Reflectance and SERS from an ordered array of gold nanorods

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Electrochimica Acta 53 (2007) 1157–1163

Reflectance and SERS from an ordered array of gold nanorods B.G. McMillan a , L.E.A. Berlouis a,∗ , F.R. Cruickshank a , P.F. Brevet b a

WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland, UK b Laboratoire de Spectrom´ etrie Ionique et Mol´eculaire, UMR CNRS 5579, Universit´e Claude Bernard Lyon 1, 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France Received 23 October 2006; received in revised form 16 February 2007; accepted 20 February 2007 Available online 25 February 2007

Abstract The optical properties of arrays of Au nanorods were studied by specular reflectance spectroscopy. The spectra were dominated by the surface plasmon modes of the Au nanoarrays superimposed on the effects of interference through the films. The longitudinal plasmon resonance moved to longer wavelength as the aspect ratio of the nanorods increased. The reflectance spectra were modelled by applying the Maxwell-Garnett approximation to a uniaxial thin film (composite Au/alumina) and this yielded a good match to the experimental data. SERS spectra on the Au nanorod arrays were recorded at different externally applied potentials and significant differences with respect to an electrochemically roughened Au electrode were revealed. These have been attributed to the nature of the composite nanoarrays, both their nanostructuring into rods and the regular arrangement of these rods. © 2007 Elsevier Ltd. All rights reserved. Keywords: Au nanorods; Surface plasmons; Reflectance; SERS; Uniaxial material

1. Introduction The optical properties of metallic nanoparticles are dominated by their surface plasmon resonances. In this work, these resonances are investigated for arrays of Au nanorods with different aspect ratios. Various methods for the synthesis of gold nanoparticles, both spheres and rods, have been described in the literature [1–9]. The method of preparation of the arrays adopted here was the template electrodeposition approach, with the gold nanorods electrochemically deposited within the pores of anodically grown porous alumina films [10]. This enables a high level of control on the shape and size of the nanoparticles so formed, with the diameter of the rod being controlled by the pore size and its length by the quantity of Au deposited into the pore. This method of production has found several applications, including low cost infrared polarizers [11] and substrates for surface-enhanced Raman scattering spectroscopy (SERS) [12,13]. However, the nature of the anodisation process on aluminium means that an electrically insulating barrier layer of alumina exists between the Al substrate and the porous layer ∗

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which does not easily allow the dependence of the SERS signal on the electrode potential to be measured. Previous studies of the optical properties of Au nanowire arrays have concentrated on their absorption spectra [1,14] and the dominant spectral features were indeed the surface plasmon modes of the metal particles. In this study, a particular application of the nanowire arrays that was of interest was their potential use as SERS susbtrates, under electrochemical control. Thus, the Au nanowire arrays embedded within alumina matrices were studied by specular reflectance as a function of the aspect ratio of the rods and the results were modelled by applying the MaxwellGarnett approximation [15] to a thin, anisotropic film composed of Au nanorods and porous alumina. Additionally, the fabrication method noted above was modified to allow direct electrical contact to the nanorods to be made and preliminary data on the SERS spectra of pyridine adsorbed onto the rods, as a function of the externally applied electrode potential, are reported. 2. Experimental The 1 mm thick, 99.999% Al foil (Alfa Aesar) used for template fabrication was cut into 7 mm × 7 mm pieces which were

