Ordered Alternating Binary Polymer Nanodroplet Array by Sequential Spin Dewetting Nandini Bhandaru, Anuja Das, Namrata Salunke, and Rabibrata Mukherjee* Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India ABSTRACT: We report a facile technique for fabricating an ordered array of nearly equal-sized mesoscale polymer droplets of two constituent polymers (polystyrene, PS and poly(methyl methacry- late), PMMA) arranged in an alternating manner on a topographically patterned substrate. The self-organized array of binary polymers is realized by sequential spin dewetting. First, a dilute solution of PMMA is spin-dewetted on a patterned substrate, resulting in an array of isolated PMMA droplets arranged along the substrate grooves due to self-organization during spin coating itself. The sample is then silanized with octadecyltrichlorosilane (OTS), and subsequently, a dilute solution of PS is spin-coated on to it, which also undergoes spin dewetting. The spin-dewetted PS drops having a size nearly equal to the pre-existing PMMA droplets position themselves between two adjacent PMMA drops under appropriate conditions, forming an alternating binary polymer droplet array. The alternating array formation takes place for a narrow range of solution concentration for both the polymers and depends on the geometry of the substrate. The size of the droplets depends on the extent of confinement, and droplets as small as 100 nm can be obtained by this method, on a suitable template. The findings open up the possibility of creating novel surfaces having ordered multimaterial domains with a potential multifunctional capability. KEYWORDS: Multifunctional surfaces, spin dewetting, alternate droplet array, silanization P olymer surfaces with ordered mesoscale topographic patterns spanning over large areas are essential for the fabrication of various state of art devices and items such as an optical microlens array for image processing, 1 optoelectronic devices, 2 nanoelectronics including flexible displays, 3 super hydrophobic and self-cleaning surfaces including biomimetic surfaces, 4 nanobiotechnology applications, and so forth. 5 The structures can be realized in various types of polymers as well as other materials such as gels, colloids, and so forth, by different patterning strategies such as optical lithography, direct write methods like focused ion beam (FIB) and electron beam lithography (EBL), soft lithography, colloidal self-assembly, spontaneous or external field mediated instability of ultrathin films, etc. Each of these techniques has its own advantages as well as disadvantages, which makes it preferred and well-suited for certain specific settings only. For example, optical lithography is widely used for microelectronics but is not well-suited for rapid prototyping of different varieties of polymers. 6 Direct write methods with charged particles such as EBL and FIB lead to ultrafine features with sub 50 nm lateral resolutions but are limited by the inherently slow serial nature of the processes. 6 Various soft lithography and nano imprint lithography (NIL) based techniques find extensive applications in patterning polymer films of different kinds due to their inherent simplicity and ease of execution. 7,8 On the other hand, non-lithographic approaches have also been utilized for generating mesoscale patterns. For example, nanosphere lithography based on self-assembly of monodis- persed nano and colloidal particles is capable of creating hexagonal closed packed array of particles on a flat substrate and more complex arrangements under appropriate templat- ing. 9,10 Various types of spontaneous or external field mediated instabilities in thin polymer films arising from interfacial interactions have also been used for patterning. 11-16 Instability mediated patterns are typically random and isotropic but can be aligned by suitable templating strategies which include using a chemically or topographically patterned substrate or a confining mask. 17-21 Ordered dewetted patterns have been successfully used for fabrication of ultrahigh density storage media, 22 single- crystalline wire based organic field effect transistors, etc. 23 Interestingly, all of the techniques discussed or cited above are capable of producing structures or assemblies with a single- component only. The surface morphology comprising ordered multicomponent domains, which are scientifically more challenging to fabricate, also finds various application in the fabrication of multifunctional coatings and in the development of exotic metamaterials. 24 To highlight the novelty of the present work, we briefly review the available approaches for creation of surfaces with Received: August 29, 2014 Revised: November 23, 2014 Published: November 24, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 7009 dx.doi.org/10.1021/nl5033205 | Nano Lett. 2014, 14, 7009-7016