Optical Field Enhancement in Au Nanoparticle-Decorated Nanorod

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Optical Field Enhancement in Au Nanoparticle-Decorated Nanorod Arrays Prepared by Femtosecond Laser and Their Tunable SERS Applications Wei Cao, Lan Jiang, Jie Hu, Andong Wang, Xiaowei Li, and Yongfeng Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13241 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Optical Field Enhancement in Au NanoparticleDecorated Nanorod Arrays Prepared by Femtosecond Laser and Their Tunable SERS Applications §

Wei Cao†, Lan Jiang*,†, , Jie Hu†, Andong Wang†, Xiaowei Li†, and Yongfeng Lu‡ †

Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute

of Technology, Beijing 100081, P.R. China §

Laser Micro/Nano-Fabrication Laboratory, Department of Mechanical Engineering, Tsinghua

University, Beijing, 100084, P. R. China ‡

Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln,

Nebraska 68588-0511, United States KEYWORDS: femtosecond laser, plasmonic nanorod, Au Nanoparticle, optical field enhancement, surface-enhanced Raman scattering (SERS)

ABSTRACT: Various Au nanostructures have been demonstrated to have an enhanced local electric field around them because of surface plasmons. Herein, we propose a novel method for fabricating Au nanoparticle-decorated nanorod (NPDN) arrays through femtosecond laser

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irradiation combined with Au coating and annealing. The nanorod cavities strongly confined light and produced enhanced optical field in response to Au nanoparticles introduction. The nanogap and diameter of the fabricated Au nanoparticles significantly affected the SERS performance, and could be simultaneously tuned with thickness-controllable Au films and substrate morphologies. The resulting Au NPDN substrate was observed to have efficient “hot spots” for tunable SERS applications. We experimentally determined that the enhancement factor of the Au NPDN substrate reached up to 8.3 × 107 at optimal parameters. Moreover, the Au NPDN substrate showed superior chemical stability, with the greatest intensity deviation of 3.2% on exposure to air for 2 months. This work provides a promising method to fabricate tunable plasmonic surfaces for highly sensitive, reproducible and chemically stable SERS applications.

1. INTRODUCTION Surface plasmons (SPs), collective electron oscillations induced on metallic nanostructures, have drawn considerable research interest because of their important applications.1-3 The nanogaps between adjacent metallic nanostructures, where localized surface plasmon resonances occur, produce strong electromagnetic (EM) coupling.4,5 These electromagnetic couplings will enhance local electric fields and have extensive applications such as super-resolution imaging,6 light harvesting,7 molecule sensing,8 and surface-enhanced Raman scattering (SERS).9 In recent years, Scientists have made significant efforts to designing enhanced optical fields (also denoted as “hot spots”) in various geometries including nanopillars,10-12 nanostars,13-16 nanorods,17-22 and nanovoids.23 These so-called ‘hot spots’, which largely increase the intensity of the local electric field around them, are vital to SERS.24


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Nanorod arrays play an important role in generating highly sensitive Raman signals, because they not only increase substrate roughness but also contribute to the production of hot spots, which exist between adjacent nanorods and markedly enhance the EM field around them.17, 21 Moreover, nanorod structures may also produce lighting rod effect resulting in a stronger local field enhancement.25 Many SERS substrates with nanorod morphologies have been fabricated using conventional approaches such as focused ion beam,26 electron beam patterning,27 oblique angle deposition,28,29 and anodic aluminum oxide templates.5,

30,31

Although certain advances

have been achieved in these methods, achieving a high density of regularly spaced nanorod substrates, which are simultaneously repeatable, highly sensitive, and cost-effective, remains difficult for the aforementioned methods. Femtosecond (fs) laser processing is an emerging field and has intrinsic advantages over traditional fabrication methods regarding controllability, economy, and repeatability. Nanostructures induced using femtosecond laser irradiation in AgNO3 solutions have recently been reported to produce enhanced optical field for SERS applications.32-34 However, during photoreduction in a metal ion solution, the production of a consecutive and uniform substrate with homogeneous signals is difficult. The major obstacle is that the photoreduced nanoparticles (NPs) randomly undergo uncontrolled growth and aggregation. Moreover, Ag structures are unstable and easily oxidized on air exposure, leading to rapid reduction in SERS enhancement factors (EFs). Although Au provides slightly lower signal enhancement than does Ag, it has excellent chemical stability. This study proposes an effective method for fabricating highly sensitive, shape-controllable, spatially uniform, and chemically stable SERS substrates through fs laser irradiation. The fabrication procedure for Au NPDN arrays is described schematically in Figure 1, and the


