Tuning Gold Nanorod-Nanoparticle Hybrids into Plasmonic Fano

Nov 24, 2010 - Figure 1a,b illustrates the side view sketch of the nanorod-film ... when the incident light is perpendicular to the nanorods (θin = 0...
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Tuning Gold Nanorod-Nanoparticle Hybrids into Plasmonic Fano Resonance for Dramatically Enhanced Light Emission and Transmission Zhang-Kai Zhou,† Xiao-Niu Peng,† Zhong-Jian Yang,† Zong-Suo Zhang,† Min Li,† Xiong-Rui Su,† Qing Zhang,‡ Xinyan Shan,‡ Qu-Quan Wang,*,† and Zhenyu Zhang§,|,⊥ †

Department of Physics, Wuhan University, Wuhan 430072, People’s Republic of China, ‡ Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China, § Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States, | Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, United States, and ⊥ ICQD, University of Science and Technology of China, Hefei, Anhui, People’s Republic of China ABSTRACT We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices. KEYWORDS Plasmonic Fano resonance, plasmon hybrid, gold percolating film, gold nanorod array, enhanced transmission, enhanced photoluminescence

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xtensive research efforts have been devoted recently to utilizing metal nanostructures to manipulate the propagation, intensity, and polarization of light,1-6 leading to the emergence of nanophotonics as a major new direction in photonics. In this emerging field, one central physical entity is plasmon, characterizing the collective excitation of conduction electrons in metal nanostructures. Many intriguing phenomena discovered recently, such as the squeezing of light into subwavelength nanoholes,7-9 and the detection of molecules trapped between nanogaps via surface-enhanced Raman scattering (SERS) with single molecule sensitivity,10-12 are tied to the coupling of incident light with plasmon modes. Such studies not only broaden our fundamental understanding of photon interaction with nanoscale systems, but also may have far-reaching technological impacts. In exploration of various intriguing plasmonic phenomena at the nanoscale, a widely studied and distinctive research emphasis is the exploitation of the coupling and hybridization of different plasmon modes supported by various elegantly fabricated metal nanostructures.13-22 Compelling examples include the plasmon coupling of a discrete mode to a continuum band, known as the “plasmonic Fano

resonance”. The Fano type absorption spectra were first reported in hole arrays in thin metal films and coaxial metallic arrays due to interferences of localized and delocalized plasmon modes.23-25 A more vivid picture of the plasmonic Fano model with three interaction regimes was convincingly demonstrated in metallic nanoparticle-film systems by tuning the film thickness.26 Recently, multiple Fano resonances in a metallic ring/disk dimer and twinned Fano resonances in the Au-Ag heteronanorod dimer were also reported.27 Most research focused on the asymmetric Fano line-shape in the absorption spectra, but enhanced emissions and Raman scattering induced by constructive interferences via the plasmonic Fano effect are seldom explored, which is of great importance for both passive and active plasmonic nanosystems. In this Letter, we investigate the optical responses of a gold nanorod array coupled with a semicontinuous gold film. Our nanosystem is very similar in principle to the metallic nanoparticle-film hybrid of Le et al.,26 but the gold nanorods have tunable long-axis surface plasmon resonance (SPR) with high quality factor (high-Q) and more efficient multiphoton luminescence (MPL) than spherical nanoparticles.28,29 Semicontinuous gold films support both localized and delocalized surface plasmon modes and have a much stronger local field enhancement than continuous thin film.30 Both the metallic nanorod array and semicontinuous film are commonly used as SERS substrates due to large local field enhancements,31-39

* To whom correspondence should be addressed. E-mail: [email protected]. Received for review: 8/1/2010 Published on Web: 11/24/2010 © 2011 American Chemical Society

