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

原子厚度的二维纳米片是下一代电子产品中很有前途的组件。然而，缺乏可扩展的制造工艺来充分展示纳米片材料的优越性能。具体来说，目前没有一种可用的技术具有必要的放置精度和拓扑控制来构建排列整齐的无褶皱纳米片堆叠。该奖项支持一种新型双液滴电流体动力印刷工艺的基础研究。研究结果可以开发一种独特的增材制造平台，用于纳米片和其他各向异性胶体颗粒（如纳米线和量子点）的图图化。这种技术对于美国保持制造业竞争力和在高性能印刷电子产品、传感器、执行器和能源设备中提出纳米片的新应用至关重要。新的双液滴电流体动力打印工艺包括首先沉积一个支撑液滴，作为Langmuir-Blodgett槽，然后是一个含有胶体二维纳米片的润湿液滴。随着支撑液滴的蒸发，二维纳米片的组装将发生。研究目标是：(1)了解溶剂表面张力和载体与润湿滴体积比对润湿滴在载体上扩散的影响；(2)了解纳米片的尺寸和浓度以及衬底润湿特性对纳米片排列的影响；(3)建立沉积纳米片的结构-性能关系。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，我们将进行双液滴打印实验。通过改变溶剂组成，溶剂表面张力在30 ~ 50 mN/m之间变化；通过改变载体和润湿滴的驱动电压和脉宽，体积比在1 ~ 100之间变化。扩展面积的时间变化将通过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。在双液滴打印实验中，纳米片的平均直径在0.2-10µm之间，湿润液滴中的纳米片浓度在0.01-1 mg/mL之间，支撑液滴的后退接触角从约0°（固定接触线）到约90°（脱落接触线）不等。组装中的纳米片排列将通过显微镜表征进行分析。拉格朗日粒子跟踪模型将用于预测纳米片的排列，其中分子动力学模拟将计算蒸发诱导流下纳米片的动力学。模拟预测将通过纳米片取向和排列方面的实验结果来验证。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件的结构（拓扑粗糙度、片与片之间的排列、纳米片之间的间隙或重叠），并使用四点探针测量其性能（电导率）。

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.