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Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays Arika Pravitasari, Maelani Negrito, Kristin Light, Wei-Shun Chang, Stephan Link, Matthew Sheldon, and James D. Batteas J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06357 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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The Journal of Physical Chemistry

Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays †

Arika Pravitasari,1 Maelani Negrito,1 ≠Kristin Light,1 Wei-Shun Chang,2 Stephan Link,2,3 Matthew Sheldon,1,4 and James D. Batteas1,4*

1

Department of Chemistry, Texas A&M University, College Station, TX 77843, USA 2 3

Department of Chemistry, Rice University, Houston, TX 77251, USA

Department of Electrical and Computer Engineering, Rice University, Houston, TX 77251, USA

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Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA

*corresponding author: e-mail: [email protected] †

Present address: Intel Corporation, Portland, OR 97124

≠

Present Address: The Chemours Company, Gregory, TX 78349

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ACS Paragon Plus Environment


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Abstract The facile assembly of metal nanostructured arrays is a fundamental step in the design of plasmon enhanced chemical sensing and solar cell architectures. Here we have investigated methods of creating controlled formations of two-dimensional periodic arrays comprised of 20 nm Au nanoparticles (NPs) on a hydrophilic polymer surface using particle lithography. To direct the assembly process, capillary force and NP concentration both play critical roles on the resulting nanostructured arrays. As such, tuning these experimental parameters can directly be used to modify the nature of the nanostructures formed. To explore this, two different concentrations of Au NP solutions (~7 x 1011 NPs/mL or 4 x 1012 NPs/mL) were used in conjunction with a fixed concentration of polystyrene microspheres (PS MS, ~ 6 x 109 PS MS/mL). Assembly at a relative humidity (RH) of 45% with the higher concentration resulted in the formation of well-defined Au nanorings of ca. 23 nm in height and 881 nm in diameter with a pitch of 2.5 µm. Assembly at 65% RH with the lower concentration of NPs resulted in Au nanodonut arrays comprised of isolated single Au NPs. To explore the extent of coupling in the well-defined structures, dark field scattering spectra were collected and showed a broad localized surface plasmon resonance (LSPR) peak with a shoulder, which fullwave electrodynamics modeling

(Finite-Difference Time Domain (FDTD) method)

attributed to be a result of pronounced particle-particle coupling along the circumference of the nanoring array.

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Introduction The unique ability of localized surface plasmon resonances (LSPRs) to concentrate light below the diffraction limit at subwavelength scales has motivated a surge in research to tailor this property for a wide array of applications.1-2 Although all conductive materials have the ability to support plasmons, noble metal nanoparticles (NPs) are of particular interest as their LSPR frequencies lie within the visible spectrum and can be tuned by varying the NP size and shape.2 However, NP size does not affect the optical properties as dramatically as NP shape.3-4 For instance, an increase in Au NP diameter from approximately 9 nm to 99 nm only results in a red shift of about 60 nm in the LSPR peak from 517 nm to 575 nm.3 Transitioning from a 15 nm Au nanosphere to a nanorod with an aspect ratio of 7.5, on the other hand, shifts the LSPR maximum from about 520 nm into the near-IR range with a wavelength of approximately 1050 nm.4 With the increase in the wet chemical synthesis methods, a variety of NP shapes such as rods,5-6 prisms,7 cubes,8 triangles,9-10 pentagons,10 nanoshells,11 and rings12 have been realized and studied. From these structures, “hot spots” of electromagnetic field enhancement were found to occur at corners and/or edges of the NPs13 and, importantly for this study,

between closely spaced NPs14-18 depending on the position of the

nanostructure relative to the polarization of incident light. The plasmonic enhancement of the local electromagnetic field is ideal for applications such as cancer treatment,6 biosensors,19 surface-enhanced Raman spectroscopy (SERS)1-2, 13-14, 20 or tip-enhanced Raman spectroscopy (TERS),1-2 nanoantennas,21 and waveguides.1-2 Due to strong local coupling, it has also been shown that the organization of smaller particles into welldefined structures with close proximity can reproduce the optical properties of individual

