Silver Nanodisk Monolayers with Surface Coverage Gradients for Use

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Silver Nanodisk Monolayers with Surface Coverage Gradients for Use as Optical Rulers and Protractors Mahmoud A. Mahmoud* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *

ABSTRACT: Colloidal silver nanodisks (AgNDs) are assembled into a monolayer with a coverage density gradient (CDG) on the surface of flat and cylindrical substrates using the Langmuir−Blodgett (LB) technique. Compressing the LB monolayers during transfer to the substrates causes the CDG assembly of the AgNDs. By functionalizing the AgNDs with poly(ethylene glycol), it is possible to control their order inside the LB monolayer assembly by changing the deposition surface pressure. Well-separated AgNDs, 2D aggregates with different numbers of particles, and highly packed 2D arrays are formed as the deposition surface pressure is increased. Localized surface plasmon resonance (LSPR) spectra collected at different separation distances from the highest coverage spot (HCS) of the CDG AgND arrays on a flat substrate are blue-shifted, and the shift increases systematically upon increasing the distance. The relationship among the LSPR peak position, the peak intensity at a fixed wavelength, and the corresponding separation distance from the HCS is fitted exponentially. A similar systematic blue shift in the LSPR spectrum of the CDG AgND monolayer on a cylindrical substrate is obtained when the substrate is rotated at different angles relative to the HCS. The fabricated CDG AgND monolayers can potentially be used for optically measuring distances and angles.

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INTRODUCTION Exciting plasmonic nanoparticles with resonance-frequency light leads to strong absorption and scattering of photons and the generation of a strong electromagnetic field.1−4 The localized surface plasmon resonance (LSPR) extinction spectral peak position of a plasmonic nanoparticle can be shifted either by changing the dielectric function of the surrounding medium or by decreasing the separation distance between multiple plasmonic particles that can couple together.5,6 These exciting properties of plasmonic nanoparticles have made them useful in many technological applications such as optical filters,7 optical switches,8−11 optical detectors,12 optical sensors,13−16 polarizers,7 wave guides,17 solar cells,18,19 and photonic crystals.12 Many of the optical applications of plasmonic nanoparticles are based on their optical response to different media. For instance, photochromic materials20−22 can alter their optical properties when exposed to UV irradiation, whereas the refractive index of electrochromic materials8,23,24 can be reversibly changed by applying an electrical potential. Coating the surface of plasmonic nanoparticles with photochromic materials20−22 or electrochromic materials8,23,24 can therefore support the reversible modulation of LSPR resonances. Other optical applications of plasmonic nanoparticles depend on changing the interparticle separation distances. The interparticle separation distance between the nanoparticles can be tuned reversibly by binding the nanoparticles to the surface of an elastic substrate,25 whereas an irreversible change in the interparticle separation distance can be realized by functionaliz© XXXX American Chemical Society

ing the nanoparticle surfaces with a reactive linker such as DNA5 or polymer.26 The different optical response of plasmonic nanoparticles induced by the change in the interparticle separation distance was used to measure changes in distance down to the nanoscale. Commonly used techniques for fabricating plasmonic nanoparticles with tailored shapes and sizes on the surface of a substrate include lithographic techniques such as electron beam lithography,27−30 nanosphere lithography,31 and optical lithography.32 The electrochemical deposition of nanoparticles on substrates, such as the template-mediated technique,33−35 and physical techniques, such as mask vapor deposition,36 were used to fabricate large-surface-area nanoparticle arrays compared to the lithographic techniques. To lower the production cost of optical devices that are based on plasmonic nanomaterials, the nanoparticles were prepared by chemical techniques37−41 and assembled into organized structures on substrates.42−46 Arrays from colloidally prepared nanoparticles were fabricated on large-area surfaces by the Langmuir− Blodgett technique (LB).42−46 The arrangement of the nanoparticles inside their arrays was controlled by changing the applied LB surface pressure47 or by changing the capping materials bound to the surface of the nanoparticles.11,44 Received: August 30, 2016 Revised: September 27, 2016

