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Water‐Dependent Photonic Bandgap in Silica Artificial Opals
Author(s) -
GallegoGómez Francisco,
Blanco Alvaro,
CanalejasTejero Victor,
López Cefe
Publication year - 2011
Publication title -
small
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.785
H-Index - 236
eISSN - 1613-6829
pISSN - 1613-6810
DOI - 10.1002/smll.201100184
Subject(s) - materials science , band gap , spheres , photonic crystal , refractive index , desorption , photonics , structural coloration , adsorption , nanotechnology , optoelectronics , chemical engineering , optics , chemistry , organic chemistry , astronomy , engineering , physics
Some characteristics of silica‐based structures—like the photonic properties of artificial opals formed by silica spheres—can be greatly affected by the presence of adsorbed water. The reversible modification of the water content of an opal is investigated here by moderate heating (below 300 °C) and measuring in situ the changes in the photonic bandgap. Due to reversible removal of interstitial water, large blueshifts of 30 nm and a bandgap narrowing of 7% are observed. The latter is particularly surprising, because water desorption increases the refractive index contrast, which should lead instead to bandgap broadening. A quantitative explanation of this experiment is provided using a simple model for water distribution in the opal that assumes a nonclose‐packed fcc structure. This model further predicts that, at room temperature, about 50% of the interstitial water forms necks between nearest‐neighbor spheres, which are separated by 5% of their diameter. Upon heating, dehydration predominantly occurs at the sphere surfaces (in the opal voids), so that above 65 °C the remaining water resides exclusively in the necks. A near‐close‐packed fcc arrangement is only achieved above 200 °C. The high sensitivity to water changes exhibited by silica opals, even under gentle heating of few degrees, must be taken into account for practical applications. Remarkably, accurate control of the distance between spheres—from 16 to 1 nm—is obtained with temperature. In this study, novel use of the optical properties of the opal is made to infer quantitative information about water distribution within silica beads and dehydration phenomena from simple reflection spectra. Taking advantage of the well‐defined opal morphology, this approach offers a simple tool for the straightforward investigation of generic adsorption–desorption phenomena, which might be extrapolated to many other fields involving capillary condensation.