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structures made of contiguous materials. As such, schemes to direct the local assembly of well-defined particles into organized architectures also offers an approach to create plasmonic nanostructures that can be further manipulated by their directed assembly.22-25 To this end, producing wafer-scale nanostructures with well-defined, controlled dimensions and periodicity quickly, without the use of expensive lithographic instruments, remains a point of interest for plasmonic applications.26 The use of colloids as a means to template pattern nanostructures has been widely used for producing high throughput arrays of nanostructures from triangles and dots9 to nanoprisms7 and rings.12, 27-30

Periodic arrays of nanorings are of particular interest here, as it has been shown that

resulting optical properties can be controlled by the ring radius, thickness and aspect ratio.12,

28-30

However, most methods used to produce the metal nanorings involve

physical vapor deposition,12 electrodeposition28 or reactive ion etching.29-30 As a result, solid rings are formed that do not include the previously observed particle-particle plasmonic coupling that provides the largest optical field enhancements.17-18 Similar to the Au architectures we demonstrate here, we have previously shown that polystyrene microspheres may be used to form arrays of CdSe quantum dot nanorings via an evaporative templating method.27 The main driving force of this method is the capillary force produced as the solvent evaporates. Ring formation results from the magnitude of the capillary force (Fcap) being greater than the sum of the opposing forces; such as adhesion (Fadh), friction (Ffric) and the double layer force (Fdl).27 In this paper, the evaporative fabrication method is explored further with the use of 20 nm 11mercaptoundecanoic acid (11-MUA)-capped Au NPs to determine the effect of relative humidity (RH) and NP concentration on the formation of nanoring arrays. Here we have

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explored the influence of NP concentration and assembly conditions on the resulting structures that can be formed on glass surfaces coated with a polymeric binder. The coupling within the NP arrays was also measured using both dark field scattering spectroscopy and numerical modeling of the resulting optical properties to deduce the extent of interparticle coupling.

Experimental Methods Synthesis of Au NPs. Au NPs were synthesized by the reduction of HAuCl4 !3H2O (Sigma Aldrich) with sodium citrate.31 The size of the citrate-stabilized Au NPs was determined to be ~ 20 nm in diameter based on the maximum surface plasmon absorbance in the UV-visible spectra (USB-ISS-UV/vis, Ocean Optics Inc.) at 523 nm. Transmission electron microscopy (TEM) images of isolated Au particles confirmed the size to be 20 ± 4 nm. Au NP Ligand Exchange to 11-MUA. Synthesized citrate stabilized Au NPs underwent further processing to exchange the capping agent to 11-mercaptoundecanoic acid (11-MUA).32 First, 1 mL of 10 mM 11-MUA in ethanol was added to 10 mL of synthesized Au NPs under vigorous stirring. Stirring was continued for at least 60 hours in the dark. The mixture was then centrifuged at 8000 rpm for 60 min (Beckman J2-21), and the supernatant was replaced with 18.2 MΩ▪cm nanopure water (NANOpure Diamond, Barnstead) that was adjusted to a pH of 11 with NaOH. The washing procedure was repeated twice to ensure removal of unbound excess ligand. The resulting Au NPs were then characterized by UV-Vis spectroscopy and TEM and stored in the dark for future use.

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Microsphere Preparation. Polystyrene (PS) microspheres (MS) of 2.5 µm in size were purchased from Duke Scientific (Fremont, CA). The spheres were repeatedly centrifuged for 5 min and re-suspended in ultrapure water to remove surfactant molecules from the solution. This centrifugation/re-suspension process was repeated 8 times. Substrate Preparation. Glass cover slides (VWR) were cleaned by sonication in ethanol for 15 minutes. Next, the clean glass slides were modified with polyvinylpyrrolidone (PVP, Sigma-Aldrich, MW: 55000). PVP-modified surfaces were prepared by soaking freshly prepared glass substrates in a 1% (w/v) PVP in ethanol solution overnight. The samples were then rinsed sequentially with ethanol and purified water for 1 minute each. Finally, the samples were dried with streaming nitrogen. Colloidal Lithography. A 1:1 mixture of ~ 6 x 109 PS MS/mL and ~7 x 1011 or 4 x 1012 Au NPs/mL was added onto the PVP modified cover slip in a drop-wise fashion (~1.5 µL droplets). The samples were then placed inside a chamber where the humidity was kept constant to specified value (within 5%) overnight. Scotch tape was used to remove the PS MS that were used as a template, leaving an array of Au nanostructures on the substrate. Nanostructure Characterization via Atomic Force Microscopy (AFM). AFM images of the Au nanostructures were taken with a combined AFM/confocal microscope (WiTec Alpha 300). All AFM images were acquired under ambient conditions in tapping mode using commercially available aluminum-coated silicon AFM tips from Nanoscience Instrument (Phoenix, AZ) with manufacturer specified nominal tip radii of less than 10 nm and nominal spring constants of 48 N/m.

