J. Phys. Chem. B 2004, 108, 16713-16716
16713
Synthesis and Optical Properties of Silver Nanowire Arrays Embedded in Anodic Alumina Membrane Rui-Long Zong, Ji Zhou,* Qi Li, Bo Du, Bo Li, Ming Fu, Xi-Wei Qi, and Long-Tu Li Department of Material Science and Engineering, Tsinghua UniVersity, Beijing 100084, P. R. China
S. Buddhudu Department of Physics, S.V.UniVersity, Tirupati -517502, India ReceiVed: June 15, 2004; In Final Form: August 9, 2004
Transparent Ag nanowire arrays embedded in anodic alumina membranes were prepared by a template-based approach combined with ac electrodeposition and subsequent etching of substrate. The optical response of the structure could be attributed to surface plasmon resonance (SPR) of Ag nanowires. When the incident light was perpendicular to the surface of the composite film, only the transverse resonant mode was excited, and a dual peak line shape appeared at about 400 nm in the optical absorption spectrum. The longitudinal resonance mode appeared at a longer wavelength when polarized light illuminated the film with an angle of incidence of about 70°, where the angle was defined with respect to the surface normal. The resonant positions and relative intensities of the two resonant modes were affected by the diameter and aspect ratio of nanowires as well as the polarization direction of incident light. In contrast to the prediction of quasistatic theory, the longitudinal resonance peak did not red shift any more while the aspect ratio was large enough.
Introduction The interaction of light with small metal particles has long been identified as an important field of research.1 Recently, it has attracted more and more attention with its applications to nonlinear optics,2 surface-enhanced Raman scattering (SERS),3,4 and plasmonics.5,6 These nanoparticle systems strongly absorb the incident light at a particular frequency due to the SPR, which is the collective resonant absorption of free electrons at the surface of metal nanoparticles. Although the optical properties of metal spheres were described by Mie’s theory in 1908, the relationship between the particle geometry and their linear optical properties has not been fully established.7 Hence, selective synthesis of metal nanoparticles with specific size, geometry, and distribution is important to those theories and applications associated with SPR. Compared with physical methods such as electron beam lithography,8-11 the template-based approach is preferred due to its simple procedure to fabricate anisotropic or ordered nanoparticle structures, in which those nanoparticles have definite size, geometry, and distribution.12-14 Among various templates, the anodic alumina membrane (AAM) template is a type of promising hard template for its uniform and ordered pore structure. Up to now, electrochemical deposition has been the most successful process of filling the AAM with wellcrystallized metal particles. Alternating current (ac) electrodeposition was used to fabricate metal nanoparticles/AAM films without stripping the substrate, and the optical properties of these opaque structures were characterized by a UV-visible reflectance spectrum.12 Direct current (dc) electrodeposition was used to fabricate transparent gold nanorods/AAM structures; however, the complexity of the process severely limited its application in other metal nanoparticles/AAM structures. Hence, only the * Corresponding author. E-mail address:
[email protected]. Phone/Fax Number: 86-010-62772975.
