Direct Visualization of Planar Assembly of Plasmonic Nanoparticles

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Interface-Rich Materials and Assemblies

Direct Visualization of Planar Assembly of Plasmonic Nanoparticles Adjacent to Electrodes in Oscillatory Electric Fields Adam Ferrick, Mei Wang, and Taylor J. Woehl Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00992 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Direct Visualization of Planar Assembly of Plasmonic Nanoparticles Adjacent to Electrodes in Oscillatory Electric Fields

Adam Ferrick, Mei Wang, and Taylor J. Woehl Department of Chemical and Biomolecular Engineering, University of Maryland, College Park

Abstract Electric 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 wellunderstood 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 ) oscillatory electric fields. We exploit the change in plasmonic NP color due to interparticle

plasmonic coupling to visualize the assembly dynamics and assembly structure of silver NPs. Planar assembly of NPs is unexpected due to strong electrostatic repulsion between NPs and indicates 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 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 longstanding unresolved questions concerning the formation of NP superlattices during electric field induced NP deposition.

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Keywords: nanoparticle assembly, directed assembly, plasmonics, electrokinetics, plasmonic coupling Introduction Arranging colloidal nanoparticles (NPs) into hierarchical structures via bottom up assembly remains a grand challenge of nanoscience research.1 Bottom-up assembly is a powerful method for functional nanomaterials design and can be achieved either through near-equilibrium self-assembly or far-

from-equilibrium directed assembly.2 Equilibrium self-assembly utilizes -scale equilibrium interparticle interactions stemming from various interparticle interactions (e.g. electrostatic, biomolecular, hydrophobic/hydrophilic interactions) to program the assembly of colloidal nanomaterials.3 While powerful in its ability to selectively form complex structures, successful self-assembly demands striking a delicate balance between interparticle interaction strength and assembly kinetics,4 which requires empirically devising complex time-dependent assembly protocols. For example, DNA hybridization induced NP assembly requires complex, days-long annealing procedures.5 Far-from-equilibrium assembly, or directed assembly, utilizes external forces and fields to drive NP assembly.2 Commonly used external driving forces include electric,6-15 magnetic,16-19 and capillary forces,20 which impose >> 1 scale forces that cause irreversible assembly. Electrophoresis (EP) is the classical first order (( )) electrokinetic force used to deposit NPs on a surface EP deposition

(EPD).21 Second order (( )) electrokinetic forces rely on induced surface charge and dipolar effects

and include dielectrophoresis (DEP),15,

22-23

induced dipole-dipole forces,24 and induced charge

electrokinetic flows,25 each of which can assemble colloids with enhanced control over particle placement

and structural ordering compared to ( ) forces.2, 8, 12, 15, 18, 24, 26 For example, micron scale colloids near a

charged electrode are well-known to assemble into planar colloidal crystals via electrohydrodynamic (EHD)27-29 and induced charge electroosmotic (ICEO)30-35 flows under the influence of AC and DC electric fields.27, 30, 33-43 Several recent literature examples demonstrate exquisite control over micron scale

colloidal assembly using ( ) forces. Using EHD fluid flow in AC electric fields, Dutcher et al.

demonstrated a non-intuitive, reversible, 2nd-order colloidal crystal phase transition in applied frequency from a hexagonally close packed to a randomly close packed state.42 Ma et al. revealed rich phase behavior of colloidal dimers in AC electric fields stemming from a complex balance of asymmetric EHD and dipole-dipole forces.44 Tang et al. used feedback control and AC dipolar interactions to assemble nearly perfect colloid crystals in quadripolar electrode cells.45

While these ( ) fluid flows induce assembly of micron sized colloids near electrode surfaces

with rich and complex phase behavior, their role in electric field directed assembly of NPs is still unclear. EHD and ICEO flows are ubiquitous across many colloidal systems, including both dielectric and metallic colloids and in aqueous and non-aqueous solvents, as long as there is free charge in the solvent. 2 ACS Paragon Plus Environment

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Interestingly, EPD of colloidal NPs onto planar electrodes often results in ordered and close-packed colloidal crystals, which is inconsistent with simple ballistic deposition of NPs that would produce disordered 3D films. Seminal experiments by Giersig and Mulvaney showing hexagonal close packed (HCP) planar colloidal crystals and monolayers of NPs formed by DC EPD suggested that in addition to EP forces oriented normal to the electrode surface, strong interparticle attraction occurs near the electrode-liquid interface.46-47 Similarly, silica NPs,48 iron oxide/silica core shell NPs,49 and silver NPs50 have all been recently observed to assemble near electrodes into close packed planar and 3D structures under DC electric fields. The question of what causes ordering and close packing of colloidal NPs near electrified interfaces remains unresolved for several decades. While EHD flow has been repeatedly implicated in electric field directed NP assembly,40, 51-55 there have been no direct in situ observations of NP assembly dynamics near charged electrodes. Direct observations are required to unequivocally reveal what causes ordering in NP assemblies formed in electric fields. More broadly, direct in situ observations

of NP assembly dynamics are needed to establish the role of ( ) electrokinetic effects during electric field directed NP assembly. This fundamental question has major implications for several exciting new directed assembly research areas—anisotropic,44, 56-62 Janus,63 and patchy NPs,3, 64-65 as well as colloidal molecules,20,

66

active soft matter,67-68 and soft microrobots,69 all undoubtedly experience ( )

electrokinetic effects in response to electric fields. In this article, we employ dark field optical microscopy to visualize in situ the behavior and

assembly dynamics of colloidal silver NPs in low frequency (< 1500 ) AC electric fields. We reveal NP assembly dynamics near an electrode surface by observing NP color changes resulting from interparticle plasmonic coupling and the formation of optically visible micron sized planar assemblies. A parametric investigation of frequency and voltage dependent NP behavior revealed three distinct behaviors: ballistic deposition, planar assembly, and defect induced assembly. We develop a scaling model that qualitatively describes the frequency and voltage dependent phase behavior in terms of AC EP

and EHD fluid flow. The results shed light on the roles of both ( ) and ( ) electrokinetic effects in the electric field directed assembly of NPs and suggest conditions under which EHD flow will impact NP assembly behavior in AC electric fields.

