Photonic Fractal Metamaterials: A Metal–Semiconductor Platform with Enhanced Volatile‐Compound Sensing Performance

Abstract

Advance of photonics media is restrained by the lack of structuring techniques for the 3D fabrication of active materials with long‐range periodicity. A methodology is reported for the engineering of tunable resonant photonic media with thickness exceeding the plasmonic near‐field enhancement region by more than two orders of magnitude. The media architecture consists of a stochastically ordered distribution of plasmonic nanocrystals in a fractal scaffold of high‐index semiconductors. This plasmonic‐semiconductor fractal media supports the propagation of surface plasmons with drastically enhanced intensity over multiple length scales, overcoming the 2D limitations of established metasurface technologies. The fractal media are used for the fabrication of plasmonic optical gas sensors, achieving a limit of detection of 0.01 vol% at room temperature and sensitivity up to 1.9 nm vol%−1, demonstrating almost a fivefold increase with respect to an optimized planar geometry. Beneficially to their implementation, the self‐assembly mechanism of this fractal architecture allows fabrication of micrometer‐thick media over surfaces of several square centimeters in a few seconds. The designable optical features and intrinsic scalability of these photonic fractal metamaterials provide ample opportunities for applications, bridging across transformation optics, sensing, and light harvesting.

Engineering Fractal Photonic Metamaterials by Stochastic Self‐Assembly of Nanoparticles

“…Aerosol techniques have been recently investigated as a new opportunity to produce functional coatings on various kind of flat or curved surfaces [43] with applications ranging from self-cleaning surfaces [44] to sensing. [25] During the aerosol deposition process, multiple particles reaching the substrate give rise to fractals that are then intertwined to each other, leading to a uniform film of fractal agglomerates. Representative morphologies of this aerosol self-assembly and deposition processes in the diffusion regime onto a silicon substrate are shown in Figure 2a-d.…”

“…The possibility to self-assemble semiconductors and metallic nanoparticles in a fractal form with high reproducibility opens the possibility to explore these novel nanoarchitectures for different applications, including, for instance, biochemical sensing and photoelectrocatalysis. These fractal structures have been recently used in optical sensing of volatile organic compounds (VOCs) [23][24][25]51] and photoelectrocatalysis. [36,37] Those works have shown the importance of stochastic fractals having self-similar properties over multiple length scales, in improving the localization of electromagnetic energy in subwavelengths volumes.…”

“…[18] The hierarchical features of Au-TiO 2 fractals, along with a high porosity (up to 98%) achieved in the DLA regime, enable the www.advancedsciencenews.com www.adpr-journal.com detection of different VOCs, achieving limits of detection of 0.01 vol% at room temperature, overperforming the results of nonfractal planar geometry. [23][24][25] Similarly, when applied as a catalyst for electrochemical CO 2 reduction reactions, Au-Bi 2 O 3 fractal structures show high selectivity to formate (HCOO À ) achieving high faradaic efficiency of 97% and high mass-specific formate current density of À54 mA mg À1 at À1.1 V versus a reversible hydrogen electrode (RHE), which is %3 times better than that of a control nonfractal Bi 2 O 3 catalyst. [36,37]…”

“…Aerosol techniques have been recently investigated as a new opportunity to produce functional coatings on various kind of flat or curved surfaces [ 43 ] with applications ranging from self‐cleaning surfaces [ 44 ] to sensing. [ 25 ]…”

“…Aiming to translate the plasmonic sensing results to the gas phase, and detect volatile organic compounds (VOCs), recently the integration of resonant plasmonic nanoparticles into tailored stochastic fractals has been investigated toward the realization of performant refractive index sensors. [23][24][25] The DLA process was used to directly self-assemble metal nanoparticles within fractal scaffolds of TiO 2 . [26,27] The stochastic nature of this process led to three-dimensionally disorganized structures, having scaleinvariant features and hierarchical porosity up to 98% consisting of meso-and nanopores.…”