Investigation by Surface-Enhanced Raman Spectroscopy of the Effect

Adsorbate-Covered Gold and Silver Island Films. E. Hesse and J. A. Creighton*. School of Physical Sciences, University of Kent, Canterbury CT2 7NH, U...
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Langmuir 1999, 15, 3545-3550

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Investigation by Surface-Enhanced Raman Spectroscopy of the Effect of Oxygen and Hydrogen Plasmas on Adsorbate-Covered Gold and Silver Island Films E. Hesse and J. A. Creighton* School of Physical Sciences, University of Kent, Canterbury CT2 7NH, U.K. Received September 28, 1998. In Final Form: February 1, 1999 The effects of radio-frequency-excited oxygen and hydrogen plasmas on gold island films with adsorbed CN-, thiophenate, or p-nitrobenzoate ions have been investigated by surface-enhanced Raman spectroscopy (SERS). An oxygen plasma caused oxidation of adsorbed CN- to Au(CN)4-, and there was evidence of oxidation of the adsorption site gold atoms at the Au/p-nitrobenzoate surface, while in a hydrogen plasma Au(CN)4- was reduced back to adsorbed CN-. No adsorbed oxidation or reduction products of thiophenate or p-nitrobenzoate were detected, however. Exposure of gold films to an oxygen or hydrogen plasma also caused partial removal of the adsorbates, though with some loss of SERS activity, and in a hydrogen plasma thiophenate was preferentially removed from a gold film with coadsorbed thiophenate and CN-. On silver films, hydrogen plasma treatment almost completely removed adsorbed CN- with only a small loss of SERS activity, but in an oxygen plasma the films were rapidly destroyed.

Introduction Exposure of surfaces to gaseous plasmas may affect their physical structure and may also bring about surface chemical changes. These surface effects of plasmas have important applications in the semiconductors industry in cleaning procedures for substrates used for thin film and semiconductor technologies and for plasma-enhanced chemical vapor deposition. Oxygen plasmas have been the most widely used for removing organic contamination from surfaces, but there is a recent interest also in hydrogen plasmas for applications where there is a requirement for surface chemical cleaning with a minimum of physical damage to the surfaces.1-5 Use has previously been made of Auger spectroscopy1,2 and SIMS5 to monitor the removal of contaminants at metallic or semiconductor surfaces by plasma treatment, and mass spectrometry has been used to identify molecules which such treatment releases into the gas phase (e.g., ref 2). To follow the changes in surface chemistry brought about by plasma treatment, however, in situ molecular analytical techniques such as surface infrared or Raman spectroscopy are required, and in this paper we investigate SERS for this type of investigation. As far as we are aware, this is the first use of SERS for this application, though a recent publication has described the use of plasmas to produce SERS active surfaces on bulk metal substrates.6 The surfaces we have examined are evaporated gold or silver island films which have been previously treated with the adsorbates CN-, or in the case of the gold films, diphenyl disulfide (DPDS) or p-nitrobenzoic acid (PNBA). CN- is a ligand whose vibration frequency is sensitive to the oxidation state of the metal atom to which it is bound, (1) Korner, N.; Beck, E.; Dommann, A.; Onda, N.; Ramm, J. Surf. Coat. Technol. 1995, 77, 731. (2) Anthony, B.; Breaux, L.; Hsu, T.; Banerjee, S.; Tasch, A. J. Vac. Sci. Technol. 1989, B7, 621. (3) Park, Y.-B.; Rhee, S.-W. Appl. Phys. Lett. 1996, 68, 2219. (4) Losurdo, M.; Capezzuto, P.; Bruno, G. J. Vac. Sci. Technol. 1996, B14, 691. (5) Burke, T. M.; Quierin, M. A.; Grimshaw, M. P.; Ritchie, D. A.; Pepper, M.; Burroughes, J. H. J. Vac. Sci. Technol. 1997, B15, 325. (6) Compagnini, G.; Pignataro, B.; Pelligra, B. Chem. Phys. Lett. 1997, 272, 453.