Experimental Section Materials and sample preparation Silver NPs and silica coated silver NPs were purchased from Nanocompsix, Inc. The NPs suspended in ethanol were washed three times in desired electrolyte by repeated centrifuging at 7000 g, decanting, and adding fresh electrolyte. Most experiments were performed with NPs that had a 70

diameter silver core, 20 silica shell, and polydispersity of ~5%. The silica shells had native 3 ACS Paragon Plus Environment

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negatively charged siloxane functionality. Bare silver NPs with negatively charged (citrate acid, lipoic acid, carboxylated polyethylene glycol thiol (COOH-PEG-SH, = 5000)) and positively charged

(branched polyethylenimine (BPEI)) stabilizing ligands and diameters ranging from 30 – 60 nm were tested as well. The silver NPs had a localized surface plasmon resonance (LSPR) wavelength of ≈

480 . Typically, the aqueous electrolyte was 1 potassium chloride (KCl) and the particle concentration was 0.1 !/#; electrolytes were prepared in type II DI water (ρ = 18.2 Ω/&). The

silica coated silver NPs had a zeta potential of '( = −33 + in 1 KCl (measured with a Malvern Zetasizer Nano). The microfluidic electrode cell is similar in design to those used in previous studies.37,

42, 70

Briefly, two transparent tin doped indium oxide (ITO) coated glass slides (Delta Technologies) served as the electrodes (Figure 1). ITO electrodes were thoroughly cleaned with soap and water, sequentially sonicated in acetone, methanol, ethanol, and DI water for 10 minutes each, and finally plasma cleaned with an ozone plasma (MTI Corp.) to remove solvent residues. We used 90 - thick double-sided tape (3M double coated tape, 9816M) with a ~1 & well cut out of the center to form the microfluidic

chamber. To prepare a sample cell, a ~5 -# drop of NPs was placed in the tape chamber and the two electrodes were sandwiched together (Figure 1a). Electrical connections to the ITO electrodes we made with copper tape. We applied sinusoidal AC potentials of amplitude ∆/ = 1 + – 5 + and frequency 1 = 50 – 1500 to the cell using an arbitrary waveform generator (Keysight 33510B); signals were measured with an oscilloscope (Keysight DSOX2002A).

Dark field optical microscopy Assembly dynamics and phase behavior were visualized using transmission dark field optical

microscopy. The microscope (Zeiss Axioscope) is equipped with an oil immersion condenser (23 = 1.4), color CCD camera (Zeiss Axiocam 503), and dry objective lenses with magnifications from 10x - 40x.

With this combination of optics, single plasmonic NPs with diameters > 30 can be visualized due to their strong plasmonic scattering and absorption in near-UV wavelengths. The color of the plasmonic NP is directly correlated to its LSPR: single silver NPs appeared blue in dark field images due to their strong scattering and absorption in the near UV, while assemblies of NPs appeared green or orange due to plasmonic coupling. Through-focal series of sample cells after assembly were acquired by taking images

at 50 steps between the focal heights of the top and bottom electrodes using the motorized objective

lens and automated software.

Electron microscopy


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NPs were imaged with scanning electron microscopy (SEM) following assembly experiments. To prepare an SEM sample, the electric field during the experiment was left on for a several minutes, after which all NPs and their assemblies attached to the electrodes. The electrode cell was dismantled and each electrode was thoroughly rinsed with DI water before the liquid sample could dry, and then dried with filtered air. This procedure removed any unattached NPs and ensured there was no NP assembly due to drying-induced capillary forces. The electrode surfaces were then plasma cleaned and imaged in an SEM (Hitachi SU-70).

RESULTS Metallic NPs (Au, Ag, Pt, Al, Cu) are unique in that they exhibit localized surface plasmon resonance (LSPR), which is due to coupling of incoming photons with free electrons in the NP. Two effects arise from the LSPR: local confinement of photons into strong electric fields on the NP surface and strong optical absorption/scattering at the plasmon resonant frequency.71 Metallic NPs exhibit visible wavelength plasmon resonances, meaning that they strongly absorb and scatter visible light and can be visualized as single particles with dark field optical microscopy. When plasmonic NPs come in close vicinity to each other (< 2.5 particle diameters), interparticle plasmonic coupling occurs and causes a red shift in the plasmon resonant wavelength and in the scattered light color observed by dark field microscopy.72-73 Red shifts in the resonant plasmon wavelength of several hundred nanometers are possible due to coupling, meaning visible color changes from blue to yellow for silver NPs can be observed by eye or with dark field microscopy.74 Coupling-induced color change observed by dark field optical microscopy is therefore a sensitive visual indicator of whether NPs exist in assemblies where they are in close vicinity to each other (< 2.5 particle diameter), or as isolated NPs. Indeed, plasmonic NPs and interparticle plasmonic coupling are commonly exploited to construct biosensors,75 molecular rulers,76 and reconfigurable optical materials.19, 26 Due to these unique properties, we hypothesized in situ dark field optical microscopy and plasmonic NPs would be enable direct visualization of the dynamics of electric field induced NP assembly (cf. Figure 1a). We chose silver NPs based on their plasmonic properties that include low dielectric loss, high Q-factor, and near-UV resonant frequency that will redshift into the visible or near-IR due to interparticle plasmonic coupling when they assemble in an electric field.71 We used silica-coated silver NPs to enhance colloidal stability, enable direct comparison of our experiments to previous studies of AC field induced assembly of micron sized dielectric colloids, and to protect the silver NPs from oxidation, which degrades their plasmonic properties.77 In a typical experiment, a colloidal suspension of NPs was subjected to an oscillatory sinusoidal applied potential and the dynamic NP behavior was imaged in real time with dark field video microscopy (Figure 1). In this initial experimental investigation, NP properties were held constant and the applied 5 ACS Paragon Plus Environment

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potential and frequency were varied to investigate their effect on assembly behavior. Several qualitative phase behaviors were observed, including ballistic NP deposition, assembly at defects, and planar assembly. We will discuss the latter first due to its implications in providing new understanding of closepacked and planar NP assembly in electric fields. Interestingly, we found that for a small range of frequencies and applied potentials, colloidal NPs assembled into discrete 2D clusters on both electrodes, with sizes ranging from hundreds of nanometers to microns. For the rest of this article, we term this behavior planar assembly. Figure 2 shows a representative time-series of images showing the dynamics of this process, with the focal plane at one of the electrode surfaces. Prior to applying the electric field, dark field microscopy images revealed a ‘sea’ of silver NPs that scattered blue light; single NPs were difficult to distinguish due to out of focus objects, but the NPs were clearly mobile and undergoing Brownian motion. A significant difference between our experiments with NPs and prior experiments with micron sized colloids is that NPs do not sediment to the bottom electrode by gravity but instead remain well dispersed throughout the liquid phase. Figure 2a shows time lapsed dark field optical images, while Figure 2b shows the mean hue of the NPs as a function of time during assembly, which quantitatively characterizes interparticle coupling-induced color changes during NP assembly. When a 1 = 200 , Δ/ = 1.5 + potential was applied to the sample cell at 5 = 10 6, the NPs were observed to change color

from blue to turquoise within a few seconds (5 = 15 6). This color change corresponded to a decrease of

the average image hue from ≈ 200° to ≈ 160°. The red shift is indicative of interparticle plasmonic

coupling, likely due to the formation of small close packed clusters of NPs. While several small individual NP clusters were visible at 5 = 15 6, the size of most NPs appeared unchanged, indicating

most clusters remained smaller than the diffraction-limited resolution of the microscope, ~300 . After

~10 seconds with the electric field on (5 = 20 6), larger visible NP clusters appeared with colors ranging

from green to yellow; this was accompanied by a further decrease of the average image hue to ≈ 110°.

