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Raman scattering by optically absorbing molecules adsorbed on “smooth” Ag and Au surfaces: Crystal violet
~
Solid State Communications, Vol.46,No.8, pp.595-599, ~____jPrinted in Great Britain.
1983.
0038-1098/83/200595-05503.00/0 Pergamon Press Ltd.
RAMAN SCATTERING BY OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES: CRYSTAL VIOLET E. Burstein* Physics Department, University of Pennsylvania, Philadelphia, PA 19104 and G. Burns and F. H. Dacol IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
(Received 28 January 1983 by G. Burns)
We report data on the resonant Raman scattering (RS) by crystal violet (CV) adsorbed on Ag and Au surfaces. The data show that the RS and luminescence by CV on a smooth Au surface is greater by an order of magnitude than that on a smooth Ag surface. These results, which are contrary to what one expects from mechanisms that depend on the complex dielectric constant of the metal substrate, are attributed to contributions that depend on the chemical bonding of the CV molecules with the metal substrate.
INTRODUCTION. The role played by surface roughness in Raman scattering (RS) by molecules adsorbed on noble metals, and in particular the role played by resonant excitation of localized plasmons in enhancing the incident and scattered EM fields, is now fairly well established 1. Unfortunately, it is not possible to obtain an accurate estimate of the magnitudes of the enhancements of the E M fields by surface roughness and, as a consequence, it is not possible to ascertain the relative magnitudes of the other contributions (e.g., electron hole pair excitations, charge-transfer excitations, vibration modulated electron density of the substrate, etc. 2) to the RS cross-section of the adsorbed molecules. A way around this difficulty is to carry out measurements of the resonance enhanced RS by adsorbed molecules on a smooth metal surface, whose RS cross-sections are sufficiently large to be observable without the enhancement of the incident and scattered E M fields. Crystal violet satisfies this criterion, as indicated by the fact that we can detect RS by a monolayer of crystal violet adsorbed on a glass substrate. The surface roughness enhanced resonant RS by crystal violet adsorbed on an anodized Ag electrode has been reported by Jeanmaire and Van Dyne 3. Hagen and coworkers 4 have used resonance enhancement to attain greater sensitivity in measur-
ing the RS by adsorbed molecules on Ag and Pt electrodes. They observed a resonance enhanced RS by p-nitroso-dimethyaniline ( p - N D M A ) molecules adsorbed on Ag and Pt electrodes, but did not observe any RS by p - N D M A adsorbed on a Au electrode. The use of resonance enhancement to increase the RS intensity is a well established procedure which has enabled one to observe "forbidden" Raman scattering in semiconductors 5 and also to observe RS by charge carrier excitations in quasi-2dimensional plasmas at semiconductor surfaces 6. Yamada and coworkers 7 have, in fact, used resonant RS to observe the RS by adsorbed molecules on non-metallic solid surfaces. We have measured the resonant RS by crystal violet (CV) adsorbed from dilute aqueous solutions on "smooth" evaporated films of Ag and Au. The films were maintained at a negative potential, from the moment of immersion to the time of removal from the solution, in order to avoid (chemical) etching of the metal surface and also to reduce any sulfide or oxide that may have formed before immersion. Our data show that the RS cross-section of CV adsorbed on a smooth Ag surface is essentially the same as that of CV adsorbed on glass. However, the luminescence of CV on a smooth Ag surface is weaker by a factor of ~ 2 0 than that of CV adsorbed on glass. On the other hand, the RS intensity of CV adsorbed on a smooth Au surface is greater by almost an order of magnitude than that of CV adsorbed on a smooth Ag surface, even at frequencies above the onset of interband transions in Au,
*Research supported in part by the N S F - M R L at the University of Pennsylvania and carried out in part while on sabbatical leave as Guggenheim Fellow at the IBM Research Center. 595
596
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
where there is little, if any, enhancement of the EM fields by residual surface roughness. Moreover, the luminescence of CV adsorbed on a smooth Au surface is greater by an order of magnitude than that of CV adsorbed on a smooth Ag surface and, surprisingly, only moderately weaker than that of CV adsorbed on a glass surface. That the RS and luminescence of CV on a smooth Au surface is greater than the RS and luminescence of CV on a smooth Ag surface is opposite to what one would expect on the b~tsis of mechanisms that depend on the magnitudes of the real and imaginary parts of the dielectric constants of the metal substrate. It provides clear-cut evidence for contributions that depend on the specific nature of the chemical bonding of CV with the metal substrate, and on the character (e.g., frequency, strength and width) of the electronic excitations of the metal-molecule complexes that are formed. CV (tri-p-dimethylaminophenyl-carbonium chloride) is a triphenyl-methane type dye whose dilute aqueous solution exhibits two overlapping absorption bands at 535nm and 605nm and a luminescence band at ~ 6 5 0 n m (Fig. 1). The resonant RS by CV in aqueous solution has been studied in some detail by Angeloni, Smulevich and Marzocchi 8. They find that the Raman spectrum consists of two- and three-component multiplets that arise from the coupled vibrations of the three
NMe2
Me2N
NMe 2 M
e
2
+ NMe 2
I
~
NMe 2
I
Crystal Violet
N
I f
I
r~bsorption
E
900 Fig. 1.
800
700 600 Wavelength (nm)
500
400
The CV structure and the adsorption and emission spectra of, CV in solution. (After F. C. Adams and W. T. Simpson, J. Molecular Spectroscopy 3, 363 (1959).
