Effects of the Substrate Refractive Index, the Exciting Light

Article pubs.acs.org/JPCC

Effects of the Substrate Refractive Index, the Exciting Light Propagation Direction, and the Relative Cube Orientation on the Plasmonic Coupling Behavior of Two Silver Nanocubes at Different Separations Nasrin Hooshmand,†,‡ Sajanlal R. Panikkanvalappil,† and Mostafa A. El-Sayed*,†,§ †

Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ‡ Department of Chemistry, Marvdasht Branch, Islamic Azad University, Marvdasht, Iran § Adjunct Professor at King Abdulaziz University, Department of Chemistry, Jeddah 22254, Saudi Arabia S Supporting Information *

ABSTRACT: In this paper, we presented a detailed theoretical investigation on the effects of changing the refractive index of the substrate and the interparticle separation distance on the electromagnetic field distribution around the face-to-face (FF) and the edge-to-edge (EE) oriented silver nanocube (Ag NC) dimers. We found that the contribution of the scattering to the absorption components is markedly decreased with increasing the substrate refractive index. This suggests that for surfaceenhanced Raman spectroscopic (SERS) applications, the use of a substrate of very low refractive index is highly recommended. However, for photothermal applications, the use of substrate of high refractive index is recommended. Furthermore, we found that the wavelengths corresponding to the dipolar and multipolar modes are nearly unaffected by increasing the interparticle separation distances above 40 nm on the high refractive index substrate. However, on the low refractive index substrate, the dipolar peaks consistently red-shifted as the separation distance decreased. We also observed that on a glass substrate, after a separation distance above 8 nm, the value corresponding to the maximum field in the FF oriented Ag NC dimer gradually increased with increasing separation distance. On the contrary, the field gradually decreased on the higher refractive index substrate (AlGaSb) above 8 nm separation. This substrate showed a larger effect on the redistribution of the dipoles and subsequent decrease in the intensities of both hot spots and cold spots. Additionally, studies on the effects of substrate location with respect to the propagation direction of the exciting light revealed that the high refractive index substrate is likely to absorb the incident light to a larger extent, thus, greatly diminishes the plasmonic enhancement of the exciting light if it propagates through the substrate first. This study is pointing toward the fact that the refractive index of the substrate has a strong effect on the intensity ratio of the multipole to the dipole modes. Substrate having high refractive index seems to relax the selection rules of light interaction with the different plasmonic multipolar characters.

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INTRODUCTION

understanding of LSPR coupling is necessary as it can provide vital information on various factors governing the resonance wavelength in individual and assembled nanostructures. The LSPR property of plasmonic nanoparticles is strongly dependent on their size, shape, composition and interparticle distance.8,18−26 Furthermore, it also exhibits a remarkable sensitivity to the proximal environments of the nanomaterial such as dielectric function of their surrounding medium and the substrate, as it can dramatically influence the plasmonic

Plasmonic nanoparticles, especially made of Ag and Au, have received much attention in recent years owing to their unique properties arising from their strong interactions with electromagnetic radiations, primarily in the visible region, and subsequent resonant excitations of the collective oscillations of surface conduction electrons known as localized surface plasmon resonance (LSPR).1−5 This can result in remarkably strong scattering and absorption properties many orders of magnitude higher than the incident electromagnetic fields. These properties can make them versatile for diverse applications such as bioimaging, labeling, optical energy transport, and chemical and biological sensing.1,6−17 In view of many applications of plasmonic nanoparticles, a fundamental © 2016 American Chemical Society

Special Issue: Richard P. Van Duyne Festschrift Received: March 9, 2016 Revised: May 25, 2016 Published: May 30, 2016 20896

DOI: 10.1021/acs.jpcc.6b02480 J. Phys. Chem. C 2016, 120, 20896−20904

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The Journal of Physical Chemistry C

