Reorientation of pentacene molecules from flat-lying to standing manners on a surface-modified amorphous SiO2 substrate investigated by molecular dynamics simulations

Reorientation of pentacene molecules from the flat-lying manner to the standing one which occurs at the early stage of thin film growth by physical vapor deposition was investigated by molecular dynamics simulations with a surface-modified amorphous SiO2 substrate. A molecular cluster comprising 50 standing pentacene molecules was placed on the substrate together with lying molecules and clusters, and the standing cluster acted as a trigger for the reorientation of lying molecules and clusters. Some lying molecules and clusters adjacent to the standing cluster stood up probably by the short-range van der Waals interaction. Some lying clusters apart from the standing cluster also stood up, and this phenomenon was possibly caused by the effect of the long-range Coulomb interaction between clusters. The number of standing molecules on the substrate increased with increasing temperature, and the temperature dependence of the orientational transition rate was discussed in view of thermal fluctuation.