Finally, micron sized aggregates formed after 15 seconds with the AC field on (5 = 25 6); here non-

spherical aggregate shapes were resolved from the dark field images, indicating their sizes had surpassed the diffraction limited resolution. Further application of the electric field caused the assemblies and clusters to irreversibly adhere to the electrode surface with no further growth observed. Higher magnification microscopic observations revealed more interesting features of AC electric field induced planar assembly (Figure 3a). In general, the rate of the assembly process varied from sample to sample and with applied voltage and frequency, but typically NPs assembled and adhered to the electrode surface within 5 – 40 69&:;6. As discussed above, formation of visible clusters was preceded by a red color shift from blue to turquoise, which was clearly observed in the first time point of the time series in Figure 3a. As time progressed, mobile NP clusters collided to form larger assemblies, which was accompanied by further red shifts in their color. After several seconds, large micron sized clusters 6 ACS Paragon Plus Environment

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formed and were immobilized on the electrode surface. Importantly, NPs and NP clusters were mobile during the initial stages of the assembly process, which indicates they were colloidally stable and did not adhere to the electrode surface during assembly. Figure 3b shows a low magnification time lapsed series of images clearly showing that NP clusters were mobile during assembly. The first three images are a time series of planar assembly at the late stages for a Δ/ = 1.5 +, 1 = 100 applied potential. These images again show a red color shift, overall increase in NP cluster size, and decrease in number of objects in the field of view over time. Supplementary movie 1 clearly shows that the growth of NP clusters at this stage was due to cluster-cluster assembly enabled by the mobility of NP clusters. The two boxed difference images on the right were obtained by subtracting the 60 s and 40 s images and the 80 s and 60 s images. Whole intact NP clusters were mobile between the time points, while NP clusters that appeared with holes at their centers were immobile, but still grew over time. The mobility of NP clusters during assembly indicates the phenomenon we observe is not simply EPD of NPs onto the electrode surface, but due to interparticle attraction between colloidally stable NPs adjacent to the electrode surface. Planar attraction between NPs indicates there is a strong attractive force between NPs near the electrode that overcomes short-ranged repulsive interparticle interactions and strong diffusive forces.

Assembly occurred exclusively at the electrolyte-electrode interfaces. Figure 4a shows the = −

plane of a through-focal series of dark field optical images of the sample cell taken after planar assembly at 1 = 200 , Δ/ = 1.5 +. While this microscope does not have confocal optics, which causes the

intensity smearing in the z-direction due to out of focus light, NP clusters clearly existed only near each electrode surface. We also disassembled the sample cell, rinsed, and dried the electrodes, and imaged them in a scanning electron microscope (SEM). There was a mixture of single NPs and 2D NP clusters with sizes up to several microns deposited on both electrode surfaces (Figure 4b). Importantly, the NP clusters were planar, close-packed, and in some cases showed hexagonal close packing, which provides further evidence that they formed by strong interparticle attraction of NPs near the electrode surface. This also suggests that planar NP assemblies experience preferential 2D growth perpendicular to the direction of the electric field. While this is expected for micron sized colloids that are confined to a plane above the bottom electrode by a balance of gravitational forces and Coulombic repulsion,78 gravitational forces are negligible for NPs, yet they still assembled into planar clusters. As discussed in the theory section below, we believe that NPs migrate to the electrode by oscillatory EP. To determine the electric field conditions under which planar assembly occurred, we performed a parametric investigation of NP behavior under various applied voltages and frequencies. We tested frequencies between 50 – 1500 and AC potentials with amplitudes ranging from 1 – 5 +. In these frequency and voltage ranges, we observed three qualitative behaviors: ballistic deposition of NPs, planar assembly, and assembly of NPs at defects (Figure 5). In a typical experiment, we applied an electric 7 ACS Paragon Plus Environment

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potential at a given frequency and amplitude for ~10 − 20 69&:;6, after which qualitative features of the sample were assessed from dark field optical microscopy observations. Ballistic deposition was characterized by irreversible deposition of NPs onto both electrode surfaces, with minimal change in the observed color of the NPs (Figure 5a, Supplementary Movie 2). The absence of any red color shift indicates the NPs were not close packed on the surface and deposited at random, which was confirmed

with SEM imaging (Figure 5d). Ballistic deposition typically occurred at low frequencies (< 500 )

and across all voltages tested. At higher frequencies (500 – 1000 ), planar assembly occurred at voltages that were frequency dependent and was distinguished based on the formation of visible NP clusters with their color red shifted to appear green or yellow (Figure 5b,e). At higher frequencies

(> 500 ), NPs concentrated near ITO surface inhomogeneities, i.e. scratches created during cleaning, which are known to alter the local electrical current and electric field in their vicinity.79 We will refer to

this behavior as defect induced assembly, which was manifested as locally increased dark field image intensity along line defects with no plasmonic color change (Figure 5c, Supplementary Movie 3). Defect induced assembly was reversible, meaning the NPs dispersed after the electric field was removed. This contrasts with the other two behaviors, which were irreversible and led to permanent NP assembly and attachment to the electrode surface. For all frequencies tests, we did not observe any type of assembly of deposition at voltages of < 1 +.

We constructed a qualitative phase diagram from these parametric experiments depicting the applied voltage and frequency regions where each behavior occurred (Figure 6). We emphasize that this is a qualitative phase diagram and not a thermodynamic equilibrium phase diagram; the behaviors characterized here occur far from equilibrium and several of them are irreversible. Focusing on the planar assembly behavior first, it is interesting to note that it only occurred over a narrow frequency band with a width and position dependent on the applied potential. For low AC potentials of 1 – 3 +, planar assembly

occurred between 100 − 300 , while at higher potential this behavior extended out to 1000 . At

frequencies higher than these NPs only assembled at defects, while at lower frequencies NPs ballistically deposited on the electrode surface.

THEORY Scaling Model for NP Behavior Our experimental observations and phase diagram suggest potential mechanisms for the observed phase regions in Figure 6. We will focus only on developing models for the ballistic deposition and planar assembly phase regions because assembly of NPs and colloids at electrode defects is well-known to occur by defect induced EHD fluid flow27, 79-80 and DEP.23, 81 Experimental observations of ballistic deposition suggest it is due to EP forces; low frequency AC electric fields are known to deposit layers of 8 ACS Paragon Plus Environment

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NPs onto electrodes.21-22, 82-83 We interpret planar assembly in terms of EHD drag forces oriented parallel to the electrode surface, which mutually entrain neighboring NPs and cause them to assemble.13, 28, 38-40 Experimental evidence supporting an EHD flow mechanism includes the mobility of NPs and clusters during assembly (Figure 3), planar assembly structure as observed by SEM (Figure 4), and assembly at both electrode surfaces (Figure 4). We develop scaling arguments for each electrokinetic effect to model the phase regions in Figure 6. First, if the maximum EP force on the NPs exceeded the repulsive Coulombic NP-electrode interaction force, we predict NPs will ballistically deposit on the electrode (red solid region, Figure 6). Secondly, if the magnitude of the EHD drag force on a NP exceeded the characteristic thermal diffusive forces and the maximum EP force is less than the repulsive columbic NPelectrode force, we predict planar assembly will occur (blue solid region, Figure 6). The EHD drag force is compared to thermal forces via the dimensionless particle Peclet number (>9), which must be >9 > 1

for planar assembly to occur. While this is a simplified picture of the phenomena we observe, we find that our scaling models produce phase regions that are qualitatively consistent with our experimental observations. Under conditions near the phase boundaries, planar assembly and defect induced assembly, or planar assembly and ballistic deposition, will occur simultaneously or in concert.

