Siloxane-functionalised surface patterns as templates for the ordered deposition of thin lamellar objects

Siloxane-functionalised surface patterns as templates for the ordered deposition of thin lamellar objects

Substrate preparation

75 mm × 25 mm microscopy slides (631-0113, VWR, Germany) are used for the evaluation of the patterning method and smaller 22 mm × 22 mm microscope coverslips (470055, Brand, Germany) as substrates for the deposition of ultra-thin sections. Before the experiments, the substrates are soaked in cleaning solution (2% Micro-90, Sigma-Aldrich, Germany) for 24 h and rinsed thoroughly with de-ionised (DI) water, followed by 10 min ultrasonic cleaning in acetone and isopropyl alcohol respectively. Subsequently, the substrates are rinsed with DI water, oven-dried for 2 h at 120 °C and stored in sealed containers. Immediately before the silanisation processes, the substrates are treated in an oxygen plasma (Diener Atto, Germany) for 10 min at 0.5 mbar to increase the number of available OH-surface groups for siloxane bonding.

PDMS stamp fabrication

Figure 2 illustrates the stamp preparation process. Laser-cut stainless steel sheet metal (size of 22 mm × 22 mm, thickness of 200 µm, PCB mask manufacturer, Beta LAYOUT, Germany), with the shape of the target pattern, are used as masters for hot embossing. The sheet metal is placed on polymethylmethacrylat (PMMA) sheets (Plexiglas, Evonik Industries, Germany) with 2 mm thickness and hot-embossed with an applied force of 2.2 kN in a mechanical press at 125 °C for 1 h. The PMMA mould is then cooled down to 80 °C within an hour and annealed at that temperature for another hour to relieve stress. To facilitate later demoulding of the final stamp, the resulting PMMA mould is treated with perfluorodecyltrichlorosilane (FDTS) (97%, Alfa Aesar, Germany). Poly(dimethylsiloxane)-based (PDMS) stamps (Sylgard 184® elastomer, Dow, Germany) are prepared by mixing elastomer and curing agent in a 10:1 weight ratio and cured at 75 °C for 2 h in the PMMA moulds. After demoulding the PDMS is rinsed with heptane to remove excess volatile components and dried in air.

Figure 2


Laser-cut sheet metal used for PMMA imprinting to create PMMA moulds to cast PDMS stamps.

Siloxane patterning of the substrate

Method I: subtractive patterning

Vapour phase silanisation: A 5% solution of 1,7-dichloro-octamethyltetrasiloxane (Cl[Si(CH3)2O]3Si(CH3)2Cl) in heptane (Sigmacote®, Sigma-Aldrich, Germany) is used for the preparation of hydrophobic coating. Complete silanisation of the substrate surface is performed by arranging 10 microscope slides vertically with 10 mm spacing in a sealed container with a volume of 900 cm3. The container is placed in an oven preheated to 55 °C to allow homogeneous heat distribution and then 150 µl of siloxane (5%) is added to perform vapor phase silanisation for 60 min at 55 °C. Subsequently, the substrates are rinsed with DI water.

Oxygen plasma patterning: The silanised surface is then covered with the PDMS stamps and exposed to an 13.56 MHz RF oxygen plasma at 0.5 mbar (Diener Atto, Germany) for 15 minutes. The stamp protects the siloxane layer at the contact surfaces and allows plasma treatment of the layer in the 200 µm high cavities as illustrated in Fig. 3.

Figure 3


Process steps of oxygen plasma treatment in cavities (I) and microcontact printing (II) to create a patterned siloxane modification as a deposition template.

Method II: additive patterning

Microcontact printing: 1,7-dichloro-octamethyltetrasiloxane (95%, Sigma-Aldrich, Germany) is used for microcontact printing experiments. The PDMS stamps with an outer dimension of 22 mm × 22 mm are inked with a freshly prepared solution of 10% chlorosiloxane in acetone (p. a., Merck, Germany) as illustrated in Fig. 3. The stamps are air-dried for 20 s and then brought into contact with the glass surface for 60 s. Subsequently, the substrate is rinsed with DI water.