Scheme 1. Pictorial Representation of the Direction of Propagation of Incident Light for (A) FF and (B) EE Oriented Ag NC Dimer

properties in a simple and discernible manner.8,18,19,27 Although the LSPR properties in general were demonstrated very well, relatively little attention has been paid to study the fundamental physics of the LSPR as well as higher order multipolar plasmon resonance bands in very closely assembled plasmonic nanoparticles dimers. Nanocubes (NCs) show strong substrate-induced LSPR coupling properties than spherical counterparts as the flat surface of the NCs makes them interact more strongly with the nearby dielectric substrate over a large area.24,28−31 Earlier studies suggest that instead of internanoparticle distance, interaction geometries can also result in unique hybridization interactions in nanoparticle dimers.32−34 It was found that the interaction of a nancube with a supporting material (substrate) strongly affected the LSPR band shift in nanocube due to their sharp corner.22 However, when the nanoparticles are attached to a solid support, the extent of the refractive index sensitivity is reduced and this reduction is directly proportional to the fraction of the nanoparticle surface in contact with the substrate.35,36 One of the important prospects of the LSPR to understand is the effect of the electromagnetic field distribution on the sensing performance of the metal nanoparticles. In view of that, Ag is the best candidate among all the metal nanoparticles due to its high plasmon response in the visible region.37 Previous investigations on Au/Au and Ag/Ag dimeric nanocubes28,29,38 using the discrete dipole approximation (DDA)18,21,39 revealed that the LSPR coupling is stronger when the two nanocubes are in close proximity to one another and the sensitivity factor is largely dependent on the interparticle separation distance, cube edge rounding effects, as well as the refractive index of the surrounding medium26,27,38,40 Here, the refractive index plays key roles in determining the LSPR bands. As the refractive index increases, the wavelength maxima of the enhanced LSPR band red shifts to a greater extent, which depend on the value of the refractive index of the medium and substrate.41,26,27,40,42,43 In this paper, we report an investigation on the effect of changing the substrate refractive index and the relative nanocube orientations with respect to one another on the LSPR properties and the electromagnetic field distributions of Ag nanocube (Ag NC) dimers separated by different interparticle distances. Here, we studied the plasmonic spectra and the field distribution around the dimers upon changing the substrate refractive index from n = 1.5 for glass to a much higher refractive index value of n = 4.6 for AlGaSb. The dependence of the refractive index sensitivity on the electromagnetic field distribution around the Ag NCs dimer as well as multipolar plasmon coupling with two different interaction geometries such as face-to-face (FF) and edge-to-edge (EE)

were also investigated as a function of the interparticle separation distances in detail. In addition, we studied the relationship between effect of increasing the substrate refractive index and the intercube separation distance on the multipolar plasmon coupling in the Ag NCs dimers arranged in FF and EE relative orientations. For substrate of high refractive index value, we studied the effect of substrate location with respect to the propagation direction of the exciting light as well. Numerical Method. Near-field interparticle coupling on the plasmon resonances in symmetrical Ag NC dimers (edge length = 42 nm) assembled in two different orientations, FF and EE, on 10 nm thick glass and AlGaSb substrates were investigated numerically. The surrounding medium was assumed to be the water. Throughout the paper, in each case, the incident light propagates from the top of the substrate (except section d) and is polarized along the dimer axis (see Scheme 1). The field analysis was performed using DDA, which is one of the most powerful theoretical techniques to model the optical properties of plasmonic nanoparticles with various sizes and shapes. The key advantage of this method is that it includes multipolar effects and finite size effects. Details on the DDA method have been described elsewhere.39,44,45 In this method, the dimers of Ag NC is represented as cubic array of several thousands of dipoles located on a cubic lattice (with volume d3). The point dipoles that are excited by an external field, their responses, and other dipole points are solved self-consistently using Maxwell’s equations. The refractive index of Ag NCs is assumed to be the same as that of the bulk metal. For this calculation, we used the DDSCAT 6.1 code developed by Draine and Flatau.39 The extinction efficiency is consider by Qext = Cext/πr2eff, where reff = (3v/4π)1/3 is the radius of a sphere having same volume as the Ag NC dimer on the substrate utilized in this study. We have calculated the field distribution around the surface of the Ag NCs dimer in both orientations (FF and EE). The plasmonic field enhancement (in log scale of |E|2/|E0|2) was calculated on the surface of a pair of cubes with the DDA technique at different excitation wavelengths. We aim to model the optical properties and plasmonic coupling behavior between the Ag NCs (edge length = 42 nm) on glass and AlGaSb substrates. The size of the slab (substrate) is limited by the method of calculation source (DDA). It was found the change in the length, width or the thickness of the slab do not alter the results.22 Here, Ag NC dimer on substrate is represented by point dipoles (1 dipole per 2 nm), which are excited by an incident photon (external field). Further, the response of each dipole to both the external field and neighboring dipoles were solved using Maxwell’s equations. We have included data for the EE oriented Ag NC dimer along the diagonal of two cubes. 20897