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Nanostructure Characterization via Dark-Field Scattering Spectroscopy. The dark-field scattering spectra of single nanorings were acquired with a custom-built microscope based on a commercial microscope (Zeiss, Observer 1m) with a transmission dark-field geometry. Unpolarized light from a halogen lamp (100 W) was focused on the sample through an oil-immersion dark-field condenser (N.A. = 1.4). The scattered light from single nanorings was collected via an air-space objective (N.A. = 0.8) and focused onto either an avalanche photodiode or a spectrometer (Princeton Instruments, SP2150) equipped with a CCD camera (Princeton Instruments, BR400). Scattering images were acquired by scanning the sample and imaging through a 50 µm pinhole which replicated a confocal scheme and only transmitted the scattered light from the region of interest. Single particle spectra were collected by guiding the scattered light toward the spectrometer/CCD camera. All single-particle spectra were corrected for the background scattering of the substrate and normalized by the lamp spectrum. The data were analyzed using a home-made Matlab script. Full-Wave Optical Simulations. Using Texas A&M University’s Intel x86-64 Linux cluster Finite-Difference Time-Domain software (FDTD Solutions, Lumerical, Inc.) was used to model the scattering spectra of an Au NP nanoring structure by solving Maxwell’s equations in every mesh point. Optical constants for Au were taken from the Johnson and Christy data,33 Konig’s data was used for the PVP layer34 and the refractive index of the glass cover slides (1.5255) was provided by the vendor (VWR). The effect on the dielectric environment caused by the capping ligand of the Au NPs constituted a small perturbation in comparison with studies of other noble metal NPs.35-36 Including the capping ligand has been shown to red-shift the LSPR peak ~5 nm, at the limit of our

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experimental spectral resoltuion.36 Therefore, to simplify the calculations and reduce computation time, the more complex optical geometry that results from explicitly modeling the capping ligand (11-MUA) was omitted. Perfect Matching Layer (PML) boundary conditions were used and two perpendicularly polarized Total-Field ScatteredField (TFSF) sources (λ = 300–900 nm) were used to simulate unpolarized light.37 To save on computational time a mesh size of 5 nm was used and around the NPs the mesh was overridden to be 0.8 nm.

Results and Discussion Au NP Nanoring Array Fabrication. A hexagonal patterned array of Au nanorings was fabricated by a previously developed procedure27 outlined in Figure 1. The concentration of PS microspheres (MS) was kept constant at 6 x 109 PS MS/mL throughout the experiment while two different concentrations of Au NPs, determined by UV-Vis spectroscopy,38 were used: 7 x 1011 and 4 x 1012 Au NPs/mL. The 20 nm Au NPs were stabilized with 11-MUA and characterized with UV-Vis spectroscopy and TEM as shown in Figure SI.1 in the Supplementary Information. Figure 2 shows the Au nanoring array formed at 45% RH with the higher Au NP concentration after rinsing with ethanol. The average height of the rings after the rinsing treatment was 23 ± 5 nm. This height was reduced from an average height of 36 ± 10 nm from the unwashed sample due to the loss of loosely bound particles (Figure SI.2). Because of the attractive van der Waals forces between the Au NPs and the PVP modified glass substrate, the patterned array is

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Figure 1. (1) A 1:1 mixture of Au NPs and polystyrene microspheres (PS MS) dispersed in water is dropcasted on a PVP modified glass substrate. (2) As the mixture dries the PS MS self-assemble into a close packed hexagonal array while the smaller Au NPs arrange themselves around the larger microspheres. (3) Scotch tape is then used to peel away the larger microspheres, (4) leaving the Au nanostructures array on the substrate.

