7906
J. Phys. Chem. C 2007, 111, 7906-7909
Optimum Length of Silver Nanorods for Fabrication of Hot Spots Geun Hoi Gu, Jurae Kim, Lily Kim, and Jung Sang Suh* Nano-Materials Laboratory, Department of Chemistry, Seoul National UniVersity, Shinlim-dong, San 56-1, Seoul 151-747, Republic of Korea ReceiVed: January 16, 2007; In Final Form: March 17, 2007
The optimum length of silver nanorods for surface-enhanced Raman scattering (SERS) was studied. Uniform silver nanorods in diameter and length were fabricated on porous anodic aluminum oxide templates prepared in sulfuric acid, and their lengths were varied by controlling the electrodeposition time. The diameter of the nanorods was approximately 37 nm, and the distance between the nanorods was very uniform at 65 nm. The nanorods were highly ordered with a hexagonal pattern and perfectly vertical with respect to the plane of the templates. A good SERS spectrum was observed for benzenethiol adsorbed at the tip of the isolated silver nanorods. The SERS intensity sensitively varied with the length of the nanorods and attained the maximum when the length was approximately 62 nm for excitation with a 632.8-nm laser line. The enhancement factor calculated by comparing the SERS intensity with the normal Raman intensity and by excluding the resonance effect of molecules was 3.9 × 107, while that calculated from the Stokes and anti-Stokes Raman data was ∼1012. Our result for the optimum length will be very useful for the fabrication of hot spots for SERS.
Introduction Recently, surface-enhanced Raman scattering (SERS) has attracted considerable attention as a probing technique for biosensors because of its high sensitivity.1-8 SERS is so sensitive that it can detect even single molecules.9-15 For the detection of single molecules, a very high surface enhancement may be required (up to 1014-1015). However, the mechanism of the huge enhancement is not clear. It is known that a very strong enhancement is obtained from particular sites, the so-called hot spots, that may be junctions between nanoparticles.16-19 To systematically study the enhancement mechanism or the hot spots, it is essential to develop a method to systematically control the physical dimensions such as the diameter, length, and aggregation of nanoparticles. Silver nanowire arrays fabricated on porous anodic aluminum oxide (AAO) templates have been used as substrates for SERS,20-25 and it has recently been suggested that they are promising nanostructures for fabricating hot spots for SERS.25 The SERS intensity of molecules adsorbed on very long nanowires (at least 5 µm) has been observed to be very weak when the nanowires are isolated; however, it is greatly increased when the nanowires are bundled together by removing the oxide layer through etching.25 The bundling of nanowires may have an effect similar to the aggregation of colloid particles. Isolated silver colloid particles have an absorption band near 390 nm due to the surface plasmon absorption.26 When silver colloid particles are aggregated, a new broad band appears in the visible region while the band near 390 nm decreases in intensity.26 The new band shifts toward red with increasing aggregation. The optical property of silver nanowires depends on their diameter and length.27 When the nanowires are relatively short, two absorption bands appear due to the transverse and longitudinal modes of the dipolar plasmon. The transverse mode is parallel to the short axis of the wires and its absorption band * To whom correspondence should be addressed. E-mail: jssuh@ snu.ac.kr.
appears in the short wavelength region; on the other hand, the longitudinal mode is parallel to the long axis and its plasmon wavelength is shifted toward red with increasing length of the nanowires. When the length becomes large, the absorption band of the longitudinal mode of the dipolar plasmon is shifted close to the infrared region and the absorption bands due to multipolar plasmons appear in the visible region.28 Therefore, the longitudinal mode of the dipolar plasmon of long nanowires is hardly exited by irradiation with a visible laser line. To fabricate hot spots for SERS using silver nanowires, we should first find the optimum length of the silver nanowires corresponding to the maximum enhancement for the laser line being used. In this study, we have fabricated relatively short silver nanorods that are well ordered and uniform in diameter and length on AAO templates, and we have determined the optimum length of silver nanorods for SERS by observing the SERS spectra for different lengths. Experimental Section A highly ordered porous AAO template was fabricated by using the two-step anodization technique.29-31 Clean aluminum sheets (99.999%, 0.5 mm thickness, Goodfellow Ltd.) were anodized in 0.3 M sulfuric acid at 10 °C and at a constant applied voltage of 25 V.30 The second anodization was performed for 3 min under the same condition as the first one. The thickness of the prepared film was approximately 500 nm. Pore widening was carried out in 0.1 M phosphoric acid at 30 °C to reduce the thickness of the bottom barrier layer. The silver nanorods were deposited in the pores of the films by applying an alternating current voltage of 14 V with a frequency of 200 Hz in ethanol solution containing 0.05 M AgNO3 (99.9999%, Aldrich) at 17 °C. A number of samples with silver nanorods of different lengths were prepared by varying the deposition time. The silver nanorods deposited on the films were immersed in ethanol solution containing 1.0 × 10-3 M benzenethiol for 20 h, washed with ethanol, and then dried for Raman measurements. Raman spectra were observed by using
10.1021/jp070384g CCC: $37.00 © 2007 American Chemical Society Published on Web 05/11/2007
Optimum Length of Silver Nanorods
Figure 1. SEM images of the silver nanorods deposited in the widened pores of AAO templates prepared in sulfuric acid solution: (a) the top view and (b-d) the cross-sectional views. The diameter of the nanorods is approximately 37 nm, and the average lengths of the nanorods are (b) 62 nm, (c) 117 nm, and (d) 164 nm.
