ARTICLE pubs.acs.org/JPCC
Tunable Fano Resonances in Three-Layered Bimetallic Au and Ag Nanoshell DaJian Wu,†,‡ ShuMin Jiang,‡ and XiaoJun Liu*,† † ‡
School of Physics, Nanjing University, Nanjing 210093, China Faculty of Science, Jiangsu University, Zhenjiang 212013, China ABSTRACT: The plasmon coupling in a three-layered Au/ SiO2/Ag nanoshell has been investigated by means of Mie scattering theory. The dipoledipole and dipolequadrupole Fano resonances have been observed in the extinction spectra of Au/SiO2/Ag nanoshells. With increase of the thickness of the middle layer, the dipoledipole Fano resonance shows a blue shift and the magnitude of this Fano profile enhances while the dipolequadrupole Fano resonance shows a red shift and reduces. With increase of the thickness of the outer shell, the dipoledipole Fano minimum shows a blue shift and its magnitude decreases while both the energy and magnitude of dipolequadrupole Fano resonance increase. All of the behaviors have been discussed with the plasmon hybridization model. In addition, the Au/SiO2/Ag nanoshell is found to show strong near-field enhancements in several regions, especially in the infrared region, which may provide effective applications in surface enhanced spectroscopy.
1. INTRODUCTION In recent years, plasmonic metal nanostructures have gained extensive interest due to the potential applications in chemical or biological sensing, surface enhanced spectroscopy, and biomedicine as well as the ability to manipulate light at nanoscale. Electromagnetic (EM) radiation irradiating on a noble metal nanoparticle can excite a coherent collective oscillation of the conduction electrons, known as localized surface plasmon resonance (LSPR), which can change sensitively with the geometrical shape and the composition.1 A particularly interesting and versatile structure is coreshell structured metal nanoparticle because the LSPR of metal nanoshell can be tuned from visible to infrared (IR) regions by varying the coreshell aspect ratio. Such a geometric “tunability” of LSPR into the near-IR region is highly beneficial for light-based biomedical diagnostics and therapeutics24 as well as surface-enhanced spectroscopy.5,6 Recently, Fano resonances in many plasmonic nanostructures have attracted great attention due to their promising applications in sensors, lasing, switching, and nonlinear devices.710 A plasmonic Fano-like profile arises from the interference between a spectrally overlapping broad resonance or continuum and a narrow discrete resonance.8 Most of the original works on plasmonic Fano resonances were reported on metallic arrays. Then some special Fano resonances have been observed in metal nanoparticle aggregates, such as heterodimers,11,12 asymmetric quadrumer,13 and symmetric septamer.9,14 The symmetric septamer is an outstanding nanostructure. The plasmon modes of septamer exhibit radiative interference effects resulting in very narrow Fano resonances, which have been found to be highly sensitive to the dielectric environment.9,14,15 Furthermore, Hao et al.16 r 2011 American Chemical Society
have found that the symmetry breaking in the concentric ring/ disk cavity enables the coupling between plasmon modes of differing multipolar order, resulting in a tunable Fano resonance. An asymmetric inner Au core in an Au nanoshell can allow the interference between superradiant and subradiant plasmon modes and hence a Fano resonance in its optical response.17 In this paper, we investigate the plasmon coupling in bimetallic gold and silver three-layered symmetric nanoshell (Au/ SiO2/Ag), consisting of an inner gold core, a middle dielectric layer, and an outer silver shell. The dipoledipole and dipole quadrupole Fano resonances have been found in the extinction spectra of Au/SiO2/Ag nanoshell by modulating the geometry. The variations of the Fano modes in Au/SiO2/Ag nanoshell have been discussed by means of plasmon hybridization theory. We further have found that the Au/SiO2/Ag nanoshell can provide strong near-field enhancements in several regions, especially in the infrared region.
