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

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 [...

In-situ study of two-dimensional dendritic growth of hexagonal boron nitride

Hexagonal boron nitride, often entitled the "white graphene" because of its large band gap, is one of the most important two-dimensional (2D) materials and frequently investigated in context with stacked arrays of single 2D layers, so called van der Waals heterostructures. Here, we concentrate on the growth of hBN on the coinage metal surface Cu(111). Using low energy electron microscopy and diffraction, we investigate the self-terminated growth of the first layer in-situ and in real time. Most prominently, we find dendritic structures with three strongly preferred growth branches that are mostly well aligned with the Cu(111) substrate and exhibit a three-fold symmetric shape. The observation of dendritic structures is very surprising since hBN was found to grow in compact, triangular-shaped islands on many other metal substrates, in particular, on transition metal surfaces where it shows a much stronger interaction to the surface. We explain the unexpected dendritic growth by an asymmetry of the bonding energy for the two possible ways a borazine molecule can attach to an existing hBN island, namely either with one of its boron or one of its nitrogen atoms. We suggest that this asymmetry originates from different dehydrogenation states of the adsorbed borazine molecules and the hBN islands. We call this mechanism "Dehydrogenation Limited Aggregation'' since it is generic in the sense that it is merely based on different dehydrogenation energies for the involved building blocks forming the 2D layer

Two-dimensional growth of dendritic islands of NTCDA on Cu(001) studied in real time

The success of future organic electronic devices distinctively depends on the electronic and geometric properties of thin organic films. Although obviously these properties are strongly influenced by the growth mechanisms, real time growth studies are relatively rare since not many experimental techniques exist that allow in situ studies in ultra high vacuum. In this context, we investigated the prototypical system 1,4,5,8-naphtalene-tetracarboxylic-dianhydride (NTCDA) on Cu(001). We used low-energy electron microscopy (LEEM) for the real-time growth study, and a variety of other techniques for investigating the geometric and electronic structure. While for similar model systems well known and well characterized growth modi occur (e.g., compact, well ordered islands or disordered, gas-like layers), for NTCDA/Cu(001) we observe the growth of dendrite-like, fractal structures. The dendritic structures arise from a strongly preferred one-dimensional growth mode forming a long-range ordered network of thin molecular chains spanning over the entire surface already at small coverages. Later in the growth process, the voids in the network structure are incrementally filled. These results are very unexpected for such a simple adsorbate system consisting of well investigated components, the properties of which were believed to be already well understood. We explain this unexpected behavior by a dendritic growth model that is supported by energetic arguments based on pair-potential calculations. These calculations give reason for the experimentally observed growth of one-dimensional structures, and therefore represent the key to a semi-quantitative understanding of this dendritic growth mode

Controlling the growth of multiple ordered heteromolecular phases by utilizing intermolecular repulsion

Metal/organic interfaces and their structural, electronic, spintronic and thermodynamic properties have been investigated intensively, aiming to improve and develop future electronic devices. In this context, heteromolecular phases add new design opportunities simply by combining different molecules. However, controlling the desired phases in such complex systems is a challenging task. Here, we report an effective way of steering the growth of a bimolecular system composed of adsorbate species with opposite intermolecular interactions—repulsive and attractive, respectively. The repulsive species forms a two-dimensional lattice gas, the density of which controls which crystalline phases are stable. Critical gas phase densities determine the constant-area phase diagram that describes our experimental observations, including eutectic regions with three coexisting phases. We anticipate the general validity of this type of phase diagram for binary systems containing two-dimensional gas phases, and also show that the density of the gas phase allows engineering of the interface structure

Momentum microscopy on the micrometer scale: photoemission micro-tomography applied to single molecular domains

Photoemission tomography (PT) is a newly developed method for analyzing angularresolved photoemission data. In combination with momentum microscopy it allows fora comprehensive investigation of the electronic structure of (in particular) metal-organicinterfaces as they occur in organic electronic devices. The most interesting aspect in thiscontext is the band alignment, the control of which is indispensable for designing devices.Since PT is based on characteristic photoemission patterns that are used as fingerprints,the method works well as long as these patterns are uniquely representing the specificmolecular orbital they are originating from. But this limiting factor is often not fulfilledfor systems exhibiting many differently oriented molecules, as they may occur on highlysymmetric substrate surfaces. Here we show that this limitation can be lifted by recording thephotoemission data in a momentum microscope and limiting the probed surface area to onlya few micrometers squared, since this corresponds to a typical domain size for many systems.We demonstrate this by recording data from a single domain of the archetypal adsorbatesystem 1,4,5,8-naphthalenetetracarboxylic dianhydride on Cu(0 0 1). This proof of principleexperiment paves the way for establishing the photoemission μ-tomography method as anideal tool for investigating the electronic structure of metal-organic interfaces with so farunraveled clarity and unambiguity

Growth and Evolution of TCNQ and K Coadsorption Phases on Ag(111)

Alkali-doping is a very efficient way of tuning the electronic properties of active molecular layers in (opto-)electronic devices based on organic semiconductors. In this context, we report on the phase formation and evolution of charge transfer salts formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) in coadsorption with potassium on a Ag(111) surface. Based on an in-situ study using low energy electron microscopy and diffraction we identify the structural properties of four phases with different stoichiometries, and follow their growth and inter-phase transitions. We label these four phases α to δ, with increasing K content, the last two of which (γ and δ-phases) have not been previously reported. During TCNQ deposition on a K-precovered Ag(111) surface we find a superior stability of δ phase islands compared to the γ phase; continued TCNQ deposition leads to direct transition from the δ to the β-phase when the K:TCNQ ratio corresponding to this phase regime is reached, with no intermediate γ-phase formation. When, instead, K is deposited on a surface precovered with large islands of the low density commensurate (LDC) TCNQ phase that are surrounded by a TCNQ 2D-gas, we observe two different scenarios: On the one hand, in the 2D-gas phase regions, very small α-phase islands are formed (close to the resolution limit of the microscope, 10-15 nm), which transform to β-phase islands of similar size with increasing K deposition. On the other hand, the large (micrometer-sized) TCNQ islands transform directly to similarly large single-domain β-phase islands, the formation of the intermediate α-phase being suppressed. This frustration of the LDC-to-α transition can be lifted by performing the experiment at elevated temperature. In this sense, the morphology of the pure TCNQ submonolayer is conserved during phase transitions