Polarized Optomechanical Response of Silver Nanodisc Monolayers

Article pubs.acs.org/JPCC

Polarized Optomechanical Response of Silver Nanodisc Monolayers on an Elastic Substrate Induced by Stretching Mahmoud A. Mahmoud* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 6, 2015 | doi: 10.1021/acs.jpcc.5b05359

S Supporting Information *

ABSTRACT: A monolayer assembly of silver nanodisks (AgNDs) was fabricated on the surface of a polydimethylsiloxane (PDMS) polymer substrate using the Langmuir−Blodgett technique. Upon stretching the PDMS substrate, the localized surface plasmon resonance (LSPR) spectrum of the AgND monolayer is blue-shifted when the incident light excitation is polarized parallel to the stretching direction. Conversely, a red shift in the LSPR spectrum of the AgND monolayer is observed in the case of light polarization orthogonal to the stretching direction. The magnitude of the shift in the LSPR spectrum is proportional to the degree of stretching of the PDMS substrate. Stretching PDMS in one direction causes its shrinking in the orthogonal direction. Consequently, the interparticle distance between individual AgNDs on the PDMS surface increases in the same direction as the mechanical stretching and simultaneously decreases in the orthogonal direction. The different optical responses of the AgND assembly on the surface of stretched PDMS when excited with different polarization directions is due to the changing strength of the plasmon field coupling, which is inversely proportional to the separation gap between the AgNDs. The experimentally measured LSPR spectra upon stretching the PDMS substrate to different lengths and varying the incident light polarization were confirmed using the discrete dipole approximation calculation technique. The same optical response was obtained for an AgND monolayer sandwiched between two PDMS substrates. Covering the surface of the AgND monolayer on the PDMS substrate with another PDMS layer on top eliminates their deformation after multiple stretching−shrinking cycles and increases its chemical stability.

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INTRODUCTION Plasmonic nanoparticles are characterized by their exciting optical properties, which are based on the localized surface plasmon resonance (LSPR) phenomenon.1−5 An LSPR spectrum and a strong plasmon field are generated upon the interaction of plasmonic nanoparticles with electromagnetic radiation of a resonant frequency.1−5 Amelioration of the efficiency of the plasmonic nanoparticles in several applications6−9 requires an enhancement of the plasmon field strength and controlling the LSPR spectral peak position.10,11 Isotropic plasmonic nanoparticles have a relatively weak plasmon field intensity, and it is difficult to tune the LSPR peak position compared with those of anisotropic shapes.3,12,13 Homogenous growth of metallic nanocrystals leads to the formation of isotropic nanoparticles.14 In contrast, anisotropic nanoparticles are prepared by inhomogeneous growth of nanocrystals.15−18 The LSPR peak position of plasmonic nanoparticles can be controlled by the following techniques: (1) by tuning the particle size, a red shift in the LSPR peak position is obtained by increasing the size of the plasmonic nanoparticles,19 (2) by changing the shape, which has a pronounced impact on the LSPR peak position and the number of peaks of the plasmonic nanoparticles,20 (3) by changing the structure of the nanoparticles from solid to hollow and decreasing the wall thickness © 2015 American Chemical Society

of the hollow nanoparticles shifts the LSPR peak to a lower energy,21 and (4) by altering the composition, e.g., the LSPR spectral peak position of silver nanoparticles appears at a higher energy compared with gold nanoparticles of an identical shape and size.12 When the shape, size, structure, and composition of the plasmonic nanoparticles are kept constant, the LSPR peak position can be tuned by either changing the dielectric function of the surrounding medium22,23 or by coupling the plasmon field of individual nanoparticles with the field of other particles in close proximity.24 The LSPR peak positions of a pair of nanoparticles shift to a lower energy as the separation distance between them is decreased.25 Plasmonic nanoparticles have been used for reversible optical modulation with an applied voltage or by exposure to UV light.23,26 Coating the plasmonic nanoparticles with a thin layer of electrochromic polymer that changes its dielectric function upon applying an electrical potential results in a shift of the LSPR peak position.23 Another technique is based on inducing the trans isomerization of a thin layer of azo-compound coating the surface of the plasmonic nanoparticles into its cis form by illumination with UV light.26 Received: June 4, 2015 Revised: July 24, 2015 Published: July 28, 2015 19359

