Plasmonic Coupling Effect in Silver Spongelike Networks

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Plasmonic Coupling Effect in Silver Spongelike Networks Nanoantenna for Large Increases of Surface Enhanced Raman Scattering Zao Yi, Xibing Xu, Jiangshan Luo, Xiaodong Jiang, Weidong Wu, You-gen Yi, and Yongjian Tang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Oct 2013 Downloaded from http://pubs.acs.org on November 1, 2013

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The Journal of Physical Chemistry

Plasmonic coupling effect in silver spongelike networks nanoantenna for large increases of surface enhanced Raman scattering Zao Yi a,b, Xibin Xu a,b, Jiangshan Luo b, Xiaodong Jiang b, Weidong Wu b, Yougen Yi a*, Yongjian Tang b** a

b

College of Physics and Electronics, Central South University, Changsha 410083, China

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China

*Corresponding author: (Yougen Yi) Email: [email protected] TEL : +86 0816 2480827 FAX: +86 0816 2480830 **Corresponding author: (Yongjian Tang) Email: myyz1984@ mail.csu.edu.cn TEL : +86 0816 2480827 FAX: +86 0816 2480830.

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The electric field enhancement of the silver spongelike networks has been described to be a systematic investigation by using three-dimensional finite-difference time-domain (3D-FDTD) simulation. Surface enhanced Raman scattering (SERS) measurements have indicated that the junction regions, the hollow nanostructured and the sharp nanotips of the broken ligaments in the silver spongelike networks act as electromagnetic “hot-spots”. The 3D-FDTD calculations have indicated that the silver spongelike networks may exhibit a high quality SERS characteristic because of the Ag chain length, chain diameters, chains gap, chains angle and sharp nanotips. A maximum enhancement factor of 3.5×1012 can be obtained with the silver spongelike networks. As potential nanoantennas, silver spongelike networks can offer an effective method to optimize plasmon coupling for synthesizing devices. Keywords: Silver spongelike network; Nanoantenna; SERS; 3D-FDTD I. INTRODUCTION Since the enhanced Raman signals have been found on the surface of metal and the metal compound with metal decoration, the surface enhanced Raman scattering (SERS) technique has been widely application for measurements, chemical and biological sensing1-3. The capability of molecular fingerprinting with ultrahigh and single molecular sensitivity has been combined by SERS technique4, 5. The chemical (CM) mechanism6 and the electromagnetic (EM) mechanism7 are two mechanisms for SERS in previous paper. The EM mechanism is recognized to contribute the most for SERS. Briefly, EM mechanism is owing to the enhanced local electric field that results in a significant increase in the cross section of the Raman scattering8-10. The surface plasmon of metal nanoparticles can induce the EM enhancement. And the EM enhancement has been influenced by the shape, dielectric environment, arrangement configuration and separation of particles, and so on

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. The “hot spots” on the metal

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surfaces are considered important factor for the enormous enhancement of the SERS14, 15. At the same time, field enhancement for SERS from metal nanostructure arrays also have been investigated by some theoretical works16-18. The “hot-spots” have been demonstrated by theoretical and experimental researches. They have been induced in the gaps between the nanoparticles. And they have major contribution to the Raman enhancement19. Many researches have demonstrated that some complex geometries can bring a strong EM coupling, and then obtain a high enhancement factor. These complex geometries are including colloidal clusters20, aggregated nanoparticles21, branchlike structures22 and chains23. These complex geometries can achieve the sufficient “hot spots” effect because of numerous contacts, by comparing with the isolated and individual nanostructure arrays. The high quality SERS substrate can be produced by these routes. The complex geometries system’s plasmon coupling phenomenon has been mentioned extensively by the theories and experiments. However, these influences of the coupling in three-dimensional (3 D) silver spongelike networks about the SERS enhancement have not been studied in detail yet. Here, a high quality SERS substrate has been prepared in this paper. We have demonstrated that this silver spongelike networks structure can be employed as an efficient SERS substrate, providing an enhancement factor as high as 3.5×1012. We have studied the effect of the enhancement factor (EF) that come from the chain diameters (D), chain length, gap, angle of crossed Ag chains and sharp nanotips through three-dimensional finite-difference time-domain (3D-FDTD) simulation. The experimental and simulation results have shown that because the homogenously distributed of “hot spots” can form, the silver spongelike networks have shown a high reproducibility and extremely SERS enhancement. II. EXPERIMENTAL METHODS

