Institut für Physik der kondensierten Materie, Heinrich-Heine-Universität Düsseldorf, D Düsseldorf, EU 2

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1 A. Otto 1, M. Lust 2, A. Pucci 2, G. Meyer 3 1 Institut für Physik der kondensierten Materie, Heinrich-Heine-Universität Düsseldorf, D Düsseldorf, EU 2 Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität, Im Neuenheimer Feld 227, D Heidelberg, EU 3 IBM Zurich Research Laboratory, Säumerstr. 4, CH-8803 Rüschlikon, Switzerland Received: February 5, 2007 Accepted (in revised form): March 8, 2007 Abstract Surface enhanced Raman scattering (SERS) by transient charge transfer (TCT) is a widely accepted mechanism. This fi rst layer effect is not active at smooth surfaces, as shown from experiments and from metallic electron-hole pair (ehp) excitations after TCT. The fi rst layer effect is rather the average over a sub-monolayer of SERS active sites (SAS), where ehp excitations are less likely. We review and present experiments using various physical methods at electrodes and at ultra high vacuum samples to prove the existence of SAS, which are probably situated within fractal monoatomic steps. As the fi rst adsorbates in ultrahigh vacuum migrate to the SAS, scanning tunnelling microscopy can reveal their atomic structure. CO on vicinal Au(211) fi rst settles down at the kink sites. NO dissociates and CO 2 is activated on a subgroup of SAS of cold-deposited Cu, these processes are only observed by SERS but not by infrared reflectionabsorption spectroscopy (IRRAS). Apparently, SERS focuses on chemical active sites, whereas IRRAS monitors the majority species, which did not react. Here we propose two possible explanations. Keywords: Quantum dots, biofunctional fluorescent microspheres, fluoroimmunoassay Résumé La dispersion Raman avec surface accentuée (SERS) par transfert de charge transitoire (TCT) est un mécanisme largement accepté. Cet effet de première couche n est pas actif avec les surfaces lisses, comme démontré par des expériences et par excitations * Author to whom correspondence should be addressed: otto@uni-duesseldorf.de après TCT de paire électron-trou métallique. L effet de première couche est plutôt une moyenne sur une sous monocouche des sites actifs (SAS) SERS, où les excitations ehp sont moins probables. Nous passons en revue et présentons des expériences utilisant une variété de méthodes physiques aux électrodes et sur des échantillons sous vide hautement poussé, afi n de prouver l existence de SAS, probablement situés à l intérieur de gradins de fractals monoatomiques. Alors que les premiers adsorbats du vide hautement poussé migrent vers les SAS, la microscopie de balayage à effet tunnel permet de révéler leur structure atomique. CO sur Au(211) vicinal se dépose d abord sur des sites à faux-pli. NO se dissocie and CO 2 est activé sur un sous groupe de SAS de Cu déposé à froid; ces processus sont seulement observés par SERS mais non par spectroscopie infrarouge réfl exion absorption (IRRAS). Il semble que SERS focalise sur des sites chimiques actifs alors que IRRAS couvre les espèces majeures, lesquelles n ont pas réagi. Nous proposons deux explications possibles. Introduction Under special conditions [1], vibrational spectra of adsorbates on metals are enhanced. Surface enhanced Raman scattering (SERS) from molecules adsorbed at metal surfaces (see also the SERS Database: Surface-Enhanced Vibrational Spectroscopy by Ricardo Aroca in the worldwide web) has at least two active enhancement mechanisms: the enhancement of the local electromagnetic field and a first layer effect - to be considered as the average over SERS active sites. The state of research in 1982 under these two aspects has been described by one of the authors [2]. Since then, the scientific fields of Mie resonances of small particles [3] and of surface plasmon polaritons [4] has matured to the field of plasmonics [5]. SERS by local field enhancement has very much profited Volume 52, No. 3, 2007

2 from the special fabrication techniques of mesoscopic and nanoscopic patterns of silver and gold particles, for instance, nanosphere lithographic massively parallel fabrication of nanoparticle arrays [6, 7, 8]. The present status of the electromagnetic mechanism of SERS is described in three articles from 2006 [9, 10, 11]. The historical roots of the chemical effect of SERS are different. Some decades ago, surface science of metals concentrated on single crystal surfaces and this established the foundations of this field, as stated in many textbooks, for instance, see reference [12]. In the mean time, disordered surfaces have come into the focus of research, for instance, in relation to metal growth processes [13], catalysis on nanostructures, e.g. CO on gold [14, 15] (highly dispersed gold has been shown to be an active catalyst for the low temperature oxidation reaction of CO). This catalytic research opens chances for the experimental work on SERS active sites and the subgroup of chemical active sites. The second big field of application of SERS in surface science of metals is interfacial electrochemistry [16] and electrocatalytic reactions (for example see reference [17]). With the electrode experiments [18, 19, 20,21, 22, 23, 24], a review [25], and the theory of Raman scattering in small metal-cluster molecule complexes [26, 27, 28, 29, 30, 31], the chemical effect by transient charge transfer [32, 33, 34, 35, 36, 37] became a well established concept. See also the careful discussion, including selection rules, by A. Creighton [38]. The early SERS work of V.V. Marinyuk has been honored in a recent comprehensive review on the adatom hypothesis [39]. Furthermore, SHG charge transfer effects have been observed [40]. The possible relations of SERS active sites with single molecule SERS and surface photochemistry are outside the scope of this article. Although the first layer effect of surface enhanced Raman scattering (SERS) by transient charge transfer is a widely accepted mechanism, the questions whether this SERS mechanism is active at smooth surfaces, as predicted also by some recent theories, need to be discussed before one can proceed with the problem of SERS active sites. In sections II and III, we review and present experiments at smooth surfaces without an apparent first layer effect and point out that the according theories have neglected electron hole pair excitations in the metal, which prevent first layer SERS. Given the absence of a first layer effect at smooth surfaces, one must conclude that the first layer effect is the average over a sub-monolayer of SERS active sites. 151 While the enhancement of the Raman intensity from a molecule by local field enhancement at the site of the molecule is directly evident, there is no such direct way to determine the nature and action of SAS. Rather, indications by many experiments with different techniques and different samples have to be considered together in sections IV to VII. Therefore, cross-referencing between the sections cannot be avoided. A subgroup of SAS seem to be chemically active as described in section VIII. Open questions concerning the hypothetical tunneling sites (section IX) and the surprising differences between the vibrational spectra of NO/Cu and CO 2 /Cu samples obtained by SERS and infrared reflection absorption spectroscopy (IRRAS) will be addressed. I) Does the first layer SERS effect due to photondriven transient charge transfer exist at a smooth surface? The first layer effect of SERS [41, 42] can be simply explained by the coherent transient electron- (or hole-) transfer being restricted to directly adsorbed molecules. However, the necessity of atomic scale roughness [43, 44] or better SERS active sites was challenged by an often cited article which postulates first layer SERS by charge transfer at smooth single crystalline Cu surfaces [45], especially in ref [46] with the title Probing photoinduced charge transfer at atomically smooth metal surfaces using surface enhanced Raman scattering. There is no doubt about transient photon driven charge transfer at smooth surfaces, as demonstrated by 2-photon emission experiment, for instance for pyridine on Cu(111) [47] and for C 6 F 6 on Cu(111) with fs-time resolution [48]. On this smooth surface, the lowest unoccupied molecular resonance of C 6 F 6 has a lifetime of 7 fs for a single monolayer (ML) which increases to 37 fs above 3 ML [48]. However, when Zylka and Otto [49] repeated the electron energy loss spectroscopy (EELS) measurements of Campion and collaborators on Cu single crystals [45], they were not able to reproduce them. These previous measurements were, without exception, performed at primary electron energies, where the first low energy electron diffraction (LEED) beam is directed nearly parallel to the surface within image states. This leads to loss structures, which change by adsorption, because the image states are shifted by the change of the work function (similar structures for pyridine on smooth silver were already reported in 1990 [50]). Of course, the EELS results in ref [49] cannot invalidate the report of SERS Canadian Journal of Analytical Sciences and Spectroscopy

