A Journey into Jamming: Phase Transitions and Edge-to-Edge Heterostructures in Langmuir Blodgett Films of Exfoliated Two-Dimensional Materials

Abstract

Two-dimensional (2D) materials hold significant promise as new electronic materials, and could enable high mobility devices, and flexible electronics. In particular, graphene and few-layer molybdenum disulfide have been the subject of significant study as new and exciting materials. Hindering the application of 2D materials is the lack of easy, scalable methods for deposition single layers of 2D materials. Top-down chemical methods like CVD can deposit large area high quality films, but remain highly destructive techniques, requiring high temperatures, harmful and caustic reagents, and high vacuum. Recent research in Langmuir Blodgett techniques have enabled large area coatings, continuous roll-to-roll coating, and expanded the repertoire of materials that can be coated. With this research has come renewed interest in the mechanics and dynamics of 2D materials coatings on the air-water interface. In this work, we expand this knowledge by undertaking a comprehensive study of 2D particle jamming on the air-water interface. Furthermore, we employ this understanding to demonstrate, for the first time, edge-to-edge heterostructure films assembled on the air-water interface. To address the challenges of in-situ characterization of a growing film, we designed a method for non-destructive, in situ film monitoring using video-kymography, in addition to traditional Langmuir film characterization techniques such as surface pressure and compression isotherms. In our findings we identify two modes of 2D material film growth on the air-water interface, an unjammed, or flowing growth mode, that is typically seen in Langmuir films of 2D materials, and a jammed growth mode, where material is condensed into a solid film by the deposition process. The key parameters that determine which mode the deposition will proceed were then identified. Within the jammed growth mode, we identify 3 phases of jammed film growth: gaseous, liquid, and solid. We then show that both the spreading dynamics of the chosen material “ink”, and the constant adding of material itself is required to progress through these stages and propose a mechanism for how a Langmuir film of 2D material can jam. Using these findings, edge-to-edge heterostructure films of reduced graphene oxide and molybdenum disulfide were assembled on the air-water interface. Critical to the assembly of these films was the ability to deposit material in the jammed growth mode, and identify when the film was in the solid phase. This serves as a proof of concept for greater and more complex multi-material films on the air-water interface.