SERS spectra of TCNQ and TTF radical ions adsorbed on Ag and Au electrodes

Surface Science 160 (1985) 87-102 North-Holland, Amsterdam

87

SERS SPECTRA OF TCNQ AND TTF RADICAL IONS ADSORBED ON Ag AND Au ELECTRODES A. GIRLANDO and G. SANDONA Institute of Physical Chemistry, 2 Via Loredan, I-35131 Padova, Italy Received 20 December 1984; accepted for publication 16 April 1985

The surface enhanced Raman (SERS) spectra of the title compounds (and of their fully deuterated analogues) adsorbed on Ag and Au electrodes are presented. The Ag SERS spectra are interpreted as due to TCNQ- and TTF +°7 species, respectively; on the Au electrode TTF is adsorbed also in the fully ionized form when the potential is brought to positive values. The SERS intensity dependence upon the change of both exciting wavelength and electrode potentials has been investigated, pointing out the prominent role of the electromagnetic SERS enhancement. On the contrary, no evidence has been found of contributions of charge-transfer mechanisms to the overall SERS intensity.

1. Introduction

Extensive experimental and theoretical investigations on surface enhanced Raman scattering (SERS) for molecules adsorbed on metal surfaces have led to encouraging progress in singling out possible mechanisms for this remarkable effect [1-3]. There is at present substantial agreement that part of the Raman intensity enhancement is caused by enhanced electromagnetic fields near the surface (electromagnetic models), but it has also been found that this mechanism is apparently unable to explain all the experimental observations. The possible importance of a "chemical" enhancement, that is, one involving a specific interaction of the adsorbed molecule with the metal, has been stressed by many authors [3]. In particular, several experimental observations have been explained in terms of charge-transfer (CT) interaction between the metal and the adsorbed molecule. However, no clear-cut experimental evidence has been given to clarify the effective role of this type of mechanism in SERS and its importance in respect to the electromagnetic one. One of the problems is that different authors make reference to different CT enhancement mechanisms [4-7]; for instance, the possible role of both adiabatic, ground-state CT [7] and photon-driven, dynamic CT [4] has been proposed. Another difficulty is that in general very little is known about the effect of CT interaction on the vibrational spectra of the molecules so far studied by SERS.

0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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A. Girlando, G. Sandonh / S E R S of T C N Q and T T F on Ag and A u electrodes

With the aim of elucidating the role of "chemical" enhancement mechanisms in SERS, and in particular of the CT one, we have undertaken a study of the tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) molecules in an electrochemical environment. These molecules easily donate or accept electrons in their frontier Tr-orbitals, and as such constitute fundamental building blocks of the family of quasi-one-dimensional organic conductors and semiconductors [8]. Their electronic structure and vibrational behaviour have been extensively studied [9,10], and the effect of the CT interaction on the vibrational spectra of the CT compounds they form has been deeply investigated [11]. As a result, the strength of the coupling of the CT electron with the molecular vibrations has been determined with good approximation for both T C N Q and T T F structures [12], making them particularly apt to the investigation of the role of CT in SERS. The latter aspect was considered only in a marginal way in the previous SERS studies of T T F and TCNQ adsorbed on island films and on colloids [13]. Furthermore, the electrochemical environment offers the advantage of allowing the control and the variation of the metal electric potential. The present paper reports the first relevant results obtained by this type of investigation.

2. Experimental The Raman spectra were recorded with a Jarrell-Ash 25-300 spectrometer, equipped with Spectra Physics krypton and argon ion lasers. The electrochemical instrumentation was: Amel model 551 potentiostat, model 556 function generator and model 731 current integrator. All the solutions were prepared using doubly distilled water and analytical grade supporting electrolytes (0.1 M) and deoxigenated by bubbling nitrogen (purity 99.999%) into the cell. The TTF, TCNQ and the corresponding salts TTF-C104 and L i - T C N Q were prepared and purified as described elsewhere [9,10]. The concentration of the TTF and TCNQ salts in the cell were of the order of 10 4 M. The working electrode was a polycrystalline Ag (assay 99.999%) or Au (assay 99.98%) rod, diameter 2 mm, sheathed in Teflon (in order to expose a well-defined area) and suspended vertically on the axis of a cylindrical glass cell. The counter electrode was a Pt wire, and all the potentials were reffered to the saturated calomel electrode (SCE). The working electrode exposed surface, made mechanically flat, was polished to a mirror finish with emery paper and alumina powder (0.03-0.05 t~m). After careful washing with distilled water, the electrode was immersed in the cell at negative potentials (typically - 0 . 6 V) and roughened by oxidation-reduction cycles (ORC) carried out with double potential sweeps. For the Ag electrode, the upper reverse potential were 0.01, 0.15 and 0.50 V, respectively with Br-, CI- and C104 salts as supporting

