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Electrochemical preparation of silver and gold nanoparticles: Characterization by confocal and surface enhanced Raman microscopy
Surface Science 597 (2005) 119–126 www.elsevier.com/locate/susc
Electrochemical preparation of silver and gold nanoparticles: Characterization by confocal and surface enhanced Raman microscopy W. Plieth a
a,*
, H. Dietz a, A. Anders a, G. Sandmann a, A. Meixner b, M. Weber b, H. Kneppe b
Technische Universitaet Dresden, Institut fuer Physikalische Chemie und Elektrochemie, D-01062 Dresden, Germany b Universitaet Siegen, Physikalische Chemie I, D-57068 Siegen, Germany Available online 11 August 2005
Abstract Localized silver and gold nanoparticles, electrochemically prepared by means of the double-pulse technique, were investigated with respect to their optical and spectroscopic properties by scanning confocal microscopy combined with surface enhanced Raman spectroscopy (SERS) and subsequent comparison with the local image of scanning electron microscopy (SEM). Analogous to the silver cluster preparation technique, controlled electrodeposition of gold nanoparticles was demonstrated, varying size from 10 to 500 nm and particle density. The maximum SERS enhancement factors found in the measurements were: (i) 1010 for silver particles and (ii) 108 for gold particles. The optical and spectroscopic data of the local nanoparticle structures investigated showed that SERS is a local phenomenon, because (i) only few particles are Raman active particles, (ii) strongest enhancements in SERS are obtained from particle agglomerates, (iii) typically the Raman radiation is emitted from irregular structures like the necks between two or more particles agglomerated. In the investigated range from 10 to 500 nm no significant influence of the particle size was observed. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Raman microscopy; Nanoparticles; Gold; Silver; Electrocrystallization
1. Introduction *
Corresponding author. Tel.: +49 351 422 69 80; fax: +49 351 422 69 81. E-mail address: [email protected] (W. Plieth).
New insight into the fundamentals of electrodeposition can be provided by studying the initial nanoparticle formation at the beginning of electrocrystallization process by means of SERS
0039-6028/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.02.042
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W. Plieth et al. / Surface Science 597 (2005) 119–126
spectroscopy. Surprising phenomena like band splitting, sometimes observed when monitoring the SERS activity of the adsorbate molecule in the process of electrocrystallization, indicated some interaction between particle growth and SERS activity [1]. To clarify the question whether SERS sites of high activity and growth centers are identical studies were focused on locally resolved Raman microscopy. Though the phenomenon of surface enhanced Raman scattering (SERS) is still under discussion, it is widely accepted that the origin of SERS is closely correlated to the enhancement of (i) the local electromagnetic field at the surface of small metallic nanoparticles and of (ii) the charge transfer between adsorbates and the metal particles [2–4]. The interest of the research was directed: (i) to the electrochemical preparation of nanoparticles of various size and particle density and (ii) to the investigation of the optical and spectroscopic properties by scanning confocal microscopy combined with SERS spectroscopy and subsequent comparison with the local SEM image. In former investigations of silver nanoparticles, it was demonstrated that the double-pulse technique is an electrochemical tool for controlling the metal deposition with respect to particle size and particle density [5,6]. This is because crystal seed formation can be exclusively transferred into the first pulse having high amplitude, and crystal growth can be conducted at a slow overvoltage without new nucleation. Based on a model on the features of the double-pulse technique, various structures of silver nanoparticles grown onto a thin ITO film covered glass plate were generated and characterized. First investigations on isolated silver particles, having a diameter of about 150 nm, showed that few nanoparticles were SERS active. The Raman spectra observed came from single carbon nanocrystallites on silver particles. It was observed, that the Raman active particles demonstrated intermittent on/off behaviour [7], as already found by Nie and Emory [8]. In fact, the SERS activity of single molecules may reach the Raman scattering cross-section of molecular fluorescence, as shown by Kneipp [9] and Nie [8] in experiments with single dye molecules adsorbed on silver colloids. These results demonstrate the
need to improve information on the features of SERS occuring at active sites of single nanoparticles which are electrochemically grown. The aim of this paper is, to contribute to the present knowledge on the SERS effect observed at isolated nanoparticles with respect to the local origin, and to determine the order of magnitude of the enhancement factor. Starting with the electrodeposition and Raman microscopic characterization of silver clusters on ITO substrates, the investigations have been also extended to gold nanoparticles.
2. Experimental 2.1. Preparation The electrodeposition of gold or silver nanoparticles on 1.32 cm2 ITO substrates was performed in separate Teflon cells of 12 ml volume using a standard three electrode setup, as previously described for silver particles [5,6]. The electrolyte used for the silver deposition contained 0.1 M KNO3, 0.1 M KCN and 0.01 M AgNO3 per liter [5,6], whereas the electrodeposition of gold was made in an acidic electrolyte of 0.005 M HAuCl4 per liter [10]. A HEKA PG284/IEC potentiostat/galvanostat was used for the double-pulse deposition. In a first pulse of high overvoltage, nucleation was forced and in a second pulse of small overvoltage of about 20 mV referred to the open circuit potential (OCP), the nucleus were directed to grow to a predetermined size. In the case of gold samples, the open circuit potential (OCP) after deposition was +820 mV vs. SCE. The pulse potentials were varied between +800 and +600 mV vs. SCE. Particles between 10 and 500 nm diameter were prepared. In the case of silver deposition, the OCP was 670 mV vs. SCE after deposition, the nucleation pulse potential E1 was 1550 mV vs. SCE and the growth pulse potential E2 was 700 mV vs. SCE. The averaged diameter of the silver cluster deposited was about 200 nm. The samples with particles were taken from the preparation bath and carefully washed and dried in the desiccators. Nevertheless, in the case of
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nitrogen cooled CCD detector. Scanning electron microscopy (SEM) images of the same sample regions were made with a Gemini 982 (Zeiss), subsequently to the optical measurements.
silver nanoparticles, carbonaceous impurities remained on the particle surface or were absorbed from the ambient atmosphere after the transfer to the Raman microscope. As revealed by the measurements, the Raman active species were graphite and nanocrystalline carbon, a typical experience in surface analysis. In the literature, the origin of carbon contamination is discussed in detail [11,12]. One general source of contamination is the metal preparation procedure. The carbon dioxide of the laboratory air might act as another source if reacting with the alkaline electrolyte on the cluster surface after removing the sample from the cluster preparation apparatus. Ions from the preparation electrolyte adsorbed on the substrate surface (e.g., CN) might also be a source for carbon, despite a careful rinsing after the deposition process. In the case of gold nanoparticles, no signals of nanocrystalline carbon could be found. Therefore, in most of the experiments, the dye Rhodamin 6G was used as SERS active molecule. It shows an additional resonance effect (surface enhanced resonance spectroscopy, SERRS). The samples were dipped into an aqueous solution of 107 M Rhodamin 6G per liter for 12 h and subsequently dried in the desiccators.
Recent investigations on the electrodeposition of silver by means of the double-pulse technique have demonstrated that the double-pulse method is a suitable technique for controlling the nanoparticle deposition, if the pulse parameters are carefully chosen and adjusted to the particle structure desired [5,6]. Whereas particle density should be controlled via the overvoltage of the nucleation pulse E1, the particle size can be enlarged by the growth time t2. Preparation regime and experimental experience was transferred to the generation of gold nanoparticles onto ITO and glassy carbon substrates. It was shown that various nanoparticle structures with particle sizes from 10 to 500 nm could be prepared [10].
2.2. Raman microscopy
3.2. Confocal microscopy and microspectroscopy
The confocal microscope used was a modified Zeiss Axiovert combined with two Avalanche photon-counting systems (SPCM-AQR-14) to measure the luminosity of a sample spot. The sample was scanned using a scan table (Physik Instrumente GmbH) with a resolution of 10 nm. The microscope was equipped with a HeNe-laser (k = 633 nm, used for Ag particles) and a frequency doubled Nd:YAG-laser (k = 532 nm), used for Au particles. In the beam path a dichroic mirror and a holographic notch filter (Kaiser Optical Systems, 6.0 O.D) were installed for rejecting back-scattered excitation light of the excitation wavelength. Two modes were applied: (i) the topography mode without using the notch filter and (ii) the Raman/fluorescence mode under use of the notch filter. Spectra were recorded with a 300 mm spectrograph (SpectraPro 300i, Acton, grating 1200 lines/mm) equipped with a liquid-
Based on the model on the features of the double-pulse parameters [5,6], samples were prepared with isolated silver clusters and low particle density (Fig. 1A and C). The particles, having a diameter of about 200 nm, were partially coagulated and formed agglomerates. In Fig. 1A, confocal images in the topography mode are shown. Subsequently, the notch filter is brought into the light beam, eliminating the laser light. Fluorescence and Raman scattered light can pass through the filter. The image of fluorescence and Raman scattering is shown in Fig. 1B. Only five distinct light spots remain in the image, which are marked with the cursor and are then compared with the topography image (Fig. 1A). An apparent enhancement factor can be obtained from the comparison of the technical data of topographic and molecular imaging. A factor of 1010 was found for silver.
3. Results 3.1. Preparation
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The result of this comparison is that the location of the Raman/fluorescence spots and the center of the clusters are not identical. The Raman/ fluorescence spots are located close to surface structures, e.g., the necks of the cluster agglomerations. To get even more insight into the surface structure around the light spots of Fig. 1B, the corresponding SEM image of the same structure could be found and is presented in Fig. 1C. There is clear evidence that the center of the Raman spot is a structure were two or more clusters are in contact. The Raman spot is caused by nanocrystalline carbon adsorbed on isolated silver particles [7]. An example of such a Raman spectrum is shown in Fig. 2. Raman bands in the region at 1350 and 1590 cm1 as well as the secondary Raman bands between 2500 and 3000 cm1 were observed in the experiments. The carbon lines agree with results reported in the literature [14]. The broad structure between 2500 and 5000 cm1 is caused by fluorescence. Fig. 3 shows that, analogous to the preparation of silver clusters, the particle size of gold can be controlled in electrodeposition. Fig. 3A shows the particle ensemble formed after the nucleation pulse, Fig. 3B the particles after the second pulse. Normally, particles formed during the first pulse (Fig. 3A), are growing in the second pulse. However, the electrode has anodic character at the
Fig. 1. Optical images of isolated silver clusters electrodeposited onto ITO covered glass by means of the double-pulse method (E1 = 1550 mV/50 ms; E2 = 700 mV/25 s) [13]. (A) Scanning confocal microscopy image (Topography mode), (B) Raman/fluorescence image of the same sample area and (C) SEM image corresponding to (A) and (B).
Fig. 2. Typical Raman spectrum of nanocrystalline carbon on a silver particle [7].
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the Nernst equilibrium potential (+820 mV). This is due to the negative shift of the reversible potential of an electrode covered by smaller nanoparticles [15,16]. Thus, dissolution of small nanocrystallites is initially observed, as seen by comparing Fig. 3A and B. This is an example of Ostwald ripening. If the overvoltage of the second pulse is substantially increased, the large particles grow under diffusion control, while the smaller nanocrystallites dissolve (diffusion coupling). Characteristic surface structures are developed (Fig. 3C). In Raman microscopy, the SERS activity of gold particles with adsorbed Rhodamin 6G was investigated. Results of clusters of the type imaged in Fig. 3C are shown in Fig. 4A–D. The most intensive Raman spot is again a cluster structure (Fig. 4B). But single clusters also show some activity. Obviously, the rough surface of the large Au clusters is the origin of the Raman/fluorescence image (Fig. 4B). Direct SERS information on the bands of Rhodamin 6G is provided by Fig. 4D measured on a Au cluster or cluster agglomerates. As expected, regions free from Au particles (Fig. 4C) do only show the bands of the substrate, glassy carbon. The maximum SERS enhancement, determined from a comparison of the technical data of topographic and spectroscopic imaging, was 108. The results can be summarized: (i) the surface enhanced Raman scattering is, at least partially, a local effect; (ii) the ‘‘hot spots’’ are mostly located at the necks between two touching particles; (iii) Raman enhancement factors of up to 1010 (for Ag) and 108 (for Au) were found; (iv) a significant influence of the particle size on the enhancement was not observed. These facts have to be taken into account in discussions on the enhancement mechanism. Fig. 3. SEM images of isolated gold clusters electrodeposited on ITO covered glass [10]. (A) single pulse (E1 = 100 mV/ 0.1 s), (B) double-pulse (E1 = 100 mV/0.1 s; E2 = +800 mV/ 90 s) and (C) double-pulse (E1 = 100 mV/0.1 s; E2 = +600 mV/90 s).
beginning of the growth pulse, if the growth potential (of +800 mV in Fig. 3B) is only few mV below
3.3. Conclusions concerning the enhancement mechanism The local resolution of surface enhanced Raman scattering may be a further step to the full understanding of the mechanism responsible for the enhancement effect. Most of the theories developed to explain the effect are based on two mechanisms:
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Fig. 4. Optical images (A,B) and Raman spectra (C,D) of isolated gold clusters electrodeposited onto glassy carbon under conditions of Fig. 3C with Rhodamin 6G adsorbate; (A) topography mode, (B) Raman-fluorescence mode, (C) background spectrum region without clusters, (D) spectra from particle covered areas.
(i) the enhancement of the local electromagnetic field in the surface region of metal particles (EM mechanism, a review is given in [2]); (ii) the interaction of adsorbed molecules with the metal surface at points of atomic-scale roughness via a chemical interaction between adsorbate and substrate (chemical mechanism, [3]). This implied the transfer of an electron from the metal particle to a higher electronic level of the adsorbed molecule [3]. The experimental results presented in this paper show, the surface enhancement of Raman scattering, at least its most intense form, is a local effect. Any explanation must take into account the local nature of the radiation. If an incoming photon hits the molecule the electrons are shifted relative to the ion positions, it becomes polarized. The decay of the polarization and the scattering of the photon is the typical elas-
tic Rayleigh scattering with a time constant of approximately 1015 s (femtoseconds). In the rare case that the scattering occurs in the moment of a transition from one vibrational state to another, the frequency of the leaving photon is changed and Raman scattering is observed. The transition moment amplitude Pfi gives the relative intensity of the scattered radiation P fi ¼ hwf jajwi i E wf is the wavefunction of the final state, wi is the wavefunction of the initial state, a is the polarizability tensor, and E the electromagnetic field. The typical cross-section of Raman scattering of 1027–1028 cm2 is much smaller than that of Rayleigh scattering (1014 cm2). If the incoming photon hits the larger structure of a silver particle, the electrons behave as a free
W. Plieth et al. / Surface Science 597 (2005) 119–126
electron gas. If the wavelength is of the appropriate dimension a surface plasmon is formed [4]. How is the situation if the incoming photon hits a structure of several silver particles (or of a metal surface with structures of similar optical properties)? The interaction of a photon with the system leads again to the excitation of surface plasmons. On a smooth surface, the field vector of the plasmon oscillation is oriented parallel to the surface. But the situation differs in regions around larger surface irregularities like the neck between two metal particles in agglomerates. This was shown by the calculations in the literature [17–19]. The described system shows a component of the field vector perpendicular to the surface. The field vector provides the basis for a strong polarization of molecular systems adsorbed in this region. The corresponding components of the polarizability tensor should be much larger then on an isolated molecule. This could be an origin of the strong enhancement from these spots. There might be another effect having its origin in a charge transfer between the adsorbate and the metal particle supported by the polarization. The charge separation connected with the plasmon generation would lead to the built-up of an electric field between the metal surface and the adsorbate. The field could have an additional effect on the scattering process leading to a forced frequency shift of the scattered photon [20]. Eventually, this phenomenon could explain the observed background enhancement.
4. Conclusion In this paper results were presented obtained on assemblies of silver and gold particles generated electrochemically with Raman microscopy. The results confirm that the extreme enhancement factors of SERS are a local effect. The ‘‘hot spots’’ were mostly connected with surface irregularities and the strongest effects were seen on points of clusters touching each other. An explanation might be that the polarizability tensor in this region has strong components perpendicular to the surface enhancing charge transfer and Raman intensity. The results clearly demonstrate, that
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the origin of the hot spots is not just an interference pattern of the electromagnetic plasmon fields [21]. No significant effect of the particle size was observed contrary to reports in the literature [22]. The understanding of spectroscopic phenomena of metal clusters is the basis for application of spectroscopy to deposition studies of SER active metals. From the different methods, SERS is the most promising one, because it makes possible the in situ monitoring of additives in the growth process. The experiments have shown that the Raman active structures are not identical with the growth centers of the deposition process. Nevertheless, the special structures showing Raman activity are irregularities which will have adsorption properties very similar to the growth centers making still the investigation of additives with SERS very attractive.
Acknowledgement This work was financed by the Deutsche Forschungsgemeinschaft in the Schwerpunktprogramm ‘‘Fundamentals of Electrochemical Nanotechnology’’.
References [1] B. Reents, G. Lacconi, W. Plieth, J. Electroanal. Chem. 376 (1994) 185. [2] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [3] A. Otto, J. Raman Spectrosc. 22 (1991) 743. [4] U. Kreibig, M. Vollmer, in: Optical Properties of Metal Clusters, Springer Series in Material Science, vol. 25, Springer, Berlin, 1995. [5] G. Sandmann, H. Dietz, W. Plieth, J. Electroanal. Chem. 491 (2000) 78. [6] M. Ueda, H. Dietz, A. Anders, H. Kneppe, A. Meixner, W. Plieth, Electrochim. Acta 48 (2002) 377. [7] P.J. Moyer, J. Schmidt, L.M. Eng, A.J. Meixner, G. Sandmann, H. Dietz, W. Plieth, J. Am. Soc. 122 (2000) 5409. [8] S. Nie, S.R. Emory, Science 275 (1997) 1102. [9] K. Kneipp, Y. Wang, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Phys. Rev. Lett. 78 (1996) 1667. [10] A. Anders, H. Dietz, W. Plieth, in preparation. [11] C.E. Taylor, S.D. Garvey, J.E. Pemberton, Anal. Chem. 28 (1996) 2401. [12] K.L. Norrod, K.L. Rowlen, Anal. Chem. 70 (1998) 4218.
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[13] M. Weber, G. Sandmann, H. Kneppe, H. Dietz, W. Plieth, A. Meixner, Chem. Phys. Lett., in preparation. [14] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [15] A. Henglein, Bunsenges. Phys. Chem. 81 (1977) 556. [16] W. Plieth, J. Phys. Chem. 86 (1982) 3166. [17] P.K. Aravind, A. Nitzan, H. Metiu, Surf. Sci. 110 (1981) 189. [18] N. Liver, A. Nitzan, J.I. Gersten, Chem. Phys. Lett. 111 (1984) 449.
[19] M. Inoue, K. Ohtaka, J. Phys. Soc. Jpn. 52 (1983) 3853. [20] W. Plieth, H. Dietz, G. Sandmann, H. Dietz, A.J. Meixner, H. Kneppe, M. Weber, in: D. Landolt, M. Matlosz, Y. Sato (Eds.), Fundamentals in Electrochemical Deposition and Dissolution, vol. 99-33, The Electrochemical Society, Pennington, New Jersey, 1999. [21] V.A. Markel, V.M. Shalaev, P. Zhang, W. Huynh, L. Tay, T.L. Haslet, M. Moskovits, Phys. Rev. B 59 (1999) 10903. [22] S. Emory, S. Nie, J. Phys. Chem. B 102 (1998) 493.
Electrochemical preparation of silver and gold nanoparticles: Characterization by confocal and surface enhanced Raman microscopy W. Plieth a
a,*
, H. Dietz a, A. Anders a, G. Sandmann a, A. Meixner b, M. Weber b, H. Kneppe b
Technische Universitaet Dresden, Institut fuer Physikalische Chemie und Elektrochemie, D-01062 Dresden, Germany b Universitaet Siegen, Physikalische Chemie I, D-57068 Siegen, Germany Available online 11 August 2005
Abstract Localized silver and gold nanoparticles, electrochemically prepared by means of the double-pulse technique, were investigated with respect to their optical and spectroscopic properties by scanning confocal microscopy combined with surface enhanced Raman spectroscopy (SERS) and subsequent comparison with the local image of scanning electron microscopy (SEM). Analogous to the silver cluster preparation technique, controlled electrodeposition of gold nanoparticles was demonstrated, varying size from 10 to 500 nm and particle density. The maximum SERS enhancement factors found in the measurements were: (i) 1010 for silver particles and (ii) 108 for gold particles. The optical and spectroscopic data of the local nanoparticle structures investigated showed that SERS is a local phenomenon, because (i) only few particles are Raman active particles, (ii) strongest enhancements in SERS are obtained from particle agglomerates, (iii) typically the Raman radiation is emitted from irregular structures like the necks between two or more particles agglomerated. In the investigated range from 10 to 500 nm no significant influence of the particle size was observed. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Raman microscopy; Nanoparticles; Gold; Silver; Electrocrystallization
1. Introduction *
Corresponding author. Tel.: +49 351 422 69 80; fax: +49 351 422 69 81. E-mail address: [email protected] (W. Plieth).
New insight into the fundamentals of electrodeposition can be provided by studying the initial nanoparticle formation at the beginning of electrocrystallization process by means of SERS
0039-6028/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.02.042
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W. Plieth et al. / Surface Science 597 (2005) 119–126
spectroscopy. Surprising phenomena like band splitting, sometimes observed when monitoring the SERS activity of the adsorbate molecule in the process of electrocrystallization, indicated some interaction between particle growth and SERS activity [1]. To clarify the question whether SERS sites of high activity and growth centers are identical studies were focused on locally resolved Raman microscopy. Though the phenomenon of surface enhanced Raman scattering (SERS) is still under discussion, it is widely accepted that the origin of SERS is closely correlated to the enhancement of (i) the local electromagnetic field at the surface of small metallic nanoparticles and of (ii) the charge transfer between adsorbates and the metal particles [2–4]. The interest of the research was directed: (i) to the electrochemical preparation of nanoparticles of various size and particle density and (ii) to the investigation of the optical and spectroscopic properties by scanning confocal microscopy combined with SERS spectroscopy and subsequent comparison with the local SEM image. In former investigations of silver nanoparticles, it was demonstrated that the double-pulse technique is an electrochemical tool for controlling the metal deposition with respect to particle size and particle density [5,6]. This is because crystal seed formation can be exclusively transferred into the first pulse having high amplitude, and crystal growth can be conducted at a slow overvoltage without new nucleation. Based on a model on the features of the double-pulse technique, various structures of silver nanoparticles grown onto a thin ITO film covered glass plate were generated and characterized. First investigations on isolated silver particles, having a diameter of about 150 nm, showed that few nanoparticles were SERS active. The Raman spectra observed came from single carbon nanocrystallites on silver particles. It was observed, that the Raman active particles demonstrated intermittent on/off behaviour [7], as already found by Nie and Emory [8]. In fact, the SERS activity of single molecules may reach the Raman scattering cross-section of molecular fluorescence, as shown by Kneipp [9] and Nie [8] in experiments with single dye molecules adsorbed on silver colloids. These results demonstrate the
need to improve information on the features of SERS occuring at active sites of single nanoparticles which are electrochemically grown. The aim of this paper is, to contribute to the present knowledge on the SERS effect observed at isolated nanoparticles with respect to the local origin, and to determine the order of magnitude of the enhancement factor. Starting with the electrodeposition and Raman microscopic characterization of silver clusters on ITO substrates, the investigations have been also extended to gold nanoparticles.
2. Experimental 2.1. Preparation The electrodeposition of gold or silver nanoparticles on 1.32 cm2 ITO substrates was performed in separate Teflon cells of 12 ml volume using a standard three electrode setup, as previously described for silver particles [5,6]. The electrolyte used for the silver deposition contained 0.1 M KNO3, 0.1 M KCN and 0.01 M AgNO3 per liter [5,6], whereas the electrodeposition of gold was made in an acidic electrolyte of 0.005 M HAuCl4 per liter [10]. A HEKA PG284/IEC potentiostat/galvanostat was used for the double-pulse deposition. In a first pulse of high overvoltage, nucleation was forced and in a second pulse of small overvoltage of about 20 mV referred to the open circuit potential (OCP), the nucleus were directed to grow to a predetermined size. In the case of gold samples, the open circuit potential (OCP) after deposition was +820 mV vs. SCE. The pulse potentials were varied between +800 and +600 mV vs. SCE. Particles between 10 and 500 nm diameter were prepared. In the case of silver deposition, the OCP was 670 mV vs. SCE after deposition, the nucleation pulse potential E1 was 1550 mV vs. SCE and the growth pulse potential E2 was 700 mV vs. SCE. The averaged diameter of the silver cluster deposited was about 200 nm. The samples with particles were taken from the preparation bath and carefully washed and dried in the desiccators. Nevertheless, in the case of
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nitrogen cooled CCD detector. Scanning electron microscopy (SEM) images of the same sample regions were made with a Gemini 982 (Zeiss), subsequently to the optical measurements.
silver nanoparticles, carbonaceous impurities remained on the particle surface or were absorbed from the ambient atmosphere after the transfer to the Raman microscope. As revealed by the measurements, the Raman active species were graphite and nanocrystalline carbon, a typical experience in surface analysis. In the literature, the origin of carbon contamination is discussed in detail [11,12]. One general source of contamination is the metal preparation procedure. The carbon dioxide of the laboratory air might act as another source if reacting with the alkaline electrolyte on the cluster surface after removing the sample from the cluster preparation apparatus. Ions from the preparation electrolyte adsorbed on the substrate surface (e.g., CN) might also be a source for carbon, despite a careful rinsing after the deposition process. In the case of gold nanoparticles, no signals of nanocrystalline carbon could be found. Therefore, in most of the experiments, the dye Rhodamin 6G was used as SERS active molecule. It shows an additional resonance effect (surface enhanced resonance spectroscopy, SERRS). The samples were dipped into an aqueous solution of 107 M Rhodamin 6G per liter for 12 h and subsequently dried in the desiccators.
Recent investigations on the electrodeposition of silver by means of the double-pulse technique have demonstrated that the double-pulse method is a suitable technique for controlling the nanoparticle deposition, if the pulse parameters are carefully chosen and adjusted to the particle structure desired [5,6]. Whereas particle density should be controlled via the overvoltage of the nucleation pulse E1, the particle size can be enlarged by the growth time t2. Preparation regime and experimental experience was transferred to the generation of gold nanoparticles onto ITO and glassy carbon substrates. It was shown that various nanoparticle structures with particle sizes from 10 to 500 nm could be prepared [10].
