Symmetry-Dependent Plasmonic Properties of Three-Dimensional

Jul 30, 2012 - appearance of crescent-shaped nanogaps at the interfaces and thus leads to a novel strong plasmon resonance mode. The finite-difference...
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Symmetry-Dependent Plasmonic Properties of Three-Dimensional Hybrid Metallic Nanostructure Arrays Zhancheng Li, Hongbin Cai, Zhonglin Han, Kun Zhang, Nan Pan, Xiaoping Wang, Xiaofang Zhai,* and Changgan Zeng* Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: We demonstrate the successful fabrication of various well-ordered 3D gold nanostructure arrays using nanosphere lithography method, and further reveal the strong dependence of the optical responses on their in-plane symmetry. For the concentrically stacked ring-cap array, its optical absorption behavior is similar to that of a ring array with the same dimension because they have the same in-plane symmetry. However, for the nonconcentrically stacked holecap array, the broken in-plane symmetry results in the appearance of crescent-shaped nanogaps at the interfaces and thus leads to a novel strong plasmon resonance mode. The finite-difference time-domain simulation shows that charge mainly assembles to the sharp edges of the nanogaps at the resonant wavelength and remarkable electric field enhancement is achieved around the sharp edges. Furthermore, the strongest resonance modes of the ring-cap array and hole-cap array show large red shift as the nanostructure size increases. The presented 3D nanostructure arrays may offer a spectrum of applications in sensing.

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INTRODUCTION Novel metallic nanoparticles exhibit unique optical properties because of the electron−photon coupling interaction between metallic nanoparticles and the incident light, which is known as localized surface plasmon resonances (LSPR).1,2 It has attracted much attention because of both fundamental and applied scientific interests, such as biological and chemical sensing,3,4 surface-enhanced Raman scattering (SERS),5 and wavelengthselective window coatings.6,7 Tuning the resonant frequency in a wide range and maximizing the field enhancement are two critical requirements for such applications. It has been demonstrated that the resonance properties of the LSPR strongly depend on the geometry of the nanostructures, such as shape and size.8 Complex nanostructures with various shapes, such as nanoring, nanocrescent, nanocap, nanograil, nanorod, and nanohole,9−16 have been fabricated to explore the versatility of the LSPR properties. Theoretical calculations and experiments have shown that the electric field may be highly localized and dramatically enhanced at the sharp corners17 or nanogaps,18 which are of great interest due to their increased sensitivity in sensing. Seeking a versatile method to fabricate various nanostructure arrays with sharp tips and nanogaps to realize large electric field enhancement is especially important for practical applications. Many efforts have been undertaken to design nanostructures with such features. Among them, the modern lithography techniques including focused-ion-beam milling and electronbeam-lithography are extensively explored,19−24 and these techniques can accurately control the size and shape of each © 2012 American Chemical Society

nanostructure in the array. However, these methods cost too much, take long production cycle, and are limited by the sample size. An alternative approach for fabricating periodic nanostructure arrays is nanosphere lithography (NSL),25,26 which has an inherent advantage of covering macroscopic areas costeffectively. Using NSL, we can get many different shaped nanostructure arrays simply by changing the angle of metal deposition or etching, alternating the process sequences, and so on. Moreover, a batch of the same shaped nanostructure arrays can be fabricated over a large area at the same time. As the fabrication process continues to improve, the fabricated structure arrays have been developed from simple triangular nanoparticles27−29 to more complex structures, such as the nanorings, split nanorings, nanobulls, and nanoneedles.30,31 Alternatively, shadow colloidal lithography, which uses nonclosed-packed monolayer and varied deposition angles, has fabricated complicated nanostructure arrays such as nanocaps,16 nanograils,13 and nanocrescents.32 Furthermore, complex dimer nanocrescent architectures33 and 3D hybrid nanostructures32,34 can also be fabricated using similar techniques based on NSL. Hybridization or coupling of plasmonic nanostructures is another way to realize large near-field enhancement.8 Recently, the coupling has been applied to crescent-shaped nanostructures, for example, nanocrescent dimer with nanosized tips in Received: May 28, 2012 Revised: July 29, 2012 Published: July 30, 2012 17781

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Figure 1. Schematic diagram for the fabrication of various-shaped nanostructure arrays.