3 152 A. Otto, M. Lust, A. Pucci, G. Meyer at a smooth surface [46]. Searching for a first layer effect of SERS from smooth surfaces needed careful Raman measurements of adsorbates on Cu-single crystals and on room temperature annealed copper films. After many sputter-annealing cycles of Cu (111), Cu (100) and Cu (110) single crystals, the ubiquitous signal of spurious carbonacious contamination [51, 52] disappeared [53]. The consequent adsorption of ethylene (C 2 ) on the 3 cleaned surfaces did not yield any Raman signal [53] (for C 2 on Cu(110) see figure 3 in [54]). These results corroborated the report of unenhanced Raman spectra of nitrogen physisorbed on Ag(111) [55]. Further experiments at smooth electrode surfaces [56] are described in section IV. II) Theoretical explanation of the absence of first layer SERS at a smooth surface In all the theoretical treatments of enhanced Raman scattering of adsorbates by transient electron or hole transfer from the metal to the molecule and back, it was always assumed that the electron (or hole) was in the same state before and after the Raman process. In this way, only molecular vibrations were excited, but not the metal. In the case of a very small cluster of metal atoms with a small number of free electrons, this is very likely. But an extended metal has a quasi-continuum of electron hole pairs excitations. It is most likely that the electron (or hole) does not return to the initial state after transient transfer to the molecule for some fs, which means that an electron hole pair is created. The process of metallic excitations by transient charge transfer without exciting a molecular vibration is the cause of surface resistance induced by adsorbates. The increase of the macroscopic Ohmic loss in DC measurements of thin films by the formation of an adsorbate layer (surface resistance) [57] reflects the creation of electron hole pairs. In inelastic laser light scattering, when a molecular vibration is excited, the electronic continuum is most likely also excited. This yields the sum of a discrete vibrational energy and a continuous electronic excitation energy for the photon energy difference between the initial laser radiation and scattered radiation. This is still a continuous spectrum of intensity versus scattered photon energy without a discrete line, and hence no Raman scattering. A more formal derivation may be found in ref [58]. The same conclusion was reached by considering dephasing of plasmonic excitations [54]. Hence, there is no first layer SERS by dynamical charge transfer at a perfectly smooth surface [59]. Recently, B. N. J. Persson et al. [60, 61] presented a chemical contribution to SERS which is closely related to Persson s very useful theories of surface resistance [57,62, 63] and of the influence of the matrix environment on the optical properties of small metallic particles [64]. Like surface resistance, the proposed new mechanism [60] is based on the electric field parallel to the surface. It is clear that the energy dissipation in DC resistance and the decrease of the dephasing time of plasmon resonances in small particles is caused by electron hole excitations by transient charge transfer from metal to adsorbate and back. But now Persson et al. [60] state: In the context of SERS (at resonance) we have no electronic excitations in the final state. If this were true, there would be no broadening of small silver particle plasmon resonance by adsorbates [64] because the energy dissipation by SERS is negligible and the dephasing of the plasmon resonance does not happen. The predicted chemical enhancement of about 102 for CO on small silver particles and of about 6000 for CO on plane smooth silver [60] is not observed. The SERS spectrum of CO of silver islands in UHV at 10K shows clearly a first layer effect for both the large signal of CO at SERS active sites at about 2110 cm -1 and for the weaker signal at 2131cm -1 of physisorbed CO [65]. In order to observe CO on Ag films, one needs cold-deposited silver films [66]. These films on cooled substrates are porous, as indicated by the surface area available for first layer adsorption of Xe by the film being much larger than the substrate surface covered by the film [67, 68], (this may also be seen in fig 7 in section VII ). The experimental relation between surface resistance, infrared, surface enhanced infrared, and surface enhanced Raman spectroscopies of CO and C 2 on copper films was discussed [69] with the Newns-Anderson model, which is the base of Persson s theories [57, 60, 62, 64]. To avoid a misunderstanding, of course the Raman signal of an adsorbate at a non-smooth surface is always enhanced by plasmonic resonances, and when it is not adsorbed at a SERS active site, it just misses the extra enhancement which is restricted to adsorbates on or at SERS active sites. III) Experimental indications for SERS active sites (SAS) and their stability a) At electrodes Pettinger et al. [70, 71] started with a smooth epitaxial Volume 52, No. 3, 2007