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the film is about 50 nm and the fractional area pAu covered by the Au nanoparticles is about 0.75 (the atomic force microscopy image of the percolating Au film is shown in Figure S1 in Supporting Information). We first investigate the plasmon coupling of short nanorod array and discontinuous films of Au, both of which have a clear absorption band near 730 nm (see Figure 2). Figure 2a is the absorption spectra of the Au nanorod array with growth time tg ) 15 s, which is recorded by using a p-polarized source with incident angle θin ) 0° and θin ) 80°, the absorption bands around ∼530 and ∼730 nm are attributed to the short-axis and long-axis SPRs, respectively. The long-axis SPR peak slightly red shifts, and the resonant intensity decreases as the incident angle decreases and totally disappears when the incident light is perpendicular to the nanorods (θin ) 0°). Figure 2b shows the optical transmission spectra of the discontinuous Au films with and without the Au nanorod array, which are recorded by using a p-polarized source with normal incidence (θin ) 0°). The deposition time td of the film is about 90s (td/tc ) 0.82). From Figure 2b, one can clearly see that the transmission of the nanorod-film hybrids are inhibited and enhanced around the short-axis and longaxis SPRs of the Au nanorods, respectively. To quantitatively analyze transmission variation, we introduce the normalized transmittance difference as ∆T/T ) (TNR-film - Tfilm)/Tfilm, where TNR-film and Tfilm represent the transmittance of the Au nanorod-film hybids and the Au films, respectively. As shown in Figure 2c, ∆T/T ) -25% for the inhibited transmission at the short-axis SPR, and ∆T/T ) +5% for the enhanced transmission at the long-axis SPR. This indicates that the enhanced transmission is induced by the interaction of the SPR of the Au film and the long-axis SPR of the Au nanorod array. A stronger transmission enhancement is observed in the percolating Au film coupled with a longer Au nanorod array. The long-axis SPR peak of the longer Au nanorod array with tg ) 40 s redshifts to ∼790 nm (see Figure 2d), and the longer Au nanorods have relatively larger differences in size, which results in a broadening of the long-axis SPR absorption. The percolating Au film with normalized deposition time td/tc ) 1.0 has a semicontinuous network nanostructure (as shown in Figure 1d) and a constant optical transmittance when λ > 800 nm (see the magenta line in Figure 2e). The transmittance of the semicontinuous Au films in the infrared region has the relationship T(λ) ) Tc - b(pAu - pc)(λ/2πaAu)1/γ,40-42 where b is a dimensionless constant, γ is the critical exponent, pc is the fractional area covered by the percolating film, and Tc is the optical percolating transmittance and measured to be about 0.23 (see Figure 2e). The transmittance T(λ) of discontinuous Au films (pAu < pc) increases as the wavelength λ increases in the near-infrared region (see Figure 2b). In contrast, T(λ) of continuous Au film (pAu > pc) decreases as λ increases (experimental data are not shown).

FIGURE 1. (a) Schematic of the standing Au nanorod array on the percolating Au film (side view). (b) The TEM image of the Au nanorods with growth time tg ) 40 s. (c) A top view of the SEM image of the AAO template loaded with Au nanorods. (d) SEM image of the percolating Au film (with deposition time td ) 110 s) on the back side of the template.

so the plasmon coupling in our nanorod-film hybrid is also expected to further enhance the local field and improve the sensitivity of SERS. In our studies, the long-axis SPR of the Au nanorod array is tuned by adjusting the rod length and the SPR of the semicontinuous Au film is tuned by controlling the sputtering deposition time. By coupling high-Q discrete plasmon modes of the gold nanorod array to low-Q continuum modes of the semicontinuous film, we uncover striking optical properties of the nanorod-film hybrids at the plasmonic Fano resonance, as reflected by the dramatically enhanced optical transmission of the semicontinuous film and 2 orders of magnitude enhancement in the avalanche MPL of the nanorod array. Our samples of the nanorod-film hybrids contain an ordered Au nanorod array standing in an anodic aluminum oxide (AAO) template and a percolating Au film formed on the backside of the template. Figure 1a,b illustrates the side view sketch of the nanorod-film hybrids and the TEM image of the Au nanorods. Figure 1c,d shows the SEM images of a top and a bottom view of the sample, respectively. The Au nanorod arrays were electrochemically grown by alternating current electrolysis in an electrolyte with Pt counter electrodes (current electrolysis 50 Hz, 5 V ac; electrolyte 0.01 M HAuCl4·4H2O and 0.1 M H2SO4 acid). From the TEM image shown in Figure 1b, the diameter dAu and length lAu of the Au nanorods with growth time tg ) 40 s are estimated to be about 20 and 60 nm, respectively. The Au films were deposited by using the sputtering method (the deposition voltage and current were 1.5 kV and 7 mA, respectively; the sputtering was carried out in argon atmosphere and the pressure of Ar+ was 5 Pa). The percolating nanostructure is formed when the sputtering deposition time td is 110 s (td/tc ) 1.0, where tc corresponds to the deposition time for growing a percolating film) as shown in Figure 1d, and the average lateral size aAu of the ellipsoidal Au nanoparticles in © 2011 American Chemical Society