Ballistic Deposition by AC EP In a low frequency AC electric field, motion of a NP via EP is oscillatory and appoximately in phase

with the electric field direction due to the small Reynolds number (?9 ≈ 0) and negligible inertial

effects.83-84 Assuming electroosmosis and diffuse layer polarization of the NPs are negligible,83 the EP

force @AB (5) on a NP in an AC electric field follows the classical Smoluchowski equation:85 where -AB =

@AB (5) = -AB (, 5),

(1)

CCD '( FE is the EP mobility, C is the relative permittivity of water, CD is the vacuum

permittivity, '( is the particle zeta potential, E is the dynamic viscosity of water, (, 5) is the time and space dependent electric field strength between the two electrodes, G = 2H1 is the angular frequency, and 1 is the applied frequency. This model assumes the thin double layer limit, i.e. I ≫ 1, which for our

experiments was I = 5.5. While this is not technically the thin double layer limit, we only seek to determine the order of magnitude EP forces in the article, so this assumption is relaxed. To determine for what voltages and frequencies we expect ballistic NP deposition occurs, we determine the maximum EP force experienced by a NP (@AB,KLM ) and compare this force to the repulsive NP-electrode Coulombic

force (@N ). To determine these forces, we model the equilibrium NP-electrode interactions in the absence of an electric field by summing Coulombic, Van der Waals, and gravitational forces between the


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electrode and NP. Using the Derjaguin approximation, the interaction potentials for Coulombic and Van der waals interactions are modeled as OP (Q) = 64CD C R

VY\ ST U VY W 5ℎ R[S ZUW 5ℎ R[S UW 9=](−IQ) V T T

O^_` (Q) = −3

L b c1 + Leb ab

b

b

+ L f RLebWg.

(2) (3)

Here Q is the surface-to-surface particle-electrode separation, is the particle radius, h is

Boltzmann’s constant, is the temperature, 9 is the elementary charge, 'V is the zeta potential of the electrode, I ij is the Debye length, and 3 is the Hamaker constant.86-87 The gravitational potential is [

Ok (Q) = Hl mn( − no p!Q, l

(4)

where n( and no are the particle and fluid density and ! is the acceleration due to gravity. Summing these interaction potentials and taking the negative spatial derivate yields the separation dependent interaction force. Table I shows the parameters we used for the scaling model. Note that there are no fitted parameters for the ballistic deposition model, but several parameters are estimated. Figure 7a shows that the maximum repulsive force is located approximately Q = 5 from the electrode. The maximum of the force curve is @N ≈ 30 ]2.

It is often assumed that colloids and NPs near charged electrodes only experience the “effective” electric field strength, which is the applied potential divided by the electrode separation. However, the effective bulk field strength is invariant with frequency and in this case would not lead to the observed frequency-dependent phase regions shown in Figure 6. Secondly, the predicted maximum EP force on a NP subjected to the effective electric field strength is orders of magnitude too small to overcome the repulsive electrode-NP electrostatic force. Together these facts suggest that the strong electric field in the polarization layer near the electrode, whose magnitude is inversely proportional to frequency,88 is responsible for the frequency dependent ballistic deposition phase behavior. Most likely, NPs transiently experience this strong electric field near the electrode surface due to their oscillatory motion in the AC field, which can have amplitudes on the order of hundreds of nanometers in low frequency AC electric

fields.83 To determine @AB,KLM we must determine ( = QAq , 5 = 0), which represents the electric field strength experienced by a NP at separation of QAq from the electrode. The inset in Figure 7a is a plot of

the NP-electrode interaction potential and shows the position of the secondary minimum at ≈ 70 . Due to the shallow shape of the potential well, NPs will exist at various separations from the electrode following Boltzmann statistics. However, for this scaling estimate we chose a single value of QAq =

70 as the average equilibrium NP-electrode separation. While the EP force oscillates in time, we expect the maximum EP force will correctly predict the conditions ballistic attachment will occur, so we

take the maximum electric field strength at 5 = 0 69&:;6.


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The electric field strength in the polarization layer depends on frequency, applied potential, ion diffusion coefficient, and distance from the electrode surfaces. We model the electric field between two

parallel co-planar electrodes at positions of = 0 and = 2ℎ separated by an aqueous 1:1 electrolyte using an analytical perturbation solution of the coupled Nernst-Planck and Poisson-Boltzmann equations solved by Hollingsworth and Saville:88 (, 5) =

∆r tu1 s

vw yz{|mv(si})p − {~|(vs)

+ Iℎ RxsW

vw

RxsW −

v w yz{|mv(si})p =](−G5). x {~|(vs)

(5)

Here ∆/ is the applied potential, 2ℎ is the electrode separation, is the mean ionic diffusion

coefficient, = −1 is the imaginary number, and 5 is time. Figure 7b shows (, 5 = 0) near the

electrode for frequencies of 50, 500, and 1500 . The validity of this perturbation solution is technically limited to applied potentials smaller than the scaled thermal potential (~25 mV). While we apply potentials much larger than this in our experiments, empirical evidence suggests this electric field

model is valid for the scaling law we develop. Several previous articles focusing on EHD flow for micron scale colloids have successfully used this perturbation solution to develop scaling laws,28, 42, 44, 70, 89 as well as one report on NP assembly by EHD flow.51 At each frequency, the electric field magnitude is locally larger several Debye lengths from the electrode surface due to accumulation of counterions in the polarization layer. Outside the polarization layer, the electric field strength decreases to the effective electric field strength,

∆r . s

Concentration polarization at a given distance from the electrode decreases as a

function of increasing frequency due to the finite mobility of the ions in solution that screen the electrode

potential. Note that for the equilibrium separation in the absence of an applied electric field, QAq =

70 , the particles reside either in or at the edge of the polarization layer when the AC electric field is applied, suggesting that some NPs will experience an enhanced electric field instead of the effective electric field strength (Figure 7b and inset). We note that the enhanced electric fields near the electrode

become unrealistically large very close to the electrode (Q < 50 ) and for very low frequencies of

1 < 50 , where the predicted field strength exceeds that due to the zeta potential drop between the

surface and bulk electrolyte, which is ~4 +/-. For this reason, we did not perform or model any experiments for 1 < 50 . We determined the EP force, @AB,KLM , using Eqn 1 with the electric field

expression in Eqn 5 evaluated at = QAq and 5 = 0 6. With predictions of @AB,KLM and @P in hand, we

model the phase behavior of the NPs shown in Figure 6 using the criterion that for frequencies and voltages where

@AB,KLM @ > 1 ballistic deposition will occur. For this criterion we obtained the modeled P

solid red phase region in Figure 6, which both follows the qualitative phase behavior, i.e. ballistic deposition occurs at increasingly larger frequencies for higher voltages and captures approximately 80% 11 ACS Paragon Plus Environment

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of the experimental phase data points. At low frequencies and large applied potentials, the NPs experience strong EP forces that create large amplitude oscillatory EP motion and effect ballistic deposition of NPs onto the electrode. As frequency is increased and voltage decreased, the NPs experienced weaker electric fields and their oscillation amplitude decreased, limiting ballistic deposition onto the electrode.

