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Electronic surface state contribution to surface enhanced Raman scattering
~
Solid State Con~nunications, Printed in Great Britain.
Vol.44,No.2,
pp.]05-107,
ELECTRONIC SURFACE STATE CONTRIBUTION
1982.
0038-1098/82/380]05-03503.00/0 Pergamon Press Ltd.
TO SURFACE ENHANCED RAMAN SCATTERING
J. Billmann and A. Otto Physikalisches
Institut
III, Universit&t (Received
DGsseldorf,
D-4000 DGsseldorf
I, Fed. Pep. Germany
April 8th 1982 by M. Cardona)
Surface enhanced Raman scattering from pyridine on silver electrodes was measured as function o£ the electrode potential and of the laser wavelength. Pronounced shifts with wavelength of the peaks in the scans of intensity versus potential are attributed to an electronic surface excitation.
A L=457,9nm
I. Introduction
1006
Inspite of extensive research during the last 5 years, the explanation of surface enhanced Raman scattering (SERS) is still controversial /I/. However, one may definitely conclude, that SERS cannot be accounted for only on the basis of electromagnetic resonances induced by supraatomie surface roughness /2/. There exists definitely a "chemical effect" whose nature still needs to be unraveled. Here, we present for the first time evidence for an electronic surface excitation which is involved in the Raman scattering process.
.
-o.ld
2. Experiment Haman spectra of pyridine adsorbed on polycrystalline silver electrodes in an 0 . 1 M KC1, 0.05 M pyridine electrolyte were recorded in the backseattering geometry. The activation of the electrodes consisted in stepping for 4 seconds to a potential of 0.28V versus SCE (saturated calomel electrode) and returning to -0.16 V. After waiting for 10- 30 minutes to allow for possible restructuring of the electrode surface, the potential was swept from -0.16 V to - 1.0 V versus SCE with 50 mV sec -I. The vibrational integrated band intensities of the pyridine SERS signal at 623 cm-1, 1006 cm -I, 1035 cm -I, 1215 cm-1 and 1594 cm -I were recorded as a function of potential for exciting laser light of wavelength 457.9 nm, 568.2 nm, 647.1 nm and 676.4 nm. In order to allow for llne shifts with potential /3/ the pass band of the double spectrometer was choosen between 6 cm -I for 457.9 nm excitation and 4 cm-1 for 676.4 nm excitation. For every vibrational band and every excitation wavelength the intensity versus potential was taken with an activated electrode, as described above. Before every new activation, the electrodes were deactivated by a chemical etch (H202 and NH4OH). As example, the intensity scans for the exciting wavelengths 457.9 nm and 647.1 nm are given in fig. I. As every scan is from a separate activation, absolute intensities cannot be compared. No significant changes in the relative intensity versus potential curves were observed for different activations.
\
i%
' -d.s . . . . Us~/i
-11o
1215 1006 /'
....
/\H
\. i! ~sg<
/
J
-0,16
i!1
]7\ ",. t"
/i \ 'k \ - QI5
_ 110 USCE/V
Figure I Intensity of adsorbed pyridlne vibrational bands at about 623 cm -I , 1006 cm -I, 1035 cm -I 1215 cm -I and 1594 cm -I versus potential of the polycrystalline activated silver electrodes measured with respect to a saturated calomel electrode (SCE). Laser wavelength 457.9 nm (a) and 647.1 nm (b).
105
106
Vol. 44, No. 2
SURFACE ENHANCED RAMAN SCATTERING
The results were independently checked for the vibrational lines 623 cm-1, 1006 cm -I, 1035 cm -I and 1215 cm -I for exciting wavelengths 457.9 n m a n d 568.2 u m b y simultaneous recording with the help of an optical multichannel analyzer system. The relative peak intensity versus potential plots were within a narrow margin the same as in fig. I, thus independently reproducing the result. The peak position of the intensity versus potential scans, averaged over several measurements is plotted versus the exciting wavenumber in fig. 2 with the exception of the vibrational band at 1035 cm -I which does not yield a clear maximum in the intensity potential scans. There is a clear displacement of the peak position with the change of exciting light. Our results corroborate the early result of Jeanmaire and VanDuyne /4/ which were recorded at a laser wavelength of 514.5 nm. The origin of the discrepancy for the vibrational band at 1594 cm -I is not clear.
