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Overgrowth of Silver Nanodisks on a Substrate into Vertically Aligned Nanopillars for Chromatic Light Polarization Mahmoud A. Mahmoud ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07311 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Overgrowth of Silver Nanodisks on a Substrate into Vertically Aligned Nanopillars for Chromatic Light Polarization Mahmoud A. Mahmoud* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 * E-mail: [email protected]

ABSTRACT

Vertically aligned and well-separated 1D silver nanopillars (AgNPLs) are prepared on a largearea quartz surface using a robust colloidal chemical technique. Silver nanodisk (AgND) monolayers were first deposited on quartz using the Langmuir-Blodgett technique, and the presence of the substrate induced asymmetric chemical overgrowth of the AgNDs into AgNPLs. The height and diameter of the prepared AgNPLs were controlled by changing the rate of the overgrowth reaction. Chloride ions were used during overgrowth to etch the silver atoms that formed sharp features on the sides of the AgNDs and to limit growth in the lateral direction. The grown AgNPLs displayed two surface plasmon resonance modes corresponding to the transverse and longitudinal electron oscillations. The intensity of the longitudinal mode increased by a factor of 9 while the intensity of the transverse mode decreased by a factor of 2.5 upon increasing the angle of incidence of the exciting light from 0° to 60°. This interesting property makes these AgNPL arrays on quartz useful as chromatic light polarizers.

KEYWORDS: Asymmetric overgrowth, chromatic polarizer, silver nanopillar, LangmuirBlodgett assembly, colloidal nanocrystal

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INTRODUCTION The exciting optical properties of small metallic nanoparticles, such as those made from gold or silver, can be attributed to their localized surface plasmon resonance (LSPR) phenomena.1-4 The interaction of plasmonic nanoparticles with electromagnetic radiation at a resonant frequency generates a sharp LSPR optical spectrum.1,2 Furthermore, the shape of the plasmonic nanoparticles has a great impact on the number of LSPR spectral peaks.5-8 Single LSPR peaks are usually observed for isotropic plasmonic nanoparticles such as nanospheres9 and nanocubes10,11 while 1D anisotropic nanoparticles such as rods,6,12 rice,13 and bones7 display multiple LSPR peaks. By changing the aspect ratio of the anisotropic nanoparticles, one can tune the peak positions in the visible and NIR electromagnetic regions. Anisotropic plasmonic nanoparticles also display different optical responses upon changing their orientation when excited with polarized light. These direction-dependent optical properties are useful in many applications such as spectral imaging,14 chromatic polarizers,15 color filtration,16,17 and data storage.18,19 In many plasmonic nanoparticle applications, binding them to a substrate is crucial for keeping them fixed during use. Due to the potential electromagnetic field coupling between nanoparticles, changing the separation distance between them also affects the optical properties during application use.20,21 In nanoparticle applications that involves a strong plasmon field and a broad LSPR spectrum, such as surface-enhanced Raman spectroscopy22,23, highly packed nanoparticle assemblies are recommended.24 Conversely, for optical application such as switching,25 optical sensing,26 and chromatic polarizers,15 narrow and sharp LSPR spectra are required. Sharp LSPR spectra cannot obtained unless the plasmonic nanoparticles are highly ordered or well-separated.25 Highly ordered nanoparticle arrays have been fabricated on substrates using different techniques such as electron beam lithography,18,19,27,28 nanosphere 2 ACS Paragon Plus Environment

