The first surface-enhanced Raman (SERS) spectrum was observed in 1974 by Fleischmann et al.1 for pyridine adsorbed on electrochemically roughened silver. However, SERS did not sustain its epidemic growth until Jeanmaire and Van Duyne reported in 1977 that the Raman intensity was magnified by a factor ca. 106 over what was reasonably expected.2 That report was corroborated by Albrecht and Creighton,3 who submitted their work for publication some six months after Jeanmaire and Van Duyne. Hundreds of papers on various aspects of SERS and associated phenomena followed, peaking in the late 1980s.
SERS enjoyed a rebirth in the mid 1990s owing to the confluence of two events: the reports by Shuming Nie4 and Katrin Kneipp5 that they were able to obtain SERS signals from single molecules, and the explosive worldwide interest in nanoscience and nanotechnology, of which SERS is a quintessential example. (A personal gauge of the currency of SERS over the past three decades is the citation pattern of my 1985 review article.6 Annual citations peaked at 61 in 1988, then slowly declined to about 40 in 1999. Thereafter they began to grow steeply, reaching 270 in 2011.) SERS remains strong today (2012) partly as a result of the keen current interest in plasmonics7 and metamaterials,8 large tracts of which grew out of SERS and its plasmonic underpinnings.
My involvement in SERS was an outgrowth of the research conducted towards my PhD under the guidance of Professor Michael J. Dignam, a pioneer in surface science and especially surface spectroscopy—and latterly even a contributor to SERS, whose lack of celebrity, despite the many novel ideas he conceived, was due to a reticence towards self-promotion. Mike Dignam bequeathed a great deal to me for which I am grateful, not least his love of theory and mathematical analysis. He died much too young in 1993.
When I began my PhD work in 1966, surface science was an ascendant and exciting field of research. Technological advances in ultra-high vacuum (UHV) made it possible to study adsorbate–surface interactions with a fair chance of avoiding contamination. I built an elaborate vacuum system in which to investigate the effect of submonolayer doses of gases on the optical properties of thin deposited metal films under UHV conditions. Because we wished to make both transmission and reflection measurements, the metal films had to be very thin. The initial results (ca. 1967–68) made it amply clear that the films' roughness often dominated their optical behaviour, especially for metals such as silver. At least this is what we conjectured, because we could not account for their optical properties with bulk metal thin-film models.
To account for the optical effects of the roughness I constructed a model in which the film consisted of a metal film with parallel sides and bulk optical constants on which rested a transition layer—an effective medium with properties that had both metal and adsorbate character whose proportions varied with adsorbate coverage. These sorts of models are routine nowadays, but in the 1960s the treatments of the optical properties of such transition layers due to subwavelength roughness (some of which were proposed by people in the budding silicon wafer processing industry) were rather naive and could not account for what I observed.
It was a paper by R. H. Doremus on island films9 that introduced me to the Maxwell–Garnett effective medium theory. I also knew the work of Gustav Mie.10 Doremus's papers alerted me to the link between the resonant dynamics of the conduction electrons (nowadays called localized surface plasmons) that lie buried within their optical constants, and the optical resonances that one observes in the extinction spectra of gold and silver colloids.
It occurred to me that modelling the roughness region as a two-dimensional version of the Maxwell–Garnett effective medium would produce results resembling what I saw experimentally, and I managed to solve the problem one Sunday afternoon in 1968. The solution incorporated two other innovations. First, it corrected for the optical constant of the metal due to the reduced electronic mean free path arising from the small size of the roughness features. That is, it replaced the mean time between electronic collisions, τB, which affects the imaginary part of the metal's dielectric function, with τ, calculated from the equation in which A is a geometric factor of the order of unity, vF is the electronic Fermi velocity and R is the particle's average radius. Second, it took account of the effect of charge donation or extraction resulting from the formation of the adsorbate–surface bond on the metal's plasma frequency (and hence on its dielectric function).11
Because the thicknesses of the films we were working with were much smaller than optical wavelengths, I expanded the expression for the transmittance and reflectance of a system consisting of a film over a substrate in orders of d/λ, keeping only first-order terms. This operation showed that in the limit of the dielectric constant of the underlying ‘bulk metal’ substrate becoming large (that is, when one goes sufficiently far towards long wavelengths) the reflectance contains no first-order contributions in d/λ for the s-polarized component.
