6628
J. Phys. Chem. 1995,99, 6628-6634
SERS Spectra of Polythiophene in Doped and Undoped States E. A. Bazzaoui, G. LCvi, S. Aeiyach, J. Aubard, J. P. Marsault, and P. C. Lacaze” Institut de Topologie et de Dynamique des Systkmes, Universitk Paris 7-Denis Diderot, CNRS URA 34, 1 rue Guy de la Brosse, F. 75005 Paris, France
Received: June 15, 1994; In Final Form: January IO, 1995@
Raman spectra of polythiophene films electrochemically deposited onto silver and platinum electrodes have been investigated using 514.5 nm laser excitation. Contrary to the case for platinum, the use of roughened silver electrodes leads to the observation of surface-enhanced Raman scattering (SERS), allowing the vibrational characterization of polythiophene in a doped state. Since these vibrational modes are very different from those obtained for the undoped species, this provides a means of evaluating the amount of defects in both states of polythiophene films. According to SERS selection rules, the polythiophene appears to be bound to the silver surface with the ring planes perpendicular to that surface.
I. Introduction Among new conducting polymers such as polyheterocyclic polythophene is of particular interest, since, due to a resonance effect of the polymer hai in,^-'^ both doped and undoped species display a great chemical and electrochemical stability in air.19,20 Raman spectroscopy has proved to be a useful method for studying the structure of conducting p o l y m e r ~ . ~ lHowever, -~~ several authors, on studying Raman spectra of polythiophene at various levels of the doping-undoping process, using 514.5 nm laser excitation, only observed slight differences between doped and undoped states in either “ex situ” 22,26*37 or “in situ” experiments. Undoped polythiophene films exhibit a very strong visible absorption band, whose maximum is located at ca. 480 nm, related to the ~t ~ t electronic * transition; on the contrary, doped species only show a very weak absorption band in this region. Therefore, when excited at 514.5 nm, the resonance effect enhances more specifically the Raman lines of the undoped species. Since the oxidized samples still contain mainly undoped species, no significant differences were detected in Raman spectra of both species when excited at 514.5 nm, but only a slight decrease in the overall Raman intensity was detected for the oxidized form. Recently, we showed that a noticeable enhancement of the doped species Raman spectra could be obtained by using an infrared laser excitation at 1064 nm, which produced the resonance of the doped species.38 In this paper, we have used surface-enhanced Raman scattering (SERS), which has been proved to be a very powerful technique to provide high quality spectra even at very low surface coverage to analyze polythiophene films. Indeed, it has been shown39.40that some metals, like silver, copper, and gold, under appropriate roughening treatments, can produce a large enhancement of Raman spectra for molecules adsorbed onto their surface. This method has opened up new perspectives in Raman spectroscopy and could be an efficient tool in the study of polymeric films,28141-43since it enables us to obtain not only selective structural insights into these polymers but also information concerning the orientation of the polymeric nuclei with respect to the metal surface. The two mechanisms commonly considered to account for SERS are connected either to a molecular enhancement of the 29332
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* To whom correspondence @
should be addressed. Abstract published in Advance ACS Absrracrs, April 1, 1995.