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FIGURE 2. (a) Absorption spectra of the Au nanorod array in an AAO template, where the nanorod growth time tg ) 15 s, and the light incident angle θin ) 0 and 80°. (b) Transmittance spectra of a discontinuous Au film with (blue line) and without (magenta line) an Au nanorod array, where the deposition time of the discontinuous film td ) 90 s, and θin ) 0°. (c) Normalized difference of the transmittance (∆T/T) of the nanorod-film hybrid involving the discontinuous film. (d) Absorption spectra of the Au nanorod array with growth time tg ) 40 s, and θin ) 0 and 80°. (e) Transmittance spectra of the percolating Au film with (blue line) and without (magenta line) an Au nanorod array with the deposition time of the percolating film td ) tc ) 110 s, and θin ) 0°. (f) Normalized difference of the transmittance (∆T/T) of the nanorod-film hybrid involving the percolating film.

The percolating Au film has a strong local field enhancement and low-Q broadband SPRs. By plasmon coupling of the percolating Au film with the Au nanorod array, transmission of the nanorod-film hybrid is significantly enhanced. Figure 2f shows that ∆T/T at 800 nm increases to about +55% as the long-axis SPR of the Au nanorod array are tuned to 790 nm. This confirms that the enhanced transmittance in the Au nanorod-film hybrid is induced by the nearfield coupling of low-Q SPR of the film and high-Q long-axis SPR of the nanorod array via plasmonic Fano interferences. We further compare the enhanced transmissions for discontinuous, semicontinuous, and continuous films. As td/ tc increases from 0.36 to 1.0, the maximal value of ∆T/T increases from -18 to +55% while the corresponding enhanced peak redshifts from ∼680 to ∼840 nm, as shown in Figure 3a. The transmission spectra of the Au nanorod array and the nanorod-film hybrids are given in Figure S2a-S2f in Supporting Information. Note that no enhanced transmission for discontinuous films with td/tc < 0.45 is observed. In this case, the SPR of the discontinuous Au films is not broad enough compared with the long-axis SPR of the © 2011 American Chemical Society

Au nanorod array, and the SPRs of the film and the nanorods could not be strongly coupled by Fano resonance. On the other hand, the transmission enhancement ∆T/T (at λ ) 800 nm) reaches the maximum value of +55% when td/tc ≈ 1.0 (see Figure 3b). These observations clearly indicate that the enhanced transmission of the nanorod-film hybrid system becomes prominent for the percolating film due to plasmonic Fano resonance. When td/tc further increases to a value larger than 1, the Au films become continuous, and ∆T/T (at λ ) 800 nm) dramatically decreases due to a weak plasmon resonance in the continuous films. Prominently enhanced transmissions are only observed when 0.64 e td/ tc e 1.09. On the contrary, transmissions of the Au films at the short-axis SPR of the Au nanorods are inhibited in the whole deposition time range 0.36 e td/tc e 1.27, the corresponding ∆T/T (at λ ) 530 nm) varies in the range -36 ∼ -59%. Conversely, the local field in the Au nanorod array is also dramatically enhanced by the percolating Au film, which is experimentally demonstrated by measuring the multiphoton-absorption-induced luminescence of the nanorod array. 51

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FIGURE 4. Nonlinear photoluminescence of the Au nanorod arrays enhanced by the percolating Au films. (a) Photoluminescence spectra of the Au nanorod-film hybrids with percolating films. (b) Excitation power dependence of the photoluminescence peak intensity of an Au nanorod array with and without a percolating Au film. The avalanche slope ν is increased and the critical excitation power Pc is decreased in the presence of a percolating Au film.