Vol. 46, No. 8
dimethyl-aniline groups, and of features corresponding to the breathing and bending modes of the three "rigid" dimethyl-aniline groups relative to the central carbon atom. EXPERIMENTAL. Ag and Au films, =1500/~ thick, were vapor deposited onto chem-mechanically polished p-type S i ( l l l ) wafers which were first coated with a 200.g. layer of Cr for better adhesion of the noble metal. In the case of Au, the freshly evaporated films were introduced into an electrochemical cell containing an 0.05 M aqueous solution of KC1 at a potential of - 0 . 0 6 V with respect to a standard calomel electrode. A PAR #179 potentiostat and a PAR #75 programmer were used for this purpose. After 100 seconds, a weak CV solution was added to attain the desired concentration of CV in the electrolyte, e.g. 10 - 5 M. After an additional 15 minutes the film was removed while maintaining the - 0 . 0 6 V potential, washed in agitated deionized water for 5 minutes and dried in a stream of dry nitrogen. The same procedure was used for the Ag films, except that 0.01M KCL was used and, after immersion at a - 0 . 0 6 V potential, the potential was set at - 1 . 1 V to attain an evolution of hydrogen, and then maintained at - 1 . 0 V until a total charge of 60 coulombs passed between the Ag film and a Pt counter electrode. CV was also deposited onto smooth glass (e.g., suprasil) slides by dipping the slides into a 10 - 5 M aqueous solution of CV, and then washing and drying as above. To obtain "quantitative" information about the effect of surface roughness, CV was also deposited onto Au and Ag surfaces which had been processed by an oxidation-reduction cycle. Adsorption isotherm measurements were carried out using both RS and luminescence to monitor the surface density of adsorbed molecules as a function of the concentration of CV in solution. The RS and luminescence signals for CV adsorbed on glass, and on smooth Ag and Au films, were found to increase with increasing concentration at concentrations below 10 - 7 M and to be relatively independent of concentration in the range from 1 0 - 7 M to 10-4M. We therefore believe that, in the case of the samples used in our studies, the CV coverage was roughly that of a monolayer. Moreover we surmise from the large size and essentially planar character of the CV molecule that the adsorbed molecules probably lie "flat" on the surface. Because of the large solubility of CV in water, we believe that it is unlikely that there is any appreciable dimer, or multilayer, formation. This is also borne out by the similarity of the Raman spectra for CV in solution to the spectra for CV adsorbed on the metal substrates. All of the spectra were measured in air using 10mW, or less, of incident laser radiation. To further decrease the intensity at the metal surfaces, the
Vol. 46, No. 8
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
597
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ENERGY SHIFT (cm-I) Fig. 2.
Typical Raman spectra, at 5145/k for CV in solution, and for CV adsorbed on anodized Ag and Au.