4 silver layer electrode and monitored the SERS intensity of the 1009 cm -1 pyridine breathing mode as a function of the amount of dissolved and re-deposited silver. An appreciable SERS signal was already found after the redeposition of one monolayer of silver. This frequency is the same as the so-called E (extra) mode of pyridine adsorbed at sites of atomic scale roughness (for a definition see ref [44]) at the silver-vacuum surface [72,73]. One should note that the positions of the strongest bands of an adsorbate at cold-deposited films and at electrodes roughened by an oxidation-reduction cycle are fairly similar, see table 1. The observation of a SERS signal from Ag(CN) 3 - complexes from silver electrodes roughened by an oxidation reduction cycle in an CN - containing electrolyte was considered as the demonstration of the importance of atomic roughness sites in SERS [81]. The cathodic quenching of SERS first reported in [82] was explained by a loss of adatoms [83, 84], and the thermal quenching of SERS at electrodes was assigned to the dissociation of Ag adatom-cl - surface complexes [85]. A comprehensive description of the relevant extensive work up to 1990 may be found in [86]. The latter publication also reports that the SERS signal recovers within 30 minutes. This probably reflects the action of the so-called exchange current given by Ag + dissolving from the electrode and settling back at some other site. This is related to the observation of roughened thin silver films by chloride induced corrosion [87, 88]. An extensive analysis of literature data from the first decade of SERS research [89] led to the first deduction 153 that the adatom concentration on colloidal silver particles depends on the preparation condition. Secondly, this concentration has a strong influence on the peak position of the SERS excitation spectrum and on the enhancement. The so-called chloride activation of SERS signals, already observed in the geminating paper by Jeanmaire and VanDuyne [74] has been reviewed in section 2 of [90]. Further strong indications of SERS-active silver -chloride-pyridine and thiocyanate complexes covering less than 3% of the surface were the competitive and cooperative adsorption on activated silver electrodes [91] (see also the local cooperative effects of cyanide and pyridine on an activated Ag electrode in an aqueous SO 4 = electrolyte [92]) and the quenching of SERS by under potential deposition of adlayers of lead [93-96], thalium [93, 96-98] and cadmium [99]. The loss of SERS by adding thiosulfate [ ] was explained by the dissolution of Ag + complexes. Silver film electrodes with (111) orientation on mica, used in the Kretschmann-ATR (attenuated total reflection) configuration [102], did not deliver any Raman signal from adsorbed p-nitrosodimethylaniline before roughening [103]. But SAS were produced by as little as one monolayer of Ag restructuring [103]. The application of the surface-plasmon resonance method with the Weierstraß prism in Otto configuration [104] to various single crystalline copper electrodes in a perchlorate electrolyte [56] results in an extra electromagnetic enhancement of about 70 with respect to the external reflection geometry used in UHV experiments [53]. For adsorbed pyridine, weak signals (with frequency Table 1 Comparison of SERS band positions with samples in UHV (E bands) and in an electrolyte, for the example of pyridine and ethylene. metal sample pyridine, CC ring ethylene CC stretch ethylene δ (CH 2 ) breathing Ag cold-deposited 1006 [72] 130 K Ag electrode 1006 [74] -0.6 V SCE Cu cold-deposited 1009 [75] 120K Cu electrode -0.5 V to -1.1 V 0.1 M KCl 1011 to 1008 [76] Au Au cold-deposited 130K ca 1009 [77] 1271 [78] 1536 [78] electrode 1010 [79] 1278/1288 [80] 1535/1545 [80] 0.1 M KClO 4 1 M H 2 SO 4 1 M H 2 SO 4 Canadian Journal of Analytical Sciences and Spectroscopy

5 154 A. Otto, M. Lust, A. Pucci, G. Meyer displaced with respect to the frequency on smooth parts of a cold-deposited Cu film [75]) were only found from the species adsorbed at the small concentration of surface defects on the copper single crystals, the big majority of adsorbed pyridine molecules did not contribute to the spectrum. In this case, no cathodic quenching and chloride activation of the SAS were observed [56]. In contrast, a polycrystalline Cu films roughened by an oxidation reduction cycle (ORC) showed cathodic quenching in 0.1M KClO 4 aqueous electrolyte, but no quenching in 0.1M KCl aqueous electrolyte [90]. This difference of the volatility between SAS in perchloric and chloride electrolyte is given by the different bonding strengths of the anions (strong for Cl -, weak for ClO 4- ). This means that the small concentration of SAS on Cu single crystals is stable, whereas on ORC roughened Cu films, the SAS are stabilized by Cl - but not by ClO 4-. On silver electrodes roughened by ORC s, Cl - cannot stabilize the SAS. While the SERS of thiocyanate on silver roughened by an ORC in 0.1M NaClO 4, 5mM thiocyanate is cathodicly quenched, this is not observed when the silver samples were mechanically polished by diamond and alumina particle slurries [105]. Apparently polished silver samples have stable SAS like Cu single crystals with surface defects [56]. mode of a monolayer of benzene. But when less than a monolayer of silver is deposited at 40 K on this silvered grating, a frequency displacing strong extra (E) breathing mode is also observed [107, 54]. b2) Silver deposition on smooth fi lms covered by pyridine Ref [108] reported spectra of pyridine adsorbed on smooth silver onto which small quantities of silver were deposited at low temperature, see fig 1. After silver deposition, the very weak band at 992 cm -1 is replaced by the strong E band at 1006 cm -1 and the N-band shoulder at 992 cm -1, is observed with increased intensity. The increase of the N band in the presence of the E band is corroborated by results in sub-section b5). This is not a classical electromagnetic effect (see section V). It was also possible to differentiate between adsorbates at SAS and on the smooth parts of the surfaces by resistance measurement with C 2 and C 2 adsorbates [109] (figure 2). At small quantities of pre-adsorbed cold-deposited Ag up to 1.5 nm average thickness, the resistance change by low gas exposures is different than b) At ultra high vacuum (UHV) surfaces In UHV, one can observe bands of adsorbed molecules at smooth terraces, which we call normal (N)-bands. One can create stabilized atomic scale roughness by cooling without the involvement of anions, cations. and water. The shifted bands, which are only observed in the presence of sites of atomic scale roughness, are called extra (E) bands by us. These conclusions were obtained from experiments with various samples. b1) silver gratings Kirtley and collaborators [106] were the first to use optical gratings covered with a silver film vapor deposited at 300 K as SERS substrates. When 2 nm of silver was deposited on top of this grating at low temperature, and after a small exposure to pyridine, a new band of the pyridine CC ring breathing vibration at 1005 cm -1 showed up (after some more exposure, a band at 992 cm -1 also appeared [106]). In 1982, this band was believed to be a manifestation of a long range effect [72], a SERS signal only from the second and further condensed layers. By using the surface-plasmon polariton resonance of a microscopically smooth silver film on an optical grating, one can observe the normal (N) Raman CC breathing Figure 1 (a) Top: Raman spectrum of a silver film, deposited at room temperature, exposed at about 40 K to 1 L of pyridine in the range of the C-C breathing mode, 1 W, integration time 2000 s. Bottom: Raman spectrum of liquid pyridine. (b) Raman spectra of the sample described in (a) for the indicated average thickness d cold of additional silver deposited on top at about 40 K. 1 W, integration times 800,400, 400,400 s. Volume 52, No. 3, 2007

6 155 Figure 2. Variation of the DC resistance of smooth and atomically rough silver films during exposure to C 2 (left) and C 2 (right). First a SERS-inactive film (T deposition = 225K, T anneal =340K) was prepared, onto which various quantities (given by average thickness d cold (nm) of Ag was deposited at 60K. Exposure to C 2 and C 6 at 60K. A monolayer of adsorbed gas was achieved at exposures given by the horizontal bars. at higher gas exposures. At thicker cold-deposited layers, the results of Holzapfel et al. [110] of a monotonous decrease (C 2 ) or increase (C 2 ) are retrieved. In section IX these results will be discussed further. b3) single crystals For example, when a Cu(977) crystal was decorated with three monolayers of Cu at 45 K and exposed to 1.5 L of ethylene, strong E bands and weak N bands were observed. After this sample was heated to 200 K, recooled to 45 K, and exposed again to 5 L of ethylene, the E bands did not reappear. In this case the SERS active sites were annealed below 200 K [111]. b4) cold-deposited fi lms Careful investigation on cold-deposited Cu samples [75] proved that the N- band originates from directly adsorbed species on non-annealable sites (probably (111) facets) and has the same frequency like the condensed species [112]. In this respect the result in Figure 3 is of interest. A cold- deposited silver film was annealed at 250 K, where it shows nearly optimum SERS [113] and re-cooled and covered with less than a monolayer of ethylene C 2. Then, the sample was further exposed to ethane (C 2 ), Figure 3. Cold-deposited silver film, annealed to 250K, and exposed at 38K to 5 L of C 2 (ethylene). The change of the intensities of the E and N modes of the CC stretch mode υ 2 and the δ(ch 2 ) scissor mode υ 3 of C 2 during post exposure to C 2 (ethane) and the CC stretch mode υ 3 of C 2 is given. N-line intensities have been enlarged, and lines are guides to the eye. which is showing up in the Raman spectrum. Although, of course, the quantity of ethylene was not increased by the post-exposure of ethane, the ethylene SERS intensities, both for the E and N bands, increased considerably [114]. This may be caused by a shift and increase of the plasmonic resonance. Of course, this would also happen by post-exposure of ethylene, in this way mimicking a long-range enhancement. The first layer effect can only be proven by different molecules in the first and second layer [41, 115] or by simultaneous SERS and resistance experiments [116]. Thermal desorption spectroscopy investigations of cold-deposited copper films [112] have shown that C 2 settles first at annealable sites (E sites), where it desorbs at about 220 K, see Figure 4, whereas C 2 contributing to the N band desorbs at about K. This desorption temperature agrees with the desorption temperature of C 2 on Cu(111) of K [117]. There are also adsorbed species, called I (invisible) species [112], desorbing between 120 and 180 K, which apparently do not contribute to the observed Raman spectrum. Maybe these are molecules whose orientation is not favorable for transient electron transfer. The discussion Canadian Journal of Analytical Sciences and Spectroscopy

7 156 A. Otto, M. Lust, A. Pucci, G. Meyer Figure 4. Thermodesorption spectra of seven copper films prepared in the same way and annealed to 200 K after various indicated exposures to ethylene at 40 K. on the I species is resumed in b7). b5) rough Cu fi lms Figure 5a presents the development of the spectra of C 2 on a 80 nm thick copper film deposited at 300 K on rough CaF 2, cooled to 40 K, exposed and measured at this temperature as function of exposure [111]. Compared to a cold-deposited film, the intensity of the E band is relatively low. The N bands appear only above 2.8 L exposure, see Figures 5a and 5b. The integrated band intensities increase less with exposure above about 8L (Figure 5b). This does not seem to be in agreement with a first layer effect, but see the discussion in b4). The right side of Figure 5b gives the band positions. The frequency of the E band falls already before the appearance of the N bands and reaches a constant value approximately above about 8L. The frequency of the N bands is constant. If a similar Cu sample is prepared at 350 K on rough CaF 2, the E bands do not show up any more (see fig 5c, left side), indicating the loss of SAS between 300 and 350K. The intensity of the N band is weaker than in the presence of E bands (compare with figure 5a). The Volume 52, No. 3, 2007 Figure 5a. Spectra of C 2 on a 80 nm thick copper film deposited at 300 K on a rough CaF 2 film of 200 nm thickness, cooled to 40 K,in the spectral range of the δ(ch 2 )-scissor mode. The band at the lower (higher) wavenumber is the E-(N-) band. Features below the E-band are discussed at the end of section IX. integrated N band and condensed layer intensity grows now linearly with exposure (fig 5c, right side), probably indicating the long-range electromagnetic enhancement (further arguments will be presented at the end of section IV). Whereas all the results in Figures 5a-c demonstrate nicely the SAS and their disappearance by heating the sample, Figure 5a does not demonstrate the first layer effect, as was discussed above in sub-section b4). b6) Metal-island fi lms On Cu island films, prepared at 390 K, the E bands of C 2 are still observed [111]. A higher annealing temperature was not possible due to technical reasons. Furthermore, the E bands of CO on a room temperature prepared silver island film are still observed [65], though a silver film heated to room temperature is SERS-inactive [113]. The difference between cold-deposited films and films on rough CaF 2 and island films may be that the first two samples can flatten, forming a sample with

