Thermal stability of self-assembled monolayers:: Influence of lateral hydrogen bonding

Temperature-programmed desorption (TPD) of self-assembled monolayers (SAMS) on gold is investigated by using in parallel mass spectrometry (MS) and infrared reflection-absorption spectroscopy (IRAS). Monolayers formed by HS(CH2)(n)-OH (n = 18, 22) and HS(CH2)(15)-CONH-(CH2CH2O)-H (EG(1)) are compared to reveal the influence of specifically introduced hydrogen-bonding groups on their thermal stability. The overall desorption process of the above molecules is found to occur in two main steps; a disordering of the alkyl chains followed by a complex series of decomposition/desorption reactions. The final step of the process involves desorption of sulfur from different chemisorption states-The amide-group-containing SAM, which is stabilized by lateral hydrogen bonds, displays a substantial delay of the alkyl chain disordering by about 50 K, as compared to the linear chain alcohols HS(CH2)n-OH. Moreover, the decomposition of the alkyls and the onset of sulfur desorption occur at a temperature that is higher by approximately 25 K as compared to the HS(CH2)(18)-OH SAM. The desorption process is also studied for two oligo(ethylene glycol)terminated SAMS, HS(CH2)(15)-X-(CH2CH2O)(4)-H (EG(4)-SAMs), where X is -CONH- and -COO- linking groups. In addition to,the molecular chain disordering, the decomposition/desorption process of the EG(4)-SAMS occurs in two steps. The first is associated with the loss of the oligomer portion and the second with the desorption of the alkylthiolate part of the molecule. Our study points out that lateral hydrogen bonding, introduced via amide groups, is a convenient way to improve the thermal stability of alkanthiolate SAMS.