whereas thiophenate and nitrobenzoate (the adsorbates resulting from exposing gold or silver surfaces to DPDS and PNBA) are fairly readily oxidizable and reducible adsorbates, respectively. These adsorbates were therefore thought to be sensitive to the kind of surface chemical changes which might occur during plasma treatment. In addition to identifying the species present at the surfaces after the plasma treatment, an objective of the investigation was also to determine whether oxygen or hydrogen plasma treatment might be useful as a cleaning technique for SERS substrates. The technique of SERS depends critically on the presence of structural features of dimensions 10-100 nm at silver or gold surfaces, and for plasma treatment to be an effective cleaning procedure, it is necessary that the plasma treatment removes adsorbates from the surface without at the same time destroying the microstructures on which the SERS phenomenon depends. Experimental Section The SERS substrates were prepared by depositing gold or silver island films from a tungsten filament evaporation source onto glass microscope slides in an Edwards model E306A vacuum coating unit. The mean thickness of the gold and silver films were 7.5 and 9.0 nm respectively, and the rate of deposition was ca. 0.1 nm s-1 at a pressure of 10-4 Pa. The glass slides had previously been cleaned by immersing them in a hot freshly prepared 1:3 mixture of H2O2 (100 volume) and concentrated H2SO4 for 5 min. For adsorption of CN- ions onto the gold or silver surfaces, the films were exposed to HCN vapor by placing them inside a bottle containing KCN in moist air for 5 min, while for adsorption of p-nitrobenzoate or thiophenate ions the gold films were dipped into a solution of 0.25 mg/g of PNBA or DPDS in methanol or acetone, respectively, after which unabsorbed PNBA or DPDS was removed by washing with the pure solvent. Plasma treatment of the films was effected by placing them in a Emitech Plasmod plasma ashing unit (Emitech, Ashford TN23 7RS, U.K.), in which a discharge was excited by a 13.56 MHz 100 W radio frequency (RF) generator at a gas pressure of 10 Pa and a flow rate of 25 cm3 (stp) per minute. The SERS spectra of the films in air were recorded with an Instruments S.A. model HR640 spectrometer with a liquid nitrogen cooled multichannel CCD detector. Raman excitation was from a He-Ne laser (632.8 nm) with an incident power of ca. 15 mW at the sample, and the Raman-scattered radiation was collected in a backscattering geometry. Curve fitting to obtain bandwidths and the wave-

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Figure 1. SERS spectra of a gold island film pretreated with HCN vapor: (a) the initial film; (b) the film after oxygen plasma treatment (3 s at 80 W); (c) the film after subsequent exposure to a hydrogen plasma (3 s at 80 W); (d) the film after reexposure to HCN vapor. numbers of band centers was by means of the Grams software package Peaksolve.

Results HCN Pretreated Gold Films. A series of spectra measured at a HCN-treated gold island film is shown in Figure 1. After the initial exposure of the film to HCN vapor, the spectrum shows a band at 2139 cm-1. The film was then exposed to an oxygen plasma at an RF power of 80 W for 3 s. After this plasma treatment, the SERS spectrum showed an initial peak at 2139 cm-1 with reduced intensity and, in addition, a new band at 2190 cm-1 with more than twice the intensity of the initial peak. Similar changes have been observed in the SERS spectra of a gold colloid in the presence of cyanide ions when exposed to oxidizing conditions7 and were attributed to the reversible oxidation of Au(CN)2- adsorbed at the aqueous gold interface to adsorbed Au(CN)4- from a comparison of the SERS data with the wavenumbers of the a1g ν(CN) vibrations of the Au(CN)2- and Au(CN)4ions in aqueous solution (2164 and 2209 cm-1, respectively).8,9 More recently, however, Kunimatsu et al.10 have provided strong evidence from in situ reflection absorption infrared spectroscopy that the cyanide species present at a gold-aqueous interface under nonoxidizing conditions is the adsorbed CN- ion rather than adsorbed Au(CN)2-. The ν(CN) wavenumber for adsorbed CN- was shown to vary with potential from 2093 cm-1 at -1.4 V (relative to the Ag/AgCl electrode) to 2140 cm-1 at +0.4 V, and the 2139 cm-1 band from the gold island film before oxidation in the oxygen plasma thus coincides with the wavenumber for adsorbed CN- near the positive end of this potential range. (7) Dorain, P. B.; Von Raben, K. U. Surf. Sci. 1985, 160, 164. (8) Jones, L. H. J. Chem. Phys. 1965, 43, 594. (9) Jones, L. H.; Smith, J. M. J. Chem. Phys. 1964, 41, 2507. (10) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R. Langmuir 1988, 4, 337.

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Since the results of Kunimatsu et al. were for a goldaqueous interface whereas the gold island films were in air, confirmation that the 2139 cm-1 band was due to adsorbed CN- was obtained by examining the effect of partially substituting CN- by 13CN-, based on the argument that if CN- is the adsorbed species, a progressive shift to lower wavenumber in the 12CN frequency is expected without band broadening as the CN- is diluted with 13CN-. If however Au(CN)2- is the adsorbed species, a broadening of the CN and 13CN stretching bands is expected near 50% isotopic exchange, due to the formation of the intermediate species Au(12CN)(13CN)- whose bands overlap the bands of Au(12CN)2- and Au(13CN)2-.10,11 Least-squares fitting of a mixed Lorentzian-Gaussian band shape to the observed SERS bands showed that indeed the 2139.0-cm-1 band shifted to 2137.4 cm-1 on isotopic dilution to 70% 12CN-, and to 2136.2 cm-1 at 35% 12CN- and that the analogous band due to 13CN- showed an isotopic dilution effect, shifting from 2088.3 cm-1 at 30% 13CN- to 2089.8 cm-1 at 65% 13CN-. This progressive shift to lower wavenumber on isotopic dilution is quantitatively almost exactly the same as that observed by Kunimatsu et al.10 and Gao et al.11 for adsorbed CN- at the gold electrodes (although their actual band frequencies are lower than for the films since their electrodes were at rather negative potentials). In addition, our spectra show no obvious broadening of the bands at partial isotopic exchange, whereas from the published ν(CN) wavenumbers for Au(12CN)2-, Au(13CN)2-, and Au(12CN)(13CN)-,9,15 an increase of ca. 8 cm-1 in the half-widths of the bands would be expected at 50% isotopic exchange if Au(CN)2were the surface species. We therefore conclude that the initial surface species formed by exposure of the gold films to HCN vapor was CN- adsorbed at the zerovalent gold surface,15 and that this was oxidized to Au(CN)4- by the oxygen plasma. It is also evident from Figure 1 that the SERS scattering cross section per CN group for the adsorbate Au(CN)4- is considerably greater than that of adsorbed CN-, since the band intensity of the Au(CN)4produced by the plasma is several times larger than the intensity lost from the adsorbed CN- band. The effect of a longer exposure to the 80 W oxygen plasma was to reduce the intensity of the Au(CN)4- band. Thus as Figure 2 shows, after 15 s exposure the band had only 25% of its intensity after 5 s in the plasma. On retreating the film with HCN vapor, however, the band was restored (Figure 2e) to slightly greater intensity than it had after the 5 s exposure. This suggests strongly that the decrease in the band intensity was due mainly to the partial removal of Au(CN)4- from the surface, rather than to a decrease in the SERS activity of the film due to changes in the island film structure. Nevertheless there was some structural change in the film, as shown by the visiblerange transmittance spectrum. Figure 3 shows that after the 15 s oxygen plasma treatment there was a broadening and reduction in the height of the plasma resonance absorption band characteristic of the gold particles centered near 840 nm. However from the good signal recovery on reexposure to HCN, the effect that this structural change has on the SERS spectrum appears to be relatively minor. (11) Gao, P.; Weaver, M. J. J. Phys. Chem. 1989, 93, 6205. (12) Scheffler, M. Surf. Sci. 1979, 81, 562. (13) Seki, H. IBM J. Res. Dev. 1993, 37, 227. (14) Jones, L. H. J. Chem. Phys. 1957, 27, 468. (15) Some of the gold island films exposed to HCN vapor also showed a weak band at 2239 cm-1 before treatment with the oxygen plasma, which slowly increased in intensity on standing the films in air. This band is close to the wavenumber characteristic of AuCN; see: Penneman, R. A.; Jones, L. H. J. Chem. Phys. 1958, 28, 169.

Adsorbate-Covered Island Films

Figure 2. SERS spectra of a gold island film pretreated with HCN vapor: (a) the initial film; (b-d) the film after oxygen plasma treatment (80 W) for 5, 10, and 15 s respectively; (e) the film after reexposure to HCN vapor.

Figure 3. Effect of plasma treatment on the visible-range transmittance spectrum of gold and silver island films pretreated with HCN vapor: gold film (a) before and (b) after exposure to an oxygen plasma (15 s at 80 W); silver film (c) before and (d) after exposure to a hydrogen plasma (10 s at 80 W).

