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The effect of self-assembled nanometric spacers on surface enhanced Raman spectra of terthiophene ultrathin films
Materials Science and Engineering C 15 Ž2001. 37–39 www.elsevier.comrlocatermsec
The effect of self-assembled nanometric spacers on surface enhanced Raman spectra of terthiophene ultrathin films Giuseppe Compagnini ) , Angela De Bonis, Rosario Sergio Cataliotti Dipartimento di Scienze Chimiche, UniÕersita` di Catania, Viale A. Doria 6, I-95125 Catania, Italy
Abstract Self-assembled monolayers ŽSAMs. of different length have been built onto silver surfaces suitable for surface enhanced Raman ŽSER. effect. The analysis of the surface enhanced Raman spectra as a function of the monolayer thickness allowed to study SER effect by changing the distance in a nanometric way. In this work, we present a case of distance dependence in which SER spectra of terthiophene are studied for molecules deposited onto SAMs previously adsorbed on roughened silver. Each spacer is able to inhibit the oligomerization of terthiophene already observed by us, when the molecules are adsorbed onto bare silver. It will also be shown how the spectral features of the deposited terthiophene gradually vary by changing the spacer thickness from 0.8 to 2.5 nm. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Self-assembled monolayers; Surface enhanced Raman
1. Introduction Self-assembled monolayers ŽSAMs. on metal surfaces have attracted a great research interest in the last decade, because of the possibility to tailor the characteristic of the new formed surfaces through the use of suitable chemical groups w1x. A possible application of these ordered structures is to separate the metal surfaces from the environment, thus preventing oxidation or corrosion phenomena, giving rise to the new surface particular physico-chemical properties. At the same time, the anchorage on the surface of the molecular tails through strong covalent bonds makes these nanometric layers resistant and long living. In the case of alkanethiols assembled on silver or gold, several papers in the last few years revealed that the length of the alkyl chain can be used as a method to vary the distance of the CH 3 head group from the metal–organic interface w1x. Some of the interesting properties of SAMs can help to solve a frequently encountered problem in the study and applications of surface enhanced Raman ŽSER. effect. It is indeed widely known that some adsorbed molecules onto noble metal surfaces show a tremendous increase Ž10 4 – 10 8 . of the Raman scattering cross-section, whose explanation is still controversial w2,3x. A major problem arises when the interaction of the molecules with the surface
)
Corresponding author. Tel.: q39-95-738-5077; fax: q39-95-337-841. E-mail address: [email protected] ŽG. Compagnini..
irreversibly destroys the chemical identity of the molecular system. Several examples can be given around this aspect, especially with molecules containing aromatic structures w4,5x. Under these regards, SAMs can be used in the investigation of dependence on the distance of the enhancement factors. Following previous results w6x, we would like to present a study of terthiophene molecules Ž3T. deposited onto roughned silver, which has been previously covered with adsorbed alkanethiol SAMs. Our starting point was the observation of a polymerization process induced when 3T molecules are directly adsorbed on rough silver, with an increase of the conjugation length w6–8x. In this respect, SAMs of different length allow us to study a distance dependence of such effect. 2. Experimental section Rough silver surfaces have been obtained as reported elsewhere w9x. These surfaces have been used for a number of SERS experiments. Some of these SER active surfaces have been covered with alkanethiol SAMs through a simple chemisorption procedure described in several other papers Žsee, for instance, Ref. w1x.. CH 3 ŽCH 2 . n SH with n s 5, 7, 11 have been used in order to vary in a nanometric way the length of the alkylic chain. Few droplets of a 10y3 M terthiophene solution in ethanol have been deposited by casting onto the surfaces and the samples have
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 1 3 - 2
38
G. Compagnini et al.r Materials Science and Engineering C 15 (2001) 37–39
been carefully cleaned, leaving only the adsorbed part of 3T left. SER and normal Raman spectra have been obtained using an ISA-Jobin Yvon U1000 double monochromator and an electro-cooled Hamamatzu phototube linked to a photon counting chain. The Spectralink acquisition unit by ISA-JY was used in encoding, by an on-line PC, the rough experimental data. The use of an argon ion laser in the 514.5-nm operating mode at low power Žaround 20 mW. avoids a thermal degradation of the samples.