- 1.0
-(m "~i]~o F.o-] -:m
-0.7
b
-0.6 -0.!
156o0
~I
,r~..°o
1215cm-
I
3 Cm-1 ~ [ 3 " ~ , ~ 5 g4cm-1 " ~ ' 1006c m-1
2o6oo
2shoo
~-LI {cm-1)
Figure 2 Peak positions of the scans intensity versus voltage as function of the laser line wavenumber for pyridine vibrational bands at about 623 cm -I (open triangles), 1006 cm -I (open squares), 1215 cm -I (open dots) and 159h cm -I (full dots). Data of Jeanmaire and VanDuyne /4/ are included within the dashed rectangle.
3. Discussion With respect to intensity versus potential scans /4/ VanDuyne /5/ discussed the possibility of a potential dependent adsorption of pyridine and chlorine and pointed out that more work was needed to understand the effect. Clearly, the strong dependence of intensity versus potential on the exciting wavelength (see fig. I) cannot be explained by adsorption - desorption or an unknown configurational change. However, desorption or rearrangement phenomena probably contribute to the loss of intensity at potentials beyond -0.8 V. The peak position in the scan intensity versus potential is plotted versus laser photon energy in fig. 3. With decreasing potential, the peak positions shift to lower laser frequencies. This rules out an explanation of the effect on the basis of an electromagnetic resonance. For increasing negative potential, the surface is increasingly negatively charged. Electron density in very small bumps would be raised in this way. This would raise the
s.o
,
,
f
,
,~ Ag(1111 4.0
~ °'° .s~/~
~o~
o/Ag (100)
3,0 @ 3
/Z.//
Ag(110~,
,,-o;f
/,
2D {110}
Ag
-t5
110
100
-1.0
111
-0.5 U( VscE)
0.0
05
Figure 3 Peak positions (marked by full triangles) of the scans intensity versus voltage as function of the exciting laser photon energy ~ for the pyrldine vibrational bands near 623 cm -I (b), 1006 cm -I (d), 1215 cm -I (a) and 1594 cm -I (c). All other signs denote transition energies for excitation into surface bands observed in electroreflectance from Ag (I00), Ag (111) and Ag (110) surfaces by Boeck and Kolb /7/, see text. Arrows at the bottom line mark the potentials of zero charge for the different silver surfaces /7/. plasma frequency and shift the electromagnetic resonances to higher photon energy - contrary to the observation. However, the result is qualitatively compatible with the concept of resonant Raman scattering involving an electronic surface excitation. The observed resonances in the intensity versus potential curves are caused by the energetic shift of electronic surface excitations with potential. The reasons for this argument are the following: A. Qualitatively similar shifts have been resported by Kolb, Ho and collaborators /6, 7/. Kolb and Boeck /7/ found pronounced structural features in the normal incidence spectra of single crystalline Ag (111), (110) and (100) surfaces. The spectral position of these features shifted with potential as given in fig. 3. Selfconsistent pseudopotential calculations of the surface electronic structure by Ho and collaborators /8, 6 / for the silver (110) and (100) surface revealed surface bands in the gaps of the projected bulk density of states above the Fermi energy (for zero potential difference between metal and electrolyte. This so-called potential of zero charge is given in fig. 3). The electroreflectance structures were assigned to optical transitions from filled bulk electronic band states to empty surface bands /8, 6, 7/. Relatively good agreement with experiment was reached for the Ag(100) surface /6/, by transition to two different surface bands. The shift of the surface bands with respect to the bulk bands due to charging of the surface was theoretically modeled by an external potential /6/. Relatively good agreement was obtained with the experimen-
Vol. 44, No. 2
107
SURFACE ENHANCED RAMAN SCATTERING
