Ni–Au magnetoplasmonic nanoparticles with a unique intertwined crystal structure showing excellent reduction and tunability of the superparamagnetic blocking temperature

Abstract

Magnetoplasmonic bimetallic nanoparticles (NPs) are inevitable components of several research domains that have widespread applications. However, bimetallic NPs that possess simultaneous magnetic and optical properties are often tremendously hard to synthesize. We report an extremely straight-forward, facile, quick, cost effective and easily scalable chemical method for the synthesis of magnetoplasmonic bimetallic nickel–gold (Ni–Au) NPs possessing a unique intertwined crystal structure, exhibiting excellent magnetic and plasmonic properties. The chemical method employs minimal number of precursors and eludes the necessity of inert gas atmospheres, high temperature refluxing procedures and templates. Through extensive crystal structure, compositional and surface characterizations by combining XRD, HR-TEM, EDX, SAED and XPS techniques, the bimetallic NPs were seen to possess an interesting structure where the atomic planes of Ni and Au were intertwined within the whole of each NP, unlike conventional core–shells or alloys. Specifically, an exhaustive HR-TEM analysis methodology applied to each visible Ni–Au NP proved vital in disclosing the intertwined nature of the Ni and Au atomic planes. The Ni–Au NPs exhibited magnetization properties that were drastically different from those of the pure metallic Ni NPs. While Ni NPs showed superparamagnetism (SPM) at temperatures much higher than room temperature, Ni–Au NPs showed an SPM blocking temperature, TB, as low as 47 K. SPM in the bimetallic system also exhibited appreciable tunability, that was demonstrated by further decreasing the TB value in the Ni–Au NPs that were synthesized using smaller sized Ni NP seeds. In addition to showing the much desired SPM behaviour at room temperature, the broad plasmonic resonance in the visible wavelength region due to the presence of Au makes the Ni–Au NPs multifunctional and thus promising for various applications. The developed synthesis method highlights the possibilities towards synthesizing magnetoplasmonic bimetallic systems with unique property-specific architectures through much less expensive, unconstrained chemical synthesis pathways.