A. Girlando, G. Sandoni~ / SERS of TCNQ and TTF on Ag and Au electrodes

89

electrolytes; for the Au electrode with C1- salt the reverse potential was 1.2 V. The charge passed in the anodic sweep was typically 30-40 m C / c m 2. The laser beam was focused to an approxymately 0.1 x 10 mm line on the electrode surface, the total incident power being 100-200 roW.

3. Results 3.1. Silver electrode

Figs. 1 and 2 show the SERS spectra of the T-FF and T C N Q species adsorbed on the Ag electrode. The observed frequencies are reported in the first column of tables 1 and 2, respectively; the tables include also the frequencies of the corresponding fully deuterated species, TTF-d 4 and TCNQd 4. The spectra have been obtained by dissolving TTF-C104 and L i - T C N Q salts in water (ca. 10 - 4 M). The neutral molecules could not be used, being practically insoluble in pure water; attempts with different solvents (including mixed ones) have failed so far, yielding non-reproducible results. Several of the classic effects usually associated with SERS [14,15] have been observed. For instance, no Raman signal or a very weak one is obtained before the oxidation-reduction cycle (ORC). On the other hand, although the ORC has in general been performed in the presence of the T T F or T C N Q salts, intense spectra are also obtained if the salts are added after the ORC. We have also noticed the electrode "passivation" in respect to SERS by going at potentials more negative than about - 1 . 2 V. Finally, the change of the supporting electrolyte (tested only in the case of TI'F) leads to spectral intensities decreasing in the order B r - -= C I - > C104-. It is also worth mentioning in the present context that with T T F photochemical effects occur with red exciting laser light (~ = 647.1 nm). Such effects are more marked when the ORC is performed with the laser beam impinging on the electrode, whereas with different wavelengths the ORC in the presence of laser light only leads to an increase of the resulting SERS spectrum. These selective photochemical effects have not been investigated further since a meaningful study would require multichannel analyzer detection to follow the rapid evolution of the phenomenon. Fig. 3 reports the intensity ( I ) of the T T F 489 cm -a band and of the T C N Q 1387 cm -1 one, respectively, as a function of the applied potential ( E ) and for different excitation wavelengths. For TCNQ, curves substantially identical to those of the 1387 cm -1 band were obtained also for the 1606 and 1203 cm -1 ones. In the case of T T F a significant I versus E curve could not be obtained with 647.1 nm excitation, due to the abovementioned photochemical effects which cause the appearance of other bands overlapping those of interest. The experiment has been performed, as usual [16], by setting the

I

r

I

1400

i

!

1200

r

!

1000 "¢,,/cm-'

!

I

800

!

I

500

,' I

'-. I

Z,O0

J

I

200

!

kj

Fig. 1. SERS spectrum from Ag electrode in ca. 10 -4 M TTF-C104, 0.1 M KBr. Exciting light: 514.5 nm. Electrode potential: - 0 . 8 V versus SCE. Intensity scale: continuous line, 103 cps; dashed line, 2 × 103 cps.

x5 L-

I

2200

1600

,,,..-----L 1-

1400

.~

2

1200

..-

.l

2

.L

1000 "O/cm -1 800

0

6 0

'

'

4130

J

L _ _

200

Fig. 2. SERS spectrum from Ag electrode in ca. 10 - 4 M LiTCNQ, 0.1 M LiCI. Exciting light: 514.5 nm. Electrode potential: - 0 . 1 5 V versus SCE. Intensity scale: continuous line, 5 x 103 cps; dashed line, 104 cps.

