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Sers excitation profile investigation of a cyanine dye adsorbed on silver colloidal particles
Volume
106, number
6
CHEMICAL
PHYSICS
11 May 1984
LETTERS
SERS EXCITATION PROFILE INVESTIGATlON
9F A CYANINE DYE ADSORBED ON SILVER COLLOIDAL
PARTICLES
K. KNEIPP and D. FASSLER CXemisny Deprttnent. Friedrich-SchilIer-Universiry. Steiger 3. DDR-6900 Jeno. German Democratic Republic Received 22 November 1983; in fmal form 10 March 1984
An excitation profiie investigation of 3,3’diethyl-2.2’-benzthiatrimethinecyanine
adsorbed on silver sol shows, for the
pure surface enhancement, a relative minimum near thedye absorption (about 550 nm) and a strong increase for yellowred wavelengths, which is esphined qualitatively in terms of an “electromagnetic” model.
1. Introduction Surfaceenhanced Raman scattering (SERS) has been the subject of intense investigation in recent years and has become an important tool for studying molecules adsorbed on metal surfaces [ 1.21. In the early years after its discovery, much attention was focused on the enhancement of Raman scattering from colourless molecules, i.e. on the enhancement of normal Raman scattering. In an effort to examine more closely the phenomenon during the past few years, dyes adsorbed on several (rough) metal substrates (electrochemical systems [3,4], silver-island films [5,6] and silver sols [7-12j)have been studied. Investigation of the Raman scattering of dye molecules near a metal surface is important from two points of view: (i) Some methodological problems connected with the Raman spectroscopic investigation of these molecules in solution, resulting especially from strong fluorescence, can be overcome [5,10,12]. (ii) The elucidation of enhanced Raman scattering of dyes can give information about the enhancement mechanisms. Very interesting, for instance, is the question of correlations between surface enhancement and the specific dye resonance [5,8,11,12]. In this paper we report on measurements of excitation profiles of a polymethine dye (3,3’diethyl2,2’-benzthiatrimethinecyaniue) adsorbed on colloidal silver. The experimental results will be discussed using 498
a model of an “‘electromagnetic” enhancement mechanism and assuming a new (increased) full width for the excited state of the adsorbed molecule compared with the solvated molecule.
2. Experimental The silver sol was prepared according to a procedure described in ref. [9]. As can be seen from transmission electron micrographs, the sol contains particles of a broad range of diameters (25-130 nm) and shows some clustering. The absorption spectrum is a broad curve with a maximum at a20 nm. The measurements were done on a 1 : 1 mixture of an ~10~~ mol/P silver sol and a 5 X 10m6 mol/Q aque-
ous dye solution. Fig. 1 shows the absorption spectrum of the sol, the aqueous dye solution and the sample mixture used in the experiments. As can be seen from fig. 1, the adsorption does not shift the absorption maximum of the dye. The experimental apparatus for Raman spectroscopy was the same as described in ref. [12]. In addition to an Ar+ laser, a Rh-6G dye laser was used for excitation in the yellow-red region. The spectra were measured in a 90” scattering geometry with a sample length of 10 mm. The samples were not stable for some hours but
Volume
106. number 6
ZOO00 -
CHEMICAL
18000
PHYSICS
LETTERS
6 000
v km-7
Fig. 1. Absorption spectra of silver sol (1). 2.5 X lo* mol/P aqueous dye solution (2). 1 : 1 mixture of silver sol with 5 X IO* mol/P dye solution (3). and calculated Raman eacitation profile (4) (see text). Optical path for (l)-(3) was 0.2 cm.
showed a sufficient stability during the time of Raman spectra recording.
The measurement of the
decrease of the Raman intensity as a function of time for different excitation wavelengths gave no indication of a dependence of this effect on the excitation wavelength. To guarantee the reproducibility, every spectrum was measured with a new sample in a defined time interval after mixing.
3.