FIGURE 3. (a) Normalized difference of the transmittance (∆T/T) of the Au films with and without an Au nanorod array. The growth time of the Au nanorods is fixed at 40 s. The sputtering deposition times (td) of the Au films are 40, 50, 70, 90, and 110 s, respectively. (b) ∆T/T (at the wavelength of 530 and 800 nm) as a function of the normalized deposition time td/tc (tc ) 110 s). The transmission at 530 nm is inhibited (∆T/T ) -36 ∼ -59%) in the whole deposition time region. In contrast, the transmission at 800 nm is significantly enhanced (∆T/T reaches about +55%) when td/tc ) 1.0.

percolating Au films when Pexc g 1.5Pc ) 240 mW. In contrast, the photoluminescence from an Au nanorod array in the absence of an Au film (as plotted in Figure 4b) shows a much weaker avalanche with a larger critical excitation power Pc ∼ 300 mW and a smaller nonlinear index ν ∼ 6.5. The photoluminescence signal from the percolating Au film is much weaker and can be neglected. In general, energy redistribution within the neighboring nanorods as nanoemitters plays a key role in the avalanche photoluminescence processes,48-50 and a larger energy redistribution rate leads to a smaller critical excitation power Pc coupled with a larger nonlinear index ν. In the avalanche photoluminescence of the Au nanorod array, the value of Pc decreases from 300 to 160 mW and ν increases from 6.5 to 11.1 due to the plasmonic coupling with the continuum of the percolating Au films. This strongly implies that the percolating Au film enhances the local field of the Au nanorods as well as the plasmonic coupling between the neighboring Au nanorods. Our present findings are expected to stimulate detailed studies of the multiple excitation, relaxation, and recombination in the avalanche photoluminescence processes of the Au nanorod arrays coupled with percolating metal films. To further reveal the underlying physical mechanism of the observed enhancements of the light transmission and emission of the Au nanorod-film hybrids, we carried out finite-difference time-domain (FDTD) simulations of the transmission spectra (Figure 5) and local field distributions (Figure 6). In the FDTD simulations, periodic boundary conditions are adopted in the x and y directions, absorption

It has been reported that the photoluminescence from noble metal nanostructures is very sensitive to local field enhancement.43-47 For example, two-photon- and three-photoninduced luminescence were observed in rough Au films and individual nanoparticles, respectively, and the fourth-order nonlinear photoluminescence was demonstrated in resonant dipole nanoantenna consisting of an Au slab dimer.3 In our studies, a p-polarized picosecond pulse laser (Ti:sapphire Mira 900, pulse-width ∼3 ps and repetition rate 76 MHz) with wavelength of 800 nm was employed as the excitation source, and the incident angle was set at 82° to efficiently excite the long-axis SPR of the standing Au nanorod array. The Au nanorod-film hybrids exhibit broadband photoluminescence spectra with main emission peaks at ∼675 nm (see Figure 4a). Most strikingly, the avalanche photoluminescence of the Au nanorod-film hybrids is observed to be strongly enhanced by the percolating film. As shown in Figure 4b, critical excitation power for the avalanche photoluminescence of the Au nanorod-film hybrid involving a percolating film is Pc ) 160 mW (corresponding to ∼12 kW/cm2), and the nonlinear index ν ) ∂ log IPL/∂ log Pexc of the photoluminescence peak intensity also dramatically increases from 2.9 to 11.1 when the excitation power Pexc g Pc. The dramatic increase of the slope represents a conversion from threephoton absorption to an avalanche process. Because of this effect, photoluminescence intensity of the Au nanorod array is enhanced more than 2 orders of magnitude by the © 2011 American Chemical Society

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FIGURE 6. FDTD simulations of local fields enhanced by the semicontinuous Au films. (a) The top view of the connecting nanoparticles of the film with nanorods, dx ) dy ) 50 nm and dz ) 10 nm for the Au nanoparticles, dAu ) 20 nm and lAu ) 50 nm for the Au nanorods, ∆ ) 10 nm. (b,c) FDTD simulated local field distributions of the Au nanorod array and nanorod-film hybrid, respectively. The wavelength is 800 nm.