optical properties of gold nanorods/AAM structures have been systematically studied by UV-visible absorption spectra.15-18 In this paper, we describe a simple process in which the ac electrodeposition is combined with subsequent etching of substrate to prepare transparent Ag nanowires (nanorods)/AAM composite films. The relationship between the diameter and aspect ratio of nanowires and their optical properties is investigated systematically by unpolarized and polarized absorption spectra. Experimental Section The AAM was fabricated by a two step aluminum anodic oxidation process, as described previously, to get a uniform pore structure.19 Because a linear relationship was found between the pore diameter of AAM and the anodic potential,20 four types of templates with different pore diameters were fabricated in oxalic acid (0.3 M) or sulfuric acid (5 wt %) solution at 20 °C at different anodic voltages. In brief, upon carefully annealing and electropolishing, a high purity (99.999%) aluminum sheet (25 × 20 × 0.3 mm) was first anodized for 6 h at a constant voltage condition. Then, the formed AAM was removed by a wet chemical etching in a mixture of phosphoric acid (6 wt %) and chromic acid (1.5 wt %) at 70 °C for about 4 h. After that, these samples were anodized for 2 h again at the same condition as the first anodization. At the end of the second anodic process, the anodic voltage was gradually decreased to 10 V and kept at this voltage for 10 min to thin the barrier layer at the pore base.21 Silver nanowires were grown from an electrolyte that contained 2 g/L AgNO3 and 2 g/L HBO3, and its pH was adjusted to 2.5 by using 1 M H2SO4. Electrodeposition (ac) was processed immediately after the second anodic procedure to keep the chemical activity of the AAM.22 During the electrodeposition, the color of the AAM changed from gold to brown with the prolonging of the deposition. After the electrodeposition,
10.1021/jp0474172 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/02/2004
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the anodic alumina membrane on one side of the sample was removed in 5 wt % NaOH, and then a part of the aluminum substrate on the same side was etched by a mixture of HCl and CuCl2.23 Finally, we got a transparent-colored thin film, which was supported by the remaining substrate. The nanowire arrays within the AAM were characterized by using SEM (JEOL JSM-6301F, Japan), TEM, SAED (JEM200CX, JEOL, Japan), and X-ray diffraction (Cu KR radiation, Rigaku, D/max-RB, Japan). The UV-visible absorption spectra were measured on a UNICAM UV 500 spectrophotometer (United States) in ambient conditions. Results and Discussion The AAM, which was fabricated in oxalic acid at 40 V, has perfect hexagonally arranged pore arrays (Figure 1a). After the ac electrodeposition, the anodic alumina channels were filled with Ag nanowires. The image of Ag nanowire arrays viewed from the bottom of the film is shown in Figure 1b, which verifies that Ag nanowires have nucleated in all pores. Both the EDX pattern (Figure 1c) and the XRD pattern (Figure 1d) confirm the existence of Ag. When the XRD pattern is compared with that of the standard powder diffraction pattern of Ag, the intensity of peak (220) is higher than that of peak (111), which is the highest intensity in the standard pattern, indicating that there was a preferred orientation during the growing of the nanowires. Since the nanowires were confined within the pores, we used the mean diameter of pores as the diameter of the nanowires. As shown in Figure 2a, the nanowires have a nearly uniform diameter, and their aspect ratio exceeds 40. An individual nanowire is shown in Figure 2b, and its SAED pattern, in Figure 2c, proves that it is a single-crystal nanowire. It is known that for a nonsphere particle different plasmon resonant modes can be excited at different dimensions.24-27 For metal nanorods, the relationship between the aspect ratio and the optical absorption peak can be described by Gan’s theory, which introduced a geometrical factor pi for different dimensions.28 The x and y axes of the nanorod are identical and correspond to the nanorod diameter (d), whereas the z axis represents the nanorod length (L). The geometrical factors pi are given by
pz )
1 - e2 1 1 + e -1 ln e2 2e 1 - e
[ (
) ]
(1)
p x ) py )
1 - pz 2
(2)
(
)
(3)
e)
L2 - d2 L2
Figure 1. (a) Top view of AAM template in 0.3 M oxalic acid at 40 V; (b) Ag nanowire arrays fabricated at ac 6.5 V viewed from the bottom, 5 wt % H3PO4 removed the barrier layer; (c) EDX pattern of Ag nanowires/AAM film; (d) XRD pattern of Ag nanowire arrays fabricated at ac 8.5 V.