DOI: 10.1021/acs.jpcc.5b05359 J. Phys. Chem. C 2015, 119, 19359−19366

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The Journal of Physical Chemistry C Interestingly, the trans form is dominated by exposure to visible light.26 The isomerization of the azo-compound changes the dielectric function and consequently the LSPR peak position.26 Plasmonic nanoparticles with a disk shape have flat faces and relatively small thicknesses.27 The 2D assembly of the nanodisks is small enough not to disturb the order of the electronic targeted for enhancement by the plasmonic nanoparticles. In addition, the possibility exists to easily tune a disk’s LSPR peak position in the visible and NIR regions by increasing the diameter of the nanodisks without altering their thickness.28,29 Although tuning the LSPR peak of silver nanorods is possible without changing their diameter and by simply increasing their length,30 the ordering of the nanorods inside their assemblies has a large impact on the optical properties after assembly on the surface of a substrate.31 Unlike the nanorod shape, nanodisks are symmetrical when assembled into 2D arrays, which eliminates the orientation effect on their optical properties. In order to utilize colloidally prepared nanoparticles in most applications, the nanoparticles have to be deposited on the surface of a substrate. The Langmuir− Blodgett technique is used to assemble the colloidally prepared nanoparticles on the surface of a substrate in a controllable manner.32,33 Herein, the plasmon field coupling between silver nanodisks (AgNDs), which are assembled into a monolayer on the surface of an elastic polydimethylsiloxane polymer (PDMS) substrate by the Langmuir−Blodgett technique, is used for optomechanical switching. The separation distance between the AgNDs inside the monolayer assembly is altered by stretching the elastic substrate, causing a resultant shift of the LSPR spectrum. Stretching the PDMS elastic substrate in one direction causes its shrinking in the orthogonal direction. When the AgND monolayer assembly is excited with polarized light, different optical responses are expected depending on the direction of polarization. When the incident light is polarized parallel to the mechanical stretching force, a blue shift in the LSPR spectrum is observed that increases linearly with the strain of the PDMS substrate. Switching the polarization to be orthogonal to the stretching force leads to a red shift in the LSPR spectrum of the AgND monolayer assembly. The red shift increases exponentially upon increasing the amount of shrinkage in the orthogonal direction. To eliminate the deformation of the AgND monolayer after multiple stretching and shrinking cycles and to protect the surface of the AgND monolayer from oxidation, a PDMS film is also deposited on the top of the nanodisks. The experimentally obtained optical results are confirmed theoretically using discrete dipole approximation (DDA) calculations. Both the LSPR spectrum and the plasmon field intensity of AgNDs on the surface of a PDMS substrate are calculated using the DDA technique for different interparticle separation distances and excitation polarization directions.

