Large-scale Patterning Of Organic Micro/nanocrystals Through Dielectrophoresis-assisted Capillary Force

In recent years, small molecule organic single crystals(OSCs) with tunable molecular design and property modulation have attracted more and more attention. OSCs show much better photonic and electrical performance than their counterpart amorphous and polycrystalline thin films due to their perfectly ordered molecular packing. It was also confirmed that the well alignment and patterning of organic nanostructures can save device costs, reduce the crosstalk between neighboring devices, and thus how to controlled align or pattern organic micro/nanocrystals is especially important. In this thesis, we introduce a facile method to produce various patterns of organic nanocrystals by combined electrical field(AEF) with the capillary force during the evaporation of the water between two electrodes.1、 Precise patterning of organic micro/nanocrystals through alternating electrical field(AEF)-assisted capillary forceA packing cell was constructed from two ITO electrodes tightened with thick double-sided tape(90μm to 100μm) were used in our experiment, the surface of the bottom ITO electrode was first pre-patterned with holes via lithography. When an aqueous dispersion of the Alq3 nanoplates was injected into this cell, a function generator was used to generate an AEF between the ITO electrodes. As an AEF was applied normal to the two ITO glass electrodes, Alq3 nanoplates quickly and selectively moved toward the photoresist-hole edge. With capillary force present during the evaporation of water, patterning of Alq3 nanoplates in the bottom ITO substrate would be achieved. The number of nanoplates deposited in the holes can be controlled by tuning the ratio between the dimensions of the holes(D) and the diameter of the nanoplates(d). This method could be used to pattern different shapes of OSCs from nanoplates to nanorods. With the combination of photolithography technique and AEFassisted capillary force method, various patterns of Alq3 nanorods including triangular and square shapes can be achieved. When bottom ITO electrode was replaced by a SiO2 /Si wafer, we can get a UV detector with the patterned nanoplates or nanorods which have a good light response to 254 nm wavelength light. This assembly method can greatly facilitate the device fabrication process and promote the integrated applications of organic nanostructures.2、Patterning of C60 nanoplates/nanorods for flexible photodetectorsA thin copper(Cu) layer was deposited on Si substrate and served as sacrificial layer in subsequent transfer processes. The gold electrode arrays were then fabricated on the sacrificial layer. After that a packing cell was constructed with the Si substrate and an ITO electrode. An aqueous dispersion of the C60 nanoplates/nanorods was injected into this cell. When an AEF was applied normally to the cell, the crystals quickly moved to the photoresist-hole edge and with the evaporation of water patterning of C60 nanoplates/nanorods in the bottom Cu substrate will be achieved. A layer of PDMS was spin-coated on the Cu substrate, and hardened at room temperature. Next, the whole structure was soaked in saturated ammonium persulfate((NH4)2S2O8) solution at room temperature to etch away the sacrificial Cu layer. After that, the device was released and floated freely on the surface of the solution. The flexible device was fabricated. The optimal on/off ratio of the photodetectors constructed from C60 nanorods reaches around 200 and 100 for C60 nanoplates. It should be noted that both the nanorods and nanoplates devices have a good light response to 540 nm wavelength light and show outstanding flexibility stability even after 50 circles of bending and relaxation. In summary, we report a simple yet efficient method for large-scale pattern of C60 single crystal on a flexible substrate.