Results and discussion
Fig. 2 shows seven Raman spectra of the adsorbed dye measured with various excitation wavelengths. In comparison with Raman measurements on the same dye in solution [ 131, the spectra show a strong suppression of fluorescence. A strong similarity between the Raman spectra of the solvated [ 131 and adsorbed molecule is observed. Fig. 3 shows the intensity ratio
Fig. 2. Raman spectra thiarrimethinecyanine lengths.
of the adsorbed measured with
corrected for v4 dependence, re-absorption
of the scattered
3,3’diethyl-2,2’-benzvarious excitation wave-
instrument
response,
light and, if rkcessary,
fluorescence background, for the eight strongest Raman lines. The same vibrations (with only small deviations in wavenumbers) can be found in the spectra of the other representatives of the homologous series in solution [ 131 as well as adsorbed on silver colloids [ 121. For blue and blue-green excitation wavelengths, the intensity ratio increases with in499
Volume
CHEMlCAL
106. number 6
PHYSICS
11 May-1984
LE-ITERS
200
ads_mol.,zad~moL hL-457.g,m, versus excitation wavelength for 3,3’-diethyl--.- 7 7’-benzthiatrimethinecyanine Fii. 3. Intensity ratio ZhL sorbed on silver x 358 cm-t, + 508 cm-‘. * 597 cm-’ , o 784 mm’, o 835 cm-‘, l 1134 cm-‘, o 1233 cm-‘, 3 1338 cm-‘.
creasing XL; near the dye absorption maximum the ratio becomes a flat curve; and for A, greater than 580 nm it shows a strong increase again. (The behaviour of the 1338 cm- 1 line seems ambiguous and therefore it will not be further interpreted in this paper.1 ._ Other authors have measured the excitation spectrum of SERS of py&line adsorbed on silver colloids, similar to our sols [14]. They also observed an in500
ad-
crease of the scattering intensity from blue to red excitation wavelengths, but without the Hat part in the curve which we found near the dye absorption. An experimental separation of the pure surface enhancement according to lads.mol./,solv.mol. AL hL between
457 and 593 nm was not possible because
Volume
CHEhllCAL
106, number 6
of the strong fluorescence ity of the dye in solution.
PHYSICS
and the small photo-stabil-
From measurements on the dye and two other homologues of it in solution, it can be concluded that the observed Raman lines are due to the chromophoric system of the polymethine [ 13,151. We transferred the results of REP measurements from the respective monomethine of the homologous series to the trimethine studied in this paper and calculated a theoreticd REP curve for it in solution, according to
t161 1
IRS T
a [(u, - q)’
+ $1
[(V, - us)2 + $1
’
(1)
which was used in ref. [ 151 for fitting the experimental values of the monomethine. In (I), ve, vL and us are the electronic absorption frequency and the exciting and scattered frequency, respectively. y is the full width of the excited state. Fig. 4 shows the relative net surface enhancement computed from the curve in fig. 3, and a dye solution excitation profile calculated with ve = 18083 cm-r,
I 1 May 1984
LETTERS
for pyrromethene adsorbed on silver colloids was measured and a minimum within the ab-
wavelength
band of the dye was found and discussed in terms of a “chemical” model. In this paper, the exci-
sorption
tation profile is discussed in an “electromagnetic” mode1 [S]. The excitation field as well as the scattered field will be enhanced by factors A(IQ) and A, respectively. In addition, the adsorbed molecules should have an increased width yads. of the excited state compared with the width 7fr_ of the solvated molecule. As in ref. [5], we assume that the purely molecular parameters are unchanged by the presence of the silver colloids. (A certain justification is given by the similarity between the Raman spectra
of solvated and adsorbed molecules.) According to an approximation of formula (1) l/v;
a I(% - vr_)’ + $1-1
and the above assumptions,
the pure surface enhance-
ment can be calculated as ads.mol. IRS solv.mol.
(v,
-
q_IZ + 7;rcc
‘+ -r&s. (r’e - Vf_)-
1IA(qyI.l(US)i? 7-
~=1500cm-~andaRamanshiftu,of1000cm-1. It can be seen from fig. 4 that there is a relative
IRS
rninimurn of surface enhancement near the dye absorption. In ref. [ 111 the surface Raman enhance-
For fitting the experimental curve of fig. 4. we computed the field intensity enhancements from the absorption spectrum of the silver sol (in the presence of
ment (relative to solution values) versus excitation
(2)
the dye) according
to [ 17 ] IeLl’ieSl’Xj_XS
ISRRS z const.