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Figure 1. (A−C) Substrate effect on the calculated extinction spectra for Ag NC dimer in EE orientation at a separation distance of 2 nm: (A) without any substrate, (B) on glass substrate (n = 1.5), and (C) on AlGaSb substrate (n = 4.6) that has relatively much higher refractive index. (D− F) Electromagnetic field distribution of Ag NC dimers in the absence of substrate (D), on a glass (E), and on AlGaSb (F). The scattering and absorption spectra corresponds to Ag NC dimer on AlGaSb showed red shift in comparison with that on the glass substrate. As seen in (B) and (C), with increasing the refractive index of the substrate, the contribution due to the scattering in the extinction spectra decreases, which suggests that for imaging applications it is better to use substrate with low refractive index.

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RESULTS AND DISCUSSION

attributed to the stronger mixing of dipolar modes with multipolar modes on the higher refractive index substrate. These results show that the high refractive index substrate leads to the lowering of symmetry of the system (which relaxes the light-matter selection rules) with subsequent mixing of dipolar and multipolar modes (see Figure 1C,F). Moreover, it is likely that the ratio of absorption to scattering largely depends on the substrate refractive index. In the EE configuration, as the refractive index of the substrate decreases, the intensity of scattering shows a higher value than absorption in the extinction spectra (Figure 1B). A similar trend was also observed for FF oriented Ag NC dimer (Figure S1). This suggests that, for SERS application, the use of substrate of low refractive index is highly recommended. However, for photothermal applications, the use of substrate of high refractive index is more desirable. It is also noted that, on higher refractive index substrate, the extinction maximum (λext) shifts more toward longer wavelength (Figure 1C). b. Dependence of the Field Distribution on the Substrate Location (Top Plane and Bottom Plane) in EE and FF Oriented Ag NC Dimer. The calculated field intensities on the top plane (away from the substrate) and the bottom plane (on the top of the substrate) of EE oriented Ag NC dimer on glass at a separation distance of 2 nm is shown in Figure 2. The images show that on the bottom plane of the substrate, the value corresponds to maximum field is relatively higher than that for the top plane (away from the substrate). Additionally, on the glass substrate, we found that the maximum field enhancement occurred in-between the two adjacent corners (at the interparticle gap) of the nanocubes. This is found to be much stronger than the maximum field enhancement at the interparticle gap in a pair of Ag NCs arranged in FF orientation (Supporting Information, Figure S2).28 The observed difference in the field distribution in EE and FF oriented Ag NCs dimers with 2 nm separation distances can be attributed to the following reasons. In the FF orientation, the substrate may try to spread the oscillating dipoles all over the faces. While, in the EE orientation, the dipoles will possibly concentrating at the small area in between the two corners of the nanocubes (see Supporting Information, Figure S2). As the field intensity is proportional to the dipolar