Figure 2. (a−d) SEM images of the ring, ring-cap, hole, and hole-cap arrays, respectively. The insets at the bottom right corner of panels a−d are the cartoons of ring, ring-cap, hole, and hole-cap, respectively. The insets at the up right corner of panels b and d show a tilted ring-cap and a tilted cap, respectively. The scale bars are 200 nm in panels a−d and 50 nm in the insets of panels b and d. The PS spheres with reduced size of 137 nm and Au film with thickness of 20 nm were adopted to fabricate all four nanostructure arrays shown here.

close proximity,33 3D stacked split rings,35 or crescents.32,34 3D hybridization of multiple structure will add another degree of freedom in the vertical direction for better tuning the resonance of LSPR. Nevertheless, advantages of this particular NSL method have not been fully exploited, especially in the complex hybrid 3D nanostructures that may exhibit novel resonance properties. In this article, we demonstrate the fabrication of several nanostructure arrays, including ring array, ring-cap array, hole array, as well as hole-cap array using a revised NSL. More importantly, their plasmonic properties are strongly dependent on their structural symmetry. When a ring array is covered by a cap array with the same dimension to form a 3D hybrid ringcap array with the in-plane symmetry unaltered, the optical absorption behavior is similar to that of the ring array. The simulation also suggests that the charge distribution does not change much if the light is normal to the surface. However, we find the symmetry breaking modifies the optical response significantly. When a cap array and a hole array with the same dimension are nonconcentrically stacked to form a 3D hybrid

hole-cap array, crescent-shaped nanogaps appear at their interface. This symmetry breaking results in a strong resonance with charge mainly assembling to the edges of the nanogaps, which is absent for the hole array. The considerably large local electric field enhancement for such a unique structure renders it an ideal platform for SERS applications.

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EXPERIMENTAL METHODS A. Sample Fabrication. The fabricating procedure of various nanopattern arrays on a macroscopic scale is shown in Figure 1a. The closely packed polystyrene (PS) sphere (200 nm in diameter) mask was prepared on a glass substrate similar to the method adopted in ref 26. Under magnetically enhanced reactive ion etching (RIE) with O2, the PS spheres can be reduced to smaller sizes (step ii). By changing the RIE duration, the sphere size can be fine-tuned in a relatively wide range. Au was subsequently deposited on the template (step iii). Various nanostructure arrays can be achieved starting from this step. By exposing to the Ar+ beam (step iv-1), Au rings were formed underneath the PS spheres due to the secondary sputtering.9 17782

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Figure 3. Plasmonic properties of the ring array and ring-cap array, respectively. (a,d) Transmittance spectra of the ring array and ring-cap array, respectively. The calculated spectra are also shown. (b,c,e,f) Simulated electric field intensity distribution of the RL and RH modes in the vertical cross section for the ring and ring-cap arrays, respectively. The insets of panels b and c and panels e and f illustrate the charge distributions for the different resonance modes of ring array and ring-cap array, respectively. PS spheres with reduced size of 152 nm and Au film with thickness of 20 nm were adopted in the fabrication.

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Then, the sample was annealed at 500 °C in high vacuum (10−5 Pa) to remove the colloidal sphere template to get a clean Au ring array (step v-1). If the duration of the Ar+ beam sputtering is not sufficiently long, then thin Au caps still remain on top of the PS spheres (step iv-2). Therefore, a ring-cap array was obtained after removing the PS mask (step v-2). When the sample just after Au deposition was annealed, a hole-cap array was obtained (step iv-3), and by further exfoliating the Au caps using adhesive tapes, we got a hole array (step v-3). B. Optical Characterization of Nanostructure Arrays. The normalized optical transmission spectra of the nanostructure arrays were measured by a UV−vis-NIR spectrophotometer (Shimadzu DUV-3700) using unpolarized light at normal incidence over a wavelength range of 400−2400 nm. C. Finite Difference Time Domain Simulation. Finite difference time domain (FDTD) (Lumerical Solutions, Canada) simulations were conducted using the structural parameters from the experiments. In the calculations, perfectly matched layer absorbing boundary conditions were adopted in the z-direction and periodic boundary conditions adopted in the X−Y plane. The plane waves are normally incident to the substrate. Because the light used in the experiment is unpolarized, the overall device resonance (R) can be treated as the superposition of the contributions of two orthogonally polarized components, that is, R = (Rx‑polarized + Ry‑polarized)/2. For the symmetrical structures, different polarized light has almost no effect on the plasmon resonance of overall device; that is, R = Rx‑polarized = Ry‑polarized. Therefore, one polarized component in the simulation can fully reflect the overall resonance of the high-symmetric ring array, ring-cap array, and hole array. However, the resonance of symmetry-broken structure is dependent on the polarized light direction. Therefore, we used two orthogonally polarized lights in the simulation for the symmetry-broken hole-cap array. The cap thickness in the ring-cap array after Ar+ beam sputtering was estimated to be 2 nm based on the best fitting between the simulation and the experiment. The cap after sputtering is still a well-defined film from the SEM image (Figure 2b). A width of 17 nm for the crescent-shaped nanogaps in the hole-cap array was adopted in the FDTD simulation.

