Highly Ordered Fe–Au Heterostructured Nanorod Arrays and Their

Article pubs.acs.org/Langmuir

Highly Ordered Fe−Au Heterostructured Nanorod Arrays and Their Exceptional Near-Infrared Plasmonic Signature Yong Zhang,†,§ Brian Ashall,‡,∥ Gillian Doyle,‡ Dominic Zerulla,*,‡ and Gil U. Lee*,† †

School of Chemistry and Chemical Biology and ‡School of Physics, University College Dublin, Dublin 4, Ireland S Supporting Information *

ABSTRACT: The potential of highly ordered array nanostructures in sensing applications is well recognized, particularly with the ability to define the structural composition and arrangement of the individual nanorods accurately. The use of heterogeneous nanostructures generates an additional degree of freedom, which can be used to tailor the optical response of such arrays. In this article, we report on the fabrication and characterization of well-defined Fe−Au bisegmented nanorod arrays in a repeating hexagonal arrangement. Through an asymmetric etching method, free-standing Fe−Au nanorod arrays on a gold-coated substrate were produced with an inter-rod spacing of 26 nm. This separation distance renders the array capable of sustaining resonant electromagnetic wave coupling between individual rods. Owing to this coupling, the subwavelength arrangement, and the structural heterogeneity, the nanorod arrays exhibit unique plasmonic responses in the near-infrared (NIR) range. Enhanced sensitivity in this spectral region has not been identified for gold-only nanorods of equivalent dimensions. The NIR response offers confirmation of the potential of these highly ordered, high-density arrays for biomedical relevant applications, such as subcutaneous spectroscopy and biosensing.

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INTRODUCTION Metallic nanorods, particularly Ag and Au nanorods, have recently attracted significant attention because of their remarkable optical characteristics based on the excitation of surface plasmon resonance (SPR).1,2 Compared to those of nanorods dispersed in solutions3−5 or placed on substrates,6−10 nanorod arrays with their long axes aligned perpendicular to the substrate exhibit strong anisotropic optical behavior that can be beneficial for spectroscopy, sensing, and imaging applications.11−16 The large number of possible applications for the arrays demands the ability to create highly periodic arrangements of uniform nanorods with minimal irregularities over large sample sizes. Flexibility is also required in terms of the nanorod composition, with multisegment nanorods the most highly coveted. The synthesis of heterogeneous nanoparticles consisting of magnetically active and plasmonically active sections has been successfully achieved via a solution-phase process.17−19 However, reproducibly organizing these anisotropic particles into vertically aligned array structures through bottom-up methods such as self-assembly processes remains challenging.20 Vapor−liquid−solid (VLS) growth has achieved the best results in producing semiconductor nanowire arrays with the size and periodicity determined by the catalytic metal alloy droplets.21 However, the VLS method shows limited applications in preparing metallic nanowire arrays and in controlling the periodic distance below 100 nm. Top-down methods such as © 2012 American Chemical Society

electron-beam lithography (EBL) allow for excellent spatial control,22 but EBL is generally limited in sample throughput and cannot easily provide a rod separation of less than 25 nm. Porous anodic alumina (PAA) template-assisted deposition offers a direct route to manufacturing metallic nanorod arrays with hexagonal 2D orders.23−25 However, the fabrication of highly ordered nanorod arrays using PAA templates is still quite challenging because of the “nanocarpet” effect during the wetchemistry etching process.26 It will be shown that excellent reproducibility in terms of the individual nanorods and the periodic arrangement has been achieved with this process while maintaining a degree of spectral tunability. In noble metals, the spectral sensitivity is predominantly controlled by the alteration of the aspect ratio of the nanorods. The optical penetration depth through biological tissue reaches its maximum in the NIR range, which has shown promising results in in vivo sensing, diagnostics, and therapy. Furthermore, SPR in the NIR range has been achieved in noble metal nanorod arrays. However, the current research is mostly based on pure plasmonic nanorod arrays with high aspect ratios.14,23 To the best of our knowledge, there does not exist a published result demonstrating the NIR plasmonic response Received: June 6, 2012 Revised: September 27, 2012 Published: October 26, 2012 17101

