Emergent Opportunities with Metallic Alloys: From Material Design to Optical Devices

Abstract

devices is the fact that their dielectric function (i.e., permittivity, ε = ε 1 + iε 2) is predefined. Thus, several research groups, including ours, have recently merged two almost orthogonal fields, photo nics, and metallurgy, to pursue metallic materials with arbitrary permit tivity. [1-5] Alloying is now a burgeoning framework for achieving materials with engineered optical properties, encom passing both nanostructures and thin films, as will be surveyed in this Review. Plasmonics exploits the interaction of incident electromagnetic fields with the col lective motion of free electrons (plasmons) in metals. This phenomenon confines the electromagnetic fields in close vicinity of the metal interfaces, dramatically enhancing the electrical field intensity surrounding the material. [6-8] Plasmons can be divided into two categories: surface plasmon polaritons (SPP) that propagate along the metal and dielectric interface, and localized surface plasmon resonance (LSPR) that are confined in a subwavelength nanostructure. For the latter, their optical scattering or absorp tion (and/or the nearby dielectrics and semiconductors) can be significantly increased. As a result, these enhancement effects are the underpinnings to a range of novel optical processes, and have found countless applications in photovoltaics, [9-11] photo catalysis, [12-14] bio and chemicalsensing, [15,16] electrooptical modu lation, [17,18] and superabsorbers. [19-21] As expected, the features of either type of plasmon are strongly dependent upon the dielectric function of the material, which is somewhat fixed in metals. Concerning materials, coinage metals, such as Au, Ag, or Cu, are widely used in photonics due to their abundant free electrons and chemical stability. [1] Other noble metals including Metallic nanostructures and thin films are fundamental building blocks for next-generation nanophotonics. Yet, the fixed permittivity of pure metals often imposes limitations on the materials employed and/or on device performance. Alternatively, metallic mixtures, or alloys, represent a promising pathway to tailor the optical and electrical properties of devices, enabling further control of the electromagnetic spectrum. In this Review, a survey of recent advances in photonics and plasmonics achieved using metal alloys is presented. An overview of the primary fabrication methods to obtain subwavelength alloyed nanostructures is provided, followed by an in-depth analysis of experimental and theoretical studies of their optical properties, including their correlation with band structure. The broad landscape of optical devices that can benefit from metallic materials with engineered permittivity is also discussed, spanning from superabsorbers and hydrogen sensors to photovoltaics and hot electron devices. This Review concludes with an outlook of potential research directions that would benefit from the on demand optical properties of metallic mixtures, leading to new optoelectronic materials and device opportunities.

Transient Structural Colors with Magnesium‐Based Reflective Filters

“…From material's perspective, structural color filters consist prevalently of conventional metals (e.g., Au, Ag, and Al) owing to their low optical loss. However, they are either nonearth abundant and CMOS‐incompatible (Au, Ag) or non‐biodegradable (Ag, Al), which often restricts their utilization in biosensing [ 28,29 ] and augmented reality. [ 30 ] Recently, magnesium (Mg) has drawn increasing research interest for nanophotonic and plasmonic applications as an earth‐abundant, biodegradable, and CMOS‐compatible alternative to conventional metals.…”

Efficient hot carrier dynamics in near-infrared photocatalytic metals

“…While hot carrier dynamics in Au-group metals (Au,Ag,Cu) covering the ultraviolet and visible spectral range have been widely studied due to their highlyenergetic efficient generation via interband excitations, [6,23,27] a microscopic understanding of the carrier relaxation processes in Pt-group metals (Pt,Pd,Ni) have received little attention [9]. These carriers are generated in the mid-infrared and near-infrared range, and subsequent engineering of the electronic and optical properties by alloying them with other noble metals has emerged as a route for the efficient generation of hot carriers as this procedure brings the positions of the d-band closer to the Fermi level, E F [28][29][30][31]. Consequently it reduces the interband energy threshold, permitting the efficient generation of long-lived hot carriers in the infrared and near-infrared region [32,33].…”

DFT-Based Approach Enables Deliberate Tuning of Alloy Nanostructure Plasmonic Properties

“…Recently, electronic, optical, and plasmonic properties of alloys have been explored using the density functional theory (DFT). , The DFT-based approach is able to circumvent the above-mentioned limitations. It requires practically no empirical data and provides a fundamental physical insight into electronic structure modification, connecting composition variation with the change in optical properties.…”