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lithography,29 and membrane templates.30-32 However, these techniques can be costly or are not easily scalable, while large scale and low cost nanoparticle production is needed for industrial applications. Colloidal chemical synthesis is the therefore one of the most valuable techniques in synthesizing large volumes of nanoparticles with controlled shape and size.7,8,33,34 LangmuirBlodgett (LB)25 and template-assisted self-assembly35 techniques have been used to assemble colloidally prepared nanoparticles into highly ordered 2D arrays on substrates. While these techniques have been used to successfully assemble colloidal isotropic or horizontally aligned nanoparticles into monolayers, vertically aligned and well-separated anisotropic nanoparticle arrays have not yet been fabricated using the LB technique. However, gold and silver nanopillar arrays were fabricated by nanosphere lithography and exhibited two LSPR spectral peaks.36,37 The assembly of anisotropic colloidal nanoparticles into vertically aligned, highly packed arrays on substrates has been demonstrated using the self-assembly technique, but the high packing inhibits many optical applications as discussed previously.24,38-40 Therefore, the aim here is to fabricate vertically aligned, well-separated silver nanopillars (AgNPLs) on transparent substrates such as quartz for optical applications. Colloidally prepared AgNDs were assembled in a monolayer on a quartz substrate using the LB technique and subsequently chemically overgrown. Unlike the overgrowth of colloidal AgNDs in solution, the overgrowth of an AgND monolayer on a substrate leads to the formation of larger hexagonal silver nanodisks. This can be attributed to the effect of the substrate, which prevents growth on the bottom face of the AgND and induces deposition of silver atoms on both the sides and the exposed face of the AgND. Vertically aligned AgNPLs were obtained when the overgrowth of AgNDs on quartz was carried out in the presence of chloride ions. The AgNPLs resulted from the deposition of silver atoms on the exposed top face of the AgNDs while Cl- etched the atoms



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located on the side walls. The diameter and the height of the AgNPLs were able to be controlled by adjusting the rate of reduction of silver ions using ascorbic acid (AA). When the rate of reduction of silver ions was increased by increasing the concentration of AA, thicker and shorter AgNPLs were formed. AgNPLs have two LSPR peaks corresponding to the longitudinal and transverse electron oscillations, and the ratio between the peak intensities of the longitudinal and transverse LSPR spectra of the AgNPLs on a substrate was changed when excitation light was incident at different angles. Consequently, it is possible to use these AgNPLs on a quartz substrate as a chromatic polarizer. EXPERIMENTAL AgNDs were prepared using the SMART technique.41 Briefly, 0.60 mL of 60 mM AgNO3 (Sigma-Aldrich, 99.9%) aqueous solution was added to 200 mL of 0.145 mM aqueous solution of polyvinyl pyrrolidone (MW=55,000 Sigma-Aldrich) (PVP). After shaking the resulting mixture, 5 mL 78 mM L-ascorbic acid (AA) (Sigma-Aldrich) was added followed by 0.12 mL of NaBH4 (5 mM, Sigma-Aldrich), and the solution was gently shaken for a few seconds. The AgNDs were precipitated out by centrifugation in 50 mL plastic tubes at 12,000 rpm for 30 minutes. The precipitated particles were then dispersed in 50 mL DI water and recentrifuged at 8,000 rpm in 15 mL plastic tubes for 20 minutes. The resulting precipitate was dispersed in 5 mL ethanol and centrifuged at 5,000 rpm in 1.7 mL tubes for 25 minutes. The precipitated nanoparticles were dispersed in 2 mL ethanol and diluted with 4 mL chloroform. The monolayer assembly of AgNDs was fabricated by depositing 2 mL of the nanoparticle solution over the surface of a Nima 611D LB trough filled with DI water using a micro-syringe. The AgND monolayer was left to dry for 30 minutes before transfer by vertical dipping to quartz and silicon substrates. A D1L-75 pressure sensor was used to measure the surface pressure of the


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AgND monolayer. After the monolayer reached equilibrium, the surface pressure was adjusted to zero. Finally, the substrate was dipped into the water sub-phase of the trough at a speed of 66 mm/minute and raised up slowly at a speed of 1 mm/minute at a fixed applied surface of 0.1 mN/m. The LSPR extinction spectrum of the AgNDs in an ethanol-chloroform mixture after 20fold dilution with ethanol is shown in Figure S1A. The volume of the AgNDs in chloroformethanol mixture used for the LB monolayer fabrication was adjusted to match the extinction intensity of the LSPR spectrum of the 20-fold diluted AgND solution. Overgrowth was conducted as follows: the AgND monolayers on the quartz substrates were immersed in 200 mL 10µM aqueous solution of NaCl for 30 min. The samples were then removed and a mixture of 4 mL 0.2 mM PVP (MW 55,000) and 0.4 mL AgNO3 6 mM was added to the solution. The AgND monolayers were immersed in the resulting solution for 5 minutes. The overgrowth reaction was initiated by adding 20 mL AA with concentrations of 1.56, 7.8, 31.2, or 78 mM to the overgrowth solution during which the samples were again removed from the solution. Finally, samples were immersed in the resulting solution for 4 hours. After the overgrowth was complete, samples were immersed in 500 mL DI water for 15 min to clean them from by-products. The samples were then removed from the DI water and left to dry. A similar overgrowth experiment was repeated by replacing the AA with 20 mL 15 mM hydroquinone (HQ, Sigma Aldrich). The overgrowth of AgND monolayers on quartz substrates in the absence of chloride ions was conducted by repeating the same procedure of the overgrowth experiment using 20 mL 7.8 mM AA. A JEOL 100C transmission electron microscope (TEM) was used to characterize the colloidal AgNDs (see TEM image in Figure S1B). Statistical analysis of 200 particles using imageJ showed that the AgNDs have a diameter of 26.8±6.1 nm and a thickness of 8.3±1.1 nm.



