Plasmonic Au/Co/Au Nanosandwiches with ... - Wiley Online Library

communications Plasmonics DOI: 10.1002/smll.200700594

Plasmonic Au/Co/Au Nanosandwiches with Enhanced Magneto-optical Activity** Juan B. Gonz lez-Daz, Antonio Garca-Martn, Jos M. Garca-Martn, Alfonso Cebollada, Gaspar Armelles,* Borja Sepffllveda, Yury Alaverdyan, and Mikael K(ll It is well known that localized surface plasmon resonances (LSPRs) greatly influence the optical properties of metallic nanostructures. The spectral location of the LSPR is sensitive to the shape, size, and composition of the nanostructure, as well as on the optical properties of the surrounding dielectric.[1] The latter effect has been used to develop different types of optical biosensors for which biological reactions near the surface of the nanostructure can be monitored through the changes in the frequency of the LSPR.[2–6] The induced electromagnetic field associated with the LSPR is greatly enhanced at the metal/dielectric interface, a phenomenon that is the basis for various types of surface-enhanced spectroscopy, such as surface-enhanced Raman scattering.[7] Furthermore, metallic nanoparticles have been shown to have light-guiding capabilities on the nanometer scale. This makes them suitable for the development of nano-optic devices.[8] The overwhelming majority of LSPR studies have focused on Au or Ag nanoparticles because these metals have suitable optical constants for application with visible wavelengths of light. However, once the morphology and composition of a nanostructure have been fixed, it is difficult to change or control the LSPR properties by external means, which would be desirable for the development of active nanoplasmonic devices. One way to overcome this problem could be to embed the metal nanostructure in an active medium, such as a liquid crystal,[9] which can be controlled by an external electrostatic field, or a ferromagnetic garnet,[10,11] which can be moderated by a magnetic field. An alternative approach could be to let the controlling field act directly on the metallic nanostructure, for instance, using nanoparticles made of ferromagnetic metals. Such metals have strong magneto-optical (MO) activity, that

[*] J. B. Gonzlez-Daz, Dr. A. Garca-Martn, Dr. J. M. Garca-Martn, Prof. A. Cebollada, Prof. G. Armelles Instituto de Microelectr"nica de Madrid Consejo Superior de Investigaciones Cientficas Isaac Newton 8 (PTM), Tres Cantos, Madrid, 28760 (Spain) Fax: (+ 34) 918 060 701 E-mail: [email protected] Dr. B. Sepffllveda, Dr. Y. Alaverdyan, Prof. M. K?ll Chalmers University of Technology Gçteborg, 41296 (Sweden) [**] Financial support from the Spanish Ministry of Science and Education (NAN2004-09195-C04 and MAT2005-05524-C02-01), and UE (NoE-Phoremost) is gratefully acknowledged.

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is, their optical properties change markedly even if the applied magnetic field is weak. Unfortunately, this high optical absorption results in a strong damping of any intrinsic LSPR that prevents the development of active plasmonic devices made solely of ferromagnetic metals. A promising route forward could be to combine ferromagnetic materials that would promote strong MO activity with noble metals that could induce plasmonic response. The large enhancement and spatial localization of the electromagnetic field associated with the LSPR suggest that a strong enhancement of the MO properties should be possible.[12] Several attempts to develop these kinds of structures have been carried out using different chemical synthesis methods to fabricate complex onion-like nanoparticles made of noble metals and ferromagnetic materials.[13–16] These systems do exhibit LSPRs, but so far no MO activity has been reported. On the other hand, continuous thin films made of Au/Co/Au trilayers were found to lead simultaneously to well-defined propagating surface plasmon polaritons and to strong MO activity at low magnetic fields.[17] Moreover, such composite structures also exhibit a magnetic-field-induced nonreciprocal effect in the surface plasmon polariton propagation,[18,19] that is, forward or backward propagating surface plasmons exhibit different wavevectors. A promising recent application of composite Au/Co thin films is the new type of high-sensitivity “magneto-plasmonic” biosensor reported in Reference [20]. In this Communication, we show that strong magneto-plasmonic effects occur in nanosandwiches composed of stacked Au/Co/Au disks. The Au/Co/Au nanosandwiches, prepared by a self-assembly process, exhibit simultaneous LSPR, magnetic, and MO properties. We show that optical and magneto-optical properties are strongly linked to the LSPR spectrum, which can be tuned by modifying the size of the nanoparticles. Moreover, there is a large enhancement of the MO properties caused by the LSPR effect. To the best of our knowledge, this is the first demonstration of a plasmonic nanostructure that can be controlled by an external magnetic field. The fabrication of the sandwich nanostructures from sputtered Au/Co/Au trilayer films was performed using colloidal lithography (CL).[21,22] In Figure 1 we show an atomic force microscopy (AFM) image corresponding to the sample obtained using polystyrene spheres of 76-nm diameter. The resulting nanoparticles have a mountainlike shape. The tip radius of the AFM is about 10 nm, therefore, the mountainlike shape is not only due to convolution effects (that would play a role in imaging the edges of the nanostructure), but also to the leftovers of the polystyrene spheres. This residual influence also explains the differences between the nominal heights of the nanosandwiches and those obtained by AFM, which are always higher than 32 nm. The plasmonic and magneto-optical properties of the fabricated nanosandwiches were analyzed by conventional UV/Vis extinction spectroscopy and magneto-optic Kerr spectroscopy, respectively. The magneto-optic Kerr effect consisted of a change in the reflectivity of the magnetic material when a magnetic field was applied. In particular, we employed a polar Kerr configuration in which the magnetic field was perpendicular to the nanosandwiches. In this con-

