Hexagonally packed arrays of self-assembled colloidal micro- and nanospheres on surfaces have been used as masks to guide deposition or etching through the interstices between the colloidal microspheres [5
]. For example, arrays of triangularly shaped metal islands can be obtained by sputter deposition of the metal [2
]. When gold is chosen as the metal, the ensuing pattern can be easily functionalized chemically with a self-assembled monolayer (SAM) of a thiol initiator, which can be subsequently amplified into polymer brushes. shows this strategy for the patterning of colloidal microspheres for the fabrication of polymer-brush microstructures. We first assembled a SMM of polystyrene latex (diameter ≈ 10 µm) on a silica substrate by gravity-induced sedimentation combined with solvent evaporation [26
], and subsequently we deposited gold into the interstices between the microspheres (). After the microsphere mask was removed by sonication, an array of hexagonally arranged triangular gold islands remained () on which we formed a SAM of thiol initiator (BrC(CH3
]. We then synthesized poly(N
-isopropylacrylamide) (PNIPAAM) brush microstructures on the islands by SI-ATRP of NIPAAM (). An AFM image of the patterned gold islands reveals a feature height of about 65 nm (). The feature size of a triangular island (≈2.3 µm) is about one quarter of the sphere diameter, and the distance between nearest-neighbor islands (≈5.3 µm) is around half of the sphere diameter, in accordance with a previous report by Haynes et al. [7
]. The resulting PNIPAAM brush height was about 350 nm, and due to polymerization also occurring at the sides of the triangles, the footprint size increased to about 2.9 µm () while the distance between nearest-neighbor islands remained about 5.3 μm. The feature size of the polymer brushes can be altered by changing (i) the size of the microspheres, (ii) the assembly of the spheres on the substrate surface, or by (iii) varying the conditions of the gold vapor deposition, to yield a range of microstructures [28
Figure 1 Schematic illustration and AFM images showing the use of CL in the fabrication of patterned polymer-brush microstructures. (A) SMM on a silica wafer serves as a template for gold deposition. (B) Removal of the microspheres by sonication. (C) Functionalization (more ...)
Colloidal microspheres have an inherently curved surface that can serve as a template for spreading alkanethiol molecules along the surface of the microspheres onto the gold substrate surface, creating a ring-shaped SAM feature around the footprint of the sphere–surface contact area. This so-called edge-spreading lithography (ESL) employing colloid microspheres as templates has been previously used to fabricate ring-shaped metal patterns [9
]. Here we replaced the octadecanethiol (ODT) molecules with thiol initiator (BrC(CH3
SH), and amplified the annular thiol initiator monolayer into ring-shaped polymer brushes (). In this patterning approach we used a SMM (sphere diameter ≈ 5 μm) to direct the transport of an alkanethiol initiator from an initiator-inked planar poly(dimethyl siloxane) (PDMS) stamp onto the gold surface (). Upon reaching the metal substrate, the thiol initiator molecules self-assemble into a patterned monolayer, which is confined by the footprint of each microsphere and the extent of lateral spreading of the thiols on the gold substrate (). Amplification of the ring-shaped initiator SAMs results in patterned, hollow cylindrical polymer brushes (–E). The inner diameter of the polymer-brush cylinders is about 900 nm. This diameter reflects the underlying ring-shaped initiator pattern and is on the order of 18% of the microsphere diameter, in close agreement with a previous report [9
]. The outer diameter of the hollow polymer-brush cylinders is about 1.5 µm, and is largely determined by the contact time of the PDMS stamp on the microsphere template, which implies that the diffusion of the thiol initiator along the surface of each microsphere depends on the contact time with the PDMS stamp [9
]. Furthermore, polymer brush microstructures may be varied by changing the concentration of the thiol initiator, or by adding inert thiol molecules [29
], which affects the thiol initiator distribution and diffusion on the gold surface.
Figure 2 Schematic illustration and AFM images showing the use of ESL for the fabrication of ring-shaped polymer-brush microstructures. (A) Arrayed SMM direct the transport of alkanethiol initiator from a planar PDMS stamp onto the gold surface (printing was carried (more ...)
Our results show that microspheres can be used to guide the spreading of a thiol initiator to form ring-shaped thiol patterns around the footprint of microspheres on the surface. While initiator-inked stamps only provide a limited thiol reservoir, the microsphere footprint could also be used as a mask for fabricating polymer-brush pillars, by inking the microsphere array with a large amount of thiol. Such an approach was first reported by Taylor and co-workers [10
], who described a simple CL technique to fabricate substrates with hexagonally patterned dots of protein surrounded by a protein-repellant layer of poly(ethylene glycol) (PEG). In that work, a self-assembled monolayer of latex spheres served as a lithographic mask to selectively graft a thin layer of PEG around the footprint of the microspheres. After removal of the spheres, a periodic pattern of holes in the protein-repellant PEG layer was exposed, and proteins could be selectively adsorbed onto the underlying surface in these holes. In a similar approach we used inert thiol to cover a SMM of polystyrene microspheres (diameter ≈ 10 μm) () to form an inert thiol SAM everywhere except in the footprint of each microsphere (), and then backfill with a thiol initiator (). Amplification of this pattern, after removal of the SMM, resulted in a periodic pattern of polymer-brush pillars (about 50 nm high and about 1.5 μm in diameter, –F). The diameters of the polymer cylinders were on the order of 15% of the microsphere diameter, in agreement with our result described above (ca. 18%).
Figure 3 Schematic illustration and AFM images showing use of colloidal microsphere lithography for patterning polymer-brush pillars. (A) A SMM, assembled on a gold substrate, serves as a mask for the inert thiol SAM pattern. (B) After ink transfer and drying (more ...)
Another type of polymer-brush microstructure can be designed by inking the microsphere arrays by thiol initiator first, to form an initiator SAM around the microspheres. This should result in a polymer-brush layer with a patterned hole-like microstructure after removal of the microspheres and subsequent amplification [11
]. Xu et al. developed a method to pattern a surface with polymer brushes during a polymerization process in a microchannel, formed between PDMS stamps and initiator-modified substrates [30
]. This so-called microchannel-confined surface-initiated polymerization technique showed that there is no polymer brush growth in the contact area of the PDMS stamp with an initiator-functionalized SAM-coated silicon wafer. This inspired us to form a SMM on thiol-initiator-coated gold substrates as a template for fabricating hole-patterned polymer brushes. We first assembled a mask of polystyrene latex particles (SMM) on a gold substrate previously covered with a SAM of thiol initiator (), and then amplified the exposed initiator by SI-ATRP of NIPAAM (). After removing the SMM, a polymer-brush thin film with a hole pattern was obtained (,D). The patterned polymer brush layer has a height of about 16 nm and a hole diameter of about 6 μm.
Figure 4 Schematic illustration and AFM images showing the use of colloidal microsphere lithography for patterning hole-like polymer-brush microstructures. (A) SMM on thiol initiator SAM-coated gold substrate. (B) Subsequent pattern amplification into polymer-brush (more ...)