1/2
where the aspect ratio is L/d. In analogy with the resonant condition of spheres, the resonant condition of the rods is
Figure 2. (a) TEM image of Ag nanowires after etching AAM; (b, c) TEM image and selected-area electron diffraction (SAED) pattern of an individual nanowire fabricated at ac 8.5 V.
metal ) -kim
theory, and the aspect ratio is chosen to vary from 1 to 6. The dependence of λmax on the aspect ratio is more pronounced for λLmax, which shifts to a longer wavelength with increasing aspect ratio, and is less pronounced for λTmax, which slightly shifts to shorter wavelength with increasing aspect ratio. The AAM is a good transparent matrix for investigating the optical properties of silver nanowires because the absorption of AAM is weak at the wavelength region of interest in the present research. We first investigated the effect of the diameter and the aspect ratio on the transverse plasmon resonance as
(4)
where m is the dielectric constant of the surrounding medium and ki () (1 - pi)/pi) is the shape-dependent screening parameter.16,29 Once the dielectric constant of metal, metal, is known, the resonant wavelength can be interpolated from the data on the bulk optical parameters for silver from Weber,30 while the dielectric constant of anodic alumina is 2.25.16 Figure 3 shows the transverse resonance wavelength λTmax and the longitudinal resonance wavelength λLmax calculated with Gan’s
Properties of Silver Nanowire Arrays
Figure 3. Theoretical calculation of the λTmax and λLmax at different aspect ratios of an Ag nanorod.
Figure 4. (a) Optical absorption spectra of Ag nanowire arrays, 5 wt % sulfuric acid under 18 V (d ) 10 nm); (b-d) 0.3 M oxalic acid under 30 V (d ) 18 nm), 40 V (d ) 30 nm), and 50 V (d ) 40 nm). All nanowire arrays grown at ac 6.5 V, 50 Hz, 40 s.
shown in Figures 4 and 5. In these experiments, the direction of unpolarized incident light was parallel to the major z axis of the nanowires. In other words, the electronic field vector was perpendicular to the major z axis. Therefore, only the transverse resonance along the minor x and y axes was excited. Figure 4 presents the absorption spectra of silver nanowires with different diameters. These spectra display a dual peak line shape, and the relative intensities of the two peaks depend on the diameter of nanowires. The two peaks can be ascribed to the transverse dipole resonance (longer wavelength) and the transverse quadrupole resonance (shorter wavelength) of silver nanoparticles, respectively.31 When the diameters of nanowires range from 10 to 30 nm (Figure 4, parts a-c), the dipole resonance peaks are stronger than the quadrupole resonance peaks. When the diameter reaches 40 nm, as shown in Figure 4d, the quadrupole resonance peak displays a distinct red shifting from 350 to 365 nm and becomes the strong peak, while the dipole resonance peak at about 400 nm is greatly suppressed and becomes the shoulder peak. This is because the quadrupole term becomes increasingly important with larger particles. Gan’s theory cannot explain these spectra, for it does not consider the effect of diameter and higher multipoles’ contributions. But, the features in these spectra are very similar to the reported spectra about silver nanospheres by Russell et al.32 In the absorption spectra, the change of the transverse dipole resonance peak λTmax of nanowires/AAO film with different aspect ratios qualitatively fits our calculations derived from
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16715
Figure 5. Optical absorption spectra of Ag nanowire arrays grown at ac 6.5 V, 50 Hz. (a) 10 s, (b) 20 s, (c) 40 s, and (d) 120 s. The diameters are equal to 30 nm.