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annealed under Ar for 5 h. Each piece was polished, first mechanically and then electrochemically at 40 V for 300 s in a 1:4 (by volume) HClO4 :ethanol solution at 5 ◦ C. On removal, the samples had a mirror-like finish. They were then anodised in 0.3 M oxalic acid solution, again at 40 V [10,16], for 20 h and the resulting disordered porous and barrier oxide films were dissolved by immersion in a 1.8 wt.% CrO3 /6 wt.% H3 PO4 aqueous solution at 70 ◦ C for 2 h [17], leaving a patterned Al surface. These prepatterned Al pieces were re-anodised for 30 min to yield alumina films having a high degree of pore ordering. To facilitate the electrodeposition of Au, it was necessary to reduce further the thickness of the ‘barrier layer’ of alumina which exists between the Al surface and the porous film. This was achieved through electrochemical means, by applying a constant current of smaller magnitude than that necessary to maintain the steady state process of film dissolution and pore formation. Here, a current density of 300 ␮A cm−2 was applied. This condition favoured film dissolution over film growth, with the voltage across the barrier layer being proportional to its thickness. The thinning was terminated when the voltage reached 6.5 V, corresponding to a thickness of ca. 8 nm [18]. Au was then electrodeposited into the pores from a 0.1 M KAu(CN)2 solution using an ac electrodeposition technique (6–10 V rms, 200 Hz). For the reflectance data, recorded over the wavelength range 400–800 nm, the light source was an Applied Photophysics 150 W Xe arc lamp interfaced with a computer controlled Applied Photophysics f/3.4 monochromator. The beam diameter was first reduced by means of an iris to ensure that the entire light incident on the sample had passed through a polarizer. A lens (focal length = 5 cm) was then used to focus the light on the sample. The reflected light was collected by a photomultiplier tube whose output was amplified by a Bentham 286 Current Amplifier before being recorded by the computer running custom written software. The spectra obtained were normalised with respect to an aluminium mirror. For the electrodes used in the SERS experiments, a different preparation procedure was used. The second anodisation period was limited to only 15 min and this allowed the Au to be electrochemically deposited throughout the entire thickness of the alumina films. Electroless silver was then deposited on top of this Au/alumina composite layer using the Petitjean method [19] where the freshly prepared solutions were cooled to 5 ◦ C to reduce the reaction rate and so allow the mixture to fully access the pores before the deposition occurred. Electrical contact was made to the deposited silver layer by means of a wire and the whole back surface of the electrode was sealed with epoxy. The Al was then dissolved away in a Cu/HCl [20] solution to leave the silver-backed Au/alumina films sealed in the epoxy. The final step involved dissolving some of the alumina in 0.1 M H3 PO4 solution to expose the tips of the nanorods. It is worth mentioning here that it is entirely possible to produce a silver nanowire array purely by this electroless deposition process. The SERS data were recorded on a Renishaw confocal Raman system employing a 632.8 nm laser. The samples were mounted within an electrochemical cell containing 0.01 M pyridine/0.1 M KCl solution and the potential was controlled by an

EG&G Model 362 scanning potentiostat. The exciting laser was focussed on the surface of the sample using a 20× objective lens and 10 spectra, each with an acquisition time of 10 s, were recorded and averaged at each potential. 3. Results and discussion Fig. 1 shows the reflectance data recorded in air from alumina samples subjected to different periods of Au electrodeposition and so, yielding nanorods with different aspect ratios a/b where a is the length of the long axis and b the length of the short axis. The main features of the spectra are the global minima and the presence of interference patterns, the latter of which arise from coherent multiple reflections within the films. For the S-polarized reflectance data, only one minimum is present, at 550 nm whereas for the P-polarized data two such minima are present, the first at 550 nm and the second at longer wavelength. These minima are a result of excitation of the two surface plasmon modes of the Au nanorods, with the transverse mode (along the nanorods’ short axes) being excited by both S- and Ppolarized light incident on the sample surface at an angle of 30◦ and the longitudinal mode (along the nanorods’ long axis) being excited only by the P-polarized light. The position of the longer wavelength minimum, and hence the longitudinal plasmon resonance, depends on the aspect ratio a/b and this effect can be observed in Fig. 1. As the period of the Au deposition time is increased, the length b of the nanorod increases and the position of the longitudinal mode is shifted to longer wavelength, with an increase in the intensity. The X-ray powder diffraction pattern for the Au deposited within the pores of a 30 min anodised sample is shown in Fig. 2. Here, the alumina film post gold electrodeposition was carefully detached, rinsed in distilled water and mounted on a stainless steel substrate for the analysis. It is evident that all the expected diffraction peaks for Au are present in the spectrum and Scherrer analysis of the data revealed a crystallite size of ∼40 nm, consistent with the pore size of the alumina. Fig. 3 shows a