polarizability of the molecule due to its adsorption onto the metal surface or to an intensification of the electromagnetic fields induced by the resonance of the metal surface electrons (surface plasmons resonance). It seems that the electromagnetic mechanism always occurs in SERS while the molecular one can manifest itself only if the molecule is specifically adsorbed onto the surface. Moreover, the SERS selection r ~ l e s predict ~ - ~ ~ that Raman modes drawing their intensity from the a, polarizability tensor component should be the most intense. Contrarily to infrared, Raman spectroscopy cannot provide direct information on the orientation of molecules with respect to the surface, since the scattered electric fields no longer have the same polarization vector as the incident one. However, it has been s h o ~ n that ~ ~ powerful , ~ ~ insights can be obtained by considering intuitively the connection between atomic vibrations and the magnitude of the Raman polarizability component. Due to its relation to the resonance frequency of the surface plasmons, the magnitude of the enhancement factor depends upon the metal properties (dielectric constant and size and shape of the protusions at the surface) and the excitation wavelength. With the commonly used laser excitation wavelength (A, = 514.5 nm) silver appeared to give rise to the highest enhancement, provided its surface has been suitably roughened by oxidation-reduction cycles (ORC). Therefore, the use of SERS to study polythiophene films requires that we overcome some experimental difficulties, since the film must be synthesized by electrochemical oxidation of thiophene or bithiophene after having performed the activation of the silver surface by ORC. Such results have been achieved in our laboratory by an appropriate choice of the solvent and the salt (see Experimental Section). In this article we analyze Raman spectra of reduced and oxidized polythiophene (PT) films electrochemically deposited onto silver and platinum electrodes. The Raman spectra of FT deposited onto platinum were undertaken essentially to obtain a standard reference to interprete the PT spectra on silver. Spectra obtained on activated silver electrodes display a clear SERS effect, allowing the observation of both reduced and oxidized forms and their assignment. The orientation of the thiophene rings with respect to the silver surface is also proposed. As far as we know, it is the first time that a FT film can be deposited onto an oxidizable metal like silver, using a new procedure set up in our laboratory. Changing the applied potential allowed the observation of both F T species (reduced and oxidized) at a very low laser power (0.5 mW) and at an excitation wavelength (Ae = 514.58 nm) where only the reduced
0022-365419512099-6628$09.00/0 0 1995 American Chemical Society
SERS Spectra of Polythiophene form is under a resonance condition. These latter conditions open up the possibility of analyzing the structure of very thin films without any photochemical and/or thermal decomposition. Polythiophene films are generally obtained by anodic electropolymerization of thiophene onto noble metals such as gold or platinum.20 However, the electrodeposition of PT film on an oxidizable metal like silver raises some important experimental difficulties associated with the much lower oxidation potential of silver with respect to the thiophene one (E'A, = 0.8 V/NHE,51E o n = 1.8 V/NHE20). Furthermore, the SERS effect can only be observed if the silver surface has been firstly roughened by ORC from ca. -0.6 to 0.2 VISCE in the presence of 0.1 M ~ ~ 1 . 5 2 This treatment, which produces a very thin layer of AgC1, makes the silver surface still more oxidizable and thus brings about some more difficulties in the electropolymerization process ( E ' ~ g ~ g = 1 0.22 V/SCES1). Nevertheless, these drawbacks can be overcome by using an electrolyte medium constituted of CH2C12 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) and biothiophene as the monomer, whose oxidation potential (1.25 V/NHE) is lower than that of the thiophene. These conditions were found to be quite suitable to deposit PT films on activated silver surfaces.
+
11. Experimental Section 11.1. Electrochemical Techniques. All electrochemical experiments were performed in a one-compartment cell using an EG & G PAR potentiostat Model 362. The working electrode was either a silver rectangular sheet (10 x 40 m2) or a glass plate (10 x 40 mm2) covered on one side by a thin platinum layer (about 500 nm) deposited by cathodic sputtering using a Balzers-Sputron II and a platinum target (Balzers, Pt 99.9% purity). The counter electrode was a stainless steel plate (15 x 40 m2), and all potentials were measured versus an Ag/AgCl electrode or a saturated calomel electrode (SCE). 11.2. Mechanical and Electrochemical Treatments of the Silver Surface. Before electrodeposition of polythiophene, the silver electrode was mechanically polished, washed twice with distilled water, and roughened by three electrochemical oxidation-reduction cycles between -0.4 and 0.15 V versus SCE with a 10 mV/s scan rate in 0.1 M KC1 aqueous solution. After this treatment, the activated silver surface was covered with a very thin brown AgCl layer. Other potassium halides such as KF, KBr, or KI have also been used to activate the silver surface. SERS spectra display different behaviors depending on the halides used to activate the surface. This point will be discussed in a forthcoming paper. 