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detailed illustration is provided in the Methods section. Briefly, a linearly polarized fs laser was used to induce a uniform periodic surface structure on silicon substrate in air (Figure 1a); the detailed experimental setup is provided in Figure S1. Subsequently, nanorod arrays were produced in water solutions with a 90° rotated polarization fs laser (Figure 1c). Finally, Au NPDN arrays were prepared by Au coating (Figure 1e) and subsequent annealing (Figure 1f). According to our review of the literature, the two-step method we propose for fabricating nanorod arrays has not been reported until now. The prepared SERS substrates were nanorod arrays decorated with abundant Au NPs, exhibiting enhanced optical field because of particle– nanorod intercoupling of localized SPs.

Figure 1. Schematic process of the Au NPDN substrate fabrication. (a) Scheme of fs laser irradiation in air. (b) Periodic surface structure fabricated through fs laser first processing in air. (c) Scheme of fs laser manufacturing in water. (d) Nanorod arrays formed in water solutions with a 90° rotated polarization fs laser. (e) Nanorod arrays coated with Au films. (f) Au NPDN arrays prepared by annealing. The red arrows in (a) and (c) represent the direction of laser polarization.


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2. RESULTS AND DISCUSSION 2.1 Au NPDN Arrays

Figure S2a depicts scanning electron microscopy (SEM) images of the ripples formed through fs laser first processing in air. As shown in this figure, the period of the ripples was approximately 560 nm. The formation mechanism of the near-subwavelength ripples can be explained as follows.35 When the Si substrate irradiated by fs laser, the electrons in the surface are excited to conduction valence with the photon energy above Si band-gap. If given enough time, the excited electrons will transfer the absorbed energy to lattice to achieve the equilibrium state. However, in our case, because of the ultra-short time scale of the fs laser, the excited electrons can’t achieve the process of energy transfer to lattice, which is typically in need of several picoseconds.36 Therefore, as the number of excited electrons increases to a critical number, the surface plasmon polaritions will be generated when the silicon surface turns into a metallic state.37 These surface plasmon polaritions electromagnetic wave will interfere with the incoming fs laser, thus forming the near-subwavelength ripples.37,38 As presented in Figure S2b, the near-subwavelength ripples were split into nanorod arrays with a 90° rotated polarization fs laser in water; the average period was estimated to be 120 nm, which breaks the light diffraction limit. According to our extensive experiments, the near-subwavelength ripples could not be split into uniform and super-diffraction-limit nanorod arrays with a 90° rotated polarization fs laser in air (Figure S3). Although some researchers have reported that the period of the ripples can be reduced by directly irradiating Si with fs laser in water,37, 39-41 the nanostructures were spatially inhomogeneous because of the generation of lots of bubbles that affected the optical field distribution (Figure S4).37 Therefore, in order to obtain uniform and super-diffraction-limit nanostructures, the two-step method we propose is imperative. The role of the first processing in


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air is to produce laser-induced near-subwavelength ripples. These near-subwavelength ripples can absorb incident light more efficiently than flat Si, which in turn promotes the laser ablation process, thus further reducing the Si ablation threshold in water.41,42 According to previous reports, the Si ablation threshold in water is decreased by a few tens of percent compared to the air environment.39, 43 In other words, the ratioφth (water)/φth (air) is approximately 70% for Si.39 However, the ratio decreased to 62.5% in our case, which can be mainly responsible for the enhanced light absorption of the ripple structure after first processing in air. Notably, the reduced Si ablation threshold in water can substantially reduce the generation of water bubbles, thus yielding a uniform substrate. The role of water is to reduce the period of nanostructures. Regarding the 120 nm nanorod formation on Si in water, the mechanism can be explained as follows.37 When the femtosecond laser irradiates the Si in water, the energy of its rising edge is mainly deposited to the Si substrate where two-photon absorption occurs. The carrier density in Si surface will gradually increase to the critical carrier density and a thin Si layer is oxidized to SiO2 layer (10-20 nm). Approaching the maximum of the fs laser intensity, the optical properties of the water transform into a metallic state when multi-photon absorption occurs, while keeping the SiO2 layer almost transparent. Subsequently, the falling edge of the fs laser excites plasmon modes extending into the SiO2 layer, which is sandwiched by the laser-excited water and Si. Consequently, the 120 nm nanorod obtained in water originates from the interference between incident laser and the surface plasmon polaritions excited in the presence of a 10-20 nm thick SiO2 layer.37