"E

v

:D

.p_

,-g

:x

A. Girlando, G. Sandoni~ / SERS of TCNQ and TTF on Ag and A u electrodes

92

Table 1 S E R S spectra of TTF and TTFd 4 radical cations on Ag and Au electrodes a,b) Assignment

TTF

TTFd 4

A g ( ~ = 514.5 nm) Au(?~ = 647.1 nm)

Ag(?~ = 514.5 nm) Au(k = 647.1 nm)

E= -0.8 V

E= -0.8V

120 256 473 489 520 621 657 745 758 950 976 1319 1450 1494 1520 1580

m m sh vs w,sh w,br w,br sh m w,br w,br w,br w,br w m vw,br

E= -0.6 V

486 m

E=0.2V

508 s

753 w

1460 vw 1491 w 1517 m w

1414 m 1508 w

120 256 471 488 518 610 651 670 742 942 971 1320 1445 1468 1488 1530

E=0.2

V

m m

ag,~7

sh vs w,sh w,br w w m w,br w,br w,br br,sh m w vw,br

504 s

ag,V6 ag,2XV7

737 w

ag,p 5

1414 m

ag~p 3

1480 w,br

ag,~2

a) Frequencies in wavenumbers. Supporting electrolyte: KC1 or KBr. Qualitative relative intensities indicated by: vs, very strong; s, strong; m, m e d i u m ; mw, m e d i u m weak; w, weak; vw, very weak; sh, shoulder; br, broad. Classification of the normal modes as in ref. [9]. b) Frequencies of the ag fundamental modes of T T F ° / T T F + : 3 0 8 3 / . . . ; 1 5 5 5 / 1 5 0 6 ; 1 5 1 8 / 1 4 3 0 ; 1 0 9 4 / 1 0 7 8 " ; 7 3 5 / 7 5 8 ; 4 7 4 / 5 1 0 ; 2 4 4 / 2 6 6 cm -1 (ref. [9]; all frequencies are from solution

samples except that marked by asterisk).

monochromator at the frequency of the maximum of the band (slitwidth greater than 6 cm-~) and varying the potential. The position of the maxima in the I versus E curves is reproducible also by using different scan rates (1 to 5 mV s-a), but depends on the direction of the potential sweep. In the case of TTF, for instance, the reversal of the potential after reaching - 1.0 V leads to a maximum shifted by about 150 mV towards less negative potentials in respect to that of fig. 3. This difference can be connected to the process of surface reconstruction a n d / o r adsorption-desorption following the dynamic Variation of the potential. In fact, by recording the full Raman band at fixed values of the potential (with adequate time intervals between the measurements) an intensity maximum at a potential about halfway between those of the cathodic and anodic sweeps is obtained. Analogous behaviour is observed in the case of TCNQ. We can therefore conclude that by varying the electrode potential the maxima of the SERS intensity occur around - 0 . 8 and - 0 . 1 V for TTF and TCNQ, respectively, and that in both cases their position is not appreciably shifted by changing the excitation wavelength. It is important to notice,

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Table 2 SERS spectra of TCNQ and TCNQd 4 radical anions on Ag electrode a,b) TCNQ

TCNQd 4

Assignment

240 350 622 730 983 1181 1203 1272 1330 1387 1504 1606 2214

240 350 620 709 874 1029

AgC1 stretch ag,1,9 ag,1,8 ag,1, 7 ag, r'6 b3g,1,45 ag,1,5

mw w vw w w vw,sh m vw vw,br ms w s w

mw w vw w w w

1272 vw 1381 1452 1570 2214

b3g , 1,44.9 ag,1,4 b3g , 1,43? ag,u 3 ag,1,2

ms vw,br s w

a) Frequencies

in wavenumbers ; exciting line: 514.5 nm. E = - 0 . 1 5 V; supporting electrolyte: LiCI. Qualitative relative intensities indicated by: s, strong; ms, medium strong; m, medium; mw, medium weak; w, weak; vw, very weak; sh, shoulder; br, broad. Classification of the normal modes as in ref. [10]. b) Frequencies of the a~ fundamental modes of T C N Q ° / T C N Q - ; 3 0 4 8 " / . . . ; 2229*/2206*; 1602"/1615; 1454"/1391; 1207"/1196; 948*/978; 711"/725; 602"/613; 334*/337; 144"/148" cm 1 (ref. [10]; all frequencies are from solution samples except those marked by asterisk).

