Adhesion to Step Edges

I then performed long time-scale simulations of many CDB molecules on the KCl(001) surface in order to gain insight into how nucleation may happen in this system. These calculations were performed using the MRS force field and a 4 layer slab representing the KCl(001) surface. The top three layers of KCl were allowed to relax along with all CDB atoms. The system was treated using the NVE microcanonical ensemble and contained a total of 49 CDB molecules on a large KCl(001) terrace which incorporated a step edge in the [100] direction. The starting point of this simulation can be seen in Figure 7.6A.

Figure 7.6: A) The starting point of a large MD simulation containing 49 CDB molecules on KCl(001). A step edge feature can be seen running down the center of the simulation cell. B) A snapshop of the MD simulation taken after 10 ns at 500 K. Some CDB molecules can be seen adhering to step edges, while others interact with each other in what resembles close-packed structures

After evolving this system for 20 ns at 500K, several CDB molecules could be observed at the step edge, as shown in Figure 7.6B. However, the other molecules exhibited very little motion during this time. Although the rate of diffusion observed for a single CDB molecule was sufficiently fast that it cannot be imaged with NCAFM, the motion of these molecules was still too slow for me to investigate growth directly using MD. Fortunately, these results still provide insight into the early stages of growth and nucleation.

Previous calculations and experimental data agree that isolated CDB molecules diffuse

rapidly across the KCl(001) surface. They must become immobilized on the surface in order to form the highly ordered films described in Chapter 6. The first possibility that I examined was the trapping of CDB molecules at step edges and kinks on the KCl(001) surface. I began by performing molecular dynamics simulations using the MRS force field of a single CDB molecule at a step edge feature on KCl(001).

Throughout 10 ps of equilibration and 20 ns of MD, the CDB molecule was observed to be stable at the step edge. Using the MD trajectory from these calculations, I then performed full DFT calculations in order to estimate the adsorption energy of a single CDB molecule at a [100] step edge on KCl(001). These DFT calculations were performed using the same methods described in Chapter 6 and represented the system as a 4 layer slab of KCl with a step edge rising one layer above the surface level. The lowest energy configuration of a CDB molecule at the step edge is shown in Figure 7.7.

Figure 7.7: The optimized adsorption geometry for a single CDB molecule at a step edge on the KCl(001) surface. The CN anchoring group simultaneously interacts with two surface cations while the hydrocarbon chain adsorbs along the step edge itself.

When an isolated CDB molecule adsorbs at a step edge, the adsorption energy increases to 4.0 eV. This value was verified using both the QM/MM hybrid scheme and a full DFT setup. The large increase in adsorption energy in this configuration confirmed that a CDB molecule that finds a step edge on the surface at room temperature will spend a significant amount of time there before desorbing. The increased adsorption energy can

be attributed largely to two interactions. The first of these is that the CN group can simultaneously interact with two cation sites on the surface rather than just one. Secondly, the hydrocarbon chain can lie along the step edge and further increase adsorption energy via vdW interactions with the surface atoms. This result is consistent with experimental images that show decorated step edges when low coverages of CDB molecules are deposited at room temperature. After one CDB molecule has been stabilized at the step edge, it becomes increasingly favourable for additional molecules to adsorb there. The lowest energy configuration of a second captured molecule adsorbing adjacent to the first one along the step edge was calculated to be 4.2 eV. CDB molecules adsorbing next to the first one on the terrace, however, were predicted to have a much smaller increase in adsorption energy resulting in a final value of 3.3 eV.

When the CDB molecule adsorbs at a kink or corner the adsorption energy increases accordingly. At such sites, the CN group can interact with a total of 3 cation sites and it becomes possible for the second hydrocarbon chain to lie along a step edge as well. One such configuration is shown in Figure 7.8

Figure 7.8: The optimized adsorption geometry for a single CDB molecule at a kink feature on the KCl(001) surface. The CN anchoring group simultaneously interacts with three sur-face cations while the two hydrocarbon chains lie along the step edges. These interactions contribute to greatly increase the adsorption energy of the molecule.

Full DFT calculations of a CDB molecule adsorbed to a kink feature on KCl(001)

estimate the adsorption energy to be 4.4 eV. These results show that molecules captured at kink features are even more stable than at step edges. These molecules were predicted to increase the adsorption energy of adjacent molecules along the step edge to 4.2 eV and ones on the clean terrace to 3.3 eV as well.