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Figure 2. SEM images of Au NPs formed on three categories of substrates coated with 5, 10, 15, and 20 nm Au films (left to right) and annealing: (a–d) flat Si, (e–h) ripple, and (i–l) nanorod substrates. Scale bars indicate 500 nm.

For comparison, flat Si and ripple substrates were coated with the same thickness of Au films (5–30 nm, dimension interval: 5 nm). Figure 2 shows SEM images of Au NPs formed on three categories of substrates after Au coating and annealing (25 and 30 nm situations are shown in Figure S5). As indicated by the SEM images in Figure 2, the substrates were nearly covered with Au NPs after annealing, and this is because most films are unstable and tend to form Au NPs when heated to a sufficiently high temperature.44 Moreover, notably, surface morphology had considerable effects on the formation of Au NPs. After annealing, the Au NPs on the flat Si substrate were larger, inhomogeneous, and irregular, whereas those on the nanorod substrate seemed to be smaller, uniform, and well-controlled in morphology when the Au film thickness was slightly increased (Figure 2). The comparison demonstrates the efficiency of our method of


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fabricating nanorod structures to obtain more uniform and shape-controllable Au NPs. To further investigate the effects of substrate morphology on Au NP size, the mean diameter, density, and standard deviation (SD) of the Au NPs on the flat Si, ripple, and nanorod substrates were determined, as illustrated in Figure 3 [Au NP-decorated Si (NPDS), NP-decorated ripple (NPDR), and NPDN substrates represent Si, ripple, and nanorod structures with Au film coating and annealing, respectively].

Figure 3. Statistical information of Au NPs. Variations in (a) mean diameter, (b) density, and (c) SD of Au NPs with varying thickness on Au NPDS (black line), NPDR (blue line), and NPDN (red line) substrates. (d–f) Histograms of NP diameter distribution on (d) Si, (e) ripple, and (f) nanorod substrates with Au film (15 nm) coating and annealing. The insets represent the corresponding SEM images.


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Figure 3a–c shows variations in the mean diameter, density, and SD of the Au NPs on different substrates, respectively. Overall, the mean diameter of the Au NPs rapidly increased with the Au film thickness (Figure 3a) because of increased Au deposition. Notably, the density of the Au NPs decreased (Figure 3b) as the size of the NPs increased, indicating the merging of the adjacent NPs during annealing. Moreover, as the deposited thickness increased, the NP diameter tended to deviate from the average, as indicated by the increasing SD (Figure 3c). At the same film thickness (15 nm), the Au NPs on the nanorod substrate showed minimum mean diameter and SD and maximum density (Figure 3d–f). This clearly indicates that substrate morphology affects the formation of Au NPs. In other words, the size of the Au NPs can be controlled by different substrates deposited with thickness-controllable Au films.

Figure 4. Formation of Au NPs on Si, ripple, and nanorod substrates.

The flat Si, ripple, and nanorod substrates were coated with Au films through electron beam evaporation. Fundamentally, the deposited Au films comprising many miniature NPs measuring a few nanometers are mainly distributed along ridges or trenches on the ripple and nanorod substrates (Figure 4, initial stage). When the Au films heated by enough temperature, they


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markedly rupture and dewet to Au islands.45 In this process, the films experience a strain because of the differential thermal expansion coefficient, given by eq 1:44 T

