Witold Piskorz

Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state.

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.

Molecular-dynamics simulations of cluster–surface collisions: Emission of large fragments

Large-scale classical molecular-dynamics simulations of (H2O)n (n = 1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].

Molecular mechanism of C–H bond cleavage at transition metal oxide clusters

Abstract The molecular mechanism of the cleavage of the C–H bond at oxide surfaces seems to be different from the oxidative addition and from the simple result of the interaction with an M–O site at the surface, representing an acid–base pair. In this Letter a novel mechanism of this process is proposed where both fragments of the cleaved C–H bond become attached to the surface oxide ions. The proton forms an OH group, the hydrocarbon fragment forms an alkoxy group. Simultaneously, the two electrons of the cleaved bond are injected into the conductivity band. The proposed mechanism is based on results of density functional theory calculations for methane molecule interacting as well with small vanadium oxide particles as cluster models mimicking fragments of the oxide surface.

On the ground electronic state of MoO+: Upgrade density functional theory calculations

Density functional theory calculations for MoO+ with the new BPW91 exchange-correlation functional are reported and compared to recent experimental results regarding electronic states and bonding in the cation. In variance to the local and previous versions of nonlocal functionals, BPW91 gives proper description of the ground state of MoO+ and ionization potentials of MoO in very good agreement with the experiment.