3. Results and discussion Fig. 1 reports four spectra which introduce the argument to be presented. Two of them are characteristic of a bulk Žsolid. 3T sample and of a polythiophene sample obtained by electropolymerization as reported elsewhere w6x. Differences found in comparing these two spectra are in agreement with what is widely known on thiophene oligomers of different length and polymers with a molecular weight distribution. The most useful spectral interval is the 1300–1600 cmy1 where the three characteristic peaks of 3T bulk ŽC`C single bond stretching at 1424 cmy1 , C5C double bond symmetric stretching at 1460 cmy1 and C5C double bond antisymmetric stretching at 1530 cmy1 . are clearly visible. Indeed, in the polythiophene spectrum we only find the 1460 cmy1 frequency as wide line, with a shoulder at higher frequency located ca. 20 cmy1 lower than the C5C antisymmetric signal observed in bulk 3T. These observations are in agreement with other results, which indicate that the increase in the number of thiophene
Fig. 1. SERS spectra of 3T adsorbed on silver with and without a SAM. Raman spectra of bulk 3T and polythiophene are shown for a comparison.
Fig. 2. SERS spectra of 3T adsorbed with and without SAMs of different length. A normal Raman spectrum of solid 3T is also shown for a comparison.
units induces a red-shift of the 1530 cmy1 band, a reduction of its intensity and an increase of the bandwidth w6–8x. On the other side, in the top of Fig. 1, we report two spectra obtained after the adsorption of 3T molecules onto SERS active silver surface, with and without a SAM Ž n s 11. alkanethiol in between. It is clearly observed how the spectral features obtained by means of this alkanethiol SAM are quite similar to the ones obtained for bulk 3T, while the spectrum of 3T adsorbed onto a naked SERS active silver surface closely resembles that obtained for native electropolymerized polythiophene. These observations can be inferred by enlightening three arguments relative to the former SER spectrum. When a SAM is used as spacer the following items are observed: Ži. the C`C single bond stretching signal appears again with remarkable intensity Žespecially when n s 11. as in bulk 3T; Žii. there is a consistent decrease of the linewidth of all bands, which tends to give a situation like to 3T and different from that of polythiophene and terthiophene adsorbed onto naked silver; Žiii. the C5C antisymmetric stretching frequency increases back reaching the value as in bulk terthiophene Ž1530 cmy1 .. In order to study in a nanometric way, the alkanethiol chain length, we also performed SER spectra using molecules with n s 5 and n s 7. This allows varying of ca. 0.14 nm, the distance from the surface of the deposited molecule, for each CH 2 group added to the tail of the alkanethiol. The spectral features of 3T adsorbed on these systems, restricted to the 1300–1600 cmy1 region, are reported in Fig. 2, where they are compared with those coming from bulk 3T and 3T adsorbed onto bare silver.
G. Compagnini et al.r Materials Science and Engineering C 15 (2001) 37–39
Fig. 3. Position and FWHM of the C5C antisymmetric stretching mode of 3T adsorbed as function of the distance from the silver surface.
The already mentioned three items, which differentiate SER spectra with and without a SAM are even more evident. It is, then, enough to space silver surfaces with n s 5 alkanethiol to induce a small shift of the C5C antisymmetric stretching band and a consistent decrease of the bandwidths. These in fact progressively decrease by increasing the distance from the metal–organic interface that is by increasing the alkanethiol chain length. This is a puzzling phenomenon that could be merely ascribed to the presence of a variety of conformational arrangements and also—even if less probably—to a variety of oligomer lengths, both decreasing with the increase of the distance from the silver interface. Since there should be a decrease of the Raman cross-section by increasing the distance from the SERS active substrate, the portion of adsorbed molecules interested to this effect progressively decreases when the SAM becomes longer. In this case, we observe that such an enhancement should be purely electromagnetic in origin w10x. It is worthy to note that several authors w11–13x refer to the red-shift of the C5C antisymmetric stretching band as the strongest evidence of an induced polymerization or at least to the increase of the conjugation length. In this respect, while 3T onto naked silver tends to polymerize w6x, terthiophene molecules on a SAM maintain their chemical individuality. Following some ellipsometric results by Jennings and Laibinis w14x, it is possible to correlate the alkanethiol lengths with the distance at which the head CH 3 group is located above the silver surface w1x. These data Ž0.14 nm for each C`C bond. have been used to plot Fig. 3, in which we report the C5C antisymmetric stretching frequency and the FWHM as function of this distance. It is straightforward to note that the frequency is almost un-
39
changed by varying the SAM spacer although it decreases at around 1508–1510 cmy1 in the case of SER spectrum obtained with 3T on naked silver. On the other side, a monotone decrease of the FWHM is observed with the increase of the distance, thus reaching values close to 3T bulk Žcompatibly with the instruments resolution.. These findings may be interpreted assuming that, even with the shortest nanometric spacer used here Ž n s 5., 3T molecule is impeded to directly interact with silver surface to produce oligomers. This is probed by the invariance of the frequency value of the antisymmetric C5C stretching which remains almost unaffected by the change of the alkanethiol chain length. However, with the shorter spacer used, the conformational situation of the three thiophene rings is randomly distributed among all possible rotameric forms w15x, thus giving large band shapes, which tend to become narrower with the longest chain Ž n s 11.. A much more complex situation has been observed with a longer alkanethiol Ž n s 17., whose puzzling aspects are actually under study.
Acknowledgements We wish to thank Prof. Giovanni Marletta for useful discussions. R.S.C. wishes to thank SuperLab for the kind hospitality during the sabbatic year 1999–2000.
References w1x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodget to Self-Assembly. Academic Press, Boston, 1991. w2x R.L. Garrel, Anal. Chem. 61 Ž1989. 401. w3x A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 Ž1998. 241. w4x G. Compagnini, B. Pelligra, B. Pignataro, Mater. Res. Soc. Symp. Proc. 501 Ž1998. 109. w5x A. DeBonis, G. Compagnini, R.S. Cataliotti, G. Marletta, J. Raman Spectrosc. 30 Ž1999. 1067. w6x G. Compagnini, A. DeBonis, R.S. Cataliotti, G. Marletta, J. Phys. Chem. 2 Ž2000. 5298. w7x U.K. Sarkar, A.J. Pal, S. Chakrabarti, T.N. Misra, Chem. Phys. Lett. 190 Ž1992. 59. w8x U.K. Sarkar, S. Chakrabarti, T.N. Misra, A.J. Pal, Chem. Phys. Lett. 200 Ž1992. 55. w9x G. Compagnini, B. Pignataro, B. Pelligra, Chem. Phys. Lett. 272 Ž1997. 453. w10x G. Compagnini, C. Galati, S. Pignataro, Phys. Chem. Chem. Phys. 1 Ž1999. 2351. w11x I. Furukawa, M. Akimoto, I. Harada, Synth. Met. 18 Ž1987. 151. w12x G. Louarn, J.P. Buisson, S. Lefrant, D. Fichou, J. Phys. Chem. 99 Ž1995. 11399. w13x E. Agosti, M. Rivola, V. Hernandez, M. Del Zoppo, G. Zerbi, Synth. Met. 100 Ž1999. 101. w14x G.K. Jennings, P.E. Laibinis, J. Am. Chem. Soc. 119 Ž1997. 5208. w15x M. Ciofalo, G. La Manna, Chem. Phys. Lett. 263 Ž1996. 73.
