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C70) nano-clusters deposited on iron surface
Vibrational Spectroscopy 39 (2005) 151–156 www.elsevier.com/locate/vibspec
SERS of gold/C60 (/C70) nano-clusters deposited on iron surface Luo Zhixun, Fang Yan * Beijing Key Lab for Nano-photonics and Nano-structure, Department of Physics, Capital Normal University, Beijing 100037, PR China Received 16 December 2004; received in revised form 30 January 2005; accepted 31 January 2005 Available online 5 March 2005
Abstract Surface enhanced Raman scattering (SERS) spectra of very good quality of gold/C60 (/C70) nano-clusters deposited on iron surface were reported by using the pyridine as a intermediate to connect and nest the C60/C70 molecule to the gap of gold nano-particles and iron substrate. The number of vibrational modes was greatly increased, especially some modes that were forbidden in Raman spectrum, appeared and even split as prediction of group theory. The enhancement factor is 106. It shows from the experiment that the ternary systems of ‘‘iron–gold/ fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and structural perturbation induced by the substrate upon the fullerene cage with the high sensitivity, in particular, the adsorption behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nano-particles and the metal surface. # 2005 Elsevier B.V. All rights reserved. Keywords: SERS; C60/C70; Gold nano-particles; Iron
1. Introduction As some unique characters, physical and chemical, of materials are usually thrown out by the material surface, especially the photoelectric and spectral characters, the study of new materials often begins with the surface characters. In fact, the discovery of superlattice structured, laminated structured, periodic, quasi-periodic structured materials and all kinds of nano-materials have recently provided abundant subject matter for surface investigation. The discovery of the high-Tc superconductivity in alkalidoped fullerene has also generated considerable interest in technological and scientific applications of fullerene-based materials, especially in the form of thin films [1–6]. Because properties of fullerene thin films depend intimately on interface configuration and adsorption patterns, the study of the adsorption behavior and the charge transfer interaction of fullerene with the metal surface is significant for understanding the film growth mechanisms and the * Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68982332. E-mail address: [email protected] (F. Yan). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.01.008
determination of the optical and electronic properties of the fullerene thin films [7–12]. Raman spectral analysis technique, using photons as probe, takes good advantages of no damages to samples and in-situ detection. Since it has applied to study the molecular structure, rapid progress has improved in material identification, organic and inorganic chemistry, biochemistry and macromolecules, catalyze, petrochemical industry, environmental sciences and so on [13]. Based on this, surface enhanced Raman scattering (SERS) results in highsensitivity and high-resolution spectra without damage to samples, and reveals rich information concerning with the interactions between the adsorbates and substrates. It is very useful to apply SERS to in situ investigation of fullerene film, by depositing fullerene systems on the electrode or metal surface. With developed method, the SERS active systems of gold/C60 (/C70) nano-clusters deposited on iron surface were prepared. It is found from the experiment that the ternary systems of ‘‘iron–gold/fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and adsorption
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L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nano-particles and the metal surface.
2. Experimental Preparation of C60/C70 was dissolved in pyridine. Colloidal gold with SERS active was prepared through a redox process based on Frens’s method [14], except that KAuCl4 substituted HAuCl4. Colloidal gold and C60/C70 solution were added by drop onto the surface of rough iron, copper, aluminum or silver. After the dryness, a series of metal piece coated with different layers of gold/C60 (/C70) nano-clusters were obtained, with the pyridine volatilization. Raman spectra excited in the near-infrared region were acquired with a FT-Raman spectrometer. A Brucker Model IFS-66 FT-Raman spectrophotometer with a line resolution of 1 cm1 and a YAG laser operated at 1064 nm was used as the excitation source at 150 mW. The concentration of the solutions of C60/C70 is 0.02 M for all.
3. Results and discussion Fig. 1a shows the near-infrared Raman spectrum of C60 solid. The high degeneracy degree in C60 molecule makes the spectra appear rather simple, in spite of the large number of atoms per molecule. The expected 10 Raman active vibrational modes obviously appear in the spectrum. Furthermore, another peak, which is usually assigned to the infrared active, appears at 568 cm1, which may
Fig. 1. (a) Raman spectrum of solid C60; (b) SERS of C60/gold clusters dropped on filter paper; (c) SERS of C60/gold clusters deposited on iron surface.
originate from the symmetry reduction by the intermolecular interaction in C60 solid. Different from the Raman spectrum of C60 solid, SERS of C60 in all systems gives much more information of vibrational for C60 molecular. Fig. 1b shows the SERS of C60/gold clusters dropped on filter paper, and Fig. 1c displays the SERS spectrum of gold/C60 nano-clusters deposited on iron surface, pyridine as the solvent media. It is remarkable in Fig. 1c that, as we can see, not only the number of vibrational modes was greatly increased, but also the significant Raman bands split as well as frequencies up and down shift. This was ascribed to the symmetry lowering and selection rule relaxation of C60 induced by the gold surface. It is extremely remarkable of the three stair highest peaks, as appearing at 270, 491 and 1462 cm1, whose SERS signal intensity was estimated to be enhanced more than 106. It is notable in the presented SERS spectra, as shown in Fig. 1b and c, that the pyridine bands are not observed even though pyridine is a very effective Raman scatterer, while it is deliberate to choose the pyridine as solvent media because of its hydrophobicity and hydrophilicity based on the structure of pyridine itself with both hydrophilic grouping and hydrophobic grouping, as well as its volatility. Fig. 2 shows the comparison of SERS spectrum of gold/ C60 nano-clusters deposited on silver (a) with that on aluminum (c), copper (b) and iron (d) surface. As one can see, they show great difference with each other, even for the three main bands. For example, the 272 cm1 mode shows three split in Fig. 2d, while the three split modes turn out to be not so obvious in Fig. 2c, and even tend to a wave packet in Fig. 2a.
Fig. 2. SERS of C60/gold clusters deposited on silver (a), aluminum (b), copper (c) and iron (d).
L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
153
frequency Hg modes, such as 1421 and 1575 cm1. But there is an exception, such as 1099 and 1250 cm1 mode split into two, respectively. There are some non-Raman active modes appearing, too. All four F1u infrared active modes appear at 513, 576, 1194 and 1421 cm1, respectively. These features can be attributed to normally forbidden modes which also become Raman-active by virtue of symmetry lowering. Among them, 576 cm1 splits into two peaks at 550 and 576 cm1, and 535 cm1 splits into two peaks at 506 and 513 cm1. A vibrational mode possibly assigned to be hyper-Raman active based on the group theory calculation, appeared, and split into two peaks at 341 and 352 cm1. This mode was measured with neutron inelastic scattering and highresolution electro-energy-loss spectroscopy method, and calculated, and attributed to Hu. The additional and split modes resulted from the symmetry lowering and selection rule relaxation by the adsorption of C60 on gold surface. Based on the group theory calculation, there are two kinds of adsorption possibility;
The bands of Raman and SERS of C60/gold nano-clusters deposited on iron are summarized in Table 1. As we can see, SERS of C60 gives much more molecular information, and it is obvious that there are many additional and split of vibrational modes. SERS spectra of C60 in our experiment are in good agreement with the conclusion of group theory this. As the normal coordinate of low-frequency modes is mostly radial along molecule, the group theory calculation concludes that there are three splits for Hg mode. In Table 1, it can be found that those Hg modes among 200–700 cm1 nearly all split as expected. For example, 273 cm1 band splits into 254, 270 and 298 cm1; while 433 cm1 band splits into 399 and 422 cm1; but it is not obvious of 703 cm1 bands split to three, two modes instead. Even it cannot be found split at 772 cm1. In the high frequency region beyond 800 cm1, the high frequency modes were ascribed to the interaction of tangential modes with the gold surface. This interaction is weaker than that in the low frequency region. As the result, there is almost no splitting observed in the four high
Table 1 SERS bands and Raman bands of C60 (cm1) Experiments
Group theory
Raman of solid C60
SERS of C60/gold clusters deposited on iron surface
Assignment
Ih
273 (s)
254 270 298 341
Hg (1)
8Hg (1)
3
T2u (1)
T2u (1)
2
433 (m)
399 (w) 422 (s)
Hg (2)
8Hg (2)
3
497 (s)
491 (s) 513 (m) 525 (m)
Ag (1) T1u (1)
2Ag (1) 4T1u (1)
1 2
568 (m)
550 (m) 576 (m)
T1u (2)
4T1u (2)
2
709 (w)
659 (w) 703 (m) 728 (w)
Hg (3)
Hg (3)
3
772 (m)
766 (s)
Hg (4) Go F1g (1) T2u (1)
Hg (4) Go F1g (1) T2u (1)
3 2 2 2
Hg (5) Gg T1u (3)
Hg (5) Gg 4T1u (3)
3 2 3
Hg (6) Hg (6) F1g
8Hg (6)
3
F1g
2
1421 (m) 1462 (vs.)
Hg (7), T1u (4) Ag (2)
8Hg (7) 4T1u (4) Ag (2)
3 1
1565 (s)
Hg (8)
8Hg (8)
3
(m) (vs.) (s) (m)
961 (w) 1025 (m) 1100 (m)
1099 (m) 1194 (w)
1249 (m)
1422 1467 1512 1574
(m) (s) (w) (m)
1246 1268 1311 1347
(m) (w) (w) (w)
Note: vs.: very strong, s: strong, m: medium, w: weak.
Number of splits C5v
C3v
3
3
3
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which is the adsorption on gold surface via pentagons (C5v) or hexagons (C3v). As calculation from molecular orbit has made known, the lowest un-occupied molecular orbit (LUMO) is three-fold degeneracy T1u, and highest occupied molecular orbit (HOMO) is five-fold degeneracy Hu. Group theory allows the 46 vibrations for the C60 molecular, given as follows: Au + 2Ag + 3F1g + 4F1u + 4F2g + 5F2u + 6Gg + 6Gu + 7Hu + 8Hg. For Ih symmetry, only the two Ag and eight Hg vibrations are Raman active, the four F1u modes are infrared active, and the 22 hyper-Raman are active (4F1u + 5F2u + 6Gu + 7Hu). Pentagonal adsorption would result in retro gradation of C60 molecule group from high symmetric Ih group to C5v group and hexagonal adsorption from Ih to C3v. Because of this, Hg, T1u and some other modes of Ih group would split into three or two modes of C5v and C3v group with lower degeneracy. For pentagon adsorption behavior, the Ih symmetry is reduced to C5V with 53 Raman active modes (19A1 + 34E1). While, for hexagons adsorption, the Ih symmetry is reduced to C3v with 89 Raman active modes (31A1 + 58E). Among them, A1 modes were the highest in intensity, indicating that C60 is probably adsorbed on the surface via pentagons of C60. Furthermore, in the lower frequency region, the splitting extent of modes predominantly corresponding to the radial motion of pentagons vibrational is stronger than that of hexagons vibrational. This also implied that C60 molecular tends to pentagon adsorption. Even from Table 1, we can find the Gu mode and Gg mode are splitting into two modes, referring to the group theory that there is difference of two or three splits between the C5V and C3V, which confirmed that the adsorption of C60 molecular are oriented on pentagons of C60 on the gold crystal surface. In fact, not only the split and additional modes in Fig. 2d excited us, it was remarkable that the signal-to-noise in Fig. 2d is the best, too. To evaluate the enhancement efficiency of this system, we quote a simple formula to calculate the enhancement factor. G¼
I SERS =Nsurf IRaman =Nbulk
(1)
Here (ISERS/Nsurf) represents the SERS intensity contributed by C60 adsorbed on substrate, and (IRaman/Nbulk) represents the normal Raman intensity of the species in solution exposed to laser light. Choosing Ag (2) (1462 cm1) as the typical band, it can be calculated that the enhancement factor G of the system is 3 106. In order to exclude the random factors from the experiments, we repeated and change the adsorbed probe molecule to C70. Fig. 3 shows SERS of C70/gold colloid deposited on iron (d), copper (c), aluminum (b) and silver (a) surface. It is interesting that Fig. 3d presents the best scene as expected. Except the similar shape of Raman bands, the split and additional modes are most obvious. So does the signal-to-noise. Not only for C60, but also for C70 systems, it is surprising to find the SERS spectra of that on silver is not so good as
Fig. 3. SERS of C70/gold clusters deposited on silver (a), aluminum (b), copper (c) and iron (d).
that on iron surface. It is notable that, especially in Fig. 2a, there are few more modes besides the three main bands of C60, and there is even a wave packet at 240 cm1 which almost covers the Raman bands of C60 at 270 cm1, only a wing left there. So what if these happen? Therefore, we infer that the different metal substrate plays different role in the induction for C60/C70 to nest in the gap of gold nanoparticles and the rough substrate. In fact, it is well shown and demonstrated in Figs. 2 and 3 that C60/C70 SERS is not only in dependent on the gold colloid, but also influenced by the rough substrate. To go deep into analysis of the mechanism of SERS of C60/C70 molecules on the systems, we propose an idealized model, in which C60/C70 molecules are nested in a coupled field made up of gold nano-particles and metal substrate. Fig. 4 presents a sketch map of the SERS active sandwich system [15–18] to visualize the behavior of the interaction of them. The base layer is metal substrate and the upper spheroidal particles are gold clusters, between them nested with spheroidal C60/C70 molecules. In brief, we divide into two parts for consideration. For the metal substrate, we introduce the old model, image field model, which attributes the enormous enhancement to the large polarizability that one calculates for certain choices of parameters when the Raman emitting system is taken to be a composite of the molecule and its conjugate-charge image in the metal. For simplicity, according to King, Van Duyne and Schatz, we assume a diagonal Raman polarizability tensor and a field polarized along the normal of the flat surface (the z-axis). The problem, therefore, becomes onedimensional.
L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
155
Fig. 4. Sketch map of the SERS active system.
The dipole moment induced in the molecule by the incident field and its image field is [19] m ¼ að~ E þ~ Eim Þ (2) ~ ~ where E and Eim are the incident and image fields, and are components of the molecular polarizability. ~ Eim is given by Eim = [(e0 e)/(e0 + e)]m/(4r3), where r is the distance between the (point)dipole and surface. Substituting and rearranging, one obtains 1 ða=4r 3 Þðe e0 Þ m¼a 1 E: (3) ðe þ e0 Þ If we consider aeff ¼ a
1 ½1 ða=4r 3 Þðe e
0 Þ=ðe þ e0 Þ
;
(4)
then it is m ¼ aeff E This expression has a pole at the frequency at which ðe e0 Þ=ðe þ e0 Þ Re a ¼1 4r 3
(5)
(6)
Clearly this is related to surface-plasmon excitation in the metal surface at the frequency on which the condition Re(e) = e0 obtains. For spheroidal gold particles, by assuming the dimensions of the surface boss to be small compared to the wavelength of light, the field about the gold nano-particles become solutions of Laplace equation, 5E = 0, rather than of the more complicated Helmholtz equation, 5E + k2E = 0. The particles are taken to be ellipsoid of revolution with semimajor and semiminor axes equal to a and b, respectively, and the adsorbed molecule was placed on the axis of cylindrical symmetry a distance H above the bump. Using spheroidal coordinates j and h and the geometrical parameter f = (a2 b2)1/2, we introduced a Raman enhancement given by 1 þ ð1 eÞj0 Q01 ðj1 Þ=½eQ1 ðj0 Þ j0 Q01 ðj0 Þ 4 (7) G¼ 1G in which j0 = a/ f, j1 = (a + H)/ f, e is a complex dielectriccomplex of the metal. Q1 is Legendre function of the second kind and G is a complex-valued quantity that depends acutely on H. In the limit of e ! 1, G = 2a/[2(a + H)]3, the term 1 G arises in the so-called image enhancement factor.
In fact, the effect of molecular orientation upon SERS should be considered. And adsorption upon a spherical metal particle and molecularly induced dipole to take an arbitrary orientation with respect to the normal to the surface should be assumed. Whether or not, the SERS mechanisms of the ternary systems need to consider both sides discussed above. It is the coupling of them that makes the good result of the experiments. Of course, it may be thought of a stronger coupling field round the C60/C70 molecules by ‘iron–gold’. It is notable that the charge transfer factors may not be neglect since SERS of other systems shows worse signal-tonoise. Actually, the frequency of SERS shift towards red in Fig. 1 may be the proof of adatom-charge-transfer. Because of the difference of Fermi energy level of metal and the LUMO of C60/C70, the charge-transfer usually occurs from metal to C60/C70, but it may come into being hybrid orbital of metal [20–22]. All these factors result in the strong interaction of C60/C70 with metals, symmetry lowering and surface selection rules, so the additional and splitting modes bands are observed. In addition, the unique characters of magnetic iron maybe an influence to SERS effect of C60/C70.
4. Conclusion A series of SERS spectrum of very good quality of gold/C60 (/C70) nano-clusters deposited on iron surface was obtained by using the pyridine as a intermediate to connect and nest the C60/C70 molecule to the gap of gold nano-particles and iron substrate. The number of vibrational modes was greatly increased, especially some modes that were forbidden in Raman spectrum, appeared and even split. Based on group theory, the additional and split modes resulted from the symmetry lowering and selection rule relaxation by the adsorption of C60 on gold surface. It shows from the experiment that the ternary systems of ‘‘iron–gold/fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and structural perturbation induced by the substrate upon the fullerene cage with the high sensitivity, in particular, the adsorption behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nanoparticles and the metal surface.
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Acknowledgments The authors are grateful for the support of this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing.
References [1] Y. Fang, Q.-J. Huang, P. Wang, X.-Y. Li, N.-T. Yu, Chem. Phys. Lett. 381 (2003) 255–261. [2] R. Schwedhelm, L. Kipp, A. Dallmeyer, M. Skibowski, Phys. Rev. B 58 (19) (1998) 13176. [3] A.P. Saab, M. Laub, V.I. Srdanov, G.D. Stucky, Adv. Mater. 10 (6) (1998) 462. [4] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi, K. Seki, N. Ueno, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 891. [5] K. Nakamoto, M.A. McKinney, J. Chem. Educ. 77 (6) (2000) 775. [6] A. Al-Mohamad, A.W. Allaf, Synth. Met. 104 (1999) 39. [7] R.A. Ganeev, A.I. Ryasnyansky, M.K. Kodirov, T. Usmanov, Opt. Commun. 185 (2000) 473. [8] Y. Li, Y. Huang, S. Du, R. Liu, Chem. Phys. Lett. 335 (2001) 524–532.
[9] Y.S. Jiang, C.G. Liu, Y.H. Shao, Prog. Phys. 15 (3) (1995) 309. [10] K.D. Gregory, L.M. Korst, P. Cane, L.D. Platt, K. Kahn, P.M. Rafailov, V.G. Hadjiev, H. Jantoljak, C. Thomsen, Solid State Commun. 112 (1999) 517–520. [11] J. Fagerstro¨m, S. Stafstro¨m, Phys. Rev. B 48 (15) (1993) 11367– 11374. [12] J. Visser, E.J.J. Groenen, Chem. Phys. Lett. 356 (2002) 43–48. [13] Proceedings of XVIIIth International Conferecne on Raman Spectroscopy, ICORS XVIII, 2004. [14] G. Frens, Nat. Phys. Sci. 20 (1973) 241. [15] H.-Z. Yu, J. Zhang, H.-L. Zhang, Z.-F. Liu, Langmuir 15 (1999) 16– 19. [16] A. Kudelski, J. Bukowska, M. Dolata, W. Grochala, A. Szummer, Mater. Sci. Eng. A 267 (1999) 235–239. [17] T. Okamoto, I. Yamaguchi, Opt. Lett. 25 (March (6)) (2000) 372. [18] M. Pattabi, K.M. Rao, S.R. Sainkar, M. Sastry, Thin Solid Films 338 (1999) 40–45. [19] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [20] R. Popescu, D. Macovei, A. Devenyi, R. Manaila, P.B. Barna, A. Kovacs, J.L. La´ba´r, Eur. Phys. J. B 13 (2000) 737–743. [21] M.R.C. Hunt, S. Modesti, P. Rudolf, Phys. Rev. B 51 (1995) 10039. [22] R. Manaila, A. Belu-Marian, D. Macovei, G. Brehm, D.Th. Marian, I. Baltog, J. Raman Spectrosc. 30 (11) (1999) 1019–1025.