8 157 Figure 5b. Integrated band intensities of E-and N-bands (left) and spectral positions (right) of the bands in Figure 1a. Fig 5c. Development of the N-band of C 2 after annealing the SERS active sites. Canadian Journal of Analytical Sciences and Spectroscopy

9 158 A. Otto, M. Lust, A. Pucci, G. Meyer a low concentration of monoatomic steps, which are mostly straight with a small concentration of kink sites. But a relatively round and smooth island just cannot exist without steps with kink sites. b7) Infrared reflection absorption spectroscopy (IRRAS) of adsorbates IRRAS on metals gives good monolayer spectra, in contrast to SERS. Whereas IRRAS of C 2 on room temperature deposited Cu films yields only the IR active modes of C 2, IRRAS of C 2 on cold-deposited copper films yields at low exposure only the Raman active lines of C 2, like SERS [118]. The excitation mechanism is very similar [118] to the SERS mechanism at SAS. This proves very nicely that the charge transfer mechanism is only active at atomic scale roughness. It seems that the I species discussed in b4) are observed in the infrared (IR) transmittance of 5.4 nm (island-like) Cu grown at 40 K on KBr (001) during C 2 exposure at 40 K [119, 120]. With increasing exposure, the sequence of appearance of the CH 2 -scissor modes is: E band, an IR band tentatively assigned to the I - species, N-band and then further growth of the infrared bands. It is a puzzle why only the SERS bands but not IR bands of the E species are observed. Is the IR field at the site of the E species screened by the electronic polarizability of nearby Cu atoms within the atomic scale roughness of the SAS, whereas an electron transfer over a fraction of one Cu atom diameter is still possible, yielding the SERS-E bands? Related problems will be discussed in section X. c) Comparison of results from electrodes and UHV samples When comparing the SERS spectra from UHV samples and electrodes, one problem is obvious: why does one not observe N bands of pyridine at an electrode? The simple answer that the N bands originate from condensed pyridine (not adsorbed in direct contact with electrode) is wrong! Figure 1a is a good indication: at an exposure of only 1 L one would not expect coverage above a monolayer. The best proof is the following: J. Grewe [112] exposed cold-deposited copper films (annealed to 200 K and re-cooled) under very small C 2 background pressure ( Torr) and slowly raised the sample temperature from 80 K to 200 K. The N band was still present at 95 K in a temperature range far above the desorption temperature of multilayers at K, see Figure 4. For these copper films the first layer effect was demonstrated also for the N species (the ethylene molecules adsorbed at (111) facets), firstly by the simultaneous quenching of the E and N intensities by the adsorption of submonolayer amounts of oxygen [121, 112] and secondly by blocking the surface by a spacer layer of pyridine. By measuring coverage and Raman intensity, Grewe et al. [112] have evaluated a ratio of the E to N Raman cross sections of 12 ± 3 for cold-deposited copper films annealed to 200 K. The absence of N bands of pyridine at electrodes cannot be explained by only pyridine settling down at SAS, in view of the detailed work on the pyridine coverage of noble metal electrodes [122]. We propose the following hypothesis: 1) N-type molecules are adsorbed next site to a SERS active site (SAS). In the ground state, they have no electronic contact to the SAS, therefore they have the vibrational frequency of the majority species. It is always the ground state vibration, which is observed in Raman spectroscopy, irrespective of the Raman mechanism. However, in the electronic excited state, the electron may enter the LUMO of the N species, which leads to an enhancement by transient charge transfer. Subtle effects of water and/or the ions of the electrolyte close this channel. 2) The signal to noise ratio is not good enough to observe the Raman signal of the majority species. 3) Only when the samples are smoothened and lose most of the SAS, the enhanced EM enhancement enables one to observe the majority of adsorbed species and the condensed species, see Figure 5c. IV) No EM resonances from active sites observed Differential reflectivity in UHV at a coverage of 1-3 monolayers of cold-deposited Ag on Ag(111) [123] and on smooth silver films [124] and during electrochemical silver deposition on smooth silver electrodes [125] yields, besides weak surface plasmon structures, only a very weak continuum of electronic excitations in the range of the employed laser radiation. The weak absorption continuum is caused by electron scattering at the residual surface roughness. It is the optical analogue of the increase of the DC resistance of a thin smooth silver film covered by submonolayers of cold-deposited silver [109]. EELS of cold-deposited copper films in the loss range 0-10 ev showed no difference between films deposited at room temperature and at low temperature [49]. One must not expect a discrete electromagnetic resonance of SAS of atomic dimensions because of the small Volume 52, No. 3, 2007

