With rapid solidification following pulsed laser melting, we have measured the dependence on interface orientation of the amount of solute trapping of several group III, IV, and V elements (As, Ga, Ge, In, Sb, Sn) in Si. The aperiodic stepwise growth model of Goldman and Aziz accurately fits both the velocity and orientation dependence of solute trapping of all of these solutes except Ge. The success of the model implies a ledge structure for the crystal/melt interface and a step-flow mechanism for growth from the melt. In addition, we have observed an empirical inverse correlation between the two free parameters (-"diffusive speeds") in this model and the equilibrium solute partition coefficient of a system. This correlation may be used to estimate values of these free parameters for other systems in which solute trapping has not or cannot be measured. The possible microscopic origin of such a correlation is discussed.

No abstract

Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab initio calculations for the interaction between the adsorbed molecules and the TiO 2 1 surface. In this study we consider two solvents, i.e. water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the perfect and defective anatase (1 0 1) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. This aspect is confirmed by ab initio molecular dynamics simulations with explicit water molecules. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (1 0 1) surface, allowing the coexistence of surface vacancy defects and adsorbed species. Infrared and photoluminescence spectra of anatase nanoparticles exposing predominantly (1 0 1) surfaces dispersed in water and ethanol support the predicted molecular-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO 2 processes in wet environments based on improved computational models with implicit solvation features.

In this work, we propose a methodology to synthesize metallic nanoparticles on textured Fluorine Tin Oxide (FTO) surface by laser irradiations of deposited Au films. In particular, the breakup of the Au films into nanoparticles (NPs) is observed as a consequence of the melting and solidification processes induced by laser irradiations. The mean Au NPs size and surface density evolution are analyzed as a function of the laser fluence. Optical characterizations of the glass/FTO/Au NPs multilayer show, in the absorption spectra, plasmonic peaks due to the Au NPs and an improvement of the light absorption efficiency from the sample with larger Au NPs. The simulated trends of the ratio between the scattering and absorption cross section suggest that the absorption efficiency dominates over the scattering efficiency in the spectral range between 200 and 600 nm. The simulation shows that, by varying the NPs radius from about 18 to 24 nm, the radiation-scattered intensity remains symmetric in forward and reverse directions. These results indicate that the surface coverage size distribution of Au NPs is the key parameter to correlate the structural and optical properties of the glass/FTO/Au NPs multilayer. Furthermore, electrical characterizations highlight a reduction in the sheet resistance of the textured FTO due to the presence of the NPs. We compare these results with those obtained for the same systems when standard furnace annealing processes are used to obtain the Au NPs on the textured FTO surface.

No abstract