Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered nanoshells

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1 Applied Physics A (8) 4:437 Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered nanoshells Jingwei Lv Haiwei Mu Xili Lu Qiang Liu Chao Liu Tao Sun 3 Paul K. Chu 4 Received: 3 September 7 / Accepted: 5 May 8 Springer-Verlag GmbH Germany, part of Springer Nature 8 Abstract The plasmonic properties of symmetry-broken Au ITO Ag multilayered nanoshells by shell cutting are studied by the finite element method. The influence of the polarization of incident light and geometrical parameters on the plasmon resonances of the multilayered nanoshells are investigated. The polarization-dependent multiple plasmon resonances appear from the multilayered nanoshells due to symmetry breaking. In nanostructures with a broken symmetry, the localized surface plasmon resonance modes are enhanced resulting in higher order resonances. According to the plasmon hybridization theory, these resonance modes and greater spectral tunability derive from the interactions of an admixture of both primitive and multipolar modes between the inner Au core and outer Ag shell. By changing the radius of the Au core, the extinction resonance modes of the multilayered nanoshells can be easily tuned to the near-infrared region. To elucidate the symmetry-broken effects of multilayered nanoshells, we link the geometrical asymmetry to the asymmetrical distributions of surface charges and demonstrate dipolar and higher order plasmon modes with large associated field enhancements at the edge of the Ag rim. The spectral tunability of the multiple resonance modes from visible to near-infrared is investigated and the unique properties are attractive to applications including angularly selective filtering to biosensing. Introduction Noble metal nanostructures have gained increasing attention due to their special optical properties and the possibility to manipulate light in some particular ways []. The ability of these nanostructures to sustain the collective excitation of electrons with an effective restoring force is known as localized surface plasmon resonance (LSPR) [, 3]. Owing * Chao Liu msm liu@6.com * Tao Sun taosun@hotmail.com.hk School of Electronics Science, Northeast Petroleum University, Daqing 6338, People s Republic of China 3 4 Institute of Materials Processing and Intelligent Manufacturing and Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 5, People s Republic of China Institute of Microelectronics, Agency of Science, Technology and Research (A*STAR), Singapore 7685, Singapore Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People s Republic of China to the dependence of LSPR on the shape and size of nanoparticles, the nature of the metal and dielectric constant of the surrounding host enable multiple strategies to design and optimize the optical properties of nanostructures [4 6]. Consequently, plasmonic nanoparticles have large potential in photovoltaics, surface-enhanced spectroscopy, bimolecular sensing, and optical metamaterials [7 9]. Similar to solid metallic nanoparticles, metallodielectric layered nanoshells have attracted considerable research interest and provide a high degree of plasmonic tunability with energies being controlled by the interaction between the sphere and cavity plasmon modes []. Reducing the symmetry of multilayered nanoshells by excising a part of the outer shell can produce a series of semi-shells including nanoeggs, nanocups and halfshells, which can enhance the interactions between plasmon modes [, ]. Symmetry-broken is proven to be suitable for excitation of higher order multipolar modes and produce plasmonic resonances from a variety of nanostructures such as nanocups and nanocrescents [3]. Furthermore, the optical properties of nanocups and nanocrescents are sensitive to angle and polarization of the incident light [4]. Qian et al. [5] observed from a unique Au nanoshell structure with two holes the Vol.:( ) 3