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dispersed in chloroform−ethanol was sprayed over the water sublayer of the trough using a microsyringe and left to dry for 30 min. Monolayers were then transferred to transparent tape substrates (2.54 cm high × 5 cm wide) adhered to silicon substrates (3 cm × 7 cm) using the vertical dipping technique. Substrates were dipped into the LB subphase at a rate of 66 mm/min. The substrates were then raised at a rate of 0.3 mm/min while the barrier of the trough was closed at a rate of 80 mm/min. LB assembly from AgNDs of of 50, 38, and 25 nm diameter were fabricated on the surface of the silicon− transparent tape substrate. The 50, 38, and 25 nm Ag ND CDGs on tape were then removed from the silicon substrates and adhered to the planar glass substrates (5 cm × 5 cm) or cylindrical glass substrates (2.3 cm diameter) in an end-to-end manner. The AgND monolayer on the silicon substrate was imaged at different spots using a Zeiss Ultra 60 scanning electronic microscope (SEM). Optical measurements were conducted using an Ocean Optics HR4000Cg-UV−NIR. The flat substrates coated with the monolayers were mounted to a translation stage to collect the measurements from different spots. The cylindrical substrate was mounted to a rotating stage, and the light excitation source was reflected through the inside wall of the substrate by a mirror fixed at its center. Transmitted light was then collected through the substrate and the AgND monolayer by the spectrometer. Discrete dipole approximation (DDA) simulations for the LSPR spectrum of an AgND with a 50 nm diameter and 7 nm thickness as well as its dimer, trimer, tetramer, and pentamer aggregates with a 7 nm interparticle distance were carried out using DDSCAT 7.3 software. The shape file used for the calculation has a dipole density of 1 dipole per 1 nm3, the dielectric function of the surroundings was taken as a mixture of the air−PEG mixture refractive index, and the separation gap between the substrate and the AgNDs was modeled as 3 nm to account for the polymer bound to the nanodisks.

The aim of this study is to fabricate colloidal silver nanodisk (AgND) monolayers with a coverage density gradient (CDG) on the surface of flat and cylindrical transparent substrates using the LB technique. AgNDs were first functionalized with poly(ethylene glycol) (PEG) to improve their assembly order and behavior. The LB film of the AgNDs was then prepared on the surface of a water sublayer of the LB trough prior to the simultaneous compression and transfer to a transparent tape substrate. The coverage density of the AgNDs on the substrate gradually increased, and aggregates composed of an increasing number of AgNDs were formed as the LB surface pressure was increased. Tape coated with the AgND CDG was then adhered to flat and cylindrical glass substrates. The LSPR spectra of the AgND CDG monolayer on the flat substrate were collected from spots separated by different distances from the highestcoverage spot (HCS). The optical measurements were also carried out for the AgND CDG monolayer on cylindrical substrates at different relative rotational angles starting from the HCS, which is designated as 0°. The LSPR spectrum of the AgNDs blue-shifted as the distance from the HCS or the rotational angle from the HCS was increased. Following the shift in the LSPR spectrum of the AgND CDG at different distances from the HCS for the flat substrate or at different angles for the cylindrical substrate will be useful for optically measuring changes in distance or rotational angle. Plasmonic nanoparticle CDGs can also potentially be used as wavelength selectors.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION LB Assembly of Colloidal AgNDs into a CDG Monolayer. Colloidal AgNDs were assembled into 2D arrays on the surfaces of different substrates at different coverage densities using the LB technique.43,49 Monolayer assemblies from the nanoparticles were first prepared on the surface of a liquid sublayer of the LB trough prior to being transferred to substrates at fixed LB surface pressures.43 In most LB assembly experiments, the surface pressure along with the coverage density of the substrate by the nanoparticles can be increased by decreasing the available surface area of the LB trough that the nanoparticles are occupying.47 LB films of PEG-functionalized AgNDs with diameters of 25, 38, and 50 nm were transferred simultaneously to the surface of silicon and transparent tape substrates by the vertical dipping technique while the monolayer was being compressed. The fabrication of the AgND monolayer with a CDG on the surface of flat and cylindrical transparent substrates is depicted in Figure 1. Figure 2A shows an SEM image of the HCS of the 50 nm AgNDs, which was transferred to the surface of the silicon substrate at the highest surface pressure. Figure S2 shows an SEM image of the AgND monolayer at a 0.2 cm separation distance from the HCS, and Figure 2B−F shows SEM images of the 50 nm AgND CDG at respective separation distances of 0.4, 0.6, 0.8, 1, and 1.2 cm from the HCS. The AgND assembly at the HCS and at a point 0.2 cm from the HCS are highly packed, with coverage densities estimated by ImageJ of 71 and 61%, respectively. AgND aggregates are not observed, and the AgNDs are separated with average interparticle separations of 10 and 12 nm for the 0 and 0.2 cm locations, respectively (Figures 2A and S2). Conversely, AgNDs of between 0.4 and 1 cm away from the HCS form separated aggregates composed of a varying numbers of AgNDs in addition to single AgNDs (Figure 2B−E). Interestingly, because of the presence of PEG