11.3. Preparation of Polythiophene Films. The electrochemical synthesis of polythiophene films on the activated silver electrode was performed by a chrono-potentiometric method at a current density of 0.5 mA cm-2 in CH2C12 (Aldrich) solution containing 0.1 M bithiophene (Aldrich) and 0.1 M TBAPF6 (Fluka) as supporting electrolyte. Two charge densities were imposed to synthesize PT films, 25 mC cm-* for XPS analysis and 2.5 mC cmP2 for SERS experiments. In the first case a thin blue polythiophene film (doped film) is obtained on the silver electrode. The film tums red (undoped film) when -0.2 V versus AgIAgC1 is applied in 0.1 M TBAPF6-CH2C12 solution. In the second case, if we suppose that the whole charge we imposed were consumed by the electropolymerizationprocess, the film thickness would be about 50 A.20 However, it must be noted that for charge densities less than 150 mC cmV2the difference between the electrode weights measured after and before the electropolymerization reaction gives negative values, a result which indicates that an
J. Phys. Chem., Vol. 99, No. 17, 1995 6629 important part of the charge is dissipated for the dissolution of the silver, and thus the film thickness should be far less than 50 A. These considerations are supported by the atomic force microscopy image of the roughned silver electrode covered with the very thin PT film, which shows that the important roughness displayed on this AFM image is mainly due to the topography resulting from the electrochemical activation treatment of the silver surface (Figure 1). In order to perform comparative studies, the same experimental conditions were used to synthesize polythiophene films on platinum electrodes. II.4. Raman Experiments. Raman spectra were recorded on a DILOR XY modular spectrometer constituted of a double monochromator used in the substractive mode (Le. with no dispersion) followed by a spectrograph and a multichannel detector. The substractive monochromator serves to select a spectral range (ca. 700 cm-'), thus eliminating the very intense Rayleigh line, and the dispersion is completed by the spectrograph. The detection system consists of an intensified 1024 diode array cooled using the Peltier effect. The main advantage of the multichannel detection is to allow short time repetitive scans with the corresponding improvement in the signal to noise ratio. For instance, in the present case, a typical spectrum requires about 50 scans of 2 s counting time each. The PT film deposited onto the electrode was excited with a focused 514.5 nm laser beam of a Spectra Physics Model 165 argon ion laser. The power was always kept very low (ca. 5 mW) to avoid the destruction of the film. An interferometric filter (3 nm, fwhm, 70 % transmission at 514.5 nm) was set in front of the focusing lens to remove the argon plasma lines. The scattered light was collected at 180" from the incident one (retro-Raman) and focused onto the entrance slit of the spectrometer. With the commonly used slits (100 pm) the spectral slit width was ca. 4 cm-l. 111. Results and Discussion III.1. Polythiophene Deposited onto Platinum Electrodes. Figure 2 compares the resonance Raman spectra of a polythiophene film, either reduced (PTS, Figure 2A) or oxidized (PToR, Figure 2B), deposited onto a platinum electrode. The only significant difference observed between these two spectra is the overall intensity of the reduced form, which is about 5 times greater than that of the oxidized one. This intensity difference can probably be attributed to a larger resonance effect of the reduced film, which is associated with a stronger UV-visible absorption at the laser excitation wavelength (514.5 nm). The wavenumbers of the observed bands for the two species and their assignments proposed on the basis of the calculated ones37are given in Table 1. These bands can be classified into three spectral regions: zone A centered at ca. 1450 cm-', zone B at ca. 1100 cm-', and zone C at ca. 700 cm-'. Each zone was investigated separately, since its analysis needed a particular band decomposition procedure. Zone A includes the most intense band (19) at ca. 1455 cm-' assigned to the symmetric stretching mode of the aromatic C=C bond ring. The band at ca. 1502 cm-' (VI), assigned to the antisymmetric stretching mode of the C=C bond ring, is rather sharp (Av = 9 cm-') for PT@ and broader (Av = 15 cm-') for PTp.". This v1 band is of particular interest when studying the conjugation length of the polymeric chain. Its position slightly shifts toward high frequencies, when the conjugation length of the oligomer source is increased: Thus it is located at ca. 1498,
Bazzaoui et al.
6630 J. Phys. Chem., Vol. 99, No. 17,1995
Microscope Scan size Set point Soan rate Nunber of samples
I
I
TM-RFM
1,447 lJM 2,535 U 1,489 Hz 512
uiew angle
-2-light
angle
Figure 1. AFM image of the roughened silver electrode covered with a very thin PT film. The charge density used in the galvanostatic process is Q = 2.5 mC cm-*.
L'2
?
0
\
'
i
1600 1400 1 i O O 1 i O O 860 660 400 Wovenumbers
/ zm-1
Figure 2. Raman spectra of polythiophene films deposited onto platinum electrodes: (A) reduced state; (B) oxidized state. Intensities are normalized.