Fig. 1. Measured reflectance spectra, using P-polarized light, as a function of the electrodeposition time (50 and 80 s) of the Au within the anodised alumina film. Dashed curve is the corresponding reflectance spectrum obtained using S-polarized light.

B.G. McMillan et al. / Electrochimica Acta 53 (2007) 1157–1163

Fig. 2. X-ray diffraction pattern recorded for the detached Au nanorods/porous alumina composite layer. The grey peaks are due to the stainless steel substrate.

scanning electron micrograph (SEM) image of the composite gold alumina layer after the surface had been exposed for 3 min in hot CrO3 /H3 PO4 to partially dissolve the alumina. The Au nanorods are clearly visible in one of the voids created by the etchant, with the hexagonal arrangement of the rods mirroring that of the surrounding unetched alumina layer. These data clearly illustrate that the Au nanorod/alumina composites are optically anisotropic and a model describing their reflectance spectra must take this into consideration. Thus, a three-layer film model, consisting of the Al substrate, the Au/alumina composite and an alumina/air layer above the composite was adopted with the Au/alumina film considered to be the only one possessing uniaxial optical properties. The ordinary and extraordinary dielectric constants describing this medium were calculated using the Maxwell-Garnett effective medium approximation. The model has been described in our earlier work [21,22] and incorporates some early and fundamental work in this field [23–25]. The schematic of the model is shown in Fig. 4. The porous alumina film grows as an array of parallel-sided pores which means that the nanorods have their long axes naturally aligned with the

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Fig. 4. Illustration of the three-layer film model used to simulate the reflectance data. The composition and refractive indices of the respective films are indicated on the figure.

external electric field applied during their formation. Furthermore, it is clear from this illustration that in these experiments, the Au deposits never extended the whole length of the pore, i.e., the electrodeposition of the gold within the pores was incomplete. The calculation for the optical constants of Layer 2 (the composite Au/alumina layer) therefore required an approach that could account for the anisotropy in the shape (related to the amount of Au deposited in the pores) and hence of the plasmon resonances of the nanorods. This was achieved by modelling the nanorods as prolate ellipsoids, which allowed geometrical factors, based on the semilong axis a and the semishort axis b to be defined. Here, εd is the dielectric constant of the alumina and εˆ m is the dielectric constant of the Au and these are incorporated into the Maxwell-Garnett effective medium approximation to give two effective dielectric constants based on the geometry of the ellipsoids [26], viz.: εˆ a,b =

(1 − qm )εd + qm βa,b εˆ m 1 − qm + qm βa,b

(1)

Here qm is the volume fraction of metal within this layer and βa,b the form factor of the ellipsoid for two axes. The ordinary, nˆ 2o , and extraordinary, nˆ 2e , refractive indices of the film are therefore:   (2) nˆ 2o = εˆ b and nˆ 2e = εˆ a The complex Fresnel coefficients for reflection at the Layer 1/Layer 2 interface, rˆ12 , and the Layer 2/Layer 3 interface, rˆ23 , can be calculated by following the approach outlined by Dignam et al. [24] and combined to give an overall reflection coefficient from the three-layer composite film system, viz. ν rˆ0123 =

Fig. 3. SEM image showing the gold nanorods in a void created through partial etching of the porous alumina in hot CrO3 /H3 PO4 .