10 159 number of conduction - electrons within the atomic environment of the SERS active site. For instance, a silver adatom at a monolayer step has only 5 nearest neighbor silver atoms and therefore on the order of 6 conduction electrons for the local polarizability, a hypothetical tetrahedral Ag 4 cluster on a (111) plane [126] and its nearest neighbors will have on the order of 11 electrons. These numbers are by far too small for a plasmonic resonance connected with such sites. V) What is the configuration of a SERS active site? E bands of ethylene are lost after annealing Cu (977) with atomic-scale roughness (produced by cold-deposition of 3 Cu monolayers) to a temperature of 200 K [111]. But SERS-active sites on vapor deposited Cu films on rough solid CaF 2 are annealed between 300 and 350 K, and copper island films do not lose these sites below 400 K [111]. These differences have been explained above by the different necessary efforts for leveling macroscopic bumps and islands. The annealing of cold-deposited copper films as a function of temperature [114] is presented in Figure 6 (top figure). The nearly total loss of the E band sets in before the N bands. The similar trend of the N bands and the Raman background at the position of the N bands has been explained with a simple model including electron scattering by surface roughness [127]. The annealing of the E bands is compared in Figure 6 to the recovery of the intensity of the elastic in phase He scattering [128] of a copper (111) crystal, onto which 0.1 monolayer (average) of Cu was deposited at 100 K during heating. The displayed annealing step was assigned to the transition from fractal to compact islands of monoatomic height (dimensions on the order of 30 nm). This transition is determined by the onset of the diffusion of adatoms length monoatomic steps (not leaving the island edges, no spreading out to the terraces) leading to straight steps and therefore to compact islands. Qualitatively similar conclusions about the same system were also drawn from high resolution low energy electron diffraction [129]. Therefore, we conjecture that the SAS are situated in fractal steps, e.g. single and multiple kink sites, adatoms, and vacancies at steps. Of course this is not in contradiction that for instance on island films the SAS cannot be annealed. When an island remains nearly rounded, it does not lose the kink-sites at its surface. So why is SERS possible for adsorbates at some of these sites? We conclude that sites at fractal steps do Figure 6. top: SERS annealing curves. The copper film was annealed from 40K in steps of about 50K after exposure to 15L of C 2. Spectra were taken at 40K after recooling and replenishing the C 2 desorbed in the previous annealing step. Intensities of E and N-bands of the modes ν 2 (CC stretch), ν 3 (CH 2 scissor) above the SERS background and SERS background at the frequencies of ν 2 N and ν 3 N below: Debye-Waller corrected in-phase n=2. He scattering intensities during annealing of 0.1 ML Cu, deposited onto Cu( 111 ) at 100 K, as a function of substrate temperature, taken from Wulfhekel et al. Canadian Journal of Analytical Sciences and Spectroscopy

11 160 A. Otto, M. Lust, A. Pucci, G. Meyer exist, where electrons are bound on the order of some fs in so-called surface resonances (sites at the surface where the extended electron wave function has a much higher amplitude than in the bulk and consequently a higher chance to be found there, which is synonymous with a longer lifetime there than at other smooth surface sites). So, even when the electron is residing for some fs in π* orbitals, it has some chance to reunite with the hole, which is waiting for some fs in the surface resonant state [58, 59]. In this way, the excitation of a molecular vibration is possible without the excitation of an electronhole pair (which in the language of solid state physics is annihilated). The adsorbate and the SERS active site would constitute together a surface complex, weakly coupled to the bulk. Surface complexes have been postulated already decades ago, see section IVa. A localized electronic surface resonance has no dispersion in surface k-space. There are some reports of excited electronic states without dispersion at step, but much work with scanning tunneling microscopy (STM) has been not done on these sites. VI Search for SAS by STM The first adsorbed molecules migrate to the sites of highest bonding enthalpy and yield the highest SERS signal, see the example of C 2 on cold-deposited Cu films [112] and CO on cold deposited silver, as demonstrated by the thermodesorption spectra [130], see Fig 7. Therefore, it might be possible to find and observe SAS with STM by looking for the adsorption site of the molecules at low exposure. It is well known that the frequency of CO adsorbed at all kinds of defects on Cu single crystal surfaces is above 2100 cm -1, see Hollin s review [131]. Among these defects are the SAS where the CO on top stretch frequency is 2102 cm -1 [132]. For the transverse frustrated translation, Akemann s SERS investigations yielded 24 cm -1 [132]. Inelastic Helium scattering spectroscopy localized this vibration on flat Cu(001) and Cu(111) at 32 cm -1 and at steps at 25.8 cm -1 [133]. This supports the assignment of the band at 2102 cm -1 at low exposures to on top adsorption at defect sites. From low temperature STM experiments it is known that at higher exposures on Cu (211) (for a ball model of that surface see Figure 8a). CO binds at the on top sites of the intrinsic steps [134]. The step edge atoms have 7 nearest neighbor Cu atoms, whereas Cu atoms within a (111) surface have the coordination number 9. An even higher binding energy was observed for single Figure 7. (a) Peak intensity of the CO stretch band in SERS from a cold-deposited silver film (at 9 K) versus exposure to CO (at 9 K). (b) CO themodesorption (TDS) spectra. The silver film was deposited at 30 K and preannealed at 110 K in order to avoid film reconstruction during TDS. Parameter is the exposure to CO at 20 K, heating rate about 4 K/s. Bilayer and multilayer desorption below 35 K is only observed at exposures above 30 L. For smooth silver films, bilayer desorption starts already at exposures of about 1.75 L. CO molecules adsorbed on single ad atoms at the lower side of steps on Cu(211) [135]. The coordination number of these adatoms is 5. A stronger bond of CO at steps on a stepped Cu(111) surface was also measured by thermodesorption spectroscopy [136]. For adsorption of CO at surface defect sites such as steps and kinks, reference [137] reports binding energies which are systematically larger than for adsorption sites within perfect terraces. Intrinsically stepped surfaces like the Cu(211) surface are particularly interesting in this case, since depending on the substrate preparation a relatively high concentration of kink sites can be easily produced. According to STM on a Cu (211) surface [134], CO completely adsorbs (at a temperature above 80 K) initially at kink sites (see Figure 8b) before occupation of step edge sites is observed. The straight lines in Figure 8b correspond to the intrinsic step edges of the Cu(211) surface. The step-step distance is nm (see fig 8a). Single CO molecules adsorbed above 80 K are mobile Volume 52, No. 3, 2007

