Abstract
Scanning probe microscopy (SPM) plays an important role in the investigation of molecular adsorption. The possibility to probe the molecule–surface interaction while tuning its strength through SPM tip-induced single-molecule manipulation has particularly promising potential to yield new insights. We recently reported experiments, in which 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules were lifted with a qPlus-sensor and analyzed these experiments by using force-field simulations. Irrespective of the good agreement between the experiment and those simulations, systematic inconsistencies remained that we attribute to effects omitted from the initial model. Here we develop a more realistic simulation of single-molecule manipulation by non-contact AFM that includes the atomic surface corrugation, the tip elasticity, and the tip oscillation amplitude. In short, we simulate a full tip oscillation cycle at each step of the manipulation process and calculate the frequency shift by solving the equation of motion of the tip. The new model correctly reproduces previously unexplained key features of the experiment, and facilitates a better understanding of the mechanics of single-molecular junctions. Our simulations reveal that the surface corrugation adds a positive frequency shift to the measurement that generates an apparent repulsive force. Furthermore, we demonstrate that the scatter observed in the experimental data points is related to the sliding of the molecule across the surface.
Highlights
The problem of the adsorption of organic molecules presents many fundamental challenges that stem mostly from the chemical complexity of organic compounds
Interesting is the possibility of probing the molecule–surface interaction while tuning its strength through a well-controlled single-molecule manipulation induced by the scanning probe microscopy (SPM) tip [6,11,1922]
Taking the difference between the red curve and the green curve obtained in [11] with the original model, i.e., without tip oscillation and surface corrugation, we discover that the inclusion of the qPlus oscillation and the surface corrugation in the model changes ∂Fz/∂z(ztip) by adding an additional negative correction term Δcorr that increases its absolute value towards the end of the manipulation
Summary
The problem of the adsorption of organic molecules presents many fundamental challenges that stem mostly from the chemical complexity of organic compounds. A detailed comparison between the simulation and the experiment, reveals systematic inconsistencies that can be attributed to three main factors that have been omitted from the initial model: i) the atomic corrugation of the surface, ii) the elasticity of the tip material, and iii) the finite amplitude of the qPlus tuning fork oscillation.
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