In 1970 I went to work for Alcan Research and Development in Kingston, Ontario. As a result, I did not get around to publishing the thesis work11 until 1973. In retrospect, writing a single, long paper rather than reporting the three discoveries in separate publications so as to underscore the potential utility of each was a strategic mistake, probably due to my failure to recognize their importance to surface spectroscopy. The electronic mean-free-path correction to the optical constants of small metal particles was independently derived by Uwe Kreibig12 and published in 1969. And the absence of first-order terms in the s-polarized spectrum of a thin dielectric film deposited on a metal was brilliantly recast as a ‘surface selection rule’ by Norman Sheppard in 1976.13
Localized surface plasmons also had an important role during my employment by Alcan, which among its voluminous holdings owned a valuable patent that described an electrochemical method for colouring anodized aluminium. The source of colours imparted to porous anodic alumina when it was subjected to alternating-current electrolysis in a metal-ion-containing electrolyte was a mystery, a fact that invited challenges to that patent. With my colleague David Goad I showed that those colours were due to ‘colloidal metal’ (later shown to be metal nanowires) deposited in the pores of the alumina. The link to the localized surface plasmon resonance was then straightforward.14 This insight was the basis of a powerful nanowire fabrication technique, still broadly used.
On returning to the University of Toronto as a faculty member I continued to investigate the optical properties of nanostructured metal surfaces. The idea of using a ‘Maxwell–Garnett film’ on a metal as a model for a nano-rough metal surface was implemented by Peter McBreen, a brilliant graduate student in my group (currently professor of chemistry at Laval University), to account for results he obtained by ellipsometric spectroscopy on cold-deposited films which showed that cold deposition produces submicroscopically rough films whose reflectance spectra contain resonances resembling the localized surface plasmon resonances of a two-dimensional film of colloidal metal atop a planar silver film. Using this model he was able to mimic the effects of the diminishing degree of roughness as the temperature of the film was increased, effects presumably brought about by surface diffusion of the metal atoms.15
At the American Chemical Society (ACS) meeting in Chicago in August of 1977 I was first made aware that the observations of Jeanmaire and Van Duyne2 signalled a new phenomenon: it was there that I heard Rick Van Duyne christen the effect SERS.16 I had spoken in one of the Catalysis Division symposia at that ACS meeting. It was the first ACS meeting I had attended, and I was proud to have been invited by the late Bob Eischens, a celebrated pioneer in the infrared spectroscopy of adsorbed molecules. Armed with the insight that we had gleaned from dealing with the optics of rough films, I knew immediately what was responsible for the striking enhancement reported by Rick in his talk; I also knew the experiment that I wanted to perform and how to carry it out.
We had been doing matrix-isolation spectroscopy on the products of reaction between metal atoms and small metal clusters and small molecular ligands such as CO and ethylene. I had been introduced to matrix isolation by my colleague Geoffrey Ozin, with whom I collaborated after my return to the University of Toronto. I had recently got a grant from the Natural Science and Engineering Research Council of Canada to build a matrix Raman apparatus, into which I designed several novel features allowing one to rotate a polished refrigerated matrix-bearing aluminium substrate to face the spectrometer at essentially any angle. I realized that by depositing the metal and the ligand sequentially onto the ultra-cold substrate (rather than simultaneously as was done in forming frozen rare-gas matrices) one could make a rough film covered by adsorbate. Cold deposited films had been known for many years to be submicroscopically rough on account of the restricted mobility of the metal atoms landing on the cold surface, and, as discussed above, we had shown that, for rough silver, the roughness showed localized surface plasmon resonances not unlike those of a two-dimensional colloidal silver film.11,15 (Years later, we showed that the boundary of the rough film was more accurately described as a self-affine interface.17)
When I returned from the ACS meeting I described my thoughts to the very small research group I had in those days: Peter McBreen, who was stationed at Erindale College, a suburban campus of the University of Toronto; Robert Lipson; and Artur Gohin, a French exchange student doing experiments and theory of Raman optical activity. The latter two were helping me establish a research laboratory on the main campus of the University of Toronto, in Toronto's city centre. Unfortunately neither the spectrometer nor the new matrix apparatus on which we could do this work was yet ready. They would not be completed until the spring of 1978, coincident, more or less, with the arrival of Dr Dan DiLella, an outstanding postdoctoral worker who had studied the vibrational spectroscopy of pyridine for his PhD. Dan was incredibly productive, alternately performing matrix resonance Raman studies on small metal dimers and clusters, and SERS experiments.
