8.3.1 Preparation of hydrophobic substrates
Hydrophobic substrates were prepared by spin-coating glass slides with either PS, poly(n-butyl acrylate-co-styrene) (P(nBA-co-S)) or poly(methyl methacrylate) (PMMA). Solu-tions in toluene with 10 –20 wt% polymer were spin-coated at 3000 rpm for 3 min and subsequent annealing at 200◦C for 5 min. SU-8 substrates were obtained by spin-coating SU-8 2050 at 3000 rpm for 30 s. After a soft bake at 65◦C for 1 min and at 95◦C for 7 min, the substrate was exposed to UV light for 2×4 s. A post exposure bake was carried out at 65◦C for 1 min and at 95◦C for 6 min. Commercially available polytetrafluoroethy-lene (PTFE) films with a thickness of 1 mm were cut into 1×1 cm2pieces and used after cleaning with ethanol.
8.3.2 Preparation of two-dimensional colloidal crystals
In this thesis, two-dimensional colloidal crystals are prepared according to the method reported by Retsch et al. (Figure 8.4).[3] This procedure can be divided into two major steps. The functionalization of glass slides, that work as parental substrates, and the assembly process at the water/air interface.
In the first step, glass substrates were cleaned in an aqueous solution of 2 vol% Hell-manexrIII in an ultrasonic bath for 15 min. After rinsing in pure water, the glass slides were deposited in ethanol and once more treated in an ultrasonic bath for 15 min (Fig-ure 8.4 a). The cleaned glass was dried under a stream of compressed air. This step is cru-cial to remove any contaminants on the glass that could influence the particle assembly.
The dried slides were then exposed to an oxygen plasma for 60 s in order to activate the glass surface for functionalization (Figure 8.4 b). The functionalization of the glass slides was accomplished with N–trimethoxysilylpropyl–N,N,N–trimethylammonium chloride.
For this purpose, an aqueous solution of the silane with a concentration of 1 vol% was prepared and stirred for 10 min prior to immersion of the glass slides. Directly after plasma treatment, the glass slides were deposited into the silane solution (Figure 8.4 c).
After 1 h the glass slides were rinsed with water and ethanol and dried in a gentle air stream. Finally, the glass slides were stored at 90◦C for 1 h to complete the condensation of the silane onto the glass surface (Figure 8.4 d). For the assembly process, particle dis-persions with 2 – 5 wt% were spin-coated onto the cationically functionalized glass slides (4000 rpm, 15 s) (Figure 8.4 e). Thereby, the particles were individually distributed on the
Figure 8.4: Schematic illustration of the procedure established by Retschet al.[3] for the preparation of two-dimensional colloidal crystals. (a) Glass slides are cleaned in an ul-trasonic bath. (b) The surface of the glass slides is activated in an oxygen plasma. (c)/(d) For the functionalization the activated glass is treated with a cationic silane, which con-denses to the glass surface at elevated temperature. (e)/(f) Aqueous particle dispersions are spin-coated onto the functionalized glass slides to yield a layer of individually dis-tributed particles. (g)/(h) Upon slow immersion of the glass slide into a solution of am-monia and SDS, the particles form a close-packed monolayer at the water/air interface.
(i) The monolayer can be collected on various hydrophilic substrates.
glass, attracted by the cationic surface groups (Figure 8.4 f). Subsequently, the coated glass slides were slowly immersed into an aqueous solution under a shallow angle (Fig-ure 8.4 g). The solution was adjusted to pH 12 by ammonia and additionally comprises 0.1 mM SDS. Upon immersion of the glass slide, the particles detached from the glass and assembled at the three-phase contact line into a hexagonal close packed monolayer (Figure 8.4 h). The freely floating monolayer could be collected from the water/air inter-face by arbitrary hydrophilic substrates. For this purpose, the substrates were submerged into the solution and lifted through the monolayer, whereby the particles attached to the surface (Figure 8.4 i).
