Search

Two-dimensional (2D) nanomaterials, such as graphene and single layer covalent organic frameworks (sCOFs) are being widely studied due to their unusual structure/property relationships. sCOFs typically feature atomic thickness, intrinsic nanoscale porosity and a crystalline lattice. Compared to other organic 2D materials, sCOFs exhibit major advantages including topological designation and constitutional tunability. This review describes the state of the art of surface-confined sCOFs, emphasizing reticular design, synthesis approaches, and key challenges related to improving quality and exploring applications.

The surface‐induced anchoring effect is a well‐developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer‐scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF‐1) can offer an anchoring effect to template C70 molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C60 system, a stepwise breakdown in ordering of C70 LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface‐confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.

The integration of molecules with irregular shape into a long-range, dense and periodic lattice represents a unique challenge for the fabrication of engineered molecular scale architectures. The tiling of pentagonal molecules on a two-dimensional (2D) plane can be used as a proof-of-principle investigation to overcome this problem because basic geometry dictates that a 2D surface cannot be filled with a periodic arrangement of pentagons, a fundamental limitation that suggests that pentagonal molecules may not be suitable as building blocks for dense films. However, here we show that the 2D covalent organic framework (COF) known as COF-1 can direct the growth of pentagonal guest molecules as dense crystalline films at the solution/solid interface. We find that the pentagonal molecule corannulene adsorbs at two different sites on the COF-1 lattice, and that multiple molecules can adsorb into well-defined clusters patterned by the COF. Two types of these dense periodic packing motifs lead to a five-fold symmetry reduction compatible with translational symmetry, one of which gives an unprecedented high molecular density of 2.12 molecules/nm2.

Over the past two decades, solution/solid STM has made clear contributions to our fundamental understanding of the thermodynamic and kinetic processes that occur in molecular self-assembly at surfaces. As the field matures, we provide an overview of how solution/solid STM is emerging as a tool to elucidate and guide the use of self-assembled molecular systems in practical applications, focusing on small molecule device engineering, molecular recognition and sensing and electronic modification of 2D materials.

Covalent organic frameworks (COFs) have gained significant attention in many applications due to their porous crystalline structures with high architectural and chemical robustness and customized topologies. A high-quality extended two-dimensional (2D) hexagonal porous COF-1 network can be obtained through cyclocondensation of three 1,4-benzenediboronic acid (BDBA) monomers by using excess water as the chemical-equilibrium-manipulating agent on a highly oriented pyrolytic graphite (HOPG) substrate. The surface-confined COF-1 can be used as template to recognize different guest molecules, allowing formation of a host/guest (H/G) structure. The intrinsic features of H/G structures lead to their wide applications in molecular recognition, catalysis, gas storage, separation and directed crystallization. Driven by the potential applications, the present work in this thesis concentrates on the preparation and properties of H/G structures based on COF-1 templates at the solution/solid interface. Firstly, scanning tunneling microscopy (STM) images show that the surface-supported two-dimensional COF-1 can act as a host architecture for C60 fullerene molecules, predictably trapping the molecules under a range of conditions. C60 adsorbs in the COF-1 template in two different sites, the top-site (T) and the pore-site (P). The fullerenes occupy the COF-1 lattice at the heptanoic acid/HOPG interface, and in dried films of the COF-1/fullerene network that can be synthesized through either drop-deposition of fullerene solution or by a dipstick-type synthesis in which the surface-supported COF-1 is briefly dipped into the fullerene solution. COF-1 can be synthesized from different solvents. When 1,2,4-trichlorobeznene (TCB) is used, STM images reveal the adsorption of TCB in the hexagonal pore of COF-1 template at TCB/HOPG interface. A well-defined loop boundary formed by a chain of pentagonal and heptagonal pores allowed us to investigate the effect of pore shape and size on TCB ad

Achieving precise control of molecular self-assembly to form designed three-dimensional (3D) structures is a major goal in nanoscale science and technology. Using scanning tunnelling microscopy and density functional theory calculations, we show that a 2D covalent organic framework (COF-1) can template solution-processed C60 guest molecules to form several solvent-dependent structural arrangements and morphologies via a 2D to 3D growth process. When 1,2,4-tricholorobenzene is used as solvent, C60 molecules form a template-defined close-packed structure. When heptanoic acid is used as solvent, a range of lower density architectures that deviate from the template-defined close packing are observed. We attribute this difference to the co-adsorption of the heptanoic acid solvent molecules, which is only achieved in the presence of the template. This work demonstrates the possibility to precisely control 3D molecular self-assembly through the synergistic combination of template and solvent effects.

This study shows that surface-supported two-dimensional (2D) porous covalent organic frameworks (COFs) can selectively bind different molecules at specific sites via different types of interactions. Scanning tunneling microscopy (STM) images collected at the liquid/solid interface reveal the adsorption of 1,2,4-trichlorobenzene (TCB) in the hexagonal pore of a COF-1 template. A well-defined loop boundary formed by a chain of pentagonal and heptagonal pores allowed the investigation of the effect of pore shape and size on TCB adsorption, suggesting that both geometrical and size effects are important in binding TCB. When both C60 and TCB are present at the solution/solid interface, the TCB molecules are selectively trapped in the pore-site, whereas fullerenes are adsorbed on the top-site of COF-1. While the former structure is stabilized by Cl⋯H hydrogen bonds, the latter is controlled by van der Waals interactions. These results suggest that COFs may offer a powerful platform for the recognition and patterning of guest molecules.

The behavior of small molecules on a surface depends critically on both molecule–substrate and intermolecular interactions. We present here a detailed comparative investigation of 1,3,5-benzene tricarboxylic acid (trimesic acid, TMA) on two different surfaces: highly oriented pyrolytic graphite (HOPG) and single-layer graphene (SLG) grown on a polycrystalline Cu foil. On the basis of high-resolution scanning tunnelling microscopy (STM) images, we show that the epitaxy matrix for the hexagonal TMA chicken wire phase is identical on these two surfaces, and, using density functional theory (DFT) with a non-local van der Waals correlation contribution, we identify the most energetically favorable adsorption geometries. Simulated STM images based on these calculations suggest that the TMA lattice can stably adsorb on sites other than those identified to maximize binding interactions with the substrate. This is consistent with our net energy calculations that suggest that intermolecular interactions (TMA–TMA dimer bonding) are dominant over TMA–substrate interactions in stabilizing the system. STM images demonstrate the robustness of the TMA films on SLG, where the molecular network extends across the variable topography of the SLG substrates and remains intact after rinsing and drying the films. These results help to elucidate molecular behavior on SLG and suggest significant similarities between adsorption on HOPG and SLG.