Figure 2. (A-C) AFM tapping mode topography images of Au nanoring arrays assembled on a PVP modified glass substrate at 45% RH with an Au NP concentration of 4 × 1012 Au NPs/mL after Scotch tape peel and ethanol rinse. (C) Zoomed in image of an Au nanoring with corresponding cross section (D). Scale bars are: 4 µm (A), 2 µm (B) and 0.2 µm (C).

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conserved after the ethanol rinsing. Without the PVP layer the majority of the fabricated Au nanorings were rinsed away. Different morphologies of Au nanostructure arrays were observed when the experimental conditions were altered. Figure 3 shows Au nanodonuts formed by using a lower concentration of Au NPs in a 65% RH after the ethanol rinsing treatment. From these images we can see that the structures are composed of what appears to be resolved individual particles, unlike the composition of nanoring structures, shown in Figure 2, which is more of a condensed agglomeration of Au NPs that were slightly wider than one NP taking into account the tip broadening. However, the LSPR peak in the scattering spectrum of a ring with a 2 NP wide thickness was broader than the experimental spectra. The average height of the nanodonut structures is 10 ± 2 nm which, as with the nanoring structures, upon washing with ethanol decreased from 12 ± 2 nm (Figure SI.3). The height of these nanodonut structures is slightly smaller in comparison to the size of a single Au NP. It is possible that the Au NPs are partially imbedded in the soft PVP matrix. The height of the polymer matrix, determined by AFM scratching, is 8 ± 1 nm

Figure 3. AFM tapping mode topography images showing Au NP donuts array on a PVP modified glass substrate. Scale bars are: 1 µm (left) and 0.3 µm (right).

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(Figure SI.4). When the experiment is conducted using a bare glass substrate without the PVP polymer, the resulting structures that survive the rinsing treatment showed an average height of a single Au NP, ~ 20 nm (Figure SI.5). From the experimental AFM images we are able to calculate the dimensions of the nanostructures, shown in Table 1. The Au nanorings have inner diameters Table 1. Au Nanoring and Nanodonut Dimensions Tabulated From AFM Images

Din (nm)

Dout (nm)

Dout/Din

Au rings

394 ± 51

881 ± 101

2.3 ± 0.3

Au donuts

250 ± 32

1387 ± 112

5.6 ± 0.6

(Din) of 394 nm, outer diameters (Dout) of 881 nm, with Dout/Din of 2.3, while the Au nanodonut array dimensions are Din = 250 nm, Dout = 1387 and Dout/Din = 5.6. The dimensions for the Au nanoring array can be approximated using the hard sphere model shown in Figure 4 to calculate rring : 𝑟!"#$ =

(𝑟!" + 𝑟!" )! − (𝑟!" − 𝑟!" )!

(1)

With rMS = 1.25 µm and rAu = 20 nm, the calculated rring is 224 nm, giving Din = 448 nm, this value agrees within the standard deviation of the experimental data. The dimensions of the nanodonut structures deviated greatly from the rring calculated by the model shown on Figure 4B. The much smaller Din of the nanodonut structures indicated that the Au NPs were able to move closer to the PS MS perimeter. Previously shown in the AFM measurement data in Figure 3 and Figure SI.4 in the Supplementary Information, the individual Au NPs are partially embedded in the PVP matrix. This is counter to the assumption made for using the hard sphere contact model

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Figure 4. Schematic of diagrams showing: (A) the forces involved in dragging the Au NP to polystyrene spheres where there is almost certainly a hydration layer that covers the entire hydrophilic Au NP, which has not been explicitly drawn, (B) a hard sphere contact model approximation to calculate rring, and (C) Au nanodonut array formation showing Au NPs partially embedded in the PVP matrix. Note that drawings are not to scale.

approximation for this system. Figure 4C depicts a much better approximation for the formation Au nanodonut arrays. Using the Din dimensions of the Au nanodonut arrays, the value x and Au NP ring height from AFM imaging, as depicted in Figure 4C, can be calculated using equation 2. The calculated value for the height of the Au NP rings in the AFM images matched the ~ 6 nm experimental value as shown in Figure 3. 𝑟!"#$% =

(𝑟!" + 𝑥)! − (𝑟!" )!

(2)

The discussed phenomena explain how humidity plays a role in the formation of the Au nanorings or nanodonut structures. In addition to the RH, the Au NP concentration serves as a determining factor in the formation of both structures. The nanodonut arrays only occur when a lower concentration of Au NPs is used under a high RH environment.