For hydrophobic uptake the substrate was immersed through the floating monolayer at an angle of 45◦ relative to the water surface and was left at the bottom of the beaker with the monolayer facing upwards. After heating the water near the glass transition temperature of the particles or the substrate for 5 min on a hotplate, the monolayers were
removed from the solution. Finally, the substrates with the colloidal assemblies were dried under ambient conditions.
8.3.3 Preparation of quasicrystalline micropatterns
The procedure for fabrication of micropatterns used in this thesis follows the concept of rapid prototyping which combines photo and soft lithography (Figure 8.5).
Figure 8.5: Schematic illustration of the rapid prototyping process including design of structures, printing of photomasks, photo lithography, soft lithography and characteri-zation.
The micropatterns feature an arrangement of circular holes with a certain diameter and distance. The positions of the holes are calculated with the generalized dual method (GDM) and visualized by computer-aided design (CAD) software (AutoCADR2017).
Prior to the actual manufacture the structures are converted into high-resolution chrome photomasks (512000 dpi) (JD Photo Data, Hitchin, United Kingdom). For the photo litho-graphic manufacture of the templates a silicon wafer is used as substrate. As the struc-tures have dimensions in the micrometer range, it is crucial to avoid contamination with e. g. dust during the manufacture. For this reason, the whole manufacture takes place in a clean room. Furthermore, the room is illuminated with yellow light to facilitate the handling of the photo resist. The polished silicon wafer is cleaned with 2-propanol and stored at 200◦C in the oven for 10 min. As the structures themselves do not adhere
di-rectly on the wafer due to their small size, an adhesion promoter in form of an additional continuous layer of photo resist is used. The cooled-off wafer is spin-coated with the photo resist SU8-2005 at 3000 rpm for 30 s. This results in a layer thickness of 5µm for the adhesion promoter. The wafer is stored on a hotplate at 95◦C for 2 min so that the solvent can evaporate from the photo resist film. Subsequently, the complete wafer is exposed to UV light with a wavelength of 365 nm for 2×2.5 s in a mask-aligner (MJB4, S ¨USS MicroTec AG, Munich, Germany). By illumination the photo initiator of the photo resist is activated and a renewed storage on a hotplate at 95◦C for 3 min cross-links the film. For the actual structure the photo resist SU8-2000.5 is spin-coated at 3000 rpm for 30 s to yield a layer thickness of 0.5µm. The film is dried on a hotplate at 95◦C for 1 min.
This time the exposure with UV light is accomplished through the chrome photomask for 2×1.5 s. Thereby only selected areas are cross-linked during the following treatment on a hotplate at 95◦C for 1 min. All non-cross-linked portions of the film are removed. For this purpose, the wafer is immersed in a bath of mr-DEV 600 and strongly agitated until all monomer is removed. Finally, the wafer is rinsed with 2-propanol and gently dried with a stream of air.
In the second preparation section the templates are replicated in PDMS via soft lithog-raphy. The liquid PDMS prepolymer is blended by mixing the base polymer with the curing agent in a 10:1 ratio. The mixture is poured onto the template and degassed at reduced pressure and ambient temperature until all bubbles have disappeared. The pre-polymer is then cured at 75◦C for 2 h. The resulting solid PDMS layer comprising circular holes with quasicrystalline symmetry is peeled off the silica wafer.
Prior to further processing the PDMS is cleaned with 2-propanol to remove dust that could impair the adhesion of metal layers. Subsequently, 50–100 nm gold and 5 nm tita-nium are vapor deposited onto the structured PDMS. In the final step, the metal layer can be transferred onto flat PDMS via contact printing. By means of plasma treatment the surfaces of the flat PDMS and the titanium layer are oxidized and activated for bond-ing. The titanol and silanol groups are brought into contact and heated to 100◦C on a hotplate to form covalent bonds. Thereby the gold-titanium layer is transferred from the structured to the flat PDMS.