a. Substrate Effect on the Plasmonic Coupling in Edge-to-Edge (EE) and Face-to-Face (FF) Orientated Ag NC Dimer. In our previous publication46 we showed that Ag NC dimer in FF orientation, at very short separation distance (2 nm), possesses two dominant plasmonic bands at lower (608 nm), and higher energy (585 nm) regions regardless of the refractive index of the substrate. Here, we investigated the effect of the refractive index of the substrate as well as the separation distance on the extinction spectra of the Ag NC dimers arranged in EE orientation in detail. Figure 1 shows the extinction spectra (both absorption and scattering) of Ag NC dimer arranged in EE manner with a separation distance of 2 nm on different substrates. Interestingly, only one enhanced plasmonic band was observed for the EE orientation. However, two plasmonic bands were observed in FF orientation (see Supporting Information, Figure S1). Here, the direction of propagation and polarization of incident light were kept constant. The incident light propagates along the x-axis (from the top of the FF and EE cubes (see the Scheme 1A,B)) and the electromagnetic field is polarized along the y-axis (interparticle axis) of the Ag NC dimer. For the comparison of the substrate effect, the extinction spectrum simulated without the substrate is also given (Figure 1A). As there was no significant difference in the refractive index of the glass (n = 1.5) and the medium used (water, n = 1.33), the Ag NC dimers showed nearly similar extinction features (Figure 1A,B) in both cases. The electromagnetic field distribution around the Ag NC dimers in each case is also given in Figure 1. We also noted that, on a low refractive index substrate, the ratio between the scattering to the absorption significantly increased in the EE orientation. In order to study this in detail, the extinction spectrum was calculated for Ag NC dimer in EE orientation on a high refractive index surface. Here we used AlGaSb (n = 4.6), which has relatively high refractive index value than glass (n = 1.5). On the high refractive index (n = 4.6) substrate, the multipole modes in the Ag NC dimer broadened and shifted to a longer wavelength compared to that observed on lower refractive index substrate. This can be 20898


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the gap between the two nanocubes (oriented in FF) increases (from 2−40 nm), the dipolar and multiplolar modes show a constant blue-shift, regardless of the separation distances (Figure 3B). However, on a low refractive index substrate such as glass (n = 1.5), even though the dipolar mode continuously blue-shifted under similar conditions as high refractive index substrate, the LSPR band corresponds to the multipole mode became nearly unchanged when the separation distance was greater than 40 nm (Figure 3A). Furthermore, the dipolar modes showed relatively larger blue-shift in low refractive index substrate (∼600−490 nm) than in high refractive index substrate (∼675−625 nm) as the separation distance is gradually reduced below 100 nm (Figure 3A,B). This suggests that, at very short distance, the contribution due to the dipole−dipole interaction between adjacent nanocubes is much stronger than the contribution resulting from the multipole interactions. However, the contribution from the multipolar interactions is greatly affected by the higher refractive index substrate. Interestingly, we noticed that the separation between dipolar and multipolar modes become larger as the distance between the Ag NCs become shorter. As the nanocubes get closer to each other, the shifts in dipolar mode becomes much larger compared to the shift in the multipolar modes due to the stronger dipole−dipole interaction as well as the fact that the substrate refractive index is not high enough to reduce their coupling interactions. At very short distances, multipolar bands loses its intensity compared to the dipolar modes and becomes weaker due to the lack of overlapping between dipolar and multipolar modes. This could result in the observed shift in dipolar modes away from the multipolar modes. Nevertheless, at larger distances, the multipolar band gains intensity and become stronger due to the stronger interaction between dipole and multipole modes as they become energetically closer in energy. On the substrate having high refractive index, a significant broadening in the dipolar modes in FF oriented Ag NC dimer was observed (the top three spectra from 40−100 nm). This could possibly due to the strong influence of the substrate and its strong coupling of the multipolar modes with the dipolar modes at large distances. When the glass was used as the substrate (n = 1.5), the coupling between multipolar and dipolar modes was much more continuous and at 2 nm the intense broad band corresponding to the dipole mode split into