The effect of self-assembled nanometric spacers on surface enhanced Raman spectra of terthiophene ultrathin films Giuseppe Compagnini ) , Angela De Bonis, Rosario Sergio Cataliotti Dipartimento di Scienze Chimiche, UniÕersita` di Catania, Viale A. Doria 6, I-95125 Catania, Italy
Abstract Self-assembled monolayers ŽSAMs. of different length have been built onto silver surfaces suitable for surface enhanced Raman ŽSER. effect. The analysis of the surface enhanced Raman spectra as a function of the monolayer thickness allowed to study SER effect by changing the distance in a nanometric way. In this work, we present a case of distance dependence in which SER spectra of terthiophene are studied for molecules deposited onto SAMs previously adsorbed on roughened silver. Each spacer is able to inhibit the oligomerization of terthiophene already observed by us, when the molecules are adsorbed onto bare silver. It will also be shown how the spectral features of the deposited terthiophene gradually vary by changing the spacer thickness from 0.8 to 2.5 nm. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Self-assembled monolayers; Surface enhanced Raman
1. Introduction Self-assembled monolayers ŽSAMs. on metal surfaces have attracted a great research interest in the last decade, because of the possibility to tailor the characteristic of the new formed surfaces through the use of suitable chemical groups w1x. A possible application of these ordered structures is to separate the metal surfaces from the environment, thus preventing oxidation or corrosion phenomena, giving rise to the new surface particular physico-chemical properties. At the same time, the anchorage on the surface of the molecular tails through strong covalent bonds makes these nanometric layers resistant and long living. In the case of alkanethiols assembled on silver or gold, several papers in the last few years revealed that the length of the alkyl chain can be used as a method to vary the distance of the CH 3 head group from the metal–organic interface w1x. Some of the interesting properties of SAMs can help to solve a frequently encountered problem in the study and applications of surface enhanced Raman ŽSER. effect. It is indeed widely known that some adsorbed molecules onto noble metal surfaces show a tremendous increase Ž10 4 – 10 8 . of the Raman scattering cross-section, whose explanation is still controversial w2,3x. A major problem arises when the interaction of the molecules with the surface
)
Corresponding author. Tel.: q39-95-738-5077; fax: q39-95-337-841. E-mail address: [email protected] ŽG. Compagnini..
irreversibly destroys the chemical identity of the molecular system. Several examples can be given around this aspect, especially with molecules containing aromatic structures w4,5x. Under these regards, SAMs can be used in the investigation of dependence on the distance of the enhancement factors. Following previous results w6x, we would like to present a study of terthiophene molecules Ž3T. deposited onto roughned silver, which has been previously covered with adsorbed alkanethiol SAMs. Our starting point was the observation of a polymerization process induced when 3T molecules are directly adsorbed on rough silver, with an increase of the conjugation length w6–8x. In this respect, SAMs of different length allow us to study a distance dependence of such effect. 2. Experimental section Rough silver surfaces have been obtained as reported elsewhere w9x. These surfaces have been used for a number of SERS experiments. Some of these SER active surfaces have been covered with alkanethiol SAMs through a simple chemisorption procedure described in several other papers Žsee, for instance, Ref. w1x.. CH 3 ŽCH 2 . n SH with n s 5, 7, 11 have been used in order to vary in a nanometric way the length of the alkylic chain. Few droplets of a 10y3 M terthiophene solution in ethanol have been deposited by casting onto the surfaces and the samples have
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 1 3 - 2
38
G. Compagnini et al.r Materials Science and Engineering C 15 (2001) 37–39
been carefully cleaned, leaving only the adsorbed part of 3T left. SER and normal Raman spectra have been obtained using an ISA-Jobin Yvon U1000 double monochromator and an electro-cooled Hamamatzu phototube linked to a photon counting chain. The Spectralink acquisition unit by ISA-JY was used in encoding, by an on-line PC, the rough experimental data. The use of an argon ion laser in the 514.5-nm operating mode at low power Žaround 20 mW. avoids a thermal degradation of the samples.