incident laser radiation was focussed by a cylindrical lens into a l m m x 4 m m image. However, even at these low intensities, some degradation of the Raman and luminescence signals with time was noted. RESULTS. Typical Raman spectra for CV molecules in dilute aqueous solution and for CV molecules adsorbed on Ag and on Au surfaces are shown in Fig. 2. The frequencies of the observed peaks in the spectra are given in Table 1. The spectra shown in Fig. 2 are those of CV adsorbed on electrochemically anodized Ag and Au surface since they have high signal to noise ratios. The solution spectrum consists of doublets and triplets that are associated with the degenerate modes of the dimethyly-aniline groups. As noted by Angeloni et al. 8, one component of each multiplet is polarized. By contrast, all of the peaks in the Raman spectra for CV molecules adsorbed on the smooth Ag and smooth Au surfaces are depolarized. Apart from some changes in the character of the peaks corresponding to the degenerate modes of the dimethyly-aniline groups, there is little change in their frequencies. On the other hand, the two low frequency peaks which are assigned to the A-symmetry breathing vibrations of the central carbon atom bonds and to the Esymmetry degenerate bending of the central carbon atom bonds, exhibit sizeable increases in frequency (Table 1). The peak of the breathing mode shifts from 208 cm -1 for CV in solution to 216 cm -1 and 234 cm - 1 for CV adsorbed on Ag and Au, respectively. The peak of the central carbon atom bond bending mode shifts from 339 cm -1 to 346 cm -1 and 359 cm -1 respectively. The larger frequency shift exhibited by the CV molecules adsorbed on Au are indicative of a stronger bonding of the CV molecules to the Au substrate than to the Ag substrate. Our data on the excitation-frequency (e.g. 488.0nm, 514.5nm, 568.2nm and 647.1nm) dependence of the RS intensity of the peaks for CV mole-
Table 1 Frequencies of the CV vibrational modes. Symm.* CV(Soln.) A E E A E E A A E E A A E E E E A
208 cm -1 339 427 445 564 729 775 814 840 919 940 1179 1305 1374 1452 1592 1624
CV I Ag
CV I Au
216 cm -1 346 424 444 570 733 763 818 840 915 943 1180 1297 1372 1445 1591 1622
234 cm -1 359 428 444 -737 762 815 840 920 945 1183 1300 1373 1448 1590 1622
*assignments by Angeloni et al. 8. cules adsorbed on Au are similar to those for CW molecules in solution. Thus, the intensities of the peaks at 234 cm -1 and 359 cm -1 exhibit a very strong enhancement (~103) in going from 488.0nm to 647.1nm. This enhancement is attributed by Angeloni et al. 8 to a F r a n c k - C o n d o n type mechanism associated with the resonance adsorption bands at 535-605nm. The enhancement of the peak at 1624 cm -1, on the other hand, is more moderate (~102 ) and is attributed to mechanisms involving the vibronic coupling of higher energy excitations at 240-300nm with the lower energy excitations at 535-605nm. Data on the relative intensities of the RS by the 1624 cm -1 mode of CV adsorbed on smooth Au
598
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
and Ag surfaces and, for comparison purposes on glass, are given in Table 2. What is particularly striking is the fact that the RS intensities for the CV molecules adsorbed on smooth Au are an order of magnitude greater than those for the CV molecules adsorbed on smooth Ag. This is the case even at excitation frequencies above the onset of interband transitions in Au at 520nm, where there is no enhancement of the EM fields by residual surface roughness. Moreover, as can be seen from the luminescence data in Table 3, the luminescence intensity of the CV molecules adsorbed on smooth Au is also greater by about an order of magnitude than that of the CV molecules adsorbed on smooth Ag. Table 2 and Table 3 also contains the data on the RS and luminescence intensities, respectively, of CV adsorbed on anodized Au and Ag surfaces. The enhancements of the RS and luminescence intensities by surface roughness, F = Irough/Ismooth , are smaller for Au than for Ag by two to three orders of magnitude. This is due, in part, to the fact that the imaginary part of the dielectric constant is larger for Au than for Ag, and in part to the fact that the amplitude of the roughness is smaller for anodized Au than for anodized Ag. We note in particular that in the case of CV adsorbed on Au there is no enhancement of the RS by surface roughness at 488.0nm, an excitation frequency which is above the onset of interband transitions. There is however an appreciable enhancement of the luminescence at 488.0nm (F ~ 7) since although the excitation frequency is above the onset of interband transitions, the emission frequency ( ~ 6 5 0 n m ) is well below the onset. DISCUSSION. The data for the RS and luminescence of adsorbed CV on Au and Ag are contrary to what one would expect on the basis of mechanisms, that depend on the magnitudes of the real and imaginary parts of the dielectric constants of the metal substrate. One would expect the enhancement of the EM fields by residual roughness to be much greater for Ag than for Au substrates. One would also expect the damping of the electronic excitations of the adsorbed molecules by the metal substrate to be greater for Au than for Ag and, as a consequence, one would expect a weaker resonance enhancement of the RS and a stronger quenching of the luminescence by CV adsorbed on Au. Also we note that the shift in the intramolecular excitation frequency of the adsorbed molecules due to the image dipole field (e.g., the image dipole contribution to the self-energy of the excitation) is, in effect, already taken into account when the RS measurements are carried out at the resonance frequency. Thus the major effect of the interaction, between the electronic excitations of the adsorbed molecules and the electronic excitations of the metal
Vol. 46, No. 8
Table 2