tal result from the Ag (100) surface. Qualitatively, the shift between surface and bulk states is analogous to the shift between surface states and bulk states of semiconductors when changing the band bending by filling or depleting surface states. Nevertheless, the details of the observed shifts which are stronger than I eV per I VSC E are still unclear. This might be due to an "overshoot" of the potential gradient between electrode and electrolyte due to the molecular structure of the adsorbed water layer /6/ or due to a nonhomogeneous distribution of the screening charge across the electrode surface. B. The shift of the pyridine Raman intensity maxima reported in this work is even larger than that of the electroreflectance structure at the most open surface, the (110)surface. (Preliminary measurements for adsorbed cyanide show a similar trend and some molecular specificity from comparison with the pyridine results.) We assign the observed shift to the shift of an unknown empty surface state with respect to the bulk Fermi level. The unknown surface state could be a surface resonance of the adsorbed pyridine in the negative ion state. The photon induced electron transfer from the metal to this negative ion resonance could account for a Raman enhancement of about 2 orders of magnitude, as calculated by Persson /9/ with a modified NewnsAnderson model for the negative ion state. The transition energies between 2 and 3 eV agree qualitatively with the energies of charge transfer excitations of pyridine on silver (111) /10/ and on coldly evaporated silver /11/ observed b¥ electron energy
loss spectroscopy /10, 11/. It remains an open question why the resonance position does depend on the vibrational frequency. The vibrational band at 1035 cm -I does not show a clear maximum in the intensity versus potential scans and a different trend than the other vibrational bands. It may be partly due to a breathing vibration of pyridine adsorbed in a different configuration (geometry of adsorption, adsorption site) than the pyridine molecules giving rise to the breathing vibration at 1006 cm -I . This point is indicated by SERS measurements of pyridine on silver electrodes /12/. The vibrational band at 1035 cm-1 nearly disappeared after 2 hours of laser irradiation (600 nm, 100 raW), whereas the 1006 cm-1 band "survived". This may be due to the exclusive disappearance of the corresponding configuration under laser irradiation. 4. Conclusion Inspite the many open questions the experimentally well documented shift of the spectral position of excitation maxima with potential can only be explained by the shift of a yet unknown electronic surface state. Judging from the width of the resonance and from the intensity versus potential scans, the resonant Raman effect involving the electronic surface state may account for an enhancement of 10 - 100 but not for the overall enhancement of 105 to 106 . Acknowledgement - This work was supported by the Deutsche Forschungsgemeinschaft (project Ot 47-10-1). We thank D. M. Kolb for allowing us to use his results prior to publication.
References /1/ /2/
/3/ /4/ /5/
/6/
S u r f a c e E n h a n c e d Raman S c a t t e r i n g , R.K. C h a n g , T. E. F u r t a k ( e d s . ) , P l e n u m 1982 A. Otto in "Light Scattering in Solids", Vol. IV, M. Cardona, G. GGntherodt (eds.), Springer R. K6tz, E. Yeager, J. Electroanal. Chem. 123, 335 (1981) D. L. Jeanmaire, R. P. VanDuyne, J. Electroanal. Chem. 84, I, (1977) R. P. VanDuyne in "Chemical and Biochemical Application of Lasers", Vol 4, Ch. 5, C. B. Moore (ed.), 1978 D. M. Kolb, W. Boeck, K.-M. Ho, S. H. Liu, Phys. Rev. Letters 47, 1921 (1981)
/7/ /8/ /9/
/10/ /11/ /12/
W. Boeck, D. M. Kolb, Surf. Science, in press K.-M. Ho, B. N. Harmon, S. H. Liu, Phys. Rev. Letters 44, 1531 (1980) B. N. J. Persson, Chem. Phys. Letters 82, 561 (1981) Ph. Avouris, J. E. Demuth, J. Chem. Phys. 75, 4783 (1981) D. Schmeisser, J. E. Demuth, Ph. Avouris, Chem. Phys. Letters, 87, 324 (1982) C. S. Allen, G. C. Schatz, R. P. VanDuyne, Chem. Phys. Letters 75, 201 (1980)
Solid State Con~nunications, Printed in Great Britain.