Figure 2. DDA calculated field intensity at the top plane [away from the substrate; (A)] and the bottom plane [on the top of the substrate; (B)] of EE oriented Ag NC dimer on glass at a separation distance of 2 nm. The maximum plasmonic field is located at the interparticle gap between the two adjacent edges of the EE oriented cubes. Compared to the top plane, the maximum field intensity is twice as high at the bottom plane, where the substrate is located.

density or number of dipoles per unit area, any variation in the number of the dipoles in FF and EE orientations may greatly affect the field distribution. We noted that the number of dipoles per unit area at the hot spots in FF orientation is very small compared to the EE orientation. The field intensity is proportional to the dipolar density or number of dipoles per unit area. Henceforth, any variation in the number of the dipoles in FF and EE orientations may greatly affect the field distribution. Moreover, the substrate also plays a significant role in redistributing the dipoles on the top plane and bottom plane of the cubes. As shown in Figure 2B, at the bottom plane, where the substrate is located, the dipole density is small on the substrate causes the dipoles in the nanocubes to spread and redistribute to the adjacent corners at the interface of two Ag NCs leading to an increase in their density compared to the intensity of dipoles on the top plane of the nanocubes. c. Dependence of the Substrate Effect on the Plasmonic Coupling in Ag NC Dimers in EE and FF Orientations at Different Separation Distances. Figures 3 and 4 summarize the dependence of the substrate effect (comparing glass with AlGaSb substrates) on the LSPR bands in Ag NC dimers with varying separation distances, orientated in FF and EE configurations. The interaction between the FF oriented Ag NC dimer becomes stronger when the nanaocubes get closer to one another at short distance around 10 nm and then becomes very strong at very short distance (2 nm).46 In the case of substrate having high refractive index (n = 4.6), as

Figure 3. DDA calculated extinction spectra for FF oriented Ag NC dimer with different separation distances on the glass (A) and AlGaSb (B) substrates. The data suggest that the dipolar mode has an almost constant wavelength maximum above 20 nm separation on the AlGaSb substrate. In contrast, the dipolar mode increases in wavelength continuously as the separation distances decreases on the glass substrate. 20899


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Figure 4. DDA calculated electromagnetic fields and field polarization vectors at different separation distances (2, 8, 20, and 100 nm) for FF oriented Ag NC dimer on the glass substrate (A) and on AlGaSb substrate (B).

Figure 5. DDA calculated extinction spectra for EE oriented Ag NC dimer on the glass (A) and AlGaSb (B) substrates. C and D shows the calculated electromagnetic field and field polarization vectors at different separation distances (2, 5, 11, and 28 nm) for EE oriented Ag NC dimers on the glass (A) and AlGaSb (B) substrates.

two bands (Figure 3A). For the separation distances greater than 4 nm, only one broad dipolar band appeared and was always coupled to one another. On the other hand, on high refractive index substrate, when the distance between the nanocubes was 40 nm, both dipolar and multipolar modes became constant. At the same time, for Ag NCs on substrate on glass, in all separation distances, the 20900


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Figure 6. DDA calculated extinction spectra for Ag NC dimers arranged in FF (A) and EE (B) configurations on substrates having various refractive indices. The intercube separation distance in each case is 2 nm. The spectra clearly show the greater stabilization effect of the substrate having the highest refractive index value on both the dipolar and higher multipolar spectral bands.

