Atomic force microscopy (AFM)
. Organosilane thin films were characterized using models 5420 and 5500 scanning probe microscopes operated in contact or tapping-mode AFM. (Agilent Technologies, Chandler, AZ). Lateral force images were acquired for either the trace or retrace views corresponding to the scan direction of the selected topography frames. The color scales of lateral-force images indicate differences in tip–surface interactions, but were not normalized for the comparison of friction changes between different tips or experiments. The tips were silicon nitride probes. Tips used with tapping-mode AFM were rectangular shaped ultrasharp silicon tips that have an aluminium reflex coating, with a spring constant of 48 N/m (Nanoscience Instruments, Phoenix, AZ). For contact-mode images, V-shaped tips (Veeco Probes, Santa Barbara, CA) with an average force constant of 0.5 N/m were used. Data files were processed by using Gwyddion open-source software, which is freely available on the internet and supported by the Czech Metrology Institute [59
]. Estimates of surface coverage were obtained for individual topography frames by manually converting images to black and white using thresholding and pixel counting with UTHSCA Image Tool [60
Preparation of latex-particle masks. Polished silicon wafers doped with boron (Virginia Semiconductor, Fredericksburg, VA) were used as substrates. Pieces of Si(111) were cleaned by immersion in a 3:1 (v/v) piranha solution for 1 h. Piranha solution consists of sulfuric acid and hydrogen peroxide, which is highly corrosive, and should be handled carefully. After acid cleaning, the substrates were rinsed with copious amounts of deionized water and dried in air. Size-sorted, monodisperse polystyrene latex mesospheres (200 nm diameter) were used as surface masks for patterning (Thermo-Fisher Scientific, Waltman, MA). Aqueous solutions of latex were cleaned by centrifugation to remove surfactants or contaminants. Approximately 300 µL of the latex solution was placed into a microcentrifuge tube and centrifuged at 15,000 rpm for 15 min. A solid pellet was formed, and the supernatant was removed and replaced with deionized water. The latex pellet was resuspended with 300 µL of deionized water by vortex mixing to prepare a 1% w/v solution. The washing process was repeated twice. A drop (10–15 µL) of the cleaned mesospheres was deposited onto clean Si(111) substrates and dried under ambient conditions (25 °C, ~50% relative humidity) for at least one hour, in order to form surface masks for nanolithography.
Particle lithography combined with vapor deposition. The masked substrates were placed into sealed glass vessels for vapor deposition of organosilane. The samples were placed on a raised platform in a jar containing 300 µL of neat octadecyltrichlorosilane (Gelest, Morrisville, PA). A vapor was generated by heating the vessel in an oven at 70 °C. After at least 6 h, the samples were removed and rinsed with ethanol and water to remove the latex masks.
Particle lithography with contact printing. For contact printing, an inked block of polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning) was used to transfer OTS to the substrate through a physical mask of latex spheres. A drop (10–12 µL) of an OTS solution in bicyclohexyl was deposited onto a clean, dry block of PDMS (2 × 2 cm2). A 30 µL volume of a 40% v/v solution of OTS in bicyclohexyl was deposited and spread evenly over the PDMS block, then quickly dried in a stream of ultra-high-purity argon. The PDMS block coated with OTS was placed on top of the masked substrate. The film of OTS was transferred from the PDMS block through the latex mask to the substrate by permeation. The areas of the Si(111) surface located directly underneath the latex particles were protected from silane deposition. After 1 h of physical contact, the PDMS block was removed. The sample was rinsed with copious amounts of deionized water. In the final step, the mask of latex particles was cleanly removed by sonication and rinsing with ethanol and deionized water. After removal of the mask, a nanostructured film of OTS was generated on the surface.
Particle lithography with immersion. For the immersion strategy of particle lithography, the masked substrates of latex were heated for 30 min at 75 °C in order to anneal the beads to the surface. Masked substrates of colloidal silica mesospheres were heated for 12 h at 140 °C. After heating, the samples were cooled for at least 20 min under ambient conditions. The mesosphere-coated substrates were then immersed into a 0.1% solution of OTS in bicyclohexyl or anhydrous toluene for 1 h. Next, the samples were removed and rinsed with ethanol and deionized water, and sonication was used to remove the latex masks.