488.~/ ~

5

TTF

6

I

-1.0

8

I

~

I

-0.6 E/V

TCNQ

4880nm

__

I

I

-0.2

I

0

I

I

-0.2 E/V

I

I

-04

I

Fig. 3. Raman intensity of the ag v6 489 cm -1 band of the TTF (left panel) and of the ag 1,4 1387 cm a band of TCNQ (right panel), both adsorbed on Ag electrode, as a function of the applied voltage ( E ) for different exciting wavelengths. The curves have been scaled appropriately to avoid crossings, and are not representative of the relative intensities observed with different wavelengths.

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A. Girlando, G. Sandonit / S E R S of TCNQ and T T F on Ag and A u electrodes

however, that the relative intensities of the bands can be affected by the change of the exciting wavelength (keeping the potential at the value of the maximum absolute intensity). In particular, the order of decreasing intensities of the TCNQ a s ~'3, ~'4 and us, as given in table 2, is altered when the exciting light is changed from 514.5. to 647.1 nm. 3.2. Gold electrode

Table 1 reports the Raman frequencies of TTF species adsorbed on the Au electrode at two different potentials. Clearly, the degree of ionicity of the TTF species is different in the two cases (see below for a complete discussion). The change of ionicity is discontinuous with the variation of the potential. As shown in fig. 4 for the spectral region 450-550 cm ], starting at 0.2 V just one band is observed (panel a). By decreasing the potential (panel b), a second band appears at lower frequency, whereas the intensity of the first one decreases. Fig. 4c shows that at - 0 . 6 V only the second band (which

I

I

E:.O.2V I

E

E:-0.6~

I

550

I

I

C#/cm-~ 450

5150

I

'

I

~/cm.~ 450

Fig. 4. SERS spectrum (450-550 cm -l region) from Au electrode in ca. 10 -4 M TTF-CIO4, 0.1 M KC1. Exciting light: 647.1 nm. Intensity scale: 2× 102 cps.

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corresponds to that observed on Ag at a similar potential) is present. This band does not disappear when the potential is returned to 0.2 V (panel d) or even at more positive postentials. Therefore the process of formation and adsorption of the species detected at negative potentials is not completely reversible. The initial situation (panel a) can be recovered by performing a new ORC. The spectra of T C N Q adsorbed on Au are rather difficult to obtain due to the presence of a very strong background luminescence which increases with time and on going to positive potentials. We have been able to detect only three Raman bands, at 1605, 1386 and 1200 cm -~ (potential: 0.0 V), which correspond to the most intense bands observed on the Ag electrode (table 2). The T T F and T C N Q spectra have been obtained only with red exciting light (?~ = 647.1 nm), according to what has been already observed for SERS spectra on the Au electrode [14]. No Raman signal could be detected with X = 514.5 nm exciting light. Considering the different response of the spectrometer/detector system at the two wavelengths and assuming for green excitation a band intensity equal to the background noise level, we estimate an enhancement factor of at least 102 for the Raman intensity on going from green to red excitation. This fact points to the importance of the electromagnetic enhancement mechanism (see below). In this context it is worth mentioning that trials to obtain Raman spectra of T T F or TCNQ on the Pt electrode have been unsuccesful, although electrochemical measurements indicate that such an electrode behaves very similarly to the Au one.

4. Discussion

4.1. Nature of the adsorbed species The vibrational spectra of TCNQ, T T F and of their radical ions have been extensively studied [9,10], constituting a reliable reference framework to interpret the SERS spectra and to identify the adsorbed species. We notice that, surprisingly enough, the SERS frequencies vary very little with potential: only shifts of 2-3 cm -1 are observed in respect to the values reported in tables 1 and 2. In the case of T C N Q the assignment of the spectra is straightforward and is reported in the last column of table 2. Apart from the 240 cm-1 band, clearly attributable to the Ag-C1 stretching mode [14], practically all the other bands can be assigned to gerade modes of the TCNQ radical anion. There is no apparent breakdown of the free-molecule selection rules and the intensity of the totally symmetric (ag) modes prevails as is often observed in ordinary Raman spectra. The above result is not surprising since the radical anion species is the stable one [17] in the electrode potential range for which SERS spectra could be obtained. In fact, the Ag electrode is oxidized at about the