spectra, field distribution, and field polarization vector for the EE oriented Ag NCs dimers on the low and the high refractive index surfaces with interparticle separation distances of 2, 5, 11, and 28 nm (Figure 5). On the substrate having lower refractive index (n = 1.5), similar to the extinction spectra of FF oriented Ag NC dimer at different separation distances, the main band corresponding to the dipolar modes significantly blue-shifted as a function of the separation distance (Figure 5A). But, on AlGaSb, the LSPR shift corresponds to the dipolar modes was minimal (Figure 5B) and remained almost unchanged at separation distance beyond 28 nm (data not shown). These results strongly reflect the vital role of substrate on plasmonic coupling in Ag NC dimer. It is likely that the substrate with high refractive index shields the internanoparticle coupling as the oscillating dipoles are strongly coupled to the substrate, which minimizes their inherent interdipolar coupling behavior. As a result, for sensing applications, the substrate with lower refractive index would be a highly recommended. Recent study shows that plasmonic “cold spots” exists at the region around the nanocubes, where the field equals to zero and hence there is no field polarization intensity devoid of any radiation.47 As these cold spots are more confined than hot spots along the edges of the nanocubes,47 they can be exploited for many applications in optics. In our study, on the glass substrate (Figure 5C), as the distance between the EE oriented Ag NC dimer decreases, the magnitude of hot spots increases and are confined largely around the adjacent corners of the nanocubes at the interface. At the same time, a very low field (“cold spots”) is ultraconfined away from these adjacent corners (Figure 5C) of the nanocubes. One possible reason for these “cold spots” could be due to the weak influence of the low refractive index substrate on the redistribution of the dipoles and the resulting enhancement in the dipolar density at the corners of the two adjacent nanocubes at the interface. However, on the substrate with higher refractive index (AlGaSb), the substrate showed a larger effect on the redistribution of the dipoles and subsequent lowering in the intensity of both the hot spots and the “cold spots” (Figure 5D). Comparing the observed results of the extinction spectra obtained with and without any substrate further validates the effect of substrate in plasmonic coupling between Ag NC dimer oriented in EE and FF configurations. It is noticed that the plasmonic band has the same trend in both EE and FF orientations either on a low refractive substrate or without a

dipolar region consistently changed with intercube separation distance (Figure 3A). The distribution and field polarization vector in the FF oriented Ag NC dimer on glass and AlGaSb substrates with separation distances 4, 8, 20, and 100 nm are shown in Figure 4. For the FF oriented Ag NC dimers on the glass substrate, the maximum enhancement occurred around the sharp corners of the cubes except at very short distance (4 nm), where the field was largely concentrated at the interface between the two cubes. We also noticed that, as the distance between the two nanocubes increases, the maximum field enhancement occurs around the corners. However, for the FF oriented Ag NC dimer on the AlGaSb substrate with 4 nm separation distance, the electromagnetic field was distributed around the corners and faces of each nanocubes (Figure 4B). Moreover, a significant enhancement in the field intensity maximum was observed at a separation distance of 8 nm. Afterward, the intensity corresponding to the maximum field dropped with increasing the separation distance (Figure 4B). On the contrary, on a glass substrate, at separation distances above 8 nm, the value corresponding to the maximum field in FF oriented Ag NC dimer gradually increased with increasing the separation distance (Figure 4A). From our study, we have identified that the electromagnetic field corresponds to the multipolar region in a single Ag NC on AlGaSb is mainly distributed all around the nanocube, including faces and corners (see Supporting Information, Figure S3). Whereas, the field corresponds to the dipolar mode is largely located at the corners. As in the case of single Ag NC on AlGaSb substrate, the FF oriented Ag NC dimer showed a significant enhancement in the characteristic electromagnetic field corresponds to the multipolar region at the interface between the two nanocubes. The enhanced field at the interface can be attributed to the strong influence of the high refractive index substrate on the coupling between the dipolar and multipolar modes between the nanocubes. Even at a separation distance of 100 nm, the field is distributed unsymmetrically around the nanocubes and an enhanced field was obvious at the interface between the nanocubes (Figure 4B). However, on the glass substrate, the Ag NC dimer at a separation distance of 100 nm did not show any noticeable coupling and the field was distributed largely around the corners as in the case of individual monomer nanocube (Figure 4A). In order to determine how the LSPR and electromagnetic field change with orientation, we calculated the extinction 20901