RESULTS AND DISCUSSION Figure 2a−d is the scanning electron microscopy (SEM) images of the ring, ring-cap, hole, and hole-cap arrays, respectively fabricated by the procedure described in the Sample Fabrication section. For the ring array shown in Figure 2a, it is clear that the outer wall is much steeper than the inner wall in a single ring. The ring-cap arrays are formed after Ar+ beam sputtering, so the caps are quite thin and corrugations are formed on the caps as shown in Figure 2b. For the hole-cap array (Figure 2d), the holes and caps are nonconcentrically stacked and crescent-shaped nanogaps develop in the hybrid structure, as indicated by the arrows. It is noted that the nanogaps are oriented randomly and the width of the size is not really uniform, which is probably due to a random “falling-over” caused by evaporation of colloidal spheres during the annealing step. Tuning the annealing temperature may help to better control the nanogaps, which is to be explored in the future. The transmittance spectrum of a ring array (Figure 3a) exhibits two resonant modes, that is, a low-energy resonance mode at 1069 nm (referred as RL) and a high-energy resonance mode at 540 nm (referred to as RH). To better understand the two resonance modes, we simulate the corresponding electric field profile and the optical transmittance spectrum of ring array using FDTD. As shown in Figure 3a, the simulated spectrum agrees well with the experimental one, and the calculated electric field distributions (Figure 3b,c and Figure S1a,b in the Supporting Information (SI)) show that the charges are distributed symmetrically around the ring center for both the RL and RH modes. The charges mainly assemble at the outer and inner edges for the RL mode, which also occurs for the randomly distributed rings. This phenomenon has been considered as a fundamental dipolar resonance mode of ring.9,10,36 Charge assembling in the RH mode is observed at the inner edges along the bridge of the ring. The RH mode is similar to the particle plasmon (pp) resonance34,37 because the resonance energy matches that of the small gold particles, and its polarization is similarly perpendicular to the contour. Nevertheless, the RH mode was extremely weak or even absent in the randomly distributed rings.9,10,36 The enhancement of RH 17783

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Figure 4. (a) Experimental and simulated transmittance spectra of the hole array. (b) Simulated transmittance spectrum of concentrically stacked hole-cap array. (c) Simulated electric field distribution in the horizontal cross section at 591 nm for the hole array. PS spheres with reduced size of 152 nm and Au film with thickness of 16 nm were adopted in the fabrication.

mode in our ring array may be associated with the magnifying effect of the well-ordered arrangement of the nanostructures.20 When a ring array is stacked concentrically by a thin cap array with the same dimension to form a 3D hybrid ring-cap array, the transmittance spectrum does not have any significant change: There are two resonance modes located at 544 and 1266 nm, respectively (Figure 3d), which is very close to that of the ring array except a little red shift. The two resonance modes are also reproduced by the simulation. The charge distribution of a ring-cap array from the simulation (Figure 3e,f and Figure S1c,d in the SI) also shows similar pattern as that of the ring array. Therefore, it seems that when a second nanostructure array is directly stacked on another nanostructure array to form a 3D hybrid nanostructure array with the in-plane symmetry unaltered, the plasmonic property does not change much when the incident light is normal to the substrate. This is further confirmed by the optical properties of other nanostructure arrays: The simulated optical spectrum of a concentric hole-cap array (Figure 4b) displays the same absorption peak position as that of a hole array with the same dimension (Figure 4a). This result can also be attributed to the similar charge distributions for the concentric hole-cap array and the hole array with the same in-plane symmetry. If the in-plane symmetry of the stacked structures is broken, then the charge distribution may vary greatly, which subsequently leads to a dramatic modulation in the optical resonance. In light of this conjecture, we fabricated a hybrid hole-cap array with the holes and caps nonconcentrically stacked. Such symmetry breaking results in the formation of the crescentshaped nanogaps, as shown in Figure 2d. Compared with the previously reported crescent-hole and crescent-particle,11,32,37,38 the crescent-shaped nanogaps in our hole-cap array (the holecap array represents the nonconcentrically stacked hole and cap array if there is no further clarification) are very narrow (ranging from 15 to 18 nm). This nanogap array may realize large electric field enhancement with the charge mainly gathering at the sharp edges. The optical spectrum of such nonconcentrically stacked holecap array is shown in Figure 5a. A weak resonance mode at 600 nm is resolved, which is very close to the resonance mode at 591 nm for a hole array. The FDTD simulation (Figure 5b and Figure 4c) confirms that these two resonance modes have similar electric field distribution. More interestingly, a strong resonance mode at 979 nm now develops for the nonconcentrically stacked hole-cap array, which is absent for a hole array. This feature should be attributed to the symmetry-breakinginduced crescent-shaped nanogaps, which may change the original charge distribution of a hole array dramatically.