Also of interest in Figure 2 is the observation that whereas the initial exposure of the film to HCN vapor resulted in the 2139 cm-1 band due to adsorbed CN- at the zerovalent gold surface (Figure 1a), the reexposure to HCN vapor after the 15 s oxygen plasma treatment regenerated the band due to Au(CN)4-, together with a shoulder at 2239 cm-1 consistent with AuCN (Figure 2e).15 Thus although cyanide is lost from the surface during the 15 s oxygen plasma treatment, it appears that the surface gold atoms remain in positive oxidation states (Au3+ and Au+), presumably as the oxides. After the 3 s treatment in the oxygen plasma, the first gold film was exposed to a hydrogen plasma for 3 s at ca. 80 W. As Figure 1c shows, the band at 2190 cm-1 due to Au(CN)4- disappeared, showing that rapid reduction

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occurred. However the 2190 cm-1 band was replaced by a peak at 2133 cm-1, slightly lower in wavenumber than the initial adsorbed CN- peak at 2139 cm-1 and with roughly one-third of its initial intensity. Figure 1d shows that on reexposing the film to HCN vapor this 2133 cm-1 peak shifted back toward its initial position at 2139 cm-1, increasing in intensity, so that in Figure 1d, where the peak is at 2136 cm-1, roughly half of the initial intensity of the 2139 cm-1 peak had been recovered. These small variations in the band position are almost identical to those noted above which occur when the coverage of adsorbed CN- is varied by isotopic dilution with 13CN-, and were clearly due to the decrease of the CN- coverage brought about by exposure to the hydrogen plasma and the subsequent restoration of the coverage on reexposure to HCN vapor, analogous to the well-known shift in vibration frequency of adsorbed CO with change in coverage.12 Thus the effect of the 3 s hydrogen plasma treatment is to reduce the Au(CN)4- produced by the oxygen plasma back to CN- adsorbed at a zerovalent gold surface, with some loss in the CN- coverage of the surface. This dependence of the wavenumber of the CN- band on coverage has a useful application, since it gives an indication of CN- coverage which does not depend on the SERS band intensity and thus helps in determining conclusively whether the decrease in SERS signal after the hydrogen plasma treatment was due to the removal of adsorbate or to the loss of SERS activity of the film. Thus exposing a gold island film which had been pretreated with HCN vapor to a hydrogen plasma (70 W) for 150 s caused the adsorbed CN- peak to shift to 2133 cm-1 and to decrease to less than 20% of its initial intensity, but on reexposure to HCN vapor the peak was restored to ca. 80% of its initial intensity. Since this reexposure to HCN was sufficient to return the adsorbed CN- peak to 2139 cm-1, one can be sure from the wavenumber of the band that the original coverage of CN- had been restored, and thus the loss of SERS intensity due to structural or surface area changes in the film was roughly 20% of the initial signal. HCN Pretreated Silver Films. Figure 4 shows the results of a similar investigation at a silver island film which had been previously exposed to HCN vapor. Before plasma treatment, the SERS spectrum of a freshly prepared silver island film exposed to HCN vapor showed bands at 2135 and 2168 cm-1 due to Ag(CN)2- and AgCN, respectively. The assignment of these bands follows from the results of Kunimatsu, Seki, and co-workers,13,16 who showed from in situ reflection absorption infrared spectroscopy that the only species present at silver electrodes at potentials more positive than -0.2 V are Ag(CN)2- and AgCN. These surface species have infrared bands at 2136 and 2167 cm-1, respectively, close to the wavenumbers of the CN stretching modes of the substances K[Ag(CN)2] (2140 cm-1)17 and AgCN (2165 cm-1).18 The wavenumbers of the infrared-active and Raman-active CN stretching modes of the Ag(CN)2- ion are almost identical,17 and the ν(CN) vibration of AgCN is both infrared and Raman active; thus, the two bands in Figure 4a are at almost exactly the expected wavenumbers for Ag(CN)2- and AgCN. Exposure of this HCN-pretreated silver film to an 80 W oxygen plasma for 5 s caused complete destruction of the island film structure, as indicated by a rapid change in the appearance of the film from blue to gray. This (16) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R. Surf. Sci. 1985, 158, 596. (17) Jones, L. H. J. Chem. Phys. 1957, 26, 1578. (18) Chantry, G. W.; Plane, R. A. J. Chem. Phys. 1960, 33, 736.

