Stripe‐Like Pattern Formation in Airbrush‐Spray Deposition of Colloidal Polymer Film

Abstract

Thin films and multilayer structures are used in a wide range of applications. To name just a few: sensor coating, [1][2][3] microelectronics, [4] solar cell technology, [5,6] and magnetic recording. [7] In the last few years, organic-based hybrid devices received strong attention from both academy and industry because of their potential for low-cost production and flexible device applications. [5] In order to improve the device performances, new approaches are needed to install thin films showing lateral order and high uniformity over large areas. Many methods have already been developed: bottom-up approaches such as solution casting, [8] spincoating, [5] inkjet printing, [9,10] dip-coating, [11] and also topdown approaches such as lithography [12,13] or vacuum deposition. [14] Recently, the novel technique of airbrush-spray deposition was used in the fabrication of organic-based multilayer devices such as diodes, [15] transistors, [16] semiconductors, [17] electrolyte films, [18,19] and solar cells. [20][21][22] The airbrush-spray coating technique allows performing fast and economic deposition of organic-based layers showing high structural uniformity over large areas and is thus of great interest in industrial applications. ExperimentalFor the polymer coating, we used a commercial airbrush-spray (Harder & Steenbeck, Evolution Silverline) with a needle diameter of 30 mm (see Fig. 1). Two different types of aqueous colloidal suspensions (provided by Kisker GbR, Steinfurt, Germany) were deposited: polystyrene (PS) colloids (initial concentration: 2.5 wt%) and carboxylated polystyrene (cPS) colloids (initial concentration: 5.0 wt%). Both types of colloids have a nominal diameter of 96 nm. Two different solvents were used for further dilution, namely deionized water and ethanol. Influence of the solvent on the film lateral ordering will be discussed later. Spray coatings were performed on flat Silicon (Si) substrates (18 Â 30 mm 2 ) at an air pressure P ¼ 3 bar in a horizontal configuration. Prior to use, the Si wafers were cleaned in a basic bath (35 mL of 25% ammoniac, 25 mL of 35% hydrogen peroxide, and 350 mL of deionized water) at a temperature of 75 8C. The cleaned substrates were further individually rinsed in deionized water and dried using nitrogen flow.Two different spray-deposition modes were examined. In the first mode, the coating is performed in one single cycle at a short spray nozzle-to-substrate distance (D SS ¼ 73 mm). In the second mode, the coating consists in three spray cycles at a larger spray nozzle-to-substrate distance (D SS ¼ 142 mm). These two distances ensure a complete coating of the substrate. By enlarging the distance, one can reduce the amount of solvent (by evaporation) impinging on the substrate. A spray cycle is 1 s, the time between two successive spray cycles (second coating mode) 5 s. The total coating time is 1 s in the first mode and 3 s in the second mode. An overview of the parameter sets used in this study is given in We use the versatile technique of Grazing incidence ...

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