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Scheme 1. Schematic Illustration of the Two Types of Etching Processesa

a

(A) Asymmetric etching process. (B) Conventional etching process. The green dots represent the aqueous etching phase. above. A third 5 h anodization step produced a highly ordered PAA template. The residual aluminum was removed by immersing the template in a saturated mercury chloride aqueous solution for 4 h. The PAA membrane was subsequently immersed in a 1 vol % phosphoric acid solution for pore widening for 12 min, resulting in a pore diameter of approximately 70 nm (Figure S1A, Supporting Information). Synthesis of Fe−Au Nanorods inside PAA Templates. A thin layer of Au was sputtered on one side of the PAA template to serve as a working electrode in an electrochemical setup (Figure S1B). It should be noted that the unique surface profile of the other side of the Au substrate (Figure S1C) provides the possibility for better adhesion between the template and the substrate. An Ag/AgCl reference electrode and a Pt wire counter electrode were utilized to form a threeelectrode configuration. The electrodeposition was controlled using a potentiostat/galvanostat (2273 Princeton Applied Research). The Fe and Au segments were deposited from freshly made iron sulfate solution (FeSO4·7H2O, 120 g/L; boric acid, 45 g/L; ascorbic acid, 1 g/ L) and a commercial electroplating solution (Orotemp 24 RTU, Technic Inc.), respectively, at −0.9 V (vs Ag/AgCl). Figure S2 (Supporting Information) presents the cross-sectional field-emission scanning electron microscopy (FE-SEM) images of the Fe−Au nanorods in PAA templates, indicating a homogeneous growth rate of each segment of the nanorod. Preparation of Fe−Au Nanorod Arrays through an Asymmetric Etching Process. Preliminary experiments showed that when the nanorod arrays were immersed in the etching solution the piece would curl after the removal of the PAA template (Scheme S1B). Therefore, to obtain highly ordered nanorod arrays, a new strategy was introduced into the etching process. The PAA templates were successfully removed with a dilute sodium hydroxide (0.01 M, Aldrich) aqueous solution in an asymmetric etching process (Scheme 1A). The etching of the PAA membrane involves a two-step mechanism: the etching of the empty section of the membrane was rapid, but the etching of the Au−Fe section of the membrane was relatively slow. In the first step, the interface between the membrane and the solution is naturally refreshed by etching for 2 to 3 min. In the second step, the interface has to be refreshed by gently shaking the solution by hand for 25−28 min. The stress in the nanorod array was

from heterostructured nanorod arrays, particularly with very small aspect ratios. In this article, we initially present a technique for the fabrication of a highly periodic, well-defined Fe−Au bisegmented, freestanding nanorod array with designated subwavelength features (e.g., a chosen rod separation of 26 nm). These highly ordered array structures were obtained on a large scale via an efficient etching method. A comprehensive characterization of these systems was accomplished via sensitive reflectance spectroscopy specifically designed for investigating polarization and angular variation. This experimental work was complimented by discrete dipole approximation (DDA) simulations of bisegmented Fe−Au nanorods, both as individual and multiple systems. The optical characteristics of the arrays were found to exhibit unique plasmonic responses accessible by incident angle and/or polarization control. The coupled plasmonic modes27 in the NIR range have been achieved here for the first time from such a heterostructured array with an aspect ratio of merely 1.3 for Au segments and have been confirmed by simulation. This unique NIR response offers a new pathway for future biosensing applications18 without the need for large aspect ratio gold nanorods.