Silver nanodisks (AgNDs) with different diameters were prepared using the simultaneous asymmetry reduction technique reported earlier.48 Briefly, to a 200 mL poly(vinylpyrrolidone) (MW = 55 000) (PVP, Sigma-Aldrich) aqueous solution (0.145 mM) in a 500 mL glass bottle, 0.60 mL of a 60 mM AgNO3 (Sigma-Aldrich) aqueous solution was added, followed by 1, 3, or 5 mL of 78.35 mM L-ascorbic acid (Sigma-Aldrich). The reaction solution was gently shaken as 0.12 mL of 5 mM NaBH4 (Sigma-Aldrich) was added, and the shaking continued for a few seconds. The AgND solution was left in the dark for 5 min before cleaning by centrifugation for 30 min at 12 000 rpm. The resulting pelleted AgNDs were dispersed in DI water, and the solution was recentrifuged at 10 000 rpm for 30 min. The precipitated AgNDs were dispersed in 30 mL of DI water, and 0.2 mL of an aqueous solution of thiolated poly(ethylene glycol) (PEG, Laysan Bio) with an average molecular weight of 2000 (MW) and a concentration of 5 mM was added. To functionalize PEG with the surface of the AgNDs, the resulting mixture was shaken for 10 h. Excess PEG was removed from AgNDs by centrifugation at 8000 rpm for 30 min, and the precipitate was then dispersed in 10 mL of ethanol (SigmaAldrich). Finally, the solution of AgNDs in ethanol was centrifuged at 6000 rpm for 20 min, and the precipitated AgNDs were dispersed in 2 mL of ethanol followed by adding 4 mL of chloroform (SigmaAldrich). Ethanol increases the dispersion of the AgNDs in chloroform because AgNDs are highly dispersed in ethanol and ethanol is highly miscible with chloroform. The LSPR spectrum of the AgNDs in the ethanol−chloroform mixture did not change when optically measured during a 2 month time period, confirming the stability of the solution. Transmission electron microscope (TEM) imaging was carried out for the AgNDs using a JEOL 100C. Figure S1 shows TEM images of AgNDs with different diameters. Statistical analysis using ImageJ software for 350 particles collected from 3 different TEM images showed that the average diameters of the AgND were 49.6 ± 5.8, 37.8 ± 5.3, and 25.3 ± 6.9 nm. The LSPR spectra of the colloidal AgNDs dispersed in water are shown in Figure S1E. The CDG LB assembly experiment was carried out using a Nima 611D LB trough filled with DI water, and a D1L-75 pressure sensor was used to measure the surface pressure. Initially, 2 mL of AgNDs B

