Roman study on the structure of cysteamine monolayers on silver

Cysteamine (2-aminoethanethiol) monolayers were spontaneously formed on silver surfaces by chemisorption from cysteamine and cystamine solutions. The present surface-enhanced Raman scattering investigation of such layers shows that the conformation of the adsorbed molecules clearly depends on the preparation procedure, surrounding liquids, pH, and applied potential. A majority of trans conformers were obtained directly after adsorption from more concentrated cysteamine solutions. Adsorption from more diluted cysteamine solutions and solutions of the corresponding disulfide, cystamine, and storage of layers containing mainly trans conformers in water lead to a predominance of gauche conformers. The portion of trans conformers could be increased by lowering the pH and applying negative potential and by the adsorption of additional cysteamine. Time-resolved analyses showed that the changes of conformations proceed with time constants of about 20 min. The slow rearrangement should be attributed to a reversible attachment of the amino group to the metal surface in addition to the chemisorption via the thiol function. The results demonstrate that special care haste be taken to obtain uniform monolayers terminated by amino groups as they are desired for various applications.