J. Phys. Chem. C, Vol. 111, No. 22, 2007 7907
Figure 2. SERS spectra of benzenethiol adsorbed at the tip of the silver nanorods (having a diameter of 37 nm) deposited on AAO templates for excitation with a 632.8-nm laser line. The Raman spectrum of Si is shown at the bottom. The length of the silver nanorods is marked on the right-hand side. The relative intensity was calculated by dividing the SERS intensity with the intensity of the silicon peak measured under the same conditions.
a micro-Raman system. The laser power incident on the sample was 2 mW, and the acquisition time was 10 s. The fabricated templates and nanorods were analyzed by using scanning electron microscopy (SEM; Philips FEG XL (30 kV)). Results and Discussion The SEM images of the top and side views of the silver nanorods deposited in the widened pores of the AAO templates prepared in sulfuric acid are shown in Figure 1. In Figure 1a, which is the top view, silver nanorods are deposited in all the pores. The diameter of the nanorods is very uniform and is approximately 37 nm; this diameter corresponds to that of the pores. In the top view, the nanorods exhibit a perfect twodimensional array with a hexagonal pattern. The distance between the nanorods is very uniform at 65 nm. The nanorod density is approximately 2.7 × 1010 nanorods/cm2. The lower part of the nanorods has a hemispheric shape. The shape of the upper part is not as uniform as that of the lower part. In the side views, some nanorods are not visible. They might have slipped out from the pores during sampling since silver particles are seen in all the pores in the top view. The alignment of the nanorods is perfectly vertical with respect to the surface of the template. The lengths of the nanorods (parts b-d of Figure 1) that are deposited for 120, 180, and 240 s are approximately 62 ( 5, 117 ( 6, and 164 ( 7.5 nm, respectively. Their lengths are fairly uniform. The SERS spectra of benzenethiol adsorbed at the tip of the silver nanorods deposited in the pores of AAO templates with a pore diameter was 37 nm for various times are shown in Figure 2. They were observed by excitation with a 632.8-nm laser line. Although the average length of the nanorods was different for each sample, the number of molecules adsorbed on each nanorod or in a unit area was almost identical since the molecules were adsorbed only at the tip of the nanorods deposited in the pores and not on the sides of the nanorods. The Raman peaks correspond to the modes of benzene ring.32 For example, the peak at 1573 cm-1 is due to ν8a, and that at 1000 cm-1 is due to ν12. The peak at 417 cm-1 is due to ν7a and contributions from the C-S stretching vibration (νCS). The intensity profiles of several peaks of Figure 2 as a function of the average length (in nanometers) of silver nanorods are shown in Figure 3. The intensity was normalized by dividing it with the intensity of the Si peak observed under the same conditions. The intensity sensitively varied with increasing length of the
Figure 3. Intensity profiles of several peaks of the SERS spectra shown in Figure 2 as a function of the length of the silver nanorods.