Planar Assembly due to EHD Flow Assembly of colloids near a charged planar electrode is thought to be driven by ICEO fluid flows,41 such as EHD fluid flow29 or a combination of EOF and ICEO flows.30, 32, 34 Here we employ the EHD flow framework originally developed by Ristenpart et al. to interpret our experimental phase diagram.28-29 In brief, a colloidal particle near the electrode locally perturbs the otherwise uniform free charge distribution and electric field. Specifically, the component of the particle’s electric dipole field oriented parallel to the electrode surface, ∥ , induces an electrical body force on the free surface charge

near the electrode, ∥ , which drives electroosmotic slip flow along the electrode. The flow occurs in the

polarization layer of the electrode, is typically directed toward the particle under most experimental conditions, and extends several particle radii along the electrode from the particle surface.29 If sufficiently strong, EHD flow entrains neighboring colloids several particle radii away and can create planar colloid crystals13, 28 or propel asymmetric colloids.89 The magnitude of the EHD fluid flow scales as follows:29 Aw ~CCD

qA∥ l r ~ v R s W cD R1 v

+

v w W+ x s

D

s\ v w . g x m es p/ \

(6)

Here Aw is the EHD velocity, is the surface charge density, ∥ is the component of the

particle electric dipole field oriented parallel to the electrode surface, is the scaled distance from the

particle surface at which the EHD flow magnitude is evaluated, and ℎ( =

QVqF is the scaled height of the

particle above the electrode surface. The parameter cannot be determined experimentally, thus we use b

as a fitting parameter and find = 4 provides the best fit between our model and experiments. We

believe this is a reasonable estimate, as = 4 corresponds to evaluating the strength of the EHD flow two particle diameters from the NP surface. The flow magnitude has a quadradic dependence on effective electric field strength, R

r W , s

because both the surface charge and electric field scale with the effective

electric field strength. EHD flow only occurs when colloids are near the electrode, within several particle radii. Because NPs do not sediment by gravity, we believe that oscillatory EP drives NPs near the electrode where they induce EHD fluid flows. D = D + D" is the dimensionless complex dipole

coefficient of the particle, which embodies the strength and direction of the particle dipole (D ) and its

phase lag with respect to the applied oscillatory field (D" ).90 The dipole coefficient is a complex function 12 ACS Paragon Plus Environment

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of particle zeta potential, particle radius, frequency, Debye length, and electrolyte ionic strength and diffusivity. We employ a low-frequency asymptotic solution to the standard electrokinetic model to determine the dipole coefficient, detailed in Shilov et al.91 This model accounts for concentration polarization of the diffuse layer surrounding the NPs using an effective surface conductivity and Stern layer conductivity. While the surface conductivity is determined analytically, the Stern layer conductivity cannot be determined analytically and thus we use it as a fitting parameter as has been done previously.89

A Stern layer conductivity of = 1 Q provided accurate phase behavior predictions with our scaling

model (typical values range from = 0 – 10 Q).89 Both the real and imaginary components of the

dipole coefficient of a dielectric 55 radius NP are negative (Figure 8a,b), indicating the EHD fluid flow is directed towards the NP surface. In the context of the scaling model in Eqn 6, D Gi decreases as

a function of frequency (Figure 8a), while D Gij is relatively constant with frequency (Figure 8b).

Overall the (Gi ) term in Eqn 6, which contains the real component of the dipole coefficient, dominates the frequency dependence of the EHD flow magnitude (Figure 8c). This is in contrast to the dipole coefficients of micron sizes colloids for which EHD flow is typically associated, where the

(Gij ) term containing the imaginary dipole coefficient (the phase lag) dominates the frequency dependence.29, 37

We characterized the interaction of a single NP with the EHD fluid flow using the particle Peclet number, >9 =

aL , ST U

(7)

which compares the velocity of convective particle transport to that of diffusive particle transport.

>9 ≫ 1 indicates convection dominates, while >9 ≪ 1 means thermal diffusion dominates. To model the

phase diagram, we establish the criterion that if >9 > 1 and

@AB,KLM @ < 1, EHD fluid flow will cause P

planar assembly of NPs at the electrode surface. Figure 6 and Figure 8d show the results of the EHD scaling model, which both describes the qualitative shape of the phase region and captures most of the

experimental data points. Note that in the predicted ballistic deposition phase region >9 > 1; however, EP motion of NPs in this regime causes them to rapidly deposit on the electrode before they assemble via EHD flow.

DISCUSSION Qualitative tests of scaling model Our scaling model makes several predictions of the effects of several parameters on NP behavior in oscillatory electric fields, including ionic strength, electrolyte type, particle size, and NP zeta potential. 13 ACS Paragon Plus Environment

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We further tested our scaling model by varying these parameters over limited ranges and comparing the experimental behavior with scaling model predictions. Specifically, silver NPs were suspended in aqueous KCl with ionic strengths of 0.5 mM, 5 mM, and 10 mM as well as 1 mM NaOH and dimethyl sulfoxide (DMSO). As expected, NPs did not assemble or ballistically deposit in either 1 mM NaOH or DMSO. Colloids are well-known to exhibit repulsion in oscillatory electric fields instead of attraction when suspended in either strong acids or bases that cause the particles to have large zeta potential,37 except for applied potentials with 1 < 25 where EHD induced assembly has been observed.43 Our

scaling model revealed that for NPs in 1 mM NaOH with a measured zeta potential of '( = −45 +, the EHD flow magnitude and particle Peclet number decrease by a factor of two across all frequencies and voltages tested in the phase diagram due to a decrease in the particle dipole field strength.90-91 The calculated electrostatic repulsive force between the NP and electrode increased by a factor of two in 1 mM NaOH, indicating ballistic deposition should not occur either due to a stronger repulsive force

between the NPs and electrode. EHD fluid flow relies on induced charge, meaning it should not occur in the absence of free ions, such as in an aprotic polar solvent like DMSO. We observed no planar assembly in DMSO under all frequencies and voltages tested, which provides further evidence planar assembly results from EHD flow and induced charge on the electrodes. Finally, the sign of the NP zeta potential should not impact EHD fluid flow or ballistic deposition because EHD flow is due to induced surface charge. Indeed, we observed lateral assembly with positively charged amine functionalized silver NPs and negatively charged NPs (Figure 9a). The ionic strength of the electrolyte had almost no effect on NP zeta potential, which varied