Raman Scattering Intensities (103 c n t / s ) of Adsorbed CV (5 mW incident laser intensity). ~'i
488.0nm
514.5nm
568.2nm
647.1nm
Smooth Au 215 cm -1 1624cm -t
.02 .21
.11 1.5
1.6 9
14 21
1 6 2 4 c m -1
.032
.10
.21
(mbl)
Glass 1 6 2 4 c m -1
.083
.10
(mbl)
(mbl)
.21
2.2
70
600
26
31
310
1900
1.5
8
29
310
1480
--
Smooth Ag
Rough Au 1 6 2 4 c m -1
Rough Ag 1 6 2 4 c m -1 F(Au) 161.54 c m - l l FtAg) 1624 cm -1
810
(mbl = masked by luminescence) Table 3 Luminescence Intensities (103 c n t / s ) of Adsorbed CV (5 mW incident laser intensity). Alum ~ 650nm. hi glass
488.0nm
514.5nm
568.2nm
647.1nm
50
70
160
120
smooth Au
9
28
100
75
smooth Ag
2
3
4
7
rough Au
66
73
390
1500
rough Ag
370
210
3000
3200
F(Au)
7.3
2.6
3.9
20
F(Ag)
190
70
750
460
substrate, is to increase the damping of the electronic excitations of the molecules. The net effect would therefore be to decrease the RS cross-section of the adsorbed molecules at resonance, and the decrease would be greater for the Au substrate than for the Ag substrate. The increase in the width of the electronic levels that occurs when molecules are adsorbed on metals also leads to a decrease in the intromolecular resonance RS cross-section of the adsorbed molecules and it therefore cannot be the cause of the differences in the observed RS intensities of CV adsorbed on smooth Au and Ag surfaces. However, since the electronic excitations of the CV molecules are already relatively broad, the decrease in the RS cross-section due to the increased broad-
Vol. 46, No. 8
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
599
ening of the CV intramolecular electronic excitations are very likely relatively small. We note also that the virtual excitation of electron-hole pairs in the metal substrates are not appreciably different for Au and Ag and we therefore do not believe that their contribution to the RS cross-sections of the adsorbed CV molecules accounts for the observed difference in the RS intensities of the Au and Ag substrates. We are left finally with differences in the contributions from intermolecular electronic excitations of the adsorbed CV-metal substrate complex as the basic cause for the difference in the RS cross-sections of CV on the two types of substrates. These excitations involve either transitions between bonding and antibonding levels or chargetransfer transitions between the substrate and the adsorbate. The difference in the RS cross-sections of CV adsorbed on Au and Ag substrates may be due either to differences in the frequencies of the "intermolecular" electronic excitation, or to differences in the oscillator strengths and widths of the excitations. The fact that the mechanisms involving the magnitude of the real and imaginary parts of the dielectric constants of the metal substrate yield appreciably smaller cross-section for CV adsorbed on Au than for CV molecules adsorbed on Ag suggests that the contribution from the intermolecular electronic excitations to the RS cross-section of the adsorbed molecule-metal substrate complex is fairly substantial.
charge-transfer and bonding-antibonding excitations), that are absent in the free molecules. The RS by adsorbed molecules is thus more appropriately viewed as the RS by adsorbate-substrate complexes. This viewpoint is highlighted by the RS of halogen ions adsorbed on Ag 9. The RS is clearly that of a halogen ion-Ag substrate complex, since the halogen ion by itself does not exhibit any vibrational RS.
F I N A L REMARKS. The absence of specific information about the electronic structure of molecules adsorbed on metal substrates, and specifically about the energies and widths of virtual bound states and bonding and antibonding states, has been a major bottleneck to efforts to elucidate the key factors (other than surface roughness enhanced EM fields) that play important roles in the surface enhanced RS by adsorbed molecules. There has been a general tendency to view the metal and the adsorbed molecules as separate entities, albeit perturbed by each others' presence, and to view the increased RS of the adsorbed molecules as being due to an enhancement by the metal substrate. It is now evident that the scattering cross-sections of the adsorbed molecules is due, in part, to contributions from intermolecular electronic excitations of the adsorbed molecule-metal substrate complex (e.g., from
3.
Acknowledgements--It is a pleasure to acknowledge discussions with P. Avouris. We also thank A. M. Torressen for preparing the Ag and Au substrates. REFERENCES 1.
2.
4. 5. 6.
7. 8.
9.
See for example: J. Gersten and A. Nitzan, J. Chem. Phys. 73, 3023 (1980); D. S. Wang, H. Chew and M. Kerker, Appl. Opt. 19, 2256 (1980); S. L. McCall, P. M. Platzman and P. A. Wolff, Phys. Rev. Lett. 77A, 381 (1980); C. Y. Chen and E. Burstein, Phys. Rev. Lett. 45, 1287 (1980); "Surface Enhanced Raman Scattering" Ed. R. K. Chang and T. E. Furtak (Plenum Press, 1982). See for example: E. Burstein, C. Y. Chen and S. Lundqvist in Light Scattering in Solids, Edited by J. L. Birman, H. Z. Cummins and K. K. Rebane (Plenum Publ. Corp. 1979) p. 479; S. L. McCall and P. M. Platzman, Phys. Rev. Lett. B22, 1660 (1980); B. N. Persson, Chem. Phys. Lett., 82, 561 (1981) and F. J. Adrian, J. Chem. Phys. 77, 5302 (1982). D . L . Jeanmaire and R. P. Van Duyne, J. ElectroanaL Chem. 84, 1 (1977). G. Hagen, B. Simic Glavski and F. Yeager, J. Electroanal. Chem. 88, 269 (1978). R . M . Martin and T. C. Damen, Phys. Rev. Lett. 26, 86 (1971). G. Abstreiter and K. Ploog, Phys. Rev. Lett. 42, 1308 (1979); A. Pinczuls, H. L. St~rmer, R. Duglo, J. M. Worlock, W. Wiegmann and A. C. Gossard, Sol. State Commun. 32, 1001 (1979). H. Yamada, Appl. Spectroscopy Rev. 17, 227 (1981). L. Angeloni, G. Smulevich and M. P. Marzocchi, J. Mol. Structure 61, 331 (1980), and J. Raman Spectroscopy 8, 305 (1979). H. Wetzel, H. Gerischer and B. Pettinger, Chem. Phys. Lett. 78, 392 (1981).
