Programmable Nanopatterns by Controlled Debonding of Soft Elastic Films Nandini Bhandaru, † Ashutosh Sharma,* ,‡ and Rabibrata Mukherjee* ,† † Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India ‡ Department of Chemical Engineering and Nanoscience Center, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India ABSTRACT: We report a facile patterning technique capable of creating nanostructures with different feature heights (h S ), periodicities (λ S ), aspect ratios (A R ), and duty ratios (D R ), using a single grating stamp with fixed feature height h P and periodicity λ P . The proposed method relies on controlling the extent of debonding and morphology of the contact instability features, when a rigid patterned stamp is gradually debonded from a soft elastic film to which it was in initial conformal contact. Depending on whether the instability wavelength (λ F scales with the film thickness h F as λ F ≈ 3h F ) and the periodicity of the stamp feature (λ P ) are commensurate or not, it is possible to obtain features along each stamp protrusion when λ F ≈ λ P or patterns that span several stripes of the stamp when λ F > λ P . In both cases, the patterns fabricated during debonding are taller than the original stamp features (h S > h P ). We show that h S can be modulated by controlling the extent of debonding as well as the shear modulus of the film (μ). Additionally, when λ F > λ P , progressive debonding leads to the gradual peeling of replicated features, which, in turn, allows possible tuning of the duty ratio (D R ) of the patterns. Finally we show that by the simultaneous modulation of A R , D R , and h S , it becomes possible to create surfaces with controlled wettability. KEYWORDS: soft lithography, nanopatterning, debonding, contact instability, elastic film ■ INTRODUCTION The development of various soft-patterning techniques including soft lithography and nanoimprint lithography in the mid-1990s opened up new vistas in the nanopatterning of soft surfaces, 1-3 which find application in the fabrication of organic electronic circuits, 4 nanobiotechnology and tissue engineering scaffolds, 5,6 hydrophobic and self-cleaning surfaces, lab-on-chip devices, microfluidic mixers, etc. 7,8 Despite the ease of implementation and low cost, most of these embossing- or imprinting-based techniques are limited by the availability of an appropriate master or stamp because most of the methods are capable of generating a perfect negative replica of the original stamp with no possible option for tuning the pattern morphology or dimension. As a result, individual litho- graphically fabricated masters are necessary to create every new pattern. In order to reduce the dependence of soft nanofabrication on the primary master, efforts are underway to develop novel techniques that are capable of generating patterns that are not a mere negative replica of the original master, based on innovative approaches such as shrinking of a patterned hydrogel, 9 swelling and deswelling of a stamp during patterning, 10 stress relaxation in a viscoelastic film, 11,12 a surface-initiated polymerization reaction in the presence of a stamp, etc. 13 Ordered patterns have also been obtained with a flat stamp by applying an external electric field 14 or a thermal gradient 15 or even in the absence of any field due to the action of attractive electrostatic forces between the mask and a thermally softened polymer film in lithographically induced self-assembly (LISA). 16 However, in all of these approaches, spacers with definite heights are necessary to maintain uniform separation between the stamp and film because the pattern morphology largely depends on the ratio between the film thickness and air gap. TheChange Figure 2B (Replace “θE” with “θ*”) Change Figure 3B, C, D (Replace “θE” with “θ*”) delicate experimental protocol renders these approaches rather difficult for bulk nanopatterning applications despite the richness of science associated with them. Of particular interest is surfaces with tall nanostructures (a few hundreds of nanometers to a few microns), which have wide application in hydrophobic and self-cleaning surfaces. The fabrication of such surfaces by any soft lithographic technique would require a stamp with tall features. However, such a stamp Special Issue: Focus on India Received: July 24, 2016 Accepted: November 1, 2016 Forum Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.6b09127 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX