Chain-Length-Dependent Reactivity of Alkanethiolate Self-Assembled Monolayers with Atomic Hydrogen

Gas–surface interactions are some of the most important yet complex chemical processes to occur, as they intrinsically involve many-body phenomena across a wide spectrum of energies and length scales. To understand these complicated interfacial interactions, we often use model systems such as thiolate self-assembled monolayers (SAMs) to study phenomena like reactivity and passivation, as these systems afford fine control over the surface parameters governing the events in question. In this study, we examine the effect of chain length on the reactivity of alkanethiolate SAMs with atomic hydrogen by monitoring morphological surface evolution throughout the reaction. These spatiotemporal data were obtained using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with directed in situ atomic hydrogen dosing. For a series of alkanethiolate SAMs 8- to 11-carbons long, we find that small increases in chain length cause disproportionately large decreases in reactivity. These reaction trends led us to develop a kinetic model characterized by two rate constants: a slow rate for hydrogen reactivity with close-packed domains, which is chain-length dependent, and a fast rate for reactivity with low-density regions, which is the same for all samples examined. In addition to reaction rates, we also tracked chain-length-dependent changes in surface morphology, notably how the size and shape of the SAMs’ etch pits evolved following hydrogen exposure. Few differences were observed in the 10C and 11C samples, while there was a significant increase in the mean etch pit area of the 8C and 9C SAMs. Overall, this study provides important quantitative insights into how surface packing and dynamic disorder of organic thin films can influence their passivation capabilities.