Solid State Communications, Vol.46,No.8, pp.595-599, ~____jPrinted in Great Britain.
1983.
0038-1098/83/200595-05503.00/0 Pergamon Press Ltd.
RAMAN SCATTERING BY OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES: CRYSTAL VIOLET E. Burstein* Physics Department, University of Pennsylvania, Philadelphia, PA 19104 and G. Burns and F. H. Dacol IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
(Received 28 January 1983 by G. Burns)
We report data on the resonant Raman scattering (RS) by crystal violet (CV) adsorbed on Ag and Au surfaces. The data show that the RS and luminescence by CV on a smooth Au surface is greater by an order of magnitude than that on a smooth Ag surface. These results, which are contrary to what one expects from mechanisms that depend on the complex dielectric constant of the metal substrate, are attributed to contributions that depend on the chemical bonding of the CV molecules with the metal substrate.
INTRODUCTION. The role played by surface roughness in Raman scattering (RS) by molecules adsorbed on noble metals, and in particular the role played by resonant excitation of localized plasmons in enhancing the incident and scattered EM fields, is now fairly well established 1. Unfortunately, it is not possible to obtain an accurate estimate of the magnitudes of the enhancements of the E M fields by surface roughness and, as a consequence, it is not possible to ascertain the relative magnitudes of the other contributions (e.g., electron hole pair excitations, charge-transfer excitations, vibration modulated electron density of the substrate, etc. 2) to the RS cross-section of the adsorbed molecules. A way around this difficulty is to carry out measurements of the resonance enhanced RS by adsorbed molecules on a smooth metal surface, whose RS cross-sections are sufficiently large to be observable without the enhancement of the incident and scattered E M fields. Crystal violet satisfies this criterion, as indicated by the fact that we can detect RS by a monolayer of crystal violet adsorbed on a glass substrate. The surface roughness enhanced resonant RS by crystal violet adsorbed on an anodized Ag electrode has been reported by Jeanmaire and Van Dyne 3. Hagen and coworkers 4 have used resonance enhancement to attain greater sensitivity in measur-
ing the RS by adsorbed molecules on Ag and Pt electrodes. They observed a resonance enhanced RS by p-nitroso-dimethyaniline ( p - N D M A ) molecules adsorbed on Ag and Pt electrodes, but did not observe any RS by p - N D M A adsorbed on a Au electrode. The use of resonance enhancement to increase the RS intensity is a well established procedure which has enabled one to observe "forbidden" Raman scattering in semiconductors 5 and also to observe RS by charge carrier excitations in quasi-2dimensional plasmas at semiconductor surfaces 6. Yamada and coworkers 7 have, in fact, used resonant RS to observe the RS by adsorbed molecules on non-metallic solid surfaces. We have measured the resonant RS by crystal violet (CV) adsorbed from dilute aqueous solutions on "smooth" evaporated films of Ag and Au. The films were maintained at a negative potential, from the moment of immersion to the time of removal from the solution, in order to avoid (chemical) etching of the metal surface and also to reduce any sulfide or oxide that may have formed before immersion. Our data show that the RS cross-section of CV adsorbed on a smooth Ag surface is essentially the same as that of CV adsorbed on glass. However, the luminescence of CV on a smooth Ag surface is weaker by a factor of ~ 2 0 than that of CV adsorbed on glass. On the other hand, the RS intensity of CV adsorbed on a smooth Au surface is greater by almost an order of magnitude than that of CV adsorbed on a smooth Ag surface, even at frequencies above the onset of interband transions in Au,
*Research supported in part by the N S F - M R L at the University of Pennsylvania and carried out in part while on sabbatical leave as Guggenheim Fellow at the IBM Research Center. 595
596
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
where there is little, if any, enhancement of the EM fields by residual surface roughness. Moreover, the luminescence of CV adsorbed on a smooth Au surface is greater by an order of magnitude than that of CV adsorbed on a smooth Ag surface and, surprisingly, only moderately weaker than that of CV adsorbed on a glass surface. That the RS and luminescence of CV on a smooth Au surface is greater than the RS and luminescence of CV on a smooth Ag surface is opposite to what one would expect on the b~tsis of mechanisms that depend on the magnitudes of the real and imaginary parts of the dielectric constants of the metal substrate. It provides clear-cut evidence for contributions that depend on the specific nature of the chemical bonding of CV with the metal substrate, and on the character (e.g., frequency, strength and width) of the electronic excitations of the metal-molecule complexes that are formed. CV (tri-p-dimethylaminophenyl-carbonium chloride) is a triphenyl-methane type dye whose dilute aqueous solution exhibits two overlapping absorption bands at 535nm and 605nm and a luminescence band at ~ 6 5 0 n m (Fig. 1). The resonant RS by CV in aqueous solution has been studied in some detail by Angeloni, Smulevich and Marzocchi 8. They find that the Raman spectrum consists of two- and three-component multiplets that arise from the coupled vibrations of the three