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represent 27% of the separated units, and single AgNDs account for 14%. At 0.8 cm from the HCS, 50% of the separated units are AgNDs dimers and 36% are AgNDs monomers. At 1.0 cm, the single particles account for 80% of the separated units whereas 17% are present as dimers. At 1.2 cm away, well-separated arrays are obtained, with a coverage density of 14% and an interparticle separation distance of 49 nm. The SEM imaging and the statistical analysis suggest that CDG AgND monolayers are successfully formed when the surface area of the LB trough occupied by the nanoparticles is decreased gradually with the simultaneous transfer of the particles to substrates. Furthermore, the uncompressed and highly compressed LB monolayers of PEG-functionalized AgNDs are composed of separated AgND arrays with respectively large and small interparticle separation distances, whereas intermediate compression of the AgND monolayer leads to the formation of separated aggregates. The LB isotherm displays the change in the surface pressure of a nanoparticle LB film upon decreasing the surface area that the nanoparticles are occupying.44 The attraction and repulsion forces between the nanoparticles control the surface pressure upon compressing the LB monolayer.44 Studying the isotherm of the LB film provides useful information about the structural change in the AgND monolayers upon compression. Figure 3A

Figure 1. Schematic depiction of the fabrication of an AgND monolayer with a CDG on the surface of flat and cylindrical transparent substrates using the LB technique.

Figure 3. (A) Langmuir−Blodgett isotherm of PEG-functionalized AgNDs with diameters of 50 nm (black), 38 nm (red), and 25 nm (blue). (B) Photograph of a glass substrate coated with a CDG of AgNDs with diameters of 50 nm (bottom zone), 38 nm (middle zone), and 25 nm (top zone). (C) Schematic for an AgND monolayer CDG on a substrate. Figure 2. SEM image of a 50 nm AgND monolayer prepared by simultaneous compression and transfer of the LB film to a substrate: (A) SEM image of the HCS and SEM images of points located a distance from the highest coverage spot of (B) 0.4, (C) 0.6, (D) 0.8, (E) 1.0, and (F) 1.2 cm.

shows the LB isotherm of AgNDs with diameters of 25, 38, and 50 nm after functionalization with PEG. Gaseous, liquid condensed, and solid phases are apparent in the LB isotherms. The surface pressure of the phase transition from the liquid condensed to the solid phase was induced at lower values as the diameter of the AgNDs increased. In the liquid condensed phase region, the change in surface pressure also increased as the diameter of the AgNDs was increased. The lowering of the surface pressure that induces the liquid condensed−solid phase transformation indicates an increase in the attraction forces between the AgNDs as their diameter is increased. Correlating the SEM images with the LB isotherm, the spots imaged 0, 0.2, 0.4, 0.6, 0.8, 1, and 1.2 cm away from the HCS correspond to available surface areas occupied by the 50 nm AgNDs of 65, 117.5, 170, 222.5, 275, 327.5, and 380 cm2, respectively. This correlation is carried out by considering the HCS “0 cm” in the CDG as corresponding to the smallest available surface area of the LB trough of 65 cm2, whereas the

on the surface of the nanodisks, the interparticle separation distance between the AgNDs forming the aggregates is fixed at roughly 7 nm, independent of the number of particles per aggregate. The coverage density was found to decrease to 30, 26, 24, and 20% when measured at spots separated from the HCS by respective increasing distances of 0.4, 0.6, 0.8, and 1 cm. The statistical analysis of the number of nanoparticles per aggregate is shown in Figure S3. For the 0.4 cm spot, only 8% of the separated units are single particles, while 60% are separated large aggregates composed of more than four AgNDs. On the 0.6 cm spot, 59% of the separated units are AgNDs dimers and trimers, aggregates of a larger number of AgNDs C