resulting AgNDs were centrifuged at 12,000 rpm for 35 min, and the precipitated particles were dispersed in DI water. The cleaned AgND solution was then centrifuged again at 10,000 rpm for 20 min; the precipitated AgNDs were dispersed in 2 mL of ethanol. Finally, 4 mL of chloroform was added to the solution of AgNDs in ethanol. The polydimethylsiloxane (PDMS) substrate was prepared by mixing 100 mL from the base with 10 mL from the curing agent of a Sylgard 184 elastomer (Dow Corning) and stirring for 2 min. The PDMS mixture was left to settle until all the bubbles disappeared, and then the mixture was poured on top of a 30 cm × 30 cm glass substrate. Finally, the PDMS film was cured at 70 °C for 12 h. The PDMS film with a thickness of 1.2 mm was cut into 3 cm × 7.5 cm pieces. The surface of the PDMS is highly hydrophobic, and hydrophilic groups were introduced by dipping the PDMS sheets in a 200 mL baker containing a mixture of 20 mL of hydrogen peroxide, 20 mL of concentrated HCl, and 100 mL of DI water for 30 min.35 The PDMS sheets were transferred to a DI water solution for washing and were dried using a dry optical tissue and a heat gun. A Nima 611D Langmuir−Blodgett trough was used to fabricate the monolayer assembly of AgNDs. The trough was filled with a water sublayer, and 2 mL of AgNDs in chloroform was cast over the water surface using a microsyringe. The trough coated with the AgND monolayer was left for 20 min to dry. The surface pressure of the AgNDs monolayer was measured by a D1L-75 model pressure sensor. The monolayer was transferred to the surface of silicon and PDMS substrates simultaneously at surface pressures of 0 and 3 mN/m by vertical dipping with a speed of 15 mm/min. Then, 1.7 mL from the PDMS base-curing agent mixture similar to that used in making the substrate was drop cast on top of the AgND monolayer bound to the surface of the PDMS substrate. The top thin film of PDMS was allowed to dry for 12 h before it was cured for 6 h at 50 °C. Optical measurements were conducted with an Ocean Optics HR4000Cg-UV-NIR; the PDMS substrate was stretched mechanically using a custom lab-built device (the photograph is in Figure S1). The colloidal AgNDs solution that was used for studying their optical properties was prepared by dispersing the precipitated AgNDs in water instead of ethanol. A Zeiss Ultra60 scanning electron microscopy (SEM) was used for imaging the AgND monolayer assembly. DDSCAT 6.1 software was used for the DDA simulations of the LSPR spectrum as well as the plasmon field intensity and the field distribution. The shape file used for the simulation contains 1 dipole per 1 nm3. Simulations were carried out for single, four, and eight AgNDs placed 1 nm away from the surface of the PDMS substrate to account for the surfactant layer thickness. The electromagnetic plasmon field was calculated for both the plane of the AgNDs close to the surface of the PDMS substrate and for the top plane of the AgNDs (far from the surface of the substrate).

EXPERIMENTAL SECTION Silver nanodisks (AgNDs) with a 42.3 ± 5.4 nm diameter and a 7.3 ± 1.4 nm thickness were prepared as reported earlier.34 Briefly, 200 mL of 0.145 mM aqueous solution of polyvinylpyrrolidone (MW = 55,000) (PVP) was prepared in a 500 mL glass bottle. Then, 0.60 mL of 60 mM AgNO3 aqueous solution was added, followed by 4 mL of 78.35 mM Lascorbic acid. To initiate the reduction of silver ions into AgNDs, 0.12 mL of sodium borohydride (5 mM) was added, and the solution was gently shaken for a few seconds. The

RESULTS AND DISCUSSION Silver Nanodisk Monolayer Assembly on the Surface of a PDMS Polymer Substrate. The Langmuir−Blodgett technique (LB) has been used successfully to fabricate 2D assemblies from colloidally prepared nanoparticles on the surface of different kinds of substrates.31,33,36,37 First, the nanoparticles are assembled into an LB monolayer on top of a liquid sublayer such as water before being transferred to the surface of the substrate by dipping.33 Figure 1A shows a photograph of the LB trough after coating the water sublayer

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LB film.36,37 The isotherm is a relationship between the surface pressure measured for the LB film and the available surface area that the AgNDs are occupying (see Figure S2).37 As most regular isotherms,33,36,37 three different phases were observed: a gaseous phase is obtained at relative surface pressure below 0.2 mN/m, a liquid condensed phase is present at surface pressures between 0.2 and 3.5 mN/m, and finally a solid phase was constructed at surface pressures higher than 3.5 mN/m. The change of the phases of the AgNDs LB film is accompanied by a remarkable change of its color. The AgND-coated surface of the trough has a violet color at 0 mN/m surface pressure, which changed into a dark blue color upon compressing the LB film to a surface pressure of 3 mN/m. The LB film was then transferred to the surface of silicon and PDMS substrates simultaneously by vertical dipping at surface pressures of 0 and 3 mN/m. The PDMS polymer is characterized by its elastic properties, optical transparency in the visible and NIR regions, and thermal and chemical stability. The exciting properties of PDMS strongly support its use as substrates for plasmonic nano-