1502, and 1505 cm-' for polythiophenes synthesized respectively from thiophene, a-bithiophene, and a-terthi~phene.~~ The frequency of this v1 band does not depend on the oxidation state of the PT film, but its width does much. The observed broadening could be attributed to changes in the conjugation length distribution in the PT chains. This conjugation length associated with n electron delocalization is favored
when the polymer rings are coplanar, owing to the maximum overlapping of the C-C inter-ring carbon pz orbitals (Figure 3). The oxidation produces distorted parts in the thus reducing the coplanarity of the rings and therefore the conjugation length. The intensity ratio I(v2)/I(v1), which has been shown23to decrease when the PT film conductivity increases, was estimated, in our case, to be respectively 17 for P T R and ~ 11 for PTJ! The very weak v3 band, assigned to the stretching mode of the C-C ring simple bond, is shown below to manifest a strong enhancement when PT is deposited onto a silver electrode. On processing the zone A bands into components of Lorentzian profiles, using a Marquard method (Spectra CalccR)),the shoulder which appears on the low-frequency side of the v2 band in the oxidized film gives rise to a weak band at ca. 1418 cm-'. This band, not detected in the reduced form of PT deposited onto platinum, can be assigned to the symmetric stretching mode of the C=C bond ring of the quinoid units in the PToR chains. The main band (v5) within zone B is assigned to in-plane deformation of the /lC-H bonds. The width of this band depends upon the oxidation state of the PT film: its value increases from 12 cm-I for P T R R to 21 cm-' for €ToR. It should be noted that when an oxidized PT film is deposited onto a roughened silver electrode, a new peak grows on the high-frequency side of the v5 band (this point will be detailed below). Zone C (Figure 4) includes, in addition to the 116 and v7 bands assigned to deformations of the PT rings, two other bands, denoted D4 and D5, which are related to distorted parts (kinks) in the polymer chains.23,25v33
J. Phys. Chem., Vol. 99, No. 17, 1995 6631
SERS Spectra of Polythiophene
TABLE 1: Band Assignments of Oxidized and Reduced Polythiophene Films Deposited onto Platinum and Silver Electrode@ lit37 PTlpt PTlAg calc freq assnmt reduced oxidized reduced oxidized assnmt 3101 stretching C-H stretching C-H ring stretching C=C
306 1 1497 1459 1359 1249 1210 1197 1039 746 697 522 420 304
1502 M-B 1502 M-Sh 1502 M-B ring stretching C=C 1455 VS-Sh 1454 VS-Sh 1455 VS-Sh ring stretching C-C 1369 VW 1367 VW 1369 VW C-H bending C-C ext stretching 1219 W 1217 W 1219 W C-C ext stretching 1177 VW 1178 VW 1171 VW C-H bending 1046 M-Sh 1048 M-B 1046 M-Sh ring deformat C-S-C 731 VW 742 VW 735 VW ring deformat C-S-C 702 W-Sh 698 W-Sh 701 M-Sh translat + rotation 563 VW translat rotation translat + rotation 298 VW 303 VW 298 VW W, weak, VW, very weak, M, medium; S , strong; VS, very strong; Sh, sharp; B, broad.