ε T = ∫ (α s − α f )dT + ε 0 ≈ (α s − α f ) ∆T + ε 0 T0

(1)

where α s and α f are the thermal expansion coefficients of the substrates and films, respectively, and ∆T and ε 0 are the change in temperature and initial strain, respectively. According to eq 1, if the coefficient of thermal expansion between Si and Au films is different, the strain will be generated and increase with the temperature.46 The Au–Si interface will produce uneven stress distribution, which facilitates the migration of the film over the stressed areas to more relaxed regions,45 resulting in the formation of Au islands (Figure 4, intermediate stage). With increasing temperature, the Au islands tend to coalesce and modify their shape to become more rounded to reduce their surface free energy (Figure 4, final stage). Typically, there are three factors determining the final morphology of the Au NPs. The first factor is the initial film thickness determined by the deposition time. The second one is the annealing condition namely temperature, holding time, and temperature increasing rate. The third factor is the intrinsic morphology of the substrates. In our cases, because the heating condition kept constant, the Au film thickness and substrates themselves mainly accounted for the different sizes of the Au NPs. As illustrated in Figure 4, the periodic nanorod structures had fewer Au deposition on a single ridge and trench than did the ripple substrate because of their smaller periodicity, which prominently influenced the ultimate size of the Au NPs. In other words, many consecutive Au films were intersected into discontinuous parts by smaller nanorod structures to prevent further migration of the deposited Au, finally contributing to the formation of smaller Au NPs on the nanorod substrate. However, with increasing Au deposition, the adjacent Au NPs formed on the


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ridges were sufficiently close to merge into larger Au NPs (Figure S5), resulting in increased mean diameter (Figure 3a). 2.2 Highly Sensitive and Tunable SERS Application of the NPDN Arrays

The SERS enhancement properties of the Au NPDN arrays were evaluated using rhodamine 6G (R6G) solutions as the target molecules; R6G is the most commonly used dye in biological experiments. After the addition of a 10 µL droplet of R6G (concentration: 10−6 mol/L), Raman spectra were measured at 10 different locations for each sample. For comparison, smooth and flat Si with 10 µL of R6G (concentration: 10−2 mol/L) was also used as a reference in Raman detections. The Raman spectra of R6G obtained on the Si, ripple, and nanorod substrates coated with varying thicknesses of Au films and annealed are presented in Figures S6, S7, and 5a, respectively. As shown in the SERS spectra in Figure 5a, no apparent signals of R6G were observed on the reference samples. Meanwhile, the Raman signals on the Au NPDN arrays initially increased with the thickness of the Au films and then started to decrease when the thickness exceeded 15 nm (Figure 5a). To explain this phenomenon, we considered the parameters of NP diameter (D) and nanogap (G) between adjacent NPs to evaluate the SERS performance. Studies have reported that the size and spacing of Au NPs significantly influence the localized SP resonance.47-50 Figures S8–10 present the FDTD simulation results of the local electric field around different diameters of a single Au NP on the flat Si substrate when not considering particle interactions. The electric field (E/E0) was enhanced initially with increasing NP diameter and then decreased as the diameter exceeded 140 nm (Figure S10). In other words, an optimal NP diameter could be obtained for maximum SERS performance, a finding that is consistent with those of previous studies.45,51 In our case, a single Au NP produced the maximum local electric field when the NP size was approximately 140 nm. Before peaking, the local


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electric field increased with the NP diameter. Considering spacing, at a constant NP diameter, the interparticle coupling effect was reported to occur when the nanogap was smaller than the NP diameter.52 That is to say, when the interparticle coupling effect occurred, SERS enhancement was sharply increased with NP diameter-to-nanogap ratio when the NP diameter was smaller than 140nm. In the current study, we defined a nondimensional number K with the particle diameter (D) divided by the nanogap (G). The detailed K values of the Si, ripple, and nanorod substrates coated with varying thicknesses of Au films and annealed are presented in Tables S1, S2, and S3, respectively. Regarding the Au NPDR and NPDN substrates, the K value initially increased with the Au film thickness before reaching the maximum at a thickness of 15 nm and then tended to decrease as the Au film thickness increased. The variation in the K value thus coincides with the Raman spectra shown in Figure 5a. Based on the aforementioned analysis, we speculate that a compromise is required to equilibrate the pros and cons of the deposited Au film thickness. The Raman spectra in Figures 5a and S7 support the existence of such a compromise in that the optimal thickness was located between excessively thin and excessively thick Au films. However, concerning the Au NPDS substrate, the K value was not adaptable any more when the thickness of Au film was more than 10 nm, because the nanogap was larger than the NP diameter (Table S1), and the interparticle coupling effect could be neglected.52 Under this circumstance, the NP diameter becomes the only factor deciding the SERS enhancement of the substrates. Therefore, the Raman signals associated with the Au NPDS substrate first decreased with the K value, and then they continued to decrease because of the loss of interparticle coupling and deviated considerably from the optimal value achieved when the NP diameter was approximately 140 nm.