In our first SERS experiment we dosed a cold-deposited silver film with CO. These experiments were begun by Dan and Rob Lipson, who transferred at about this time to the group of Boris Stoicheff in physics, and went on to celebrity as dean of science at the University of Victoria in British Columbia. The experiment worked the very first time it was tried, and Dan performed numerous experiments with a range of adsorbates (ethylene, propylene and benzene) over the next 12 months.
Experiments with cold deposited films (the substrate temperature was held at about 10 K) had many advantages. Not the least of these were that one could determine the adsorbate dose (by calibrating the film thickness by using interference fringes), and one could condense multiple monolayers onto the rough silver film, allowing the SERS spectrum of the first layer to be distinguished from the Raman spectra of the subsequent layers, whose spectroscopic features were often slightly shifted in frequency from those of the adsorbed molecules.
In the spring of 1978 I wrote a paper in which I jotted down my early ideas regarding the relationship between SERS and the excitation of localized surface plasmons (LSPs) in micro-rough silver.18 Focusing on the original reports on SERS from electrochemically roughened silver, I suggested that the submicroscopic silver roughness features (nano-features in today's idiom) were subwavelength and sustained plasmons, and it was the plasmon resonance that was responsible for the enhancement. I also made several predictions: (i) that intense SERS would be observed primarily with nanostructured systems composed of the coinage metals (and the alkalis) because only nanoparticles of those metals would possess the high-Q LSP resonances required for large enhancement; (ii) that subwavelength nanostructure was an essential ingredient for observing SERS; and (iii) that true colloids of those metals should also produce SERS. Milton Kerker,19 Gersten and Nitzan20 (who first enunciated the so-called |EL|4 paradigm for SERS enhancement) and Horia Metiu21 (who was the first to point out that interparticle coupling resulted in unusually intense local enhancements—the key to our current understanding of single-molecule SERS) were gracious enough to acknowledge in their early papers that they undertook their calculations based on my proposals.18 And all three predictions were borne out within a year of the appearance of that paper. Alan Creighton was the first to demonstrate SERS from silver and gold colloids,22 ascribing the enhancement to localized surface plasmon excitation.
A parenthetical anecdote comes to mind in the context of SERS from alkalis. A brilliant graduate student, Kevin Maynard, who joined my group in 1984, and subsequently become highly successful in the energy industry, was tasked with investigating SERS on cold-deposited alkali films. Kevin began his work on the aforementioned matrix machine, which was a high-vacuum—but not UHV—system, depositing alkali by using commercial alkali getters. By adsorbing CO on rough Li, Na, Rb and Cs, he got often deeply coloured surfaces with intense and rich SERS spectra that he speculated were due to anionic species (perhaps resembling the well-known cyclic anions: deltate (C3O32−), squarate (C4O42−), croconate (C5O52−) and rhodizonate (C6O62−)),23 which Kevin surmised were products of the surface reaction between CO and a residual oxygen impurity mediated by the reactive nano-rough alkali surface. Those reaction products disappeared when he switched to our UHV machine (which had been built by Bob Wolkow, now a celebrated surface scientist and scanning tunnelling microscopy expert at the University of Alberta). Kevin went on to study some of the rich surface science that resulted when cold-deposited silver was promoted with potassium.24 As a result we never got around to publishing those earlier results. Because alkali-metal SERS substrates have not been studied widely (despite their very strong SERS activity) I include one of Kevin's unpublished spectra (figure 1) showing the SERS spectrum obtained when CO was deposited on a cold-deposited (that is, nano-rough) K surface. (A spectrum of benzene on Li was, however, reported.6)
Probably the very first gathering to discuss the theory of SERS, a joint US–Japan seminar organized in February of 1979 by Eli Burstein (an eminent condensed-matter physicist from the University of Pennsylvania), was held in a rather seedy motel on the beach in Santa Monica that, as far as the manager remembered, had never hosted a conference before. I arrived early on the Sunday and, with a few other friends, helped Eli clear out the mattresses and other items stored in the ‘conference room’ in time for that evening's session. It was at that meeting that I first met Walter Kohn, who had just been named director of the recently formed Institute for Theoretical Physics (now the Kavli Institute for Theoretical Physics) at the University of California, Santa Barbara. The motel's manager, who wore a bathing suit, a T-shirt and sandals throughout the meeting, recognized Walter's picture in the Los Angeles Times and became convinced that we were involved in something top secret. Why else would we be ‘hiding out’ in a motel like his?
