Controlling Crystallization of Two-dimensional Materials at Solid-liquid Interfaces

Abstract

Two-dimensional (2D) materials have been widely explored in biological, quantum, and energy-related applications due to their unique chemical, physical, and mechanical properties. The novel properties and their technological potential have driven the development of numerous synthesis strategies for 2D materials over the past decades. However, current synthesis strategies still rely heavily on practitioner expertise and trial-and-error (Edisonian) approaches. In this work, we propose solid–liquid interfaces as a powerful platform for controlling interfacial energies and structures, which can subsequently be used to direct the growth of 2D materials. This approach leverages the symmetry-breaking and 2D confinement effects of the interfaces. The overarching goal is to establish scientific design principles that enable adaptive control over the structures, phases, and crystallographic orientations of 2D materials, as well as to access stable or metastable 2D phases that cannot be synthesized in bulk solution. By tuning parameters such as intermolecular interactions, molecule–substrate interactions, entropic contributions, and external electric fields (E-fields), the structure of the solid-liquid interface can be modulated; thereby leading to variations in the interfacial energy landscapes that determine the stability of phases, the thermodynamic barrier to their nucleation, and the kinetics of desolvation and attachment. In this way, nucleation, growth, and assembly of 2D materials at interfaces can be precisely controlled. To test the hypothesis, a set of projects were chosen to individually investigate the various factors governing interfacial free energies. In the first project, patchy proteins, L-rhamnulose-1-phosphate aldolase (RhuA), with tunable interactions, shapes, and electrostatic patchiness, were selected as a model system to understand the effects of molecule-molecule interactions, the entropic drivers related to shape complementarity, and external E-fields. Using in-situ AFM, we observed the 2D assembly of distinct phases of β-cyclodextrin (CD) and azobenzene (Azo) modified RhuA (CDRhuA and AzoRhuA, respectively) at mica-water interfaces, as well as the transitions between them. For AzoRhuA, the presence of appropriate long functional groups and weak inter-protein interactions enables the formation of multiple polymorphs and between which phase transitions occur via two distinct pathways. However, when functional groups become excessively long or bulky, as with CDRhuA, densely packed alternating structures emerge via an unusual assembly pathway involving protein adsorption and reconfiguration. In this system, entropy related to shape complementarity dominate the assembly outcome. To probe the balance of forces governing these behaviors, we performed coarse-grained (CG) simulations in which we varied the relative contributions of protein-substrate interactions and protein-protein interactions, including both enthalpic and entropic forces. The simulations reveal that formation of the alternating pattern depends on a delicate balance between enthalpy and entropy, with the entropy of the bulky CD side groups and RhuA main bodies serving as the dominant driving force among the combined contributions. Finally, because the site-modified proteins possess intrinsic dipole moments, they provide an opportunity to study how coupling between external E-fields and the protein dipoles biases the thermodynamics and kinetics of nucleation. Moreover, the coupling between the E-field and the protein dipole moment might be a “knob” to manipulate the enthalpic term in the free energy to enable the controllable competition between system entropy and enthalpy. In the second project, we investigated the assembly of a peptide known to form 2D crystalline films on MoS2 on three representative van der Waals (vdW) substrates: WS2, MoS2, and highly oriented pyrolytic graphite (HOPG) to explore the effects of molecule-substrate interactions on assembly. Using in-situ AFM, we found that assembly of the model system, MoSBP1 peptides (YSATFTY), is substrate-dependent, resulting in multilayers on WS2, monolayers on MoS2, and multiple coexisting phases on HOPG. On WS2, the higher negative charge, strong long-range forces, and extensive hydration layering appear to promote multilayer stacking. In contrast, MoS2 has stronger short-range interactions with the peptides but much weaker long-range interactions and hydration structure, which may favor monolayer formation. Molecular dynamics simulations predict a corresponding switch from monolayer to multilayer aggregates of the adsorbed monomers, reflected in their relative mobilities. On hydrophobic HOPG, peptides bind most strongly and remain as monomers with high surface mobility. The peptide dimers comprising the basic unit of the crystals are more compact on HOPG, which has a smaller lattice constant than WS2 or MoS2, suggesting strain contributes to stabilizing multiple phases. Together, these results provide mechanistic insights into how surface charge, hydration structure, and lattice structures of vdW substrates govern peptide assembly. We also found that differences in strain and electronic states of twisted vdW materials may influence the epitaxial relationship between substrates and peptides. In the third project, the knowledge gained from the model systems was applied to control the interfacial energy of the substrate-electrolytes and Zn-electrolyte interfaces using additives, addressing an application-driven challenge: the suppression of zinc dendrites in batteries. We used the in-situ electrochemical (EC) AFM to directly observe the interfacial evolution during Zn electrodeposition and polymer adsorption on Cu substrates in the presence of varying concentrations of ZnSO4 and polyethylene oxide (PEO), one of the simplest and most widely used polymers. Contrary to previous literature assumptions which emphasize the binding to the growing Zn crystal surfaces or Zn2+ ions, our results demonstrate that PEO smooths Zn films by promoting nucleation of (002)-oriented Zn platelets through interactions with the Cu substrate. Density functional theory simulations support this finding by showing that PEO adsorption on Cu modifies the interfacial energy of Zn/Cu/electrolyte interfaces, favoring the stabilization of Zn (002) on the Cu substrate, as well as confines Zn electrodeposition to a narrow near-surface region. These findings elucidate a novel design principle for electrode smoothing, emphasizing the importance of substrate selection paired with polymer additives that exhibit an attractive interaction with the substrate, but minimal interaction with growing crystals, offering a mechanistic perspective for improved battery performance. Building on this principle and our established platforms for studying complex electrochemical interfaces, we further examined how polymer chemical structures and anions in solutions influence polymer-induced electrode flattening using sequence-defined peptoids as a model system.