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EXPERIMENTAL SECTION

Synthesis of Porous Anodic Alumina (PAA) Templates. The PAA templates were synthesized by a three-step anodization method (1), modified from the method established by Masuda et al.2 A highpurity (99.999%) piece of aluminum (Aldrich) was electropolished in a mixture of 5 vol % sulfuric acid (98%, Aldrich), 95 vol % phosphoric acid (85%, Aldrich), and 20 g/L chromium oxide (Aldrich) at 20 V and 85 °C for 2 min. This polished piece of aluminum was anodized in 0.42 M oxalic acid (Aldrich) at 40 V and 0 °C for 11 h. The alumina layer was removed from the sample using an aqueous mixture of chromium oxide, phosphoric acid, and water (45 g/L/3.5 vol %/96.5 vol %) at room temperature for 5 h. A second 6 h anodization and 5 h stripping step was conducted under the same conditions as described 17102

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Discrete Dipole Approximation Simulations. Simulations investigating the scattering of light from individual, double, and trigonal Fe−Au nanorod targets were carried out at the Phaeton Cluster Facilities (University College Dublin) using DDSCAT version 7.0.28 To simulate the nanorods accurately, a number of initial parameters, including material optical constants29,30 and target geometries, were defined in accordance with the experimental setup. The simulations presented here were carried out at a scattering angle of 55°, and the light source was unpolarized because this is the general practice for visible spectrum experiments. Each of the four Fe−Au targets (single, double, trigonal, and hexagonal) and the hexagonal Au consist of a lattice containing 104 < N < 105 dipoles. Four Fe−Au nanorod configurations were analyzed that are summarized in Table 1. The first was a free-standing single Fe−Au

linked to the etching speed in this second step, and an optimized etching speed was found in 0.01 mg/mL NaOH solution. After three thorough washings of the nanorod array in water, the array was attached to a substrate and allowed to dry. Electron and Optical Microscopy. The top-view and crosssectional scanning electron microscopy (SEM) images were acquired with an FE-SEM (Hitachi S-4300). The crystal structure of the nanorods was characterized with transmission electron microscopy (TEM, JEOL 2100) and selected-area electron diffraction (SAED). The single-crystalline structure of the Au segments was confirmed by their SAED pattern, as can be seen in Figure S3B (Supporting Information). Typical diffraction rings in the SAED pattern for the Fe segments, shown in Figure S3C (Supporting Information), revealed their polycrystalline structure. The photograph in Figure 1A was acquired with a Nikon Eclipse 80i upright microscope equipped with a Nikon Digital Sight DS-Fi1 color CCD camera, a 100 W Hg light source, and a 10× Nikon objective lens.

Table 1. Detailed Parameters Used for DDA Simulations target

effective radius (nm)

number of dipoles

interdipole separation (nm)

single Fe−Au double Fe−Au trigonal Fe−Au hexagonal Fe− Au

63.447 81.73 133.64 122.60

10 403 22 680 96 512 74 340

4.7 4.7 4.7 4.7

nanorod that matched its dimensions to the experimentally examined rods. The purpose of this single-rod simulation was to identify plasmonic modes associated with isolated Fe−Au nanorods, as shown in Figure S6A (Supporting Information). The second configuration was composed of two closely neighboring Fe−Au nanorods, as shown in Figure S6B. This double-rod configuration was used to investigate and identify coupled modes between neighboring nanorods mimicking an experimental separation of 26 nm. The third target configuration of the three nanorods again has an interrod spacing of 26 nm in a trigonal arrangement, as shown in Figure S6C. The purpose of this target was to show that by increasing the number of rods in close proximity to each other, the coupled plasmonic modes become more pronounced. Finally, a hexagonal arrangement that can be used as a repeatable unit cell to simulate larger sections was simulated (Scheme 2).