The proceedings of that conference, which were published in Solid State Communications,25 reported several important early ideas about SERS. In my contribution I expanded my thinking on the way in which inter-particle coupling affected the plasmon resonance in a rough silver surface and realized that, as a result, the SERS enhancement would trend upward towards the red for coupled particles.26
In the summer of 1979 I began a sabbatical with my family in Bordeaux, France (where my daughter Leslie was born), taking with me the data for our first three or four papers on SERS27–29 plus several dealing with resonance Raman spectroscopy of matrix-isolated transition metal dimers and trimers. Approximately a month after we arrived in France I left my pregnant wife and two small children to attend the ACS meeting in Washington DC, where I presented more or less the totality of the work we had done up to that point in an invited talk.30 Two points seemed to make the greatest impact on the audience. The first was that we observed very strong SERS from benzene and ethylene, two molecules that did not contain nitrogen. Some early SERS researchers had mused that the phenomenon was somehow connected to molecules containing nitrogen. The second point was that very intense bands that were normally Raman forbidden were observed in the SERS spectrum of benzene adsorbed on rough silver.29 The appearance of normally forbidden bands usually implies symmetry reduction; for example, reducing the symmetry of benzene when it forms a metal–benzene π-complex often activates normally forbidden bands. However, even in those cases the intensity of the normally forbidden bands is never very great; moreover, the band frequencies are often significantly shifted on account of the formation of the strong metal–ligand bond. In contrast, for benzene adsorbed on rough silver the Raman band shifts were modest, whereas the intensities of the forbidden bands were rather big.
We proposed an alternative explanation for the appearance of normally forbidden Raman bands in SERS. We noted that electric field strengths decay rapidly in the vicinity of a metal surface. As a result, terms in the expression for the transition dipole that depend on the spatial rates of change of the electric field should be significantly intensified for molecules residing near metal surfaces.31 For a free molecule those terms contribute negligibly in comparison with the terms that depend directly on the electric field strength rather than the spatial rate of change of the electric field, because, in free space, significant changes in field intensity occur over a distance of the order of half a wavelength, whereas near a metal surface the electric field strength varies rapidly. We went on to show that the Raman bands of benzene that became intense corresponded precisely to those that belonged to the same irreducible representations as the components of the (third-rank) tensor that is activated in a spatially rapidly varying field. We further reasoned that if this is true for Raman, it should also be true for other forms of spectroscopy, and proved that it was indeed so for the fluorescence spectrum of a conjugated molecule deposited on silver, whose quadrupole-allowed transitions became intensified.32
While in Bordeaux I wrote the CO, ethylene and benzene papers (as well as two matrix-isolation papers). In retrospect, sending the handwritten drafts to my assistant in Canada for typing, together with hand-drawn figures that had to be drafted in India ink, then mailing back my corrections before submitting the result to the journal using transatlantic air mail makes me wonder how those papers were ever completed. But it does explain some of the reasons why work largely completed in 1978 was not submitted for publication until 1980.
Because we were able to distinguish the spectrum of chemisorbed CO from that of bulk CO overlayers deposited over that first layer, and to quantify the number of bulk CO molecules by recording interference fringes as a function of dose, we were able to report one of the first careful quantitative determinations of the (average) SERS enhancement (106) in that CO SERS paper.28 A good-quality SERS excitation spectrum was also reported in that paper that showed that for the cold-deposited rough films the most intense SERS was obtained by exciting in the yellow region of the spectrum, indicating that the surface nanofeatures were strongly interacting.
