High-Resolution Imaging of Molecular and Nanoparticles Assemblies with Kelvin Force Microscopy

High-resolution studies of self-assemblies of semifluorinated alkanes molecules F₁₂H₈ and F₁₄H₂₀ [F_nH_m = CF₃(CF₂)_n(CH₂)_mCH₃], and CdTe particles were performed with single-pass Kelvin force microscopy. Surface potential contrast, which is related to the strength and orientation of molecular dipoles, empowers the characterization of self-organized structures. Lamellar structures, ribbons and toroids of F₁₄H₂₀ and F₁₂H₈ were observed on graphite and the differences of surface potential were interpreted in terms orientation of –CH₂–CF₂– dipoles. A gradual sublimation of F₁₂H₈ molecules allowed a visualization of top and bottom molecular layers on the substrate. Prior to the sublimation a part of lamellae of the bottom layers was transformed into the ribbons. The surface potential data suggest that this transition proceeds with the reorientation of the molecular chains from the horizontal to vertical direction. Self-assembly of CdTe nanoparticles into nanowires was monitored upon drying on mica. The process is accompanied by drastic changes of surface potential. The formed nanowires exhibit strong positive surface potential that assumes a structural transition with a formation of strong dipole moment in these self-assemblies.