We proposed a bottom-up approach for the fabrication of magnetic–plasmonic nanostructures that exhibit tunable plasmon enhanced hots spots and Fano resonance behavior. The nanostructures are formed from the self-assembly of magnetic–plasmonic core–shell nanoparticles. The magnetic core enables magnetophoretic control of particles during assembly, while the plasmonic shell provides interesting and useful optical behavior. We demonstrate proof-of-concept using a combination of Monte Carlo analysis to predict self-assembly and full-wave computational analysis to study the optical properties of the assembled structures. Our analysis demonstrates that viable structures can be assembled using a magnetic template-assisted self-assembly protocol and that flexible tunability of the optical response can be achieved due to the strong sensitivity of nanogap hot spots and Fano resonance features on distinct geometric parameters and the surrounding medium. We demonstrate the self-assembly and Fano resonance response of a heptamer nanostructure formed from Fe3O4@Au nanoparticles and discuss its performance for biosensing. The ability to fabricate such nanostructures using bottom-up methods holds potential for numerous novel applications. Moreover, the photonic modeling approach demonstrated here broadly applies to arbitrary particle geometries, material properties, and assemblies and can be used for the rational design of such applications.

We introduce an innovative additive manufacturing method that involves the drop-on-demand (DOD) printing of molten metal droplets to build three-dimensional (3D) metal structures of arbitrary shape. This technique is based on magnetohydrodynamic (MHD) droplet generation. Specifically, a transient magnetic field, generated by an electrically-pulsed external coil, induces a circulating current in molten aluminum that back couples to the applied field and creates a Lorentz force density (effective pressure) inside the printhead droplet ejection chamber. This effective pressure causes the ejection of a liquid metal droplet through a nozzle. Arbitrary 3D metal structures are printed in a layer-by-layer fashion. We present a commercial MHD-based printing system under development by Vader Systems (www.vadersystems.com) and introduce two computational models that predict system performance. We discuss the underlying physics of droplet generation and the thermo-fluidic aspects of droplet deposition, coalescence and solidification. We demonstrate good agreement between our computational models and measured data.

We study the optical response of monodisperse colloids of core-shell plasmonic nanoparticles and introduce a computational approach to optimize absorption for photothermal applications that require dilute colloids of non-interacting particles with a prescribed volume fraction. Since the volume fraction is held constant, the particle concentration is size-dependent. Optimization is achieved by comparing the absorption spectra of colloids as a function of particle size and structure. We demonstrate the approach via application to colloids of core-shell SiO2@Au and Fe3O4@Au nanoparticles with particle sizes that range from 5–100 nm and with the incident wavelength varying from 600–1200 nm. The absorption spectra are predicted using Mie theory and the analysis shows that there is a unique mix of parameters (core radius, shell thickness, wavelength) that maximize absorption, independent of the value of volume fraction. We show that lossy Fe3O4 cores produce a much broader absorption peak with much less sensitivity to variations in particle structure and wavelength than lossless SiO2 cores. This approach can be readily adapted to colloids of nanoparticles with arbitrary materials, shapes and structure using appropriate numerical methods to compute the absorption spectra. As such, it is useful for the rational design of colloids and process variables for a broad range of photothermal applications.

We demonstrate a computational study used to evaluate drop-on-demand printability of liquid metals via a contactless magnetohydrodynamic (MHD) pumping method. We show that the ejection regimes of pure liquid metal droplets can be categorized using two dimensionless quantities: We and a new dimensionless quantity S=Ha2Ca. By plotting We vs S, a linear relationship emerges which relates the velocity through the ejection orifice to the applied magnetic flux density. Additionally, satellite-free droplet generation is shown to be bounded by the ranges 1000≲S≲2000 and 10≲We≲20. These ranges, coupled with the linear We vs S relationship, allow one to predict the critical magnetic flux necessary to eject a satellite-free liquid metal droplet for any liquid metal with a very low viscosity to surface tension ratio (Oh<0.005). We discuss the physics underlying the MHD ejection process and relate the pump action to the dimensionless quantities. We use an MHD finite element model to parametrically sweep through applied magnetic fields and explore two-phase ejection of Al, Cu, Fe, Li, Sn, Ti, Zn, and Zr droplets from a 200 μm orifice. The model is validated using experimental high speed video ejection of Zn and Al, and the reported relationship between We and S can be used to connect the input flux density to the resulting ejection regime.