Figure 1. Fe−Au nanorod arrays showing highly ordered structures. (A) Top-view SEM image illustrating the nanorod arrays from a highly uniform hexagonal arrangement. The inset image shows a representative photograph of reflected light from a 100 × 100 μm2 area of the nanorod arrays and serves to illustrate the uniformity of the nanorod array on this scale. (B) Tilted SEM image revealing the segmented nanorod structure of the Au−Fe arrays (i.e., the Au and Fe sections can be identified by the intensity of the back-scattered electrons). The inset image shows a representative SEM image of the Au substrate after the removal of the template and prior to the deposition of Fe metal. This exposed interface was obtained by sonicating the nanorod array.

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RESULTS AND DISCUSSION Figure 1 presents the field-emission scanning electron microscopy (FE-SEM) images of the Fe−Au nanorod arrays. The top-view SEM image (Figure 1A) illustrates that the nanorod arrays are highly regular in a hexagonal arrangement,

Magnetometry. The magnetic properties of the nanorod arrays were analyzed by superconducting quantum interference device (SQUID) magnetometry (Quantum Design, MPMS XL 7T). Figure S4A,B (Supporting Information) present representative hysteresis loops of the arrays and the magnified area (smoothed) of the loops near the origin, with the applied field oriented parallel and perpendicular to the nanorod's long axis, respectively. Spectroscopy and Optical Characterization. To characterize the optical response of the nanorod arrays, reflectance spectra were acquired over a range of illumination angles and polarization states using a precision, triple-axis goniometric table. The experimental setup is illustrated in Figure S5 (Supporting Information). A white-light source (100 W halogen lamp) was focused and collimated to give a beam of white light of 5 mm diameter. A polarizer (extinction ratio of 5000:1) could also be inserted into the beam path when required. The reflected light was analyzed with a fiber optic coupled spectrometer (Photon Control SPM-002) that had the sample goniometer at its fulcrum. Spectral reflectance measurements for unpolarized, perpendicularly polarized, and parallel polarized illumination were acquired for illumination angles of between 15 and 60° in 2.5° steps. The intensities of all spectra were normalized against the illumination light source. This process removed both the spectral profile of the light source and the spectrometer (i.e., the spectrometer's sensitivity decreases in both the UV and IR directions).

Scheme 2. Schematic Illustration of the Hexagonal Unit Target for the DDA Simulations

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and the tilted cross-sectional image (Figure 1B) reveals the Fe− Au segmented nanorod structure. The fabricated rods have a diameter (D) of 70 ± 0.9 nm, a periodic midpoint−midpoint distance (d) of 96 ± 1.1 nm, an iron segment length (LFe) of 186 ± 6.8 nm (aspect ratio of 2.7), and a gold segment length (l) of 92 ± 2.5 nm (aspect ratio of 1.3). The direct contact between the nanorod and the substrate and the strong mechanical strength of the interface were desirable in avoiding potential contamination caused by the catalyst used in VLS growth18 and in protecting against the nanocarpet effect during the etching process.23 The resulting nanorod arrays have an asymmetric structure with the Au end of the nanorods exposed to the media and the Fe end anchored to the substrate. When the separation between adjacent nanorods is on the order of 1−100 nm, the capillary pressure (ΔP = γ/r, where γ is the surface tension of the liquid and r is the rod separation) can be high enough to cause the nanorods to deform easily. This nanocarpet effect was eliminated by applying an asymmetric etching method as described in the Experimental Section. The etching of the PAA membrane involves a two-step mechanism, as shown in Scheme 1, in which the empty membrane is first rapidly removed while the section filled with Au−Fe is removed more slowly. The nanocarpet effect arose from the etching process rather than from the conventional liquid evaporation process because it was found that the excess stress during the etching process is significantly greater than the surface tension when the liquid evaporates. It should be noted that the effect from liquid evaporation is negligible because of the relatively small aspect ratio of the nanorods and the strong adhesion between nanorods and the substrate. Also, the nanorod arrays exhibit good flexibility as a result of having a thin (300−500 nm) Au layer as the substrate. These factors enabled the reliable and reversible transfer of the monolithic nanorod arrays onto glass, silicon, or sapphire. The inset photograph in Figure 1A clearly shows the apparent uniform green color of the nanorod arrays observed in the far field, which is consistent with its characteristic spectra where the peak at approximately 530 nm dominates in the visible range (Figure 2). The presented nanorod arrays possess an ultrahigh density of 1.25 × 1014 rods/m2, with each nanorod normal to the substrate surface and arranged in a hexagonal 2D order. The single-crystalline structure of Au and the polycrystalline structure of Fe have been revealed, respectively (Supporting Information). Furthermore, compared to Ni nanorod arrays,31 the high saturation magnetization (ca. 200 emu/g) and soft magnetic (the squareness, i.e., the ratio between the remanent and saturated magnetization, Mr/Ms < 0.1) behavior of the Fe segment allows for the controlled manipulation of the monolithic nanorod arrays in a 2D manner (i.e., on the surface of an aqueous solution in applications such as the detection of heavy metal ions32). Optically, the Fe segment also functions as a spacer, minimizing the potentially unwanted coupling between the Au nanorod and the Au substrate. One interesting application of the above-described nanorod arrays is based on their potential to support a strong plasmonic response.33 Indeed, in the mindset of nano-optics, the abovedescribed material falls into the classification of plasmonic metamaterials.34,35 These materials consist of subwavelengthsized components capable of supporting surface plasmons (SPs), which are surface-bound electromagnetic waves confined to a metal/dielectric interface. These occur as a result of a resonant interaction between an illuminating electromagnetic