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S4C,D shows SEM images collected from the 3.2 and 4.8 cm spots for the 25 nm AgND CDG. The arrangement of the AgNDs with diameters of 38 and 25 nm inside their solid and gaseous phases is similar to that of the AgNDs with a 50 nm diameter. Shape analysis of the AgNDs was carried out using the SEM images as shown in Figure S5. Three different shapes are observed: circular, triangular, and distorted (any other shape). Although the shape distribution of the 50 nm AgNDs is not as great as that of the 38 and 25 nm AgNDs, the LSPR spectrum still remains narrow. Figure 3B depicts the glass substrate after sticking three different coated tapes with CDG monolayers of AgNDs with diameters of 50, 38, and 25 nm. Three different zones are clearly observed, corresponding to each AgND diameter, and the color change and color density are decreased in each zone from bottom to top, corresponding to a decrease in the deposition surface pressure. Figure 3C shows a schematic depiction of the substrate coated with an AgND monolayer CDG. Optical Ruler Based on an AgND Monolayer CDG. The electromagnetic field of plasmonic nanoparticles can couple strongly with that of neighboring particles in close proximity.51 This plasmon field coupling leads to a shift in the LSPR spectral peak to an energy lower than that of the individual nanoparticle.52 The strength of the overall electromagnetic field resulting from the plasmon field coupling in a collection of nanoparticles and the wavelength shift of the LSPR spectrum depend on both the number of particles and the interparticle separation distance between them.27,53,54 Interestingly, the CDG monolayer of AgNDs offers the possibility to tune both the number of particles per aggregate and the interparticle separation distance between the AgNDs. AgNDs are 2D anisotropic plasmonic nanoparticles characterized by a strong LSPR spectral peak resulting from the electron oscillation along the plane of the nanodisk (in-plane) and a very weak out-ofplane plasmon mode. AgNDs were chosen for the current study for the following reasons: AgNDs have a narrow LSPR spectral peak compared to that of other nanoparticles as a result of the similarity of the width-to-height aspect ratio, the sensitivity of the AgNDs to the change in the separation distance is high because of strong plasmon field confinement along the sides, and the LSPR spectra of the AgNDs can be easily tuned in the visible and NIR regions by increasing the AgND diameter. Figure 4A shows the LSPR spectrum of a 50 nm AgND monolayer CDG collected at separation distances from the HCS of 0, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 cm. Increasing the distance from the HCS results in a blue shift of the LSPR spectral peak followed by a decrease in the peak intensity. Figure 4B shows the relationship between the LSPR peak wavelength positions collected from different spots on the 50 nm AgND monolayer CDG on a flat glass substrate and the corresponding separation distance from the HCS. The LSPR peak position−distance relationship is fitted to an exponential function with a decay constant of 0.76 ± 0.09 cm (eq 1). The relationship between the peak intensity of the LSPR spectrum of the 50 nm AgND CDG at the 720 nm wavelength and the separation distance from the HCS is shown in Figure 4C. The LSPR peak intensity is found to decrease by increasing the distance from the HCS in an exponential manner with a decay constant of 1.2 ± 0.1 cm (eq 2). The decrease in the LSPR intensity at a single wavelength is attributed to the decrease in the coverage density and the blue shift in the LSPR peak position as the distance from the HCS is increased. This

spot at 1.2 matches the largest available area of the trough of 380 cm2 (Figure 3A). Because the speeds of the barrier and the dipper of the LB trough were fixed, the intermediate points can be divided equally between these two extremes. Using the LB isotherm to correlate available surface areas with surface pressures, the spots in the CDG imaged at 0, 0.2, 0.4, 0.6, 0.8, 1, and 1.2 cm were transferred to the surface of the substrate at respective surface pressures of 6.5, 3.5, 2.5, 1.4, 0.65, 0.2, and 0 mN/m. These deductions suggest that the area between the HCS and 0.2 cm of the AgND monolayer is transferred to the substrate from the solid phase of the monolayer whereas the area between 0.4 and 1 cm is transferred from the liquid condensed phase of the LB film. Areas of the AgND monolayer located at separation distances larger than 1 cm are transferred to the substrate from the gas phase. As-prepared AgNDs are capped with PVP; therefore, aggregates were not obtained upon compressing the LB films.25 However, the interparticle separation distance between the AgNDs capped with PVP did decrease upon compressing the LB film without changing the order of the AgNDs.50 This is due to the repulsion force between the PVP molecules bound to the sides of the AgNDs preventing their aggregation. Conversely, after functionalizing the AgNDs with PEG, aggregates of different numbers of AgNDs are formed depending on the LB applied surface pressure. Unlike PVP, which is located on the side of the AgNDs, PEG chains binds through the thiol group to both the faces and the sides of the AgNDs. When the LB films of the PEG-functionalized AgNDs were compressed, the aggregates in the liquid condensed phase were found to have an increased number of particles per aggregate as the surface pressure was increased. Second, an unexpected increase in the interparticle separation distance between the nanodisks from ∼7 nm in the liquid-condensed phase aggregates to ∼12 nm in the solid phase 2D AgND arrays occurred. It was reported recently that gold nanocubes functionalized with 2K PEG were ordered into highly packed arrays inside their LB film with an interparticle separation distance of ∼4 nm.44 The distance between the gold nanocubes remained constant after compressing their LB film, but the width of the nanocube arrays was increased by increasing the applied surface pressure. The width of the gold nanocube arrays, the number of particles per AgND aggregate, and the interparticle separation distance between either gold nanocubes or AgNDs are all varied to minimize the internal energy of the LB film. The internal energy of the LB film is controlled by both the attraction and repulsion forces between the nanoparticles and the PEG chains bound to their surface.44 These forces greatly depend on the geometry of the nanoparticles, which explains the larger observed separation distance between the AgNCs of ∼7 nm compared to ∼4 nm obtained for the gold nanocube arrays. As the LB film of the AgNDs is compressed, the mutual attraction force between different aggregates leads to an increase in the interparticle separation distance between the AgNDs from ∼7 to ∼12 nm in order to minimize the strain in the LB film and lower the AgND-PEG system internal energy. The strain resulting from the compression of the LB film can also be lowered by other means as in the case of the highly packed gold nanocube arrays functionalized with 2K PEG, which upon compression cracked into smaller arrays.44 SEM images of the CDG composed of 38 nm AgNDs for the 1.4 and 3 cm spots are shown in Figure S4A,B, whereas Figure D