[
(E$L(EZ)S
1
Abs,AbsS,
(3)
where Abst and AbsS are the fractional absorptions and eL = (Ed),_ + i(el)L and es = (el)S + i(e& are the metal die!ectric functions at the excitation (x;) and
Raman-scattered
@S) wavelengths,
respectively.
Curve (4) in fig. 1 shows the calculated SER exciration profde of a 1000 cm-* Raman band according
to formula (3) with the dielectric function silver from ref. [ 181. Using this excitation
Fig. 4. Relative
net surface enhancement
benzthiatrimethinecyanine tation wavelength (-). ment (- - -) (see text).
of 3.3’diethyL2.2’adsorbed on silver sol versus exci-
and calculated
net surface enhance-
data of
profile for the field enhancements, the experimental curve of fig. 4 can be fitted in a first approximation with formula (2), setting 7ads. = 3rfrce = 4500 cm-l (dotted curve in fig. 4; for XL = 496 run the theoretical curve was fitted to the experimental value). In summary, the results indicate that the observed excitation profde of SEW of a dye absorbed on silver 501
Volume
106, number
CHEMICAL
6
colloids, in particular
the relative minimum
PHYSICS
within
the dye absorption and the enhancement in the yel-
low-red region, can be qualitatively explained in an “electromagnetic” model. Further investigation of the different behaviour of the different Raman lines is underway. Too small theoretical values compared with the experimental curve at longer excitation wavelengths could give a hint to an additional “chemical” mechanism. Acknowledgement
LETTERS
ArQca and R.O. Loutty, J. Raman Spectry. 12 (1982) 262. Chem. Phys. Letters 74 (1980) 125; 79 I71 ME. Lippitsch. (1981) 244. S. Efrima. B. Katz and 2. Priel, I81 A. Bachackashvilli. Chem. Phys. Letters 94 (1983) 571. 191 P.C. Lee and D. hleisel. J. Phys. Chem. 86 (1982) 3391. Opt. Spectry. 52 I101 A.V. Baranov and Y.S. Bobovitch. (1982) 385; Soviet Phys. JETP Letters 35 (1982) 149. IllI M.E. Lippitsch and F.R. Aussenegg, in: Springers series in chemical physics, Vol. 33. Surface studies with lasers (Springer, Berlin. 1983) p. 41. 1121 K. Kneipp, G. Hinzmann and D. Fassler, Chem. Phys. Letters 99 (1983) 503. Wemcke. K. Lenz, H.-J. 1131 A. Lau. K. Kneipp.W.
I61 R.
Weigmann,
We are grateful to Mr. G. Hinzmann for preparing the sols, to Dr. E. DBpel and Dr. W. Dietel for their support concerning the dye laser and to Mr. L. Horn for the electron micrographs.
1141 1151
I’61
References
1171 [ 11 A. Otto. Appl. Surface Sci 6 (1980) 309. 12 1 TE. Furtak and J. Reyes, Surface Sci. 93 ( 1980) 35 I_ 131 D.L. Jeanmaire and RP. van Duyne, J. Electroanal. Chem. [4]
J.R.
84 (1977)
Campbell
1.
and J.A.
Creighton,
143 (1983) 353. [S 1 DA. Weitz. S. Caroff, J.1. Cersten Chem. Phys. 78 (1983) 5324.
J. Elecuoanal.
Chem.
and A. Nitzan.
J.
I1 May 1984
1181
D. Pas&x
and G. Hinzmann,
Advan.
Mol.
Relasation Interaction Processes 24 (1982) 27. R. Dornhaus and RX. Chang. Appl. Surface Sci. 6 (1980) 345. K. Kneipp, C. Hinzmann and D. Fasder, Wiss. Z. Friedrich-Schiller-Universitit Jena, Math.-Naturwiss. R. 31 (1982) 1119. M.L.A. Temperini, 0. Sala and H-l. Bernstein, J. Raman Spectry. 7 (1978) 295. DA. Weitz. S. Garoff and T.J. Gramila, Opt. Letters 7 (1982) 168; JA. Creighton, in: Springers series in chemical physics, Vol. 33. Surface studies with laser (Springer, Berlin, 1982) p. 55. P.B. Johnson and R.W. Christy. Phys. Rev. 86 (1972) 4370.