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A Zeiss Ultra60 scanning electron microscope (SEM) was used to image the samples by sputtering a thin (~5 nm) film of gold on top of the samples in oxygen. A DI (Digital Instruments) Dimension-3000 atomic force microscope (AFM) was used for topography and height characterization of the 2D nanoparticle arrays. Scans were taken in tapping mode using a silicon tip with a resonant frequency of 120 kHz, and samples were scanned at 1 Hz with a resolution of 512 lines and 512 pixels/line. Optical measurements were conducted with an Ocean Optics HR4000Cg-UV-NIR. For optical measurement at different tilting angles, the fiber optic guide of the spectrometer was fixed while the substrate was tilted. RESULTS AND DISCUSSION LB Assembly of Colloidal Silver Nanodisks on the Surface of Different Substrates LB assembly of colloidally prepared plasmonic nanoparticles into 2D arrays with controlled order improves their applications.25 Fabrication of the nanoparticle arrays by the LB technique involves the formation of a monolayer from the nanoparticles on the water-air interface which is then transferred to the surface of a substrate.42,43 Two critical issues should be addressed for LB assembly: whether changing the substrate has an effect on the order and the coverage of the nanoparticle arrays and whether nanoparticles are embedded on the surface of the water sub-layer or are surrounded by air more than water. AgNDs have a shape that is well suited in studying the distribution of the nanoparticles on the surface of the trough and after transfer to a substrate due to the following characteristics. First, their LSPR spectrum is narrow so it is possible to visually examine the arrays for a change in color, which would indicate a change of the dielectric function of the surrounding medium. Second, the disk shape has the highest possible surface area that can be exposed to a substrate compared with other shapes of similar volume. For instance, ~ 32% of the surface area of an AgND with a diameter of 26.8 nm


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and a height of 8.3 nm is exposed to the substrate, which suggests that the substrate or water sub-layer should have a large influence on their assembly and optical properties. Figure 1A depicts an AgND monolayer on the surface of an LB water sub-layer. The red color of the AgND colloidal solution in water (Figure 1B) changes to pale yellow when organized into a monolayer on the top of the trough. This drastic change in color indicates a large decrease of the dielectric function of the surrounding medium, which cannot be obtained unless the AgNDs on the top of the LB trough are exposed to air rather than to a water medium. This suggests that the surface tension of the water sub-layer is stronger than the attraction force between the AgND and water. When the AgND films were transferred from the LB trough to quartz substrates, the color changed to a darker yellow that resulted from an increase of the refractive index of the substrate (1.46 for quartz compared to 1.33 for water) (Figure 1C). Figure S2 shows the LB isotherm of the 26.8±6.1 nm diameter AgND monolayer. As in the case of most regular isotherms, three different phases are observed: the gaseous phase obtained at relative surface pressures below 0.2 mN/m, the liquid condensed phase located at surface pressures between 0.2 and 3.7 mN/m, and the solid phase at surface pressures higher than 3.7 mN/m. The effect of changing the substrate on the order of the AgNDs inside their 2D arrays was studied by transferring the monolayers to quartz and silicon substrates simultaneously. Figure 1D and E show the SEM image of AgND monolayers assembled on the surface of silicon and quartz substrates respectively. The SEM images of AgND arrays on the surface of quartz and silicon substrates are similar, confirming that the substrate has no influence on the nanoparticle distribution inside the LB film. AFM images of an AgND monolayer deposited on quartz are shown in Figure 1F and Figure 1G. Based on the AFM images, the average height of the monolayer is 12.0±1.8 nm. The difference between the 8.3±1.1 nm thickness of the as prepared