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These desired silver spongelike networks24 may be prepared under solvothermal condition. The condition includes alkaline pH solution of glucose and AgNO3. In this research, the typical SEM images of the sample at 16,000× and 160,000× magnifications are shown Fig. 1a and b. The high magnification image has shown that Ag nanowires form 3 D networks by interconnected (Fig. 1 b). In this paper, we pay attention to the SERS characteristic of silver spongelike networks rather than the preparation processes. In SERS spectra measurements, first of all, by using an accurate pipette, we dropped a 10 µL droplet of R6G aqueous solution (different concentration) on the samples. Secondly, let the samples dried in air at ambient temperature, in order to gain a uniform molecule membrane over an area of about 10 mm2. In this experiment, we prepared three samples with the same about SERS-active substrates. In addition, each sample was used to select ten different points in order to detect the R6G probes which confirm the reproducibility and stability of these samples. We used Renishaw 2000 model confocal microscopy Raman spectrometer which includes a CCD detector and a holographic notch filter to measure SERS spectra. Leica DMLM system was adopted in the microscope attachment. The laser beam was focused onto a spot about 1 µm in diameter by using a 50×objective. The SERS was excited by using radiation of 514.5 nm that from an air-cooled argon ion laser which was about 7.2 mW at the position of samples. The recorded time of the Raman spectra is 20 s. All of the Raman spectra were recorded by using baseline corrected and noise filtered 25. III. SIMULATION METHODS We used a commercial FDTD calculations version 7.5.3 for 3D-FDTD simulations26. The boundary conditions of the simulation domain are perfectly matched layer absorbing boundaries. The calculation region is 0.3 × 0.3 × 0.1 µm3, and the cell size is 1 × 1 ×1 nm3. We installed propagation directions of

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plane waves with 514.5 nm wavelength parallel with the x axis 27. In order to closely match the experimental condition, the model of Ag chain was introduced. The Fig. 2 shows the schematic geometries of the structures studied. For simplicity, we used connected Ag nanospheres with different diameter (D) as the calculation models to simulate the electric field enhancement distribution for the silver spongelike networks (distance between centres (L)), as shown in the Fig. 2(a). The parameter of D is the diameter of sphere. The parameter L is L = 0.75 D, it is the distance between centers of spheres. The Fig. 2(b-d) show these models of Ag chains with various gaps, crossed Ag chains with different angle and conical Ag chain, respectively. We assumed the electric field parallel with the x axis, in each case. We set the simulation time to be 300 fs, and input pulse width is 14.70 fs in order to ensure the fields decay completely. We set the refractive index of surrounding medium as 1.0 for air. These Ag arrays were free-floating in air, no substrates. The Ag’s dielectric constant came from the experimental data of Palik 28. We express the near-field intensity enhancement images. They are set up by dividing the electric field strength (E2 = Ex2 + Ey2 + Ez2) around the Ag chain. In the simulation, first of all, we calculate the local electric field. And then evaluate the field intensity for each mesh through integration. At last we compare the computation with the EF values which come from the measured Raman spectra. IV. RESULTS AND DISCUSSION The large surface electric field enhancement for SERS could be induced by the localized surface plasmon resonance. The SERS can be the effectively sensitive diagnostic which used for self-assembled Ag NPs29. Ag spongelike networks at different grown time were used as the SERS samples in order to estimate SERS contribution. The contribution came from the “hot spots” of Ag spongelike networks. Fig. S1 shows the TEM images and SPR absorption spectrum of the Ag spongelike networks that prepared at different grown time. The detailed analyses have been reported in

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our previous paper 24. Fig. 3 presents the SERS spectrum of R6G, the concentration is 1×10-13 M, with different samples at different grown time. We have assigned the spectral traits of R6G in detail previously so we will not repeat here again28. As shown, the R6G on the Ag particles - modified substrate (3 h) displayed very light signal. However, the R6G on the sample (6 h) displayed a medium SERS signal. Compared to the response of the sample (6 h), the sample (9 h) displayed a much stronger SERS response. According to the results about the SERS characteristic for the SERS substrates which we have studied previous30, 31, we can roughly estimate the enhancement factor (EF) for the Ag NPs (3h), Ag chains (6h) and Ag spongelike networks (9h) are 2.2×109, 4.3×1010 and 3.5×1012 by comparing the peak at 1649 cm-1. The R6G’s SERS signals on the Ag spongelike networks -modified substrates are about 1000 times stronger than the R6G’s SERS signals on the isolated Ag NPs substrates. Due to these enhancement factors, they are remarkable for an ensemble-averaged measurement32, 33. The SERS mechanism includes electromagnetic field enhancement mechanism and chemical effect mechanism. The chemical effect can offer a factor of 102. However, the enhanced electromagnetic field can offer an enhancement factor of up to 107-1012 32. In our present research, the SERS signals which from the Ag spongelike networks - modified substrates were much stronger than SERS signals for Ag NPs. Here, we think the local electromagnetic field enhancement is the mainly reason for SERS enhancement. The SERS enhancement probably related to the following factors. First of all, the strong intense electromagnetic field can be produced in the gaps that come from the adjacent Ag nanoparticles. And the electromagnetic field can effectively improve the Raman signal of probe molecules. These probe molecules locate in the gaps that come from the adjacent Ag nanoparticles34. Secondly, the influence of nanopores should be considered, also. The nitrogen adsorption/desorption isotherm (at 77