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To explain this we postulate the following. At higher RH values, the evaporation of the liquid capillary proceeded over a longer time period in comparison to the system with a low RH value. The lower concentration of Au NPs, when compared to the high Au NP solution with the same volume, contained a higher percentage of water in the capillary. In our system, the difference in the concentrations of the Au NP solutions was five-fold. But, as the system evaporates down to the volume of the liquid capillary bridge, the originally five-fold more concentrated Au NPs will be even more concentrated than the lower Au NPs concentration. The slower evaporation rate in the higher RH environment combined with the system of lower Au NP concentration and higher water content further increased the evaporation time of the system. This allowed for the Au NPs to be partially embedded in the PVP matrix, as shown by the AFM data in Figure 3. This also allowed for the individual NPs to move closer to the PS perimeter, resulting in the smaller Din for the nanodonut arrays. The higher concentration of Au NPs has a higher ionic strength originating from the negatively charged 11-MUA ligand and NaOH in the solution. The ionic strength relates to the Debye length (i.e. electrostatic shielding between the particles), where the Debye length κ-1 is inversely proportional to the ionic strength I. The reduced Debye length is due to the screening effect from an increased ionic strength as the Au NP concentration is increased.39 Hence, in the case of the lower concentration of Au NPs, the repulsive force between each NP is greater, preventing the formation of a condensed ring structure. As stated previously, for the formation of the ring to occur, Fcap must be greater than the sum of all the opposing forces (Fadh+Fdl+Ffric),27 also shown in Figure 4. It is possible that in the low concentration system, the repulsive double layer force between

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the Au NPs was large and counteracted Fcap. As observed, the Dout of the nanodonut arrays was also larger in comparison to the nanoring arrays, in agreement with the postulated increasing repulsion force between each individual NP. Varying both the RH and Au NP concentration can influence the formation of these nanostructures as shown in Figure 5. The low concentration of Au NPs appears to form good condensed ring structures for a lower RH, and transform into donut structures

Figure 5. AFM topographic images of Au nanostructures as the relative humidity, RH, (controlled within 5%) and Au NP concentration are varied. All scale bars are 2 µm.

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at higher RH. While the higher concentration of Au NPs form uniform ring structures at ambient (~45%) RH, jagged ring structures at a lower RH of 30% and less uniform ring structures at a higher humidity of 65%. Supporting our previous argument, Au NPs in lower concentration are able to self-assemble into ring nanostructures at a lower RH compared to the high concentration of Au NPs. The higher water content in the capillary of the system with the lower concentration requires a lower RH for the ring formation to occur. On the other hand, when the higher Au NPs system was assembled under a low 30% RH, the system dried too quickly before the NPs were able to form uniform ring structures around the PS MS creating jagged ring structures with non-uniform aggregation of Au NPs on its side.

Au NP Nanoring Array Plasmon Resonance. It has been shown that polygonal structures of Au NPs exhibit an increase in intensity and red-shift in plasmon resonance spectral peak position.18 For solid rings, a single peak is produced as a result of coupling between the inner and outer ring walls whose position can be shifted as a function of nanoring size and thickness.12, 28, 30 Dark-field scattering spectra were taken of a single Au NP nanoring array, shown in black in Figure 6, which consist of a broad peak with a shoulder. Full-wave optical modeling (Finite-Difference Time Domain method) was used to solve Maxwell’s Equations to determine the underlying interactions responsible for the LSPR peak shape and spectral position. As can be seen in the dark-field scattering spectra and numerical simulation in Figure 6, the main position of the LSPR peak, 530 nm for a single Au NP (green), is red-shifted to ~595 nm in the dark-field scattering spectra and 582 nm in the simulation. By changing the distance between NPs in the modeled ring

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(blue and red traces), it was found that coupling between particles has a strong influence on the optical response of the nanoring structure. A difference of only 3 nm in NP spacing red shifts the LSPR about 30 nm leading to a spectral position in good agreement

Figure 6. Comparison of numerical simulation LSPR scattering spectra for a single Au NP (green) and an Au nanoring with NP separation distances of

~4 nm (blue) and ~1 nm (red) to a representative

experimental dark-field scattering spectrum of a single Au nanoring (black).