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Figure 5. Raman spectra of R6G obtained on (a) nanorod substrate coated with different thicknesses of Au film and annealed and (b) different types of substrates, namely Si, ripple, and nanorod substrates, after the deposition of 15 nm film and annealing. (c) Variations in the average EF with Au film thickness calculated on three categories of substrates: Au NPDS, NPDR, and NPDN substrates.


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To justify the benefit of the Au NPDN arrays fabricated in this study, the Raman spectra on different categories of substrates, namely Si, ripple, and nanorod substrates, after the deposition of 15 nm films and annealing were also analyzed (Figure 5b) under the same experimental condition. The Au NPDR substrate produced higher signals than the Au NPDS substrate because of the light-trapping effect of the ripple structure, which can efficiently couple the incident photon to the three-dimensional microstructure to excite the molecules.53 Despite these factors, the Au NPDR substrate still yielded much weaker signals than the Au NPDN substrate because it had a greater period, which inhibits the production of narrower “nanogaps” existing between adjacent ridges. To quantitatively investigate the EFs on different substrates, the EFs were calculated using the Raman peak at 1361 cm−1 (the assignments of the main bands of R6G are shown in Table S4) through the classic formula54 shown in eq 2:

EF =

I surf

N surf

/

I vol Nvol

(2)

where I surf and I vol indicate the Raman intensities on SERS-active substrates and reference samples, respectively; N surf and Nvol represent the number of molecules detected on SERSactive substrates and reference samples, respectively. Herein, we assume that the effective areas to adsorb R6G molecules are identical for the reference and SERS substrates. Therefore, the accumulating molecules are proportional to the concentration of the target molecules. Figure 5c indicates the variations in the average EF with Au film thickness, as calculated on the three categories of substrates: Au NPDS, NPDR, and NPDN substrates. The EF of the Au NPDN substrate showed a sudden increase and a subsequent sharp decrease with increasing Au film thickness (Figure 5c). In particular, in the case of 15 nm Au deposition and annealing, the


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calculated EFs of the Au NPDS, NPDR, and NPDN substrates were 8.5 × 105, 2.5 × 107, and 8.3 × 107, respectively, indicating highly sensitive and tunable SERS applications.

Figure 6. (a) Raman mapping image of 12 µm × 12 µm area using the integrated intensity at a band of 1361±10 cm-1 on NPDN substrate with 15 nm Au deposition and annealing. (b) Variations of Raman spectra over time on the nanorod substrate of 15 nm Au deposition and annealing. The reference represents the Raman signal obtained on smooth and flat Si with 10 µL of R6G (concentration: 10−2 mol/L).

For practical applications, the reproducibility and stability of SERS substrates are as important as their EF and are sometimes even considered the most accurate indicators in multiple measurements. Studies have reported that an enhancement of 106–109 is high enough for single molecule detection;55,56 thus, the EF of the Au NPDN substrate fabricated in the current study is adequately high for molecular-level measurements. Accordingly, the reproducibility and chemical stability of SERS substrates have become more imperative. To investigate the reproducibility of the substrates, Raman mapping image of 12 µm × 12 µm area using the integrated intensity at 1361±10 cm-1 was obtained on NPDN substrate with 15 nm Au deposition and annealing (Figure 6a). The relative standard deviation (RSD) of the Raman intensity at 1361 cm-1 is estimated to be ~15% over 169 grid cells with an equal 1 µm spacing, demonstrating


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highly homogeneous signals on our NPDN substrates. Meanwhile, the Au NPDN substrate showed superior chemical stability, with the greatest intensity deviation of 3.2% on exposure to air for 2 months (Figure 6b). 2.3 Optical field enhancement based on FDTD simulation