NMe2
Me2N
NMe 2 M
e
2
+ NMe 2
I
~
NMe 2
I
Crystal Violet
N
I f
I
r~bsorption
E
900 Fig. 1.
800
700 600 Wavelength (nm)
500
400
The CV structure and the adsorption and emission spectra of, CV in solution. (After F. C. Adams and W. T. Simpson, J. Molecular Spectroscopy 3, 363 (1959).
Vol. 46, No. 8
dimethyl-aniline groups, and of features corresponding to the breathing and bending modes of the three "rigid" dimethyl-aniline groups relative to the central carbon atom. EXPERIMENTAL. Ag and Au films, =1500/~ thick, were vapor deposited onto chem-mechanically polished p-type S i ( l l l ) wafers which were first coated with a 200.g. layer of Cr for better adhesion of the noble metal. In the case of Au, the freshly evaporated films were introduced into an electrochemical cell containing an 0.05 M aqueous solution of KC1 at a potential of - 0 . 0 6 V with respect to a standard calomel electrode. A PAR #179 potentiostat and a PAR #75 programmer were used for this purpose. After 100 seconds, a weak CV solution was added to attain the desired concentration of CV in the electrolyte, e.g. 10 - 5 M. After an additional 15 minutes the film was removed while maintaining the - 0 . 0 6 V potential, washed in agitated deionized water for 5 minutes and dried in a stream of dry nitrogen. The same procedure was used for the Ag films, except that 0.01M KCL was used and, after immersion at a - 0 . 0 6 V potential, the potential was set at - 1 . 1 V to attain an evolution of hydrogen, and then maintained at - 1 . 0 V until a total charge of 60 coulombs passed between the Ag film and a Pt counter electrode. CV was also deposited onto smooth glass (e.g., suprasil) slides by dipping the slides into a 10 - 5 M aqueous solution of CV, and then washing and drying as above. To obtain "quantitative" information about the effect of surface roughness, CV was also deposited onto Au and Ag surfaces which had been processed by an oxidation-reduction cycle. Adsorption isotherm measurements were carried out using both RS and luminescence to monitor the surface density of adsorbed molecules as a function of the concentration of CV in solution. The RS and luminescence signals for CV adsorbed on glass, and on smooth Ag and Au films, were found to increase with increasing concentration at concentrations below 10 - 7 M and to be relatively independent of concentration in the range from 1 0 - 7 M to 10-4M. We therefore believe that, in the case of the samples used in our studies, the CV coverage was roughly that of a monolayer. Moreover we surmise from the large size and essentially planar character of the CV molecule that the adsorbed molecules probably lie "flat" on the surface. Because of the large solubility of CV in water, we believe that it is unlikely that there is any appreciable dimer, or multilayer, formation. This is also borne out by the similarity of the Raman spectra for CV in solution to the spectra for CV adsorbed on the metal substrates. All of the spectra were measured in air using 10mW, or less, of incident laser radiation. To further decrease the intensity at the metal surfaces, the
Vol. 46, No. 8
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
597
'
'
CRYS'ALTVIOLET" ON
'
'
'
'
I
I
ROUGIHAg
I
I
i
i
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I
l--Z
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rn r~
CRYSTAL VIOLET ON
A
,
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I
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CRYSTAL VIOLET
Z
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t~
0
I
I
" ~'~,'T"k~*tw~
200
400
600
~
~1
~
800
~N/V'~A..~"
I W'~'~v~"
I000
1200
I
"
1400
I
1600
ENERGY SHIFT (cm-I) Fig. 2.
