Step-wise decomposition process of azobenzene self-assembled monolayers

The step-wise decomposition of 4-(12-(dodecyldithio)dodecyloxy)azobenzene (AzoC24) in self-assembled monolayer (SAMs) on Au(1 1 1) was observed by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) under the ultra-high vacuum (UHV) condition. This decomposition process only occurred after the formation of the SAM.The TD spectra clearly showed two steps of thermal decomposition of the azobenzene moiety. At approximately 450 K, fragments of m/e = 77 and 105 were clearly observed. These fragments were decomposed species obtained by the breakage of the C-N bonds of the azobenzene moiety. At about 490 K, other fragment of m/e = 93 assigned to the phenoxy ion was detected. In order to examine the decomposition process, we measured the S 2p and N 1s XPS of the SAM at various temperatures. The results suggest that diazonium moiety is the first to be decomposed and the remaining structure is desorbed together with breakage of C-O bond between the phenoxy moiety and alkyl chain with increasing temperature.