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K) and Horvath-Kawazoe (HK) pore distribution curve of the Ag spongelike networks are shown in Fig. 4. From the curve we know that the adsorption quantity increases with increase in the pressure. And the adsorption quantity becomes indeﬁnite at the high pressure region. These phenomenon indicate that the adsorption takes place firstly in the pores of the nanomaterials, and later on the surface of the Ag spongelike networks. According to the IUPAC definition, we think this adsorption isotherm is the type II adsorption curve. The Ag spongelike networks’ surface area is 35 m2/g by using the Brunauer-Emmett-Teller (BET) method. Here, in our study, the Ag spongelike networks’ pore diameter is about 2~4 nm. Because of these nanopores, the electromagnetic coupling can become stronger at the spongelike networks films. Through the local optical coupling, these nanopores can induce lots of “hot spots” when the gap width is suitable. In our case, the junction of the branched chains can serve as the hot sites. In the spongelike networks, the ligaments are welded by many Ag nanoparticles which can supply lots of grain boundaries. So, in this case, lots of good activity sites can been provide by these grain boundaries for SERS sensors. The globoid metal nanostructure SERS substrates can provide interparticle fusion for enhanced electromagnetic field only. However, because of having lots of nanoparticle junctions, the Ag spongelike networks substrates can provide lots of “hot sites” for surface plasma, in order to “rough surface” EM enhancement. Obviously, here form more “hot spots” for SERS enhancement when spheroidal NPs begin growing chainlike nanostructures. At these “hot spots”, Ag spongelike networks’ plasmon coupling can provide extremely intense local electromagnetic field, therefore produce strong SERS signals. Ag spongelike networks substrates can make the largest SERS signals because they have lots of considerable coalescence of nanostructures. However, owing to have fewer junctions, Ag chainlike nanostructures substrates can provide smaller SERS signals. In addition, extremely localized electromagnetic fields can be provided by the broken ligaments’ sharp nanotips.

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And the extremely localized electromagnetic fields can produce ultrahigh SERS enhancements35 because the sharp nanotips have additional lightning rod effect36.The detailed shape of the Ag spongelike networks has been reported in our previous paper24. Fig. 5 shows the SERS spectrum of R6G with different concentration that is 10-12, 10-13, 10-14, 10-15 and 10-16 M on the Ag spongelike networks with a growth time 9h. The SERS signal intensities will decrease when the concentration decreases. However, we can still watch the signals at a concentration as low as 10-16 M. Therefore, this silver spongelike networks structure could be employed as an efficient SERS substrate in this study. Through measuring the Raman activity with different storage time, the stability of the Ag spongelike networks - modified substrate has been studied. After the substrates were drop-coated with R6G solution, a Raman signal could be got by them even though 2 months later. In Fig. 6, it represents the aging of the SERS spectrum, which used for R6G (1×10-13 M), on spongelike networks. After a period of 2 months, there is just a very small amount to reduce in the SERS intensity. Therefore, for detecting R6G, the spongelike networks -modified substrate shows the high activity and stability of SERS. The 3D-FDTD method was applied to compute the electromagnetic field distribution which surrounds the laser-illuminated Ag spongelike networks by numerically solving Maxwell’s equations37, in order to further indicate the physical mechanism of SERS enhancements of the Ag spongelike networks. Fig. 7 (a~h) shows the electric field enhancement distribution that comes from Ag chains (Ag particle numbers are 1, 2, 3, 4, 5, 6, 7 and 8, respectively.). The Ag chain diameter is 40 nm. The input light is polarized along the x-axis. When the incident light’s polarization is parallel to the long axis of the Ag chains, the maximal electric field strength (|E|/|E0|)2 (E is the local fields, E0 is the input fields.) is found to be about 5, 11, 26, 84, 485, 168, 40 and 21 for eight models (The color bars of Fig. 7

