Nanoantennas report dissipative assembly in oscillatory electric fields

Understanding driving forces for dissipative, i.e., out of equilibrium, assembly of nanoparticles from colloidal solution at liquid-solid interfaces provides the ability to design external cues for reconfigurable device response. Here electrohydrodynamic flow (EHD) at an electrode-liquid interface is investigated as a dissipative driving force for tuning optical response. EHD results from an oscillatory electric field in a liquid cell between two electrodes and drives assembly of gold nanoparticles (NP) into two-dimensional clusters on electrode surfaces. Clusters are chemically crosslinked during assembly to freeze assemblies for electron microscopy characterization in order to understand how to 'nucleate' cluster formation. Electron microscopy images show deposition with a potential having an amplitude of 5 V and frequency of 100 Hz produces surfaces with isolated NP, which can seed EHD flow. A second deposition step at 5 V and 500 Hz produces a high density of quadramers on surfaces. When exciting near the local surface plasmon resonance of the Au NP clusters formed during assembly, Au NPs serve as in situ nanoantenna reporters of assembly and disassembly. Surface enhanced Raman scattering (SERS) measurements of Au NP capped with 4-mercaptobenzoic acid show order of magnitude signal enhancements occur during cluster formation in the presence of an oscillatory field, which occurs on a time scale of seconds. Confocal fluorescence spectroscopy is used to monitor the dissipative assembly of Au NP over multiple cycles. Results provide insight on how electrical stimuli and seeding local perturbations affects formation of NP clusters and resultant optical response provides insight on how to tune response of optically active surfaces.