Figure 1. (A) Photograph of the Langmuir−Blodgett trough coated with a monolayer of AgNDs when the trough is not compressed (0 mN/m relative surface pressure). (B) Photograph of the AgND monolayer assembly fabricated on the surface of a stretchable PDMS substrate at a surface pressure of 3 mN/m. (C) SEM image of the AgND monolayer assembled on the surface of a silicon substrate at a surface pressure of 3 mN/m. The AgNDs are well separated, and no aggregation was observed.

with a monolayer of AgNDs. The LB isotherm monitors the interactions between AgNDs while they are arranged into an

Figure 2. (A) Schematic depiction of an AgND monolayer assembly on the surface of a PDMS substrate before and after stretching. LSPR spectra of an AgND monolayer assembly on the surface of the PDMS substrate measured for different stretching amounts: (B) when the incident light excitation was polarized parallel to the stretching force and (C) when the orthogonal light polarization was used while the PDMS was stretched at different amounts. Upon stretching the PDMS substrate, a blue shift in the LSPR spectrum was observed under parallel incident light polarization, while a red shift was obtained under orthogonal light polarization. (D) Relationship between the LSPR peak positions and the percent change of the length of the PDMS substrate. (E) Simulated LSPR spectra of four AgNDs on the surface of the PDMS substrate calculated for different directions of polarization and at different separation distances. The theoretical results agreed well with the experimental results. 19361


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Figure 3. Plasmon field contour calculated by the DDA technique for four AgNDs placed 1 nm away from the surface of a PDMS substrate: (A) The AgNDs on the surface of the unstretched substrate were excited at 606 nm with light polarized in the same direction as the stretching direction. (B) The PDMS substrate was strained 11% from its original length, causing a 3.5% shrinkage in the orthogonal direction, and the calculation was carried out at 584 nm excitation with parallel polarized light. (C) Simulations conducted while the PDMS was stretched by 26% at a 573 nm excitation wavelength with parallel polarized light. (D) An unstretched PDMS substrate, where the AgNDs were excited by 606 nm unpolarized light. (E) The PDMS was strained by 11%, and orthogonally polarized light with a 619 nm wavelength was used for excitation. (F) The AgNDs were excited with 655 nm light polarized in the orthogonal direction of the stretching force that caused a 26% expansion of the PDMS substrate.

coverage, shifting the LSPR spectrum to a lower energy.31 Fortunately, the PDMS substrate is elastic, which makes it possible to change the interparticle separation distance between the individual AgNDs inside the assembly reversibly upon stretching. The LSPR peak of an AgND monolayer assembly fabricated on the surface of a PDMS substrate at a surface pressure of 3 mN/m (53% coverage density) is at 605 nm (see Figure 3 A). When PDMS is stretched in one direction, it shrinks in the orthogonal direction of the applied mechanical force. Figure 3A shows a schematic depiction of an AgND monolayer bound to the surface of a substrate before and after stretching. Consequently, the interparticle separation distance between the individual AgNDs inside the monolayer assembly on the surface of the PDMS substrate is expected to increase in the same direction of the applied force. Conversely, the interparticle separation distance between the AgNDs in the orthogonal direction of the applied force is expected to decrease. On the basis of this speculation, optical measurements of the monolayer assembly of AgNDs on a PDMS substrate were conducted for varying amounts of PDMS stretching and with incident light polarizations parallel and orthogonal to the stretching force. Figure 2B shows the LSPR spectrum of a monolayer assembly of AgNDs on the PDMS substrate measured for different strains when the incident light is polarized parallel to the direction of stretching force. A 26.9% expansion of the PDMS substrate is accompanied by a blue shift of the LSPR spectrum of the AgND monolayer assembly on its surface from 605 nm before stretching to 572 nm afterward. The LSPR spectrum of the AgND monolayer assembly on the PDMS measured for different strains of the substrate when the excitation light was polarized orthogonal to the stretching force displayed a red shift, which also increased by increasing the amount of strain (see Figure 2C). The LSPR spectrum was red-shifted from 605 nm when measured for an

