Nanoscale ordering of the liquid/solid interface using scanning tunneling microscopy
The control of the lateral assembly and spatial arrangement of micro- and nano-objects at interfaces is often a prerequisite for potential applications in the field of nanoscience technology. To create two-dimensional (2D) patterns, one can take advantage of ‘active’ manipulation techniques such as (optical) lithography and scanning-probe microscopy (SPM) techniques. Molecular self-assembly methods—the assembly of molecules without guidance or management from an outside source—provide an alternative approach to building defined structures with dimensions on the nanometer scale.
Self-assembly is a natural phenomenon that can be observed in many biological, chemical, and physical processes.1 Chemisorption—where a substrate molecule adheres to a surface through the formation of a chemical bond, as opposed to a weak physical force (physisorption)—modifies substrate properties such that they significantly differ from those of the ‘naked’ substrate. Chemisorbed self-assembled monolayers2 are thus of prime interest for technological applications. In contrast to chemisorption, physisorption is not very suitable for making ‘permanent’structures. Nevertheless, physisorbed adlayers are perfect model systems to investigate the interplay between molecular structure and the formation of ordered assemblies in two dimensions with high spatial resolution. The dynamic nature of liquid/solid interfaces (where molecules adsorb and desorb) provides an environment in which two-dimensional crystals can be formed by self-healing.
Scanning tunneling microscopy (STM) is one of the preferred techniques to investigate the ordering and properties of these self-assembled layers (usually monolayers), not only under ultrahigh vacuum conditions, but also at the liquid/solid or air/solid interface.3–5 In STM, a metallic tip is brought very close to a conductive substrate (with the self-assembled physisorbed monolayer on top). By applying a voltage between both conductive media, a tunneling current through a classically impenetrable barrier is induced between the two electrodes. For imaging purposes, the tip and substrate are scanned precisely relative to one another and the current is accurately monitored as a function of the lateral position. The contrast in STM images reflects both topography and electronic effects.
A first important factor in obtaining highly regular 2D patterns is the control of molecular conformation upon physisorption. In collaboration with Jan van Esch at the University of Groningen, we recently designed a 2D ‘turn element’, allowing the structure to change direction.6 The successful folding of the molecule critically depends on design features that optimize both molecule-molecule and molecule-substrate interactions. Though physisorption is clearly a weaker interaction than chemisorption, the substrate often acts as a template for the 2D ordering. Successful folding (an intramolecular feature) and molecular organization (an intermolecular feature), were achieved by optimizing molecular symmetry and non-covalent interactions (see Figure 1).
An often underestimated, though obviously important, factor at the liquid/solid interface is the role played by the solvent. Typical organic solvents have a low vapor pressure, are electrochemically inert and have a lower affinity for the substrate than for the compound of interest. However, there is increasing evidence for their effect on ordering. For instance, we discovered, in collaboration with Virgil Percec at the University of Pennsylvania, that the choice of solvent has a major impact.7 Where alkylated solvent molecules carry a phenyl group(1-phenyloctane), some solvent molecules are co-adsorbed in very regular fashion. Meanwhile, amphiphilic molecules of similar size carrying a hydroxyl group(OH) (1-octanol) or carboxylic-acid group (COOH) (1-octanoic acid) do not co-adsorb. Understanding the role of solvent allows the 2D crystals to be fine tuned.
What are these regular 2D assemblies of molecules good for? One could, for example, use them as templates for the subsequent deposition of other nano-objects. Another option is to take advantage of the 2D-crystallinepacking of the molecules to carry out chemical reactions that require strict pre-organization of monomers (distance-orientation), as done for the topochemical polymerization of diacetylenes. This reaction is unique as it can be induced by UV illumination as well as through manipulation with the STM tip.8,9 The latter approach allows carrying out the polymerization reaction with nanometer precision. We are doing this work in collaboration with K. Müllen at MPI Mainz.10
Scanning tunneling microscopy is more than a tool to make images. It is a powerful methodology to investigate self-assembly at the liquid/solid interface where it provides insight into the ordering, dynamics, reactivity, and electronic properties of adsorbates. Moreover, it can also act as an active manipulation tool. Much is to be learned about the effect of solvent and substrate on the self-assembly properties. In addition, the use of potential control to direct the self-assembly in a controlled way at the water/solid interface will open many more possibilities for even water-insoluble organic compounds.