Figure 2. Optical reflectance spectra of the Fe−Au nanorod arrays under a range of incident angles and polarization states. (I) Unpolarized light as illumination. (II) Perpendicularly (blue) and parallel (red) polarized light as illumination. The spectra were individually normalized to unity and were normalized against a background spectrum.

wave and collective local surface charge density oscillations of the free electrons of the metal.36 As an initial characterization of the optical response of the nanorod arrays, reflectance spectra were acquired over a range of illumination angles and polarization states, as shown in Figure 2. The spectra exhibit the classical dual resonance behavior associated with the transverse and longitudinal modes occurring in the gold segment of the nanorods. These plasmonic modes are evident in the unpolarized incident light spectra (Figure 2I) centered at approximately 530 nm (Ae) and at approximately 665 nm (Be) for the transverse and longitudinal SP modes, respectively. The general behavior of these two eigenmodes is consistent with the response of the dipolar modes supported by isolated nanorods (Figure 4). The peaks in the NIR range (Ce, De, and Ee) demonstrate the presence of additional plasmonic resonant modes. It will become clear that the underlying source of these modes is not simply attributed to one mechanism but rather is a complex behavior resulting from a variety of factors. The response encompasses contributions from the primary modes; the transverse and longitudinal modes, from the secondary modes; the interaction of the plasmonic modes between the particles (i.e., interparticle coupling); and resonances not associated with the geometric axes of the nanorods. The behavior is further complicated by the presence of the iron 17104

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segment and the gold substrate. The system was specifically tailored to achieve these secondary coupled plasmonic modes by the nature of its highly ordered array structure and significant subwavelength inter-rod spacing of 26 nm.37 This metamaterial plasmonic system not only displays resolved coupling modes but also is highly sensitive to angular and polarization variations. Changing the polarization state not only offers a sensitive detection method but also further reconfirms these modes as possessing a coupled plasmonic nature. The perpendicular and parallel polarized results are shown in Figure 2II. At lower incident angles, the peaks in the NIR range are almost overlapped and out of the range of detection whereas at higher incident angles these peaks shift to shorter wavelengths and become well separated, identified as Ce (at around 875 nm), De (at around 905 nm), and Ee (at around 985 nm) modes. An overview of the spectral behavior can be seen in Figure 3. This is a contour plot of the reflectance

Figure 4. (I) Experimental scattering results (black). (II) Simulation of the scattering by a single Fe−Au nanorod (blue), a double Fe−Au nanorod target (red), and a trigonal Fe−Au nanorod target (green). (III) Simulation of the scattering by a Fe−Au hexagonal unit target. (IV) Simulation of the scattering by the Au segment only of the hexagonal unit target. Both simulated and experimental spectra are presented here for an unpolarized illumination angle of 55°.

