Surface spectroscopy, sample preparation SERS

Sampling in surface-enhanced Raman and infrared spectroscopy is intimately linked to the optical enhancement induced by arrays and fractals of hot metal particles, primarily of silver and gold. The key to both techniques is preparation of the metal particles either in a suspension or as architectures on the surface of substrates. We will therefore detail the preparation and self-assembly methods used to obtain films, sols, and arrayed architectures coupled with the methods of adsorbing the species of interest on them to obtain optimal enhancement of the Raman and infrared signatures. Surface-enhanced Raman spectroscopy (SERS) has been more widely used and studied because of the relative ease of the sampling process and the ready availability of lasers in the visible range of the optical spectrum. Surface-enhanced infrared spectroscopy (SEIRA) using attenuated total reflection coupled to Fourier transform infrared spectroscopy, on the other hand, is an attractive alternative to SERS but has yet to be widely applied in analytical chemistry. 

The observation and understanding of SERS are clearly very important developments in the study of surface chemistry and surface physics. The combination of molecular information and extraordinary sensitivity provides a valuable probe of surface structure and behavior. Out of the broad study of SERS by both chemists and physicists have emerged several approaches to using SERS for chemical analysis. A common analytical situation involves preparation of a SERS active substrate by one of several methods, then exposure of the substrate to a liquid or gaseous sample. Subsequent Raman spectroscopy of the adsorbed layer provides the analytical signal, enhanced by whatever chemical or field enhancement is provided by the adsorbate-substrate interaction. The current and next section are not intended to address SERS substrates comprehensively, but several of analytical interest are described. 

If an enhancement of five to six orders of magnitude can be achieved routinely, TERS for small molecules, which are not in resonance with the laser line, is within reach. These molecules have cross sections of the order of (dff/dfJ) 10 cm sr and are barely seen as adlayers on smooth interfaces by normal Raman spectroscopy. To test this, benzenethiol was chosen, which assumes an essentially vertical orientation to the surface due to its thiol group. The adlayer preparation is quick and easy the adsorption occurs from an etha-nolic solution, and a self-assembled monolayer is formed on previously flame-annealed gold or platinum surfaces. Fig. 10.26 shows TERS spectra for benzenethiol at Au(llO) and Pt(llO) surfaces. The comparison of the spectra reveals the characteristic benzenethiol bands but with sHghtly different band positions and relative intensities for the two samples and a nearly 20-fold lower intensity level for benzenethiol at Pt(llO). The comparison of these data with SER spectra for... 

Surface-enhanced Raman spectroscopy (SERS) provides a means of obtaining the Raman spectmm of a monolayer. Although this method requires careful preparation of a roughened surface (necessary for intensity enhancement), and the absorbance may vary from sample to sample, it is very sensitive to the functionalities located close to the metal surface. The enhancement factor depends inter alia on the distance between the functional group and the metal surface on one hand and the surface coverage on the other. A typical distance at which the enhancement factor decreases to half of its initial value in a well-packed gold-thiol monolayer is about 3.5-7. This technique provides useful... 

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