la8b00992_si_002.avi (2.32 MB)
Download fileDirect Visualization of Planar Assembly of Plasmonic Nanoparticles Adjacent to Electrodes in Oscillatory Electric Fields
media
posted on 2018-05-04, 00:00 authored by Adam Ferrick, Mei Wang, Taylor J. WoehlElectric
field-directed assembly of colloidal nanoparticles (NPs)
has been widely adopted for fabricating functional thin films and
nanostructured surfaces. While first-order electrokinetic effects
on NPs are well-understood in terms of classical models, effects of
second-order electrokinetics that involve induced surface charge are
still poorly understood. Induced charge electroosmotic phenomena,
such as electrohydrodynamic (EHD) flow, have long been implicated
in electric field-directed NP assembly with little experimental basis.
Here, we use in situ dark-field optical microscopy and plasmonic NPs
to directly observe the dynamics of planar assembly of colloidal NPs
adjacent to a planar electrode in low-frequency (<1 kHz) oscillatory
electric fields. We exploit the change in plasmonic NP color resulting
from interparticle plasmonic coupling to visualize the assembly dynamics
and assembly structure of silver NPs. Planar assembly of NPs is unexpected
because of strong electrostatic repulsion between NPs and indicates
that there are strong attractive interparticle forces oriented perpendicular
to the electric field direction. A parametric investigation of the
voltage- and frequency-dependent phase behavior reveals that planar
NP assembly occurs over a narrow frequency range below which irreversible
ballistic deposition occurs. Two key experimental observations are
consistent with EHD flow-induced NP assembly: (1) NPs remain mobile
during assembly and (2) electron microscopy observations reveal randomly
close-packed planar assemblies, consistent with strong interparticle
attraction. We interpret planar assembly in terms of EHD fluid flow
and develop a scaling model that qualitatively agrees with the measured
phase regions. Our results are the first direct in situ observations
of EHD flow-induced NP assembly and shed light on long-standing unresolved
questions concerning the formation of NP superlattices during electric
field-induced NP deposition.
History
Usage metrics
Categories
Keywords
NP superlatticesInduced charge electroosmotic phenomenaphase regionsinterparticle forcesEHD fluid flowOscillatory Electric Fields Electric field-directed assemblyinterparticle plasmonicPlanar assemblysurface chargeEHD flow-induced NP assemblyplasmonic NPselectrokinetic effectsPlanar Assemblyfield-directed NP assemblyfrequency-dependent phase behaviorinterparticle attractionDirect VisualizationPlasmonic Nanoparticles Adjacentplasmonic NP colorassembly structurenanostructured surfacesfrequency rangefield-induced NP depositionassembly dynamicsfield directionsilver NPsNP assembly