+
1502 S-B 1437 VS-B 1360 S-B
VI
1227 W-B 1180W 1052 M-B 742 W 693 M-Sh 563 VW
v4
vz v3
v5 v6
v7
303 VW
TABLE 3: Film Quality Evaluation for Oxidized and Reduced Polythiophene Deposited onto Platinum and Silver Electrodes PTPt PTlAg reduced oxidized reduced oxidized 0.55 0.80 1.25 I(D~YI(v~) 0.35 I(WI(v7)
Figure 3. Schematic representation showing the maximum overlapping of the C-C inter-ring carbon pr orbitals. TABLE 2: Kink Mode Frequencies of Oxidized and Reduced Polythiophene Films Deposited onto Platinum and Silver Electrodes PTPt PWAg litZ5 reduced oxidized reduced oxidized assnmt 652 644 649 644 D5 682 680 678 682 678 D4 1155 1149 1159 1160 1159 D3 1177
1177
1178
1171
1180
Dz
The vg band related to the C-S-C deformation is in fact constituted of an overlap of two peaks, the intensities of which strongly depend on the oxidation state of the PT film (see below). The positions of the kink modes observed in the present work are compared in Table 2 with those reported by Furukawa et a1.25 The intensity ratios between the Dz, D3, Dq, and D5 kink bands and the v7 band have been used by several authors23~25,30,33 as
0.35
0.40
0.40
1.10
indicators of the amount of defects within the polymer chains. Therefore, in order to evaluate the PT film quality, a decomposition of the C spectral region into five bands with Lorentzian shapes was performed, from which the intensity ratios Z(D4)l Z(v7) and Z(Dg)/Z(v7) were estimated and reported in Table 3. These ratios confirm that the inter-ring distortions are more pronounced for the oxidized sample than for the reduced one. 111.2. Polythiophene Deposited onto Silver Electrodes. The electrodeposition of PT films onto silver plates instead of a platinum one is of double interest when analyzing the PT Raman spectra. First, all the Raman bands observed when PT films are deposited on platinum are found again when silver electrodes are employed (Figure 5). However, owing to the SERS effeci, the signal to noise ratio increases by about a factor 10 when using silver electrodes. Second, significant differences are pointed out between spectra of the reduced (PTR*~,Figure 5A) and oxidized (PT‘)*g, Figure 5B) polythiophene films deposited onto silver while almost no difference was observed in the case of platinum (Figure 2). As above, Raman spectra were divided into the three zones (A, B, and C) which were analyzed separately.
L
730
680
3 / “-1
Figure 4. Decomposition of the zone C bands into components of Lorentzian profiles in the case of the Pt electrode: (A) PTR? (B) PTo”.
6632 J. Phys. Chem., Vol. 99, No. 17, 1995
Bazzaoui et al.
Figure 6. Orientation of quinonic units with respect to the silver surface. magnetic enhancement is of the same order of magnitude as the thickness we have estimated for our films; the SERS selection rules remain pertinent in our experiments. This conclusion is contradictory to that proposed by Tourillon et al.,49 who suggest, from NEXAFS spectroscopy, that the undoped poly(3-methylthiophene) films lie flat on the metal ' 1600 1400 i i o o 10'00 a i o 660 400 20c surface while long 3-alkylpolythlophene cycles are perpendicular Wavenumbers / cm- 1 to it. Without entering into a scientific controversy, we can attribute Figure 5. Raman spectra of polythiophene films deposited onto silver the discrepancy between our conclusions and those of Tourillon electrodes: (A) reduced state; (B) oxidized state. Intensities are et al. by the following remarks. (i) Unsubstituted polythiophene normalized. and 3-methylpolythiophene behave d i f f e r e n t l ~ .(ii) ~ ~The electropolymerization processes were performed under different In zone A, the v2 symmetric C=C stretching band of PToAg experimental conditions: The starting material was bithiophene films is particularly affected by oxidation: While its maximum in our case and monoalkylthiophene in ref 49, and the is lowered by 18 cm-l, its width increased from 26.5 cm-' in P T Rto~ 80.5 ~ cm-I in PToAg. The very weak v3 band in P T R ~ ~ , electrolytes and electrodes used in each case were different. Lastly, when oxidation is performed, some cycles might be associated with C-C ring vibration, is strongly enhanced in slightly distorted, but it is unlikely that the cycle's orientation PToAg. According to the forthcoming arguments, this observais turned upside down. Therefore it is likely that the cycle tion is a first clue to support the hypothesis that the polymer planes of both reduced and oxidized species make an important chain lies on the silver surface with the ring planes making a angle with the silver surface. significant angle with the silver surface. The wavenumber of the VI band remains unchanged at ca. This result can be explained by considering a bithiophene 1502 cm-', but its relative intensity increases from the reduced unit in the PT chain,37which has the C2vsymmetry. During an to the oxidized forms. in-plane (AI) vibration wherein the nuclei are restricted to the The shoulder appearing in the oxidized sample between the PT plane, it is likely that the changes in the a, polarizability VI and v2 bands at ca. 1480 cm-' is assigned to the C=C tensor component should