SERS of gold/C60 (/C70) nano-clusters deposited on iron surface Luo Zhixun, Fang Yan * Beijing Key Lab for Nano-photonics and Nano-structure, Department of Physics, Capital Normal University, Beijing 100037, PR China Received 16 December 2004; received in revised form 30 January 2005; accepted 31 January 2005 Available online 5 March 2005
Abstract Surface enhanced Raman scattering (SERS) spectra of very good quality of gold/C60 (/C70) nano-clusters deposited on iron surface were reported by using the pyridine as a intermediate to connect and nest the C60/C70 molecule to the gap of gold nano-particles and iron substrate. The number of vibrational modes was greatly increased, especially some modes that were forbidden in Raman spectrum, appeared and even split as prediction of group theory. The enhancement factor is 106. It shows from the experiment that the ternary systems of ‘‘iron–gold/ fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and structural perturbation induced by the substrate upon the fullerene cage with the high sensitivity, in particular, the adsorption behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nano-particles and the metal surface. # 2005 Elsevier B.V. All rights reserved. Keywords: SERS; C60/C70; Gold nano-particles; Iron
1. Introduction As some unique characters, physical and chemical, of materials are usually thrown out by the material surface, especially the photoelectric and spectral characters, the study of new materials often begins with the surface characters. In fact, the discovery of superlattice structured, laminated structured, periodic, quasi-periodic structured materials and all kinds of nano-materials have recently provided abundant subject matter for surface investigation. The discovery of the high-Tc superconductivity in alkalidoped fullerene has also generated considerable interest in technological and scientific applications of fullerene-based materials, especially in the form of thin films [1–6]. Because properties of fullerene thin films depend intimately on interface configuration and adsorption patterns, the study of the adsorption behavior and the charge transfer interaction of fullerene with the metal surface is significant for understanding the film growth mechanisms and the * Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68982332. E-mail address: [email protected] (F. Yan). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.01.008
determination of the optical and electronic properties of the fullerene thin films [7–12]. Raman spectral analysis technique, using photons as probe, takes good advantages of no damages to samples and in-situ detection. Since it has applied to study the molecular structure, rapid progress has improved in material identification, organic and inorganic chemistry, biochemistry and macromolecules, catalyze, petrochemical industry, environmental sciences and so on [13]. Based on this, surface enhanced Raman scattering (SERS) results in highsensitivity and high-resolution spectra without damage to samples, and reveals rich information concerning with the interactions between the adsorbates and substrates. It is very useful to apply SERS to in situ investigation of fullerene film, by depositing fullerene systems on the electrode or metal surface. With developed method, the SERS active systems of gold/C60 (/C70) nano-clusters deposited on iron surface were prepared. It is found from the experiment that the ternary systems of ‘‘iron–gold/fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and adsorption
152
L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nano-particles and the metal surface.
2. Experimental Preparation of C60/C70 was dissolved in pyridine. Colloidal gold with SERS active was prepared through a redox process based on Frens’s method [14], except that KAuCl4 substituted HAuCl4. Colloidal gold and C60/C70 solution were added by drop onto the surface of rough iron, copper, aluminum or silver. After the dryness, a series of metal piece coated with different layers of gold/C60 (/C70) nano-clusters were obtained, with the pyridine volatilization. Raman spectra excited in the near-infrared region were acquired with a FT-Raman spectrometer. A Brucker Model IFS-66 FT-Raman spectrophotometer with a line resolution of 1 cm1 and a YAG laser operated at 1064 nm was used as the excitation source at 150 mW. The concentration of the solutions of C60/C70 is 0.02 M for all.
3. Results and discussion Fig. 1a shows the near-infrared Raman spectrum of C60 solid. The high degeneracy degree in C60 molecule makes the spectra appear rather simple, in spite of the large number of atoms per molecule. The expected 10 Raman active vibrational modes obviously appear in the spectrum. Furthermore, another peak, which is usually assigned to the infrared active, appears at 568 cm1, which may
Fig. 1. (a) Raman spectrum of solid C60; (b) SERS of C60/gold clusters dropped on filter paper; (c) SERS of C60/gold clusters deposited on iron surface.
originate from the symmetry reduction by the intermolecular interaction in C60 solid. Different from the Raman spectrum of C60 solid, SERS of C60 in all systems gives much more information of vibrational for C60 molecular. Fig. 1b shows the SERS of C60/gold clusters dropped on filter paper, and Fig. 1c displays the SERS spectrum of gold/C60 nano-clusters deposited on iron surface, pyridine as the solvent media. It is remarkable in Fig. 1c that, as we can see, not only the number of vibrational modes was greatly increased, but also the significant Raman bands split as well as frequencies up and down shift. This was ascribed to the symmetry lowering and selection rule relaxation of C60 induced by the gold surface. It is extremely remarkable of the three stair highest peaks, as appearing at 270, 491 and 1462 cm1, whose SERS signal intensity was estimated to be enhanced more than 106. It is notable in the presented SERS spectra, as shown in Fig. 1b and c, that the pyridine bands are not observed even though pyridine is a very effective Raman scatterer, while it is deliberate to choose the pyridine as solvent media because of its hydrophobicity and hydrophilicity based on the structure of pyridine itself with both hydrophilic grouping and hydrophobic grouping, as well as its volatility. Fig. 2 shows the comparison of SERS spectrum of gold/ C60 nano-clusters deposited on silver (a) with that on aluminum (c), copper (b) and iron (d) surface. As one can see, they show great difference with each other, even for the three main bands. For example, the 272 cm1 mode shows three split in Fig. 2d, while the three split modes turn out to be not so obvious in Fig. 2c, and even tend to a wave packet in Fig. 2a.