12 161 observed at low CO exposures of cold-deposited Cu films [132] is bonded to kink sites. VII Chemically active sites In SERS, activated species N 2 -δ and CO 2 - have been observed besides the neutral species [138], see Figures 9 and 10, but after exposing cold-deposited Cu to NO, only dissociation products of NO can be seen [139], as shown in Figure 11. Whereas the anionic CO 2 - is bonded stronger to the surface than the neutral species, the CO 2 - activating sites are annealed below 200 K [140]. The same holds for N 2 [138], see Figure 9. Furthermore, the NO dissociation is no longer seen by SERS after annealing the copper sample to about 150K [141]. The loss of the chemical activity by annealing the cold-deposited Cu films to 200K is clearly below the loss of the SAS at 250K, see Figure 6. Maybe there are more types of active sites, with the chemical active sites being a subgroup of them. Figure 8a. Ball model of the Cu(211) surface. The distance between intrinsic steps is nm. Horizontal chains from left to right are along the (0, -1, 1) direction. Copper atoms in the step edge are labeled A, at the kink site: B. along the intrinsic steps and settle down at the kink sites, i.e. at the endpoints of the intrinsic steps, see Figure 8b. These kink sites have the coordination number of 6. Imaging of CO molecules in the STM is accompanied by a strongly tip dependant contrast formation. In general, using a metallic tip, CO molecules appear as a depression (with a small centre protrusion), while using a CO terminated tip they appear as a single protrusion. On the intrinsic steps, CO molecules are observed as dark depressions with a metallic tip [134]. In the case of the adsorption on a single adatom, a single protrusion is observed, but the height of the whole complex is smaller than the height of the adatom alone. A similar effect in the appearance of the CO molecules at kink sites, using a metallic tip, can be observed in Figure 8b. The height of the kink atom CO complex is lower than that of the clean step edge. At low enough temperatures, where no CO diffusion is observed, molecular manipulation of single CO molecules can be applied to reversibly occupy kink sites with a single CO molecule. Kink atom - CO complexes formed this way have the same appearance in the STM images. In summary, it is very likely that the CO SERS band VIII Hypothetical tunneling sites The DC resistance and the ratio of the real surface and the apparent surface (roughness factor) of cold-deposited SERS active Ag films decrease approximately in the same way during annealing from 50 to 330 K by a factor of about 20 [68, 116]. In other words, the resistance is approximately proportional to the internal surface. This was rationalized with the hypothesis that the internal surface (open to adsorption) and the scattering centers for the electrons at the Fermi surface propagating from crystal grain to crystal grain are given by porous grain boundaries, which are eventually transformed into compact grain boundaries by annealing. The higher resistance of a porous grain boundary with respect to a compact grain boundary may be given by narrow junctions and by narrow tunneling gaps of atomic dimensions. Holzapfel et al. [110] detected in relatively thick cold-deposited films a decrease in resistance by adsorption of molecules with a π* orbital (for instance C 2 ) and an increase of resistance for molecules without π* orbital (for instance C 2 ). This is also evident in Figure 2 at 4 nm of cold-deposited Ag. The eventual increase of the surface roughness factor is reflected by the increasing exposure to form a monolayer. A decrease is not observed at smooth surfaces, see also Figure 2. Only molecules contributing to the resistance change in thick cold deposited Ag films contributed to SERS [116]. Clearly, for the small cold-deposited silver quantities Canadian Journal of Analytical Sciences and Spectroscopy

13 162 A. Otto, M. Lust, A. Pucci, G. Meyer Figure 8b. STM plot of a non-perfect Cu(211) surface after a small exposure to CO, decorating some of the end points of the step edge Cu chains in (0, -1, 1) direction, (from left down to right up). Some of the CO molecules are indicated by an arrow. Volume 52, No. 3, 2007

14 163 Figure 10. Comparison of SERS and infrared reflection-absorption spectra (IRRAS) of linear CO 2 and activated CO 2 -on cold-deposited Cu films (40 K). Bands between 645 and 768 cm -1 are due to OCO bending modes, bands at 1182 and 1368 cm -1 are due to the symmetric CO stretch modes and bands between 2323 and 2346 cm -1 are due to the antisymmetric CO stretch mode of linear CO 2. Figure 9. Bottom: Raman spectrum of a copper film deposited at 40 K and exposed to 8 L N 2, at 40 K. Top: Raman spectrum of a copper film, deposited at 40 K, then annealed at TA= 200 K for 120s and exposed to 8 L N 2, at 40 K. Figure 11. Comparison of of the IRRAS and SERS spectrum of cold-deposited Cu exposed to NO. The weak N 2 O bands in the IRRAS spectrum appear only in the second monolayer. Canadian Journal of Analytical Sciences and Spectroscopy