silver nanorods. In Figure 3, the maximum intensity is observed for the silver nanorods whose average length is approximately 62 nm. It is concluded that the optimum length for SERS is approximately 62 nm for excitation with a 632.8-nm laser line. In our preliminary tests, the optimum length seemed to be shorter than 62 nm for excitation with a 514.5- or 488-nm laser line. We calculated the enhancement factor by comparing the intensity of the 1573-cm-1 peak in the SERS spectrum with that in the normal Raman spectrum by using a technique similar to that reported previously.33 The silver nanorods with a diameter and length of approximately 37 and 62 nm, respectively, were used for observation of the SERS spectrum. By assumption that molecules with an individual cross-sectional area of 0.147 nm2 were adsorbed as a monolayer at the tip of each nanorod with a diameter of ∼37 nm, approximately 6.92 × 106 molecules were sampled by the laser beam with a beam diameter of 2 µm. In fact, 32.4% of the surface area was occupied by the hexagonal array of nanorods in which the nanorod diameter was 37 nm and the distance between the nanorods was 65 nm. The normal Raman spectrum was observed for a 100-µm-thick cell filled with pure benzenethiol liquid that had a density of 1.08 g/cm3. The molecular mass of benzenethiol is 110.18 g/mol. The probe volume was approximated as a cylinder with a diameter of 2.0 µm and a height of 100 µm. Under these conditions, 1.85 × 1012 molecules were sampled. The normal Raman spectrum and the SERS spectrum of benzenethiol that were obtained by using a low-magnification objective lens (10×) and in an acquisition time of 100 s are shown in Figure 4. The
7908 J. Phys. Chem. C, Vol. 111, No. 22, 2007
Figure 4. Comparison of the normal Raman spectrum of pure benzenethiol liquid with the SERS spectrum of benzenethiol adsorbed at the tip of the silver nanorods; both the spectra were observed by excitation with a 632.8-nm laser line under the same conditions. The normal Raman spectrum is magnified 50 times to normalize the intensity of the peak at 1575 cm-1.
intensity of the SERS peak is very similar to that of the normal Raman peak magnified 50 times. From this data of the relative intensity and the number of molecules sampled from the normal Raman and SERS measurements, the enhancement factor is calculated to be approximately 1.3 × 107. This value is somewhat smaller than the values reported in the literature.34,35 However, we wish to emphasize that the measured enhancement factor contains no contribution from the resonance effect of molecules since benzenethiol has no resonance effect when it is excited with a 632.8-nm laser line. Further, the contribution from the excitation of the transverse mode of the surface plasmon was intentionally excluded; for our measurement geometry, the longitudinal mode could be inefficiently excited. These features are discussed below. The SERS spectra are measured using a micro-Raman system and in the normal way by laser beam irradiation perpendicular to the surface of the sample template placed on the sample stage. In this case, the polarization direction of the laser beam is parallel to the short axis of the nanorods, while it is perpendicular to the long axis. Therefore, in this geometry, the transverse mode of the surface plasmon can be excited by irradiation but the longitudinal mode cannot be excited. However, for our silver nanorods, the contribution of the transverse mode may be very small or negligible because the diameter of a nanorod is approximately 37 nm, which is considerably smaller than the optimum length of approximately 62 nm. Further, the nanorods are well separated from each other, and the gap between them is approximately 28 nm. Therefore, there is almost no enhancement from a lateral interaction like that in bundling. For the micro-Raman system, a lens having a very short focal length is used to focus the laser beam and to simultaneously detect the Raman signals. In this case, the part of the laser beam at the edge will be refracted with a wide angle. The part of the refracted laser beam will irradiate the nanorods at some angle. Therefore, the longitudinal mode of the surface plasmon of the nanorods can be excited since the refracted laser beam has a polarization component parallel to the long axis of the nanorods. By this argument, the intensity of the refracted laser beam will increase with the magnification of the lens used. Therefore, when the magnification of the lens is increased, the increase in the SERS intensity will be considerably more than that expected from an increase in the detection angle. To verify this, we measured the Raman spectra of Si and the SERS spectra of benzenethiol adsorbed at the tip of the nanorods with an
Gu et al.
Figure 5. (a) Anti-Stokes and (b) Stokes SERS and (c) anti-Stokes and (d) Stokes normal Raman spectra of benzenethiol. Normal Raman spectra were observed for pure benzenethiol liquid, and SERS spectra were observed for benzenethiol adsorbed at the tip of the silver nanorods. The base lines were subtracted for easy comparison. The signal accumulation time for the normal Raman spectra was 10 times longer than that for SERS spectra.
