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

原子级薄的2D纳米片是下一代电子产品的有前途的组件。然而，缺乏可扩展的制造工艺来充分展示纳米片材料的上级性质。具体地，目前没有可用的技术具有必要的放置精度和拓扑控制来构建未起皱的纳米片的对齐堆叠。该奖项支持对新型双液滴电流体动力印刷工艺的基础研究。研究结果可以开发出独特的增材制造平台，用于图案化纳米片以及其他各向异性胶体颗粒（例如，纳米线和量子点）。这种技术对于美国在制造业中保持竞争力并在高性能印刷电子，传感器，执行器和能源设备中带来纳米片的新应用至关重要。新的双液滴电流体动力学打印过程涉及首先沉积作为Langmuir-Blodgett槽的支持液滴，然后是包含胶体2D纳米片的润湿液滴。随着载体液滴蒸发，将发生2D纳米片的组装。本研究的目的是（1）了解溶剂表面张力和载体与润湿液滴的体积比对润湿液滴在载体液滴上铺展的影响;（2）了解纳米片尺寸和浓度以及基底润湿性质对纳米片排列的影响;（3）建立沉积纳米片的结构-性质关系。石墨烯和二硫化钼纳米片将用于本研究。为了实现第一个目标，将进行双液滴打印实验。通过改变溶剂组成，溶剂表面张力将在30-50 mN/m之间变化，并且通过改变载体和润湿液滴的驱动电压和脉冲宽度，体积比将在1至100之间变化。扩散面积的时间变化将通过具有几十微秒分辨率的高速摄影来测量。第二个目标将通过实验研究和计算机模拟来实现。对于双液滴印刷实验，纳米片尺寸的平均直径将在0.2-10 μm之间变化，润湿液滴中的纳米片浓度在0.01-1 mg/mL之间，并且载体液滴的后退接触角将从具有钉扎接触线的约0°变化到具有脱钉接触线的约90°。将通过显微镜表征来分析组装中的纳米片对齐。将创建拉格朗日粒子跟踪模型用于预测纳米片排列，其中分子动力学模拟将计算蒸发诱导流动下的纳米片动力学。模拟预测将通过纳米片取向和对齐方面的实验结果进行验证。为了实现第三个目标，将使用电子显微镜和原子力显微镜测量沉积的纳米片组装体的结构（就拓扑粗糙度、片与片的对准、纳米片之间的间隙或重叠而言），并且将使用四点探针测量性质（导电性）。

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 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.