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Figure 7. (A, B) DDA calculated extinction (middle) and electromagnetic field distribution of EE (A) and FF (B) oriented Ag NC dimer (at a separation distance of 2 nm) on a strong absorbing substrate (AlGaSb), where the incident exciting light propagates through the strong absorbing substrate first before exciting the nanoparticles surface plasmon.

found that the substrate with high refractive index shields the interparticle dipolar coupling to a larger extent, which has been attributed to the strong interaction of the oscillating dipoles in nanoparticles with the substrate and subsequent reduction in their inherent interdipolar coupling behavior. Compared to high refractive index substrate, a marked decrease in the absorption to the scattering components was observed in the low refractive index substrate, which suggests the use of substrate having minimum value of their refractive index in SERS applications. Unlike lower refractive index substrate, the dipolar and multipolar modes in Ag NC dimer on a high refractive index substrate were found to be nearly unaffected by the separation distances beyond 40 nm. More interestingly, we found that the intensity corresponds to the maximum field in FF oriented Ag NC dimer on glass substrate increased with increasing separation distances. However, a consistent decrease was observed on the higher refractive index (AlGaSb) substrate beyond 8 nm. We also demonstrated that the high refractive index substrate plays a significant role in redistributing the dipoles and subsequent weakening in the intensity of both hot spots and cold spots. Furthermore, when the incident light propagates from the bottom of the substrate, it is likely that the high refractive index substrate strongly absorbs the incident exciting light thus greatly diminishes the plasmonic enhancement of the exciting light. Our study is pointing toward the fact that the substrate refractive index has a vital role in redistributing the dipoles, which can significantly affect the LSPR and electromagnetic field distribution around the nanoparticles.

substrate (only with water in the medium; Figure 6). Our study is pointing toward the fact that the substrate refractive index has a vital role in redistributing the dipoles, which can significantly affect the LSPR and electromagnetic field distribution around the nanoparticles, and thus the observed spectra. d. Effect of the Location of High Refractive Index Substrate with Respect to the Propagation Direction of the Exciting Light on the Field Enhancement and the Plasmonic Spectra. In all the aforementioned surface plasmon enhancements experiments, the exciting light first excites the surface plasmon of the nanoparticles used then passes through the substrates. For substrates with small refractive index, the excitation order is unlikely to change the observed field distribution even when the excitation sequence is reversed. This is expected not to be true for experiments with substrates of high refractive index. In the present experiments, the effects of the location of the high refractive index substrate in absorbing the exciting incident light was studied by simulating the extinction spectra and electromagnetic field distribution around EE (Figure 7A) and FF (Figure 7B) oriented Ag NC dimer at a separation distance of 2 nm, where the incident light is propagated from the bottom of the AlGaSb substrate first before it excites the surface plasmon of the nanocubes as shown in Figure 7. In both cases, the high refractive index substrate strongly absorbs the incident exciting light thus greatly diminishes the plasmonic enhancement of the exciting light. The broad band observed between 500 and 600 nm could be due to the absorption of the substrate (AlGaSb) itself, which is evident from the spectra shown in Figure 7.

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CONCLUSIONS We studied numerically the dependence of the refractive index, interparticle separation distance and direction of propagation of incident light on the electromagnetic field distribution around FF and EE oriented Ag NC dimer on a low (n = 1.5) and high (n = 4.6) refractive index substrates. By analyzing the LSPR and electromagnetic field distribution around the nanoparticles, we

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02480. Additional DDA calculated fields and depolarization vectors (PDF). 20902


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 404-894-0292. Fax: 404-894-0294. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF Division of Chemistry (CHE) Grant 1306269. REFERENCES

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Effects of the Substrate Refractive Index, the Exciting Light

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