particles for different optical applications. Technically, because of the strong hydrophobicity of the PDMS surface, assembly of a hydrophilic AgND monolayer by the LB technique with a hydrophilic water sublayer is not possible. Hydrophilic hydroxyl groups were introduced to the surface of PDMS by hydrogen peroxide acidic oxidation.35 Figure 1B shows a photograph of a 3 cm × 7.5 cm PDMS substrate coated with an AgND monolayer assembly fabricated at an LB surface pressure of 3 mN/m (liquid condensed phase). The color of the AgND assembly on the PDMS substrate is blue. SEM was used to examine the homogeneity of the AgND monolayer assembly after being transferred to the substrate surface. PDMS is not conductive as required for SEM imaging, and therefore, the AgND monolayer was transferred to the surface of silicon substrates simultaneously with PDMS. Figure 2A and Figure S3 show the SEM images of the AgND 2D assembly fabricated on the surface of silicon substrates at surface pressures of 3 and 0 mN/m, respectively. The percent surface coverage of the substrate with AgNDs was determined by ImageJ and discovered to be 10.5 and 53% for monolayers fabricated at 0 and 3 mN/m, respectively. It is clear from the SEM image that the AgNDs are separated and do not contact one another even at high AgND surface coverage. AgNDs have 1/3{422} facets on the top and bottom faces, and the side edges are bounded by a mixture of {111} and {100} facets.38 PVP capping used during the synthesis of the AgNDs is selectively bound to the {100} facets39 located on the side of the AgNDs. The PVP capping molecules bound to the side surface of the AgNDs eliminate their aggregation, as seen from the SEM images. Consequently, no aggregation was observed. Optical Properties of the AgND Monolayer Assembly on the Surface of a PDMS Substrate. The plasmon field coupling of plasmonic nanoparticles organized into 2D assemblies greatly influences their optical properties.31 The strength of the field coupling increases upon increasing the 19362