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Figure 4. SERS spectra of a silver film pretreated with HCN vapor: (a) the initial film; (b) the film after hydrogen plasma treatment (10 s at 80 W); (c) the film after reexposure to HCN vapor.

destruction of the film was accompanied by the complete loss of the SERS spectrum, which was not restored by reexposure to HCN vapor. In contrast, exposure of an identical film to a hydrogen plasma (80 W) for 10 s also caused complete loss of the SERS spectrum (Figure 4b), but as Figure 4 shows, when the film was reexposed to HCN vapor, the SERS spectrum (particularly the band at 2135 cm-1 due to Ag(CN)2-) was substantially restored. On the basis of the total band area due to Ag(CN)2- and AgCN in Figure 4, the recovery of the SERS signal after the hydrogen plasma treatment and reexposure to HCN was about 60%. The hydrogen plasma treatment also caused a slight broadening of the plasmon resonance of the film as shown in the transmission spectrum in Figure 3, and there was a change in the appearance of the film from blue toward gray. As for the gold film, this suggests there was some change in the island film structure caused by the hydrogen plasma, and that the unrecovered loss of SERS signal was due to this structural change. To investigate this further, scanning electron micrographs of the films were recorded before and after exposure to the hydrogen plasma (Figure 5), and these showed that the most obvious effect on the island film structure was an increase in the lateral dimensions of the silver islands from ca. 30-50 nm to 100-150 nm. These dimensional changes are similar to changes seen by Van Duyne et al. in atomic force microscopy (AFM) images of 8.0 nm mean thickness silver island films after annealing the films at 600 K for 60 min.19 From the depth profiling capability of the AFM images, this earlier study showed that the annealing process resulted in coagulation of the islands, in which there was an increase both in the projected size of the islands in the plane of the glass substrate and in the particle height. This coagulation was accompanied by a blue shift of the plasmon resonance in the optical transmission spectrum. Thus there is a qualitative (19) Van Duyne, R. P.; Hulteen, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101.

Figure 5. SEM of a silver island film (bottom), and of the film after 10 s exposure to a 80 W hydrogen plasma (top).

Figure 6. SERS spectrum of a gold island film pretreated with DPDS: (a) the initial film; (b) the film after oxygen plasma treatment (3 s at 70 W); (c) the film after retreatment with DPDS.

resemblance both in the micrographs and the transmission spectra between the changes brought about by the hydrogen plasma and those caused by annealing, suggesting that an important effect of the exposure to the hydrogen plasma was heat input. DPDS Pretreated Gold Films. Figure 6 gives the SERS spectrum of a gold island film after dipping into the

Adsorbate-Covered Island Films

Figure 7. SERS spectra showing the effect of a hydrogen plasma on coadsorbed CN- and thiophenate on a gold island film: (a) the initial film after pretreatment with HCN vapor and (b) after dipping in DPDS solution; (c) the film after exposure to a hydrogen plasma (5 s at 50 W); (d) the film after redipping in DPDS solution; (e) the film after then reexposing to HCN vapor.

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Figure 8. SERS spectrum of a gold island film pretreated with PNBA: (a) the initial film; (b) the film after oxygen plasma treatment (10 s at 10 W); (c) the film after retreatment with PNBA.

DPDS solution and subsequent washing, together with spectra showing the effect of exposing the film to an oxygen plasma. DPDS is known to undergo S-S bond cleavage on adsorption at silver island film surfaces, and the initial spectrum, Figure 6a, confirms that the adsorbed species at the gold island film surface was C6H5S-, with bands at 1571 s, 1478 w, 1070 s, 1020 s, 997 s, 695 w, 475 w, and 420 m cm-1, (s, m, and w denote strong, medium, and weak bands), almost identical to those of thiophenate at silver surfaces.20-22 After the film was exposed to an oxygen plasma for 3 s at 70 W, the thiophenate bands were reduced to less than 20% of their original height, as shown in Figure 6b, but no new bands appeared and there were no shifts in any of the thiophenate bands. The film was then redipped in the DPDS solution, and as Figure 6c shows, the SERS peak heights were almost fully restored. Thus the effect of the oxygen plasma was to remove most of the thiophenate from the gold island film surface without significant loss of the SERS activity of the film, and there was no SERS evidence for any oxidation products of thiophenate remaining adsorbed at the surface. Thiophenate appears to adsorb more strongly on gold than CN-, since the SERS band of adsorbed CN- is replaced by thiophenate bands when a HCN pretreated gold island film is immersed in DPDS solution, and in addition CN- does not adsorb well on a gold surface which had been previously exposed to DPDS. It is therefore interesting that thiophenate was found to be preferentially removed from a gold island film on which there was coadsorbed thiophenate and CN- when the film was treated with a hydrogen plasma. To prepare the initial film, a freshly prepared gold island film was first exposed to HCN vapor to give a strong SERS band at 2137 cm-1 due to adsorbed CN- (Figure 7a). The film was then dipped into a solution of DPDS in acetone (1.0 mg/mL) and unadsorbed DPDS removed by washing with acetone. New bands in the 1600-400 cm-1 region (Figure 7b) confirmed that thiophenate was coadsorbed on the film, but in addition, the CN- band was largely transformed into the