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were commonly characterized by a peak associated with the LSPR. The effects induced by the Co were found by comparison to the Au/Co/Au and Au 60-nm diameter disks; the LSPR peak broadened and was shifted to higher energy. Both shifts and broadenings of the LSPR peak induced by Co with respect to pure Au have previously been observed in core/shell nanoparticles made of ferromagnetic and noble metals. For different systems, the peak is blue-[16] or redshifted,[13,14] which suggests that the shift depends on the actual structure of the nanoparticles. However, broadening of the LSPR peak was always observed and has been ascribed to the high absorption of Co. On the other hand, the extinction peak of the Au/Co/Au nanoparticles shifted towards lower energy as we increased the disk diameter. This behavior was similar to that observed for pure noble metal nanoparticles, reflecting its plasmonic origin. This is best viewed in the inset of Figure 2, where the energetic positions of the absorption peaks for the Au/Co/Au nanoparticles is depicted. The effect of incorporating Co into the Au nanoparticles was not only to induce a shift and a broadening of the LSPR, but also to introduce MO activity that was absent in pure Au. This effect has not been reported for any other composite system developed so far. As an example, Figure 3 shows the effect of a magnetic field, applied perpendicular to the sample plane, on the polarization state of reflected light for the Au/Co/Au nanoparticles of 60-nm diameter. In particular, we present the magnetic-field dependence of the Figure 1. Three- and two-dimensional AFM images of the Au/Co/Au nanoparticle sample obtained with 76-nm-diameter polystyrene spheres.

figuration, the light reflected by the magnetized sample was subject to a rotation of the polarization plane and a change in ellipticity state with respect to the incident linearly polarized light. In Figure 2 we present the extinction spectra for samples of Au/Co/Au nanoparticles of 60-, 76-, and 110-nm diameter. We compared these spectra to that of a sample containing Au nanodisks of 60-nm diameter and 32-nm height. As can be observed, the extinction spectra of all the samples

Figure 2. Extinction spectra of the samples. In the inset, the energy position of the extinction peaks as a function of the diameter of the polystyrene sphere is shown. small 2008, 4, No. 2, 202 – 205

Figure 3. Polar Kerr loop of the Au/Co/Au nanoparticle sample obtained with 60-nm-diameter polystyrene spheres. A sketch of the experimental configuration is also presented.


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communications ellipticity (hysteresis loop of the Kerr ellipticity) at normal incidence. These results were obtained using the experimental set-up described in Reference [23]. It is noteworthy that, as expected, no magnetic-field-induced changes of the polarization state of the reflected light were observed for the pure Au nanoparticles. As we increased the magnetic field, the Kerr ellipticity increased and reached a saturation value for a magnetic field of around 12000 Oe. This value corresponded to the magnetic field needed to saturate the magnetization of the Co layer of the nanoparticles. The value of the ellipticity at saturation depended on the wavelength of the incident light, as is shown in Figure 4a, where we present its wavelength dependence compared to that of a continuous Au/Co/Au layer with identical Au and Co thicknesses. For the nanosandwich sample, the Kerr ellipticity spectrum had a peak in the same energetic region as the extinc-

tion peak. Moreover, as we increased the size of the nanoparticles, the position of the maximum of the Kerr ellipticity was red-shifted, as the extinction peak was (see Figure 4a). Therefore, we can associate the peak of the ellipticity spectrum with the LSPR of the Au/Co/Au nanoparticles. In Figure 4b the polar Kerr rotation spectrum is shown. This spectrum has an S-shape structure that was also located in the same energetic region as the extinction peak. For comparison, in the same figure we present the polar Kerr rotation and ellipticity spectra of the continuous trilayer system with identical Au and Co thicknesses. These spectra were completely different, which pinpointed the effect of the LSPR on the MO properties of the Au/Co/Au nanoparticles. Furthermore, although the amount of Co in the continuous layer was nearly 5 times higher than that of the nanoparticle layer, the magnitudes of the MO effects were similar, which indicated that the LSPR present in the nanoparticles led to a large enhancement in MO activity. To understand the origins of the MO effects observed and their correlation with the LSPR excitations, we calculated the polar Kerr ellipticity and rotation spectra for an array of Au/Co/Au nanoparticles with identical dimensions to those studied experimentally. This was done using a scattering matrix formalism adapted to treat materials with MO activity[24] that allows an exact solution for the wave propagation in ordered arrays. This formalism took into account all the interactions between the different nanoparticles and substrate effects, allowing us to calculate the full reflectivity Jones matrix ! rss rsp r¼ ð1Þ rps rpp where s and p represent the two polarizations. From this matrix we can obtain the complex Kerr rotation that is defined as F ¼ q þ if ¼ rps rpp ð2Þ