3. Results and discussion Fig. 1 reports four spectra which introduce the argument to be presented. Two of them are characteristic of a bulk Žsolid. 3T sample and of a polythiophene sample obtained by electropolymerization as reported elsewhere w6x. Differences found in comparing these two spectra are in agreement with what is widely known on thiophene oligomers of different length and polymers with a molecular weight distribution. The most useful spectral interval is the 1300–1600 cmy1 where the three characteristic peaks of 3T bulk ŽC`C single bond stretching at 1424 cmy1 , C5C double bond symmetric stretching at 1460 cmy1 and C5C double bond antisymmetric stretching at 1530 cmy1 . are clearly visible. Indeed, in the polythiophene spectrum we only find the 1460 cmy1 frequency as wide line, with a shoulder at higher frequency located ca. 20 cmy1 lower than the C5C antisymmetric signal observed in bulk 3T. These observations are in agreement with other results, which indicate that the increase in the number of thiophene
Fig. 1. SERS spectra of 3T adsorbed on silver with and without a SAM. Raman spectra of bulk 3T and polythiophene are shown for a comparison.
Fig. 2. SERS spectra of 3T adsorbed with and without SAMs of different length. A normal Raman spectrum of solid 3T is also shown for a comparison.
units induces a red-shift of the 1530 cmy1 band, a reduction of its intensity and an increase of the bandwidth w6–8x. On the other side, in the top of Fig. 1, we report two spectra obtained after the adsorption of 3T molecules onto SERS active silver surface, with and without a SAM Ž n s 11. alkanethiol in between. It is clearly observed how the spectral features obtained by means of this alkanethiol SAM are quite similar to the ones obtained for bulk 3T, while the spectrum of 3T adsorbed onto a naked SERS active silver surface closely resembles that obtained for native electropolymerized polythiophene. These observations can be inferred by enlightening three arguments relative to the former SER spectrum. When a SAM is used as spacer the following items are observed: Ži. the C`C single bond stretching signal appears again with remarkable intensity Žespecially when n s 11. as in bulk 3T; Žii. there is a consistent decrease of the linewidth of all bands, which tends to give a situation like to 3T and different from that of polythiophene and terthiophene adsorbed onto naked silver; Žiii. the C5C antisymmetric stretching frequency increases back reaching the value as in bulk terthiophene Ž1530 cmy1 .. In order to study in a nanometric way, the alkanethiol chain length, we also performed SER spectra using molecules with n s 5 and n s 7. This allows varying of ca. 0.14 nm, the distance from the surface of the deposited molecule, for each CH 2 group added to the tail of the alkanethiol. The spectral features of 3T adsorbed on these systems, restricted to the 1300–1600 cmy1 region, are reported in Fig. 2, where they are compared with those coming from bulk 3T and 3T adsorbed onto bare silver.
G. Compagnini et al.r Materials Science and Engineering C 15 (2001) 37–39
Fig. 3. Position and FWHM of the C5C antisymmetric stretching mode of 3T adsorbed as function of the distance from the silver surface.
