Photoisomerization mechanism of azobenzene in the lowest excited state S 1 ͑n ‫ء‬ ͒ is investigated by ab initio molecular dynamics ͑AIMD͒ simulation with the RATTLE algorithm, based on the state-averaged complete active space self-consistent field method. AIMD simulations show that cis to trans isomerization occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types, namely, clockwise and counterclockwise rotation pathways. On the other hand, trans to cis isomerization occurs via conventional rotation pathway where two phenyl rings rotate around the NN bond. The quantum yields are calculated to be 0.45 and 0.28Ϯ 0.14 for cis to trans and trans to cis photoisomerizations, respectively, which are in very good agreement with the corresponding experimental results.

Triboemission of hydrocarbon molecules such as methane and ethylene induces chemical and mechanical wear of diamond-like carbon.

It is known that the wear of silicon-based materials is due to the tribochemical reaction with water at the sliding interface, but the detailed mechanisms remain under debate. In this study, we used a first-principles molecular dynamics method to investigate the tribochemical wear mechanism. When a small amount of water was present at the sliding interface, the formation of interfacial bridge bonds connecting the two surfaces was observed. These bonds transmitted shear force to the surfaces that induced strain therein. The strained surface Si–O bonds subsequently reacted with water (Si–O–Si + H2O → Si–OH + Si–OH), that is, the hydrolysis reaction occurred. Because the hydrolysis reaction resulted in dissociation of the surface Si–O bonds, water promoted tribochemical wear. However, when a large amount of water was present, it separated the two surfaces. The water thereby suppressed the formation of interfacial bridge bonds and in turn the hydrolysis of Si–O bonds and thus tribochemical wear. Our results indicate that water could either promote or suppress tribochemical wear, depending on how much was present. We suggest that the previously reported humidity dependence of the tribochemical wear of silicon-based materials can be explained in terms of these contrasting roles of water.

Electrolytic corrosion of boron-doped diamond (BDD) electrodes after applying a high positive potential to decompose organic compounds in aqueous solution was studied. Scanning electron microscopy images, Raman spectra, and glow discharge optical emission spectroscopy revealed that relatively highly boron-doped domains were primarily corroded and relatively low boron-doped domains remained after electrolysis. The corrosion due to electrolysis was observed especially in aqueous solutions of acetic acid or propionic acid, while it was not observed in other organic compounds such as formic acid, glucose, and methanol. Electron spin resonance measurements after electrolysis in the acetic acid solution revealed the generation of methyl radicals on the BDD electrodes. Here, the possible mechanisms for the corrosion are discussed. Dangling bonds may be formed due to abstraction of OH groups from C-OH functional groups by methyl radicals generated on the surface of the BDD electrodes. As a result, the sp diamond structure would be converted to the sp carbon structure, which can be easily etched. Furthermore, to prevent electrolytic corrosion during electrolysis, both the current density and the pH condition in the aqueous solution were optimized. At low current densities or high pH, the BDD electrodes were stable without electrolytic corrosion even in the acetic acid aqueous solution.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.