Vol.44,No.2,
pp.]05-107,
ELECTRONIC SURFACE STATE CONTRIBUTION
1982.
0038-1098/82/380]05-03503.00/0 Pergamon Press Ltd.
TO SURFACE ENHANCED RAMAN SCATTERING
J. Billmann and A. Otto Physikalisches
Institut
III, Universit&t (Received
DGsseldorf,
D-4000 DGsseldorf
I, Fed. Pep. Germany
April 8th 1982 by M. Cardona)
Surface enhanced Raman scattering from pyridine on silver electrodes was measured as function o£ the electrode potential and of the laser wavelength. Pronounced shifts with wavelength of the peaks in the scans of intensity versus potential are attributed to an electronic surface excitation.
A L=457,9nm
I. Introduction
1006
Inspite of extensive research during the last 5 years, the explanation of surface enhanced Raman scattering (SERS) is still controversial /I/. However, one may definitely conclude, that SERS cannot be accounted for only on the basis of electromagnetic resonances induced by supraatomie surface roughness /2/. There exists definitely a "chemical effect" whose nature still needs to be unraveled. Here, we present for the first time evidence for an electronic surface excitation which is involved in the Raman scattering process.
.
-o.ld
2. Experiment Haman spectra of pyridine adsorbed on polycrystalline silver electrodes in an 0 . 1 M KC1, 0.05 M pyridine electrolyte were recorded in the backseattering geometry. The activation of the electrodes consisted in stepping for 4 seconds to a potential of 0.28V versus SCE (saturated calomel electrode) and returning to -0.16 V. After waiting for 10- 30 minutes to allow for possible restructuring of the electrode surface, the potential was swept from -0.16 V to - 1.0 V versus SCE with 50 mV sec -I. The vibrational integrated band intensities of the pyridine SERS signal at 623 cm-1, 1006 cm -I, 1035 cm -I, 1215 cm-1 and 1594 cm -I were recorded as a function of potential for exciting laser light of wavelength 457.9 nm, 568.2 nm, 647.1 nm and 676.4 nm. In order to allow for llne shifts with potential /3/ the pass band of the double spectrometer was choosen between 6 cm -I for 457.9 nm excitation and 4 cm-1 for 676.4 nm excitation. For every vibrational band and every excitation wavelength the intensity versus potential was taken with an activated electrode, as described above. Before every new activation, the electrodes were deactivated by a chemical etch (H202 and NH4OH). As example, the intensity scans for the exciting wavelengths 457.9 nm and 647.1 nm are given in fig. I. As every scan is from a separate activation, absolute intensities cannot be compared. No significant changes in the relative intensity versus potential curves were observed for different activations.
\
i%
' -d.s . . . . Us~/i
-11o
1215 1006 /'
....
/\H
\. i! ~sg<
/
J
-0,16
i!1
]7\ ",. t"
/i \ 'k \ - QI5
_ 110 USCE/V
Figure I Intensity of adsorbed pyridlne vibrational bands at about 623 cm -I , 1006 cm -I, 1035 cm -I 1215 cm -I and 1594 cm -I versus potential of the polycrystalline activated silver electrodes measured with respect to a saturated calomel electrode (SCE). Laser wavelength 457.9 nm (a) and 647.1 nm (b).
105
106
Vol. 44, No. 2
SURFACE ENHANCED RAMAN SCATTERING
The results were independently checked for the vibrational lines 623 cm-1, 1006 cm -I, 1035 cm -I and 1215 cm -I for exciting wavelengths 457.9 n m a n d 568.2 u m b y simultaneous recording with the help of an optical multichannel analyzer system. The relative peak intensity versus potential plots were within a narrow margin the same as in fig. I, thus independently reproducing the result. The peak position of the intensity versus potential scans, averaged over several measurements is plotted versus the exciting wavenumber in fig. 2 with the exception of the vibrational band at 1035 cm -I which does not yield a clear maximum in the intensity potential scans. There is a clear displacement of the peak position with the change of exciting light. Our results corroborate the early result of Jeanmaire and VanDuyne /4/ which were recorded at a laser wavelength of 514.5 nm. The origin of the discrepancy for the vibrational band at 1594 cm -I is not clear.
