The mechanism for the stabilization and surfactant properties of epitaxial silicene.

Using real-time in situ scanning tunneling microscopy and density functional theory simulations, we have studied the growth of Si films on Ag(111) beyond the silicene monolayer, evidencing the existence of metastable phases and an original growth mechanism. Above monolayer Si coverage, an initial structure forms, which is identified as an Ag-free Si bilayer with additional Si adatoms. With further deposition, this structure is replaced by a distinct bilayer structure covered by Si trimers and Ag atoms. The formation of these bilayers follows counterintuitive dynamics: they are partially inserted within the Ag substrate and form by expelling, from the underlying substrate, the atoms that reinsert below the adjacent silicene layer. The growth is therefore characterized by an unexpected "surfactant competition" between Ag and silicene: while silicene is a metastable surfactant for the Ag(111) surface, Ag plays the role of a surfactant for thicker diamond-like Si islands. In spite of being thermodynamically unfavoured, the silicene monolayer is, thus, a remarkably stable structure because of the high kinetic barrier for the growth of thicker layers.

Critical Au Concentration for the Stabilization of Au-Cu Nanoparticles on Rutile against Dissociation under Oxygen.

Controlling aging of catalysts is of crucial importance to preserve their properties, in particular for bimetallic nanoparticles (NPs) where reaction can modify the composition. Herein, we have studied the stability upon oxygen exposure of gold-copper NPs supported on rutile. We have used in situ scanning tunneling microscopy to follow the evolution of individual Au, Cu and Au-Cu NPs with various compositions grown on the TiO2(110) surface, during each step from their nucleation to their modification with oxygen. We demonstrated a direct relation between the stability of the nanoparticles and their Au concentration. Whereas pure Cu nanoparticles dissociate under O2, Au-Cu NPs containing at least 20% Au are stable. This is explained by a modification of the local density of states of Cu atoms upon alloying.

Probing the Si-Si dimer breaking of Si(100)2x1 surfaces upon molecule adsorption by optical spectroscopy.

The adsorption of atoms and molecules of several gases of the Si(100)2x1 silicon reconstructed surface is investigated by surface differential reflectance spectroscopy. This UV-visible optical spectroscopy makes possible the discrimination between two adsorption modes, depending on whether or not the adsorption leads to breaking the Si-Si dimers. The observation of two different optical features is assigned to the bonding on dangling bonds or to the breaking of dimers, and gives access to the adsorption mode of hydrogen, water, oxygen, and pyridine. Moreover, the technique being quantitative, we can determine the total amount of dimers involved in the adsorption and monitor the adsorption kinetics.

Monitoring the transitions of the charge-induced reconstruction of Aau(110) by reflectance anisotropy spectroscopy.

Missing-row reconstructions on Au(110) immersed in electrolytes have been studied by in situ reflectance anisotropy spectroscopy. Transitions between the 1 x 3, 1 x 2, and 1 x 1 surface structures were monitored as a function of the applied potential. A kinetic model allowed us to reproduce the data satisfactorily. These results confirm the theoretical predictions showing that the surface charge determines the surface reconstruction. The transition potentials and the activation barriers were determined.