Computational Modeling of Tensile Stress Effects on the Structure and Stability of Prototypical Covalent and Layered Materials.

Understanding the stability limit of crystalline materials under variable tensile stress conditions is of capital interest for technological applications. In this study, we present results from first-principles density functional theory calculations that quantitatively account for the response of selected covalent and layered materials to general stress conditions. In particular, we have evaluated the ideal strength along the main crystallographic directions of 3C and 2H polytypes of SiC, hexagonal ABA stacking of graphite and 2H-MoS 2 . Transverse superimposed stress on the tensile stress was taken into account in order to evaluate how the critical strength is affected by these multi-load conditions. In general, increasing transverse stress from negative to positive values leads to the expected decreasing of the critical strength. Few exceptions found in the compressive stress region correlate with the trends in the density of bonds along the directions with the unexpected behavior. In addition, we propose a modified spinodal equation of state able to accurately describe the calculated stress-strain curves. This analytical function is of general use and can also be applied to experimental data anticipating critical strengths and strain values, and for providing information on the energy stored in tensile stress processes.

Assessing the Surface Oxidation State of Free-Standing Gold Nanoparticles Produced by Laser Ablation.

The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose we performed the experiment on a controlled free-standing nanoparticle beam produced by combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68% of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au nanoparticles.

Vibrational spectra of triiodomesitylene: combination of DFT calculations and experimental studies. Effects of the environment.

A study of the internal vibrations of triiodomesitylene (TIM) is presented. It is known from X-rays diffraction at 293 K that the molecule has nearly D(3h) symmetry because of the large delocalization of the methyl protons. By using Raman and infrared spectra recorded at room temperature, a first assignment is done by comparing TIM vibrations with those of 1,3,5-triiodo- and 1,3,5-trimethyl-benzene. This assignment is supported by DFT calculations by using the MPW1PW91 functional with the LanL2DZ(d,p) basis set and assuming C(3h) symmetry. The agreement between the calculated and experimental frequencies is very good: always better than 97% for the observed skeletal vibrations. The calculations overestimate the methyl frequencies by 7%, and experiment shows only broad features for these excitations. Because a neutron diffraction study had established that the TIM conformation at 14 K is not exactly trigonal, new theoretical calculations were done with C(s) symmetry. This shows that all previous E' and E'' modes of vibration are split by 2-12 cm(-1). This is confirmed by infrared, Raman, and inelastic neutron scattering spectra recorded below 10 K. Apart from two frequencies, all the TIM skeleton vibrations have been detected and assigned by using C(s) symmetry. For the methyl vibrations, experiment has confirmed the splitting of the previously degenerate modes; only some small discrepancies remain in the assignment. This is partly due to the difference of the model conformation used in the calculations and the crystallographic one. All these results confirm that each of the three methyl groups has not only its own tunnel splitting but also a different specific spectroscopic behavior for all the molecular modes.