Vapor Deposition of Hybrid Organic–Inorganic Dielectric Bragg Mirrors having Rapid and Reversibly Tunable Optical Reflectance Mustafa Karaman, †,§ Steven E. Kooi, ‡ and Karen K. Gleason* ,†,‡ Department of Chemical Engineering and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemical Engineering, Selcuk UniVersity, Konya 42075, Turkey ReceiVed October 30, 2007. ReVised Manuscript ReceiVed January 7, 2008 Rapid (0.3 s) and reversible biomimetic response of flexible dielectric mirrors was achieved by alternating inorganic (titania) and organic (poly(2-hydroxyethyl methacrylate, pHEMA) layers. Tunable reflectance bands in the visible range resulted from water swelling of the un-cross-linked pHEMA layers, without affecting the optical thickness of the high refractive index inorganic layer, which is in precise analogy to the structural color changing mechanism employed by many natural species. Larger refractive index contrast than accessible for all organic mirrors allow the desired reflectivity to be achieved with fewer layers and hence less overall thickness. The observed optical responses quantitatively match model predictions and are completely reversible. There is no loss in reflectivity intensity upon swelling. Hybrid heterostructures were grown within a single hot-wire chemical vapor deposition (CVD) chamber, resulting in smooth and uniform nanoscale layers of high interfacial quality. To the best of our knowledge, this is the first ever combination of an inorganic thin film with a fully functional polymer thin film having interfacial smoothness at the nanoscale. The room-temperature solventless HWCVD process is scalable to large area roll-to-roll deposition and is compatible with deformable substrates such as paper and plastic. 1. Introduction Fascinating structural coloration effects are observed in animals possessing iridophores, cells that contain stacks of thin platelets separated by gel-like cytoplasm. 1–6 Ideally, the thickness of each platelet and the cytoplasm separating them is regulated such that the reflections from each interface interfere constructively, generating a well-known one- dimensional photonic structure, a Bragg mirror. The eye of a scallop, Pecten maximus, contains 30–40 layers of high- index material guanine (n ) 1.83) separated by low-index cytoplasm (n ) 1.33). 3 The distinctive reflective stripes of a tropical fish Paradise whiptail are also made up of guanine- cytoplasm-based iridophores and rapid (∼0.25 s) and revers- ible color transitions are caused by swelling and shrinking of spaces between the guanine plates, induced by osmotic movements of water in cytoplasm. 2 Natural photonic struc- tures, such as those observed in butterfly scales, can have highly selective vapor response to individual vapors and hence potential technological applications for sensing. 7 Thin film Bragg mirrors and other specialized thin film optical filters are manufactured commercially onto rigid substrates using inorganic materials, such as silica (SiO 2 ) and titania (TiO 2 ). 8 Typically, layer thicknesses are precisely controlled by vapor deposition, a synthesis method capable of giving high growth rates uniformity over large areas. 9 Incorporation of organic materials allows creation of de- formable and tunable photonic structures. 10–12 In all organic mirrors, the refractive index contrast is low, necessitating a large numbers of layers to achieve high reflectivity. 11 Additionally, cross-linking of the adjacent polymer layers is necessary to prevent dissolution during the wet fabrication processes, which further decreases the index contrast. The resultant thick and highly cross-linked structures introduce diffusion limitations on the response time during swelling and shrinkage cycles. In a recent study, 10 the duration required for a complete shift in an all-organic Bragg structure in the reflectance band in the presence of an organic solvent was several tens of hours. Convertino and co-workers showed the possibility of tunable hybrid Bragg reflectors made from Teflon and gold as promising sensing elements. 12 The presence of metal clusters significantly improves the solvent absorbing power of the organic Teflon part, which is * Corresponding author. E-mail: kkg@mit.edu. Tel: (617) 253-5066. Fax: (617) 258-5042. † Department of Chemical Engineering, Massachusetts Institute of Technology. § Selcuk University. ‡ Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology. (1) Parker, A. R. J. Opt. A: Pure Appl. Opt. 2000, 2, R15–R28. (2) Mathger, M. L.; Land, M. F.; Siebeck, U. E.; Marshall, N. J. J. Exp. 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Mater. 2008, 20, 2262–2267 10.1021/cm703107d CCC: $40.75  2008 American Chemical Society Published on Web 02/20/2008