average length of approximately 62 nm by using three lenses whose magnifications were 10, 50, and 100. The SERS intensity was calibrated by dividing it with the intensity of the Si peak measured under the same conditions. The calibrated SERS intensities were 3.5, 10.5, and 10.5 for the three lenses with magnifications of 10, 50, and 100, respectively. For the lenses with magnifications of 50 and 100, the relative intensity was the same. This may mean that the laser beams focused by these lenses have similar magnitudes of the polarization component parallel to the long axis of the nanorods. In any case, the relative intensity measured by the 50× or 100× lens is approximately three times greater than that measured using the 10× lens. In a previous study, we calculated the enhancement factor based on the observed Raman data by using a lens with a magnification of 10. The calculated enhancement factor was approximately 1.3 × 107. Therefore, the calculated enhancement factor based on the measured Raman data for a higher magnification lens is expected to be approximately 3.9 × 107. It should be mentioned that, in the measurement of the enhancement factor, we have used the 10× lens because with a high magnification lens, the laser beam does not assume a cylindrical form in the liquid sample. In such a case, it is difficult to estimate the number of molecules irradiated by the laser beam. With the geometry of the AAO sample plate placed vertically on the sample stage and with orientation of the nanorods in the direction of polarization of the laser beam, the Raman signal will be enhanced purely by the excitation of the longitudinal mode of the surface plasmon. Unfortunately, with these conditions, we could not quantitatively and reliably measure the Raman spectrum because the silver nanorods were too short (∼60 nm) to place reproducibly at the center of the laser beam. Finally, we have calculated the enhancement factor from the Stokes and anti-Stokes data by using the reported method.36 We have measured both the normal Stokes Raman spectrum of liquid benzenethiol and Stokes SERS of benzenethiol adsorbed on silver nanorods under the same conditions; we also measured their anti-Stokes spectra. These spectra are shown in Figure 5. The anti-Stokes to Stokes SERS signal ratio for the 1573-cm-1 band, PaSERS/PsSERS, is normalized by the anti-Stokes to Stokes signal ratio for the normal Raman spectrum measured under the same conditions and at the same temperature: K(νm) ) (PaSERS(νm)/PsSERS(νm))/(PaRS(νm)/PsRS(νm)). The calculated value of K(νm) from four measured intensities of the 1573-cm-1 band
Optimum Length of Silver Nanorods was in the range of 1.1-1.3. In the steady state and for weak saturation conditions, K(νm) is given as
K(υm) ) σSERS(υm)τ1(υm)ehυm/kTnL + 1 where σSERS is the effective SERS cross section, τ1 the lifetime of the first excited vibrational state, and nL the photon flux density of the excitation laser beam. For the peak at 1573 cm-1, the exponential term is approximately 2 × 103 at 300 K. Since the laser intensity used in the measurements was approximately 105 W/cm2, nL is approximately 3 × 1023 photons s-1 cm-2. Therefore, for K(νm) ) 1.2, the product σSERS(νm)τ1(νm) is on the order of 10-28 cm2 s. Since that σRS(νm)τ1(νm) is on the order of 10-40 cm2 s for normal Raman scattering,35 the enhancement factor is on the order of 1012. This value is considerably larger than the value of 4.9 × 107 that is calculated from the measured intensities of normal Raman spectra and SERS. In the latter case, it is assumed that the molecules are adsorbed as a monolayer at the tip of the nanorods. By a simple comparison, only 0.005% of the molecules in the sample contribute to the observed SERS signal. It should be mentioned that the high enhancement factor of the order of 1014-1015 has been observed only for molecules like Rhodamine 6G that have a resonance effect.36 When excited with a 632.8-nm laser line, benzenethiol does not have any resonance effect. Conclusion In summary, we have fabricated very short silver nanorod arrays, by alternating current electrodeposition, on AAO templates that were prepared by anodization in sulfuric acid and then their pores were widened. The length of the silver nanorods was varied by controlling the deposition time. The resultant silver nanorods were very homogeneous with regard to their diameter and were well ordered and almost perfectly aligned vertically with respect to the template. A good SERS spectrum was observed for the molecules adsorbed at the tip of the isolated silver nanorods. The enhancement factor was approximately 3.9 × 107. However, the enhancement factor calculated from the Stokes and anti-Stokes Raman data was ∼1012. The SERS intensity sensitively varied with the length of the nanorods, and it attained the maximum when the length was approximately 62 nm for excitation with a 632.8-nm laser line. From our results, it is concluded that the optimum length of silver nanorods consisting of hot spots is approximately 62 nm for excitation with a 632.8-nm laser line. This result will be very useful for the fabrication of hot spots for SERS. Acknowledgment. The authors are grateful for the financial support of KOSEF through CNNC, of the National R&D Project for Nano Science and Technology, and of the BK21 program. References and Notes (1) Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536.