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of particles per aggregate. The LSPR spectra of single, dimer, trimer, tetramer, pentamer, and hexamer 50 nm AgNDs separated by 7 nm were calculated using the discrete dipole approximation (DDA) technique. The calculated LSPR spectrum red shifts and its spectral width is increased by increasing the number of AgNDs as shown in Figure 4D. As observed from the TEM images in Figure 2, the AgND pentamers and hexamers were organized into different structures, and DDA calculations were carried out for the three pentamer and four hexamer configurations (Figure S7). Although the calculated LSPR spectral shapes of the AgND pentamer and hexamer greatly depend on the specific configuration of the nanodisks, the spectra are centered at ∼740 and 750 nm for the pentamer and hexamer, respectively. For the 0.4 cm spot, 60% of the separated clusters of particles are large aggregates composed of more than five AgNDs. The broad, low-energy calculated LSPR spectrum of the large aggregates confirms the broad LSPR spectrum measured at the 0.4 cm spot. For a spot 0.6 cm away, 59% of the separated AgND units are dimers and trimers, which have narrower and higher-energy LSPRs than do larger aggregates. However, the presence of larger aggregates (27%) causes the experimental spectrum at 0.6 cm away to appear slightly broader. The narrow LSPR spectrum 0.8 cm away is attributed to the presence of 50% AgND dimers and 36% single AgNDs. Both the AgND single particle and dimer have a narrow LSPR spectrum. Similarly, the sharper LSPR spectrum measured 1.0 cm away from the HCS is based on 80% single AgNDs and 17% dimers. The LSPR spectrum measured at the 1.2 cm spot where the AgNDs are well-separated is found to be sharp. The advantages of the controlled aggregation of the PEGfunctionalized AgNDs compared to the ordered 2D AgND arrays obtained for the LB assembly of AgNDs capped with poly(vinylpyrrolidone) are 2-fold.25 First, the controlled aggregation of AgNDs eliminates the delocalized, far-field− near field coupling that can take place for highly ordered AgND arrays. The plasmon field coupling of the near field of each AgND with the fields of other AgNDs inside the arrays causes broadening of the LSPR spectrum and retardation of the plasmonic energy. Second, the interparticle separation distance between the AgNDs inside the aggregates is small, which increases the strength of the plasmon field coupling. Optical measurements were also conducted for CDG monolayers composed of the 38 and 25 nm AgNDs on glass (Figure 2B). Figure S6A,B shows the LSPR spectra of the CDG monolayer composed of AgNDs with diameters of 38 and 25 nm measured at different separation distances from the HCS of the 50 nm AgND CDG (Figure 2B). The first measured spot on the 38 nm AgND monolayer is separated by 1.4 cm from the 50 nm AgND HCS. The LSPR spectrum of the 38 nm AgND CDG blue shifts from 624 to 565 nm when measured at different spots between 1.4 and 3 cm. The blue shift of the LSPR spectrum increases by increasing the separation distance between the measured spot and the HCS in an exponential relationship with a 1.67 ± 0.19 cm decay constant, as shown in Figure 4B. Figure 4C depicts the relationship between the peak intensity of the LSPR spectrum of the CDG 38 nm AgNDs at 623 nm and the corresponding separation distance from the HCS. The distance-dependent peak intensity relationships are fitted exponentially with a decay constant of 2.38 ± 0.81 cm. The relationship between the LSPR peak position of the 25 nm AgND monolayer CDG and the corresponding separation distance from the HCS is also exponential with a decay

