Exploiting Native Al₂O₃ for Multispectral Aluminum Plasmonics

Abstract

Aluminum, despite its abundance and low cost, is usually avoided for plasmonic applications due to losses in visible/infrared regimes and its interband absorption at 800 nm. Yet, it is compatible with silicon CMOS processes, making it a promising alternative for integrated plasmonic applications. It is also well known that a thin layer of native Al 2 O 3 is formed on aluminum when exposed to air, which must be taken into account properly while designing plasmonic structures. Here, for the first time we report exploitation of the native Al 2 O 3 layer for fabrication of periodic metal−insulator−metal (MIM) plasmonic structures that exhibit resonances spanning a wide spectral range, from the nearultraviolet to mid-infrared region of the spectrum. Through fabrication of silver nanoislands on aluminum surfaces and MIM plasmonic surfaces with a thin native Al 2 O 3 layer, hierarchical plasmonic structures are formed and used in surface-enhanced infrared spectroscopy (SEIRA) and surface-enhanced Raman spectrocopy (SERS) for detection of self-assembled monolayers of dodecanethiol. KEYWORDS: aluminum plasmonics, metal−insulator−metal cavities, surface-enhanced Raman spectroscopy, surface-enhanced infrared spectroscopy, nanoparticles, hierarchical structures R ecent advancement in plasmonics enabled the development of better performing plasmonic materials for the ultraviolet (UV) 1−5 and infrared (IR) 6−13 portion of the light spectrum. Typically gold (Au) and silver (Ag) are the most common materials used to fabricate nanostructures to study novel plasmon-enhanced materials and enable optical phenomena such as negative refraction, 14,15 transformation optics, 16 surface plasmon sensors, 17,18 surface-enhanced Raman spectroscopy (SERS), 19,20 surface-enhanced infrared absorption spectroscopy (SEIRA), 21,22 and plasmon-enhanced solar cells and detectors. 23,24 Au has an internal band transition around 500 nm, which limits utilization of gold toward the UV portion of the visible spectrum. 25 Due to its chemically inert properties, stability, and tailorable binding to biomolecules, Au is widely used for surface plasmon resonance sensor applications working at visible wavelengths closer to the NIR regime. Ag is considered the optimal material for plasmonic applications in the visible spectrum due to its low loss compared to other metals. 25 However, Ag suffers from atmospheric sulfur contamination and oxidation. 26 Aluminum arises as a promising material for UV 27,28 and deep UV plasmonic applications 3,4,29−31 owing to its high plasma frequency. Al has high losses from the visible to IR range as well as an interband absorption around 800 nm, which makes it less favorable as a NIR plasmonic material. 1,25,32 Still, localized plasmon resonances in Al have been demonstrated in several geometries, including nanoparticles, 27,30,33 triangles, 3,28,34 discs, 4,35,36 rods, and nanoantennas. 31,37,38 The relative abundance of Al can be advantageous for the design of plasmonic absorbers in solar energy conversion or for i...

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