The already mentioned three items, which differentiate SER spectra with and without a SAM are even more evident. It is, then, enough to space silver surfaces with n s 5 alkanethiol to induce a small shift of the C5C antisymmetric stretching band and a consistent decrease of the bandwidths. These in fact progressively decrease by increasing the distance from the metal–organic interface that is by increasing the alkanethiol chain length. This is a puzzling phenomenon that could be merely ascribed to the presence of a variety of conformational arrangements and also—even if less probably—to a variety of oligomer lengths, both decreasing with the increase of the distance from the silver interface. Since there should be a decrease of the Raman cross-section by increasing the distance from the SERS active substrate, the portion of adsorbed molecules interested to this effect progressively decreases when the SAM becomes longer. In this case, we observe that such an enhancement should be purely electromagnetic in origin w10x. It is worthy to note that several authors w11–13x refer to the red-shift of the C5C antisymmetric stretching band as the strongest evidence of an induced polymerization or at least to the increase of the conjugation length. In this respect, while 3T onto naked silver tends to polymerize w6x, terthiophene molecules on a SAM maintain their chemical individuality. Following some ellipsometric results by Jennings and Laibinis w14x, it is possible to correlate the alkanethiol lengths with the distance at which the head CH 3 group is located above the silver surface w1x. These data Ž0.14 nm for each C`C bond. have been used to plot Fig. 3, in which we report the C5C antisymmetric stretching frequency and the FWHM as function of this distance. It is straightforward to note that the frequency is almost un-
39
changed by varying the SAM spacer although it decreases at around 1508–1510 cmy1 in the case of SER spectrum obtained with 3T on naked silver. On the other side, a monotone decrease of the FWHM is observed with the increase of the distance, thus reaching values close to 3T bulk Žcompatibly with the instruments resolution.. These findings may be interpreted assuming that, even with the shortest nanometric spacer used here Ž n s 5., 3T molecule is impeded to directly interact with silver surface to produce oligomers. This is probed by the invariance of the frequency value of the antisymmetric C5C stretching which remains almost unaffected by the change of the alkanethiol chain length. However, with the shorter spacer used, the conformational situation of the three thiophene rings is randomly distributed among all possible rotameric forms w15x, thus giving large band shapes, which tend to become narrower with the longest chain Ž n s 11.. A much more complex situation has been observed with a longer alkanethiol Ž n s 17., whose puzzling aspects are actually under study.
Acknowledgements We wish to thank Prof. Giovanni Marletta for useful discussions. R.S.C. wishes to thank SuperLab for the kind hospitality during the sabbatic year 1999–2000.
References w1x A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodget to Self-Assembly. Academic Press, Boston, 1991. w2x R.L. Garrel, Anal. Chem. 61 Ž1989. 401. w3x A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 Ž1998. 241. w4x G. Compagnini, B. Pelligra, B. Pignataro, Mater. Res. Soc. Symp. Proc. 501 Ž1998. 109. w5x A. DeBonis, G. Compagnini, R.S. Cataliotti, G. Marletta, J. Raman Spectrosc. 30 Ž1999. 1067. w6x G. Compagnini, A. DeBonis, R.S. Cataliotti, G. Marletta, J. Phys. Chem. 2 Ž2000. 5298. w7x U.K. Sarkar, A.J. Pal, S. Chakrabarti, T.N. Misra, Chem. Phys. Lett. 190 Ž1992. 59. w8x U.K. Sarkar, S. Chakrabarti, T.N. Misra, A.J. Pal, Chem. Phys. Lett. 200 Ž1992. 55. w9x G. Compagnini, B. Pignataro, B. Pelligra, Chem. Phys. Lett. 272 Ž1997. 453. w10x G. Compagnini, C. Galati, S. Pignataro, Phys. Chem. Chem. Phys. 1 Ž1999. 2351. w11x I. Furukawa, M. Akimoto, I. Harada, Synth. Met. 18 Ž1987. 151. w12x G. Louarn, J.P. Buisson, S. Lefrant, D. Fichou, J. Phys. Chem. 99 Ž1995. 11399. w13x E. Agosti, M. Rivola, V. Hernandez, M. Del Zoppo, G. Zerbi, Synth. Met. 100 Ž1999. 101. w14x G.K. Jennings, P.E. Laibinis, J. Am. Chem. Soc. 119 Ž1997. 5208. w15x M. Ciofalo, G. La Manna, Chem. Phys. Lett. 263 Ž1996. 73.