2 437 Page of two-dimensional angular selectivity on optical properties of the nanoshell. Hao et al. [6] studied plasmonic nanocavities consisting of a disk and ring. They showed broader dipolar modes and large local electric field enhancement at the edge of the nanoshell rim rendering them promising in surface-enhanced Raman spectroscopy (SERS) applications [7]. Recently, research on the optical properties of metallic nanoshells consisting of typically a silica interlayer has been spurred by advanced nano-optical characterization. In the Au SiO Au nanoshells, the interaction between the plasmon resonance mode of the inner Au core and outer Au shell produces coupling that can generate first and higher order resonances, which can be explained by plasmon hybridization method [8]. The Au SiO Au multilayered nanoshells with the Au core offset from the center and the excised Au shell have been reported recently [9]. In comparison with the typical dielectric materials SiO, indium tin oxide (ITO) possess highly favorable optical properties such as near-metallic conductivity and photo penetrability, which potentially contributes to support more multiple plasmonic resonances than SiO interlayer over a broad range of frequencies []. Based on these attractive properties, it is worth to exploring the influence of the geometry and angular selectivity on the optical response of symmetry-broken Au ITO Ag multilayered nanoshells when ITO is selected as dielectric interface layer. In this work, the optical properties of the symmetrybroken Au ITO Ag multilayered nanoshells by shell cutting are investigated by the finite element method (FEM) and plasmon hybridization method. The effects of polarization of the incident light and geometrical parameters on the LSPR of multilayered nanoshells are studied based on the plasmon hybridization theory. The electric field enhancement contours and surface charge distributions of the symmetry-broken Au ITO Ag multilayered nanoshells are analyzed based on transverse coupling and axial coupling. Theory J. Lv et al. Figure presents the schematic of the of symmetry-broken Au ITO Ag multilayered nanoshells. The radii of the Au core, middle ITO layer, and outer Ag shell are r, r and r 3, respectively, and the cutoff height of the outer Ag nanoshell is h. The incident radiation propagates along the x-axis and the polarization vector of the incident light is along the y-axis. The symmetry-broken Au ITO Ag multilayered nanoshells are set to a β rotation angle along the z-axis. The simulation is performed with the COMSOL Multiphysics software with Optics module. The simulated region is surrounded by a perfectly matched layer to absorb scattered light and the dielectric functions of gold, ITO, and silver are obtained from Palik s handbook [] and SOPRA N&K database []. The embedding medium is air in all cases. The plasmon hybridization theory is adopted to describe the plasmon properties of the symmetry-broken Au ITO Ag multilayered nanoshells [3]. There are two distinct resonance modes induced by different polarizations of the incident light [4]. The transverse mode is induced when the incident light is polarized perpendicular to the symmetry axis of the outer nanocup and the axial mode is generated when the polarization direction of the incident light is parallel to the symmetry axis of the outer nanocup [5]. With the reduced symmetry of the nanoshells, the selection rule for the same angular momentum is relaxed and new plasmon resonance modes appear because the multipolar primitive sphere plasmon modes can hybridize with all the multipolar cavity plasmon modes [6]. With regard to the transverse mode shown in Fig. a, the dipolar mode ω i for hybridization of a sphere ω S and cavity ω C can interact with not only the dipole mode of the outer nanocup ω, but also the quadrupolar, octupolar, and higher-order modes of the outer nanocup ω, l =, 3,,. This interaction creates two resonance modes, higher energy antisymmetric l and Fig. Schematic illustration of the symmetry-broken Au ITO Ag multilayered nanoshells: a schematic of the Au ITO Ag multilayered nanoshells in the rectangular coordinate system and b mid-sectional view of the Au ITO Ag multilayered nanoshells z y β (a) x (b) E D A B r F C r r 3 h 3

3 Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered Page 3 of 437 (a) (b) ω i l = l = l = l = The Transverse Mode Ε ω i l = l = l = l = The Axial Mode Ε ω i l = l = ω ω i l = l = ω Fig. Energy level diagrams describing plasmon hybridization in the symmetry-broken Au ITO Ag multilayered nanoshells: a transverse coupling and b axial coupling lower energy symmetric. Furthermore, the quadrupolar coupling modes (, ) and higher-order modes (, ) can be considered as the interaction l l between the quadrupolar mode ω i and higher-order modes ω i l of the sphere with all multipoles of the nanocup ω. l As shown in Fig. b for the axial mode, the dipolar mode ω i, quadrupolar mode ω i, and higher-order modes of ω i l of the sphere can interact with all the multipole of the outer nanocup ω, l =, 3,,. This interaction generates new resonance modes, that is, antisymmetric modes and symmetric modes, l l l =, 3,,. Fig. 3 Energy diagrams of plasmon hybridization of nanoshells: a extinction spectrum of the Ag nanoshell with ITO as the dielectric core, b extinction spectrum of the symmetry-broken Au ITO Ag multilayered nanoshells, and c extinction spectrum of the Au nanosphere (a) (b) (c) Extinction (a.u.) 3

