Conformational order of n-dodecanethiol and n-dodecaneselenol monolayers on polycrystalline copper investigated by PM-IRRAS and SFG spectroscopy

Self-assembled monolayers (SAMs) of n-dodecanethiol (C12H25SH) and n-dodecaneselenol (C12H25SeH) on polycrystalline copper have been elaborated with the purpose of achieving densely packed and crystalline-like assemblies. By combining the surface sensitivity of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), the effect of the self-assembly time (15 min, 30 min, 1 h, 2 h and 24 h) on the formation of n-dodecanethiol and n-dodecaneselenol monolayers on untreated and electrochemically reduced polycrystalline copper has been investigated. On electrochemically reduced copper, PM-IRRAS spectroscopy shows that both molecules are able to form well organized layers. SFG spectroscopy indicates that the C12H25SeH SAMs are slightly better ordered than those achieved with C12H25SH. On untreated copper, the two molecules lead to different film organizations. Both PM-IRRAS and SFG indicate that C12H25SH SAMs are of the same film quality as those obtained on electrochemically reduced copper. On the contrary, C12H25SeH monolayers are invariably poorly organized at the molecular level.