Au−Cu Alloy Nanoparticles with Tunable Compositions and Plasmonic Properties: Experimental Determination of Composition and Correlation with Theory

For plasmonic alloy nanoparticles, theoretical modeling and experimental characterization are both central to our capabilities involving predictable synthesis and targeted applications. This article uses composition-tunable colloidal Au−Cu nanoparticles as a model system for exploring the issue of reliable experimental determination of composition in plasmonic alloy nanoparticles and correlation of this experimental data with theoretical predictions. Highly uniform spherical Au1-x Cu x alloy nanoparticles were synthesized with compositions ranging from x = 0 to 0.5. The particle compositions were analyzed independently using both powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), which represent two of the most common nanoparticle composition analysis techniques. The plasmon resonance frequencies, determined experimentally for each sample using UV−vis spectroscopy, red shift with increasing copper content as expected. These experimentally determined plasmon resonance frequencies were then compared to the values predicted theoretically based upon the XRD and EDS composition measurements. Although EDS and XRD are both found to be acceptable methods for experimentally determining the composition, careful data analysis suggests that XRD composition measurements are more accurate for smaller values of x, whereas EDS measurements are more accurate for larger values of x. In addition, some discrepancies between the experimentally determined plasmon resonance frequencies and those predicted by theory suggest inaccuracies in using a simple linear mixing rule to determine the dielectric constant of the Au−Cu alloys.