2. ELECTROMAGNETIC SCATTERING BY A THREELAYERED METAL NANOSHELL The plasmon resonances and their couplings in a three-layered nanoshell can be quantified by the extinction spectrum. The EM scattering by a three-layered nanoshell can be understood with Mie scattering theory.18,19 The EM waves are expanded to spherical partial waves using vector spherical harmonics and then Maxwell’s boundary conditions are applied to resolve the Received: September 30, 2011 Revised: October 22, 2011 Published: October 27, 2011 23797
dx.doi.org/10.1021/jp209446p | J. Phys. Chem. C 2011, 115, 23797–23801
The Journal of Physical Chemistry C
ARTICLE
unknown expansion coefficients of the scattered and interior waves. The required parameters used in calculation are the radii of inner core, middle dielectric layer, and outer shell, r1, r2, and r3, and the permittivities for inner core, middle layer, outer shell and embedding medium, ε1, ε2, ε3, and ε4, respectively. For metal nanostructures, the dielectric constants have real and imaginary frequency-dependent components, which should be affected by the scattering of conduction electrons on the surfaces of nanoparticle. A modified Drude dielectric function should be used to describe the optical responses of gold core and silver shell. Thus, ε1 and ε3 can be expressed as18 ε1 ðωÞ ¼ 1
ε3 ðωÞ ¼ 1
ω21p ω2 þ iωγ1 ω23p ω2 þ iωγ3
þ χ1∞
ð1Þ
þ χ3∞
ð2Þ
where the background susceptibilities χ1∞ and χ3∞ arise from the electron polarizability and interband transition. ω1p and ω3p are the bulk plasma frequencies for inner core and outer shell, respectively. When the size-dependent electron scattering becomes important, the bulk collision frequencies can be modified as γ1 ¼ γ1f þ
V1f a1
ð3Þ
γ3 ¼ γ3f þ
V3f a3
ð4Þ
where γ1f and γ3f are the bulk collision frequencies for inner core and outer shell, respectively, V1f and V3f are the Fermi velocities for inner core and outer shell, respectively, and the reduced electron mean free paths a1 = 2r1 and a3 = r3 r2. The parameters can be obtained by fitting the dielectric function to a particular frequency range of bulk dielectric data.20 According to Mie theory, the extinction efficiency Qext can be expressed as21 Qext ¼
2 ∞ ð2n þ 1ÞReðan þ bn Þ ðkrÞ2 n ¼ 1
∑
ð5Þ
Here, k is the wavenumber outside the particle and Re means the real part. an and bn are the scattering coefficients.17,18
3. RESULTS AND DISCUSSION Figure 1 shows the extinction spectra of Au/SiO2/Au (solid line), Ag/SiO2/Au (dashed-line), and Au/SiO2/Ag (dottedline) nanoshells suspended in water (ε4 = 1.7689). Here, the radii of inner core, middle layer, and outer shell are fixed at 20, 30, and 50 nm, respectively. The dielectric constant of SiO2 is ∼2.04. For three-layered metal/dielectric/metal nanoshell, the plasmon resonances can be understood simply as hybridization between a nanosphere plasmon with radium of r1 and a nanoshell plasmon with thickness of (r3 r2).22 There are three hybridized plasmon modes with increased energy in order: an antisymmetric bonding ω (an antisymmetric coupling between the plasmon mode ωs in inner core and the symmetric mode ω in outer nanoshell), a symmetric antibonding mode ω+ (an symmetric coupling between ω s and ω ), and ω + mode (an antisymmetric coupling between ωs and the antisymmetric mode ω+ in outer nanoshell). Bardhan et al.22 have designated the third mode ω+ as a nonbonding mode because of its extremely weak strength.
Figure 1. The extinction spectra of Au/SiO2/Au (solid line), Ag/SiO2/ Au (dashed line), and Au/SiO2/Ag (dotted line) nanoshells suspended in water (ε4 = 1.7689). Here, r1, r2, and r3 are fixed at 20, 30, and 50 nm, respectively.
