Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations

Temperature-dependent molecular orientations in the initial growth processes of pentacene on amorphous SiO2 surface with different substrate temperatures have been investigated using molecular dynamics simulations. As the substrate temperature ranges from 270 K to 600 K, there exists a transition behavior for pentacene cluster from the normal-oriented, ordered configuration to the lateral-oriented, disordered one as measured by the decreased average orientation angle and order parameter, showing the significant effect of the substrate temperature on the molecular orientation. The transition behavior is related to the strength relationship between molecule-molecule interactions and molecule-substrate interactions. During the optimal temperature range between 300 K and 350 K, the pentacene molecules tend to form the normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions, which is affected by the substrate temperature in a greater degree than the molecule-substrate interactions. When the temperature is lower than 300 K, the ordering of pentacene cluster becomes a little worse. A higher substrate temperature results in the lateral orientation with the weakening of the molecule-molecule interactions. Then the further intensification of molecular thermal motion gradually makes the molecules separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.