Molecule-Surface Interactions

The CDB molecules were synthesized and deposited onto KCl(001) and imaged using NCAFM. However, since this set of experiments was performed at room temperature, I could not unambiguously identify the adsorption geometry and site, as I did previously with the Co-Salen molecule, due to thermal effects. In order to better understand the balance between the molecule-molecule and molecule-surface interactions, I began by characterizing

the adsorption of CDB molecules onto KCl(001). The molecule was positioned in various initial configurations on the KCl(001) surface, as described in the previous section. All atoms within the CDB molecule and the top 3 surface layers were allowed to relax. The lowest energy configuration of a single CDB molecule on KCl(001) is shown in Figure 6.3.

Figure 6.3: The theoretical minimum energy configuration of a single CDB molecule on KCl(001). K surface cations are shown in green while Cl anions are shown in purple. Within the CDB molecule, C atoms are shown in yellow, H atoms in white, O atoms in red, and N atoms in blue. The molecule lies flat on the surface with CN groups positioned above surface cations. The lowest energy configuration for the hydrocarbon arms corresponds to the cation rows on the surface.

The CDB molecule adsorbs onto KCl(001) with the axis of the molecule (defined as the line connecting the two CN groups) oriented along the [110] direction of the surface.

In this configuration, the CN functional groups are able to interact with cations on the surface. In addition to this lowest energy adsorption geometry, the CDB molecule can be rotated around one CN endgroup in either direction in order to fit the other CN endgroup above nearby surface cation sites without much change in adsorption energy. This is due to the fact that, although the distance between the cations slightly increases, the molecule is still flexible enough to efficiently anchor to the surface. I partitioned the total adsorption energy in this system into contributions from electrostatic interactions and vdW interactions, as with Co-Salen molecules in previous chapters. The total adsorption energy was calculated to be 3.12 eV with a DFT contribution of 0.7 eV and a vdW contribution of

2.42 eV (calculated using semi-empirical dispersion corrections [20]). This partitioning of the adsorption energy indicates that more than 75% of the total adsorption energy can be attributed to vdW interactions with the surface. However, due to the homogeneous nature of the surface, these interactions are not site specific and do not influence the adsorption geometry of the molecule as much as more localized interactions. The geometry of the adsorbed molecule is instead governed by the electronic interactions between polar groups and the surface.

Further analysis of the adsorption of CDB molecules onto KCl(001) using Mulliken [180]

population analysis or Bader charge analysis [181] reveals that a charge transfer within this system is negligible. Furthermore, an evaluation of the work function across the simulation cell reveals a negligible work function change. Finally, I examined an electronic density of states plot of a single CDB molecule adsorbed onto KCl(001), as shown in Figure 6.4.

Figure 6.4: The electronic density of states plot of a CDB molecule adsorbed onto KCl(001).

The total density of states has been projected into surface (Black) and molecule states (Red). The HOMO of the molecule lies nearly 2 eV above that of the surface and the LUMO of the molecule lies nearly 2 eV below that of the surface.

These results predict that the CDB molecule on KCl(001) can be characterized as physisorption, since the HOMO of the molecule lies nearly 2 eV above that of the surface and the LUMO of the molecule lies nearly 2 eV below that of the surface, as shown in Figure 6.4. Furthermore, it is important to note that these results were also used to validate the QM/MM scheme employed to simulate monolayer structures; a 1 eV reduction in the HOMO-LUMO gap of KCl(100) did not have any effect on the electronic structure of the adsorbed system.

In order to further characterize the different molecule-surface contributions involved in the CDB adsorption, I performed additional DFT calculations using the same setup on rep-resentative molecular fragments (cyanophenyl, decyloxy chains) adsorbed to the KCl(001) surface. The strongest molecule-surface interaction comes from interactions between the CN functional group of cyanophenyl and cation sites on the surface. The cyanophenyl molecular fragment adsorbs with N above a cation site while the attached benzene ring adsorbs parallel to the surface plane with combined adsorption energy of 0.7 eV. My results are consistent with previous calculations that give the adsorption energy of a physisorbed benzene ring as 0.5 eV [238] and the adsorption energy of a cyano group on KBr(001) as 0.2 eV [230, 232]. The adsorption energy of the CN group increases above anion vacancies, divacancies, and at step edges and kinks where it can simultaneously interact with more cations. This interaction also represents the strongest electronic interaction present in the system.

The second most significant electronic contribution to the adsorption energy is the in-teraction between O atoms of the decyloxy chains and the surface. In the unfragmented molecule the central phenyl ring adsorbs in a tilted configuration to minimize steric in-teractions with adjacent rings. This means that only one O atom can interact with the surface at a time and results in a net interaction that is much weaker than the CN inter-actions with the surface K ion sites. This is supported by the fact that the O atom to cation interaction was not observed in some low energy configurations (within 0.1 eV of the lowest energy monomer configurations).

Finally, I considered the interaction between the hydrocarbon chains and the KCl(001) surface. Due to rumpling on ionic surfaces, relaxing the surface resulted in the cations being displaced towards the surface (below the surface plane) while the anions were dis-placed away from the surface (above the surface plane) in agreement with the prior calcula-tions [239] and experiments [36]. However, this does not noticeably change the adsorption energy of hydrocarbon chains on the surface. The differences in energy that I calculated by placing a decycloxy chain on the surface with various chain configurations were minor (<0.05 eV). This predicts that at room temperature, the hydrocarbon chains are expected to be quite mobile in contrast to the anchoring groups.

These results show that most components of the molecule-surface interactions within my system are not site dependent and only have minor effects on the adsorption geometry of CDB molecules on the surface. The only exceptions are the interactions between the CN functional group and cations on the KCl(001) surface and a much less prominent interaction between the CDB O atom and a surface cation. These CN groups can be thought of as the anchoring groups of the molecule. Furthermore, small energy differences between chain orientations on the surface hint that the dynamic properties of the CDB molecule on KCl may be important, especially at room temperature.