Figure 5. (a) Experimental and simulated transmittance spectra of the nonconcentrically stacked hole-cap array. (b−d) Simulated electric field distribution in the horizontal cross section for the nonconcentrically stacked hole-cap array at 600 and 990 nm with polarization parallel to the short axis of the gap and 768 nm with polarization perpendicular to the short axis of the gap, respectively. The insets of panels c and d illustrate the charge distributions for the different field polarizations. PS spheres with reduced size of 152 nm and Au film with thickness of 16 nm were adopted in the fabrication.

For the symmetry-broken hole-cap array, the electric field with polarization parallel and perpendicular to the short axis of the crescent-shaped nanogaps at normal incidence should excite different charge distributions and thus lead to different resonance mode. This is confirmed by the FDTD simulations, as shown in Figure 5a: a resonance mode develops at 990 nm when the electric field is parallel to the short axis of the gap (denoted as parallel mode), whereas a resonance mode develops at 768 nm when the electric field is perpendicular to the short axis of the gap (denoted as perpendicular mode). Because unpolarized light was adopted in the experiment, both resonant modes should be observed. However we cannot distinguish these two modes experimentally and only observe a resonance feature centered at 979 nm, as shown in Figure 5a. This may be due to the fact that the shapes and sizes of the fabricated nanogaps are not uniform, which differ from the uniform structure model adopted in the FDTD simulations. A broadening of the resonances caused by difference gap sizes may lead to a smearing of the two resonances that are not resolved. Figure 5c,d shows the simulated electric field distribution of the parallel mode and perpendicular mode, respectively, and, 17784

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interestingly, considerably large electric field enhancement of | E|/|E0| ≈ 104 for the perpendicular mode and |E|/|E0| ≈ 3 × 103 for the parallel mode is achieved. In contrast, the maximum field enhancement of a hole array is relatively weak; that is, |E|/| E0| ≈ 102. The large field enhancement should be attributed to the sharp edges of the nanogaps. Both resonances are dipolar resonances from the FDTD simulations; that is, the charge oscillates along the short axis of the nanogaps for the parallel mode while along the long axis for the perpendicular mode. This is similar to the ‘pp’ resonances observed in the crescent particles and crescent holes.37 Such strong electric field may find a lot of applications in biological and chemical sensing. Furthermore, we investigate how the nanostructure size affects the plasmonic properties, whereas the periodicity in the arrays does not change (200 nm). Figure 6a shows the

of the ring-cap array and hole-cap array show remarkable red shift as the nanostructure size increases. The presented wellordered nanostructure arrays, especially the hybrid nanostructure arrays, might provide an ideal platform for the practical application based on the localized electric enhancement.

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ASSOCIATED CONTENT

S Supporting Information *

FDTD simulations of the ring array and ring-cap array. 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] (X.Z.); [email protected] (C.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate support from the Fundamental Research Funds for the Central Universities (Grant No. WK2340000011), NSFC (Grants Nos. 10974188, 91021018, 20933006, 11104258, and 11034006), ‘‘One-hundred-person Project’’ of CAS, NKBRPC (Grant No. 2009CB929502), SRFDP (20113402110046), NCET, and CPSFFP (Grant No. 20100470837).

Figure 6. (a) Transmittance spectra of the nonconcentrically stacked hole-cap arrays with various diameters of PS spheres adopted in the fabrication. (b) Wavelength of the strongest resonance as a function of the diameter of PS sphere for different nanostructure arrays. Au film with thickness of 20 nm was adopted in the fabrication.

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transmittance spectra of the nonconcentrically stacked hole-cap arrays with varied sizes. A significant red shift of the strongest resonance mode is observed when the hole-cap size is increased. The red shift has a linear dependence on the size of the PS spheres adopted in the fabrication. Such linear dependence of the red shift is also observed for other nanostructure arrays, as shown in Figure 6b. Nevertheless, the slope in the linear relationship varies for different nanostructure arrays. For example, for the hole array, the red shift is only 24 nm per 30 nm increase of PS diameter, whereas for the hole-cap array and ring-cap array, the red shift exceeds 100 nm per 30 nm increase in PS diameter. The latter is much higher than those previously reported.39 In particular, the red shift even reaches 187 nm per 30 nm increase in PS diameter for the ring-cap array. Such red shift may be due to the increased contribution of retardation effect as the nanostructure size increases.39

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CONCLUSIONS We have successfully fabricated various 3D metallic nanostructure arrays using NSL method and find that their plasmonic properties depend on their in-plane symmetry. For the concentrically stacked ring-cap array, its optical response almost resembles that of a ring array with the same dimension. However, when a cap array and a hole array with the same dimension are stacked nonconcentrically to form a hybrid holecap array with crescent-shaped nanogaps at the interfaces, the optical spectrum shows a new strong resonance mode that is absent in the hole array. FDTD simulations show that the charge mainly assembles to the sharp edges of the nanogaps and considerably large electric field enhancement arises at the resonant wavelength. Moreover, the strongest resonance modes 17785

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