Adsorption Behaviors of Mixed Monolayers of n-Alkanes at the Liquid-Solid Interface

Identifying Target Molecule and Trace Amount of the Byproduct by Two-Dimensional Self-Assembly with Different Solution Concentrations

Scanning tunneling microscopy (STM) is a powerful way to realize the recognition of self-assembled nanostructures on the atomic scale. In this article, dihexadecyl 6,9-bis((4-(hexadecyloxy)phenyl)ethynyl) phenanthro[9,10-c]thiophene-1,3-dicarboxylate (D-PT) and dihexadecyl 6-bromo-9-((4-(hexadecyloxy) phenyl)ethynyl)phenanthrol[9,10-c]thiophene-1,3-dicarboxylate (S-BrPT) with different substituents were chosen as the target system. D-PT with four side chains as the target molecule and S-BrPT with three side chains and a bromine substituent as the byproduct were mixed in a molar concentration ratio of 20:1. The effect of solution concentration on the molecular self-assembly of the mixture was investigated by STM at the hexadecane/HOPG interface. At high concentrations, only D-PT molecules formed a dimer pattern resulting from the intermolecular van der Waals force and self-adaption. Further diluting the solution, D-PT formed the coexisting dimer and linear structures, in which the linear pattern was formed via solvent coadsorption. At low concentrations, S-BrPT molecules forming N-shaped dimers appeared and filled the linear structure fabricated by D-PT molecules. With further decrease in the concentration, S-BrPT molecules formed N-shaped dimers covering almost half of the surface area, resulting from the C-Br···π and Br···H-C bonds. At very low concentrations, S-BrPT molecules formed N-shaped dimers to arrange the matrix architecture due to the coadsorption of more hexadecane molecules. Density functional theory (DFT) calculations demonstrated that the stronger intermolecular C-Br···π and Br···H-C bonds were significant factors in determining the formation of N-shaped dimers and the stability of this nanostructure. This work enriches the diversity of self-assembled motifs and provides a strategy to characterize different symmetric molecules with trace amounts in a mixed system by STM.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Electrografting and Langmuir-Blodgett: Covalently Bound Nanometer-Thick Ordered Films on Graphite

We present two different molecular organizations obtained from octadecylamine (ODA) molecules on a highly oriented pyrolytic graphite (HOPG) surface: (i) self-organized physisorbed ODA molecules lying flat on the surface and (ii) a strongly electrografted compact crystalline monolayer of ODA molecules standing up on the surface. This new structure is obtained by combining the Langmuir-Blodgett transfer of an ODA Langmuir film onto HOPG with oxidative electrografting. The presence of an organic film on HOPG is characterized by attenuated total reflectance-infrared spectroscopy and Raman spectroscopy, while atomic force microscopy and scanning tunneling microscopy allow the observation of the two molecular organizations with adsorbed molecules lying flat on HOPG or strongly grafted in an upright position on the HOPG surface. Interestingly, the second molecular organization preserves a hexagonal symmetry and its lattice parameters are intermediate between those of ODA Langmuir films and that of the HOPG underlying surface. The functionalization of surfaces with organic films is a major issue in the design of sensors with biomedical applications or organic electronics and energy storage devices and these structures may find applications in these fields.

Molecular patterns of oligopeptide hydrocarbons on graphite

Graphitic materials including graphene, carbon nanotubes and fullerenes, are promising for use in nanotechnology and biomedicine. Non-covalent functionalization by peptides and other organic molecules allows changing the properties of graphitic surfaces in a controlled manner and represents a big potential for fundamental research and applications. Recently described oligopeptide-hydrocarbon derivative N,N'-(decane-1,10-diyl)bis(tetraglycineamide) (GM) is highly prospective for the development of graphitic interfaces in biosensor application as well as in structural biology for improving the quality of high-resolution atomic force microscopy (AFM) visualization of individual biomacromolecules. However, molecular organization of GM on graphitic surfaces is still unknown. In this work, the molecular model of GM at the water/highly oriented pyrolytic graphite (HOPG) interface has been developed basing on the high-resolution AFM and full-atom molecular modeling data. This model explains two periodicities observed in AFM images by GM self-assembly on a HOPG surface with formation of the stacks with the lateral shifts. The obtained results reveal the particular patterns and dynamics of GM molecules adsorbed on graphite and unravel the puzzle of peptide self-assembly on graphitic surfaces.

On the formation of concentric 2D multicomponent assemblies at the solution–solid interface

We report on the design and fabrication of a four-component supramolecular network consisting of three concentric shells around a central guest.

Complexity of two-dimensional self-assembled arrays at surfaces

The developing field of complexity in self-assembled systems on surfaces is discussed.