Flexible Transparent Metal/Polymer Composite Materials Based on Optical Resonant Laminate Structures Sudarshan Narayanan, Jihoon Choi, † Lisa Porter, and Michael R. Bockstaller* Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: Suitable design of periodic metal/polymer composite materials is shown to facilitate resonant tunneling of light at absorbing wavelengths and to provide a means to significantly reduce optical absorption losses in polymer-based metallodielectric composite structures. The conditions for resonant tunneling are established based on the concept of “photonic band edge alignment” in 1D-periodic systems. For the particular case of a four-layer gold/polystyrene laminate structure, it is shown that the matching of the lower band edge of the 1D-periodic structure with the plasma frequency of the metal component facilitates the increase of optical transmission by about 500% as compared to monolithic film structures of equal total thickness. The effect of sheet thickness on the optical properties of thin metal films is determined and shown to be an important prerequisite for the reliable prediction of resonant metallodielectric structures. The resonant 1D-periodic metal/polymer heterostructures are shown to retain the flexural stability of the polymer matrix and thus could find application as flexible transparent conductors in areas such as “plastic electronics”. KEYWORDS: photonic crystal, composite, transparent conductor, plastic electronics, thin metal film ■ INTRODUCTION Metallodielectric/polymer-matrix composite materials have received attention as a material platform for applications ranging from broadband power limiters for electromagnetic shielding to antistatic coatings and optical materials. 1−3 A general drawback associated with the addition of metals to polymer matrices is the increase of absorption thatin most circumstancesprevents the application of metal/polymer composite materials in areas where optical transparency is required. The strong absorption of metals arises due to fully occupied d-states in conjunction with the high free electron density close to the metals’ Fermi levels, thus giving rise to interband and plasma absorption of incident electromagnetic waves. 4 The pronounced optical absorption of metals limits the distance that light (or other electromagnetic waves) of practical wavelengths can travel through a metal without incurring significant loss. The latter is often described in terms of the “skin depth” δ that corresponds to the distance along which the intensity of the wave |E| 2 decreases to 1/e of its value at the surface (where e denotes the Euler number). The small skin depth (δ ≅ 20−40 nm) of noble metals such as gold or silver in the visible wavelength range thus limits the application of metallic elements to ultrathin films if optical transparency is to be retained. An elegant approach to reduce optical losses in metal film structures was presented by Scalora and co-workers who demonstrated that the transmittance of laminated metallodielectric structures can be increased by several orders of magnitude as compared to monolithic metal film structures by taking advantage of a phenomenon called “optical resonant tunneling”. 5,6 The latter refers to the effect of a suitably engineered periodic grating structure to redistribute the electric field of an incident electromagnetic wave such that at absorbing frequencies the field is concentrated within the nonabsorbing dielectric component while being depleted from the absorbing metal regions. As a consequence, light traversing the optically resonant structure experiences reduced absorption. To date, the application of resonant tunneling to enhance the transmittance of metal-hybrid structures has focused on ceramic/metal (MgF 2 /Ag) structures that allow for particularly efficient tunneling due to the favorable mismatch of the dielectric constants of the respective constituents (see discussion below). However, the brittle mechanical character- istics of ceramics in conjunction with the (often) weak interfacial bonding render ceramic/metal composite materials sensitive to mechanical damage or thermal-induced delamina- tion that limit the application of ceramic/metal laminate structures. 7 In this contribution, we establish “design criteria” for the fabrication of optically resonant polymer/metal laminate structures and we demonstrate the fabrication of metal/ polymer composite materials with 5-fold increase of optical transparency (as compared to the respective monolithic structures) that retain the mechanical flexibility and robustness that is characteristic of polymer materials. The excellent electron conductivity of laminated polymer/metal hybrid structures that is imparted by the continuous metal component could render the resulting “transparent metallodielectric nanocomposites” a platform for innovative material technolo- gies in areas such as plastic electronics, power limiting, or Received: December 21, 2012 Accepted: April 23, 2013 Published: April 23, 2013 Research Article www.acsami.org © 2013 American Chemical Society 4093 dx.doi.org/10.1021/am303211g | ACS Appl. Mater. Interfaces 2013, 5, 4093−4099