Another milestone in the application of plasmonics to SERS occurred in the early 1990s with the insight (largely due to M. I. Stockman and V. M. Shalaev) that for fractal aggregates of metal particles the interactions of the individual nanoparticles' dipolar plasmonic resonances couple to form highly localized normal modes.33 This provided a theoretical basis for the conjecture34 that the enhancement on SERS-active systems is often highly spatially heterogeneous so that average 106-fold enhancements must arise from the contribution of rather few ‘hot spots’ where the local enhancement was much larger than 106-fold ‘diluted’ by the response of many rather poorly enhancing sites. The predicted SERS localization was proved by direct near-field imaging of the surface of compacted fractal silver nanoparticle aggregates.35
In conclusion, the plasmonic explanation for SERS, which has so far successfully accounted for the most seminal aspects of that effect, was first proposed in 1978.18 The field of plasmonics and major aspects of our current interest in metamaterials seem to have evolved from that. It was Jeanmaire and Van Duyne's 1977 report that alerted us to the fact that in SERS we were looking at inexplicably large Raman signals. For this reason I do not fully concur with the sentiment expressed in a recent reminiscence,36 ‘that the first experimental observations of the phenomenon [SERS] constitute its discovery.’ The retrograde motion of the planets was known in ancient time, yet we credit Copernicus with the heliocentric explanation. Similarly in SERS: the Raman spectrum of molecules adsorbed on rough silver was unquestionably first observed by Fleischman et al.1 But the insight that the spectrum was inordinately enhanced must be credited to Jeanmaire and VanDuyne.2
↵1 M. Fleischmann, P. J. Hendra and A. J. McQuillan, ‘Raman spectra of pyridine adsorbed at a silver electrode’, Chem. Phys. Lett. 26, 163–166 (1974).
↵2 D. L. Jeanmaire and R. P. Van Duyne, ‘Surface Raman spectroelectrochemistry. 1. Heterocyclic, aromatic, and aliphatic amines adsorbed on anodized silver electrode’, J. Electroanal. Chem. 84, 1–20 (1977).
↵3 M. G. Albrecht and J. A. Creighton, ‘Anomalously intense Raman spectra of pyridine at a silver electrode’, J. Am. Chem. Soc. 99, 5215–5217 (1977); J. A. Creighton, ‘Contributions to the early development of surface-enhanced Raman spectroscopy’, Notes Rec. R. Soc. 64, 175–183 (2010).
↵4 S. Nie and S. R. Emory, ‘Probing single molecules and single nanoparticles by surface-enhanced Raman scattering’, Science 275, 1102–1106 (1997).
↵5 K. Kneipp, Y. Wang, H. Kneipp, I. Itzkan, R. R. Dasari and M. S. Feld, ‘Population pumping of excited vibrational states by spontaneous surface-enhanced Raman scattering’, Phys. Rev. Lett. 76, 2444–2447 (1996).
↵6 M. Moskovits, ‘Surface-enhanced spectroscopy’, Rev. Mod. Phys. 57, 783–826 (1985).
↵7 S. A. Maier, M. L. Brongersma, P. G. Kik, S. Melzer, A. A. G. Requicha and H. A. Atwater, ‘Plasmonics—a route to nanoscale optical devices’, Adv. Mater. 13, 1501–1505 (2001).
↵8 D. R. Smith, J. B. Pendry and M. C. K. Wiltshire, ‘Metamaterials and negative refractive index’, Science 305, 788–792 (2004); J. B. Pendry, ‘Negative refraction makes a perfect lens’, Phys. Rev. Lett. 85, 3966–3969 (2000); V. M. Shalaev, W. S. Cai, U. K. Chettiar, H. K. Yuan, A. K. Sarychev, V. P. Drachev and A. V. Kildishev, ‘Negative index of refraction in optical metamaterials’, Optics Lett. 30, 3356–3358 (2005); D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser and S. Schultz, ‘Composite medium with simultaneously negative permeability and permittivity’, Phys. Rev. Lett. 84, 4184–4187 (2000).