- 1.0
-(m "~i]~o F.o-] -:m
-0.7
b
-0.6 -0.!
156o0
~I
,r~..°o
1215cm-
I
3 Cm-1 ~ [ 3 " ~ , ~ 5 g4cm-1 " ~ ' 1006c m-1
2o6oo
2shoo
~-LI {cm-1)
Figure 2 Peak positions of the scans intensity versus voltage as function of the laser line wavenumber for pyridine vibrational bands at about 623 cm -I (open triangles), 1006 cm -I (open squares), 1215 cm -I (open dots) and 159h cm -I (full dots). Data of Jeanmaire and VanDuyne /4/ are included within the dashed rectangle.
3. Discussion With respect to intensity versus potential scans /4/ VanDuyne /5/ discussed the possibility of a potential dependent adsorption of pyridine and chlorine and pointed out that more work was needed to understand the effect. Clearly, the strong dependence of intensity versus potential on the exciting wavelength (see fig. I) cannot be explained by adsorption - desorption or an unknown configurational change. However, desorption or rearrangement phenomena probably contribute to the loss of intensity at potentials beyond -0.8 V. The peak position in the scan intensity versus potential is plotted versus laser photon energy in fig. 3. With decreasing potential, the peak positions shift to lower laser frequencies. This rules out an explanation of the effect on the basis of an electromagnetic resonance. For increasing negative potential, the surface is increasingly negatively charged. Electron density in very small bumps would be raised in this way. This would raise the
s.o
,
,
f
,
,~ Ag(1111 4.0
~ °'° .s~/~
~o~
o/Ag (100)
3,0 @ 3
/Z.//
Ag(110~,
,,-o;f
/,
2D {110}
Ag
-t5
110
100
-1.0
111
-0.5 U( VscE)
0.0
05
Figure 3 Peak positions (marked by full triangles) of the scans intensity versus voltage as function of the exciting laser photon energy ~ for the pyrldine vibrational bands near 623 cm -I (b), 1006 cm -I (d), 1215 cm -I (a) and 1594 cm -I (c). All other signs denote transition energies for excitation into surface bands observed in electroreflectance from Ag (I00), Ag (111) and Ag (110) surfaces by Boeck and Kolb /7/, see text. Arrows at the bottom line mark the potentials of zero charge for the different silver surfaces /7/. plasma frequency and shift the electromagnetic resonances to higher photon energy - contrary to the observation. However, the result is qualitatively compatible with the concept of resonant Raman scattering involving an electronic surface excitation. The observed resonances in the intensity versus potential curves are caused by the energetic shift of electronic surface excitations with potential. The reasons for this argument are the following: A. Qualitatively similar shifts have been resported by Kolb, Ho and collaborators /6, 7/. Kolb and Boeck /7/ found pronounced structural features in the normal incidence spectra of single crystalline Ag (111), (110) and (100) surfaces. The spectral position of these features shifted with potential as given in fig. 3. Selfconsistent pseudopotential calculations of the surface electronic structure by Ho and collaborators /8, 6 / for the silver (110) and (100) surface revealed surface bands in the gaps of the projected bulk density of states above the Fermi energy (for zero potential difference between metal and electrolyte. This so-called potential of zero charge is given in fig. 3). The electroreflectance structures were assigned to optical transitions from filled bulk electronic band states to empty surface bands /8, 6, 7/. Relatively good agreement with experiment was reached for the Ag(100) surface /6/, by transition to two different surface bands. The shift of the surface bands with respect to the bulk bands due to charging of the surface was theoretically modeled by an external potential /6/. Relatively good agreement was obtained with the experimen-
Vol. 44, No. 2
107
SURFACE ENHANCED RAMAN SCATTERING