Fig. 2. SERS of C60/gold clusters deposited on silver (a), aluminum (b), copper (c) and iron (d).
L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
153
frequency Hg modes, such as 1421 and 1575 cm1. But there is an exception, such as 1099 and 1250 cm1 mode split into two, respectively. There are some non-Raman active modes appearing, too. All four F1u infrared active modes appear at 513, 576, 1194 and 1421 cm1, respectively. These features can be attributed to normally forbidden modes which also become Raman-active by virtue of symmetry lowering. Among them, 576 cm1 splits into two peaks at 550 and 576 cm1, and 535 cm1 splits into two peaks at 506 and 513 cm1. A vibrational mode possibly assigned to be hyper-Raman active based on the group theory calculation, appeared, and split into two peaks at 341 and 352 cm1. This mode was measured with neutron inelastic scattering and highresolution electro-energy-loss spectroscopy method, and calculated, and attributed to Hu. The additional and split modes resulted from the symmetry lowering and selection rule relaxation by the adsorption of C60 on gold surface. Based on the group theory calculation, there are two kinds of adsorption possibility;
The bands of Raman and SERS of C60/gold nano-clusters deposited on iron are summarized in Table 1. As we can see, SERS of C60 gives much more molecular information, and it is obvious that there are many additional and split of vibrational modes. SERS spectra of C60 in our experiment are in good agreement with the conclusion of group theory this. As the normal coordinate of low-frequency modes is mostly radial along molecule, the group theory calculation concludes that there are three splits for Hg mode. In Table 1, it can be found that those Hg modes among 200–700 cm1 nearly all split as expected. For example, 273 cm1 band splits into 254, 270 and 298 cm1; while 433 cm1 band splits into 399 and 422 cm1; but it is not obvious of 703 cm1 bands split to three, two modes instead. Even it cannot be found split at 772 cm1. In the high frequency region beyond 800 cm1, the high frequency modes were ascribed to the interaction of tangential modes with the gold surface. This interaction is weaker than that in the low frequency region. As the result, there is almost no splitting observed in the four high
Table 1 SERS bands and Raman bands of C60 (cm1) Experiments
Group theory
Raman of solid C60
SERS of C60/gold clusters deposited on iron surface
Assignment
Ih
273 (s)
254 270 298 341
Hg (1)
8Hg (1)
3
T2u (1)
T2u (1)
2
433 (m)
399 (w) 422 (s)
Hg (2)
8Hg (2)
3
497 (s)
491 (s) 513 (m) 525 (m)
Ag (1) T1u (1)
2Ag (1) 4T1u (1)
1 2
568 (m)
550 (m) 576 (m)
T1u (2)
4T1u (2)
2
709 (w)
659 (w) 703 (m) 728 (w)
Hg (3)
Hg (3)
3
772 (m)
766 (s)
Hg (4) Go F1g (1) T2u (1)
Hg (4) Go F1g (1) T2u (1)
3 2 2 2
Hg (5) Gg T1u (3)
Hg (5) Gg 4T1u (3)
3 2 3
Hg (6) Hg (6) F1g
8Hg (6)
3
F1g
2
1421 (m) 1462 (vs.)
Hg (7), T1u (4) Ag (2)
8Hg (7) 4T1u (4) Ag (2)
3 1
1565 (s)
Hg (8)
8Hg (8)
3
(m) (vs.) (s) (m)
961 (w) 1025 (m) 1100 (m)
1099 (m) 1194 (w)
1249 (m)
1422 1467 1512 1574
(m) (s) (w) (m)
1246 1268 1311 1347
(m) (w) (w) (w)
Note: vs.: very strong, s: strong, m: medium, w: weak.
