Engineering Fractal Photonic Metamaterials by Stochastic Self‐Assembly of Nanoparticles

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.

Keywords

• fractals
• scale invariant
• self-similar
• ultra-porous