Magnetic and Optical Properties of Au–Co Solid Solution and Phase-Separated Thin Films and Nanoparticles

Abstract

The chemical composition and morphology of Au x Co1–x thin films and nanoparticles are controlled via a combination of cosputtering, pulsed laser-induced dewetting (PLiD), and annealing, leading to tunable magnetic and optical properties. Regardless of chemical composition, the as-deposited thin films and as-PLiD nanoparticles are found to possess a face-centered cubic (FCC) Au x Co1–x solid-solution crystal structure. Annealing results in large phase-separated grains of Au and Co in both the thin films and nanostructures for all chemical compositions. The magnetic and optical properties are characterized via vibrating sample magnetometry (VSM), ellipsometry, optical transmission spectroscopy, and electron energy loss spectroscopy (EELS). Despite the exceptionally high magnetic anisotropy inherent to Co, the presence of sufficient Au (72 atom %) in the Au x Co1–x solid solution results in superparamagnetic thin films. Among the as-PLiD nanoparticle samples, an increased Co composition leads to a departure from traditional ferromagnetism in favor of wasp-waisted hysteresis caused by magnetic vortices. Phase separation resulting from annealing leads to ferromagnetism for all compositions in both the thin films and nanoparticles. The optical properties of Au x Co1–x nanostructures are also largely influenced by the chemical morphology, where the Au x Co1–x intermixed solid solution has significantly damped plasmonic performance relative to pure Au and comparable to pure Co. Phase separation greatly enhances the quality factor, optical absorption, and electron energy loss spectroscopy (EELS) signatures. The enhancement of the localized surface plasmon resonances (LSPRs) scales with the reduction in Co composition, despite EELS evidence that excitation of the Co portions of a nanoparticle can provide a similar, and in some instances enhanced, LSPR resonance compared to Au. This behavior, however, is seemingly limited to the LSPR dipole mode, while higher-order modes are greatly damped by a Co aloof position. This observed magneto-plasmonic functionality and tunability could be applicable in biomedicine, namely, cancer therapeutics.

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

“…The Rietveld refinements of the XRD patterns indicate a cell parameter of 4.0038 ± 0.0002 Å (sample A), 4.0043 ± 0.0007 Å (sample B) and 4.0328 ± 0.0006 Å (sample C). These cell parameters are contracted compared to pure Au (4.079 Å), which is consistent with the formation of a solid solution where Co, which has a smaller atomic size (1.25 Å) than Au (1.44 Å) 51 , is a random substitutional impurity in the gold lattice 49 , 51 , 52 . A bimetallic compound forming a solid solution with the same lattice of the pure metal is a textbook case for the application of Vegard’s law on the linear dependence of the cell parameter to the alloy composition 58 .…”

“…Here we consider the prediction and quantitative assessment of the optical properties in alloys of plasmonic (Au) and magnetic (Co) elements. Au is the most chemically and physically stable plasmonic element, biocompatible and renowned for its surface chemistry allowing versatile functionalization 34 , 47 – 49 . Besides, Au has interesting catalytic properties for a series of photocatalytic and photo-electrocatalytic reactions of strategic importance for developing a sustainable economy 34 , 47 – 49 .…”

“…Au is the most chemically and physically stable plasmonic element, biocompatible and renowned for its surface chemistry allowing versatile functionalization 34 , 47 – 49 . Besides, Au has interesting catalytic properties for a series of photocatalytic and photo-electrocatalytic reactions of strategic importance for developing a sustainable economy 34 , 47 – 49 . Co is an earth-abundant element with the second highest atomic magnetic moment in its pure metallic state (1.72 μ B ), possesses plasmonic properties when not oxidized, and exhibits the catalytic properties of the transition metals of the 4 th period 50 , 51 .…”

Synthesis of Bimetallic Nanoparticles and Applications—An Updated Review

“…The highly magnetic properties of Au-Fe and Ni-Co bimetallic nanoparticles makes them suitable to use as contrast agent for CT and MRI diagnosis/prognosis imaging [224] and theranostic agent for tumors [225]. Similarly, Cu-Fe have been used for enhanced chemodynamic therapy [226], Pd-Pt, Au-Co, Au-Co, Ag-Cu, Au-Pt have been used for cancer therapeutics and anticancer activity [21,[227][228][229][230] and Au-Bi have been used for the inhibition of tumor cells [231]. In preventive medicine, the use of bimetallic nanoparticles is quite extensive and many of them are used as antimicrobial agents, antioxidants, antidiabetic, anti-Alzheimer, anti-inflammatory and drug delivery.…”

Rationalization of the sub-surface segregation in nanoalloys of weakly miscible metals

“…For AuCo, experimental works report more rounded shapes for nanoalloys (but still with an fcc structure) than geometrically well-defined ones. [32][33][34][35] In this case, the TOh shape can be considered as the geometrical one closest to a sphere. From the point of view of the theoretical approach, the most important thing is to have in the model different sites with characteristic coordination numbers in order to rationalize the phenomena of segregation at the atomic scale.…”