Gan’s theory in Figure 3. As shown in Figure 5, because the aspect ratio of nanowires increases with the prolonging of electrodeposition, the transverse dipole resonance peak λTmax blue shifts with the increase of deposition time. In particular, the peak shifts dramatically at the initial growing stage (Figure 5, parts a-c). After 40 s of growing, however, the blue shifting of λTmax slows down and becomes almost invisible. Because of the prolonging of deposition, the aspect ratio approaches infinity (L/d f ∞). Equations 1-3 lead to e ) 1/2 and kx,y ) 1. So, λTmax has a limit in blue, and at the wavelength the relationship between the dielectric constant of silver and the surrounding medium is metal ) -m. These spectra also give information about reflection on different interfaces. When the aspect ratio of nanowires is small, the reflection of the film is strong compared with the absorption. As shown in Figure 5a, the spectrum displays intensive oscillatory signals, which are caused by the interference of the probe beam between the top face of the anodic alumina template and the interface of the Ag nanowires layer and AAM layer. So, from these oscillatory signals we can calculate the thickness of the layer that does not contain the Ag nanowires. With the growing of nanowires, the absorption becomes intensive and the full width at half-maximum (fwhm) is so wide that the color of the membrane gradually becomes deeper. This wide fwhm results from the wide distribution of the length of nanowires.16 Because of the anisotropy, the absorption spectra of Ag nanowires/AAM film presented an intensive longitudinal resonance at a longer wavelength when the film was tilted and illuminated by a polarized light. A polarizer was placed in front of the sample to adjust the polarization direction. When the polarizer was rotated every 90°, the longitudinal resonance peak changed from the maximum to the minimum (Figure 6a) and the transverse resonance peak displays a reverse changing in intensity. With the increase of the aspect ratios, the screening parameter of the longitudinal resonance kz approaches infinity; thus, the longitudinal resonance peak λLmax shifts toward a longer wavelength continually. In the research of Bohmer et al.,28 the optical properties of colloidal gold nanorods were studied, and the longest λLmax exceeded 1600 nm. Figure 6b shows a series of polarized absorption spectra of Ag nanowires/AAM film, in which the aspect ratios increase with the prolonging of electrodeposition. But, unlike the prediction of Gan’s theory, the λLmax does not shift toward a longer wavelength, as the aspect ratio is large enough. Recently, Yang et al.33 prepared a Langmuir-Blodgett silver nanowires monolayer with an aspect
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Zong et al. however, the changing of the longitudinal resonance peak does not agree with the theoretical predictions because the longitudinal resonance peak does not shift to a longer wavelength, again in a larger aspect ratio. With the variety of the embedded materials and the adjustability of the structure, the metal nanowires/AAM structure will have promising applications in many fields related to the surface plasmon resonance of metal nanoparticles. Acknowledgment. This work was supported by National Science Foundation of China under the grants 50172025 and 50272032, and also the Ministry of Sciences and Technology of China through the 973 project under the grant 2001CB6104. We express our grateful thanks to them for their support. References and Notes
Figure 6. (a) Absorption spectra of composite films at different polarized directions; (b) absorption spectra of composite films with different electrodeposition times.
ratio of about 45, and they also found that the λLmax was only at about 500-600 nm. Hence, the absorption spectra agree with the Gan’s theory only qualitatively.34 Three factors affect the application of the quasistatic theory to the structure. The first is the diameter of nanowires. Gan’s theory only considers the dipole resonance, but in our experiments the diameters of nanowires are so large that the multipole contributions cannot be neglected. The second is the length of nanowires. While the particle size is compatible with or even larger than the wavelength of incident light, scattering effects have to be considered. The third is the volume fraction of the metal nanoparticles. When nanorods were dispersed into the liquid, the volume fraction of metal in those colloids was only about 10-5 to 10-6.28 In our Ag nanowires/ AAM composite film, however, the volume fraction of metal is at the level of 10-2 to 10-1. In this case, the electromagnetic interaction between nanowires cannot be neglected. Conclusions Using the AAM template combined with AC electrodeposition, we have improved the process of preparing transparent Ag nanowires/AAM film. The optical properties of the film have been systematically characterized by UV-visible absorption spectroscopy. The transverse plasmon resonance displays a dual peak spectrum, which is ascribed to the multipole contributions, and the resonant position varies with the diameter and aspect ratio of nanowires. When a polarized light illuminated the film with an angle of incidence of about 70°, a longitudinal resonance peak appears at a longer wavelength. In our experiment,
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