Au(CN)4- band at 2179 cm-1, showing that the electrons required for the cleavage of DPDS to give adsorbed thiophenate had come from the oxidation of adsorbed CNto Au(CN)4-. When the film was exposed to a hydrogen plasma (5 s at 50 W), this Au(CN)4- was reduced back to adsorbed CN- as shown by the disappearance of the band at 2179 cm-1 and the growth of the band at 2137 cm-1 (Figure 7c), but there was only a small decrease (ca. 10%) in the total area of this doublet band compared to the corresponding band in Figure 7b. Thus the total amount of cyanide species at the surface was approximately unchanged by the hydrogen plasma treatment. In contrast, the bands due to adsorbed thiophenate were reduced by the plasma treatment to less than 10% of their intensity in Figure 7b. When the film was redipped in the DPDS solution, however, these thiophenate bands were almost completely restored, and there was a reduction in the intensity of the 2137 cm-1 band due to partial displacement of adsorbed CN- by thiophenate (Figure 7d). Finally, when the film was immersed in HCN vapor, there was little further change (Figure 7e). PNBA Pretreated Gold Films. Figure 8a shows the initial SERS spectrum of a gold film after dipping in a PNBA solution in methanol and subsequent washing. As discussed by Roth et al. the spectrum is that of the adsorbed p-nitrobenzoate anion,23 for which a band assignment has been published by Ernstbrunner at al.24 The dominant bands in Figure 8a are at the following wavenumbers (cm-1): 1597 (8a, νring), 1376 (νsym(CO2-)), 1344 (νsym(NO2)), 1109 (19a,νring), 865 (δNO2 - δCO2-), and of interest also is a weak band at 528 cm-1 due to NO2 and CO2- rocking.24 Exposing the film for 10 s to a 10 W oxygen plasma (Figure 8b) resulted in a new band at 1328 cm-1 and a decrease in the p-nitrobenzoate bands to about one-third of their initial height. No other new bands appeared, suggesting that the other bands of the new surface species are coincident with the bands of p-nitrobenzoate, but there were in addition increases in the relative intensities of the bands at 865 and 528 cm-1. Since 1328, 865, and 528 cm-1 are close to or coincident with the wavenumbers of

(20) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. (21) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307. (22) Joo, T. H.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57.

(23) Roth, P. G.; Venkatachalam, R. S.; Boerio, F. J. J. Chem. Phys. 1986, 85, 1150. (24) Ernstbrunner, E. E.; Girling, R. B.; Hester, R. E. J. Chem. Soc., Faraday Trans. 1978, 74, 1540.