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(a~h). The units are (V/m)2)). Comparing with the isolated Ag nanosphere, the chain’s field intensity is much stronger than them, the reason is that the polarization direction is along the length of the chain (longitudinal plasmon). For SERS, it is widely believed that |(|E|/|E0|)|4 is the factor for the Raman intensity increases38. The maximum SERS enhancements on the eight models of Ag chains correspond to 2.5×101, 1.2×102, 6.8×102, 7.1×103, 2.4×105, 2.8×104, 1.6×103 and 4.4×102, separately. This result shows that compare with the isolated Ag nanosphere which is consistent in agreement with our experimental results (Fig. 3), the Ag chains have better SERS enhancement ability. When irradiating draw near to longitudinal plasmon resonance, it revealed extremely electromagnetic field at Ag chain. There are two contributions for the electromagnetic field: one is dipolar plasmon resonance, and the other is so-called “the lightning rod effect”. The high curvature of nanorod ends can provide the strong electromagnetic fields. And the high enhancements that come from ‘‘the lightning rod effect’’ have significant contribution for SERS39. The highest intensity will appear on the chains when the electric field intensity at the “hot spots”. However, the “hot spots” do not have the necessary relationship with the number of spheres for the Ag chain. When the number of nanospheres increased in the Ag chain, the intensity of the “hot spots” does not always increase automatic, as shown in Fig. 7. The change manner is the sigmoid by the drastically and monotonically. With the number of nanospheres up to n=5, the intensity of the “hot spots” can provide a maximal electric field strength |E|2 (485). However, when the contacting nanospheres’ numbers continue to increase, the intensity begins to decrease. That is to say, the results show that five spheres are the “optimal aggregation number” for a linear aggregate of Ag nanostructures when the diameter is 40 nm. The Ag chain with five spheres can provide the hottest spot for electric field intensity at 514.5 nm excitation. Compare with the EF of the isolated Ag nanospheres, the EF of the Ag chains is about four orders of magnitude at the same excitation.

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With different polarization directions, the incident light could excite various plasmons of the same mode 40. Fig. 8 shows the arrows which are the different polarization directions: (a) 30º, (b) 60º, (c) 90º. As shown in Fig. 7(e), the incoming light is polarized along the x-axis. And the polarization parallel to the Ag chain’s long axis. Along the long axis’ direction, the field distribution is symmetrical. However, as shown in Fig. 8, the field distribution’s symmetric nodes could convey one side of the Ag chain to the other side when the polarization angle is changed. Therefore, the Ag chain’s surface plasmonic field distribution could be clearly controlled by the incident light’s polarization. Meanwhile, when the polarization direction become small, the Ag chain’s EF could be increased 2~3 orders of magnitude. When its polarization direction is changed, the localized surface plasmon (LSP) coupling will be newly distributed between the two Ag nanoparticles. The EF enhancement was suppressed by the LSP coupling “delocalization” and distribution, the reason is that a parallel orientation of LSP coupling, which coupling with polarization, is essential for getting extremely light intensity. Because the tensor mode polarization is the key for nonlinear optical phenomena, this will be more preferable for EF enhancement

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plasmonic field distribution clearly. Fig. 9 presents calculated the relationship of maximal electric field enhancement that the different diameter of chains and the connected nanoparticle number. The electric field enhancement distribution of the different diameter Ag chains with the different Ag particle numbers are shown in Fig. S2 - Fig. S4. The EF of four models increase in a series: 50 nm < 20 nm < 30 nm < 40 nm, on the basis of the 3D-FDTD calculation results of the maximum field enhancement ((|E|/|E0|)2). Moreover, compared with the Ag chain (D=50 nm), the EF of Ag chain (D=40 nm) can be increased by one order of magnitude. The aspect ratio of the gold nanorods can affect the absorption intensity of the longitudinal