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LSPR spectrum of the eight AgNDs is located at wavelengths longer than that experimentally obtained, and generally, the simulated LSPR spectrum of the eight AgNDs did not match any of the measured spectra of the AgNDs monolayer at any strain amount of the PDMS substrate. However, the calculated LSPR spectrum of the four AgNDs on the PDMS substrate agreed well with the experimentally measured results. The correlation between the optical measurements and the DDA simulations suggest that the coupling between the AgNDs inside the monolayer on the PDMS substrate is localized between each neighboring particles. One possible reason for the plasmon field coupling not extending over the AgNDs monolayer could be based on the unequal separation gaps between the AgNDs inside the monolayer and the different size distributions of the AgNDs. A similarity of size and separation gaps of the AgNDs is most likely required for linear and 2D plasmon field coupling to occur. Plasmon Field of AgNDs Calculated on the Surface of PDMS Substrates by the DDA Technique. The DDA technique is used to simulate the plasmon field coupling and to calculate the plasmon field distribution of plasmonic nanoparticles.3 Figure 3 shows the plasmon field contours of four AgNDs on the surface of the PDMS substrate calculated for different stretching percentages. The plasmon field contours calculated for the plane of the AgNDs close to the substrate at an incident light polarization parallel to the stretching direction is shown in Figure 3A−C. It is clear that the plasmon field of each neighboring pair of AgNDs couple together in the same direction of polarization. The intensity of the plasmon field was found to decrease as the stretching percent was increased, due to the increase of the separation gap between the nanodisk pair. Although the separation gap in the orthogonal direction decreases by increasing the stretching percent, the plasmon field coupling in the orthogonal direction is forbidden.10 Unpolarized light can interestingly excite the four AgNDs simultaneously. Figure 3D shows the plasmon field contours of four AgNDs when placed on the surface of unstretched PDMS and excited by unpolarized light. The plasmon field intensity of the four AgNDs placed at equal separation distances on the surface of PDMS when excited with polarized light is much stronger than when unpolarized light was used for excitation. Finally, when the polarization of the incident light was switched to be orthogonal to the stretching force, the plasmon field of the orthogonal AgND pairs undergoes coupling. As the separation gap between the AgNDs was decreased in the orthogonal direction upon stretching, the plasmon field intensity was increased (see Figure 3E and F). Effect of the PDMS Substrate on Plasmon Field Distribution of AgNDs. The substrate has proven to have a great impact on the plasmon field of plasmonic nanoparticles.43−46 Generally, the substrate breaks the symmetry of the top−bottom field distribution of the nanoparticles, and the effect of the substrate increases by increasing both the dielectric constant of the substrate44 and the surface area of the nanoparticles facing the substrate.45 The effect of the substrate on the plasmon field of highly symmetrical plasmonic nanoparticles such as silver nanocubes has been discussed intensively in the literature.43,44,46 AgNDs are two-dimensional anisotropic nanoparticles,47 which are characterized by the D∞h symmetry and the presence of two dipolar plasmon resonance modes. These two plasmon modes result from plasmon oscillations along the plane of the nanodisk (in-plane surface plasmon resonance) and perpendicular to the plane (surface

unstretched substrate to 649 nm after a 26.9% strain was induced, which was accompanied by a 9.8% shrinkage in the orthogonal direction. Figure 2D shows the relationship between the LSPR spectral peak position of the AgND monolayer assembly on a PDMS substrate and the elongation or compression of the PDMS substrate. The data shown on the left part of Figure 2D was obtained from optical measurements of the AgND assembly on PDMS when collected with an incident orthogonal light polarization, while the results of the parallel light polarization are shown in the right part of the figure. Negative strain values in the right part of Figure 2D indicate compression of the PDMS in the orthogonal direction. It is clear that the relationship between the LSPR peak positions when measured under orthogonal light polarization and the compression percentage is exponential. In contrast, the relationship between the LSPR peak position and stretching when the polarization is switched to be parallel to the applied force is linear. In summary, different optical responses of the AgND monolayer assembly on the surface of PDMS were observed for different incident light polarizations. This is attributed to the increase of the separation distance between the individual AgNDs in the direction of the applied force, while the separation between the AgNDs decreases in the orthogonal direction of the stretching. Similar optical measurements were conducted for the AgND monolayer assembly on a PDMS surface using unpolarized light excitation for different strains (see Figure S4). The fwhm of the measured LSPR spectrum was found to increase as the PDMS was stretched. Stretching the substrate changes the interparticle separation distance between the AgNDs unequally, and unlike using polarized light that excites the nanodisks in one direction, unpolarized light excites the AgNDs in all directions. The unpolarized light induces inhomogeneous plasmon field coupling of the AgNDs, and this can cause broadening of the LSPR spectra. The electromagnetic fields of plasmonic nanoparticles organized into either linear40 or branched41 chains couple together along the chains. However, the LSPR spectrum of the chain of nanoparticles is controlled by their numbers and the separation gap between them.41,42 The plasmon field coupling extends over a long distance, up to a few microns in a linear chain of plasmonic nanoparticles separated by similar separation gaps.42 Conversely, the plasmon field was observed to locally couple in the connected linear chain of the nanoparticles with equal interparticle separation distances but not between portions of the chain with asymmetric separation distances.42 On the basis of this idea, it is useful to examine whether the plasmon field coupling is localized or extended along the AgND monolayer. In order to determine the number of AgND plasmon fields that couple inside the AgND monolayer, discrete dipole approximation simulation (DDA) calculations were carried out for the LSPR spectra of small AgND 2D arrays composed of four and eight particles. The simulated spectra were then compared with experimentally obtained optical results. Figure 2E shows the calculated LSPR spectrum of four AgNDs with a 7 nm thickness and a diameter of 42 nm on the surface of a PDMS substrate. The calculations were carried out before and after expansion of the PDMS by 7.3, 11, 18.3, and 25.6%, which were accompanied by contractions in the orthogonal direction of 2.3, 3.5, 7, and 9% respectively. Figure S5 shows the calculated LSPR spectrum of eight AgNDs placed on a 25.6% stretched PDMS substrate, which was 9% orthogonally contracted as well. The simulated 19363