Typical Raman spectra, at 5145/k for CV in solution, and for CV adsorbed on anodized Ag and Au.
incident laser radiation was focussed by a cylindrical lens into a l m m x 4 m m image. However, even at these low intensities, some degradation of the Raman and luminescence signals with time was noted. RESULTS. Typical Raman spectra for CV molecules in dilute aqueous solution and for CV molecules adsorbed on Ag and on Au surfaces are shown in Fig. 2. The frequencies of the observed peaks in the spectra are given in Table 1. The spectra shown in Fig. 2 are those of CV adsorbed on electrochemically anodized Ag and Au surface since they have high signal to noise ratios. The solution spectrum consists of doublets and triplets that are associated with the degenerate modes of the dimethyly-aniline groups. As noted by Angeloni et al. 8, one component of each multiplet is polarized. By contrast, all of the peaks in the Raman spectra for CV molecules adsorbed on the smooth Ag and smooth Au surfaces are depolarized. Apart from some changes in the character of the peaks corresponding to the degenerate modes of the dimethyly-aniline groups, there is little change in their frequencies. On the other hand, the two low frequency peaks which are assigned to the A-symmetry breathing vibrations of the central carbon atom bonds and to the Esymmetry degenerate bending of the central carbon atom bonds, exhibit sizeable increases in frequency (Table 1). The peak of the breathing mode shifts from 208 cm -1 for CV in solution to 216 cm -1 and 234 cm - 1 for CV adsorbed on Ag and Au, respectively. The peak of the central carbon atom bond bending mode shifts from 339 cm -1 to 346 cm -1 and 359 cm -1 respectively. The larger frequency shift exhibited by the CV molecules adsorbed on Au are indicative of a stronger bonding of the CV molecules to the Au substrate than to the Ag substrate. Our data on the excitation-frequency (e.g. 488.0nm, 514.5nm, 568.2nm and 647.1nm) dependence of the RS intensity of the peaks for CV mole-
Table 1 Frequencies of the CV vibrational modes. Symm.* CV(Soln.) A E E A E E A A E E A A E E E E A
208 cm -1 339 427 445 564 729 775 814 840 919 940 1179 1305 1374 1452 1592 1624
CV I Ag
CV I Au
216 cm -1 346 424 444 570 733 763 818 840 915 943 1180 1297 1372 1445 1591 1622
234 cm -1 359 428 444 -737 762 815 840 920 945 1183 1300 1373 1448 1590 1622
*assignments by Angeloni et al. 8. cules adsorbed on Au are similar to those for CW molecules in solution. Thus, the intensities of the peaks at 234 cm -1 and 359 cm -1 exhibit a very strong enhancement (~103) in going from 488.0nm to 647.1nm. This enhancement is attributed by Angeloni et al. 8 to a F r a n c k - C o n d o n type mechanism associated with the resonance adsorption bands at 535-605nm. The enhancement of the peak at 1624 cm -1, on the other hand, is more moderate (~102 ) and is attributed to mechanisms involving the vibronic coupling of higher energy excitations at 240-300nm with the lower energy excitations at 535-605nm. Data on the relative intensities of the RS by the 1624 cm -1 mode of CV adsorbed on smooth Au
598
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
and Ag surfaces and, for comparison purposes on glass, are given in Table 2. What is particularly striking is the fact that the RS intensities for the CV molecules adsorbed on smooth Au are an order of magnitude greater than those for the CV molecules adsorbed on smooth Ag. This is the case even at excitation frequencies above the onset of interband transitions in Au at 520nm, where there is no enhancement of the EM fields by residual surface roughness. Moreover, as can be seen from the luminescence data in Table 3, the luminescence intensity of the CV molecules adsorbed on smooth Au is also greater by about an order of magnitude than that of the CV molecules adsorbed on smooth Ag. Table 2 and Table 3 also contains the data on the RS and luminescence intensities, respectively, of CV adsorbed on anodized Au and Ag surfaces. The enhancements of the RS and luminescence intensities by surface roughness, F = Irough/Ismooth , are smaller for Au than for Ag by two to three orders of magnitude. This is due, in part, to the fact that the imaginary part of the dielectric constant is larger for Au than for Ag, and in part to the fact that the amplitude of the roughness is smaller for anodized Au than for anodized Ag. We note in particular that in the case of CV adsorbed on Au there is no enhancement of the RS by surface roughness at 488.0nm, an excitation frequency which is above the onset of interband transitions. There is however an appreciable enhancement of the luminescence at 488.0nm (F ~ 7) since although the excitation frequency is above the onset of interband transitions, the emission frequency ( ~ 6 5 0 n m ) is well below the onset. DISCUSSION. The data for the RS and luminescence of adsorbed CV on Au and Ag are contrary to what one would expect on the basis of mechanisms, that depend on the magnitudes of the real and imaginary parts of the dielectric constants of the metal substrate. One would expect the enhancement of the EM fields by residual roughness to be much greater for Ag than for Au substrates. One would also expect the damping of the electronic excitations of the adsorbed molecules by the metal substrate to be greater for Au than for Ag and, as a consequence, one would expect a weaker resonance enhancement of the RS and a stronger quenching of the luminescence by CV adsorbed on Au. Also we note that the shift in the intramolecular excitation frequency of the adsorbed molecules due to the image dipole field (e.g., the image dipole contribution to the self-energy of the excitation) is, in effect, already taken into account when the RS measurements are carried out at the resonance frequency. Thus the major effect of the interaction, between the electronic excitations of the adsorbed molecules and the electronic excitations of the metal
