Precisely tailoring surface chemistry of layered materials is a growing need for fields ranging from electronics to biology. For many applications, the need for noncovalently adsorbed ligands to simultaneously control interactions with a nonpolar substrate and a polar solvent is a particular challenge. However, biology routinely addresses a similar challenge in the context of the lipid bilayer. While conventional standing phases of phospholipids (such as those found in a bilayer) would not provide spatially ordered interactions with the substrate, here we demonstrate formation of a sitting phase of polymerizable phospholipids, in which the two alkyl chains extend along the surface and the two ionizable functionalities (a phosphate and an amine) sit adjacent to the substrate and project into the solvent, respectively. Interfacial ordering and polymerization are assessed by high-resolution scanning probe measurements. Water contact angle titrations demonstrate interfacial pKa shifts for the lipid phosphate but not for the amine, supporting localization of the phosphate near the nonpolar graphite surface.

As functionalized 2D materials are incorporated into hybrid materials, ensuring large-area structural control in noncovalently adsorbed films becomes increasingly important. Noncovalent functionalization avoids disrupting electronic structure in 2D materials; however, relatively weak molecular interactions in such monolayers typically reduce stability toward solution processing and other common material handling conditions. Here, we find that controlling substrate temperature during Langmuir-Schaefer conversion of a standing phase monolayer of diynoic amphiphiles on water to a horizontally oriented monolayer on a 2D substrate routinely produces multimicrometer domains, at least an order of magnitude larger than those typically achieved through drop-casting. Following polymerization, these highly ordered monolayers retain their structures during vigorous washing with solvents including water, ethanol, tetrahydrofuran, and toluene. These findings point to a convenient and broadly applicable strategy for noncovalent functionalization of 2D materials in applications that require large-area structural control, for instance, to minimize desorption at defects during subsequent solution processing.

Noncovalent monolayer chemistries are often used to functionalize 2D materials. Nanoscopic ligand ordering has been widely demonstrated (e.g., lying-down lamellar phases of functional alkanes); however, combining this control with micro- and macroscopic patterning for practical applications remains a significant challenge. A few reports have demonstrated that standing phase Langmuir films on water can be converted into nanoscopic lying-down molecular domains on 2D substrates (e.g., graphite), using horizontal dipping (Langmuir-Schaefer, LS, transfer). Molecular patterns are known to form at scales up to millimeters in Langmuir films, suggesting the possibility of transforming such structures into functional patterns on 2D materials. However, to our knowledge, this approach has not been investigated, and the rules governing LS conversion are not well understood. In part, this is because the conversion process is mechanistically very different from classic LS transfer of standing phases; challenges also arise due to the need to characterize structure in noncovalently adsorbed ligand layers <0.5 nm thick, at scales ranging from millimeters to nanometers. Here, we show that scanning electron microscopy enables diynoic acid lying-down phases to be imaged across this range of scales; using this structural information, we establish conditions for LS conversion to create hierarchical microscopic and nanoscopic functional patterns. Such control opens the door to tailoring noncovalent surface chemistry of 2D materials to pattern local interactions with the environment.

Integrating functionalized 2D materials into multilayer device architectures increasingly requires understanding the behavior of noncovalently adsorbed ligands during solution processing. Here, we demonstrate that the headgroup dynamics of polymerized monolayers of functional alkanes can be controlled to modify surface wetting and environmental interactions. We find that headgroup dynamics are sensitive to the position of the polymerizable diyne group; thus, the polymerization process, typically used to stabilize the noncovalent monolayer, can also be used to selectively destabilize chain-chain interactions near the headgroups, making the headgroups more solvent-accessible and increasing surface hydrophilicity. Conversely, interactions with divalent ions can be used to tether headgroups in-plane, decreasing surface hydrophilicity. Together, these results suggest a strategy for the rational design of 2D chemical interfaces in which the polymerization step reconfigures the monolayer to promote the desired environmental interactions.