First principles calculations of the optical and plasmonic response of Au alloys and intermetallic compounds

Accurate prediction of the optical properties of nanoalloys with both plasmonic and magnetic elements

The alloying process plays a pivotal role in the development of advanced multifunctional plasmonic materials within the realm of modern nanotechnology. However, accurate in silico predictions are only available for metal clusters of just a few nanometers, while the support of modelling is required to navigate the broad landscape of components, structures and stoichiometry of plasmonic nanoalloys regardless of their size. Here we report on the accurate calculation and conceptual understanding of the optical properties of metastable alloys of both plasmonic (Au) and magnetic (Co) elements obtained through a tailored laser synthesis procedure. The model is based on the density functional theory calculation of the dielectric function with the Hubbard-corrected local density approximation, the correction for intrinsic size effects and use of classical electrodynamics. This approach is built to manage critical aspects in modelling of real samples, as spin polarization effects due to magnetic elements, short-range order variability, and size heterogeneity. The method provides accurate results also for other magnetic-plasmonic (Au-Fe) and typical plasmonic (Au-Ag) nanoalloys, thus being available for the investigation of several other nanomaterials waiting for assessment and exploitation in fundamental sectors such as quantum optics, magneto-optics, magneto-plasmonics, metamaterials, chiral catalysis and plasmon-enhanced catalysis.

Recent Developments in Plasmonic Alloy Nanoparticles: Synthesis, Modelling, Properties and Applications

Despite the traditional plasmonic materials are counted on one hand, there are a lot of possible combinations leading to alloys with other elements of the periodic table, in particular those renowned for magnetic or catalytic properties. It is not a surprise, therefore, that nanoalloys are considered for their ability to open new perspectives in the panorama of plasmonics, representing a leading research sector nowadays. This is demonstrated by a long list of studies describing multiple applications of nanoalloys in photonics, photocatalysis, sensing and magneto-optics, where plasmons are combined with other physical and chemical phenomena. In some remarkable cases, the amplification of the conventional properties and even new effects emerged. However, this field is still in its infancy and several challenges must be overcome, starting with the synthesis (control of composition, crystalline order, size, processability, achievement of metastable phases and disordered compounds) as well as the modelling of the structure and properties (accuracy of results, reliability of structural predictions, description of disordered phases, evolution over time) of nanoalloys. To foster the research on plasmonic nanoalloys, here we provide an overview of the most recent results and developments in the field, organized according to synthetic strategies, modelling approaches, dominant properties and reported applications. Considering the several plasmonic nanoalloys under development as well as the large number of those still awaiting synthesis, modelling, properties assessment and technological exploitation, we expect a great impact on the forthcoming solutions for sustainability, ultrasensitive and accurate detection, information processing and many other fields.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

The Quest for Zero Loss: Unconventional Materials for Plasmonics

There has been an ongoing quest to optimize the materials used to build plasmonic devices: first the elements were investigated, then alloys and intermetallic compounds, later semiconductors were considered, and, most recently, there has been interest in using more exotic materials such as topological insulators and conducting oxides. The quality of the plasmon resonances in these materials is closely correlated with their structure and properties. In general gold and silver are the most commonly specified materials for these applications but they do have weaknesses. Here, it is shown how, in specific circumstances, the selection of certain other materials might be more useful. Candidate alternatives include Ti_x N, VO₂ , Al, Cu, Al-doped ZnO, and Cu-Al alloys. The relative merits of these choices and the many pitfalls and subtle problems that arise are discussed, and a frank perspective on the field is provided.

Optical Properties of Au-based and Pt-based Alloys for Infrared Device Applications: A Combined First Principle and Electromagnetic Simulation Study

Due to the rapid progress in MEMS-based infrared emitters and sensors, strong demand exists for suitable plasmonic materials for such microdevices. We examine the possibility of achieving this goal by alloying other metals with the noble metals Au and Pt, which have some drawbacks, such as low melting point, structural instability, and high costs. The six different metals (Ir, Mo, Ni, Pb, Ta, and W) which possess good properties for heat resistance, stability, and magnetism are mixed with noble metals to improve the properties. The optical properties are calculated by density functional theory and they are used for further investigations of the optical responses of alloy nanorods. The results show that the studied alloy nanorods have wavelength selective properties and can be useful for infrared devices and systems.

Combinatorial Thin Film Sputtering Au xAl1- x Alloys: Correlating Composition and Structure with Optical Properties

The Au-Al alloy system was investigated via a combinatorial thin film sputtering method for its potential as a plasmonic material. Au _xAl_{1- x} combinatorial libraries were cosputtered from Au and Al elemental targets and the composition, phase, and dielectric function of a ∼350 nm film was determined using energy dispersive spectroscopy (EDS), grazing incidence X-ray diffraction (GIXRD), and spectroscopic ellipsometry, respectively. The phase evolution and optical properties were analyzed after annealing various compositions under a vacuum. The phases present matched the expected phases based on the published Al-Au binary phase diagram at all compositions. Interestingly, the mixed phase Al-AuAl₂ region showed the most optical tunability, where a maximum in the real part of the dielectric function progressively shifted to higher energy for increasing gold concentration. For almost pure AuAl₂, the imaginary component is largely reduced in the visible range and is comparable to that of pure Al in the UV region. A 20-nm-thick film with composition Au_0.74Al_0.26 was studied using a (scanning) transmission electron microscope with an in situ laser heating system. The structures of the as-deposited and laser annealed films were determined using selected area diffraction and the bulk plasmon of AuAl₂ and Al realized with electron energy loss spectroscopy. Last, the Au-rich solid solution region was investigated as a surface enhanced Raman spectroscopy (SERS) substrate using the benezenethiol (BT) molecule. Good SERS intensity was maintained up to 30% Al addition where enhancements of 10⁵ to 10⁷ were still observed.

The effect of vacancies on the optical properties of AuAl₂

AuAl2 is an intermetallic compound with a vivid purple colour attributable to a bulk plasmon energy in the visible part of the spectrum. However, the colour of as-deposited thin films is not as strong and only develops upon annealing. Density functional theory calculations of the dielectric function are presented for a variety of vacancy types and concentrations. The results support the view that the effect of annealing on colour is correlated with a reduction in concentration of Al vacancies. The effect of vacancies on the optical properties can be understood as arising from the complex interplay between interband transitions around the Fermi level and the plasmon energy.