Figure 4. (A) LSPR spectra of a 50 nm AgND monolayer CDG on a flat glass substrate measured at different distances from the HCS. (B) The LSPR peak position of a monolayer CDG of AgNDs with diameters of 50 nm (black), 38 nm (red), and 25 nm (blue) plotted against the corresponding separation distances from the HCS. (C) The LSPR peak intensity for the AgND CDG with diameters of 50 nm (black) calculated at 720 nm, 38 nm (red) calculated at 623 nm, and 25 nm (blue) calculated at 599 nm versus the corresponding distance from the HCS. Both the LSPR peak position−distance and peak intensity−distance curves are fitted to an exponential relationship. (D) Simulated DDA LSPR spectra of an AgND single particle, dimer, trimer, tetramer, pentamer, and hexamer.

observation makes it possible to optically measure changes in distance by following the change in the intensity of the LSPR peak of the AgNDs at a single wavelength. LSPR peak position = 638 + 81e(distance from HCS/0.76)

(1)

LSPR peak intensity = 0.3 + 1.4e(distance from HCS/1.2)

(2)

The shift in the LSPR spectral peak of the AgND monolayer CDG when measured at different separation distances from the HCS can be examined more clearly by matching the optical spectrum with the corresponding SEM image. The LSPR spectra measured at the 0 and 0.2 cm spots corresponding to highly packed AgND arrays with average interparticle separation distances of 10 and 12 nm are intense and slightly broad. The small separation distances between the AgNDs strongly supports the coupling of the plasmon field of each particle with the fields of other nanoparticles inside the array (delocalized plasmon field coupling).55 The observed blue shift of the LSPR upon moving from the HCS to a spot 0.2 cm away results from an increase in the interparticle separation distance between the AgNDs inside the arrays. The area between the 0.4 and 1 cm spots on the CDG is composed of aggregates of different sizes and comparable separation distances of approximately 7 nm (Figures 2B−E and S3), with the plasmon field localized inside each aggregate. From the results, it was observed that the LSPR spectrum blue shifts and its width decreases upon increasing the separation distance from the HCS, which was also accompanied by a decrease in the number E

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Langmuir constant of 3.63 ± 0.91 cm as shown in Figure 4B. An exponential curve with a decay constant of 4.22 ± 1.90 cm is obtained when the LSPR peak intensity at 499 nm is plotted against the separation distance from the HCS (Figure 4C). In general, the decay constant of the exponential expression correlated to the LSPR peak position or peak intensity and the distance from the HCS is found to decrease by increasing the diameter of the AgNDs. This can be attributed to the increases in the plasmon field intensity and domain that result from a larger-diameter AgND. The wavelength of the transmitted light from the glass substrate coated with the three zones composed of CDG monolayers with AgND diameters of 50, 38, and 25 nm can be tuned between 720 and 468 nm upon changing the excited spot from the HCS to just 4.8 cm away. Consequently, such a device can be used as a wavelength selector. AgND Monolayer CDG on a Cylindrical Substrate. Previous results were obtained for AgND monolayers deposited on a tape substrate adhered to a flat glass substrate. The advantage of using transparent tape as a substrate for the AgND monolayer is that in addition to its flexibility it can be adhered to other substrates with different shapes or roughness. AgNDs with different diameters were assembled into CDG monolayers on a tape substrate, which were then adhered to the surface of a cylindrical glass substrate in sequential order of the 50, 38, and 25 nm diameters. Figure S8A shows the LSPR spectrum of the 50 nm AgND CDG on a cylindrical substrate measured at different rotational angles starting from the HCS (0°) to 80°. When the angle of rotation was increased to 90°, the measured LSPR spectrum was collected for the HCS of the 38 nm AgNDs. Figure S8B shows the LSPR spectrum of 38 nm AgNDs collected when the angle of rotation was changed from 90 to 180°. The LSPR spectrum of the 25 nm AgNC monolayer CDG was obtained when the cylindrical glass substrate was rotated by an angle of between 190 and 270° as shown in Figure S8C. The LSPR spectrum is blue-shifted as the substrate is rotated between the angles of 0 and 270°. Figure 5A shows the relationship between the LSPR peak position of the CDG AgNDs and the corresponding angle of rotation. Each AgND CDG zone with the same diameter was fit exponentially. The decay constant of the exponential equation was found to decrease by increasing the diameter of the AgNDs. The relationship between the peak intensities at 720, 623, and 499