106, number
6
CHEMICAL
PHYSICS
11 May 1984
LETTERS
SERS EXCITATION PROFILE INVESTIGATlON
9F A CYANINE DYE ADSORBED ON SILVER COLLOIDAL
PARTICLES
K. KNEIPP and D. FASSLER CXemisny Deprttnent. Friedrich-SchilIer-Universiry. Steiger 3. DDR-6900 Jeno. German Democratic Republic Received 22 November 1983; in fmal form 10 March 1984
An excitation profiie investigation of 3,3’diethyl-2.2’-benzthiatrimethinecyanine
adsorbed on silver sol shows, for the
pure surface enhancement, a relative minimum near thedye absorption (about 550 nm) and a strong increase for yellowred wavelengths, which is esphined qualitatively in terms of an “electromagnetic” model.
1. Introduction Surfaceenhanced Raman scattering (SERS) has been the subject of intense investigation in recent years and has become an important tool for studying molecules adsorbed on metal surfaces [ 1.21. In the early years after its discovery, much attention was focused on the enhancement of Raman scattering from colourless molecules, i.e. on the enhancement of normal Raman scattering. In an effort to examine more closely the phenomenon during the past few years, dyes adsorbed on several (rough) metal substrates (electrochemical systems [3,4], silver-island films [5,6] and silver sols [7-12j)have been studied. Investigation of the Raman scattering of dye molecules near a metal surface is important from two points of view: (i) Some methodological problems connected with the Raman spectroscopic investigation of these molecules in solution, resulting especially from strong fluorescence, can be overcome [5,10,12]. (ii) The elucidation of enhanced Raman scattering of dyes can give information about the enhancement mechanisms. Very interesting, for instance, is the question of correlations between surface enhancement and the specific dye resonance [5,8,11,12]. In this paper we report on measurements of excitation profiles of a polymethine dye (3,3’diethyl2,2’-benzthiatrimethinecyaniue) adsorbed on colloidal silver. The experimental results will be discussed using 498
a model of an “‘electromagnetic” enhancement mechanism and assuming a new (increased) full width for the excited state of the adsorbed molecule compared with the solvated molecule.
2. Experimental The silver sol was prepared according to a procedure described in ref. [9]. As can be seen from transmission electron micrographs, the sol contains particles of a broad range of diameters (25-130 nm) and shows some clustering. The absorption spectrum is a broad curve with a maximum at a20 nm. The measurements were done on a 1 : 1 mixture of an ~10~~ mol/P silver sol and a 5 X 10m6 mol/Q aque-
ous dye solution. Fig. 1 shows the absorption spectrum of the sol, the aqueous dye solution and the sample mixture used in the experiments. As can be seen from fig. 1, the adsorption does not shift the absorption maximum of the dye. The experimental apparatus for Raman spectroscopy was the same as described in ref. [12]. In addition to an Ar+ laser, a Rh-6G dye laser was used for excitation in the yellow-red region. The spectra were measured in a 90” scattering geometry with a sample length of 10 mm. The samples were not stable for some hours but
Volume
106. number 6
ZOO00 -
CHEMICAL
18000
PHYSICS
LETTERS
6 000
v km-7
Fig. 1. Absorption spectra of silver sol (1). 2.5 X lo* mol/P aqueous dye solution (2). 1 : 1 mixture of silver sol with 5 X IO* mol/P dye solution (3). and calculated Raman eacitation profile (4) (see text). Optical path for (l)-(3) was 0.2 cm.
showed a sufficient stability during the time of Raman spectra recording.
The measurement of the
decrease of the Raman intensity as a function of time for different excitation wavelengths gave no indication of a dependence of this effect on the excitation wavelength. To guarantee the reproducibility, every spectrum was measured with a new sample in a defined time interval after mixing.
3.
