Multidimensional scanning probe microscopy study of molecular surface adsorption energy landscapes

Scanning probe microscopy permits a characterization of the diffusion of atoms or molecules on surfaces, which is governed by diffusion energy barrier and entropy difference between ground and excited states. With our home-built, cantilever based UHV, low temperature scanning force microscope, the diffusion of benzene-based molecules with flexible and rigid hydrocarbon chains, namely,1,3,5-Triethylbenzene (TEB) and 1,3,5-Trimethylbenzene (TMB), respectively, measured at temperatures between 6.38 and 10.68 K on Cu (111) have been studied. An Arrhenius analysis revealed that the molecule with the flexible hydrocarbon arms (TEB) showed an enhanced diffusion rate which we can attribute to its higher configurational entropy of its ethyl chains. Moreover, we used multidimensional AFM to map the vertical and lateral forces required to manipulate both molecules. Compatible with the lower diffusion energy barrier of the TEB molecule, the force required to manipulate the TEB was much smaller than that of the TMB molecule. In addition, the measured flexural and torsional frequency shift data permitted a detailed analysis of the manipulation process.