15 164 A. Otto, M. Lust, A. Pucci, G. Meyer Table 2. Frequency of the ν 3 δ(ch 2 ) scissor mode of C 2 adsorbed on different Cu samples. C 2 δ(ch 2 ) scissor mode cold-deposited Cu rough Cu film on CaF 2 Cu island film frequency (cm -1 ) reference [112] [111] [145] E band, low exposure E band, high exposure N band, at all exposures Figure 12. (a) Schematic representation of the way in which the overlap of electron density distortions at adjacent island rims may result in an overall smoothing of the electron density contours for small islands. (b) A schematic representation of step heights as measured by Thermal Energy Atomic Scattering and by Low Energy Electron Diffraction. Note that the classical turning point of thermal energy atoms is actually located nm above the surface. of 0.15 nm in Figure 2, porous grain boundaries and tunneling gaps are not expected. In ref [109] Persson et al. gave a good explanation of the decrease of resistance by C 2 seen in Figure 2 with covalent bonding between C 2 and silver adatoms. But this model could not explain the increase of resistance by C 2 adsorption. It was proposed that the resistance increase by C 2 was caused by preventing the lateral smoothening of the electronic density profile (Smoluchowski effect [142,143]) at sites of atomic scale roughness, see Figure 12, thus increasing the resistance [109]. Loosely speaking, C 2 would accept some electron density within its π* orbital, and when adsorbed at sites of atomic scale roughness in a proper way, it would smooth the profile, but C 2 adsorbed at the same site would roughen the profile. To test this idea, it would be interesting to measure whether the resistance change by adsorbed CO is positive. Accord ing to the STM results in section VI, CO adsorbs on the edges, which can be considered as partly positively charged. In the language of chemistry, the edges or generally the protrusions are Lewis acid sites, defined as sites accepting electron pairs [144]. In this picture, CO would act against the Smoluchowski type smoothening, thus increasing the resistance. Tunneling sites within porous grain boundaries as SAS [110] postulated two years before the work described above [109] gave a respective explanation. For instance, C 2 at a tunneling site decreases the resistance by resonant tunneling through the π* orbital whereas the only channel for tunneling through C 2 is the high lying σ* orbital [110]. However, it has to be noticed that the formation of porous grain boundaries needs a certain average thickness of the cold-deposited films. Molecules bridging a tunneling site would have electronic contact to both grains, not just one to one active site. Therefore, one may expect different molecular vibration frequencies in this case. But nothing indicating this has been observed for SERS of C 2 adsorbed in thick cold-deposited copper films (assumed to have tunneling sites, called TS ), rough Cu films on CaF 2 and Cu island films (both assumed to have no TS), see the observed frequencies in Table 2. In summary, molecules in TS sites remain so far a hypothesis. Nevertheless, we should mention that during annealing of a cold-deposited Cu film covered with C 2, the E band was lost between 210 and 230 K (in agreement with the TDS data in Figure 3), but a band at about 1227 cm -1, which is weakly apparent in Figure 1a, persisted [146]. The shoulder at about 1260 cm -1 was annealed between 135 and 180 K [146]. Both features are also present in Figure 1a. IX Comparison of SERS and IRRAS spectra of NO, CO 2, and N 2 on Cu The results to be compared were obtained by different people at different times with different equipment, Volume 52, No. 3, 2007

16 165 Table 3. Preparation conditions of cold-deposited films. IRRAS N 2, NO, CO 2 [147] SERS N 2 [138] SERS NO [141] SERS CO 2 [138, 140, 148] substrate copper plate +35 nm Cu in UHV at RT copper plate +50 nm Ag at RT in UHV copper plate +100 nm Cu at RT in UHV Cu deposition temperature Cu film thickness Cu deposition rate K 25 nm 0.75 nm/minute 10, 40 K nm 0.5 nm/s 30 K 100 nm 0.4 nm/s Copper plate 40 K nm 0.1 nm/s therefore the cold-deposited Cu films were not prepared in the same way, see Table 3. The question that arises is whether the comparison makes sense. All cold-deposited Cu films have a thickness of the order of the light penetration depth (skin depth) d p in bulk copper, which may be estimated in the spectral ranges of mid-infrared and red laser light with the Drude dielectric function as c/ω p, (ω p is the plasma frequency), which is constant throughout all of this spectral range. Using the data for Cu at the laser wavelength nm [149] and 1800 cm -1 [150] gives values of d p of 28.6 and 24.4 nm, respectively. Most likely, the region of the cold-deposited copper films beyond a depth of 50 nm does not play a role in SERS. Detailed experiments on the role of thickness are available for cold-deposited silver. The maximum of the desorption peak of Xe on smooth silver at 95 K increased with the mass thickness d cold of the cold-deposited Ag film on the smooth silver. For d cold = 30 nm, it has reached 123 K, and up to d cold = 100 nm it still rises by an additional 5 K [68]. This means that at d cold = 30 nm, not much is needed to have the full collection of surface sites of special atomic configuration. As seen by film resistance, the thick film properties are already reached at d cold = 4 nm, see Figure 5. Beyond that thickness, the resistance dependence on the exposure to C 2 is only decreasing. The different film preparations may lead to changes in the ratio of electromagnetic and chemical contributions to SERS, but a full suppression of a special kind of surface site can be excluded, see for example the same C 2 signals on very differently prepared Cu substrates in Table 2. The differences of SERS and IRRAS of CO 2 [147] (Figure 10) and NO [139] (Figure 11) on cold-deposited Cu are not well understood. Whereas IRRAS shows the symmetric and antisymmetric vibrations of dimerized NO and weak bands of N 2 O only after formation of the first layer of NO, SERS does not reveal the NO bands, but only the NO-dissociation products N 2, N 2 -δ [138], (Figure 11, compare to Figure 9), N 2 O and atomic O. It can be excluded that the difference is caused by laser heating or photochemical effects [139]. Apparently, the electromagnetic enhancement without the site-specific electron effect is not enough to observe the abundant NO dimers. In this case, SERS focuses on the chemical active sites. CO 2 IRRAS and SERS also deliver completely different results, see Figure 10. IRRAS detects the infrared antisymmetric CO stretch vibration at about cm -1 and the degenerate CO 2 bending vibration as a doublet at 653 and 665 cm -1. Employing the infrared surface selection rules [151] would indicate a mixed parallel and perpendicular or an oblique adsorption configuration. The doublet of the bending vibration is caused by lifting the degeneracy of the bending mode. The frequency position of the observed SERS spectra of CO 2 is given in Table 4, the assignment to an electronic charge transfer mechanism via the 2 Π u shape resonance [152] will be presented elsewhere [153]. Why are the SERS bands of CO 2 not observed in IRRAS, whereas the SERS lines of C 2 show up in IRRAS? Is this caused by an electronic screening effect, or by the difference of the photo energies of infrared and visible spectral range? In this sense, the visible excitation would be in resonance, and the IR excitation would be out of resonance. The Raman bands of C 2 observed by IRRAS has the shape resonance B 2g at 1.8eV, and the absorption maximum in Ag-C 2 clusters is at about 2.25eV [156], which is the charge transfer energy for the free molecule, calculated accurately [157] at ev. The best value for adsorbed C 2 for comparison with SERS and IRRAS would be obtained by two-photon photoemission of C 2 adsorbed on Cu, but we do not know a respective experiment. The shape resonance for CO 2 is at 3.6 ev [152], but the shape resonance of N 2 is at 2.3 ev [158], which Canadian Journal of Analytical Sciences and Spectroscopy