Figure 3. Contour plot of the unpolarized reflectance spectra for the complete angular range. The contours are sampled at 0.1 intervals. Again, the intensities of the spectra are individually normalized to unity, as in Figure 2. The areas of red denote high intensity, and the blue regions indicate low scattering levels. This is shown in the scale bar on the right of the image.

found in the simulation. This arises from the setup of the target orientation in the simulation. The unpolarized light scattering is calculated on the basis of the contributions of both parallel and perpendicular polarized light. The perpendicular polarized light shows intensity levels comparable to the experimental results whereas the coupled modes are not present in the parallel polarization results. Thus, the intensity of the coupled modes was reduced for the simulation of the scattering of the unpolarized light. Also noteworthy is the emergence of more pronounced coupled modes with the addition of rods to the simulations. This strongly conveys the existence of coupling between the nanorods when separated at this distance of 26 nm. Schematics of the angular orientation of the targets are shown in Figure S6. Figure S7 (Supporting Information) presents the experimental and simulated results of the Fe−Au targets at perpendicular and parallel polarized illumination, respectively. The experimental spectra display additional secondary modes, the source of which will be discussed shortly. In the simulation for a single nanorod, the peaks labeled as A1 and B1 arise from the transverse and longitudinal modes, respectively, and C1 and D1 are secondary plasmonic resonance states. The introduction of a second nanorod into the target causes these peaks (C2 and D2) to red shift into a more distinguishable albeit broad peak, suggesting that there is strong electromagnetic coupling between adjacent nanorods. This is further confirmed by the simulation of a trigonal arrangement of the nanorods (Figure 4II). Peaks C3 and D3 become more defined, and the

spectra as a function of angle and intensity for the unpolarized illumination state. The regions of highest intensity are denoted in red, and blue represents the opposite. In comparison to the plasmonic eigenmodes (localized SP responses and transverse and longitudinal modes), the coupled SP modes show a significant angular dependence in spectral positions, generally exhibiting a blue shift in wavelength position with an increasing angle of incidence. This supports the understanding that these NIR plasmonic modes arise from collective resonance behavior and delocalized characteristics.38 The assignment of modes C, D, and E as secondary coupled SP modes was further confirmed by DDA32 simulations, as presented in Figure 4II,III,IV, which is a simulation method accepted as an excellent tool for simulating the spectral response of nanosystems.39,40 This numerical method tool also allowed the effect of the iron on the plasmonic response to be determined. Typically with Au-only nanorods two distinguishable resonances dominate because of the geometry of the rod, namely, the aspect ratio and the physical dimensions. It is evident from the simulations that the wavelength position of the transverse mode labeled as An (n = 1, 2, 3) is in good agreement with experimental mode Ae. The presence of a gold background in the physical nanorod structure accounts for the relatively small shift between the experimental and simulated values. Excellent correlation also exists for longitudinal modes. The observed intensity of the coupled modes in the experimental spectra is significantly stronger than that 17105

