Surface-enhanced Raman scattering: From noble to transition metals and from rough surfaces to ordered nanostructures

In the mid-1970s, surface-enhanced Raman scattering (SERS) was discovered which impacted on surface science and spectroscopy because of its extremely high surface sensitivity. However, SERS had not developed as many people had hoped to be a powerful surface diagnostic technique that can be widely used because of some obstacles. For example, only three noble metals Au, Ag, and Cu could provide large enhancement, severely limiting the widespread applications involving other metallic materials of both fundamental and practical importance. In this article, emphasis is put on the recent work of our group to directly generate SERS on net transition metals (e.g., Pt, Ru, Rh, Pd, Fe, Co, Ni, and their alloys) by developing various roughening procedures and optimizing the performance of the confocal Raman microscope. An approach of replacing the randomly roughened surface with ordered nanorod arrays of transition metals is introduced as a promising class of highly SERS-active substrates. The surface enhancement factor for transition metals was calculated, which ranged from 1 to 4 orders of magnitude. The applications of SERS in surface adsorption; electro-catalysis, and corrosion of transition-metal-based systems demonstrated several advantages of in situ surface Raman spectroscopy. A preliminary theoretical approach, considering the electromagnetic and chemical contributions, is presented to explain the SERS behavior of transition metal electrodes and nanorod arrays. It has been shown that SERS together with other surface-enhanced optical phenomena could be one of important issues not only in surface science but also in nanoscale science. Prospects and further developments in this exciting field are discussed with emphasis on the emerging experimental methodology.