Thermal Stability of Alkanethiolate and Aromatic Thiolate Self-Assembled Monolayers on Au(111): An X-ray Photoelectron Spectroscopy Study

Applications of functional self-assembled monolayers (SAMs) rely to a large extent on their stability, with the thermal stability being of particular importance. In this context, thermal stability of archetypical, non-substituted alkanethiolate (AT) and aromatic thiolate (ArT) SAMs on gold was studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy, taking several monolayers with different lengths of the molecular backbones as representative examples and heating them in UHV. Evaporated, grain-structured Au(111) was used as the support, in view of broad practical relevance of this substrate type. AT and ArT SAMs were found to be stable up to temperatures of similar to 373 and similar to 383-393 K, respectively, followed by extensive molecular desorption and decomposition upon further heating. The decomposition was mediated by the cleavage of C-S bonds, which represented the so-called "weak or weakest link" in the systems. This process contributed minorly to the temperature-induced degradation of AT SAMs but was the dominant degradation channel for the ArT case. The residual films exhibited similar to 10% (AT) or similar to 35% (ArT) of the initial packing density and represented a superposition of the laying-down phase and atomically adsorbed sulfur, with a possible coexistence of a restructured, standing-up phase in the ArT case.