ˆν

ˆν

ν + rˆ ν e−2iγ + rˆ ν e−2i(γ+δ ) + r ν rˆ ν rˆ ν e−2iδ r01 12 23 01 12 23

ν rˆ ν e−2iγ + r ν rˆ ν e−2i(γ+δˆ ) + rˆ ν rˆ ν e−2iδˆ 1 + r01 12 01 23 12 23 ν

ν

(3)

where the superscript ν denotes either the P- or S-polarization. The quantities γ and δˆ ν are the phase changes suffered by the light during a single traversal of Layers 1 and 2, respectively, and are introduced to account for the interference effects observed due to multiple reflections. Unlike the approach taken by Goad and Moskovits [23] it was felt to be unnecessary to suppress these interference effects since they are clearly represented in

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Fig. 5. Simulated reflectance spectra as a function of the aspect ratio of the Au nanorods. The aspect ratios and polarization states used are indicated on the figure.

our experimental data. The reflectance of the system is then simply given by ν Rν0123 = |ˆr0123 |2

(4)

Fig. 5 gives the reflectance spectra, calculated from the model for both P- and S-polarized light. It also shows the spectra calculated for P-polarized incident light for different aspect ratios of the nanorods. The bulk dielectric constants of Au [27] and Al [28] were used in the calculation, with the refractive index of alumina being taken as 1.62 [23]. As can be seen from Fig. 5, the main features of the experimental data are replicated, with both longitudinal and transverse plasmon modes for the P-polarized case being clearly represented as well as the interference effects. It is worth noting that the latter arise mainly from light transmission through Layer 1. Furthermore, it is clear that the observed experimental trend regarding the wavelength dependence of the longitudinal plasmon mode on the aspect ratio, as presented in Fig. 1, is also replicated by the model. Electroreflectance experiments were also performed on these composite systems and have been reported elsewhere [12]. The results were also satisfactorily described within the present general framework of the three-layer composite film. The SERS experiments were carried out using a 0.01 M pyridine solution with 0.1 M KCl as a background electrolyte. This solution was chosen as the SERS from pyridine adsorbed on Au has been well characterised in the past and thus provided a suitable reference system for comparison with our work. The initial experiment employed a 1 mm diameter Au disc electrode which was electrochemically roughened by completing 25 oxidation/reduction cycles in 0.1 M KCl solution [29]. Each cycle involved scanning the potential from −0.3 to 1.2 V at 1 V s−1 and scanning back to −0.3 V at 0.5 V s−1 after a 1.2 s delay. The saw tooth ramp was repeated every 30 s. The roughened electrode was then mounted within a special cell and the 632.8 nm laser was focussed onto its surface. The SERS spectra recorded at various electrode potentials on this roughened Au surface are shown in Fig. 6 with the assignment of the spectral features reported in Table 1 [30]. These data compare

favourably to previously recorded SERS spectra for pyridine on gold under the same conditions, apart from one exception. The peak at 1339 cm−1 is not generally observed in SERS experiments on Au and has thus been only tentatively assigned to the ␯14 vibrational mode. The only previously reported SERS band in this region was from an Fe electrode [31]. It has long been considered that the adsorbed pyridine molecules are N-bonded perpendicular to the metal surface [31] and this is verfied here as all the observed enhanced vibrational modes correspond to in-plane perturbations with the strongest bands having their induced dipoles perpendicular to the metal surface. The dependence of the SERS intensity on the electrode potential is also in accordance with the previously reported data on these systems [32]. Figs. 7 and 8 show the SERS spectra recorded as a function of electrode potential from the specially prepared composite Au nanorod/alumina sample (as discussed in Section 2) that had been etched for 90 min in phosphoric acid solution in order to expose the nanorods. These spectra exhibit several key differences when compared to those recorded from the Au disc electrode. First and foremost, an additional peak at ca.1020 cm−1 appears and becomes a dominant feature of the spectra. This peak is attributed to the bond formed between the nitrogen lone pair and the metal when the pyridine is chemisorbed. Although this peak has been reported in the SERS spectra recorded at Ag electrodes [33,34] it is unusual to observe it when Au is used as the substrate. It has indeed only been recently observed by Abdelsalam et al. from an Table 1 Assignation of peaks in SERS experiments for pyridine adsorbed on gold [30] Label