between −30 + and −35 + for ionic strengths of 0.5 − 5 KCl. The NP zeta potential decreased in magnitude to −21 + in 10 KCl. Either defect induced assembly, or no observable behavior,

occurred over all frequency and voltages tested in Figure 6 for NPs in 0.5 KCl, while NPs

ballistically deposited onto the electrode surface under these conditions in 5 and 10 KCl. We observed no significant planar assembly at any of these ionic strengths. Our model indicates that the change in Debye length is likely responsible for the observed NP behavior. As ionic strength increases and Debye length decreases, the equilibrium distance of NPs from the electrode decreases and NPs are subjected to stronger electric fields in the polarization layer (cf. Figure 7b). NPs in 0.5 KCl did not

ballistically deposit due to the larger Debye length and larger predicted equilibrium NP-electrode separation of ~100 , which prevents NPs from experiencing strong electric fields inside the

polarization layer. According to eqn. 6, the EHD flow magnitude and particle Peclet number scale with the Debye parameter as Aw ~1 + I l , supporting the absence of planar assembly at lower ionic strengths.


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NP sizes as small as 10 nm in diameter have been observed to form close-packed and sometimes ordered superlattices in response to AC and DC electric fields.46, 51 Particle size impacts many aspects of the competing electrokinetic effects in this system, including diffusive forces and the magnitude of the EHD flow. We tested bare silver NPs with diameters of 30 and 60 in 1 mM KCl and found that each underwent planar assembly at applied potentials of 5 +, 200 and 4 +, 200 , respectively,

nearly double the voltage needed to assemble 110 NPs at these frequencies (Figure 9). The

assembled 30 NPs were green in color (Figure 9a), compared to the orange and yellow colored

assemblies of 110 NPs. This was because the smaller NPs experienced less interparticle plasmonic

coupling, leading to a smaller red shift in their LSPR.73 Our scaling model is qualitatively consistent with

the observation that stronger electric fields were needed to assemble smaller NPs with EHD flow, as the Peclet number scales as particle size squared (Eqn 7). Because the EHD flow magnitude increases as applied voltage squared, the applied voltage must be increased an amount proportional to the decrease in particle size to leave the particle Peclet number unchanged.

SUMMARY Using dark field optical microscopy, we have shown the first direct in situ observations of planar assembly of NPs into close packed assemblies adjacent to an electrode, which we interpreted in terms of EHD fluid flow. We present strong experimental and theoretical evidence for the existence of EHD fluid flow induced planar assembly of NP suspensions with particles sizes as small as 30 . Our experiments

reveal dynamics of the assembly process, which proceeds via assembly of small clusters of NPs, which then assemble via cluster-cluster attachments into micron-sized planar assemblies. Critically, NPs and clusters thereof remain mobile during assembly, indicating the observed process is not classical EPD. Parametric experiments demonstrate the effects of frequency and voltage on qualitative assembly behavior and reveal AC EP to be an important competing electrokinetic effect. Planar assembly only occurs in a narrow frequency band, the position and width of which depends on applied voltage, and directly competes with AC EPD of NPs onto the electrode surface, which explains why direct observations of EHD induced NP assembly have been elusive over the past decades. Our proposed mechanism suggests that the electric field in the polarization layer plays an essential role in controlling the phase behavior of the NPs in AC fields. Importantly, our scaling model provides predictions of when EHD fluid flow will induce formation of close-packed NP assemblies in oscillatory electric fields. This approach could also enable sensitive measurement of the interparticle separation within NP assemblies from their color as observed by dark field microscopy, for example by using a combination of microscopy, single particle UV-VIS spectroscopy, and simulations. More broadly, our results shed light on a decades-long unanswered question of the underlying mechanism for the formation of planar close 15 ACS Paragon Plus Environment

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packed NP assemblies in electric fields40, 46-47 and opens new research avenues for using induced charge electrokinetics to controllably assemble NPs on surfaces.

FIGURES

Figure 1. (a) Schematic drawing (not to scale) of sample cell and dark field optical microscopy for visualizing AC electric field induced NP assembly. Dashed lines represent the color of scattered light from single NPs and planar assemblies, indicating that single silver NPs appear blue in the dark field image, while planar NP assemblies display red shifted color. (b) Schematic drawing of the dominant forces, AC EP and EHD fluid flow, controlling NP behavior near an electrode in an AC electric field. The streamlines show the approximate flow pattern of EHD fluid flow.


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Figure 2. Dynamics of planar assembly of 110 nm silica coated silver NPs. a) Time-lapsed series of

dark field optical microscopy images showing the planar assembly of NPs in a Δ/ = 1.5 +, 1 = 200

applied AC potential. The electric field was applied at 5 = 10 6 (red dashed line in (b)). b) The average hue of the NPs in the images from a movie of the NP assembly in (a) as a function of time. Hue is a quantitative measurement of color, determined from the RGB values of the original color images. The

inset shows that ℎ9 = 0° corresponds to red, ℎ9 = 120° corresponds to green, and ℎ9 = 240° corresponds to blue. The color of the data points in (b) corresponds to the average color of the NPs in that frame, while the positions of the white arrows indicate the times points in (a).


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Figure 3. Dynamics of planar assembly. (a,b) Cropped time-lapsed series of dark field optical microscopy images showing planar assembly of silica coated silver NPs. NPs and assemblies remained mobile during assembly, indicating that they remain colloidally stable during planar assembly. The applied potential in each experiment was Δ/ = 3 +, 1 = 200 .


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Figure 4. NPs assemble exclusively at electrode surfaces into 2D close packed planar assemblies. (a) Optical z-stack constructed from a through focal series of dark field optical microscopy images taken of

silica-coated silver NPs after planar assembly with a Δ/ = 1.5 +, 1 = 300 applied AC potential. The distance between the two blue dashed lines is 90 -. (b) SEM images of the ITO electrode surface after

planar assembly of silica-coated silver NPs in a Δ/ = 2 +, 1 = 300 applied AC potential.


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Figure 5. Frequency and voltage dependent NP assembly behavior. Dark field optical microscopy images (top row) and SEM images (bottom row). (a,d) Ballistic deposition, (b,e) planar assembly, and (c)

defect induced assembly. In this case, planar assembly was induced with a Δ/ = 1.5 +, 1 = 100 applied potential, ballistic deposition with Δ/ = 2 +, 1 = 200 , and defect induced assembly with a

Δ/ = 2.5 +, 1 = 500 potential.


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Figure 6. Qualitative phase diagram of NP behavior as a function of applied potential and frequency. The data points were derived from experimental images of NPs following application of the

given potential and frequency for ~10 − 20 seconds (cf. Figure 5). The shaded red and blue regions are

the results of the scaling models, with = 4 and ℎ( = 1.27 in Eqn 6.