be larger when the PT plane is vibration of the aromatic units. perpendicular to the silver surface than when it is parallel to it The broad band appearing as a shoulder in the PToAg (z axis normal to the silver surface). Since the SERS selection spectrum at ca. 1600 cm-' could presumably arise from the rule p r e d i ~ t s that ~ , ~ the ~ vibrations belonging to the same stretching of a quinoidic C=C bond which appears as a defect irreducible representation as a, are the most enhanced and in the doped species. This C=C mode is not detected on the according to the foregoing arguments, the vibrations occurring platinum electrode but is observed on the silver-roughened perpendicular to the silver surface should be more intense than electrode, owing to its intensity enhancement by the SERS those parallel to it. effect. This suggests that the C=C groups should adopt an In the reduced form of the PT film, the C-C ring bond is orientation, with respect to the silver surface, such that parallel to the surface whatever the orientation of the PT plane significant changes of the polarizability tensor component a, is with respect to the silver surface, thus leading to a weak band. are produced when C=C vibration occurs. Such an orientation, When the PT film is oxidized, the C-C ring bond of the proposed in Figure 6, is consistent with the hypothesis that the doped unit makes an angle with respect to the surface, thus cycles of the PT chain are perpendicular to the silver surface. leading to a rather important variation of the a, component Moreover, the assumption that the sulfur should be close to and then allowing a significant enhancement. On the other hand, the silver surface is also supported by the observation of SERS if the ring planes were parallel to the metal surface, only weak of thiols in which the sulfur links the silver.54 enhancement could occur for both doped and undoped units, since the C-C bond would always be parallel to the surface. In zone B, on going from P T Rto~ PToAg ~ films, the v5 band Furthermore, if the polythiophene cycles were parallel to the (C-H bending) is shifted from 1046 to 1052 cm-' and widened silver surface, we should expect some bands corresponding to from 12.5 to 22.5 cm-'. A new band, not reported in the vibrations perpendicular to the cycles to appear. .As no such literature, appears as a shoulder at ca. 1068 cm-'; it can be bands can be detected, we can conclude to an orientation of assigned to a structural defect. the PT rings with respect to the silver surface wherein the cycle *Inaddition to bands assigned to cycle deformation modes v6 planes make a significant angle with the metal surface. This and v7, zone C displays bands related to kink modes and a band orientation is consistent with the sulfur reactivity. at 844 cm-l, which is only apparent in the oxidized sample. Moreover, it has been shown, from SERS, on nonconducting This band already observed in the IR spectra55arises probably spacers of variable t h i ~ k n e s s ~that ~ . ~ the * range of the electrofrom the PF6- ion bound to oxidized PT units to balance positive
J. Phys. Chem., Vol. 99, No. 17, 1995 6633
SERS Spectra of Polythiophene
Figure 7. Decomposition of the zone C bands into components of Lorentzian profiles in the case of the Ag electrode: (A) PTR*~; (B) PToAg.
'
1600 1400 1200 1000 800
600
Wavenumbers
400 200
/
1600 1400 1200 1000 800
cm-1
600 400 20C
WavenLmbers
/
cm-1
Figure 8. Raman spectra of polythiophene films in the reduced state: (A) deposited onto a platinum electrode; (B) deposited onto a silver electrode. Intensities are normalized.
Figure 9. Raman spectra of polythiophene films in the oxidized state: (A) deposited onto a platinum electrode; (B) deposited onto a silver electrode. Intensities are normalized.
charges, and it is enhanced by the SERS effect, since it is not detected on the platinum surface. The presence of this ion was indeed detected in the oxidized sample by XPS measurements. The kink bands can be used to characterize the oxidation state of the film, since they are much more prominent in the oxidized samples (Figure 7). The intensity ratios, given in Table 3, are thus illustrative of the defect increase in oxidized films. Furthermore, the single broad band, observed at ca. 735 cm-' for the reduced sample, appears, in the oxidized one, as constituted, in fact, of two overlapping bands at 730 and 742 cm-' (Figure 7). These two bands assigned to the C-S-C deformation of the cycle (742 cm-') and kink (730 cm-l) are rather enhanced. 111.3. Comparison of the Raman Spectra of PT Films Deposited on Silver and Platinum. Raman spectra of reduced PT films deposited on silver and platinum electrodes are almost superimposable: They display the same bands with identical positions and equal relative intensities (Figure 8). The only visible difference takes place in the signal to noise ratio, which is about 10 times greater for PT deposited on silver than for a deposit on platinum. This suggests that the orientation of the cycles with respect to the surface should be the same for both metals; otherwise the relative intensities would not be the same.