4 437 Page 4 of 3 Results and discussion Figure 3 shows the energy level diagrams of the Ag ITO nanoshells, symmetry-broken Au ITO Ag multilayered nanoshells, and Au nanosphere. We consider the symmetrybroken Au ITO Ag multilayered nanoshells with r /r /r 3 = /4/5 nm and h = 3 nm. Panels (a) and (c) show the extinction spectra of the symmetry-broken ITO Ag nanoshells and individual Au nanosphere and Panel (b) displays the spectrum of the interacting and hybridized system. It confirms that the symmetry-broken ITO Ag nanoshells and Au nanosphere as bonding and antibonding combinations of different multipolar resonance modes. The interaction between the dipole bonding mode ω of the nanoshells and primitive dipole resonance mode l = of the Au core leads to a dipole bonding mode at.7 ev and dipole anti-bonding mode at 3.73 ev. Hybridization between the quadrupolar bonding modes of the nanoshell and spherical Au core gives rise to the quadrupole mode ω,, and at.7,.9, and.44 ev, respectively. On account of the finite speed of the incident light, 3 when a wave hits a circular structure from the side, only part of the symmetry-broken nanoshells is polarized and polarization of the surface charges can expand in a series of multipolar resonance modes [7]. To investigate the influence of polarization of the incident light on the extinction efficiency, the extinction spectra of the symmetry-broken Au ITO Ag multilayered nanoshells under radiation with different polarized incident light are presented in Fig. 4. The transverse coupling and axial J. Lv et al. coupling correspond to the multilayered nanoshells with rotation angle β = and β = 9 along the z-axis, respectively. In the extinction spectrum of the transverse coupling, five resonance modes at.7,.7,.9,.44, and 3.73 ev can be interpreted as the,,,, and 3 ω+ modes, respectively. In the spectra of the axial coupling, the dipole modes and and quadrupole modes and are observed at.7, 3.73,., and.3 ev, respectively. At.7 ev, there is no shift of the and modes during rotation of 9 and the intensity of the mode reaches a maximum, whereas the intensity of the mode maintains a minimum value. When the angle of polarization of the incident light is changed by 9, the spectrum dips at. and.3 ev for the transverse coupling almost change to the quadrupolar resonance peak of and. It can be concluded that the plasmon resonances observed from the symmetry-broken Au ITO Ag multilayered nanoshells are sensitive to light polarization. Figure 5 shows the extinction spectra of the symmetrybroken Au ITO Ag multilayered nanoshells with different Au core radii in transverse coupling. Specifically, the radii of the Au core r = 5,, 4, 6 and 3 nm are considered when other parameters are fixed. It can be seen that multiple plasmonic modes show a regular variation as the radius of Au core increases from 5 nm to 3 nm and the spectral line (r = nm) exhibit more multiple plasmonic modes when comparing with spectra of other radii of the Au core. The Extinction Efficiency (a.u.) transverse mode axial mode Fig. 4 Extinction spectra of the symmetry-broken Au ITO Ag multilayered nanoshells with different polarization direction of the incident light Extinction Efficiency (a.u.) /4/5 /4/5 4/4/5 6/4/5 3/4/ Fig. 5 Extinction spectra of the symmetry-broken Au ITO Ag multilayered nanoshells with different Au core radii in transverse coupling 3