with dark-field scattering measurements. Based on the molecule length of the 11-MUA capping ligand of ~2 nm including the Au-S bond, assuming an all-trans configuration of the ligands and no overlap between ligands of neighboring NPs, the smallest separation NP separation would be ~4 nm.40-41 However, studies have shown that alkyl chain capping ligands on NPs will form gauche defects to fill the space resulting from the curvature of the particle and maximize nearest neighbor intermolecular interactions.42-43 Therefore, assuming a collapsed chain of the alkyl chain ligands, a NP spacing of ~1 nm is not unreasonable. As the separation between NPs decreases, the rings begin to take on characteristics of a solid ring structure that can be seen as the appearance of the shoulder

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in the LSPR peak and in the electric field profiles (Figure 7). This collective behavior where the NP plasmons are coupled over the entire ring is possible because of the

Figure 7. Comparison of numerical simulation electric field profiles for an Au nanoring with NP separation distances of ~4 nm (A) and a solid Au nanoring (B) and their corresponding simulated normalized scattering cross sections (C, D) with respect to the simulated scattering cross section of a single gold nanoparticle (green) and the averaged scattering intensity from 12 individual Au NP nanorings in a 5 µm × 5 µm area seen in Figure S6.

relatively small ring diameter, as for NP rings with larger diameters of many micrometers plasmon coupling was found to be localized, extending only over tens of NPs.23 However

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the optical field intensity between particles within the ring with a maximum electric Field Enhancement Factor (FEF, Equation 3) of about 4.5 × 103 is much larger than can be achieved with contiguous metal nanoring structures with an FEF of around 16. 𝐹𝐸𝐹 =

!! !!""#

(3)

The FEF is the ratio of the enhanced electric field near the surface of the NP to the incident electric field. Where El is the maximum value of the local electric field and Eappl is the electric field applied on the system.

Conclusions Capillary force is the main driving force for the formation of the Au nanoring arrays via particle lithography. The balance between Fcap and the rest of the opposing forces (Fadh, Fdl, and Ffric) plays an important role in this formation. As the RH and the concentration of Au NPs are varied, the balance between these forces shift, which alters the morphologies of the formed nanostructures. Uniform nanoring structures were assembled at 45% RH by using a 4 x 1012 Au NPs/mL concentration, and at a lower RH of 30% using a lower Au NP concentration of 7 x 1011 Au NPs/mL. However, increasing the RH to 65% and using the lower Au NP concentration, the morphology of the nanostructures was altered from nanorings to nanodonuts. The higher RH combined with the lower Au NP concentration resulted in a higher water content of the system which lead to a slower evaporation process of the capillary bridge and, thus, separation of the PS MS from the substrate. This presumably also applies to the Au NPs where, during the evaporation process, they are separated from the substrate by a capillary liquid bridge. This separation reduces the Fcap acting on the system, not allowing the ring structure to

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form. Instead, a less condensed donut structure is produced. The Au NP nanorings produced here exhibit a plasmon resonance peak at ~595 nm, resulting from interparticle LSPR coupling, with a shoulder at around 646 nm due to coupling between the inner and outer walls of the nanoring much like that of a solid ring structure. This method offers a cost-effective avenue of producing a periodic nanostructure whose optical properties can be easily tuned. Moreover, the use of NPs to form the ring structure provides an additional parameter that can be utilized to tune the LSPR for specific applications through particle-particle spacing.

Supporting Information. UV-Vis Spectrum and TEM image of synthesized Au NPs, AFM images with cross sections of both concentrations of Au NPs before the ethanol rinsing treatment and an AFM scratch experiment of the PVP layer.

Acknowledgments

The authors wish to acknowledge the many outstanding contributions to the field

of physical chemistry made by Dr. Miquel Salmeron. We also acknowledge the Texas A&M Supercomputing Facility (http://sc.tamu.edu/) for providing the computing resources used in conducting the numerical modeling research reported in this paper. We gratefully acknowledge support of this work by the National Science Foundation CHE1213802 and CHE-1611119 to J.D.B., M.S. acknowledges support from the Robert A. Welch Foundation (A-1886) and S.L. acknowledges support from the Robert A. Welch Foundation (C-1664) and the Air Force (MURI FA9550-15-1-0022).

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References 1. 2. 3. 4.

5. 6.

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