Nanorod structures have been reported to show highly sensitive Raman signals because of the formation of nanogaps between adjacent nanorods, thus considerably enhancing the electric field.17, 21 In the present study, we fabricated a nanorod substrate covered with abundant Au NPs, which yielded optical field enhancement in two aspects. First, the nanorod structure itself stimulated the electric field enhancement because of the nanogap effect. Figures 7a and 7b present the comparison of a local electric field induced by a plane wave (λ = 532 nm) incident vertically on the ripple and nanorod substrates based on FDTD simulation. The gap between adjacent ridges on the nanorod substrate was approximately 30 nm, whereas that on the ripple substrate was approximately 100 nm. Apparently, the nanorod substrate produced higher electric enhancement than did the ripple substrate because it promoted the production of narrower nanogaps between adjacent ridges. Considering that the SERS signals are proportional to the biquadrate of the electric field, the Raman spectra enhancement was more remarkable on the nanorod substrate than on the ripple substrate (Figure 5b). This comparison demonstrates the efficiency of our method of fabricating nanorod structures to obtain highly sensitive substrates. Second, Au NPs also contributed to local electric field enhancement because of collective electron oscillations at Au–Si interfaces. Figure 7d shows the FDTD simulation of a single Au NP (diameter = 20 nm) on a flat Si substrate. As illustrated in the figure, the single Au NP boosted the intensity of the local electric field by several times at the Au–Si interface, where the light energy could be focused to the junction through the intercoupling of localized SPs.


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Although the single Au NP on the flat Si substrate already produced a considerably enhanced local electric field, notably, the local electric field was markedly improved when the Au NP was placed inside the nanorod cavity (Figure 7e). Particle-in-cavity architectures have been reported to produce optical field enhancement at particle–cavity interfaces when one of the cavities is resonant with the cavity-depressed NP mode.57,58 In the present study, the fabricated nanorod substrate was decorated with Au NPs on ridges or cavities. Therefore, it can be concluded that the SERS performance is mainly decided by interparticle coupling on ridges and optical field enhancement in cavities.

Figure 7. FDTD simulations of local electric field induced by a plane wave (λ = 532 nm) incident vertically on different structures: (a) ripples and (b) XY section of nanorods. (a) and (b) have the same scale bar and polarization direction. (c) The YZ section of a single nanorod cavity. (d) A single Au NP (diameter = 20 nm) on a flat Si substrate; (e) A single Au NP (diameter = 20 nm) in the nanorod cavity. (c), (d), and (e) have the same scale bar and polarization direction. The black arrows represent the direction of laser polarization.


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CONCLUSION This study presents an effective method for fabricating highly sensitive, shape-controllable,

and chemically stable SERS substrates through fs laser irradiation combined with Au coating and annealing. The prepared SERS substrates were nanorod arrays decorated with abundant Au NPs. The size of the Au NPs can be well-controlled using different substrates deposited with thickness-controllable Au films. The nanorod structure facilitates the formation of the smallest Au NPs in that periodic nanorod structures have fewer Au deposition on a single ridge and trench than do ripple substrates because of their smaller periodicity. Therefore, at the same Au film thickness, the Au NPDN substrate always showed the minimum mean diameter, SD, and maximum density compared with the Au NPDS and NPDR substrates. The Raman signal on the Au NPDN substrate had an optimal intensity at an Au film thickness of 15 nm. To explain this phenomenon, we considered the parameters of NP diameter (D) and nanogap (G) to evaluate the SERS performance. A compromise was considered to equilibrate the positive and negative effects of the deposited film thickness. Regarding the Au NPDN substrate, the optimal EF reached up to 8.3 × 107, demonstrating adequate intensity for single molecule detection. The relative standard deviation (RSD) of the Raman intensity at 1361 cm-1 on the Au NPDN substrate is estimated to be ~15% over 169 grid cells. Moreover, the Au NPDN substrate exhibited chemical stability, with the greatest intensity deviation of 3.2% on exposure to air for 2 months. The FDTD simulation indicated that the nanorod substrate produced higher electric enhancement than the ripple substrate because it promoted the production of narrower nanogaps between adjacent ridges. Furthermore, the nanorod cavities strongly confined light and produced optical field enhancement in response to Au NP introduction. Therefore, the SERS performance of Au NPDN substrates is mainly decided by interparticle coupling on ridges and optical field


