Resonance-induced cluster-mobility: Dynamics of a finite Frenkel-Kontorova model

Statics and dynamics of a finite incommensurate Frenkel-Kontorova chain are studied in the framework of both continuum and discrete models. Since the ground state of the finite chain is not degenerate it cannot freely slide along the substrate, i.e., the finite chain does not possess the “supermobility” of the infinitely long incommensurate chain. Instead, the finite chain shows a “resonance mobility”. Simulating the behaviour of the finite chain in an external field we observe resonances around the lowest frequency in the acoustic branch of the chain spectrum. This frequency depends on the chain size and the degree of incommensurability and corresponds to vibration “in-phase” of all solitons in the chain. The resonance leads to a significantly increased diffusive mobility of clusters of a specific size. This selectively enhanced cluster-mobility persists also in the presence of stochastic forces and friction.     

Oxygen-related band gap state in single crystal rubrene

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.     

Monitoring the non-radiative relaxation time of PpIX solution with Au nanoparticles using Photoacoustic Spectroscopy

In this work we have used the Photoacoustic Spectroscopy (PAS) to determine in vitro the non-radiative relaxation time (NRRT) of a protoporphyrin IX (PpIX) standard solution and samples of PpIX(1), PpIX(2) and PpIX(3) with Au nanoparticle concentrations of 0.001008, 0.00504 and 0.01008 mmol in 25 mL of water respectively. We have used PpIX disodium salt (DS) solution of 25% HCl. The results show that the NRRT average values, obtained for each one of the solution were: τ = 29 ± 0.001, 84 ± 0.001 and 62 ± 0.009 ms for PpIX(1), PpIX(2) and PpIX(3), respectively. These values were compared with some NRRT of triplet states reported in the literature for molecules with tetrapyrrolic structure, increasing the NRRT considerably. From each solution it was obtained its PAS signal phase as a function of the light modulation frequency from 17 to 80 Hz. UV-vis spectrophotometer, photoluminescence spectroscopy and Transmission Electron Microscopy (TEM) were used in order to obtain the optical absorption spectra, the photoluminescence intensities, and the gold nanoparticle sizes respectively. Our investigations are devoted to improve the thermal treatments of drugs the porphyrins as photosensitizers used in image photodynamic therapy.     

Electromigration of Aluminium through Quasi Bamboo‐Like Grain Blocked Silicide Interconnects

MoSi₂ interconnects between electrical aluminium contact pads were exposed to a high direct current density of 1 MA cm^‐2 during 2000 hours. Thermal annealing of the interconnects prior to the experiment generated grains of almost the width of the line, providing for bamboo‐like grain blocked polycrystalline clusters along the line acting as migration stopping sites. Aluminium migration out of the cathode contact pad is observed with pronounced precipitations on the interconnect side walls. SEM and EDX are applied to identify sites and nature of the electromigration product.