Compared with Au/SiO2/Au and Ag/SiO2/Au nanoshells, Au/ SiO2/Ag nanoshell has an additional peak at 2.91 eV, as shown in Figure 1. The peaks at 1.92 and 2.41 eV correspond to the dipole peaks for ω and ω+ modes, which are well-known as the subradiant and superradiant dipole modes.17,22 The peak at 2.91 eV means the subradiant quadrupole mode. A Fano minimum appears at ∼2.03 eV, which is due to the interference between subradiant and superradiant dipole modes. Such a dipoledipole Fano resonance also can be observed in Au/SiO2/Au or Ag/SiO2/ Au, which has already been reported by some previous papers.16,17 Another distinct Fano minimum can be found in the Au/SiO2/Ag nanoshell at 2.80 eV, which arises from the coupling of the subradiant quadrupole mode with the superradiant dipole mode. Figure 2 shows the electric field distributions of Au/SiO2/Ag nanoshells calculated at (a) 2.80 eV (dipolequadrupole Fano minimum), (b) 2.41 eV (superradiant dipole peak), and (c) 2.91 eV (subradiant quadrupole peak). In Figure 2b, the large electrical field occurs along the incident polarization and only locates within a few nanometers of the shell surface, which should be interpreted by the dipole plasmon resonance.23 At 2.80 eV, the electrical field distribution on the outer shell also shows the classic dipole resonance pattern (similar to that shown in Figure 2b). At the same time, the inner core exhibits a quadrupolar pattern (similar to the pattern in the core, as shown in Figure 2c). Such electrical field distribution clearly shows the mixed dipolar quadrupolar character of the dipolequadrupole Fano resonance.17 For the Au/SiO2/Ag nanoshell, the plasmon dipole mode of inner Au core is at 2.30 eV and plasmon resonance of Ag nanoshell can decrease from ∼3.10 eV to a very small value by decreasing the Ag shell thickness. Therefore, it is easy to obtain a strong interference between the plasmon resonance modes in Au core and Ag shell by devising the geometry. Figure 3 shows the extinction spectra of Au/SiO2/Ag nanoshell with different r2 value. Here, r1 and r3 are fixed at 20 and 50 nm, respectively. The increased r2 value should lead to the decrease of the outer shell thickness and the increase of the separation between inner core and outer shell. The increased separation suppresses the coupling between inner core and outer shell and makes the blue shift of the ω mode and the red-shift of the ω+ mode. In addition, the decreased outer shell thickness reduces the energy of the ω mode in the Ag nanoshell,24 which will make the red shifts of the ω and ω+ modes. When the r2 value is small, with increasing r2 value, the influence of the decreased shell thickness on the ω mode is weaker than that due to the increased separation. In this case, the subradiant dipole peak shows a blue shift, which will enhance the coupling between the subradiant and superradiant dipole modes. Thus, as shown in 23798
dx.doi.org/10.1021/jp209446p |J. Phys. Chem. C 2011, 115, 23797–23801
The Journal of Physical Chemistry C
ARTICLE
Figure 2. The distributions of the electric field enhancement of Au/SiO2/Ag nanoshell calculated at (a) 2.80 eV (dipolequadrupole Fano minimum), (b) 2.41 eV (superradiant dipole peak), and (c) 2.91 eV (subradiant quadrupole peak). Here, r1, r2, and r3 are fixed at 20, 30, and 50 nm, respectively.
Figure 3. The extinction spectra of Au/SiO2/Ag nanoshell with (a)r2 = 25 nm, (b)r2 = 30 nm, (c)r2 = 35 nm, (d)r2 = 40 nm, and (e)r2 = 45 nm. DDF and DQF mean the dipoledipole and dipolequadrupole Fano modes, respectively. Here, r1 and r2 are fixed at 20 and 50 nm, respectively.
Figure 4. The extinction spectra of Au/SiO2/Ag nanoshell with (a) r3 = 35 nm, (b) r3 = 40 nm, (c) r3 = 45 nm, (d) r3 = 50 nm, and (e) r3 = 55 nm. DDF and DQF mean the dipoledipole and dipolequadrupole Fano modes, respectively. Here, r1 and r2 are fixed at 20 and 30 nm, respectively.
