Detection and Stabilization of a Previously Unknown Two-Dimensional (Pseudo)polymorph using Lateral Nanoconfinement.

We report on the detection and stabilization of a previously unknown two-dimensional (2D) pseudopolymorph of an alkoxy isophthalic acid using lateral nanoconfinement. The self-assembled molecular networks formed by the isophthalic acid derivative were studied at the interface between covalently modified graphite and an organic solvent. When self-assembled on graphite with moderate surface coverage of covalently bound aryl groups, a previously unknown metastable pseudopolymorph was detected. This pseudopolymorph, which was presumably "trapped" in between the surface bound aryl groups, underwent a time-dependent phase transition to the stable polymorph typically observed on pristine graphite. The stabilization of the pseudopolymorph was then achieved by using an alternative nanoconfinement strategy, where the domains of the pseudopolymorph could be formed and stabilized by restricting the self-assembly in nanometer-sized shallow compartments produced by STM-based nanolithography carried out on a graphite surface with a high density of covalently bound aryl groups. These experimental results are supported by molecular mechanics and molecular dynamics simulations, which not only provide important insight into the relative stabilities of the different structures, but also shed light onto the mechanism of the formation and stabilization of the pseudopolymorph under nanoscopic lateral confinement.

Phase selectivity triggered by nanoconfinement: the impact of corral dimensions.

By using a novel protocol to spatially confine molecules in well-defined small 2D areas, the so-called nanocorrals, we show using scanning tunneling microscopy (STM) how this kind of confinement affects self-assembled molecular network (SAMN) formation at a liquid-solid interface. The 2D lateral confinement, imposed by the size of the nanocorrals, has a clear impact on the phase selectivity of a molecule that can form both low-density and high-density SAMNs, the high-density phase being promoted by the confinement.

Biasing Enantiomorph Formation via Geometric Confinement: Nanocorrals for Chiral Induction at the Liquid-Solid Interface.

Nanocorrals created by scanning probe lithography on covalently modified graphite surfaces are used to induce a chiral bias in the enantiomorphic assembly of a prochiral molecule at the liquid/graphite interface. By controlling the orientation of the nanocorrals with respect to the underlying graphite surface, the nanocorral handedness can be freely chosen and thus a chiral bias in molecular self-assembly is created at an achiral surface solely by the scanning probe lithography process.

Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates Using Diazonium Chemistry.

A network of self-assembled polystyrene beads was employed as a lithographic mask during covalent functionalization reactions on graphitic surfaces to create nanocorrals for confined molecular self-assembly studies. The beads were initially assembled into hexagonal arrays at the air-liquid interface and then transferred to the substrate surface. Subsequent electrochemical grafting reactions involving aryl diazonium molecules created covalently bound molecular units that were localized in the void space between the nanospheres. Removal of the bead template exposed hexagonally arranged circular nanocorrals separated by regions of chemisorbed molecules. Small molecule self-assembly was then investigated inside the resultant nanocorrals using scanning tunneling microscopy to highlight localized confinement effects. Overall, this work illustrates the utility of self-assembly principles to transcend length scale gaps in the development of hierarchically patterned molecular materials.

The impact of grafted surface defects and their controlled removal on supramolecular self-assembly.

We demonstrate the use of covalently modified graphite as a convenient and powerful test-bed for the versatile investigation and control of 2-D crystallization at the liquid solid interface. Grafted aryls act as surface defects and create barriers to supramolecular self-assembly. An easily tunable grafting density allows for varying the effect of such defects on supramolecular self-assembly. Finally, the defects can be locally removed, triggering monolayer reconstructions and allowing in situ investigations of thermodynamically unstable or metastable morphologies.