1 Proceedings LOPE-C 2010 Evaporation-assisted self-assembly of functional layers upon inkjet printing Enrico Sowade# (1), Christian Belgardt# (2), Jens Hammerschmidt (1), Thomas Baumgärtel (2), Thomas Blaudeck (1), Harald Graaf (2), Reinhard R. Baumann (1), Christian von Borczyskowski (2) (1) Institute for Print and Media Technology, (2) Center for Nanostructured Materials and Analytics, Chemnitz University of Technology, Chemnitz, Germany Abstract 1. In this paper, we report about two different approaches to combine (i) bottom-up self-assembly and (ii) inkjet printing as a manufacturing scenario for a structured surface modification. First, related to the earlier findings of Park and Moon [1] and Perelaer et al. [2], we highlight printing trials with polymer microspheres of about 300 nm particle diameter stabilized in an aqueous suspension using a Dimatix DMP-2831 materials printer with 10 pl print heads in single-nozzle and multi-nozzle operational mode. By fine-tuning both ink formulation (formamide, ethylene glycol) and the surface energy of the substrate (corona, alcyl silane self-assembled monolayers) it turns out that it is possible to generate ordered monolayers of spheres with a structure definition on the sub- millimeter scale and grain sizes of several microns (as investigated by optical and scanning electron microscopy). In contrast to a recent report [3], applying a specific functional surface coating to the microspheres is not necessary. Our results are in line with current activities to employ convective or evaporation-driven self assembly to control pattern morphologies [4], but relate to inkjet as a lab-scale manufacturing technique for structured deposition. Second, we present the structured modification of silicon oxide surfaces with said alcyl silane (octadecyl trichlorosilane, OTS) self- assembled monolayers (SAMs) by inkjet printing and we compare it with a static standard method for the formation of SAMs [5]. Traditionally, in order to reach a high-quality OTS-SAM with densely packed alkyl chains in all-trans conformation, long reaction times are needed [6]. Our results indicate that a structured hydrophobization of the silicon oxide surface by inkjet printing is possible using OTS diluted in toluene at millimolar concentrations. The SAM formation is assumed to be governed by an evaporation- driven self-assembly of the OTS molecules on the surface. Introduction For five decades now, inkjet printing has been emerged towards a perspectively reliable, flexible and considerably fast additive deposition technique for the production of microstructures. In ‘functional printing’ such as large-area electronics, the patterned and scalable in size deposition of layers is a challenging task. In contrast to graphical printing, the degree of order of the nanoscopic building blocks of the materials is important for the functionality of the layer. Whereas graphical printing does not consider morphologies smaller than the resolution limit of human recognition, morphologies on the micron and sub-micron scale in the end define the performance of devices. 2. 2.1. Printed Microspheres State of the Art First publication about inkjet printing of microspherical and submicrospherical particles date back to the early last decade. In principle, two main strategies to deal in this field can be derived: (i) attempts to obtain colloidal aggregates with photonic functionalities for various applications and (ii) fundamental research to investigate determining factors of evaporation-driven self-assembly. The state of the art is determined in substance by the following milestones: inkjet printing of droplets containing silica microspheres on different substrates [1a], of droplets containing polysterene microspheres with different diameter on silicon producing also 3D structures (microdomes) [1b, c], introducing a variable ink composition and solids contents and using hydrophilic silicon [1d], inkjet printing of silica microspheres with different diameter to investigate the influence of the contact angle [2], and inkjet printing of large-area patterned photonic crystals consisting of functionalized core-shell microspheres [3]. Our work has the aim to extend this strategy for a contemporarily widely used laboratory multi- nozzle DOD piezo-inkjet printer. 2.2. Substrates and Inks Commercially available square microscope glass slides from VWR International {Darmstadt, Germany; hydrolytic borosilicate glass with a size of (18x18) mm, thickness (0.13- 0.16) μm} were used as substrates. All substrates were cleaned in ethanol and then rinsed with deionized water (DI water) and subsequently dried under a flow of air. The following surface modifications were applied: (i) no further treatment (“untreated”); chemical treatments: (ii) surfactant treatment, (iii) hexamethyldisilazane (HMDS) treatment, (iv) octadecyltrichlorosilane (OTS) treatment; physical treatment: (v) corona treatment. The chemical treatments were done in chemical bath deposition according to known methods. Corona treatment was performed using a high voltage discharge (2.3 kV). Furthermore, PET was used as substrate. Highly monodisperse polystyrene microsphere particles suspended in an aqueous environment were used as inks. The suspensions were obtained from Duke Scientific {Palo Alto, CA, USA; 0.1 % solids content, (300±5) nm mean particle diameter; (57.3±0.9) mN/m surface tension} and BS-Partikel GmbH {Wiesbaden, Germany; 2.0 % solids content, mean particle diameter (305.0±8) nm, (46.8±0.8) mN/m surface tension}. The surface tension of the inks was determined by a Dataphysics OCA20 (Filderstadt, Germany) contact-angle measurement system (see Section 3.4). The inks were stored at 8 °C and before printing treated ultrasonically to prevent agglomerations. 2.3. Inkjet Printing The colloidal inks containing polystyrene microspheres were printed on the differently treated glass slides by using a