Abstract
This project investigates the spectral responses of Au:Ag alloy nanoparticles to understand how modifications influence their spectra. To calculate it, we used the JavaScript Mie Scattering Calculator, a dielectric function, and ANSYS Lumerical Finite Difference Time Domain (FDTD) electromagnetic simulations under the presence of a substrate and silver oxide shells. Lastly, we measured the nanoparticles in the lab. We found that phenomena like redshift and FWHM increase as radius increases and nanoparticles oxidize, and each nanoparticle shows specific trends depending on their composition.
Introduction
Gold (Au) and silver (Ag) nanoparticles are particularly efficient in their interaction with light, producing highly localized electric fields that are used to enhance fluorescence, making them a great point of focus for Localized Surface Plasmon Resonance (LSPR) studies. This project investigates the spectral responses of Au:Ag alloy metal nanoparticles to understand how distinct structural and environmental modifications influence their extinction, absorption, and scattering profiles.
Experimental Details
First, we used the JavaScript Mie Scattering Calculator to calculate the spectra for gold and silver nanoparticles. Then, we generated a dielectric function based on a model for Au:Ag alloys to compare absorption, scattering, and extinction with different gold percentages and radii. To more accurately model the experimental system and quantify the influence of different environmental changes, we ran electromagnetic simulations using the ANSYS Lumerical Finite Difference Time Domain (FDTD) method and tested additional effects: the presence of a substrate with the refractive index of glass and silver oxide shells. Lastly, we measured the scattering spectrum of nanoparticles made up of 25% gold and 75% silver to visualize their experimental absorbance and scattering.
Results
Mie Scattering Simulations
For exclusively Ag nanoparticles, as radius increases (30 nm to 50 nm), radiative damping, phase retardation, and redshift phenomena are exhibited, and the LSPR peak shifts to longer wavelengths. The scattering resonance for exclusively Au nanoparticles is strongly damped by interband transitions that increase toward the short wavelength side of the resonance.
For nanoparticles with a set composition and increasing radius, absorption and scattering increases. However, when size is constant and composition is modified (25% to 75% gold), absorption decreases and the scattering resonance redshifts and broadens as interband transitions grow in at the short wavelength side of the spectrum.
Finite Difference Time Domain
Because Mie theory is an analytical solution to Maxwell’s equations for an ideal sphere in homogeneous medium, we need an alternative method to predict the spectra of nanoparticles on surfaces and with more complex geometries. We used the FDTD method, which iteratively solves Maxwell’s equations for an arbitrary geometry using discretized voxels. FDTD simulations showed trends identical to Mie theory methods. However, when we measure our nanoparticles on top of glass, we notice a decrease in absorption and scattering cross sections. This can happen because there is a dipole-image dipole interference.
Substrate and Shell
Simulations with the substrate and a silver oxide shell showed an increase in FWHM and redshift as the shells grew larger.
Figure 1. FDTD simulated spectra of a 30-nm, 25% gold alloy spherical nanoparticles with 1, 2, and 3 nm thick silver oxide shells.
In the lab, the scattering levels of nanoparticles with a 25% concentration of gold were more redshifted than expected, we attributed this to the silver oxide shells that form around majority-silver nanoparticles as time passes, and an increase in relative gold concentration as the silver oxidizes.
Figure 2. Experimental darkfield scattering spectrum of a nominally 50-nm, 25% gold nanosphere on a glass substrate.
Conclusion
The scattering and absorption spectra of Au:Ag alloy metal nanoparticles is dependent on their Au:Ag molar ratio, environment, and size. Our results demonstrate phenomena like redshift and full width at half maximum increase as radius increases and nanoparticles oxidize, but composition determines spectral trends overall.
Acknowledgements
Words are not enough to express how grateful I am to my mentor, Dr. Lee. His support has been paramount in every step of the way. Since day one, he provided me with time, resources, and advice that helped me feel ready to succeed not only in this research project, but also in my journey as a first-generation college student. His belief in my abilities helped me feel confident handling unexpected results and other setbacks, and I will forever appreciate his help in solidifying my identity in STEM. I’d also like to extend gratitude towards Dr. Prabhakar and the American Chemical Society for giving me the opportunity to explore chemistry in ways that I would have never thought possible.
References
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