As we have suggested above, i.e. the ring planes of the undoped units make an important angle with the silver surface, we can conclude that it is the same for the cycles of reduced PT films on platinum. This orientation is moreover corroborated by the fact that all the observed bands are assigned to in-plane vibrations only. The Raman spectra of oxidized PT films deposited onto silver are quite different from those on platinum (Figure 9). Besides the broadening and the shift of the bands which have already been discussed, some rather intense bands, denoted D1, D2, D3, D4, and D5, characteristic of structural defects appear when oxidized PT films are synthesized on silver. These bands exhibit a noticeable SERS effect, implying that the defect is close to the silver surface or even bound to it. Then, we assume that the oxidized film is bound to the silver surface by doped units which act as anchor points, the planes of the cycles making an important angle with the metal.
IV. Conclusion In this paper, we have shown that the Raman spectra of polythiophene films excited at 514.5 nm depend strongly on the nature of the substrate on which the film is deposited.
6634 J. Phys. Chem., Val. 99, No. 17, 1995
The use of an activated silver surface, as electrode, has allowed the observation of SER spectra of PT films. The spectrum of the oxidized f o m , excited at 514.5 nm, is quite different from those observed until now at this excitation wavelength. Thus, even if the resonance effect on the aromatic units is weaker, the SERS enhancement affects specifically the quinoic cycles and the structural defect vibrations which appear to act as anchor points for the binding of the oxidized film onto the metal surface. The spectrum of oxidized polythiophene synthesized on a silver electrode can then be considered as arising from three contributions: vibrations of aromatic rings, quinoic cycles, and kink modes, like distortions and a-B coupling. On a platinum electrode, we have mainly observed the resonance Raman spectrum corresponding to the aromatic units of which the UV-visible electronic transitions match the laser emission. Moreover, using a band decomposition procedure we have shown that spectra of oxidized samples display a weak band at ca. 1418 cm-’ assigned to the stretching vibration of C=C double bonds of quinonic cycles. Furthermore, the bands associated with structural defects are more intense, relative to the normal modes, in the oxidized than in the reduced species. References and Notes (1) Tourillon, G.; Gamier, F. J . Electroanal. Chem. 1982, 135, 173. (2) Afanas’ev, V. L.; Nazarova, I. B.; Khidekel, M. L. Chem. Abstr. 1981, 94, 4306. (3) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. Phys. 1982, 21, 567. (4) Hotta, S.; Hosaka, T.; Shimotsuma, W. Synth. Met. 1983, 6, 317. (5) Waltman, R. J.; Bargon, I.; Diaz, A. J. J . Phys. Chem. 1983, 87, 1459. (6) Gamier, F.; Tourillon, G.; Garzard, M.; Dubois, J. C. J . Electroanal. Chem. 1983, 148, 299. (7) Tourillon, T.; Gamier, F. J . Phys. Chem. 1983, 87, 2289. (8) Tanaka, S.; Sato, M.; Khaeriyama, K. Makromol. Chem. 1984,185, 1295. (9) Tanaka, S.; Sato, M.; Khaeriyama, K. Polym. Commun. 1985, 26, 303. (10) Roncali, J.; Gamier, F. J . Chem. Soc., Chem. Commun. 1986, 783. (11) Berlin, A,; Pagani, G. A,; Sannicolb, F. J. J . Chem. Soc., Chem. Commun. 1986, 1663. (12) Sugimoto, R.; Gu,H. B.; Hayashi, S.; Yoshino, K. Synth. Met. 1987, 18, 247. (13) Yashima, H.; Kobayashi, M.; Lee, K. B.; Chung, D.; Heeger, A. J.; Wudl, F. J . Electrochem. SOC.1987, 134, 46. (14) Ikenue, Y.; Chiang, J.; Patil, A. 0.;Wudl, F.; Heeger, A. J. J . Am. Chem. SOC. 1988, 110, 2983. (15) Ikenue, Y.; Uotani, N.; Patil, A. 0.;Wudl, F.; Heeger, A. J. Synth. Met. 1989, 30, 305. (16) Gratzl, M.; Hsu, D. F.; Riley, A. M.; Janata, J. J . Phys. Chem. 1990, 94, 5973. (17) Otero, T. F.; Rodriguez, J.; de Larreta-Azelain, E. Polymer 1990, 31, 220. (1 8) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J . Am. Chem. Soc. 1990, 112. 7869.
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