Measurement of the Binding Energies of the Organic-Metal Perylene-Teracarboxylic-Dianhydride/Au(111) Bonds by Molecular Manipulation Using an Atomic Force Microscope

Based on single molecule manipulation experiments in a combined scanning tunneling microscope/frequency modulated atomic force microscope, we quantify the individual binding energy contributions to an organic-metal bond experimentally. The method allows the determination of contributions from, e.g., local chemical bonds, metal-molecule hybridization, and van der Waals interactions, as well as the total adsorption energy.

Figure 1

Scheme of the experimental setup. A single PTCDA molecule on Au(111) is contacted by a tip attached to a qPlus sensor. The oscillating sensor is retracted from and approached to the surface [28]. In this way, the molecule is repeatedly detached from the surface and brought back. Changes in the resonance frequency of the sensor reflect changes in the stiffness of the junction.

Figure 2

(a) Histogram ($\sqrt{N}$) of 226 $\Delta f$ traces acquired during lifting and lowering of PTCDA. All curves have been aligned at feature $B$. The $\Delta f$ signal during tip approach was subtracted [20]. The $z$ axis is shifted to match the simulated curves in panel (b). Dotted lines mark regions 1 and 2 within which the correspondence between experiment and simulation is calculated (reduced ${\chi }^2$). Distinct features of the histogram are labeled $A_1$, $A_2$, and $B$. (b) Histogram ($\sqrt{N}$) of 4166 $\Delta f$ traces simulated with different parameter sets $P$ that have passed the preselection criteria (see text). (c) The energy dissipation of the qPlus sensor as well as the experimental variance ${\sigma }^2$ show a peak around $z=16\AA$ (see text).

Figure 3

(a) Plot of the correspondence between individual simulations and experiment (reduced ${\chi }^2$) versus the adsorption energy of PTCDA for all sets $P$ with ${\chi }^2<3$. (b) and (c) Plots of ${\chi }^2$ versus the energy contributions of the different potentials (and different types of atoms). “Carbon” stands for all atoms except ${\mathrm{O}}_{\mathrm{carb}}$. Dotted lines are a guide for the eye.

Figure 4

(a) Comparison between simulated and experimental $\Delta f$ curve for PTCDA and NTCDA on Au(111). Parameter set $P^{*}$, derived for PTCDA (see text), was used for both simulations. The experimental curves are cut in the region of an instable junction (Fig. 2), and both parts are separately aligned with the simulation. (b) Binding potential as a function of molecule-substrate separation calculated with set $P^{*}$ for PTCDA and NTCDA. The molecule is relaxed on the surface and subsequently moved to different heights without further relaxation.