96

A. Girlando, G. Sandon& / SERS of TCNQ and TTF on Ag and Au electrodes

same potential at which the T C N Q neutral molecule should be formed. On Au, this potential region could not be studied, as mentioned before; on the other hand, it has been shown that neutral TCNQ transforms to T C N Q - when adsorbed on Au colloids [13]. At potentials more negative than - 0 . 3 V the T C N Q - Raman bands have practically disappeared (see fig. 4), in accordance with the fact that below this potential the dianion species should be the prevailing one [17]. The SERS spectra recorded at these potentials are not well reproducible, nor can the bands of TCNQ 2- be clearly identified. Apart from the unlikely adsorbtion of a dianion on the electrode at these negative potentials, the situation is complicated by the possible formation of protonated products [18]. A deeper investigation of the SERS spectra in this potential region is required; here, we limit ourselves to stating that between 0.0 and - 0 . 3 V, TCNQ is adsorbed on Ag or Au electrodes as a fully ionic species. The determination of the degree of ionicity (or oxidation state, hereafter p) of adsorbed T T F is more difficult than in the TCNQ case. It is well know that T T F easily forms "mixed valence" salts, i.e. salts in which the T T F molecule has a p value intermediate between 0 and 1 [19,20]. Furthermore, previous SERS studies of neutral T T F adsorbed on Ag and Au colloids or island films have shown the rather surprising result that the TTF is oxidized on the surface to at least two different ionic species [13]; on Au colloids, the SERS spectra have been interpreted as due to T T F +°3 and T T F +. In the interpretation of the SERS spectra presented in this paper it is convenient to start from those relevant to the Au electrode (table 1). The bands detected at 0.2 V closely match the frequencies of the totally symmetric (ag) modes of the fully ionic T T F [9]. By going to - 0 . 6 V this species transforms (with some degree of irreversibility, see fig. 4) into another one which closely corresponds to that detected on the Ag electrode at similar potentials (table 1). The bands at 486 and 1517 cm -1 on Au (at 489 and 1520 cm -1 on Ag, the latter band shifting to 1488 cm -a on deuteration) can be safely assigned to T T F ag u6 and P2 modes, respectively. Both these modes have been shown to display a nearly linear frequency dependence on p [21]; by using them as probes for the charge of T T F adsorbed on the electrodes at negative potentials, and taking the average weighted for the magnitude of the ionization frequency shift, we obtain a p value of 0.65 + 0.1. This finding is not surprising in light of the already mentioned tendency of T T F to form mixed valence salts (p varying between 0.6 and 0.8) with the CI-, Br- and C10 4 ions used as supporting electrolytes [19,20]; quite unusual is instead the weakness of the T T F ag ~3 mode in the SERS spectra. As a matter of fact, the SERS spectra resemble the ordinary Raman ones of TTF-Br0.76 or TTF-C10.78 powders [20], with the only exception of the ag u3 mode which in these salts displays an intense band around 1440 cm-1. On the other hand, alternative interpretations of the SERS spectra at negative potentials (for instance in terms of neutral TTF) cannot be proposed, presenting difficulties, much greater than the above one. We there-

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97

fore attribute the spectra to a T T F +0"7 species, with some still unclear factors which drastically weaken the intensity of the ag ~3 mode. As shown in table 1, we associate this mode to the broad 1460 cm -1 (Au) and 1450 cm -1 (Ag) bands, in agreement with the estimated charge of 0.65 . From the SERS spectra we are led to conclude that the fully ionic T T F species is adsorbed on the electrode at positive potentials, whereas at potentials as negative as - 0 . 8 V the adsorbed species is a partially ionic one. The fully neutral species has not been identified in the electrode potential range that can be inspected by SERS. This finding apparently contradicts what is known from electrochemical studies of T T F employing a Pt electrode in acetonitrile or a carbon paste electrode in water [22]. According to these studies, T T F + would be stable only above 0.3 V, and T T F ° below 0.0 V, the intermediately charged T T F +°7 being the species present in between. On the other hand, our experimental conditions are quite different from the above, and it is known that the electrochemical behaviour of T T F is strongly dependent on the nature of the supporting electrolyte and on the solubilities of the formed T T F salts [22]. Apparently, under our experimental conditions, the intermediate-ionicity species is the most stable on the electrode, being also reoxidized with difficulty; the presence of neutral q-TF at negative potentials cannot be excluded by SERS (a weak band is often but not always appearing at 472 c m - l ) , but is very difficult to confirm. The SERS spectra have allowed us to detemine the ionicity of the T C N Q and T T F adsorbed species. On the other hand, we are unable to establish if the species is adsorbed as such or if it gives rise to self-dimers or other aggregates on the surface. It is well known, in fact, that a m o n o m e r - d i m e r equilibrium is established in solutions of both T C N Q and TTF; however, the R a m a n spectra of the monomer and of the dimer display only slight frequency differences [9,10]. In this situation we cannot establish if the band detected at 120 cm -1 for T T F on Ag, present with both KCI and KBr supporting electrolytes, is due to an A g - T T F or to a T T F - T T F vibration. 4.2. S E R S enhancement mechanisms