5 Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered Page 5 of 437 ω mode blue-shifts slightly as the Au core radii increase, while the and modes red-shift. The red shift of 3 both the antisymmetric and symmetric coupling modes ω+ and are ascribed to the phenomenon that a 3 larger inner Au core radius decreases the intermediate ITO layer thickness and increases the plasmon interaction between the Au core and outer Ag nanocup [8]. The mode at.7 ev weakens and disappears as the radius of the Au core increases. The mode at 3.73 ev is attributed to the outer Ag nanocup and no shift is observed as the radius of the Au core increases. The energy range of modeling and simulation of symmetry-broken Au ITO Ag multilayered nanoshells are ev, which corresponds to the wavelengths range of 73 5 nm. Hence, the plasmon resonance of the symmetry-broken Au ITO Ag multilayered nanoshells can be tuned to the near-infrared region by simply changing the radius of the Au core. Figure 6 presents the relationship between the extinction spectra of the symmetry-broken Au ITO Ag multilayered nanoshells with different Au core radii with regard to axial coupling. The influence of the radii of the Au core r = 5,, 4, 6 and 3 nm on the optical properties of nanoshells are investigated when the cutoff height and radii of the middle ITO layer r and outer shell r 3 are fixed. As the radius of the Au core increases, the plasmonic interaction between the Au core and outer Ag nanocup changes, resulting in a blue shift of the lower energy antisymmetric coupling mode and red shift of the higher energy symmetric coupling mode. With a larger Au core radius, there is greater coupling ω+ between the plasmon modes of the core and shell, resulting in the red shift of the higher energy symmetric coupling mode [9, 3]. Meanwhile, the dipole lower energy antisymmetric coupling mode and higher energy symmetric coupling mode are almost unchanged. This effect is exactly balanced by the blue shift caused by interactions with the lower energy antisymmetric coupling mode, which increases with core radius size [3]. It can be concluded that the dipole modes and are more sensitive to the bonding mode ω of the outer Ag nanoshell, whereas the quadrupole modes of and depend more on the mode ω i of the inner Au core. Figure 7 shows the local electric field enhancement spectra of the symmetry-broken Au ITO Ag multilayered nanoshells for transverse coupling at different positions which are marked as Point A to Point F in Fig.. The radii of the multilayered nanoshells are fixed at r /r /r 3 = /4/5 nm and h is 3 nm. The ratio between the local electric field of the symmetry-broken Au ITO Ag multilayered nanoshell surface and external incident field can be expressed as E / E [3]. On account of plasmon coupling between the inner Au core and outer Ag shell, the symmetry-broken Au ITO Ag multilayered nanoshells exhibit five hybridization modes from point A to point F. It can be seen from inset (b) that the intensity of the local electric field enhancement at point A is larger than that at points B to F and the quadrupole mode at.7 ev can be excited, indicating the strongest plasmonic coupling interaction between the ITO interlayer and Ag cutoff section nanoshell. The large field enhancement at the edge of the Ag rim is useful to surface-enhanced Raman spectroscopy Extinction Efficiency (a.u.) /4/5 /4/5 4/4/5 6/4/5 3/4/5 E/E point B point C point D point E point F E/E (b) point A point B point C point D point E point F Fig. 6 Extinction spectra of the symmetry-broken Au ITO Ag multilayered nanoshells with different Au core radii in axial coupling Fig. 7 Electric field enhancement spectra at different positions of the symmetry-broken Au ITO Ag multilayered nanoshells in the transverse coupling mode 3

6 437 Page 6 of (SERS). The lower energy mode at.7 ev corresponds to symmetrical coupling between the bonding shell plasmon and sphere plasmon and the higher energy mode at 3.73 ev stands for antisymmetric coupling between ω+ the antibonding shell plasmon and sphere plasmon. Therefore, these two modes have greater dipole moments and the peaks of the local electric field enhancement correspond to symmetric and antisymmetric coupling between the bonding and antibonding shell plasmons. The sphere plasmon is intense and can be observed (Points D and E). At Point C, a weaker near-field is observed due to transverse coupling compared to the electric field enhancement at points A to E. The local electric field enhancement curves at different points of the symmetry-broken Au ITO Ag multilayered nanoshells show that the larger enhancement observed from the and modes are mainly attributed to LSPR of 3 the inner Au core (Points B and F). Figure 8 shows the representative electric field distributions of the symmetry-broken Au ITO Ag multilayered nanoshell in transverse coupling calculated in terms of five plasmonic resonance modes. The electric field enhancement at.7 