Two-dimensional cysteine and cystine cluster networks on Au(111) disclosedby voltammetry and in situ scanning tunneling microscopy

Microscopic structures for molecular monolayers of L-cysteine and L-cystine assembled on Au(111) have been disclosed by employing electrochemistry and in situ scanning tunneling microscopy (STM). High-resolution STM images sh ow that the adlayers of both cyteine and cystine exhibit highly-ordered net worklike clusters with (3 root 3 x 6)R30 degrees structure. By combining th e surface coverage estimated from voltammetric data, each cluster is demons trated to include six individual cysteine molecules or three cystine molecu les. As a comparison, no cluster structure is observed for the 1-butanethio l adlayer prepared and examined under the same conditions as those for cyst eine and cystine. This suggests that intermolecular and intramolecular hydr ogen bonds among adsorbed cysteine or cystine molecules could be responsibl e for the origin of the cluster-network structures for the adlayers. Severa l models are proposed and used to explain the in situ STM observations in d etail.