Engineering surfaces to explore 2D crystallization

Surfaces and interfaces play a pivotal role in chemistry and physics, because they break phase symmetry. Often ordering effects occur near the surface, which deeply modify the physical properties and chemical reactivity. Thermodynamically, liquid crystals are rather simple systems because their liquid character allows them to relax rapidly and reach equilibrium. A far more complex and richer situation occurs for crystals, for which the structure results form an interplay of thermodynamics and kinetics. Crystals are arrays of atoms, ions or molecules arranged in a periodic way, i.e., with long-range order. Among all types of crystals, those formed out of organic molecules are the ones that exhibit the largest diversity of crystal structures because organic molecules are themselves inherently complex. Despite decades of research, the crystallization of organic molecules remains mysterious in many aspects. It is impossible to predict, a priori, the crystal structure of a given organic molecule. In addition to thermodynamics, kinetics also plays a fundamental role because most of the crystal phases are formed upon heterogenous nucleation. Over the years, scientists have tried to tailor the structure and chemistry of solid surfaces to control polymorphism. Since surfaces appear to play a very crucial role, scientists have started to explore the occurrence of substrate-induced polymorphism (SIP), i.e., the formation of polymorphs that exist only in the vicinity of solid substrates and that differ from the bulk phase, which can be either be liquid, liquid crystalline or crystalline. When defining SIPs, it must be stated that they are different from self-assembled monolayers (SAMs), as SIPs extend over several molecular layers at least. Another line of research, although conceptually connected to substrate-induced phases, is the self-assembly of molecules physisorbed on highly-regular surfaces (2D crystallization), such as graphene which is an individual layer of graphite. Unprecedented control of the structure of two-dimensional (2D) has been reached. Structure polymorphism in 2D self-assembled organic monolayers deserves careful attention, as processing conditions such as the concentration of building blocks and the choice of the solvent can significantly affect the nanoscale structure of the films. Furthermore, the substrate has a significant effect on the formation and stabilization of polymorphs. The project intends to marry substrate-induced polymorphism and 2D crystallization, which are two complementary, but currently disconnected, lines of research, in order to gain a fundamental understanding of polymorphism at the interface with solid substrates.