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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 has to be taken to obtain uniform monolayers terminated by amino groups as they are desired for various applications.

Two strategies are compared for the formation of self-assembled monolayers (SAMs) of silicon phthalocyanines on gold. Silicon phthalocyanines were synthesized with thiol anchoring groups in either eight peripheral side chains (the "octopus") or with one short thiol in an axial position (the "umbrella"). Both approaches gave phthalocyanines capable of forming SAMs on gold surfaces. The orientation and coverage of the phthalocyanines were compared using ellipsometry, X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman scattering (SERS). The octopus silicon phthalocyanines form poorly organized SAMs in which the phthalocyanine (Pc) rings are strongly tilted with respect to the gold surface. On average, between 3 and 4 of the thiol "arms" fail to bind to the gold surface, even when limiting coverage is achieved after 7 days of soaking. The film thickness is 22 ( 5 Å. In contrast, the umbrella silicon phthalocyanine produces close-packed SAMs within 1 h in which the Pc rings lie parallel to the gold surface. The average thickness of the later SAMs is 11 ( 3 Å, and each phthalocyanine ring occupies an average area of 284 Å 2 .