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Figure 4. (A) LSPR spectrum measured for colloidal AgNDs dispersed in water (red) and for a well-separated AgND monolayer on a PDMS substrate (black). (B) DDA-calculated LSPR spectrum of a single AgND in surrounding water medium (red) and a single AgND placed 1 nm away from the surface of a PDMS substrate.

the percent surface coverage of an AgND with a 42.3 ± 5.4 nm diameter facing the substrate is 38%, which is higher than the 16.7% of a silver nanocube of similar volume. Moreover, the surface-area-to-volume ratio of the 42.3 ± 5.4 nm AgND is 38%, which is larger than the 28% of a nanocube with a similar volume. Consequently and in principle, the substrate is expected to have a greater effect on breaking the symmetry of the plasmon field distribution of the AgNDs. In fact, the plasmon field calculated for a single AgND on a PDMS substrate (LSPR spectrum in Figure 4B) at both the 547 and 530 nm modes showed a higher intensity closer to the surface of the substrate compared with that on top of the AgND (see Figure S6). The substrate has proven to have a great influence on the LSPR spectrum and the plasmon field distribution of silver nanocubes.43 However, the surface plasmon resonance was split by the substrate into two plasmon modes, one showing a high field intensity close to the substrate and a low intensity on the top of the nanocube, while the opposite plasmon field distribution was observed for the second plasmon mode.43 Opposite to what was expected, the substrate has a lower effect in breaking down the spectral degeneracy of the inplane plasmon resonance of AgND than in the case of silver nanocubes. This can be attributed to the fact that the symmetry of the silver nanocubes is higher than that in the case of an AgND, even though the surface area of AgNDs touching the substrate is higher than that of silver nanocubes. Figure S7 shows the calculated plasmon field contours on the top of four AgNDs while placed on the surface of a PDMS substrate. The plasmon field was calculated for the four AgNDs for different strains of the PDMS substrate when the incident light was polarized parallel to the stretching force. Similar to the calculation carried out near the surface of the substrate, the plasmon field intensity on the top of the AgNDs (far from the PDMS) was found to decrease as the PDMS substrate was stretched. Moreover, the difference between the plasmon field intensity on the top of the AgNDs (Figure S7A−C) and that calculated close to the surface of the substrate (Figure 3A−C) is less than that in case of the single AgND on PDMS (Figure S6). Consequently, the plasmon field distortion of the AgND by the substrate is decreased due to the interparticles plasmon field coupling.