FIGURE 5. FDTD simulations of transmission spectra enhanced by an Au nanorod array. (a) The top view of the nanorod-film hybrid structure. The bright and dark yellow circles represent Au nanorods and nanoparticles, respectively. The surface-to-surface gap ∆ between the nanorods and nanoparticles is 10 nm. The period is 50 nm, the diameter dAu ) 20 nm for the Au nanorods. The large Au nanoparticles in the film have the diameters dx ) dy ) 50 nm and dz ) 10 nm, which are contacted each other to form a connecting network. The small Au nanoparticles in the film have the diameters dx ) dy ) 32 nm and dz ) 10 nm. The dashed lines represent the periodic boundaries in the x and y directions in the simulations. (b) FDTD calculated transmission spectra of the semicontinuous Au film with and without an Au nanorod array (the Au nanorod length lAu ) 0, 40, and 50 nm, respectively). (c) Normalized differences of the transmittance of the semicontinuous Au film with an Au nanorod array (lAu ) 40, 50, and 80 nm).

can clearly see that the transmission is enhanced around the SPR of the films (ranging from 650 to 920 nm) and is decreased around the short-axis SPR of the nanorod array (∼530 nm). The calculated ∆T/T at 800 nm apparently redshifts as the Au rod-length increases, which qualitatively reproduces the experimental results in Figure 2. Random distributions of the nanoparticles would broaden the enhanced transmission peak in ∆T/T (the FDTD calculations with the unit cell consisting of 4 different sizes of Au NPs are given in Figure S3 in Supporting Information). We also carry out discrete dipole approximation (DDA) calculations for the extinction spectra of 6 × 6 arrayed Au nanorods with incident angle 90° and the corresponding nanorod-film hybrid with incident angle 0° (see Figure S4a,S4b in Supporting Information). The DDA calculations clearly reveal that the central wavelengths of the nanorod-film hybrids are very close to the long-axis SPRs of the Au nanorod array, which further confirms strong plasmonic Fano coupling between the gold nanorod array and semicontinuous film. Figure 6 shows that the local field enhancement of the Au nanorod array is significantly enhanced by the percolating Au film at 800 nm. Note that the maximal local field is near the far ends of the Au nanorod array, and the enhance-

boundary conditions in z direction, and the dielectric constant of gold are taken from Johnson and Christy’s article.51 Two simplified Au nanorod-film hybrid structural models (shown in Figures 5a and 6a, respectively) for the percolating system were used; both nanorods and nanoparticles are periodically arrayed with hexagonal symmetry (as imposed by the AAO template used in the experiments) and each of the nanoparticles is on the central axis of the nanorods. Even though the nanostructure and the absorption spectra of the periodically arrayed nanoparticles are different from the percolating film consisting of less ordered nanoparticles, the fundamental plasmon-coupling mechanism in the nanorod-film hybrids is very similar. Figure 5 shows the transmission spectra of the percolating Au film with and without the Au nanorod array, and one © 2011 American Chemical Society

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ment factor reaches about 150%. These simulation results and the experimental observations indicate that both the transmission and local fields of the metal nanorod-film hybrids are significantly enhanced, and the dominate mechanism is plasmonic Fano resonance. In summary, we have shown two striking phenomena arising from the plasmonic coupling and interference between a percolating Au nanoparticle film and a standing Au nanorod array. The transmittance of the percolating Au film is enhanced ∼55% by the Au nanorod array. Conversely, the local field in the Au nanorod array is enhanced as well, leading to over 102 times the enhancement of the photoluminescence when Pexc > 1.5Pc. The high-Q plasmons in the Au nanorod arrays are enhanced by the low-Q plasmons of the percolating Au films. The nonlinear enhancement effect arising from the plasmonic Fano resonance in this nanorod-film hybrid system may bring about promising applications in the design of various plasmonic nanodevices, such as enhancing the sensitivity of SERS substrates, improving the efficiency of nanoantenna arrays, and building plasmonic lasers.

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Acknowledgment. We gratefully thank Professor Qi-Kun Xue, Professor Qi-Huang Gong, and Professor Ying Gu for helpful discussions, Dr. Li Zhou and Dr. Xue-Feng Yu for sample characterizations, and Mr. Jia Li for help on the FDTD simulations. This work was supported in part by NSFC (10874134), National Basic Research Program of China (2007CB935300, 2011CB922200), the U.S. DOE (Division of Materials Sciences and Engineering, Office of Basic Energy Sciences), and by the U.S. NSF (DMR-0906025).

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Supporting Information Available. Atomic force microscopy (AFM) images of the percolationg Au film, measured transmittance spectra of the Au island films with and without Au nanorod array, and the DDA calculations of the Au nanorod and Au nanorod-film hybrid are presented. This material is available free of charge via the Internet at http:// pubs.acs.org.

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DOI: 10.1021/nl1026869 | Nano Lett. 2011, 11, 49-–55