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AgNDs determined from the TEM and the AFM-determined height of the AgNDs can be attributed to the roughness of the quartz substrate and the presence of the PVP capping layer, which can be present under the AgNDs after monolayer assembly. B

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Figure 1 A) Photograph of an LB trough coated with a 26.8 nm diameter AgND monolayer. B) Photograph of colloidal AgNDs dispersed in water. C) Photograph of an AgND monolayer fabricated at an LB surface pressure of 0.1 mN/m on the surface of a 1 in. x 4 in. quartz substrate. SEM image of an AgND monolayer transferred by vertical dipping at a surface pressure of 0.1 mN/m to the surface of D) a silicon substrate, E) a quartz substrate. F) AFM image of an AgND monolayer on a quartz substrate. G) Topographical AFM image of the AgND monolayer on the quartz substrate.


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Asymmetric Overgrowth of AgNDs Induced by Quartz Substrates and Cl- Ions The LB technique can be used to assemble colloidal nanoparticles of isotropic and anisotropic shapes into 2D arrays of different structures different substrates, although assembly of 1D anisotropic nanoparticles such as rods or wires in a vertical manner is challenging.25 However, anisotropic nanoparticles can also be obtained from asymmetric overgrowth of nanocrystals, which can be chemically induced by using foreign ions.44, With this goal in mind, AgND monolayers on quartz substrates were overgrown with different growth conditions. Figures 2A-C respectively show the SEM, AFM, and topographical AFM images of AgNDs on quartz after overgrowth using 7.8 mM AA. The circular AgNDs became hexagonal after overgrowth, and the diameter and the thickness were increased to 55.5±5.9 nm and 23.6±4.2 nm respectively. This suggests that silver atoms deposited on both the sides and the top face of the AgND during overgrowth. Comparatively, the overgrowth of colloidal AgNDs in solution leads to silver atoms depositing only on faces, resulting in truncated right bipyramids.41 AgNDs on quartz were also overgrown in the presence of Cl- at different concentrations of AA. Figure 2D-F show the SEM and AFM images of AgNDs on quartz after overgrowth using 7.8 mM AA. Silver nanopillars (AgNPLs) with a diameter of 32.5±4.8 nm and a height of 67.8±6.9 nm were formed. The SEM and AFM images of the resulting AgNPLs when using 78 mM AA in the presence of Cl- are shown in Figure 3A-C. Shorter AgNPLs with a 43.9±7.2 nm diameter and a 36.2±4.1 nm height were obtained. Lowering the concentration of the AA used during the overgrowth of AgNDs in the presence of Cl- ions led to an increase in height and a decrease in diameter of the AgNPLs. The rate of overgrowth of the AgNDs on quartz in the presence of Cl- was decreased by lowering the concentration of AA. In summary, the overgrowth technique presented in this study was used to fabricate vertically aligned, well-separated AgNPLs over a large area on



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quartz substrates (see Figures S3A, B, and C for lower magnification SEM images of nanoparticles shown in Figures 2A and D, and Figure 3A respectively). Figure S4 shows topographical cross sections along the center of a hexagonal AgND obtained from the overgrowth of AgNDs in the absence of Cl- and AgNPLs resulting from the overgrowth of AgNDs using 7.8 and 78 mM AA. A

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Figure 2 A) SEM image, B) AFM image, and C) topographical AFM image of hexagonal AgNDs overgrown using 7.8 mM AA. D) SEM image, E) AFM image, and F) topographical AFM image of tall AgNPLs overgrown using 7.8 mM AA in the presence of Cl- ions. A

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Figure 3 A) SEM image, B) AFM image, and C) topographical AFM image of short AgNPLs overgrown using 78 mM AA in the presence of Cl- ions.