nm for AgNDs with respective diameters of 50, 38, and 25 nm and the angle of rotation is shown in Figure 5B.

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CONCLUSIONS An optical ruler and optical protractor were fabricated from colloidally prepared AgNDs on the surfaces of flat and cylindrical substrates. SEM imaging of the CGD AgND monolayer showed that AgND aggregates were not observed in either the gaseous or solid-state regions, whereas aggregates with a variable number of nanoparticles dependent on the surface pressure were obtained in the liquid condensed phase. The AgNDs were well separated in the gaseous phase, whereas in the solid phase highly packed 2D AgND arrays were formed. The interparticle separation distance between the AgNDs inside the aggregates deposited in the liquid condensed phase was roughly 7 nm. This distance was increased to approximately 12 nm upon compressing the LB film to the solid phase. Optical measurements of the CDG AgNDs at different separation distances from the HCS showed an exponential relationship between the LSPR peak position and the distance from the HCS. A similar exponential relationship was obtained between the LSPR peak position of the cylindrical CDG AgND and the rotational angle from the HCS. Similar optical responses and assemblies were observed for AgNDs with smaller diameters of 38 and 25 nm, although the decay constant of the exponential optical relationship increased as the diameter of the AgNDs was increased. Moreover, the applied LB surface pressure required to induce the phase transformation from the liquid condensed phase to the solid phase was decreased by increasing the diameter of the AgNDs. DDA calculations of AgND monomers and higher-order aggregates supported the experimentally observed spectra. The systemic change in the optical properties of the flat and cylindrical CDG AgNDs due to changing the distance or the angle of rotation makes this assembly useful as an optical ruler and protractor. The LB assembly of nanoparticles with a CDG introduced in this study can be easily generalized. A CDG of plasmonic nanoparticles of any size, shape, composition, and organization inside a monolayer can be used as optical rulers and protractors after initial calibration.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03211. TEM image of as-prepared AgNDs with different diameters and statistical analysis for the diameters of the AgNDs. LSPR spectra of AgNDs dispersed in water and of the CDG AgND monolayer. SEM images of monolayer assembly. Statistical analysis for the separated units in the SEM images of the AgND monolayer of CDG. Shape analysis of AgNDs. Simulated DDA LSPR spectra of an AgND pentamer and hexamer with different configurations. (PDF)

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Figure 5. (A) Relationship between the value of the LSPR peak position of CDG AgND monolayers on a cylindrical glass substrate and the corresponding angle of rotation from the HCS. The monolayers are composed of AgNDs with diameters of 50 (black), 38 (red), and 25 nm (blue). (B) LSPR peak intensity for the CDG monolayer composed of AgNDs with diameters of 50 nm (black) calculated at 720 nm, 38 nm (red) calculated at 623 nm, and 25 nm (blue) calculated at 599 nm plotted versus the angle of rotation.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. F

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-FG02 09ER46604.

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DOI: 10.1021/acs.langmuir.6b03211 Langmuir XXXX, XXX, XXX−XXX