Surface wrinkling by chemical modification of poly(dimethylsiloxane)-based networks during sputtering Michelle D. Casper, * a Arif ¨ O. G¨ ozen, a Michael D. Dickey, b Jan Genzer b and Jon-Paul Maria a Wrinkling is an important mechanical phenomenon that generates periodic topographical patterns across a surface. This paper presents experimental evidence that surface wrinkles, which form consequent to thin film magnetron sputtering of either indium tin oxide (ITO) or aluminum on poly(dimethylsiloxane) networks (PDMS-N) made from a commercial Sylgard-184 kit, result from chemical modification of the PDMS-N surface as opposed to extrinsic thermomechanical stresses originating from differential thermal expansion. X-ray photoelectron spectroscopy results reveal that the PDMS-N surface becomes depleted in carbon and concurrently enriched in oxygen relative to silicon due to sputtering. This silica-like surface layer possesses intrinsic compressive stress that leads to wrinkle formation during the first z5 seconds of sputtering. The wrinkles maintain their periodicity irrespective of the thickness of the ITO film formed during subsequent deposition. Furthermore, upon removal of the ITO layer, the wrinkles persist with their periodicity unchanged. A narrow sputtering pressure window between 2 and 12 mTorr generates wrinkles. Pressures below this range cannot sustain a radio frequency plasma, while pressures above this range provide sufficient thermalization of kinetic energy as to eliminate the energetic bombardment that modifies the PDMS-N. This study provides a new understanding of the origins of wrinkling in sputtered films on polymeric substrates and creates opportunities to manipulate the topography produced by spontaneous surface wrinkling. Introduction Electronic and photonic materials rely routinely on topo- graphically patterned micro- and nano-structures to endow functionality and/or enhance performance. 1 In systems composed of hard and so multi-layers, spontaneous surface wrinkling can be transformed from an unwanted mechanical instability to a naturally occurring opportunity that produces corrugated surfaces. Wrinkling occurs by subjecting a bilayer comprising a stiff skin resting on and adhering strongly to a deformable substrate to stress. This stress can be released through the bending and deformation of the layers to form regularly spaced corrugations. By controlling the directionality of the compressive stress, wrinkling can be used to produce different types of periodic surfaces, ranging from randomly- oriented to well-aligned wrinkles, as a consequence of the relaxation of isotropic and anisotropic stresses, respectively. 2,3 In the case of uniaxial stress, both compressive stress as well as stress in elongation can lead to wrinkling. In many cases, wrinkled surfaces are formed by depositing a rigid lm (e.g., vapor deposited metal or metal oxides) onto a compliant polymeric substrate, such as a poly(dimethylsilox- ane) network (PDMS-N). 2,4–11 Alternatively, the rigid lm may be generated by chemically modifying the surface of a PDMS-N substrate (e.g., oxidation via exposure to oxygen plasma). 10,12–18 Ultimately, the wrinkling instability occurs by subjecting the bilayer system to some form of extrinsic or intrinsic stress, i.e., thermal expansion mismatch strain due to heating or mechanical stretching, or swelling of the substrate by exposure to a solvent. 19–21 While the stress can result from a variety of sources, the common requirement is surface compression. Here we report a study of surface wrinkling in a system consisting of an indium thin oxide (ITO) lm deposited on a PDMS-N substrate. This system forms wrinkles (1) in situ during the rst 5–10 seconds of sputter deposition, (2) with charac- teristic wavelengths independent of lm thickness, and (3) with topography that perseveres aer removing the deposited ITO lm. These results are in contrast to multiple studies that attribute wrinkles formed by physical vapor deposition of rigid lms on elastomeric substrates to the stresses arising from a Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27606, USA. E-mail: mdcasper@ncsu.edu; Fax: +1 919 515 3419; Tel: +1 919 513 4660 b Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27606, USA Cite this: Soft Matter, 2013, 9, 7797 Received 8th April 2013 Accepted 26th June 2013 DOI: 10.1039/c3sm50966d www.rsc.org/softmatter This journal is ª The Royal Society of Chemistry 2013 Soft Matter, 2013, 9, 7797–7803 | 7797 Soft Matter PAPER