Number of splits C5v
C3v
3
3
3
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which is the adsorption on gold surface via pentagons (C5v) or hexagons (C3v). As calculation from molecular orbit has made known, the lowest un-occupied molecular orbit (LUMO) is three-fold degeneracy T1u, and highest occupied molecular orbit (HOMO) is five-fold degeneracy Hu. Group theory allows the 46 vibrations for the C60 molecular, given as follows: Au + 2Ag + 3F1g + 4F1u + 4F2g + 5F2u + 6Gg + 6Gu + 7Hu + 8Hg. For Ih symmetry, only the two Ag and eight Hg vibrations are Raman active, the four F1u modes are infrared active, and the 22 hyper-Raman are active (4F1u + 5F2u + 6Gu + 7Hu). Pentagonal adsorption would result in retro gradation of C60 molecule group from high symmetric Ih group to C5v group and hexagonal adsorption from Ih to C3v. Because of this, Hg, T1u and some other modes of Ih group would split into three or two modes of C5v and C3v group with lower degeneracy. For pentagon adsorption behavior, the Ih symmetry is reduced to C5V with 53 Raman active modes (19A1 + 34E1). While, for hexagons adsorption, the Ih symmetry is reduced to C3v with 89 Raman active modes (31A1 + 58E). Among them, A1 modes were the highest in intensity, indicating that C60 is probably adsorbed on the surface via pentagons of C60. Furthermore, in the lower frequency region, the splitting extent of modes predominantly corresponding to the radial motion of pentagons vibrational is stronger than that of hexagons vibrational. This also implied that C60 molecular tends to pentagon adsorption. Even from Table 1, we can find the Gu mode and Gg mode are splitting into two modes, referring to the group theory that there is difference of two or three splits between the C5V and C3V, which confirmed that the adsorption of C60 molecular are oriented on pentagons of C60 on the gold crystal surface. In fact, not only the split and additional modes in Fig. 2d excited us, it was remarkable that the signal-to-noise in Fig. 2d is the best, too. To evaluate the enhancement efficiency of this system, we quote a simple formula to calculate the enhancement factor. G¼
I SERS =Nsurf IRaman =Nbulk
(1)
Here (ISERS/Nsurf) represents the SERS intensity contributed by C60 adsorbed on substrate, and (IRaman/Nbulk) represents the normal Raman intensity of the species in solution exposed to laser light. Choosing Ag (2) (1462 cm1) as the typical band, it can be calculated that the enhancement factor G of the system is 3 106. In order to exclude the random factors from the experiments, we repeated and change the adsorbed probe molecule to C70. Fig. 3 shows SERS of C70/gold colloid deposited on iron (d), copper (c), aluminum (b) and silver (a) surface. It is interesting that Fig. 3d presents the best scene as expected. Except the similar shape of Raman bands, the split and additional modes are most obvious. So does the signal-to-noise. Not only for C60, but also for C70 systems, it is surprising to find the SERS spectra of that on silver is not so good as
Fig. 3. SERS of C70/gold clusters deposited on silver (a), aluminum (b), copper (c) and iron (d).
that on iron surface. It is notable that, especially in Fig. 2a, there are few more modes besides the three main bands of C60, and there is even a wave packet at 240 cm1 which almost covers the Raman bands of C60 at 270 cm1, only a wing left there. So what if these happen? Therefore, we infer that the different metal substrate plays different role in the induction for C60/C70 to nest in the gap of gold nanoparticles and the rough substrate. In fact, it is well shown and demonstrated in Figs. 2 and 3 that C60/C70 SERS is not only in dependent on the gold colloid, but also influenced by the rough substrate. To go deep into analysis of the mechanism of SERS of C60/C70 molecules on the systems, we propose an idealized model, in which C60/C70 molecules are nested in a coupled field made up of gold nano-particles and metal substrate. Fig. 4 presents a sketch map of the SERS active sandwich system [15–18] to visualize the behavior of the interaction of them. The base layer is metal substrate and the upper spheroidal particles are gold clusters, between them nested with spheroidal C60/C70 molecules. In brief, we divide into two parts for consideration. For the metal substrate, we introduce the old model, image field model, which attributes the enormous enhancement to the large polarizability that one calculates for certain choices of parameters when the Raman emitting system is taken to be a composite of the molecule and its conjugate-charge image in the metal. For simplicity, according to King, Van Duyne and Schatz, we assume a diagonal Raman polarizability tensor and a field polarized along the normal of the flat surface (the z-axis). The problem, therefore, becomes onedimensional.
L. Zhixun, F. Yan / Vibrational Spectroscopy 39 (2005) 151–156
155
Fig. 4. Sketch map of the SERS active system.
The dipole moment induced in the molecule by the incident field and its image field is [19] m ¼ að~ E þ~ Eim Þ (2) ~ ~ where E and Eim are the incident and image fields, and are components of the molecular polarizability. ~ Eim is given by Eim = [(e0 e)/(e0 + e)]m/(4r3), where r is the distance between the (point)dipole and surface. Substituting and rearranging, one obtains 1 ða=4r 3 Þðe e0 Þ m¼a 1 E: (3) ðe þ e0 Þ If we consider aeff ¼ a
1 ½1 ða=4r 3 Þðe e
0 Þ=ðe þ e0 Þ
;
(4)
then it is m ¼ aeff E This expression has a pole at the frequency at which ðe e0 Þ=ðe þ e0 Þ Re a ¼1 4r 3
(5)
(6)
Clearly this is related to surface-plasmon excitation in the metal surface at the frequency on which the condition Re(e) = e0 obtains. For spheroidal gold particles, by assuming the dimensions of the surface boss to be small compared to the wavelength of light, the field about the gold nano-particles become solutions of Laplace equation, 5E = 0, rather than of the more complicated Helmholtz equation, 5E + k2E = 0. The particles are taken to be ellipsoid of revolution with semimajor and semiminor axes equal to a and b, respectively, and the adsorbed molecule was placed on the axis of cylindrical symmetry a distance H above the bump. Using spheroidal coordinates j and h and the geometrical parameter f = (a2 b2)1/2, we introduced a Raman enhancement given by 1 þ ð1 eÞj0 Q01 ðj1 Þ=½eQ1 ðj0 Þ j0 Q01 ðj0 Þ 4 (7) G¼ 1G in which j0 = a/ f, j1 = (a + H)/ f, e is a complex dielectriccomplex of the metal. Q1 is Legendre function of the second kind and G is a complex-valued quantity that depends acutely on H. In the limit of e ! 1, G = 2a/[2(a + H)]3, the term 1 G arises in the so-called image enhancement factor.
