AYUNTAMIENTO JAUMAVE, TAM

When Sir Chandrasekhra Venkata Raman first discovered the phenomenon that bears his name in 1928, the „instrumentation‟ was crude, because he used sunlight as the source and a telescope as the collector.58 The scattering was detected by his eyes, which was most remarkable. Later, much effort was expended in trying to improve the components of Raman instrumentation in every possible way. In the early stages, the development of better excitation sources was paramount. Different kinds of lamps which contained helium, bismuth, or lead were invented, but were found unsatisfactory due to the low light intensities until mercury lamp sources were designed for Raman use in the 1930s. These were then

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modified by Hibben using a mercury burner.59 Spedding and Stamm60 used a version of this design with a cooled version in 1942 and these developments were further progressed by Rank and McCartney61 in 1948. The Raman instrument became commercial thanks to the development of the mercury excitation system, which consisted of four lamps surrounding the Raman tube, by Hilger Co. In 1952, Welsh et al. improved the Hilger lamp by introducing one with a four-turn helix of Pyrex tubing.62 The practicality of using different elements for coloured materials was tested in the next decade before laser sources were developed for use in Raman spectroscopy in 1962. Eventually, Ar+ and Kr+ lasers became available and the Nd- YAG laser superseded these.63

The detection systems for Raman measurements have also made astonishing progress in their sensitivity. From the original measurements using photographic plates, to the first photoelectric Raman instrument reported in 1946 by Rank and Wiegand64, to the usage of a cooled photomultiplier in 1950, scientists who had ambitions for Raman spectroscopy had never given up on improving the instrument. Developments in the optical train started in the early 1960s. After discovering the efficiency of removal of stray light with a double monochromator rather than a single one, a triple one was also introduced. Now, Raman spectra can also be obtained by Fourier transform (FT) spectroscopy.65

However, the upper limit of differential Raman scattering sectionsfor molecules is about 10-29 cm-2·sr-1, which means, the corresponding Raman intensity for a monolayer of an adsorbate is less than 1 count per second (cps) with traditional Raman spectrometer systems.66 Therefore, it was out of the detection limit of all Raman spectrometers for most adsorbates since Raman intensities from molecules adsorbed on surfaces were so low.

Then the extensive study of the structure and dynamics of molecules adsorbed on surfaces using Raman spectroscopy began… by accident! In 1974, surface enhanced Raman scattering (SERS) from pyridine adsorbed on electrochemically roughened silver was first reported by Martin Fleischmann et al. at the Department of Chemistry in the University of Southampton in the UK.67 This initial publication has been cited more than 2100 times. Later in 1977, there were two groups who independently discovered that neither the surface area nor the concentration of the adsorbed species could account for so much of the enhanced signal as believed by Fleischmann et al.67. Jeanmaire and Van Duyne proposed an electromagnetic enhancement theory,68 while Albrecht and Creighton proposed a charge-transfer theory,69 both of which will be introduced in detail in section 2.4.5.

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Although the exact mechanism of the enhancement effect of SERS is still a matter of debate, it has never impeded its exploration and application. Since then, the effect of surface enhanced intensity from molecules on other metal surfaces has been found including lithium, sodium and potassium.70, 71 The coinage metals, namely silver, copper and gold gave rise to the strongest and most reliable SERS activity because of their special optical properties.72 Because of its high sensitivity, SERS is a useful probe for studies of in situ gas-solid and liquid-solid interfaces at the molecular level.73-76 Robust research activities have expanded from an electrochemical environment to air and indeed UHV conditions.77 All possible metallic morphologies, such as smooth surfaces, sphere segment void structure, films coated on roughened substrates, colloids and powders, and even catalysts supported on insulator granules became the challenging targets and were conquered one by one.76, 78-91 However, only the few coinage metals mentioned above could provide great enhancement of the Raman effect. Due to this particular restriction, Raman spectroscopy was not used as widely as IR spectroscopy in surface science and electrochemistry and certainly never for single crystal electrodes other than the coinage metals.

Nevertheless, researchers had never given up on expanding the application of Raman spectroscopy to the study of other metallic and even non-metallic surfaces. Some of these workers spent much effort in studies of electrocatalytically interesting transition metals in particular by coating SERS-active Ag or Au electrodes with ultrathin films of Ni, Co, Fe, Pt, Pd, Rh and Ru.92-97 By either electrochemical deposition or laser ablation, SERS spectra of adsorbed species on these films could be obtained thanks to the long range effect of the electromagnetic enhancement created by the SERS-active substrate.90 Although it provided a new way to explore different metal surfaces and gain information on various interfaces, the enhancement effect could be weakened if the coated film was too thick. Therefore, only a few atomic layers of the film thickness would give rise to the strongest effect. A problem often encountered however was incomplete coverage of the SERS active metal support resulting in „pin holes‟ of high SERS activity dominating the spectrum rather than the coating metal Hence, it was not possible to avoid the adsorbate binding to the exposed substrate and generating strong signals. This difficulty of preparing pinhole free, ultrathin films with good stability limited the wider application of SERS.

Once these difficulties were overcome however, studies over a wide range of different metals and potentials became more and more realistic.76, 98 Nowadays, SERS has become a general

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technique for researchers to study as many reactions as possible occurring at the electrified interface at a molecular level.