Review Cite This: ACS Photonics 2019, 6, 240−259
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Quantifying Figures of Merit for Localized Surface Plasmon Resonance Applications: A Materials Survey Brock Doiron,*,† Moń ica Mota,† Matthew P. Wells,‡ Ryan Bower,‡ Andrei Mihai,‡ Yi Li,† Lesley F. Cohen,† Neil McN. Alford,‡ Peter K. Petrov,‡ Rupert F. Oulton,† and Stefan A. Maier†,§ †
Department of Physics, Imperial College London, London, United Kingdom Department of Materials, Imperial College London, London, United Kingdom § Nanoinstitut München, Chair in Hybrid Nanosystems, Faculty of Physics, Ludwig-Maximilians Universität München, München, Germany Downloaded via LANCASTER UNIV on April 13, 2019 at 13:47:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedented capabilities in optoelectronic devices, medical treatments and nanoscale chemistry, due to a strong enhancement of light-matter interactions. As we continue to explore novel applications, we require a systematic quantitative method to compare suitability across different geometries and a growing library of materials. In this work, we propose application-specific figures of merit constructed from fundamental electronic and optical properties of each material. We compare 17 materials from four material classes (noble metals, refractory metals, transition metal nitrides, and conductive oxides) considering eight topical LSPR applications. Our figures of merit go beyond purely electromagnetic effects and account for the materials’ thermal properties, interactions with adjacent materials, and realistic illumination conditions. For each application we compare, for simplicity, an optimized spherical antenna geometry and benchmark our proposed choice against the state-of-the-art from the literature. Our propositions suggest the most suitable plasmonic materials for key technology applications and can act as a starting point for those working directly on the design, fabrication, and testing of such devices. KEYWORDS: plasmonics, Mie theory, material characterization, hot electron devices, photothermal applications
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plasmonic applications have been demonstrated experimentally, they have yet to be integrated into widespread industrial settings, as is seen with photonics and electronics with the exception of surface enhanced Raman scattering (SERS) substrates.8 This is primarily due to the incompatibility with industrial fabrication techniques due to the relatively low melting temperatures of conventional plasmonic metals resulting in diffusion into silicon9 and the difficulty of patterning inert metals using gas phase chemical etching.10 In addition, the large absorptive losses at visible wavelengths result in pronounced heating, which can speed up the degradation of surrounding components.11 Recently, there has been particular interest in alternative materials and the development of nonradiative applications12,13 to circumvent these issues without compromising performance. We divide the diverse range of materials currently being investigated into four classes: noble metals, refractory (hightemperature stable) metals, transition metal nitrides, and
he strong interaction between light and the free electrons of metals gives rise to extraordinary phenomena not possible with conventional dielectric photonic systems. The oscillating electric field of light can resonantly drive these free electrons in what is called a surface plasmon resonance (SPR) due to the generation of interfacial charges that coherently exchange energy with the electromagnetic field. From the coining of the term “surface plasmon” as a quantization of plasma oscillation in the late 1950s in thin metal foils,1 plasmonics has developed into an active area of research.2 With recent developments in nanofabrication techniques, it is possible to fabricate metallic particles with sizes below the wavelength of light that still readily interact with incident radiation. This excitation is termed a localized surface plasmon resonance (LSPR) distinguished from a propagating surface plasmon polariton (SPP) in thin metal films. The ability to confine and control light on length scales below the diffraction limit was unprecedented and stimulated a surge of proposed novel applications3 including integrated photonic circuits,3 imaging,4,5 and sensing.6,7 Although these and many other © 2019 American Chemical Society
Received: October 1, 2018 Published: January 18, 2019 240
DOI: 10.1021/acsphotonics.8b01369 ACS Photonics 2019, 6, 240−259
ACS Photonics
Review
loss while still maintaining plasmonic properties in the visible regime. With the advantages and limitations of each material class and the range of material properties within each class, it is clearly not straightforward to determine the best route forward in the development of commercially viable plasmonic applications. This issue is well-recognized within the plasmonics community and has resulted in proposals of universal “quality factors” for localized and propagating SPRs, which simply consider the ratio of the real and imaginary parts of the bulk dielectric permittivity.34,35 Although useful as a first step, this fails to capture the interaction of the particle with its environment, the effects of geometry, and specifications of particular applications. Since 2015, several groups have addressed this issue, quantifying and comparing figures of merit for localized heating36 and solar energy harvesting.37 However, such investigations have relied on the electrostatic approximation, which limits the validity of the estimations to spheres below 10 nm is size due to the exclusion of retardation effects.38 Such small nanoparticles are challenging to fabricate in planar geometries but are also not relevant for many applications where particle sizes exceed 100 nm. As such, a more rigorous electromagnetic treatment is required that can account for the geometric effects underpinning one of the most critical advantages of plasmonics: tuning the resonance by particle size. In this work, we propose figures of merit for broad industrially relevant applications, where the material selection and geometry of plasmonic devices are simultaneously optimized using only standard optical thin film characterization as input. First, using conventional Drude-Lorentz fitting procedures of the measured spectroscopic ellipsometry data, we can extract properties of both free and bound electrons while accounting for both material quality and the presence of interfacial oxide layers. Next, using Mie theory calculations, we analytically describe the dipolar resonance mode of a metallic sphere for a range of radii and wavelengths as well as the corresponding scattering and absorption cross sections. As these two processes underpin a considerable number of plasmonic applications, we review the underlying physics of each application then develop a figure of merit for the considered applications using these cross sections. The simplicity of this approach (shown schematically in Figure 1) allows for a straightforward and systematic characterization of each application with directly applicable results as well as easy expansion to new materials and related applications.
conductive oxides. The extensive range of physical properties (hardness, heat tolerance, thermal dissipation) and electronic characteristics (free carrier concentration, effective mass) are taken into account to examine materials which will facilitate plasmonic devices that can be efficiently implemented across the ultraviolet (UV), visible, and infrared (IR) regimes. The summary of the operation ranges and advantageous properties is summarized in Table 1. The most widely used and wellTable 1. material Properties and Operation Rangesa
a
Each class of materials possess advantages including low loss, thermal stability, and a tunable carrier concentration. As a first step, the potential materials can be selected based on the required properties and the operation range (UV, visible, or near-IR).
studied materials in plasmonics are silver (Ag), gold (Au), and copper (Cu). These noble metals were originally preferred due to their stability and high conductivity resulting in sharp resonances. However, these materials are also plagued by low melting temperatures (