In fact, the effect of molecular orientation upon SERS should be considered. And adsorption upon a spherical metal particle and molecularly induced dipole to take an arbitrary orientation with respect to the normal to the surface should be assumed. Whether or not, the SERS mechanisms of the ternary systems need to consider both sides discussed above. It is the coupling of them that makes the good result of the experiments. Of course, it may be thought of a stronger coupling field round the C60/C70 molecules by ‘iron–gold’. It is notable that the charge transfer factors may not be neglect since SERS of other systems shows worse signal-tonoise. Actually, the frequency of SERS shift towards red in Fig. 1 may be the proof of adatom-charge-transfer. Because of the difference of Fermi energy level of metal and the LUMO of C60/C70, the charge-transfer usually occurs from metal to C60/C70, but it may come into being hybrid orbital of metal [20–22]. All these factors result in the strong interaction of C60/C70 with metals, symmetry lowering and surface selection rules, so the additional and splitting modes bands are observed. In addition, the unique characters of magnetic iron maybe an influence to SERS effect of C60/C70.
4. Conclusion A series of SERS spectrum of very good quality of gold/C60 (/C70) nano-clusters deposited on iron surface was obtained by using the pyridine as a intermediate to connect and nest the C60/C70 molecule to the gap of gold nano-particles and iron substrate. The number of vibrational modes was greatly increased, especially some modes that were forbidden in Raman spectrum, appeared and even split. Based on group theory, the additional and split modes resulted from the symmetry lowering and selection rule relaxation by the adsorption of C60 on gold surface. It shows from the experiment that the ternary systems of ‘‘iron–gold/fullerene’’ are very effective and active. It provides convenience for probing the C60/C70 vibrational structure, the physical properties and structural perturbation induced by the substrate upon the fullerene cage with the high sensitivity, in particular, the adsorption behavior, the interaction of fullerene with the metal surface and the SERS mechanism of molecules nested between the gold nanoparticles and the metal surface.
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Acknowledgments The authors are grateful for the support of this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing.
References [1] Y. Fang, Q.-J. Huang, P. Wang, X.-Y. Li, N.-T. Yu, Chem. Phys. Lett. 381 (2003) 255–261. [2] R. Schwedhelm, L. Kipp, A. Dallmeyer, M. Skibowski, Phys. Rev. B 58 (19) (1998) 13176. [3] A.P. Saab, M. Laub, V.I. Srdanov, G.D. Stucky, Adv. Mater. 10 (6) (1998) 462. [4] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi, K. Seki, N. Ueno, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 891. [5] K. Nakamoto, M.A. McKinney, J. Chem. Educ. 77 (6) (2000) 775. [6] A. Al-Mohamad, A.W. Allaf, Synth. Met. 104 (1999) 39. [7] R.A. Ganeev, A.I. Ryasnyansky, M.K. Kodirov, T. Usmanov, Opt. Commun. 185 (2000) 473. [8] Y. Li, Y. Huang, S. Du, R. Liu, Chem. Phys. Lett. 335 (2001) 524–532.
[9] Y.S. Jiang, C.G. Liu, Y.H. Shao, Prog. Phys. 15 (3) (1995) 309. [10] K.D. Gregory, L.M. Korst, P. Cane, L.D. Platt, K. Kahn, P.M. Rafailov, V.G. Hadjiev, H. Jantoljak, C. Thomsen, Solid State Commun. 112 (1999) 517–520. [11] J. Fagerstro¨m, S. Stafstro¨m, Phys. Rev. B 48 (15) (1993) 11367– 11374. [12] J. Visser, E.J.J. Groenen, Chem. Phys. Lett. 356 (2002) 43–48. [13] Proceedings of XVIIIth International Conferecne on Raman Spectroscopy, ICORS XVIII, 2004. [14] G. Frens, Nat. Phys. Sci. 20 (1973) 241. [15] H.-Z. Yu, J. Zhang, H.-L. Zhang, Z.-F. Liu, Langmuir 15 (1999) 16– 19. [16] A. Kudelski, J. Bukowska, M. Dolata, W. Grochala, A. Szummer, Mater. Sci. Eng. A 267 (1999) 235–239. [17] T. Okamoto, I. Yamaguchi, Opt. Lett. 25 (March (6)) (2000) 372. [18] M. Pattabi, K.M. Rao, S.R. Sainkar, M. Sastry, Thin Solid Films 338 (1999) 40–45. [19] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [20] R. Popescu, D. Macovei, A. Devenyi, R. Manaila, P.B. Barna, A. Kovacs, J.L. La´ba´r, Eur. Phys. J. B 13 (2000) 737–743. [21] M.R.C. Hunt, S. Modesti, P. Rudolf, Phys. Rev. B 51 (1995) 10039. [22] R. Manaila, A. Belu-Marian, D. Macovei, G. Brehm, D.Th. Marian, I. Baltog, J. Raman Spectrosc. 30 (11) (1999) 1019–1025.