plasmon resonance).48 In order to study the effect of the substrate on the optical properties of nanodisks, the LSPR spectrum of a well-separated AgND monolayer on a PDMS substrate, fabricated at a surface pressure of 0 mN/m, was compared with that of colloidal AgNDs. The large interparticle distance between the AgNDs eliminates potential plasmon field coupling between neighboring AgNDs. Three LSPR spectral peaks and a shoulder were obtained for colloidal AgNDs (see Figure 4A). Two of the peaks that appeared at 625 and 412 nm are assigned to the in-plane dipole and out-of-plane plasmon resonance, respectively. A narrow LSPR peak at 337 nm corresponds to an out-of-plane quadrupole plasmon resonance, and the slight shoulder at 370 nm is for the in-plane quadrupole plasmon resonance.27 The LSPR spectrum in Figure 4A measured for the well-separated AgND monolayer on the surface of the PDMS substrate showed a clear difference compared with that of colloidal AgNDs. The low-energy, sharp plasmon peak of the colloidal AgNDs is symmetrical and has a Lorentzian nature compared to the asymmetric Lorentzian nature of the AgNDs on PDMS. The effect of the substrate on the optical properties of AgNDs studied experimentally was examined using DDA calculations. Figure 4B shows the simulated LSPR spectra of a single 42 nm AgND with a 7 nm thickness in the colloidal state and when placed 1 nm away from the surface of the PDMS substrate. Interestingly, as observed from the measured LSPR spectrum, the low-energy, sharp plasmon peak of the simulated LSPR spectrum of AgND on the substrate is more symmetric and narrower than that of the colloidal AgNDs. The substrate induces the breakdown of spectral degeneracy of the AgNDs.49 As AgNDs surrounded with water are characterized by D∞h symmetry, a highly degenerate single in-plane plasmon mode is formed.47 This highly degenerate single plasmon mode causes the pumping of the plasmon excitation energy into a single plasmon mode, which increases the oscillator strength and decreases the width of the LSPR spectrum.47 When the substrate is introduced, it induces a new lower C∞υ symmetry of the AgND by disturbing the distribution of the oscillating conduction band electrons. This lowers the value of the oscillator strength and causes the increase in width of the LSPR spectrum of the AgNDs on a substrate. Comparing the surface area of AgNDs exposed to the surface of the substrate with that of silver nanocubes of similar volume, 19364




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CONCLUSIONS A silver nanodisk monolayer assembly was fabricated on the surface of an elastic PDMS substrate at a moderately high surface coverage using the Langmuir−Blodgett technique. The separation distance between the AgNDs inside the 2D assembly with the high coverage is small enough to enable plasmon field coupling, and the strength of the plasmon field coupling as well as LSPR shift amount is sensitive to the distance separating the AgNDs. Stretching the elastic PDMS substrate of the AgND 2D assembly changed the separation distance between the individual nanodiscs inside the assembly, with mechanical elongation of the PDMS substrate from one direction accompanied by its shrinking in the orthogonal direction. Upon stretching the PDMS substrate, a blue shift in the LSPR spectrum of the AgND monolayer assembly was observed when the light used for excitation was polarized parallel to the stretching force. The degree of blue shift was found to increase linearly with the substrate strain. When the incident light was polarized in the orthogonal direction of the stretching force, a red shift in the LSPR spectrum was observed by an amount that increased exponentially with an increase in stretching. Theoretical calculations using DDA simulations were carried out to confirm the experimental optical results, and the experimental and theoretical results accorded well with one another. In summary, mechanical stretching of the elastic PDMS substrate manipulates the optical properties of the AgNDs assembled into a monolayer on its surface. The device fabricated in this study can be used to measure mechanical forces, being able to detect changes in distance based on the LSPR shift after calibration, or as an optical modulator that is able to reversibly alter the wavelength of light.

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REFERENCES

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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.jpcc.5b05359. Photography of the setup that used to measure the optical properties of AgND monolayer assembly on the surface of PDMS substrate; Langmuir−Blodgett isotherm of AgNDs measured on top of a water sublayer; SEM image of the AgND monolayer assembled on the surface of a silicon substrate; LSPR spectrum of AgND monolayer assembly on the surface of PDMS; LSPR spectrum and plasmon field contours of eight AgNDs placed on stretched PDMS; plasmon field contours calculated for a single AgND placed on the surface of a PDMS substrate; and calculated plasmon field contours on top of four AgNDs while placed on the surface of a PDMS substrate (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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. 19365


Article


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Polarized Optomechanical Response of Silver Nanodisc Monolayers

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