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Table I. Scaling model parameters. Particle radius () Dielectric constant of water (CP ) Debye parameter (I ij ) Electrode spacing (2ℎ) Stern layer conductivity ( )† EHD flow parameter (b)‡ Temperature ( ) Particle zeta potential ('( ) Electrode zeta potential ('V )‡ Hamaker constant (3)‡

†

Fitted parameter

‡

Estimated parameter

55 80 1.04 · 10¡ ij 90 - 1 Q 4 298 ¢ −33 + −30 + 0.5 · 10iD


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Figure 7. Scaling model for ballistic deposition of NPs. (a) Interaction force between a 110 sphere

and flat plate with no electric field applied. @P is defined as the maximum interaction force. Inset is the corresponding interaction potential showing the position of the secondary minimum. (b) The electric field

strength at 5 = 0 6 as a function of distance from the positively charged electrode and frequency

(1 = 50 , blue dashed-dotted curve; 1 = 500 , green dashed curve; 1 = 1500 , red solid curve)

for an applied potential of Δ/ = 3 +. Inset is the electric field evaluated at = 70 as function of

frequency. (c) The maximum AC EP force m−@AB,KLM p on NPs as a function of applied frequency, for the

electric field strength at 5 = 0 6. The applied potential is Δ/ = 3+. The dashed line indicates the constant repulsive electrostatic force between the NP and electrode in the absence of an electric field.


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Figure 8. Dipole coefficients and scaling model for planar assembly via EHD flow. The real (a) and imaginary (b) dipole coefficients, and (c) the frequency dependent dipolar term in eqn. 6 as a function of

frequency for a 55 dielectric particle. (d) Plot of the regions in which ballistic deposition (blue) and

planar assembly (red) are predicted to occur from the scaling model. Note that >9 > 1 throughout the entire blue region, but NPs deposit ballistically prior to planar assembly occurring. = 4 and ℎ( =

1.27 in Eqn 6.


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Figure 9. Planar assembly of bare silver NPs in oscillatory electric fields. (a) Planar assembly of 30 diameter aminated silver NPs functionalized with BPEI ligands (Δ/ = 5 +, 1 = 200 ) and (b) 60 diameter silver NPs functionalized with COOH-PEG-SH ( = 5000) (Δ/ = 4 +, 1 =

200 ).


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62. Yang, Y.; Chen, G. D.; Martinez-Miranda, L. J.; Yu, H.; Liu, K.; Nie, Z. H., Synthesis and LiquidCrystal Behavior of Bent Colloidal Silica Rods. Journal of the American Chemical Society 2016, 138 (1), 6871. 63. Zhang, J.; Grzybowski, B. A.; Granick, S., Janus Particle Synthesis, Assembly, and Application. Langmuir 2017, 33 (28), 6964-6977. 64. Lunn, D. J.; Finnegan, J. R.; Manners, I., Self-assembly of "patchy" nanoparticles: a versatile approach to functional hierarchical materials. Chemical Science 2015, 6 (7), 3663-3673. 65. Bharti, B.; Rutkowski, D.; Han, K.; Kumar, A. U.; Hall, C. K.; Velev, O. D., Capillary Bridging as a Tool for Assembling Discrete Clusters of Patchy Particles. Journal of the American Chemical Society 2016, 138 (45), 14948-14953. 66. Ma, F. D.; Wu, D. T.; Wu, N., Formation of Colloidal Molecules Induced by Alternating-Current Electric Fields. Journal of the American Chemical Society 2013, 135 (21), 7839-7842. 67. Shields, C. W.; Velev, O. D., The Evolution of Active Particles: Toward Externally Powered SelfPropelling and Self-Reconfiguring Particle Systems. Chem 2017, 3 (4), 539-559. 68. Zhang, J.; Luijten, E.; Grzybowski, B. A.; Granick, S., Active colloids with collective mobility status and research opportunities. Chemical Society Reviews 2017, 46 (18), 5551-5569. 69. Han, K.; Shields, C. W.; Diwakar, N. M.; Bharti, B.; Lopez, G. P.; Velev, O. D., Sequence-encoded colloidal origami and microbot assemblies from patchy magnetic cubes. Science Advances 2017, 3 (8). 70. Woehl, T. J.; Chen, B. J.; Heatley, K. L.; Talken, N. H.; Bukosky, S. C.; Dutcher, C. S.; Ristenpart, W. D., Bifurcation in the Steady-State Height of Colloidal Particles near an Electrode in Oscillatory Electric Fields: Evidence for a Tertiary Potential Minimum. Physical Review X 2015, 5 (1). 71. Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W. Y.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. N., Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications. Chemical Reviews 2011, 111 (6), 3669-3712. 72. Jain, P. K.; El-Sayed, M. A., Plasmonic coupling in noble metal nanostructures. Chemical Physics Letters 2010, 487 (4-6), 153-164. 73. Jain, P. K.; El-Sayed, M. A., Universal scaling of plasmon coupling in metal nanostructures: Extension from particle pairs to nanoshells. Nano Letters 2007, 7 (9), 2854-2858. 74. Willets, K. A.; Wilson, A. J.; Sundaresan, V.; Joshi, P. B., Super-Resolution Imaging and Plasmonics. Chemical Reviews 2017, 117 (11), 7538-7582. 75. Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P., Biosensing with plasmonic nanosensors. Nature Materials 2008, 7, 442. 76. Sönnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P., A molecular ruler based on plasmon coupling of single gold and silver nanoparticles. Nature Biotechnology 2005, 23, 741. 77. Kuzma, A.; Weis, M.; Flickyngerova, S.; Jakabovic, J.; Satka, A.; Dobrocka, E.; Chlpik, J.; Cirak, J.; Donoval, M.; Telek, P.; Uherek, F.; Donoval, D., Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles. Journal of Applied Physics 2012, 112 (10). 78. Fagan, J. A.; Sides, P. J.; Prieve, D. C., Vertical motion of a charged colloidal particle near an AC polarized electrode with a nonuniform potential distribution: Theory and experimental evidence. Langmuir 2004, 20 (12), 4823-4834. 79. Ristenpart, W. D.; Jiang, P.; Slowik, M. A.; Punckt, C.; Saville, D. A.; Aksay, I. A., Electrohydrodynamic Flow and Colloidal Patterning near Inhomogeneities on Electrodes. Langmuir 2008, 24 (21), 12172-12180. 80. Nadal, F.; Argoul, F.; Kestener, P.; Pouligny, B.; Ybert, C.; Ajdari, A., Electrically induced flows in the vicinity of a dielectric stripe on a conducting plane. European Physical Journal E 2002, 9 (4), 387-399. 81. Moutet, P.; Deram, P.; Sangeetha, N. M.; Ressier, L., Dynamics of DiEP-Force-Directed Assembly of NaYF4 Colloidal Nanocrystals into Tunable Multilayered Micropatterns. Journal of Physical Chemistry Letters 2014, 5 (17), 2988-2993. 30 ACS Paragon Plus Environment

