Mechanical Response and Energy-Dissipation Processes in Oligothiophene Monolayers Studied with First-Principles Simulations

Abstract

We present an extensive first-principles study of the interaction between a silicon oxide nanoasperity and a sexithiophene monolayer in order to investigate the individual molecular processes responsible for the energy dissipation during atomic force microscope (AFM) operation. Our approach includes not only ground-state calculations of the tip–sample interaction, but an extensive set of molecular dynamics simulations at room temperature to include the folding deformation modes that are necessary to describe properly the mechanical response on the molecular layer. With this large computational effort, we have characterized the complex configuration space of the combined tip–sample system in a function of the distance, position and orientation of the tip. We identify surface adhesion as the relevant short-range dissipation mechanism. The system is trapped, due to the presence of energy barriers that cannot be overcome even with the available thermal energy, in different bonding configurations corresponding to local energy minima during the approach and retraction of the tip. These energy barriers are responsible for breaking the adiabaticity and thus lead to force hysteresis and energy dissipation. The quantitative agreement between our calculations and experimental results for the mechanical strength and the dissipated energy supports the use of combined theoretical–experimental dynamic AFM studies in order to gain a fundamental understanding of the microscopic mechanisms involved in energy dissipation.