Self-assembly of (S)-glutamic acid on Ag(100): a combined LT-STM and ab initio investigation.

Self-assembly of organic molecules at metal surfaces is of greatest importance in nanoscience; in fact, it opens new perspectives in the field of molecular electronics and in the study of biocompatible materials. Combining an experimental low-temperature scanning tunneling microscopy investigation with ab initio calculations, we succeeded to describe in detail (S)-glutamic acid adsorption on Ag(100) at T = 350 K. We find that (S)-glutamic acid organizes in a squared structure and, at variance with the majority of cases reported in literature, it adsorbs in the neutral form, 4.6 A above the surface plane. The interaction with the poorly reactive Ag substrate is only due to weak van der Waals forces, while H-bonds between carboxyl groups and the formation of a OCOH-OCOH-OCOH-OCOH cycle at the vertex of the squares are the main responsible for the self-assembly.

Oxygen adsorption on gold nanofacets and model clusters.

We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra.

Experimental and density functional theory study of the vibrational properties of 2-mercaptobenzimidazole in interaction with gold.

The vibrational properties of the 2-mercaptobenzimidazole (MBI) molecule in interaction with gold were examined by a combined approach of FTIR measurements and density functional theory (DFT). A complete assignment of the 42 normal modes of MBI has been performed on the basis of DFT calculations at the B3PW91 level in complement to the Raman and FTIR spectra. Calculations demonstrated that, on the deprotonated MBI molecule, the negative charge is localized on the sulfur atom, favoring the formation of a gold-sulfur bond upon reaction of MBI with gold. This was confirmed by the very good agreement between the calculated spectrum and the experimental spectra of different gold-MBI compounds, indicating that the vibrational properties of adsorbed MBI are chiefly determined by the coordination through the sulfur atom.

Periodic DFT study of the structural and electronic properties of bulk CoAl2O4 spinel.

In this study, structural and electronic properties of CoAl2O4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co2+ and Al3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al3+ at tetrahedral sites (and Co2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl2O4 can be interpreted as dominated by the local behavior of Co2+ at octahedral sites.

DFT study of oxygen adsorption on modified nanostructured gold pyramids.

Periodic DFT calculations are used to predict and investigate the adsorption behavior of molecular oxygen on Au, Au/Pt, and Pt surfaces. To obtain an array of pyramids containing surface atoms with the lowest possible coordination number, a nano-modified surface consisting of a symmetrically "modified" (100) surface was used. The effect of atom substitution (organized alloying) is investigated. The adsorption of molecular oxygen on a pure gold pyramid is exothermic by 0.77 eV for the end-on adsorption mode. In the case of a pure platinum pyramid, the end-on adsorption mode was found to dissociate; however, a side-on geometry was encountered with an energy of adsorption of 2.3 eV. This value is in line with the fact that the adsorption energy of small molecules does not vary much on Pt surfaces with different indices. Additionally, some geometrically related trends of the surface deformation in relation to its composition and after adsorption of molecular oxygen are highlighted.