ω (cm−1 )

Vibration mode

Vibration type

a b c d e f

1009 1034 1066 1216 1339 1599

␯1 ␯12 ␯18a ␯9a ␯14 ␯8

Ring breathing C–H in-plane deformation C–H in-plane deformation C–H in-plane deformation Asymmetric C–H stretching Ring stretching

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Fig. 6. SERS spectra of an electrochemically roughened Au disc electrode at different potentials in a solution containing 0.01 M pyridine and 0.1 M KCl (peak positions are given in Table 1).

Fig. 7. SERS spectra of the Au nanorods array at positive potentials (vs. SCE) in a solution containing 0.01 M pyridine and 0.1 M KCl.

Fig. 8. SERS spectra of the Au nanorods array at negative potentials (vs. SCE) in a solution containing 0.01 M pyridine and 0.1 M KCl.

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ordered nanoporous Au electrode [35]. Their nanostructured Au electrode can be considered to be equivalent to a ‘photographic negative’ of our system in that Au nanopores rather than nanorods were employed. It can also be clearly seen from the data in Figs. 7 and 8 that the overall SERS intensity increases as the potential is decreased to −300 mV. Furthermore, the peak associated with the N–Au band becomes the dominant feature of the spectra over the potential range between +200 and −100 mV. Outwith this range, the ␯1 vibrational mode is the dominant feature as is observed for the electrochemically roughened Au disc electrode system. At −500 mV, the Au–N peak appears as a shoulder on the now dominant ␯1 band with an overall reduction in the SERS intensity. Additionally, the peak at 1339 cm−1 in the Au disc electrode spectra was absent from these data which is more in line with the previously reported SERS spectra from Au. The observed behaviour of the peak at 1020 cm−1 is significantly different to previously reported experiments showing this feature, as it usually only appears as a weak band and is never the dominant peak in spectra recorded on Au substrates. Its dependence on electrode potential also does not follow the same trend as that reported in the literature [32,35]. The strength of the interaction between the metal and the pyridine remains unchanged, as any alteration of the bond strength would lead to a change in the vibrational frequency. This is not found in our data. Furthermore, since the relative peak intensities in this region of the spectrum vary with the applied potential, it cannot be the case that simply more or less pyridine is adsorbing (i.e. a change in coverage is occurring) as the potential is changed. The theoretical basis to explain the large enhancements (∼1011 ) of the SERS signal at such ordered nanoarrays was laid by Garcia-Vidal and Pendry [36], Aravind et al. [37] as well as others [38]. Garcia-Vidal and Pendry [36] for example showed that large local enhancements in the Raman signal of adsorbed molecules could result from the excitation by the incident radiation of the plasmon localised between two nanocyclinders. The magnitude of the electric field produced was strongly dependent on the geometry of the region where this coupling occurs. Furthermore, Moskovits and coworkers [39] demonstrated that the SERS signal from rafts of silver nanowires exhibited a strong dependence on the polarization state of the exciting light, with a 10-fold increase in the intensity for light polarized with the electric field perpendicular to the long axis of the nanowires. In this configuration, a molecule adsorbed in the region between such two nanowires in the raft would experience a very intense field brought about by the close proximity of the conjugate charges on the individually polarized nanowires. It could well be that in our experiments the pyridine was to be found in such interstices between the nanorods (as well as in other positions) but it would have been difficult using our experimental configuration to ensure that the electric vector was indeed normal to the long axis of the rods in all the experiments carried out and reported here. Also, the large (∼106 ) enhancement above that due a roughened surface, expected due to the close proximity of the nanorods was also not found in our data. On the other hand, the clear effect of the applied electric potential on the SERS signal of the pyridine on the nanorod array would suggest that

