A Versatile and Simple Method for the Preparation of Substrates for Surface-Enhanced Infrared Spectroscopy of Monolayers.

b adsorbed on surfaces that can enhance the absorption and emission of electromagnetic radiation. Surface-enhanced Raman scattering (SERS) is a wellinvestigated phenomenon and an established surface analytical tool (1). The surface enhancement of infrared spectra was first observed and investigated by Hartstein et al. in 1980 (2a), but it is still developing and has not yet matured into a reliable analytical technique. Self-assembled monolayers (SAMs), especially alkanethiols on gold and silver surfaces, have been investigated intensely in the recent past (3, 4) for their potential applications in molecular technologies as well as for their importance in the fundamental understanding of interfacial properties. Essential tools for the characterization of monolayers have been methods of electrochemistry (4a), ellipsometry (4b), wettability (4c), reflection absorption infrared spectroscopy (4d–f ), sum-frequency spectroscopy (4g), X-ray photoelectron spectroscopy (4h), rare gas atom scattering (4i), ion-surface collisions, scanning tunneling (4j), and atomic force microcopies (4k). Although each technique has focused on a few of the aspects of self-assembly, infrared spectroscopy has shed light on most of the issues concerning their structure. However, the poor sensitivity of the reflection absorption infrared spectroscopic technique in its normal mode of operation has substantially hindered its applicability to monolayers (4d–f ). Both SERS (1) and surface-enhanced infrared (SEIR) spectroscopy (2), although surface sensitive, can be applied only to roughened surfaces. For these spectroscopies, preparation of reliable substrates is the most important requirement. In earlier studies of surface-enhanced Raman scattering of SAMs carried out in our laboratory (5), we have successfully used a simple method for the preparation of surface-enhanced Raman active gold as well as silver films by sputter-depositing gold or silver on oxidized aluminum foils. The same method has been used here for the preparation of silver substrates for the surfaceenhanced infrared studies. We also prepared silver surfaces where it was coated on a smooth silicon surface in order to compare the extent of enhancement of IR absorption. Thin silver films on surfaces with nanometer scale roughness are known to be composed of silver islands (2), which can cause enhancement in the vibrational spectra of adsorbed molecules. Among the several ways to induce microscopic roughness on substrates, such as mechanical polishing, electrochemical etching, and ion sputtering, one of the simplest, particularly on a metallic specimen, is to oxidize it. Here, oxidized aluminum foils were used as substrates to coat the thin silver films. The conditions employed change the quality of the films formed. Commercial aluminum foils of 0.02-mm thickness, used for packaging, were employed for that purpose. The foils were oxidized by heating them at a temperature of 500°C in air for 5 h. These foils were coated with silver (99.9% pure) in an Edwards sputter coater used for the preparation of scanning electron microscopy (SEM) specimens. Evaporation was carried out at a pressure of 0.01 Torr of Ar (discharge conditions: 1.3 kV, 5 mA). The aluminum foil was sheeted on the substrate holder which was 10 cm in diameter. After sputtering for about 5 min, a film of about 300 Å thickness was formed. By keeping all other conditions the same, silver films of different thickness were prepared by varying the coating time from 5 to 35 min. Maximum enhancement was found in the IR studies when the coating was done for 5 min. These films were cut into appropriate size and used for the preparation of SAMs. Silver films on smooth silicon surfaces were also