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.

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(1 1 1). 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(1 1 1) 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.

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 a 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.