Figure 3ac, with increasing r2 value, the dipoledipole Fano minimum shows a blue shift and the Fano profile is enhanced. When the r2 value is large enough, with further increasing r2 value, the influence of the decreased shell thickness on the subradiant dipole mode is stronger than that due to the increased separation. The large red shift of subradiant dipole mode happens. In this case, the subradiant dipole mode cannot couple with the superradiant dipole mode and hence the dipoledipole Fano resonance is absent, as shown in spectra d and e of Figure 3. On the other hand, with increasing r2 value, the red shift of subradiant
quadrupole mode is larger than that of superradiant dipole mode and hence the red shift of the dipolequadrupole Fano resonance. The decreased shell thickness should narrow the superradiant dipole peak and subradiant quadrupole peak, which reduces the magnitude of dipolequadrupole Fano mode, as shown in Figure 3. We further investigate the dependences of the two Fano modes in Au/SiO2/Ag nanoshell on r3 value. Here, r1 and r2 are fixed at 20 and 30 nm, respectively. In this case, the thickness of the middle layer is fixed and then the coupling strength 23799
dx.doi.org/10.1021/jp209446p |J. Phys. Chem. C 2011, 115, 23797–23801
The Journal of Physical Chemistry C
ARTICLE
photon energy and Ag shell thickness. Here, the Ag shell radius is fixed at 50 nm. Compared with SiO2/Ag nanoshell, it is obviously observed that the Au/SiO2/Ag nanoshell can provide strong near-field enhancement in several regions, especially in the infrared region, which may provide effective application for surface enhanced spectroscopy.
Figure 5. (a) The extinction spectrum (solid line) and the corresponding local field factor (dotted line) of Au/SiO2/Ag nanoshell. The contour plots of LFF in (b) Au/SiO2/Ag nanoshell and (c) SiO2/Ag nanoshell as a function of photon energy and shell thickness.
between inner core and outer shell is also fixed. When r3 = 35 nm, the outer Ag shell is very thin (5 nm) and the plasmon dipole resonance of the Ag shell is far away from that of the inner Au core. The subradiant and superradiant dipole modes couple directly to the incident light and the Fano shape is absent in the extinction spectrum, as shown in Figure 4a. The increased r3 value should increase the energy of ω mode and the phase retardation. The increased energy of the ω mode results in the large blue shift of subradiant dipole mode. The increased phase retardation should increase radiative damping, which broadens the subradiant and superradiant peaks. When the r3 value increases to 45 nm, the subradiant dipole mode couples with the superradiant dipole mode and hence the dipoledipole Fano resonance appears, as observed in Figure 4c. With further increase of the r3 value, the dipoledipole Fano minimum shows a blue shift and the magnitude of this Fano mode is reduced, as shown in spectra ce of Figure 4. Meanwhile, the dipole quadrupole Fano resonance shows a blue shift and the Fano profile is enhanced. The difference trend in Fano magnitude between dipoledipole and dipolequadrupole resonances with increasing r3 value is ascribed to the larger blue shift of the subradiant quadrupole mode compared to that of the superradiant dipole mode. Thus, the increased outer shell thickness can greatly enhance the dipolequadrupole Fano resonance. Finally, we discuss the near-field enhancement properties in Au/SiO2/Ag nanoshell. In Figure 5a, the solid line shows the extinction spectrum of Au/SiO2/Ag nanoshell and the dottedline represents the variation of the corresponding local field factor (LFF). Here, r1, r2, and r3 are fixed at 20, 30, and 50 nm, respectively. It is well-known that the plasmon resonance can induce a very strong electrical field near the metal nanoparticle. We assume the maximum of the near-field enhancement |E B/ B E in|max as the local field factor (LFF), which locates on the outer surface of the particle and at the poles along the incident polarization. The surprising discovery is that three large LFF peaks appear