2.2. Raman microscopy
3.2. Confocal microscopy and microspectroscopy
The confocal microscope used was a modified Zeiss Axiovert combined with two Avalanche photon-counting systems (SPCM-AQR-14) to measure the luminosity of a sample spot. The sample was scanned using a scan table (Physik Instrumente GmbH) with a resolution of 10 nm. The microscope was equipped with a HeNe-laser (k = 633 nm, used for Ag particles) and a frequency doubled Nd:YAG-laser (k = 532 nm), used for Au particles. In the beam path a dichroic mirror and a holographic notch filter (Kaiser Optical Systems, 6.0 O.D) were installed for rejecting back-scattered excitation light of the excitation wavelength. Two modes were applied: (i) the topography mode without using the notch filter and (ii) the Raman/fluorescence mode under use of the notch filter. Spectra were recorded with a 300 mm spectrograph (SpectraPro 300i, Acton, grating 1200 lines/mm) equipped with a liquid-
Based on the model on the features of the double-pulse parameters [5,6], samples were prepared with isolated silver clusters and low particle density (Fig. 1A and C). The particles, having a diameter of about 200 nm, were partially coagulated and formed agglomerates. In Fig. 1A, confocal images in the topography mode are shown. Subsequently, the notch filter is brought into the light beam, eliminating the laser light. Fluorescence and Raman scattered light can pass through the filter. The image of fluorescence and Raman scattering is shown in Fig. 1B. Only five distinct light spots remain in the image, which are marked with the cursor and are then compared with the topography image (Fig. 1A). An apparent enhancement factor can be obtained from the comparison of the technical data of topographic and molecular imaging. A factor of 1010 was found for silver.
3. Results 3.1. Preparation
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The result of this comparison is that the location of the Raman/fluorescence spots and the center of the clusters are not identical. The Raman/ fluorescence spots are located close to surface structures, e.g., the necks of the cluster agglomerations. To get even more insight into the surface structure around the light spots of Fig. 1B, the corresponding SEM image of the same structure could be found and is presented in Fig. 1C. There is clear evidence that the center of the Raman spot is a structure were two or more clusters are in contact. The Raman spot is caused by nanocrystalline carbon adsorbed on isolated silver particles [7]. An example of such a Raman spectrum is shown in Fig. 2. Raman bands in the region at 1350 and 1590 cm1 as well as the secondary Raman bands between 2500 and 3000 cm1 were observed in the experiments. The carbon lines agree with results reported in the literature [14]. The broad structure between 2500 and 5000 cm1 is caused by fluorescence. Fig. 3 shows that, analogous to the preparation of silver clusters, the particle size of gold can be controlled in electrodeposition. Fig. 3A shows the particle ensemble formed after the nucleation pulse, Fig. 3B the particles after the second pulse. Normally, particles formed during the first pulse (Fig. 3A), are growing in the second pulse. However, the electrode has anodic character at the
Fig. 1. Optical images of isolated silver clusters electrodeposited onto ITO covered glass by means of the double-pulse method (E1 = 1550 mV/50 ms; E2 = 700 mV/25 s) [13]. (A) Scanning confocal microscopy image (Topography mode), (B) Raman/fluorescence image of the same sample area and (C) SEM image corresponding to (A) and (B).
Fig. 2. Typical Raman spectrum of nanocrystalline carbon on a silver particle [7].
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the Nernst equilibrium potential (+820 mV). This is due to the negative shift of the reversible potential of an electrode covered by smaller nanoparticles [15,16]. Thus, dissolution of small nanocrystallites is initially observed, as seen by comparing Fig. 3A and B. This is an example of Ostwald ripening. If the overvoltage of the second pulse is substantially increased, the large particles grow under diffusion control, while the smaller nanocrystallites dissolve (diffusion coupling). Characteristic surface structures are developed (Fig. 3C). In Raman microscopy, the SERS activity of gold particles with adsorbed Rhodamin 6G was investigated. Results of clusters of the type imaged in Fig. 3C are shown in Fig. 4A–D. The most intensive Raman spot is again a cluster structure (Fig. 4B). But single clusters also show some activity. Obviously, the rough surface of the large Au clusters is the origin of the Raman/fluorescence image (Fig. 4B). Direct SERS information on the bands of Rhodamin 6G is provided by Fig. 4D measured on a Au cluster or cluster agglomerates. As expected, regions free from Au particles (Fig. 4C) do only show the bands of the substrate, glassy carbon. The maximum SERS enhancement, determined from a comparison of the technical data of topographic and spectroscopic imaging, was 108. The results can be summarized: (i) the surface enhanced Raman scattering is, at least partially, a local effect; (ii) the ‘‘hot spots’’ are mostly located at the necks between two touching particles; (iii) Raman enhancement factors of up to 1010 (for Ag) and 108 (for Au) were found; (iv) a significant influence of the particle size on the enhancement was not observed. These facts have to be taken into account in discussions on the enhancement mechanism. Fig. 3. SEM images of isolated gold clusters electrodeposited on ITO covered glass [10]. (A) single pulse (E1 = 100 mV/ 0.1 s), (B) double-pulse (E1 = 100 mV/0.1 s; E2 = +800 mV/ 90 s) and (C) double-pulse (E1 = 100 mV/0.1 s; E2 = +600 mV/90 s).