Mechanistic Study of the Asymmetric Overgrowth of the AgNDs To examine the mechanisms behind the AgND overgrowth observations, thermodynamic pathways are first considered. The AgND sides have a higher surface energy than that of the top flat face due to three primary causes. First, the top faces of the AgNDs are bounded by a low surface energy {111} facet (0.553 eV) while the sides are bounded with, in addition to the {111} facet, a slightly more energetic {100} facet (0.653 eV).45-49 Second, a high energy twinning defect exists in the sides of the AgNDs as a result of a stacking fault generated during formation. Lastly, the side walls of the AgNDs are convex in nature in contrast to the flat top face, and in



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accordance with the Gibbs-Thomson effect, the convex surface has a higher surface energy than the flat one. Based on the conducted experiments, the overgrowth mechanism of AgNDs into AgNPLs can be summarized in the schematic in Figure 4. The overgrowth process involves silver atom generation resulting from the reduction of silver ions by AA followed by the deposition of silver atoms on the surface of the AgNDs. Based on the surface energies of the {111} and {100} facets, thermodynamic growth results in silver atoms being preferentially deposited on the sides of the AgNDs, thereby increasing the surface area of the top face bounded with the low energy {111} facet. Indeed, AgNDs on quartz overgrew into bigger hexagonal nanodisks with 29 and 12 nm increases in diameter and thickness respectively, demonstrating a preference for lateral rather than vertical growth. In this case, vertical growth may also be influenced by the presence of a polymer capping agent. During AgND synthesis, PVP is used as a capping agent, which selectively binds to the {100} facet located on the sides and stabilizes the prepared nanodisks.41 The {100} facets should theoretically be completely covered with PVP chains, although multiple washing steps of the prepared AgNDs could detach some PVP molecules. The adsorbed PVP molecules remaining on the sides of the overgrown AgNDs may lead to increased deposition of silver atoms on the top face. The effect of the PVP capping agent presence on the overgrowth of the AgNDs is discussed further in detail later. Next, the effects of the Cl- ions on the overgrowth process were considered. Halide ions such as Cl-, Br-, or I- chemisorb specifically on the {100} facets of colloidal silver nanoplates, inducing the deposition of silver atoms on the {111} facets.46,47 The binding strength of the halide ions to the Ag surface is increased in the order of Cl-900 nm. Chem. Commun. 2009, 7170-7172. (47) Kim, M. H.; Kwak, S. K.; Im, S. H.; Lee, J.-B.; Choi, K.-Y.; Byun, D.-J. Maneuvering the growth of Silver Nanoplates: use of Halide ions to Promote Vertical Growth. J. Mater. Chem. C 2014, 2, 6165-6170. (48) An, J.; Tang, B.; Zheng, X.; Zhou, J.; Dong, F.; Xu, S.; Wang, Y.; Zhao, B.; Xu, W. Sculpturing Effect of Chloride Ions in Shape Transformation from Triangular to Discal Silver Nanoplates. J. Phys. Chem. C 2008, 112, 15176-15182. (49) Tang, B.; Xu, S.; An, J.; Zhao, B.; Xu, W.; Lombardi, J. R. Kinetic Effects of Halide Ions on the Morphological Evolution of Silver Nanoplates. Phys. Chem. Chem. Phys. 2009, 11, 10286-10292. (50) Sulek, M. W.; Sas, W.; Wasilewski, T.; Bak-Sowinska, A.; Piotrowska, U. Polymers (Polyvinylpyrrolidones) As Active Additives Modifying the Lubricating Properties of Water. Ind. Eng. Chem. Res. 2012, 51, 14700-14707. (51) Mahmoud, M. A. Polarized Optomechanical Response of Silver Nanodisc Monolayers on an Elastic Substrate Induced by Stretching. J. Phys. Chem. C 2015, 119, 19359-19366.



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(52) O’Brien, M. N.; Jones, M. R.; Kohlstedt, K. L.; Schatz, G. C.; Mirkin, C. A. Uniform Circular Disks With Synthetically Tailorable Diameters: Two-Dimensional Nanoparticles for Plasmonics. Nano Lett. 2015, 15, 1012-1017. (53) Huang, Y.; Kim, D.-H. Dark-Field Microscopy studies of Polarization-Dependent Plasmonic Resonance of Single Gold Nanorods: Rainbow Nanoparticles. Nanoscale 2011, 3, 3228-3232. (54) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Spectroelectrochemistry of Colloidal Silver. Langmuir 1997, 13, 1773-1782.


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