M-H Choi, H. Noh, S.B. Kaemmer
Park Systems inc.,
United States
Keywords: Van der Waals forces, atomic force microscopy, self-assembly, F14H20 superstructure, PinPoint mode, non-contact mode
Summary:
Van der Waals (vdW) forces are significant intermolecular forces that come into play when molecules are in close proximity.1,2 These forces are particularly relevant for non-polar or weakly polar molecules, such as those in SFAs. VdW forces are often modeled using the Lennard-Jones Potential, providing a simplified representation of attractive and repulsive forces between particles (atoms).3, 4 Thus, the Lennard-Jones Potential is one of the centerpieces for applying force field and dispersion interactions in molecular dynamic (MD) simulations to study the behavior and interactions of atoms and molecules over time.2 The fundamental principle behind high-resolution imaging in Atomic Force Microscopy (AFM) lies in the precise control of vdW forces during the interaction between atoms at the AFM probe tip and the sample surface.5, 6 Operating AFM in the repulsive force regime in the Lennard-Jones Potential generates strong vdW forces between AFM tip and sample surface, while the attractive regime is characterized by comparatively weak forces.3 AFMs ability for precise force control on a sub-microscopic scale becomes particularly crucial when considering the influence of vdW forces on self-assembled small entities. F14H20, despite its electrically neutral nature, possesses localized charge asymmetry that makes it a compelling subject for self-assembly investigations. When deposited onto a surface, F14H20 molecules, with a 3.1 D dipole along the chain, showed a –0.8 V surface potential in Kelvin Probe studies on LB layers of various FnHm variants,7, 8 indicating vertically oriented molecular chains with fluorinated parts facing the air. Previously, a morphological shift in the superstructure of F14H20 surface micelles, transitioning from elongated to spiral shapes, was reported. This shift occurred as the occurrence of elongated micelles decreased with an increase in surface pressure during the macroscale transfer process.9 In this study, we investigated the effects of direct repulsive force interaction on F14H20 aggregates (superstructure formation) using a nanomechanical technique, an AFM mode called PinPoint mode.10,11 Applying direct repulsive forces, enabled precise control of minuscule van der Waals forces, allowing observation of significant transformations in F14H20 surface micelle aggregates. Subsequentially, we analyzed time-dependent structural changes, demonstrating how direct repulsive forces induced rapid structural modifications, while attractive forces measured in Non-contact mode12 resulted in slower self-assembly transformations. And the molecular structure with high polarity after transformations were confirmed with contact potential measurement by using Kelvin probe force microscopy (KPFM). This approach provides new insights into the force-mediated control of molecular self-assembly processes, contributing to our understanding of how semifluorinated alkanes like F14H20 order in specific polymorphisms. The high-resolution imaging capabilities of AFM allowed us to observe these structural changes at the nanoscale, revealing details about the formation and transformation of F14H20 superstructures