Tunable Anisotropy in Inverse Opals and Emerging Optical Properties


Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. 2014;26 (4) :1622-1628.


Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the possible structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method for introducing anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol−gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure. Further, we prevent the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy and infrared spectroscopy studies clearly identify increasing degrees of sol−gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.


■ ACKNOWLEDGMENTS K.R.P. acknowledges support from a National Science Foundation Graduate Research Fellowship and a National Defense Science and Engineering Graduate Fellowship from the Department of Defense. N.V. acknowledges funding from the Leopoldina Fellowship Program. M.K. acknowledges support from the Alexander von Humboldt Foundation. C.C.P. acknowledges the support of an Edward, Frances and Shirley B. Daniels and Wyss Fellowship while at the Radcliffe Institute for Advanced Study (2012−2013). The authors thank Dr. Ian Burgess and Dr. Caitlin Howell for helpful discussions and Grant England and Jack Alvarenga for help with the angular reflectance and infrared instruments, respectively. This work was funded with support from the Air Force Office of Scientific Research (AFOSR) via Grant FA9550-09-0669-DOD35CAP. This work was performed in part at the Center for Nanoscale Systems (CNS) at Harvard University, which is supported by National Science Foundation Grant ECS-0335765.

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Last updated on 05/04/2018