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enhancement in cavities. This study broadens the scope for further functionality of plasmonic devices for applications in SERS, light harvesting, biological imaging and catalysis. 4

METHODS

4.1 Fabrication of Au NPDNs

We performed substrate nanofabrication experiments by using a mode-locked Ti: Sapphire laser system (Spectra Physics, Inc., Santa Clara, California, USA; 800 nm, 35 fs, 1 kHz). An objective lens (Olympus MPLFLN 10×; numerical aperture [NA] = 0.3) was used to focus the Gaussian profile laser beam perpendicular to the Si substrate. The Si sample was placed on a computer-controlled, six-axis motion stage (M-840.5DG, PI, Inc., Karsruhe, Germany). The resolution of the stage in the XY directions and Z direction are 1 µm and 0.5 µm, respectively. The laser power was controlled using a graduated neutral density filter. The direction of the fs laser polarization was adjusted by a half-wave plate. First, the linearly polarized fs laser irradiation was applied to achieve a periodic surface structure in air by using high-pressure nitrogen gas. The scan direction was perpendicular to the laser polarization. The laser fluence, scan speed, and interval were 0.32 J/cm2, 160 µm/s, and 2 µm, respectively. Second, the nanorod arrays was produced in water solutions with a 90° rotated polarization fs laser. To reduce the effects of vibration produced by the motion stage, at this time, the scan velocity was reduced to 20 µm/s. The total laser fluence was reduced to 0.2 J/cm2 because of the reduced Si ablation threshold after the first processing, which can substantially reduce the generation of water bubbles, thus yielding a uniform substrate. Finally, an electron beam evaporation coating system (DENTON Explorer 14 Coating System) was used to varnish the processed nanorod substrate with Au films with different thicknesses. The film thickness can be accurately controlled by


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modulating the evaporation time. A muffle furnace (GHA12/300, Carbolite) was used to conduct the annealing process. Before achieving the maximum temperature of 1065 °C, the temperature increase rate was set at 40 °C/min and the soaking time was set at 1.5 h to induce the annealing. 4.2 Sample Characterization

SEM images of the samples were obtained through a scanning electron microscope (XL30S– FEG, FEI, Inc., Hillsboro, Oregon, USA) with an acceleration voltage of 10 kV. The morphology of the substrates was characterized using an atomic force microscope (Dimension Edge PSS, Bruker, Inc., Karsruhe, Germany). 4.3 Raman Measurements and Data Processing

SERS measurements were performed by identifying R6G molecules by using a Renishaw InVia Reflex spectrometer. Prior to the measurements, 10 µL of R6G was dropped on all the samples, followed by an air-drying process for at least 2 h. A concentration of 10−2 mol/L R6G was detected on the flat Si substrate for reference (excitation power: 6 mW) and 10−6 mol/L for the Au NPDR and NPDN substrates (excitation power: 0.06 mW). To preferably acquire Raman signals on Au NPDS, 10−5 mol/L R6G was adsorbed for measurements. For each sample, the Raman spectra were obtained from 10 locations through a Renishaw Raman microscope at an excitation wavelength of 532 nm by using a 50× microscope objective (NA = 0.5). Finally, the spectra were baseline-corrected and then averaged by professional Raman software (Wire 3.2). Figure S11 shows the SERS spectra of NPDN substrate after deposition of 15 nm Au film and annealing without correcting the baseline.


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4.4 FDTD Simulations

Numerical simulation was conducted using commercial software (FDTD solution). The calculation was accomplished on a single- and multiple-period segment of the nanostructures with a uniform 0.5 nm mesh. The simulation region was terminated using periodic boundary conditions along X and Y directions and using perfectly matched layer boundaries in the Z direction of plane wave propagation. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website. Additional data of experimental setup, SEM characterizations, Raman measurements and FDTD simulations. AUTHOR INFORMATION Corresponding Author *(L.J) E-mail: [email protected], [email protected] ORCID Lan Jiang: 0000-0003-0488-1987 Notes The authors declare no competing financial interest.


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Abstract Graphic


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Optical Field Enhancement in Au Nanoparticle-Decorated Nanorod

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