J. Phys. Chem. C, Vol. 111, No. 22, 2007 7909 (2) Shafer-Peltier, K. E.; Haynes, C. L.; Glucksberg, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 2003, 125, 588. (3) Grubisha, D. S.; Lipert, R. J.; Park, H. Y.; Driskell, J. D.; Porter, M. Anal. Chem. 2003, 75, 5936. (4) Nam, J. M.; Stoeva, S. I.; Mirkin, C. A. J. Am. Chem. Soc. 2004, 126, 5932. (5) Lyandres, O.; Shah, N. C.; Yonzon, C. R.; Walsh, J. T., Jr.; Gluckberg, M. R.; Van Duyne, R. P. Anal. Chem. 2005, 77, 6134. (6) Kneipp, J.; Kneipp, H.; Rice, W. L.; Kneipp, K. Anal. Chem. 2005, 77, 2381. (7) Pavan, Kumar, G. V.; Ashok Reddy, B. A.; Arif, M.; Kundu, T. K.; Narayana, C. J. Phys. Chem. B 2006, 110, 16787. (8) Premasiri, W. R.; Moir, D. T.; Klempner, M. S.; Krieger, N.; Jones, G., II; Ziegler, L. D. J. Phys. Chem. B 2005, 109, 312. (9) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (10) Kneipp, K.; Wang, Y.; Kneipp, H.; Perlman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (11) Kneipp, K.; Kneipp, H.; deinum, G.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Appl. Spectrosc. 1998, 52, 175. (12) Kneipp, K.; Kneipp, H.; Kartha, V. B.; Manoharan, R.; Deinum, G.; Itzhan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. E 1998, 57, R6281. (13) Svedberg, F.; Li, Z.; Xu, H.; K1all, H. Nano Lett. 2006, 6, 2639. (14) Habuchi, S.; Cotlet, M.; Gronheid, R.; Drix, G.; Michiels, J.; Vanderleyden, J.; De Schryver, F. C.; Hofkens, J. J. Am. Chem. Soc. 2003, 125, 8446. (15) Le Ru, E. C.; Meyer, M.; Etchegoin, P. G. J. Phys. Chem. B 2006, 110, 1944. (16) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569. (17) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173. (18) Xu, H.; Aizpurua, J.; Kall, M.; Apell, P. Phys. ReV. E 2000, 62, 4318. (19) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261. (20) Joo, Y.; Suh, J. S. Bull. Kor. Chem. Soc. 1995, 16, 808. (21) Suh, J. S.; Lee, J. S. Chem. Phys. Lett. 1997, 281, 384. (22) Jeong, D. H.; Zhang, Y. X.; Moskovits, M. J. Phys. Chem. B 2004, 108, 12724. (23) Sauer, G.; Brehm, G.; Schneider, S.; Graener, H.; Seifert, G.; Nielsch, K.; Choi, J.; Gosele, U.; miclea, P.; Wehrspohn, R. B. J. Appl. Phys. 2005, 97, 024308. (24) Wang, H. H.; Liu, C. Y.; Wu, S. B.; Peng, C. Y.; Chan, T. H.; Hsu, C. F.; Wang, J. K.; Wang, Y. L. AdV. Mater. 2006, 18, 491. (25) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200. (26) Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Jana, S.; Pande, S.; Vo-Dinh, T.; Jiang, H.; Pal, T. J. Phys. Chem. B 2006, 110, 13436. (27) (a) Wang, D. S.; Chew, H.; Kerker, M. Appl. Opt. 1980, 19, 2256. (b) Kuwata, H.; Tamaru, H.; Esumi, K.; Miyano, K. Appl. Phys. Lett. 2003, 83, 4625. (c) Suzuki, M.; Maekita, W.; Wada, Y.; Nakajima, K.; Kimura, K.; Fukuoka, T.; Mori, Y. Appl. Phys. Lett. 2006, 88, 203121. (28) Laurent, G.; Felidj, N.; Aubard, J.; Levi, G.; Krenn, J. R.; Henau, A.; Schider, G.; Leitner, A.; Aussenergg, F. R. J. Chem. Phys. 2005, 122, 011102. (29) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (30) Masuda, H.; Hasegwa, F.; Ono, S. Electrochem. Soc. 1997, 144, L127. (31) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys. 1998, 37, L1340. (32) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570. (33) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Duyne, R. P. V. J. Phys. Chem. B 2005, 109, 11279. (34) Williamson, T. L.; Guo, X.; Zukoski, A.; Sood, A.; Da´z, D. J.; Bohn, P. W. J. Phys. Chem. B 2005, 109, 20186. (35) Su, K. H.; Durant, S.; Steele, J. M.; Xiong, Y.; Sun, C.; Zhang, X. J. Phys. Chem. B 2006, 110, 3964. (36) Kneipp, K.; Wang, Y.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1996, 76, 2444.