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plasmonic signals instead of using high-aspect-ratio plasmonic nanorods.35 Furthermore, an additional degree of freedom with regard to tuning the plasmonic response of the Fe−Au segmented nanorod arrays can be obtained by controlling and manipulating the internal or external magnetic field.41

spectral position red shifted further, approaching the experimental data. It was seen previously in Figure 1A containing the SEM image of the nanorods that they are specifically arranged in a highly ordered repeating hexagonal unit pattern. This pattern was replicated in the DDA simulations for both (Scheme 2) the Fe−Au array (Figure 4III) and the Au segment only of the array (Figure 4IV). The simulated spectra for the Au segment only shows a reduced level of scattering with features at approximately 780 and 820 nm. This is followed by a dramatic fall off in intensity as the IR range is approached. The intended absence of the gold substrate from the simulations implies that these modes must be attributed to interparticle coupling. As a result, modes C5 and D5 are the transversely coupled and longitudinally coupled modes, respectively. In comparison to the Au segment only simulations, the Fe−Au hexagonal unit results show a broader response in the region of 770−950 nm. The peaks seen in the Au segment only simulations are red shifted in the case of the Fe−Au system to 797 and 894 nm, respectively, with an additional mode at 948 nm. The effect of the iron segment is thus to red shift the modes while providing a broader spectral response in this region than for the Au segment only equivalent. By observing the perpendicular polarized results (Figures S7 and S8 in the Supporting Information), it is clearly evident that this broad response in the NIR range is achieved only in the presence of the iron. Comparing the experimental data with the simulations of the hexagonal target reveals two facts. First, the wavelengths positions of all peaks (including peaks C, D, and E in the NIR) are well predicted by the simulations, and thus these peaks do not result as a consequence of the gold substrate that has been deliberately left out in the simulations that are shown. However, the intensity of the NIR region is clearly underestimated in these simulations. This can be explained by further simulations that take the gold substrate explicitly into account and show a broad featureless but high-intensity background in the region of 700−950 nm (see Au substrate only and hexagonal Au nanorods on gold substrate simulations in the Supporting Information). The overall response is thus a highly complex plasmonic response. Although DDA is the optimum choice for modeling this behavior, it allows a simulation of only the full spectral response. The numerous plasmonic processes cannot be examined in isolation, and thus the assignment of modes arises from a comparison of different nanorod arrangements as shown in Figure 4. Subsequently, the peaks labeled C, D, and E are highly likely to arise from the transversely coupled mode, the longitudinally coupled mode, and a secondarily coupled mode, respectively. The modes are much more pronounced in the experimental results than the simulations, but the simulations have shown that increased rod numbers improve the wavelength matching of the peaks noticeably. With reference to the intensity levels, it must be considered that the experimental nanoarray is almost infinite in size in comparison to the target of seven nanorods, and any primary or secondary modes that exist in the experimental situation will be much more pronounced than those in the simulations. However, excellent wavelength correlation between the predicted and detected wavelength positions is nonetheless achieved. From the above discussion, the demonstration of the extraordinary optical characteristic of the Fe−Au nanorod arrays suggests a new application that uses heteronanostructures and their subwavelength features to generate NIR

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CONCLUSIONS We have demonstrated that highly ordered magnetic− plasmonic Fe−Au nanorod array structures can be produced by templated PAA membrane synthesis. An asymmetric etching method was successfully used to create the desired nanorod arrays with exemplary periodicity and individual rod duplicability. These heterostructured nanorod arrays have controlled subwavelength features that result in a tunable plasmonic responses in the visible−NIR range, as confirmed both experimentally and theoretically. The coupled SP modes in the NIR range could be very useful in biosensing applications, and the array structure provides an ideal model for studying the interparticle plasmonic coupling between adjacent nanorods.

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ASSOCIATED CONTENT

* Supporting Information S

Synthesis of PAA templates, crystal structures of the nanorods, magnetic properties of the nanorod arrays, and DDA simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Present Addresses §

School of Physics, Trinity College Dublin, Dublin, Ireland. School of Physics, Dublin Institute of Technology, Dublin, Ireland. ∥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Electron Microscopy Laboratory (UCD) and the Research IT Service (UCD) for their technical support. Supporting Information is available online from the author. This work was supported by the Science Foundation of Ireland through grants 08/IN/2972 and 08/ENE/1198.

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REFERENCES

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dx.doi.org/10.1021/la302290v | Langmuir 2012, 28, 17101−17107