at 1.88 eV (LFF ∼ 7.303), 2.33 eV (LFF ∼ 6.815), and 2.91 eV (LFF ∼ 6.586), which arise from the subradiant dipole, superradiant dipole, and subradiant quadrupole modes, respectively. In addition, we compare the near-field enhancement of Au/SiO2/Ag nanoshell (see Figure 5b) with that of SiO2/Ag nanoshell (see Figure 5c). Figure 5b shows the contour plot of LFF in Au/SiO2/Ag nanoshell as a function of photon energy and Ag shell thickness. Here, the radii of inner core r1 and outer shell r3 are fixed at 20 and 50 nm, respectively. Figure 5c shows the contour plot of LFF in SiO2/Ag nanoshell as a function of
4. CONCLUSIONS We have investigated the dipoledipole and dipolequadrupole Fano resonances in concentric Au/SiO2/Ag nanoshells. It is found that the resonant energy and intensity of two Fano resonances can be tuned by changing the geometric parameters, i.e., the radii of the inner core, middle dielectric layer, and outer shell. Furthermore, we demonstrate that Au/SiO2/Ag nanoshells can provide the excellent near-field enhancement compared to the classical two-layered metal nanoshells. Thus, we believe that Au/SiO2/Au particles may also have the advantage over SiO2/Au nanoshells. It is anticipated that three-layered nanoshells may provide a new route to design plasmonic effects, which are useful to the practical applications of multilayer nanoshells in optical devices and biomedicine. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China under Grant No. 2012CB921504 and the National Natural Science Foundation of China under Grants 11174113, 10904052, and 10874088. ’ REFERENCES (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549–13554. (3) Gobin, A. M.; Lee, M. H.; Halas, N. J.; James, W. D.; Drezek, R. A.; West, J. L. Nano Lett. 2007, 7, 1929–1934. (4) Choi, M. R.; Stanton-Maxey, K. J.; Stanley, J. K.; Levin, C. S.; Bardhan, R.; Akin, D.; Badve, S.; Sturgis, J.; Robinson, J. P.; Bashir, R.; Halas, N. J.; Clare, S. E. Nano Lett. 2007, 7, 3759–3765. (5) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. ACS Nano 2008, 4, 707–718. (6) Wu, D. J.; Liu, X. J. Appl. Phys. Lett. 2010, 97, 061904. (7) Hao, F.; Nordlander, P.; Sonnefraud, Y.; Van Dorpe, P.; Maier, S. A. ACS Nano 2009, 3, 643–652. (8) Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. Nat. Mater. 2010, 9, 707–715. (9) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Science 2010, 328, 1135–1138. (10) Yang, Z. J.; Zhang, Z. S.; Zhang, L. H.; Li, Q. Q.; Hao, Z. H.; Wang, Q. Q. Opt. Lett. 2011, 36, 1542–1544. (11) Bachelier, G.; Russier-Antoine, I.; Benichou, E.; Jonin, C.; Del Fatti, N.; Vallee, F.; Brevet, P. F. Phys. Rev. Lett. 2008, 101, 197401. (12) Yang, Z. J.; Zhang, Z. S.; Zhang, W.; Hao, Z. H.; Wang, Q. Q. Appl. Phys. Lett. 2010, 96, 131113. (13) Fan, J. A.; Bao, K.; Wu, C.; Bao, J. M.; Bardhan, R.; Halas, N. J.; Manoharan, V. N.; Shvets, G.; Nordlander, P.; Capasso, F. Nano Lett. 2010, 10, 4680–4685. 23800
dx.doi.org/10.1021/jp209446p |J. Phys. Chem. C 2011, 115, 23797–23801
The Journal of Physical Chemistry C
ARTICLE
(14) Bao, K.; Mirin, N.; Nordlander, P. Appl. Phys. A: Mater. Sci. Process. 2010, 100, 333–339. (15) Mirin, N. A.; Bao, K.; Nordlander, P. J. Phys. Chem. A 2009, 113, 4028–4034. (16) Hao, F.; Sonnefraud, Y.; Van Dorpe, P.; Maier, S. A.; Halas, N. J.; Nordlander, P. Nano Lett. 2008, 8, 3983–3988. (17) Mukherjee, S.; Sobhani, H.; Lassiter, J. B.; Bardhan, R.; Nordlander, P.; Halas, N. J. Nano Lett. 2010, 10, 2694–2701. (18) Wu, D. J.; Liu, X. J. Appl. Phys. B: Laser Opt. 2009, 97, 193–197. (19) Wu, D. J.; Xu, X. D.; Liu, X. J. J. Chem. Phys. 2008, 129, 074711. (20) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370–4379. (21) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (22) Bardhan, R.; Mukherjee, S.; Mirin, N. A.; Levit, S. D.; Nordlander, P.; Halas, N. J. J. Phys. Chem. C 2010, 114, 7378–7383. (23) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (24) Prodan, E.; Nordlander, P. J. Chem. Phys. 2004, 120, 5444–5454.
23801
dx.doi.org/10.1021/jp209446p |J. Phys. Chem. C 2011, 115, 23797–23801