ev corresponding to the hybridization mode is presented in Fig. 8a. The strong electric field enhancement is almost confined inside the ITO dielectric interlayer between the Au core and outer Ag nanoshell. The hot spots J. Lv et al. in the nanoshells are located between the Ag nanocup opening and ITO interlayer due to the strong interaction of the adjacent ITO interlayer and outer nanocup. The local electric field is symmetrically distributed on both sides of the ITO dielectric interlayer and Ag nanocup opening. With regard to the mode at.7 ev, the electric field enhancement is mainly distributed in the ITO interlayer and intersecting part of ITO interlayer and Ag nanocup opening, as shown in Fig. 8b. When the incident light impinges the multilayered nanoshell, part of the energy diffuses into the outer Ag nanocup and moves towards the inner Au core from which it is reflected back and trapped inside the ITO interlayer [33]. For the nanoshells at.9 ev corresponding to the mode, the strongest enhancement appears on the surface of the Ag nanocup opening and inner Au core, indicating the dominant role of the inner Au core in LSPR of the nanoshells. With respect to the nanoshells at hybridization mode at 3.44 ev, a strong electric field exists at the Ag nanocup opening. The electric field enhancement at 3.73 ev corresponding to the mode is illustrated in Fig. 8e. Owing to the different types of charges on the inner and outer surfaces of the Ag shell, the hot spots are densely located close to the surface of the Ag nanocup. Figure 9 displays the surface charge distributions of the symmetry-broken Au ITO Ag multilayered nanoshell in transverse coupling at specific energies characteristic of the Fig. 8 Electric field enhancement contours around the symmetry-broken Au ITO Ag multilayered nanoshells at the energy of,,, ω, and in the 3 transverse coupling spectrum (a).7ev (b).3.7ev.39 (c).5 (d) 5.7.9eV.39.44eV.39 (e) eV.39 3

7 Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered Page 7 of 437 Fig. 9 Surface charge distributions of the symmetry-broken Au ITO Ag multilayered nanoshells in the transverse coupling spectrum Ag shell ITO interlayer Au core 4.7 ev -4.7 ev -.9 ev ev ev - coupled modes. The charge density distribution indicates that the peak at.7 ev corresponds to the dipolar resonance mode which is considered a dipolar dipolar mode. The dipole moment in both the inner core and outer nanocup oscillates out of phase, resulting in decreased radiative damping and spectral narrowing [34]. Opposite charges are found on the surface along the E field direction in which the incident light is polarized. The charge polarity on the inner Au core is oppositely aligned with the outer Ag nanocup. This phenomenon corresponds to the low-energy configuration of the multilayered nanoshell and indicates that the plasmon resonance is the low-energy bonding mode from the core shell interaction. The and modes at.7 and.9 ev stand for the quadrupolar resonance resulting from coupling between the dipolar mode of the Au core and quadrupolar mode of the Ag nanocup. The distribution of the quadrupolar mode appears spatially distorted caused by the admixture of a dipole component polarized 3

8 437 Page 8 of ω along the horizontal direction [35]. For the quadrupoleoctupole resonance at.44 ev, the surface charge on 3 the Au core exhibits a quadrupolar pattern with a distinct separation of positive (red) and negative (blue) charges. The six nodes show up in the charge distribution of the nanocup characteristic of an octupole. The mode at 3.73 ev is an anti-bonding state formed by the interaction of the dipolar nanocup and dipolar Au core with a small admixture of the octupolar shell mode. The charge distribution exhibits an alternating half-ring shape stacked from the top to the bottom of the nanoshell and the charge polarity is the same along the inner and outer surfaces of the Au core and Ag nanocup. Figure presents the contours of the electric field enhancement of the symmetry-broken Au ITO Ag multilayered nanoshells in axial coupling. The electric field enhancement corresponding to the mode at.7 ev is shown in Fig. a. The electronic enhancement is mainly distributed on the Ag nanocup opening and a few nanometers outward of the nanoshells and the symmetry of the distribution of hot spots depends on polarization of the incident light compared with the electric field enhancement in transverse coupling. The electric field