Different growth modes of molecular adsorbate systems and 2D materials investigated by low-energy electron microscopy

Abstract

In this thesis, the growth of two organic molecules, PTCDA and NTCDA, and of the 2D material hBN on copper surfaces are investigated. The particular focus of this work lies on the interplay of different interaction mechanisms occurring for these systems which lead to completely different growth modes including dendrite-like, fractal growth modes and compact island formation. $\textbf{Organic Molecules}$ In the first part of this thesis, the growth of PTCDA and NTCDA on the Cu(001) surface are investigated. Although these two molecules are chemically closely related, they exhibit two different growth modes on this metal surface. For PTCDA on Cu(001), it is well known that the attractive intermolecular interaction of the PTCDA molecules, caused by the quadrupole moment of the molecule, leads to the growth of compact islands at already very low coverage [54, 66]. This process is quantified in this work by determining three important growth parameters which influence the island formation: the critical cluster size, the cohesion energy of two PTCDA molecules and the diffusion barrier of the adsorbed molecules. By analyzing island size distributions within the aggregation regime and applying methods developed for atomic nucleation on surfaces, it was possible to determine the critical cluster size i for temperatures between 300K and 390K. This parameter corresponds to the number of molecules in the largest cluster of molecules which is not yet stable. The fact that for temperatures below 317K two molecules are already forming a stable cluster (i = 1) enabled to calculate the diffusion barrier for individual molecules on this surface: E$_{D}$ = (0.45 ± 0.21) eV. With increasing temperature, one expects an increase of the critical cluster size. However, the case of i = 2 is experimentally not observed; instead at temperatures above 317K, four molecules are needed to form a stable cluster (i = 3). This direct change in critical cluster size from 1 to 3 is explained by the specific geometric conditions for the case of PTCDA/Cu(001). Furthermore, using pair-potential calculations it is possible to determine a second crucial energy for layer growth: the cohesion energy of two molecules which amounts to E$^{(2)}_{B}$ = (0.89 ± 0.34) eV. In contrast to PTCDA, NTCDA exhibits a completely different growth mode on the same substrate in the submonolayer regime for temperatures at and above room temperature. Clear indications are found for a dendrite-like, fractal growth mode. This finding is based on BF-LEEM measurements indicating that no compact [...