Collaborative Research: Dual-droplet Electrohydrodynamic Printing of 2D Nanosheets

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Atomically thin, 2D nanosheets are promising components for next-generation electronics. However, there is a lack of scalable manufacturing processes to fully showcase the superior properties of nanosheet materials. Specifically, no currently available technique has the requisite placement accuracy and topology control to build aligned stacks of unwrinkled nanosheets. This award supports fundamental research on a novel dual-droplet electrohydrodynamic printing process. Research results can enable the development of a unique additive manufacturing platform for patterning nanosheets, as well as other anisotropic colloidal particles (e.g., nanowires, and quantum dots). Such technology is crucial for the US to stay competitive in manufacturing and bring forth novel applications of nanosheets in high-performance printed electronics, sensors, actuators, and energy devices. The new dual-droplet electrohydrodynamic printing process involves first depositing a support droplet which acts as a Langmuir-Blodgett trough, followed by a wetting droplet containing colloidal 2D nanosheets. Assembly of the 2D nanosheets will occur as the support droplet evaporates. The research objectives are (1) to understand the effects of solvent surface tensionand volume ratio of the support and wetting droplets on the spreading of the wetting droplet over the support droplet; (2) to understand the effects of nanosheet size and concentration, and substrate wetting properties on the alignment of nanosheets; and (3) to establish the structure-property relationships of the deposited nanosheets. Graphene and Molybdenum disulfide nanosheets will be used in this study. To achieve the first objective, the dual-droplet printing experiments will be conducted. Solvent surface tension will be varied between 30-50 mN/m by changing solvent composition, and volume ratio will be varied from 1 to 100 by changing the driving voltage and pulse width for both support and wetting droplets. The temporal change of spreading area will be measured by high-speed photography with a few tens of microseconds resolution. The second objective will be achieved by both experimental study and computer simulation. For dual-droplet printing experiments, nanosheet size will be varied between 0.2-10 µm in mean diameter, nanosheet concentration in the wetting droplet between 0.01-1 mg/mL, and the receding contact angle of the support droplet will be varied from about 0° with a pinned contact line up to ~90° with a depinned contact line. The nanosheet alignment in the assembly will be analyzed by microscopy characterization. A model of Lagrangian particle tracking will be created for prediction of nanosheet alignment, where molecular dynamics simulation will compute nanosheet dynamics under the evaporation-induced flow. Simulation predictions will be verified by experimental results in terms of nanosheet orientation and alignment. To achieve the third objective, the structure (in terms of topological roughness, sheet-to-sheet alignment, gaps or overlaps between nanosheets) of the deposited nanosheet assembly will be measured using electron microscopy and atomic force microscopy, and the property (conductivity) will be measured using four-point probe.