beginning of the growth pulse, if the growth potential (of +800 mV in Fig. 3B) is only few mV below
3.3. Conclusions concerning the enhancement mechanism The local resolution of surface enhanced Raman scattering may be a further step to the full understanding of the mechanism responsible for the enhancement effect. Most of the theories developed to explain the effect are based on two mechanisms:
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Fig. 4. Optical images (A,B) and Raman spectra (C,D) of isolated gold clusters electrodeposited onto glassy carbon under conditions of Fig. 3C with Rhodamin 6G adsorbate; (A) topography mode, (B) Raman-fluorescence mode, (C) background spectrum region without clusters, (D) spectra from particle covered areas.
(i) the enhancement of the local electromagnetic field in the surface region of metal particles (EM mechanism, a review is given in [2]); (ii) the interaction of adsorbed molecules with the metal surface at points of atomic-scale roughness via a chemical interaction between adsorbate and substrate (chemical mechanism, [3]). This implied the transfer of an electron from the metal particle to a higher electronic level of the adsorbed molecule [3]. The experimental results presented in this paper show, the surface enhancement of Raman scattering, at least its most intense form, is a local effect. Any explanation must take into account the local nature of the radiation. If an incoming photon hits the molecule the electrons are shifted relative to the ion positions, it becomes polarized. The decay of the polarization and the scattering of the photon is the typical elas-
tic Rayleigh scattering with a time constant of approximately 1015 s (femtoseconds). In the rare case that the scattering occurs in the moment of a transition from one vibrational state to another, the frequency of the leaving photon is changed and Raman scattering is observed. The transition moment amplitude Pfi gives the relative intensity of the scattered radiation P fi ¼ hwf jajwi i E wf is the wavefunction of the final state, wi is the wavefunction of the initial state, a is the polarizability tensor, and E the electromagnetic field. The typical cross-section of Raman scattering of 1027–1028 cm2 is much smaller than that of Rayleigh scattering (1014 cm2). If the incoming photon hits the larger structure of a silver particle, the electrons behave as a free
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electron gas. If the wavelength is of the appropriate dimension a surface plasmon is formed [4]. How is the situation if the incoming photon hits a structure of several silver particles (or of a metal surface with structures of similar optical properties)? The interaction of a photon with the system leads again to the excitation of surface plasmons. On a smooth surface, the field vector of the plasmon oscillation is oriented parallel to the surface. But the situation differs in regions around larger surface irregularities like the neck between two metal particles in agglomerates. This was shown by the calculations in the literature [17–19]. The described system shows a component of the field vector perpendicular to the surface. The field vector provides the basis for a strong polarization of molecular systems adsorbed in this region. The corresponding components of the polarizability tensor should be much larger then on an isolated molecule. This could be an origin of the strong enhancement from these spots. There might be another effect having its origin in a charge transfer between the adsorbate and the metal particle supported by the polarization. The charge separation connected with the plasmon generation would lead to the built-up of an electric field between the metal surface and the adsorbate. The field could have an additional effect on the scattering process leading to a forced frequency shift of the scattered photon [20]. Eventually, this phenomenon could explain the observed background enhancement.
4. Conclusion In this paper results were presented obtained on assemblies of silver and gold particles generated electrochemically with Raman microscopy. The results confirm that the extreme enhancement factors of SERS are a local effect. The ‘‘hot spots’’ were mostly connected with surface irregularities and the strongest effects were seen on points of clusters touching each other. An explanation might be that the polarizability tensor in this region has strong components perpendicular to the surface enhancing charge transfer and Raman intensity. The results clearly demonstrate, that
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the origin of the hot spots is not just an interference pattern of the electromagnetic plasmon fields [21]. No significant effect of the particle size was observed contrary to reports in the literature [22]. The understanding of spectroscopic phenomena of metal clusters is the basis for application of spectroscopy to deposition studies of SER active metals. From the different methods, SERS is the most promising one, because it makes possible the in situ monitoring of additives in the growth process. The experiments have shown that the Raman active structures are not identical with the growth centers of the deposition process. Nevertheless, the special structures showing Raman activity are irregularities which will have adsorption properties very similar to the growth centers making still the investigation of additives with SERS very attractive.
Acknowledgement This work was financed by the Deutsche Forschungsgemeinschaft in the Schwerpunktprogramm ‘‘Fundamentals of Electrochemical Nanotechnology’’.
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