Results and discussion
Fig. 2 shows seven Raman spectra of the adsorbed dye measured with various excitation wavelengths. In comparison with Raman measurements on the same dye in solution [ 131, the spectra show a strong suppression of fluorescence. A strong similarity between the Raman spectra of the solvated [ 131 and adsorbed molecule is observed. Fig. 3 shows the intensity ratio
Fig. 2. Raman spectra thiarrimethinecyanine lengths.
of the adsorbed measured with
corrected for v4 dependence, re-absorption
of the scattered
3,3’diethyl-2,2’-benzvarious excitation wave-
instrument
response,
light and, if rkcessary,
fluorescence background, for the eight strongest Raman lines. The same vibrations (with only small deviations in wavenumbers) can be found in the spectra of the other representatives of the homologous series in solution [ 131 as well as adsorbed on silver colloids [ 121. For blue and blue-green excitation wavelengths, the intensity ratio increases with in499
Volume
CHEMlCAL
106. number 6
PHYSICS
11 May-1984
LE-ITERS
200
ads_mol.,zad~moL hL-457.g,m, versus excitation wavelength for 3,3’-diethyl--.- 7 7’-benzthiatrimethinecyanine Fii. 3. Intensity ratio ZhL sorbed on silver x 358 cm-t, + 508 cm-‘. * 597 cm-’ , o 784 mm’, o 835 cm-‘, l 1134 cm-‘, o 1233 cm-‘, 3 1338 cm-‘.
creasing XL; near the dye absorption maximum the ratio becomes a flat curve; and for A, greater than 580 nm it shows a strong increase again. (The behaviour of the 1338 cm- 1 line seems ambiguous and therefore it will not be further interpreted in this paper.1 ._ Other authors have measured the excitation spectrum of SERS of py&line adsorbed on silver colloids, similar to our sols [14]. They also observed an in500
ad-
crease of the scattering intensity from blue to red excitation wavelengths, but without the Hat part in the curve which we found near the dye absorption. An experimental separation of the pure surface enhancement according to lads.mol./,solv.mol. AL hL between
457 and 593 nm was not possible because
Volume
CHEhllCAL
106, number 6
of the strong fluorescence ity of the dye in solution.
PHYSICS
and the small photo-stabil-
From measurements on the dye and two other homologues of it in solution, it can be concluded that the observed Raman lines are due to the chromophoric system of the polymethine [ 13,151. We transferred the results of REP measurements from the respective monomethine of the homologous series to the trimethine studied in this paper and calculated a theoreticd REP curve for it in solution, according to
t161 1
IRS T
a [(u, - q)’
+ $1
[(V, - us)2 + $1
’
(1)
which was used in ref. [ 151 for fitting the experimental values of the monomethine. In (I), ve, vL and us are the electronic absorption frequency and the exciting and scattered frequency, respectively. y is the full width of the excited state. Fig. 4 shows the relative net surface enhancement computed from the curve in fig. 3, and a dye solution excitation profile calculated with ve = 18083 cm-r,
I 1 May 1984
LETTERS
for pyrromethene adsorbed on silver colloids was measured and a minimum within the ab-
wavelength
band of the dye was found and discussed in terms of a “chemical” model. In this paper, the exci-
sorption
tation profile is discussed in an “electromagnetic” mode1 [S]. The excitation field as well as the scattered field will be enhanced by factors A(IQ) and A, respectively. In addition, the adsorbed molecules should have an increased width yads. of the excited state compared with the width 7fr_ of the solvated molecule. As in ref. [5], we assume that the purely molecular parameters are unchanged by the presence of the silver colloids. (A certain justification is given by the similarity between the Raman spectra
of solvated and adsorbed molecules.) According to an approximation of formula (1) l/v;
a I(% - vr_)’ + $1-1
and the above assumptions,
the pure surface enhance-
ment can be calculated as ads.mol. IRS solv.mol.
(v,
-
q_IZ + 7;rcc
‘+ -r&s. (r’e - Vf_)-
1IA(qyI.l(US)i? 7-
~=1500cm-~andaRamanshiftu,of1000cm-1. It can be seen from fig. 4 that there is a relative
IRS
rninimurn of surface enhancement near the dye absorption. In ref. [ 111 the surface Raman enhance-
For fitting the experimental curve of fig. 4. we computed the field intensity enhancements from the absorption spectrum of the silver sol (in the presence of
ment (relative to solution values) versus excitation
(2)
the dye) according
to [ 17 ] IeLl’ieSl’Xj_XS
ISRRS z const.
