Abstract
The scale‐invariant features of fractal‐structured materials offer significant opportunities for the manipulation of short‐ and long‐range light–matter interactions in a 3D space, with recent photonics applications including biomolecular sensing and visible‐blind photodetectors. The development of synthesis methods for the large‐scale fabrication of fractal metamaterials with tuneable hierarchy bears significant potential and is the focus of many research fields. Among various fabrication routes, Brownian's motion‐driven coagulation of nanomaterials, below their sintering temperature, leads to fractal‐like structures presenting self‐similar properties at different length scales. Herein, an in‐depth investigation of the properties of fractal metamaterials obtained via the scalable self‐assembly of hot aerosols of TiO2, Bi2O3, and Au‐Bi2O3 nanoparticles, chosen as representative photonic materials, is reported. The fractal properties of these aerosol‐synthesized nanoparticle powders and thin films are systematically investigated via small‐angle X‐ray scattering (SAXS), image analysis, and theoretical modeling. It is demonstrated that in the diffusion‐limited aggregation (DLA) regime the fractal dimensions are preserved and in the range of 1.75–1.83 during the formation of the nanoparticle agglomerates, independently of the material. These findings provide a flexible platform for the engineering of macroscale 3D nanomaterials with hierarchical properties with potential applications ranging from energy harvesting to photocatalysis and sensing.
Highlights
In nanophotonics, precise arrangements of the nanostructures, terns, such as snowflakes, lightning bolts, heartbeat, and pulmonary vessels.[6]
We investigate the synthesis mechanisms that lead to the formation of fractal agglomerate, and we investigate the relationship between numerical density, fractal dimensions, and morphology
We demonstrate that the self-similar properties arising by the stochastic nature of the deposition process are preserved during the formation of fractal films, for various materials, providing a flexible platform for the scalable engineering of tailored fractal photonic materials
Summary
Precise arrangements of the nanostructures, terns, such as snowflakes, lightning bolts, heartbeat, and pulmonary vessels.[6]. Using photon scanning tunneling microscopy (PSTM), they have directly imaged the localization of dipolar eigenmodes due to the localized surface plasmons and they have shown a high sensitivity of these modes to frequency and polarization This peculiarity makes metallic fractal systems excellent candidates for applications where the strength and the spatial localization of the electric fields are of particular importance, including the study of nonlinear phenomena and surfaceenhanced Raman spectroscopy (SERS).[19,20] illumination with suitable frequency of fractal aggregates can induce localization of dipolar modes in subwavelength regions smaller than the cluster size, leading to a high energy density which, in turn, results in giant enhancement of many optical processes. We introduce a computational model to predict the morphology of these powder fractals and thin film clusters, generated in the DLA regime achieving comparable structures to those obtained experimentally.[38,39] We demonstrate that the self-similar properties arising by the stochastic nature of the deposition process are preserved during the formation of fractal films, for various materials, providing a flexible platform for the scalable engineering of tailored fractal photonic materials
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