changes in the polarizability of the individual nanorods could also be effected by this means. 4. Conclusions In this work, the optical properties of arrays of Au nanowires were studied by specular reflectance spectroscopy as a function of their aspect ratio. The spectra were dominated by the surface plasmon modes of the Au nanoarrays superimposed on the effects of interference through the films. The position of the longitudinal plasmon mode moved to longer wavelength as the nanorod aspect ratio increased. The reflectance spectra were modelled by application of the Maxwell-Garnett approximation to a thin uniaxial film and this has yielded a good match to the experimental data. SERS spectra on the Au nanorods arrays recorded at different potentials has revealed significant differences with respect to those on electrochemically roughened Au electrodes and these have been attributed to the nature of the composite systems formed by nanorods assemblies. Strong localised enhancement of the electromagnetic field could occur in certain positions within this ordered nanostructure, giving rise to increased intensity of the SERS signal for pyridine adsorbed in between the Au nanorods. References [1] C.A. Foss, M.J. Tierney, C.R. Martin, J. Phys. Chem. 96 (1992) 9001. [2] D. Almawlawi, C.Z. Liu, M. Moskovits, J. Mater. Res. 9 (1994) 1014. [3] C.A. Foss, G.L. Hornyak, J.A. Stockert, C.R. Martin, J. Phys. Chem. 98 (1994) 2963. [4] Y. Yu, S.-S. Chang, C.-L. Lee, C.R.C. Wang, J. Phys. Chem. 101 (1997) 6661. [5] B.M.I. van der Zande, M.R. B¨ohmer, L.G.J. Fokkink, C. Sch¨onenberger, J. Phys. Chem. 101 (1997) 852. [6] S.-S. Chang, A.-W. Shih, C.-D. Chen, W.-C. Lai, C.R.C. Wang, Langmuir 15 (1999) 701. [7] N.A.F. Al-Rawashdeh, M.L. Sandrock, C.J. Seugling, C.A. Foss, J. Phys. CHem. 102 (1998) 361. [8] S. Link, M.B. Mohammed, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3073. [9] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065. [10] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [11] S. Saito, M. Miyagi, Appl. Opt. 28 (1989) 3529. [12] Z.Q. Tian, B. Ren, D.Y. Wu, J. Phys. Chem. B 106 (2002) 9463. [13] J.L. Yao, G.P. Pan, K.H. Xue, D.Y. Wu, B. Ren, D.M. Sun, J. Tang, X. Xu, Z.Q. Tian, Pure Appl. Chem. 72 (2000) 221. [14] S.L. Pan, D.D. Zeng, H.L. Zhang, H.L. Li, Appl. Phys. A 70 (2000) 637. [15] J.C. Maxwell-Garnett, Philos. Trans. Roy. Soc. Lond. 205 (1906) 247. [16] S. Shingubara, O. Okino, Y. Sayama, H. Sakaue, T. Takahagi, Jpn. J. Appl. Phys. 36 (1997) 7791. [17] A.W. Brace, P. Sheasby, Technology of Anodising Aluminium, 2nd Ed., Technicopy Ltd., 1979. [18] J.P. O’Sullivan, G.C. Wood, Proc. Roy. Soc. Lond. A 317 (1969) 511. [19] J.B. Sidgwick, Amateur Astronomer’s Handbook, Faber and Faber Ltd., 1980. [20] Y. Zhao, M. Chen, Z. Yanan, T. Xu, W. Liu, Mater. Lett. 59 (2005) 40. [21] B.G. McMillan, L.E.A. Berlouis, F.R. Cruickshank, D. Pugh, P.F. Brevet, Appl. Phys. Lett. 86 (2005) 211912. [22] B.G. McMillan, L.E.A. Berlouis, F.R. Cruickshank, P.F. Brevet, J. Electroanal. Chem. 599 (2007) 177. [23] D.G.W. Goad, M. Moskovits, J. Appl. Phys. 49 (1978) 2929.

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