tal result from the Ag (100) surface. Qualitatively, the shift between surface and bulk states is analogous to the shift between surface states and bulk states of semiconductors when changing the band bending by filling or depleting surface states. Nevertheless, the details of the observed shifts which are stronger than I eV per I VSC E are still unclear. This might be due to an "overshoot" of the potential gradient between electrode and electrolyte due to the molecular structure of the adsorbed water layer /6/ or due to a nonhomogeneous distribution of the screening charge across the electrode surface. B. The shift of the pyridine Raman intensity maxima reported in this work is even larger than that of the electroreflectance structure at the most open surface, the (110)surface. (Preliminary measurements for adsorbed cyanide show a similar trend and some molecular specificity from comparison with the pyridine results.) We assign the observed shift to the shift of an unknown empty surface state with respect to the bulk Fermi level. The unknown surface state could be a surface resonance of the adsorbed pyridine in the negative ion state. The photon induced electron transfer from the metal to this negative ion resonance could account for a Raman enhancement of about 2 orders of magnitude, as calculated by Persson /9/ with a modified NewnsAnderson model for the negative ion state. The transition energies between 2 and 3 eV agree qualitatively with the energies of charge transfer excitations of pyridine on silver (111) /10/ and on coldly evaporated silver /11/ observed b¥ electron energy
loss spectroscopy /10, 11/. It remains an open question why the resonance position does depend on the vibrational frequency. The vibrational band at 1035 cm -I does not show a clear maximum in the intensity versus potential scans and a different trend than the other vibrational bands. It may be partly due to a breathing vibration of pyridine adsorbed in a different configuration (geometry of adsorption, adsorption site) than the pyridine molecules giving rise to the breathing vibration at 1006 cm -I . This point is indicated by SERS measurements of pyridine on silver electrodes /12/. The vibrational band at 1035 cm-1 nearly disappeared after 2 hours of laser irradiation (600 nm, 100 raW), whereas the 1006 cm-1 band "survived". This may be due to the exclusive disappearance of the corresponding configuration under laser irradiation. 4. Conclusion Inspite the many open questions the experimentally well documented shift of the spectral position of excitation maxima with potential can only be explained by the shift of a yet unknown electronic surface state. Judging from the width of the resonance and from the intensity versus potential scans, the resonant Raman effect involving the electronic surface state may account for an enhancement of 10 - 100 but not for the overall enhancement of 105 to 106 . Acknowledgement - This work was supported by the Deutsche Forschungsgemeinschaft (project Ot 47-10-1). We thank D. M. Kolb for allowing us to use his results prior to publication.
References /1/ /2/
/3/ /4/ /5/
/6/
S u r f a c e E n h a n c e d Raman S c a t t e r i n g , R.K. C h a n g , T. E. F u r t a k ( e d s . ) , P l e n u m 1982 A. Otto in "Light Scattering in Solids", Vol. IV, M. Cardona, G. GGntherodt (eds.), Springer R. K6tz, E. Yeager, J. Electroanal. Chem. 123, 335 (1981) D. L. Jeanmaire, R. P. VanDuyne, J. Electroanal. Chem. 84, I, (1977) R. P. VanDuyne in "Chemical and Biochemical Application of Lasers", Vol 4, Ch. 5, C. B. Moore (ed.), 1978 D. M. Kolb, W. Boeck, K.-M. Ho, S. H. Liu, Phys. Rev. Letters 47, 1921 (1981)
/7/ /8/ /9/
/10/ /11/ /12/
W. Boeck, D. M. Kolb, Surf. Science, in press K.-M. Ho, B. N. Harmon, S. H. Liu, Phys. Rev. Letters 44, 1531 (1980) B. N. J. Persson, Chem. Phys. Letters 82, 561 (1981) Ph. Avouris, J. E. Demuth, J. Chem. Phys. 75, 4783 (1981) D. Schmeisser, J. E. Demuth, Ph. Avouris, Chem. Phys. Letters, 87, 324 (1982) C. S. Allen, G. C. Schatz, R. P. VanDuyne, Chem. Phys. Letters 75, 201 (1980)