[
(E$L(EZ)S
1
Abs,AbsS,
(3)
where Abst and AbsS are the fractional absorptions and eL = (Ed),_ + i(el)L and es = (el)S + i(e& are the metal die!ectric functions at the excitation (x;) and
Raman-scattered
@S) wavelengths,
respectively.
Curve (4) in fig. 1 shows the calculated SER exciration profde of a 1000 cm-* Raman band according
to formula (3) with the dielectric function silver from ref. [ 181. Using this excitation
Fig. 4. Relative
net surface enhancement
benzthiatrimethinecyanine tation wavelength (-). ment (- - -) (see text).
of 3.3’diethyL2.2’adsorbed on silver sol versus exci-
and calculated
net surface enhance-
data of
profile for the field enhancements, the experimental curve of fig. 4 can be fitted in a first approximation with formula (2), setting 7ads. = 3rfrce = 4500 cm-l (dotted curve in fig. 4; for XL = 496 run the theoretical curve was fitted to the experimental value). In summary, the results indicate that the observed excitation profde of SEW of a dye absorbed on silver 501
Volume
106, number
CHEMICAL
6
colloids, in particular
the relative minimum
PHYSICS
within
the dye absorption and the enhancement in the yel-
low-red region, can be qualitatively explained in an “electromagnetic” model. Further investigation of the different behaviour of the different Raman lines is underway. Too small theoretical values compared with the experimental curve at longer excitation wavelengths could give a hint to an additional “chemical” mechanism. Acknowledgement
LETTERS
ArQca and R.O. Loutty, J. Raman Spectry. 12 (1982) 262. Chem. Phys. Letters 74 (1980) 125; 79 I71 ME. Lippitsch. (1981) 244. S. Efrima. B. Katz and 2. Priel, I81 A. Bachackashvilli. Chem. Phys. Letters 94 (1983) 571. 191 P.C. Lee and D. hleisel. J. Phys. Chem. 86 (1982) 3391. Opt. Spectry. 52 I101 A.V. Baranov and Y.S. Bobovitch. (1982) 385; Soviet Phys. JETP Letters 35 (1982) 149. IllI M.E. Lippitsch and F.R. Aussenegg, in: Springers series in chemical physics, Vol. 33. Surface studies with lasers (Springer, Berlin. 1983) p. 41. 1121 K. Kneipp, G. Hinzmann and D. Fassler, Chem. Phys. Letters 99 (1983) 503. Wemcke. K. Lenz, H.-J. 1131 A. Lau. K. Kneipp.W.
I61 R.
Weigmann,
We are grateful to Mr. G. Hinzmann for preparing the sols, to Dr. E. DBpel and Dr. W. Dietel for their support concerning the dye laser and to Mr. L. Horn for the electron micrographs.
1141 1151
I’61
References
1171 [ 11 A. Otto. Appl. Surface Sci 6 (1980) 309. 12 1 TE. Furtak and J. Reyes, Surface Sci. 93 ( 1980) 35 I_ 131 D.L. Jeanmaire and RP. van Duyne, J. Electroanal. Chem. [4]
J.R.
84 (1977)
Campbell
1.
and J.A.
Creighton,
143 (1983) 353. [S 1 DA. Weitz. S. Caroff, J.1. Cersten Chem. Phys. 78 (1983) 5324.
J. Elecuoanal.
Chem.
and A. Nitzan.
J.
I1 May 1984
1181
D. Pas&x
and G. Hinzmann,
Advan.
Mol.
Relasation Interaction Processes 24 (1982) 27. R. Dornhaus and RX. Chang. Appl. Surface Sci. 6 (1980) 345. K. Kneipp, C. Hinzmann and D. Fasder, Wiss. Z. Friedrich-Schiller-Universitit Jena, Math.-Naturwiss. R. 31 (1982) 1119. M.L.A. Temperini, 0. Sala and H-l. Bernstein, J. Raman Spectry. 7 (1978) 295. DA. Weitz. S. Garoff and T.J. Gramila, Opt. Letters 7 (1982) 168; JA. Creighton, in: Springers series in chemical physics, Vol. 33. Surface studies with laser (Springer, Berlin, 1982) p. 55. P.B. Johnson and R.W. Christy. Phys. Rev. 86 (1972) 4370.