One question that has to be answered before undertaking the discussion on SERS mechanisms is whether the high intensity displayed by the T F F and T C N Q ions adsorbed on the electrode is really due to a "surface enhancement" effect. In fact, both ions display electronic transitions in the visible region [9,10], so that the high intensity might originate from the normal resonance R a m a n effect, possibly from a thick film formed over the electrode. As a matter of fact, R a m a n spectra of ca. 5/~m A g - T C N Q films (formed by direct reaction of T C N Q with Ag) can be obtained without difficulty [23]. Although we do not have performed coverage measurements, several pieces of evidence indicate that the spectra discussed in the present paper can be considered as

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A. Girlando, G. Sandonit / SERS of TCNQ and TTF on Ag and Au electrodes

" t r u e " SERS spectra. Apart from the empirical observations classically associated with the SERS which we have already mentioned in section 3.1, the most convincing proof comes from a comparison of the results obtained on Ag and Au electrodes with green (~ = 514.5 nm) and red (~ = 647.1 nm) exciting laser light. On Ag, the red excitation leads to spectra only slightly more intense than those obtained with green light; furthermore, changes of the relative intensities of the bands have been observed, which do not agree with that reported for normal resonance Raman of the bulk compounds [23,24]. This is particularly evident in the case of the TCNQ ag P3, /~4 and u5 modes, whose decreasing relative intensities, as given in table 2, are appreciably altered when red excitation is used. On Au, the change from green to red light leads to an intensity enhancement factor of at least 102 , the molecular species being clearly the same as those adsorbed on Ag; in addition, the same kind of behaviour is observed for all the three species adsorbed ( T C N Q - , T T F +°7 and TTF+), irrespective of the likely differences in their electronic spectra. The above arguments lead us to conclude that "surface effects" play a major role in the Raman intensity enhancement of adsorbed T C N Q and TTF. Among these effects the importance of electromagnetic field enhancement through surface plasmon polarition (SPP) excitation is evidenced [14]. In fact, the spectra on Au are observed only for exciting light below the SPP resonance frequency (2.5 eV), whereas Pt, which cannot support SPP excitations, does not give a detectable Raman signal. Of course, also other SERS enhancement models, like the "image field effect", predict this kind of behaviour; however, these alternative models have not yet received independent positive experimental support [1-3]. We are now faced with the problem of ascertaining the presence of other enhancement mechanisms, and in particular of the CT one. As already mentioned in section 1, under the broad label of CT several SERS models have been proposed [3,7], and it is not always easy to assess precisely what consequences should be expected from such an enhancement mechanism. Rather than analyze or compare the various CT models, in what follows we shall briefly examine what specific effects should in general expected for a CT enhancement mechanism, in such a way that its possible presence can be distinguished from that of other ones. SERS CT models can be divided into ground- and excited-state mechanisms [7]. In the former, the intensity enhancement is a consequence of a (groundstate) CT between metal and adsorbate; in the second, the CT interaction is viewed as a dynamic, photon-driven process, in which the enhancement mechanism in some way resembles the resonance Raman effect. Since a CT electronic state is involved, in any case the CT models imply at some stage an electron-phonon coupling. In the field of CT crystals the coupling of the CT electron with the intramolecular vibrations has been very conveniently expressed [11] in terms of the linear electron-molecular vibration (e-mv) cou-