↵9 R. H. Doremus, ‘Optical properties of thin metallic films in island form’, J. Appl. Phys. 37, 2775–2781 (1966); R. H. Doremus, ‘Plasma resonances in small metallic particles’, J. Appl. Phys. 35, 3456–3457 (1964).
↵10 G. Mie, ‘Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen’, Annln Phys. 25, 377–445 (1908).
↵11 M. J. Dignam and M. Moskovits, ‘Influence of surface-roughness on transmission and reflectance spectra of adsorbed species’, J. Chem. Soc. Faraday Trans. II 69, 65–78 (1973).
↵12 U. Kreibig and C. Von Frags, ‘Limitation of electron mean free path in small silver particles’, Z. Phys. 224, 307–323 (1969).
↵13 H. A. Pearce and N. Sheppard, ‘Possible importance of a metal–surface selection rule in interpretation of infrared-spectra of molecules adsorbed on particulate metals—infrared spectra from ethylene chemisorbed on silica-supported metal catalysts’, Surf. Sci. 59, 205–217 (1976).
↵14 D. G. W. Goad and M. Moskovits, ‘Colloidal metal in aluminum oxide’, J. Appl. Phys. 49, 2929–2934 (1978).
↵15 P. H. McBreen and M. Moskovits, ‘Optical properties of silver films deposited at low temperatures’, J. Appl. Phys. 54, 329–335 (1983).
↵16 R. P. VanDuyne, D. L. Jeanmaire and C. S. Allen, ‘Molecular characterization of electrode surfaces by Raman and resonance Raman spectroscopy’, Abstr. Pap. Am. Chem. Soc. 174, 4 (1977).
↵17 C. Douketis, T. L. Haslett, Z. Wang, M. Moskovits and S. Iannotta, ‘Self-affine silver films and surface-enhanced Raman scattering: linking spectroscopy to morphology’, J. Chem. Phys. 113, 11315–11323 (2000); C. Douketis, Z. Wang, T. L. Haslett and M. Moskovits, ‘Fractal character of cold-deposited silver films determined by low-temperature scanning tunneling microscopy’, Phys. Rev. B 51, 11 022–11 031 (1995).
↵18 M. Moskovits, ‘Surface roughness and the enhanced intensity of Raman scattering by molecules adsorbed on metals’, J. Chem. Phys. 69, 4159–4168 (1978).
↵19 M. Kerker, D. S. Wang and H. Chew, ‘Surface enhanced Raman scattering (SERS) by molecules adsorbed at spherical particles’, Appl. Opt. 19, 4159–4174 (1980).
↵20 J. Gersten and A. Nitzan, ‘Electromagnetic theory of enhanced Raman-scattering by molecules adsorbed on rough surfaces’, J. Chem. Phys. 73, 3023–3037 (1980).
↵21 P. K. Aravind and H. Metiu, ‘The enhancement of Raman and fluorescent intensity by small surface roughness. Changes in dipole emission’, Chem. Phys. Lett. 74, 301–305 (1980).
↵22 J. A. Creighton, C. G. Blatchford and M. G. Albrecht, ‘Plasma resonance enhancement of Raman scattering by pyridine adsorbed on silver or gold sol particles of size comparable to the excitation wavelength’, J. Chem. Soc. Faraday Trans. II 75, 790–798 (1979).
↵23 P. S. Santos, J. H. Amaral and L. F. C. De Oliveira, ‘Raman spectra of some transition metal squarate croconate complexes’, J. Mol. Struct. 243, 223–232 (1991); L. F. C. de Oliveira and P. S. Santos, ‘Chromophore-selective resonance Raman spectra of copper(II) croconate and rhodizonate complexes with nitrogenous counterligands’, J. Mol. Struct. 263, 59–67 (1991).
↵24 K. J. Maynard and M. Moskovits, ‘A surface enhanced Raman study of carbon dioxide coadsorption with oxygen and alkali metals on silver surfaces’, J. Chem. Phys. 90, 6668–6679 (1989).
↵25 Solid State Communications volume 32, issue 1 (October 1979).
↵26 M. Moskovits, ‘Enhanced Raman scattering by molecules adsorbed on electrodes—a theoretical model’, Solid State Commun. 32, 59–62 (1979).