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82. Gardeshzadeh, A. R.; Raissi, B.; Marzbanrad, E., EP deposition of SnO2 nanoparticles using low frequency AC electric fields. Materials Letters 2008, 62 (10-11), 1697-1699. 83. Kloepper, K. D.; Onuta, T. D.; Amarie, D.; Dragnea, B., Field-induced interfacial properties of gold nanoparticles in AC microEP experiments. Journal of Physical Chemistry B 2004, 108 (8), 2547-2553. 84. Fagan, J. A.; Sides, P. J.; Prieve, P. C., Vertical oscillatory motion of a single colloidal particle adjacent to an electrode in an ac electric field. Langmuir 2002, 18 (21), 7810-7820. 85. Obrien, R. W.; White, L. R., EP MOBILITY OF A SPHERICAL COLLOIDAL PARTICLE. Journal of the Chemical Society-Faraday Transactions Ii 1978, 74, 1607-1626. 86. Russel, W. B.; Schowalter, W. R.; Saville, D. A., Colloidal Dispersion. Cambridge UP: Cambridge, 2001. 87. Hunter, R., Foundations of Colloid Science. Oxford UP: Oxford, 2009. 88. Hollingsworth, A. D.; Saville, D. A., A broad frequency range dielectric spectrometer for colloidal suspensions: cell design, calibration, and validation. Journal of Colloid and Interface Science 2003, 257 (1), 65-76. 89. Ma, F. D.; Yang, X. F.; Zhao, H.; Wu, N., Inducing Propulsion of Colloidal Dimers by Breaking the Symmetry in Electrohydrodynamic Flow. Physical Review Letters 2015, 115 (20). 90. Mangelsdorf, C. S.; White, L. R., Dielectric response of a dilute suspension of spherical colloidal particles to an oscillating electric field. Journal of the Chemical Society-Faraday Transactions 1997, 93 (17), 3145-3154. 91. Shilov, V. N.; Delgado, A. V.; Gonzalez-Caballero, F.; Grosse, C., Thin double layer theory of the wide-frequency range dielectric dispersion of suspensions of non-conducting spherical particles including surface conductivity of the stagnant layer. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2001, 192 (1-3), 253-265.


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Figure 1. (a) Schematic drawing (not to scale) of sample cell and dark field optical microscopy for visualizing AC electric field induced NP assembly. Dashed lines represent the color of scattered light from single NPs and planar assemblies, indicating that single silver NPs appear blue in the dark field image, while planar NP assemblies display red shifted color. (b) Schematic drawing of the dominant forces, AC electrophoresis and EHD fluid flow, controlling NP behavior near an electrode in an AC electric field. The streamlines show the approximate flow pattern of EHD fluid flow. 66x62mm (300 x 300 DPI)

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Figure 2. Dynamics of planar assembly of 110 nm silica coated silver NPs. a) Time-lapsed series of dark field optical microscopy images showing the planar assembly of NPs in a ∆φ = 1.5 V, f = 200 Hz applied AC potential. The electric field was applied at t = 10 s (red dashed line in (b)). b) The average hue of the NPs in the images from a movie of the NP assembly in (a) as a function of time. Hue is a quantitative measurement of color, determined from the RGB values of the original color images. The inset shows that hue = 0° corresponds to red, hue = 120° corresponds to green, and hue = 240° corresponds to blue. The color of the data points in (b) corresponds to the average color of the NPs in that frame, while the positions of the white arrows indicate the times points in (a). 103x179mm (300 x 300 DPI)


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Figure 3. Dynamics of planar assembly. (a,b) Cropped time-lapsed series of dark field optical microscopy images showing planar assembly of silica coated silver NPs. NPs and assemblies remained mobile during assembly, indicating that they remain colloidally stable during planar assembly. The applied potential in each experiment was ∆φ = 3 V, f = 200 Hz. 73x30mm (300 x 300 DPI)


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Figure 4. NPs assemble exclusively at electrode surfaces into 2D close packed planar assemblies. (a) Optical z-stack constructed from a through focal series of dark field optical microscopy images taken of silica-coated silver NPs after planar assembly with a ∆φ=1.5 V, f=300 Hz applied AC potential. The distance between the two blue dashed lines is 90 µm. (b) SEM images of the ITO electrode surface after planar assembly of silicacoated silver NPs in a ∆φ = 2 V, f = 300 Hz applied AC potential. 40x18mm (300 x 300 DPI)


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Figure 5. Frequency and voltage dependent NP assembly behavior. Dark field optical microscopy images (top row) and SEM images (bottom row). (a,d) Ballistic deposition, (b,e) planar assembly, and (c) defect induced assembly. In this case, planar assembly was induced with a ∆φ = 1.5 V, f = 100 Hz applied potential, ballistic deposition with ∆φ = 2 V, f = 200 Hz, and defect induced assembly with a ∆φ = 2.5 V, f = 500 Hz potential. 84x56mm (300 x 300 DPI)


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Figure 6. Qualitative phase diagram of NP behavior as a function of applied potential and frequency. The data points were derived from experimental images of NPs following application of the given potential and frequency for ~10 - 20 seconds (cf. Figure 5). The shaded red and blue regions are the results of the scaling models, with b = 4 and hp = 1.27 in Eqn 6. 67x49mm (300 x 300 DPI)


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Figure 7. Scaling model for ballistic deposition of NPs. (a) Interaction force between a 110 nm sphere and flat plate with no electric field applied. Fc is defined as the maximum interaction force. Inset is the corresponding interaction potential showing the position of the secondary minimum. (b) The electric field strength at t = 0 s as a function of distance from the positively charged electrode and frequency (f = 50 Hz, blue dashed-dotted curve; f = 500 Hz, green dashed curve; f = 1500 Hz, red solid curve) for an applied potential of ∆φ = 3 V. Inset is the electric field evaluated at z = 70 nm as function of frequency. (c) The maximum AC EP force (-F(EP,max)) on NPs as a function of applied frequency, for the electric field strength at t = 0 s. The applied potential is ∆φ = 3 V. The dashed line indicates the constant repulsive electrostatic force between the NP and electrode in the absence of an electric field. 53x14mm (300 x 300 DPI)


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Figure 8. Dipole coefficients and scaling model for planar assembly via EHD flow. The real (a) and imaginary (b) dipole coefficients, and (c) the frequency dependent dipolar term in eqn. 6 as a function of frequency for a 55 nm dielectric particle. (d) Plot of the regions in which ballistic deposition (blue) and planar assembly (red) are predicted to occur from the scaling model. Note that Pe > 1 throughout the entire blue region, but NPs deposit ballistically prior to planar assembly occurring. b = 4 and hp = 1.27 in Eqn 6. 167x305mm (300 x 300 DPI)


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Figure 9. Planar assembly of bare silver NPs in oscillatory electric fields. (a) Planar assembly of 30 nm diameter aminated silver NPs functionalized with BPEI ligands (∆φ = 5 V, f = 200 Hz) and (b) 60 nm diameter silver NPs functionalized with COOH-PEG-SH (MW = 5000) (∆φ = 4 V,f = 200 Hz). 45x22mm (300 x 300 DPI)


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Table of Contents Graphic 45x27mm (300 x 300 DPI)


Direct Visualization of Planar Assembly of Plasmonic Nanoparticles

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