Pattern characterisation

Contact angle analysis

The hydrophobicity of siloxane coatings created by vapour phase silanisation is characterized with a drop shape analyzer (DSA 100, KRÜSS, Germany) by measuring the mean static contact angle (CA) between distilled water and the substrate surface. Two measurements using the sessile drop method are performed on each substrate surface. For the measurement, a droplet volume of 10 µl is dispensed and the resulting contour fit to a Young-Laplace model for contact angles >10° and to a circle model for smaller contact angles. The baseline is set manually and measurements are done 10 s, 30 s, 1 min and 2 min after drop deposition.

Characterisation of the fluid containment properties of the channel templates

In order to assess the fluid containment properties of the resulting channel templates, the hydrophilic channels are filled with an increasing amount of water and inspected optically. The fluid inlet is placed close to the beginning of the channels and slowly (10 µl/min) filled with DI water using a syringe pump (NE-1000, New Era Pump Systems Inc, USA). Images are taken at 0.88 µm resolution with an optical microscope (INM 200, Leica, Germany). Using the refraction at the water surface in top illumination, spreading of the wetting fluid into the wettable channel area is clearly visible, allowing automated analysis with image processing tools (canny edge detection algorithm, scikit-image library). The contour of the spread fluid is fit against a straight nominal shape to estimate the length and width of the hydrophilic channels. The root of the quadratic fitting error (standard deviation) is used to estimate the deviation from the nominal shape. The channels are then filled with water until the fitting error increases steeply, which indicates the maximum water storage capacity.

ToF-SIMS surface analysis

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is performed in a ToF.SIMS5-100 (ION-TOF GmbH, Germany). Ultra-High Vacuum (UHV) base pressure is <5 × 10−9 mbar. For high mass resolution the Bi source is operated in a bunched mode providing short ({{
primary ion pulses at 25 keV energy and a lateral resolution of approx. 4 µm. The short pulse length of 1.5 ns allows high mass resolution. Charge compensation during spectrometry is necessary due to the highly insulating nature of the glass substrates. Therefore, an electron flood gun providing electrons of 21 eV is applied, and the secondary ion reflectron tuned accordingly. Images with a field of view of several square millimeters are obtained by scanning the sample stage and are recorded with a 10 µm pixel size. For high lateral resolution imaging at the channel edge, in order to determine the patterning fidelity, a non-bunched primary ion mode is used. With nominal mass resolution the signals of SiO2, SiO3, and SiO3H are recorded with 256 × 256 pixel in a field of view of 500 µm × 500 µm. The pattern fidelity at the channel edge is obtained by adding the glass and siloxane signals parallel to the edge.

Deposition of ultramicrotome sections

Blocks of epoxide resin are prepared using a two-component epoxide embedding kit (EpoFix kit, Science Services, Germany) with a volume ratio 15: 2 of resin to hardener and cured for 2 days at room temperature. The blocks are trimmed to a typical slightly trapezoidal shape with 0.85 mm width on the short side and 1.4 mm height using a 90° diamond trimming knife (Trim 90, Diatome, Switzerland). The block is then cut into 60-nm thin sections using an ultramicrotome (Powertome XL, RMC, USA) and a modified diamond knife (Ultra Jumbo, Diatome, Switzerland) with an extra wide boat to allow lateral movement of the substrate across the complete length of the diamond knife. Sectioning is performed with standard settings as recommended by the knife manufacturer. The substrates are patterned according to the subtractive process by plasma-induced siloxane degradation and submerged into the DI water-filled reservoir of the modified diamond knife boat. Using a motorised 3-axis handling system, the substrates are then lifted to allow dewetting in the hydrophobic containment areas and the formation of a thin water film on the hydrophilic channels. Afterwards the channels of the template are aligned with the section ribbon and further sectioning is performed to push the section ribbon inside the channel. When a channel is filled with sections, the section ribbon is separated manually, and the substrate moved laterally to align the next empty channel with the end of the section ribbon. The process of cutting, separation and substrate shifting is then repeated to fill the remaining channels with sections. After all channels are filled, the final deposition of the sections on the substrate is accomplished by slowly lifting the substrate out of the water reservoir and allowing the remaining water to air dry.