As a bottom-up alternative, self-assembly - the spontaneous organization of molecules into stable, structurally well-defined aggregates - has been put forward as a possible paradigm for generating nanoscale templates under ambient conditions or at the liquid-solid interface. Various parameters influence the outcome of the self-assembly process such as solute concentration, temperature, the choice of solvent and substrate. Recently surface-confined two-dimensional (2D) molecular networks, especially those with void spaces, so-called "2D porous networks", attract a lot of interest. We investigate the self-assembly of "simple" systems, but also of multicomponent mixtures.
Understanding how a stereogenic center influences conformations at the molecular scale and organization at the supramolecular level is an elusive and intriguing challenge in a number of scientific disciplines. We focus on surface-confined systems and study how the chiral nature of molecules affects the ordering at the liquid-solid interface. What drives molecules to form two-dimensional chiral patterns? What happens when equimolar mixtures of enantiomers are crystallized or are physisorbed at a surface? And what about the role of the solvent at the liquid-solid interface?
These STM 'pictures' of self-assembled monolayers at the liquid-solid interface might give the impression that these molecules are captured and 'frozen' on the surface. This is definitely not the case. Several spontaneous dynamic phenomena are known to occur at the liquid/solid interface including adsorption-desorption of molecules, reorientation (translation and rotation) of molecules and Ostwald ripening which describes the growth of larger particles at the expense of smaller particles. Our interest is in understanding what kind of dynamics are occurring in these self-assembled layers, how are they affected (by molecular design, etc.), and what are the relevant time-scales. This is of crucial importance to design stable supramolecular patterns or to create highly-responsive surfaces
Scanning probe microscopy allows the investigation of reactions on surfaces with submolecular resolution, even at the liquid-solid interface. Our interest is the investigation of carefully chosen reactions on surfaces and their visualization by scanning probe techniques, and in particular scanning tunneling microscopy .The motivation is three-fold. We want to understand the role of the substrate on surface-confined chemical reactions as some reactions only take place on the surface. In addition, we use the surface as a support to form new molecules, e.g. 2D polymers, that cannot be formed in solution. Furthermore, by carrying out reactions on surfaces, we can basically modify the properties of these surfaces.
Recently, we started the investigation of molecular self-assembly in well-define small areas, the so-called “nanocorrals”. The nanocorrals with different size, shape, and orientation are created on covalently modified highly oriented pyrolytic graphite surfaces using scanning probe nanolithography, i.e., nanoshaving. We discovered for some molecular systems that when nanoshaving is performed in situ, at the liquid−solid interface, the orientation of the self-assembled monolayers is affected. Careful choice of the nanoshaving direction with respect to the substrate symmetry axes promotes alignment of the supramolecular lamellae within the corral. Self-assembly occurring inside corrals of different size and shape reveals the importance of geometric and kinetic constraints.
By using non-covalent as well as covalent functionalization strategies, we aim at changing the properties of 2D materials such as MoS2, graphene, and many others.
For instance, the concepts developed in our group in relation to molecular self-assembly on surfaces, in particular on graphite, are now being evaluated in view of the unique properties of graphene. Graphene is a single atom thick crystal composed of carbon arranged in a honeycomb lattice. One current key challenge in graphene research is to tune its charge carrier concentration, i.e., p- and n-doping graphene. Functionalization of graphene by physisorbed self-assembled monolayers (SAMs) of organic molecules is a promising approach to achieve uniform and controlled doping. We are investigating the potential of molecular self-assembly on graphene to control doping.
Also covalent functionalization strategies can change the properties of 2D materials. Recently we implemented a protocol for the controlled functionalization of graphite and graphene using radical chemistry, avoiding multilayer formation. Interestingly, STM gives a nanoscale view on the degree of functionalization, showing each grafting site. The degree and density of grafting can be controlled. An interesting development is the controlled removal of the grafted molecules using a scanning probe microscopy tip as a broom, restoring the pristine nature of the graphite or graphene substrate. Furthermore, it is possible to protect graphite and graphene surfaces from this radical attack by physisorbed self-assembled monolayers.
Using sensitive scanning probe microscopy techniques, optical nanoscopy techniques, or a combination of both, we investigate a variety of biological materials on surfaces. Current research activities focus on DNA adsorption and (multi)protein-DNA interactions. The latter projects address several questions in contemporary biochemical and biomedical research, e.g. towards the understanding of integration steps of the HIV genome into the human genome. Several of these studies are carried out in collaboration with groups from the medical school, as well as with colleagues from the Physics and Astronomy department.