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the stretching, bending, and rocking modes of the NO2 or CO2- groups, the effect of the oxygen plasma treatment on the adsorbate molecules thus appears to cause only of a minor change which affects only these binding groups. A possible interpretation is that, as was the case with adsorbed CN-, the oxygen plasma increases the oxidation state of the gold adsorption sites rather than oxidizing the adsorbate molecules. Consistent with this interpretation, when the film was redipped in PNBA solution, the original p-nitrobenzoate bands were restored to slightly more than their initial height, but in addition there was also a considerable increase in the new band at 1328 cm-1 (Figure 8c). This increase in the 1328 cm-1 band is interpreted as due to the adsorption of p-nitrobenzoate at additional oxidized gold surface sites formed in the oxygen plasma, similar to the Au3+ or Au+ sites which the SERS evidence suggests were present at a gold/CN- surface after an oxygen plasma treatment. Consistent with this interpretation, the effect of a hydrogen plasma (10 s at 10 W) on a gold film treated with PNBA was similarly to diminish the intensity of the p-nitrobenzoate SERS bands, but without giving rise to the 1328 cm-1 band or to the changes in the relative intensities of the bands at 865 and 528 cm-1. Discussion These results show that there are chemical changes in adsorbed monolayers on gold island films brought about by brief exposure to an oxygen or hydrogen plasma which may be identified by SERS. The chemical effects that were observed were changes in the oxidation state of the adsorption site gold atoms at the Au/CN- and Au/pnitrobenzoate surfaces, and there was also evidence that positive oxidation state gold atoms remain at the surface after oxygen plasma treatment even though the adsorbate may have been removed. There was no evidence from the SERS spectra for any oxidation or reduction products of thiophenate or p-nitrobenzoate remaining adsorbed at the surface, however. The results also show that it is possible to remove a large fraction of the adsorbate molecules from silver or gold island films by plasma treatment without seriously reducing the SERS activity of the films. For the gold films, the use of hydrogen plasmas showed no obvious advantage in adsorbate removal vs film damage relative to oxygen plasmas, though adsorbate removal was more rapid with an oxygen than with a hydrogen plasma. The SERS band intensities from the gold films were reduced to typically ca. 20% of the initial intensities at the plasma power levels and exposure times used here, and there was recovery of more than 70% of the SERS signal on reexposing the gold films to the adsorbate. In the case of silver films, although oxygen plasmas rapidly destroyed the films, exposure of a film pretreated with HCN to a hydrogen plasma caused complete removal of the adsorbate SERS signal, and on reexposure to HCN, the SERS bands were restored to (25) Sigmund, P. Phys. Rev. 1969, 184, 383. (26) I. Sorli, I.; Petasch, W.; Kegel, B.; Schmid, H.; Liebl, G. J. Microelectron. Electron. Components Mater. 1996, 26, 35. Shohet, J. L., In Encyclopedia of Physical Science and Technology, 2nd ed.; Meyers, R. A., Ed.; Academic Press: London, 1992; Vol. 13, p 33.

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about 60% of their initial intensity. These levels of adsorbate removal and SERS signal recovery obviously fall short of the 100% levels that would be desirable for a cleaning procedure for island film SERS substrates. However it must be noted that no attempt was made to optimize the plasma processing, and it seems possible that, with further work, exposure conditions could be found that would have less effect on the island film structure, thereby allowing longer exposure to the plasmas and more complete adsorbate removal. There are several effects of plasma treatment which could result in a decrease in the intensity of SERS bands, namely the removal of adsorbates by thermal desorption or by chemical reaction with the plasma, partial destruction of the island film microstructure due to surface diffusion and sputtering, and structural and electronic changes affecting the metallic surface sites. The increase in particle size observed in the scanning electron micrographs of a silver island film after hydrogen plasma treatment (Figure 5) suggests that it is the heat input to the film, rather than sputtering loss, which is the main cause of structural changes in the films during hydrogen plasma processing. This is supported by estimates of the sputter rates, which depend on the relative masses of the gaseous ions of the plasma and the atoms of the substrate, and are thus very small for hydrogen plasmas. Estimation of sputter rates requires knowledge of the percent ionization and the energy of ions in the plasma,25 and taking 1% ionization and 30 eV as upper limits for these quantities,26 the estimated removal rate upper limits for our equipment are 0.013 and 0.015 nm/s for oxygen plasma treatment and 0.0009 and 0.0012 nm/s for hydrogen plasma treatment of gold and silver surfaces, respectively. Thus although sputtering may have contributed significantly to structural changes in the films in the case of the oxygen plasmas, its effects appear to be insignificant for such short exposures to hydrogen plasmas. We are at present uncertain of whether it is through chemical or thermal effects that the plasmas remove the adsorbates from the surfaces, though the preferential removal of adsorbed thiophenate from a gold film in the presence of coadsorbed CN- by a hydrogen plasma, when competitive adsorption experiments show that thiophenate is the stronger adsorbate, suggests that a chemical interaction may be mainly responsible for the removal of thiophenate by hydrogen plasmas. In such cases where removal is by a chemical process, it may be possible to achieve surface cleaning without significantly affecting the film microstructure by the technique of remote plasma processing,3,4 in which the substrate is placed down stream of the plasma region so as to allow surface reactions with relatively long-lived plasma species without the heating or sputtering effects of the plasmas. Acknowledgment. We gratefully acknowledge support from a Research Grant from the Engineering and Physical Sciences Research Council, U.K., and thank the Central Microstructure Facility, Rutherford Appleton Laboratory, Didcot, U.K., for help with scanning electron microscopy (Figure 5). LA9813578