Figure 4. a,b) Experimental polar Kerr ellipticity and rotation spectra, respectively, of the 60-nm-diameter (circles) and 110-nm-diameter (squares) Au/Co/Au sample and the continuous Au/Co/Au layer (triangles). c,d) Theoretical polar Kerr ellipticity and rotation spectra for two different disk arrangements: 60-nm-diameter disks in triangular (thin continuous line) and square (thin dotted line) lattices; 110-nmdiameter disks in triangular (thick continuous line) and square (thick dotted line) lattices. For comparison, the experimental spectra of the continuous Au/Co/Au layer are also presented (thinner continuous line). e) Position of the absorption and Kerr ellipticity peaks as a function of the nanoparticle size. Dots and lines represent experimental and theoretical positions, respectively, in triangular lattice (continuous) or in square lattice (dashed). In the inset, a sketch of the theoretical disk arrangement is presented.

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where q is the Kerr rotation and f is the Kerr ellipticity. The nanoparticles were modeled as sandwiches of three disks with diameters that corresponded to those of the polystyrene spheres. The thickness of each disk corresponded to that of the experimental Au and Co layers. We have considered two types of geometries for the arrangement of the disk: triangular and square. The distance between the disks (lattice parameter) is determined by the surface coverage of the polystyrene spheres (Figure 4 shows a sketch of the arrangements). The optical properties of the Au and Co that were used came from Reference [19] and the MO constants of Co were extracted from the polar Kerr rotation and ellipticity spectra for the continuous Au/Co/Au film. In Figure 4c and d we present theoretical results for the 60- and 110-nm Au/Co/Au systems. It is evident that both types of arrangement gave similar results. Both the evolution of the MO spectra when we changed from a continuous to a nanoparticle layer (i.e., the appearance of the S-shaped structure in the polar Kerr rotation, the peak in the ellipticity spectra, and the enhancement of the MO activity) and the red-shift of the spectra were reproduced when we increased the disk diameter. However, the values for the theoretical spectra


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were higher than the experimental ones. This effect was attributed to an overestimation of the amount of Co inside the nanoparticles. This reduction could have resulted from the existence of a nonferromagnetic layer on the peripheral surface of the nanoparticles because Co was laterally exposed to air and, therefore, could be slightly oxidized. We have also calculated the absorption spectrum of the disks layer and, in Figure 4e, we present the theoretical position of the absorption and ellipticity peaks for the two arrangements as a function of the disk diameter. The theoretical results reproduced the trend observed for the experimental results: a red-shift of both peaks as we increase the size of the nanoparticles and a red-shift of the position of the ellipticity peak with respect to the absorption peak. Therefore, the structure observed in the polar Kerr rotation and ellipticity spectra was indeed related to the LSPR of the Au/Co/Au nanoparticles. The red-shift of the position with respect to the absorption peak results from the different wavelength dependence of the optical and MO constants. In summary, large-scale active nanoparticles exhibiting optical properties that can be controlled by an external magnetic field were obtained by colloidal lithography of Au/ Co/Au thin films. The optical and magneto-optical properties were governed by the LSPRs of the nanoparticles and could be tuned by modifying the size or shape of the nanoparticles. In addition, the LSPR enhancement of the electromagnetic field in the cobalt layer induced a large increase in the MO effects. These effects lead to the possibility of developing new types of active optical devices based on LSPRs. Additionally, the subwavelength size and the strong localization of the electromagnetic field for the nanoparticles make them promising candidates for the development of high-sensitivity magnetoplasmonic nanosensors with multiplexing capabilities.

Experimental Section Nanosandwich fabrication: Metal trilayer films with 6 nm of Au, 10 nm of Co, and 16 nm of Au are sputtered onto glass substrates. First, the top metal surface was coated with a layer of poly(diallyldimethyl-ammonium chloride) (MW 200000–350000, Aldrich) that made the surface positively charged and facilitated electrostatic adsorption of negatively charged sulfate latex spheres (Interfacial Dynamics Corporation, U.S.A.). The surface was then rinsed with Milli-Q water, dried with nitrogen, and exposed to a 0.2 wt % aqueous solution of latex spheres. After rinsing again with Milli-Q water and drying with nitrogen, the top metal surface became patterned with a short-range ordered array of latex spheres. The particle array was then used as a mask for directed Ar-ion-beam etching (Oxford model 300 Ion Beam Etching System, 500 V, 200 mA, 240 s).[22] The diameter of the latex spheres (60, 76 or 110 nm) directly controlled the diameter of the nanosandwiches. The equilibrium coverage, which defined the average distance between the spheres on the surface, was controlled by electrostatic repulsion through the surface charge of the spheres. In this study,

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we used surface coverages of 20–30% and the total area covered by the nanosandwiches was at least 1 cm2.

Keywords: ferromagnetic materials · magneto-optical activity · nanoparticles · optics · plasmonics

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