Abstract
When fabricating photonic crystals from suspensions in volatile liquids using the horizontal deposition method, the conventional approach is to evaporate slowly to increase the time for particles to settle in an ordered, periodic close-packed structure. Here, we show that the greatest ordering of 10 nm aqueous gold nanoparticles (AuNPs) in a template of larger spherical polymer particles (mean diameter of 338 nm) is achieved with very fast water evaporation rates obtained with near-infrared radiative heating. Fabrication of arrays over areas of a few cm(2) takes only 7 min. The assembly process requires that the evaporation rate is fast relative to the particles' Brownian diffusion. Then a two-dimensional colloidal crystal forms at the falling surface, which acts as a sieve through which the AuNPs pass, according to our Langevin dynamics computer simulations. With sufficiently fast evaporation rates, we create a hybrid structure consisting of a two-dimensional AuNP nanoarray (or "nanogrid") on top of a three-dimensional polymer opal. The process is simple, fast, and one-step. The interplay between the optical response of the plasmonic Au nanoarray and the microstructuring of the photonic opal results in unusual optical spectra with two extinction peaks, which are analyzed via finite-difference time-domain method simulations. Comparison between experimental and modeling results reveals a strong interplay of plasmonic modes and collective photonic effects, including the formation of a high-order stopband and slow-light-enhanced plasmonic absorption. The structures, and hence their optical signatures, are tuned by adjusting the evaporation rate via the infrared power density.
Highlights
Periodic nanostructures have a wide range of applications including antireflection coatings,[1] self-cleaning films,[2] sensors,[3,4] metamaterials,[5] and devices for energy harvesting.[6]
Stabilized gold nanoparticles (AuNPs), approximately 10 nm diameter, in water were blended with spherical acrylic copolymer particles (338 nm) in water.[22]
The dispersions were cast as films and dried over a range of evaporation rates, which are expressed as the velocity at which the water surface falls downward during drying and which were determined experimentally
Summary
Periodic nanostructures have a wide range of applications including antireflection coatings,[1] self-cleaning films,[2] sensors,[3,4] metamaterials,[5] and devices for energy harvesting.[6]. According to previous reports, the method typically suffers from long drying times, which are conventionally used in colloidal crystallization from liquids to obtain a high degree of periodicity and order.[16] This time inefficiency presents a major obstacle to the widespread adoption of the horizontal deposition method. Hybrid structures combine aspects of plasmonics and photonics.[21] The strong synergetic effect of plasmon excitation and the photonic band gap results in materials with advanced optical functionalities that enable the control of the propagation, emission, and extinction of light on the nanoscale. We show how the ordering of the nanostructure benefits from shorter fabrication times, which contradicts the commonly held belief that slow drying with long assembly times is needed to obtain structures with a high degree of periodicity. We show that the surface morphology of the hybrid structures obtained can be adjusted by changes in the evaporation rate without the need for a sophisticated experimental setup or additional processing steps
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