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99

piing constants, gi: g, = ( O ~ / O Q i ) o ,

where ~ is the energy of the molecular orbital involved in the CT and Qi is the i th normal coordinate. This definition has been used indeed by Persson [6] in his excited-state CT model of SERS. Not all the models of CT SERS mechanism consider so explicitly the e - m v coupling [5]; on the other hand, also if the coupling with the molecule-metal vibration is important, it is known that for the large 7r-molecules we are considering here the e - m y coupling cannot be disregarded [11]. Thus the relative intensities of the intramolecular modes should be connected with the magnitude of the e - m v coupling. In Persson's model, in fact, the "chemical enhancement" factor results to be proportional to g Z. For non-degenerate electronic ground states g, can be different from zero only if Q~ belongs to the totally symmetric representation of the free-molecule symmetry group; thus only totally symmetric vibrations should be enhanced. However, also nontotally symmetric modes have been observed in SERS spectra, so that the Persson and other analogous "two-step" models (i.e. where the photon directly drives the electron from the metal to the molecule) have been criticized in favour of a "four-step" model, in which the photon is first coupled to electron-hole pairs in the metal [3]; explicit expressions for the SERS enhancement factor have not been worked out in this case. Apart from this point, one of the most convincing proofs in favour of excited-state CT SERS models has been the shift of the maxima in the curve of Raman intensity versus applied potential with the change of exciting wavelength [16]. The variation of the applied potential in fact changes the position of the Fermi energy level in the metal in respect to the energy of the CT orbital in the molecule, thus changing the position of the optimum resonance for a given exciting light energy. Ground-state CT SERS models have not been worked out as extensively as the excited-state ones; the most recent one by Lippitsch [7] does not consider explicitly the effect of e - m v coupling. On the other hand, the physical basis of Lippitsch's model is quite similar to that of the ordinary CT crystals [11], for which the effect of the e - m v interaction on the vibrational spectra is a well known and important one. In particular, it has been shown [11] that, whenever there is intermediate charge transfer between the two CT partners, the intramolecular totally symmetric frequencies are downshifted ( in respect to the values in the absence of e - m y coupling) by an amount once again related to

g?.

In summary, one would be able to state that a CT SERS mechanism is operating when at least one of these three effects is observed: (i) the I versus E curve shifts with exciting wavelength; (ii) the relative intensities of the totally symmetric SERS bands are connected to the square of the (independently

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determined) e - m v coupling constants; (iii) the frequencies of the totally symmetric modes are downshifted (by an amount related to gi) in respect to the expected values. As has already been shown by fig. 3, the effect (i) above has not been observed in the case of adsorbed T T F +°7 or T C N Q . We therefore attribute the occurence of the maxima in fig. 3 to the variation of the surface coverage, 0: following Otto [3], we may assume that the overall R a m a n enhancement G is given by (wL is the laser frequency): a = GEM(COL, CO)OCT(COL, CO, E)GRRS(COL, c o ) 0 ( e ) , where GEM is the SERS enhancement due to the electromagnetic mechanism, GRRS is the term due to the normal resonance Raman when col is close to a localized electronic transition of the adsorbate, and GCT is the enhancement term. Now, since the ionicity of the ground state does not vary appreciably with the potential (see section 4.1) the localized electronic transitions of the adsorbate are not expected to vary, so GRRS is constant with the potential, as is GEM, and does not affect the position of the maximum. On the other hand, the insensitivity of the I/E curve to coL and to co (for different vibrational modes) put in evidence that the maximum is due essentially to 0 ( E ) : the contribution of GCT is either not relevant or masked by 0(E). The latter possibility should, however, be operative for both compounds, which appears unlikely. It is not difficult to realize, on the other hand, that even the effects (ii) and (iii) expected for a SERS CT mechanism are not observed. As explained in section 1, T T F and T C N Q are molecules particularly apt to this kind of inspection, since the values of their e - m y coupling constant have already been determined experimentally and shown to be practically independent of the charge at the molecule [12]. It is therefore easy to show that the relative intensities of the SERS spectra are not correlated with the known g values. This fact is particularly evident for the T T F + ° v a g v3 mode, which has by far the largest g value in respect to the other T T F ag modes, and whose SERS intensity is instead the weakest one (this is true independently from the assignment proposed in table 1). Always considering the T T F +°v species, the frequency downshift can also be ruled out by recognizing that the T T F ag v2 and v6 modes give a consistent indication of the O value, despite the fact that the latter mode has an e - m v coupling constant considerably greater than the former [12]. From the above considerations we are led to the conclusion that CT SERS mechanisms (excited- and ground-state) apparently do not play a significant role in the obtained SERS spectra of T C N Q and TTF. This finding appears rather surprising, considering the importance of the CT interaction in most of the molecular solids in which these molecules are involved. One possible and rather obvious explanation for its negligible role in the present case could be that T T F and T C N Q are not bound to the metal through their ~r-orbitals, but