enhancement of the nanoshells at. ev corresponding to the mode is presented in Fig. b. Antisymmetric coupling between the plasmon resonance of the Au core and outer Ag nanocup leads to a strong anisotropic local electric field distribution in the middle ITO dielectric layer and Ag nanocup opening. For the mode at.3 ev, the maximum enhancement occurs at hot spots at which a strong electromagnetic field exists. The electric field J. Lv et al. ω+ enhancement of the symmetry-broken Au ITO Ag multilayered nanoshell around the inner Au core is more concentrated at the hot spots and it is presented with a deep red color. Concerning the mode at 3.73 ev, the electric field on the surface of the outer Ag shell and nanocup opening is stronger than that of the ITO interlayer and Au core. When the incident light is axially polarized, the corresponding extinction spectra exhibit four distinct peaks and the plasmon modes may be assigned by examining the charge distribution on the surface of the nanoshells, as shown in Fig.. At.7 ev, bonding-type hybridization between the oppositely aligned Ag shell dipolar mode and Au core dipolar mode gives rise to a dipolar mode in the combined symmetry-broken Au ITO Ag multilayered nanoshells. Meanwhile, the dipolar distributions appear skewed, indicating a large multipolar interaction to attenuate and shift the resonance mode. The and modes at. and.3 ev are the bonding and antibonding states formed by the interaction of the dipolar nanocup and dipolar Au core with a small admixture of the quadrupolar shell mode. The charge distribution associated with this mode is skewed to one side, revealing that the mode is excitable in the axial polarization since retardation effects in this size regime allow the two sides of the outer Ag nanocup to be oppositely polarized with respect to each other. The broken symmetry of the Ag nanocup geometry relaxes the three-fold degeneracy of the dipolar mode for a spherically symmetric nanoshell [36] giving rise to splitting into the two observed dipolar modes, antisymmetric mode at.7 ev and symmetric mode at 3.73 ev. Fig. Electric field enhancement contours around the symmetry-broken Au ITO Ag multilayered nanoshells at the frequencies of,, ω+ and, in the axial coupling spectrum (a) 8. (b).7ev.59.ev (c) 9.4 (d) 6..3eV eV.59 3

9 Localized surface plasmon resonance properties of symmetry-broken Au ITO Ag multilayered Page 9 of 437 Fig. Surface charge distributions of the symmetry-broken Au ITO Ag multilayered nanoshells in the axial coupling spectrum Ag shell ITOinterlayer Au core 5.7eV ev ev ev - 4 Conclusion The optical extinction properties of symmetry-broken Au ITO Ag multilayered nanoshells in both the transverse mode and axial mode coupling are investigated theoretically by the finite element method. The hybridized modes of the multilayered nanoshells depend on the incident light polarization and coupling of the plasmon resonance modes between the Au core and symmetry-broken Ag nanocup is explained by plasmon hybridization. The mixed modes induced by the broken symmetry Ag nanocup introduce a dipolar plasmonic mode into higher order multipolar modes rendering them visible in the extinction spectra. Polarization of the incident light has a large influence on the extinction spectra and different polarization creates distinctly different surface charge distributions. The extinction modes of the multilayered nanoshells can be conveniently tuned to the near-infrared region by changing the radius of the Au core. The electric field enhancement is mainly distributed in different regions of the multilayered nanoshells and a large field enhancement at the edge of the Ag rim can be observed. The distinctive spectral properties of the perforated Au ITO Ag multilayered nanoshells facilitate the design of an effective platform for LSPR sensing. Acknowledgements This work was supported by the National Natural Science Foundation of China (547469), Natural Science Foundation of Heilongjiang Province (E7), Northeast Petroleum University Innovation Foundation For Postgraduate (YJSCX7-34NEPU) as well as City University of Hong Kong Applied Research Grant (ARG) No and Strategic Research Grant (SRG) No The authors acknowledge the valuable comments and discussions with Prof. Xianli Li of the Northeast Petroleum University. References. S.J. Barrow, A.M. Funston, X.Z. Wei, P. Mulvaney, Nano Today 8, 38 (3) 3

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