↵27 M. Moskovits and D. P. DiLella, ‘Enhanced Raman spectra of ethylene and propylene adsorbed on silver’, Chem. Phys. Lett. 73, 500–505 (1980).
↵28 D. P. DiLella, A. Gohin, R. H. Lipson, P. H. McBreen and M. Moskovits, ‘Enhanced Raman spectroscopy of CO adsorbed on vapor-deposited silver’, J. Chem. Phys. 73, 4282–4295 (1980).
↵29 M. Moskovits and D. P. DiLella, ‘Surface enhanced Raman spectroscopy of benzene and benzene-d6 adsorbed on silver’, J. Chem. Phys. 73, 6068–6075 (1980).
↵30 M. Moskovits, D. Dilella, P. McBreen and R. Lipson, ‘Enhanced Raman scattering by molecules adsorbed on metals’, Abstr. Pap. Am. Chem. Soc. (September), 31 (1979).
↵31 J. K. Sass, H. Neff, M. Moskovits and S. Holloway, ‘Electric field gradient effects on the spectroscopy of adsorbed molecules’, J. Phys. Chem. 85, 621–623 (1981).
↵32 M. Moskovits and D. P. DiLella, ‘Intense quadrupole transitions in the spectra of molecules near metal surfaces’, J. Chem. Phys. 77, 1655–1660 (1982).
↵33 V. M. Shalaev and M. I. Stockman, ‘Fractals—optical susceptibility and giant Raman scattering’, Z. Physik D 10, 71–79 (1988); M. I. Stockman, ‘Inhomogeneous eigenmode localization, chaos, and correlations in large disordered clusters’, Phys. Rev. E 56, 6494–6507 (1997); V. M. Shalaev, R. Botet, J. Mercer and E. B. Stechel, ‘Optical properties of self-affine thin films’, Phys. Rev. B 54, 8235–8242 (1996); V. M. Shalaev and M. I. Stockman, ‘Optical properties of fractal clusters (susceptibility, giant scattering by impurities)’, Soviet Phys. JETP 65, 287–294 (1987); V. A. Markel, L. S. Muratov, M. I. Stockman and T. E. George, ‘Theory and numerical simulation of optical properties of fractal clusters’, Phys. Rev. B 43, 8183–8195 (1991); M. I. Stockman, V. M. Shalaev, M. Moskovits, R. Botet and T. F. George, ‘Enhanced Raman scattering by fractal clusters: scale-invariant theory’, Phys. Rev. B 46, 2821–2830 (1992).
↵34 P. Hildebrandt and M. Stockburger, ‘Surface-enhanced resonance Raman spectroscopy of rhodamine 6G adsorbed on colloidal silver’, J. Phys. Chem. 88, 5935–5944 (1984); B. Pettinger, K. Krischer and G. Ertl, ‘Giant Raman scattering cross section for an adsorbed dye at Ag colloids associated with low EM field enhancement’, Chem. Phys. Lett. 151, 151–155 (1988).
↵35 M. Moskovits, L. L. Tay, J. Yang and T. Haslett, ‘SERS and the single molecule’, Topics Appl. Phys. 82, 215–227 (2002); M. Moskovits, L. Tay, J. Yang and T. Haslett, ‘SERS and the single molecule: near-field microscopy and spectroscopy’, Proc. SPIE, 4258, 43–49 (2001); V. A. Markel, V. M. Shalaev, P. Zhang, W. Huynh, L. Tay, T. L. Haslett and M. Moskovits, ‘Near-field optical spectroscopy of individual surface-plasmon modes in colloid clusters’, Phys. Rev. B 59, 10903–10909 (1999); D. P. Tsai, J. Kovacs, Z. Wang, M. Moskovits, V. Shalaev, J. S. Suh and R. Botet, ‘Photon scanning tunneling microscopy images of optical excitations of fractal metal colloid clusters’, Phys. Rev. Lett. 72, 4149–4152 (1994).
↵36 A. J. McQuillan, ‘The discovery of surface-enhanced Raman scattering’, Notes Rec. R. Soc. 63, 105–109 (2009).
- This journal is © 2012 The Royal Society