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plasmon resonance42. However, in the present study, it seems difficult to discovery a good connection between the maximum field enhancement and the aspect ratio of the Ag chain. Fig. 10 shows the schematic and simulated maximal field enhancement ((|E|/|E0|)2) of the smooth Ag nanowire. From the electromagnetic field distribution of the system, one can find that the local plasmon field distribution of smooth Ag nanowire is different from Ag chain. The maximal electric field strength (|E|/|E0|)2 of the smooth Ag nanowire has remarkable weaken compared with the Ag chains (as shown in fig. 7 e). The reason for this is the additional charge separation at the notches that come from the joint of Ag nanostructures. It can be displayed that the field strength in the notches becomes singular if the notches possess perfect edges. This singular behavior is fatal for obtaining a reliable absolute value for the electrical field enhancement by means of simulation. Therefore, the Ag chain (D=40 nm) is likely to be more appropriate for a higher field enhancement. So, the Ag chain (D=40 nm) can provide stronger SERS enhancement factor. The local field enhancing has a key and necessary factor that plasmonic coupling between adjacent nanoparticles43. The “hot spots” will appear at the gap area of the dimer when two or more metal nanoparticles contact to each other44. Here, lots of gaps can be formed among the closely region Ag nanowires and the nanopores in our experiments. From the Fig. 4, we known that the pore diameter of the Ag spongelike networks spread in the range of 2~4 nm. Therefore, lots of gaps appear at the Ag spongelike networks. Fig. 11 shows maximal electric field enhancement of the FDTD models with different gaps. As shown in Fig. 11a, the maximal electric field strength (|E|/|E0|)2 of 4.8×103 can be obtained when a small gap appeared between two Ag chains (such as 2 nm). However, the maximal field strength’s value is about 900 when the distance of gap was 5 nm (Fig. 11c). Compared with the right column of Fig. 11a, the value is about 18.8% of the maximal electric field strength with 2 nm gap.

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The plasmonic coupling between the nanoparticles with suitable gap can provide the high electric field. The high electric field can offer the largest Raman scattering enhancements for some molecules presenting in the gap. These SERS ‘‘hot spots’’ with very sensitivity can achieve the single molecule detection. However, as shown the Fig. 11d, the maximal field strength’s value is about 170 when the distance of gap was increased to 10 nm. Compared with the right column of Fig. 7f, the value is not significantly improved over that obtained from isolated Ag chain, representing weak field enhancement within the gap. It means that the plasmon interaction is quite weak in such case due to the field attenuation deviating from either side of the gap. Here, we found an interesting phenomenon that the maximal electric field strength (|E|/|E0|)2 of the Ag chains is much bigger than the Ag nanoparticles with the same gap distance (3 nm), as shown Fig. 11b and Fig. 11f. Compare with the EF of the Ag nanospheres (EF=2.3×104), the EF of the Ag chains (EF=2.3×107) is about three orders of magnitude at the same gap distance. We believe this is most possibly on account of the Ag chains’ dipolar plasmon resonance and “the lightning rod effect”. Due to the electric field reduction, the gap region between the Ag chains could be considered as “cold spots” 45. For plasmon enhanced spectroscopies, these results are very significant on elongated metal nanostructure assemblies. The reason is that, under resonant excitation, the gap regions between elongated nanoparticles do not usually form “hot spots”. Optical signal enhancements could just be realized through exciting the bonding plasmon mode. The plasmon interaction effect will appear when the gap distance is decreased to ~10 nm or less. The hot spot numbers start to increase drastically. As a result, the Raman signal intensity starts to increase. This is the strong interaction case: due to the small distance between the chains and nanohole, localized surface plasmons from each structure are strongly coupled and thereby significantly enhance the field in the gap, providing a great number of hot spots to active the Raman molecule.

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The high magnification SEM image (Fig. 1) has shown that Ag nanowires form 3 D networks by interconnected. Through lots of small Ag NPs, Ag spongelike networks can be linearly welded lead to a larger number of grain boundaries. Towards either catalysis, these grain boundaries can provide lots of high chemical activity sites24. When two or more Ag chains are fitted together forming 3 D networks, we calculated the Ag chains’ surface plasmonic field distribution from different angle, in order to approach the actual situation. Four kinds of crossed Ag chains with different angle are shown in the Fig. 12, including 30°, 60°, 90° and 120° (The angle is along the x axis). Here, the Ag chain has seven Ag particles. And the diameter of Ag chain is 40 nm. From the color bars of Fig. 12 (a~c), we know that the maximal electric field strength (|E|/|E0|)2 is 49, 43, 850 and 145 for four models. From the electromagnetic field distribution of the four models, we know that, by the coupling with different angle, these symmetrical crossed Ag chains can provide a homogeneous local plasmon field. Four models’ EF could be discovered to increase in a series: 30°

Plasmonic Coupling Effect in Silver Spongelike Networks

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