several favorable paths between these minima. The calculations consistently explain the experimental observations at low and ambient temperature.

The growth of N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DiMe-PTCDI) on KBr(001) has been studied by frequency modulation atomic force microscopy. At low coverages, DiMe-PTCDI forms molecular wires with a length of up to 600nm; at higher coverages, wire networks were found. The height of the wires is two or more molecular layers; if the sample is cooled to 250K during evaporation, also wires of one layer are found. All wires grow along the [110] and [1¯10] directions of the KBr(001) substrate. There is no wetting layer of molecules. Atomic resolution of the substrate could be achieved between the wires. Due to their size and shape, the molecular nanowires might act as a model system for organic electronics research on insulating substrates.