Manipulating Exfoliated 2D Materials at the Air-Water Interface: Towards Large-area Coatings and Applications

Abstract

The aim of this thesis is to develop a versatile, large-area coating process for two-dimensional (2D) materials which preserves single-layer or exfoliated sheets and provides comprehensive investigation of the controlling factors for the deposition process. Compared to other approaches, traditional Langmuir film methods not only are economically suitable for transition to large-scale manufacturing but also permit retention of the exfoliated form and freedom of sheet rearrangements. However, challenges remain in existing Langmuir film-based traditional and continuous coating process: 1). Significant material loss due to water-miscible solvent; 2). External forces required for film densification and transfer. In this thesis, we expand upon my MASc work which focuses on the development of a solvent-spreading assisted Langmuir film method which enables large batch film transfer process or a continuous one for monolayer graphene-based materials. With the goal of applying the coating process for films in flexible, optoelectronic devices such as in solar cells, we refined the procedure for preparing spreading dispersion of exfoliated molybdenum disulfide (MoS2). During investigation of the chemical and colloidal stability of the precursor, chemically exfoliated MoS2 (ce-MoS2) dispersions in water, we found that ce-MoS2 degrades rapidly to form soluble molybdates under alkaline conditions and exposure to both light and air. Building upon this fundamental study, the modified spreading dispersion retains almost 100% of metallic MoS2 and contains ~40% increase in single-layer content. Then, we investigated the structure of the aggregates formed upon spreading each drop of the ink on the air-water interface. We observed cluster structure transitions from island-like domains to more linear networks in three different nanosheets as dispersion concentration is reduced. Regardless of assembly method, we found that cluster structure impacts the attainable density of transferred Langmuir films. Finally, since the spreading pressure of solvent provides the driving force for film assembly and transfer, we evaluated the spreading profiles of various combinations of organic solvents and their impacts on Langmuir film formation. We uncover that the formulation of mixing polar and water-immiscible solvents not only largely reduces the loss of materials into the subphase water but also induces a sharp increase in spreading pressure which facilities the formation of a solid film as well as further densification and transfer of the floating film to substrates in a R2R deposition system. We also develop a multi-dripper system to showcase the strategy of coating films with arbitrary widths. When conducting the R2R deposition, we unveil the importance of using wetting substrate to maximize the effectiveness from the spreading pressure for achieving continuous, uniform coatings. In the end, we attempt to utilize the Langmuir film of ce-MoS2 in the perovskite solar cell to demonstrate the potential use of our coating process.