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101

through the S and N atoms, respectively. The CT interaction, if present, would then occur between the T T F and T C N Q themselves, as it does in segregated stack solids or in self-dimers [25]. It remains true in any case that intense SERS spectra have been obtained also without apparent contribution of the metal-molecule CT enhancement mechanisms.

5. Conclusions In the present paper we have reported and interpreted the first complete SERS spectra of the T T F and T C N Q radical ion species in an electrochemical environment. The electromagnetic field enhancement through SPP excitation has been shown to contribute significantly to the SERS intensity. On the other hand, no evidence has been found of contributions through CT interaction, at least such as it is expected from the proposed models [5-7]. In other words, it appears that CT enhancement meachanisms are not a necessary prerequisite for SERS. On the other hand, this finding does not necessarily imply that the electromagnetic field enhancement is the only factor leading to the SERS spectra of these compounds: other "chemical" enhancements [3] might play a role. The fact that the relative intensities of the SERS bands are rather (or strikingly) different from those observed in resonance (or pre-resonance) R a m a n spectra of the bulk compounds suggests that the coupling of SPP's (or of other photon-driven electronic excitations in the metal) to the localized molecular electronic transitions (that for T C N Q and T T F ions occur in the visible region) should be taken into account. Further studies are in progress to investigate this possibility.

Acknowledgements This work has been supported by the National Research Council and by the Ministry of the Education of Italy. We thank M.R. Philpott (IBM, San Jose) for helpful discussions in the early stage of the work; the continuous and enlightening scientific exchange with C. Pecile and E. Vianello (Universith di Padova) is also gratefully acknowledged.

References [1] R.K. Chang and T.E. Furtak, Eds., Surface Enhanced Raman Scattering (Plenum, New York, 1982). [2] R.P. Cooney, M.R. Mahoney and A.J. McQuillan, in: Advances in Infrared and Raman Spectroscopy, Vol. 9, Eds. R.J.H. Clark and R.E. Hester (Heyden, London, 1982) ch. 4. [3] A. Otto, in: Light Scattering in Solids, Vol. IV, Eds. M. Cardona and G. G~ntherodt (Springer, Berlin, 1984) ch. 6.

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[4] A. Otto, J. Billmann, J. Eickmans, U. Erti~rk and C. Pettenkofer, Surface Sci. 138 (1984) 319, and references therein. [5] F. Adrian, J. Chem. Phys. 77 (1982) 5302; H. Ueba, S. Ichimura and M. Yamada, Surface Sci. 119 (1982) 433; K. Arya and R. Zeyher, Phys. Rev. B24 (1981) 1852. [6] B.N.J. Persson, Chem. Phys. Letters 82 (1981) 561. [7] M.E. Lippitsch, Phys. Rev. B29 (1984) 3101, and references therein. [8] See, for instance, J.S. Miller, Ed., Extended Linear Chain Compounds, Vol. II (Plenum, New York, 1982). [9] R. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Phys. 71 (1979) 2282, and references therein. [10] R. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Soc. Faraday If, 74 (1978) 235, and references therein. [11] A. Girlando, R. Bozio, C. Pecile and J.B. Torrance, Phys. Rev. B26 (1982) 2306; A. Girlando, A. Painelli and C. Pecile, Mol. Crystals Liquid Crystals 120 (1985) 17. [12] A. Painelli, A. Girlando and C. Pecile, Solid State Commun. 52 (1984) 801. [13] C.J. Sandroff, D.A. Weitz, J.C. Chung and D.R. Herschbach, J. Phys. Chem. 87 (1983) 2127; C.J. Sandroff and D.R. Herschbach, Langmuir 1 (1985) 131. We are grateful to C.J. Sandroff for kindly making a preprint of the above paper available to ns.

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