Vol. 46, No. 8
Table 2
Raman Scattering Intensities (103 c n t / s ) of Adsorbed CV (5 mW incident laser intensity). ~'i
488.0nm
514.5nm
568.2nm
647.1nm
Smooth Au 215 cm -1 1624cm -t
.02 .21
.11 1.5
1.6 9
14 21
1 6 2 4 c m -1
.032
.10
.21
(mbl)
Glass 1 6 2 4 c m -1
.083
.10
(mbl)
(mbl)
.21
2.2
70
600
26
31
310
1900
1.5
8
29
310
1480
--
Smooth Ag
Rough Au 1 6 2 4 c m -1
Rough Ag 1 6 2 4 c m -1 F(Au) 161.54 c m - l l FtAg) 1624 cm -1
810
(mbl = masked by luminescence) Table 3 Luminescence Intensities (103 c n t / s ) of Adsorbed CV (5 mW incident laser intensity). Alum ~ 650nm. hi glass
488.0nm
514.5nm
568.2nm
647.1nm
50
70
160
120
smooth Au
9
28
100
75
smooth Ag
2
3
4
7
rough Au
66
73
390
1500
rough Ag
370
210
3000
3200
F(Au)
7.3
2.6
3.9
20
F(Ag)
190
70
750
460
substrate, is to increase the damping of the electronic excitations of the molecules. The net effect would therefore be to decrease the RS cross-section of the adsorbed molecules at resonance, and the decrease would be greater for the Au substrate than for the Ag substrate. The increase in the width of the electronic levels that occurs when molecules are adsorbed on metals also leads to a decrease in the intromolecular resonance RS cross-section of the adsorbed molecules and it therefore cannot be the cause of the differences in the observed RS intensities of CV adsorbed on smooth Au and Ag surfaces. However, since the electronic excitations of the CV molecules are already relatively broad, the decrease in the RS cross-section due to the increased broad-
Vol. 46, No. 8
OPTICALLY ABSORBING MOLECULES ADSORBED ON "SMOOTH" Ag AND Au SURFACES
599
ening of the CV intramolecular electronic excitations are very likely relatively small. We note also that the virtual excitation of electron-hole pairs in the metal substrates are not appreciably different for Au and Ag and we therefore do not believe that their contribution to the RS cross-sections of the adsorbed CV molecules accounts for the observed difference in the RS intensities of the Au and Ag substrates. We are left finally with differences in the contributions from intermolecular electronic excitations of the adsorbed CV-metal substrate complex as the basic cause for the difference in the RS cross-sections of CV on the two types of substrates. These excitations involve either transitions between bonding and antibonding levels or chargetransfer transitions between the substrate and the adsorbate. The difference in the RS cross-sections of CV adsorbed on Au and Ag substrates may be due either to differences in the frequencies of the "intermolecular" electronic excitation, or to differences in the oscillator strengths and widths of the excitations. The fact that the mechanisms involving the magnitude of the real and imaginary parts of the dielectric constants of the metal substrate yield appreciably smaller cross-section for CV adsorbed on Au than for CV molecules adsorbed on Ag suggests that the contribution from the intermolecular electronic excitations to the RS cross-section of the adsorbed molecule-metal substrate complex is fairly substantial.
charge-transfer and bonding-antibonding excitations), that are absent in the free molecules. The RS by adsorbed molecules is thus more appropriately viewed as the RS by adsorbate-substrate complexes. This viewpoint is highlighted by the RS of halogen ions adsorbed on Ag 9. The RS is clearly that of a halogen ion-Ag substrate complex, since the halogen ion by itself does not exhibit any vibrational RS.
F I N A L REMARKS. The absence of specific information about the electronic structure of molecules adsorbed on metal substrates, and specifically about the energies and widths of virtual bound states and bonding and antibonding states, has been a major bottleneck to efforts to elucidate the key factors (other than surface roughness enhanced EM fields) that play important roles in the surface enhanced RS by adsorbed molecules. There has been a general tendency to view the metal and the adsorbed molecules as separate entities, albeit perturbed by each others' presence, and to view the increased RS of the adsorbed molecules as being due to an enhancement by the metal substrate. It is now evident that the scattering cross-sections of the adsorbed molecules is due, in part, to contributions from intermolecular electronic excitations of the adsorbed molecule-metal substrate complex (e.g., from
3.
Acknowledgements--It is a pleasure to acknowledge discussions with P. Avouris. We also thank A. M. Torressen for preparing the Ag and Au substrates. REFERENCES 1.
2.
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7. 8.
9.
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