Thiolate self-assembled monolayers (SAMs) provide platforms for easily customizable organic interfaces, making them an excellent model system for studying the chemical properties of organic thin films. In particular, their reactions with atomic gas species yield important information about gas-surface interactions in organic films, specifically how static and dynamic disorder influence passivation. We are currently investigating the reactions of these SAMs with atomic hydrogen (H), using an angle-directed atomic gas source and <i>in situ</i> ultra-high vacuum scanning tunnelling microscopy (UHV-STM). First, a series of alkanethiolate samples of varying chain length (8- to 11-carbon atoms long) were reacted with H, resulting in the monolayers’ conversion from close-packed standing-up phase to lower density lying-down phase. It was found that small increases in chain length caused disproportionately large decreases in reactivity at room temperature. This reaction progression was described by an exponential function with two rates: a slow rate for hydrogen reactivity with standing-up phase, which is dependent on chain length, and a fast rate for low-density phase reactions, which is the same for all samples. Chain length-dependent changes in surface morphology were also observed: short chain SAMs experienced significant etch pit reconstruction over the course of reaction, while longer chain SAMs did not. Finally, the effect of temperature on the reactivity of octanethiolate and decanethiolate SAMs were examined by carrying out their reaction with atomic hydrogen at lower temperatures (110 K, 250 K and 270 K). The SAM’s reactivity was greatly reduced with decreased temperature, with longer overall reaction times at 270 K and 250 K and no visible reaction occurring at 110 K under the same flux conditions. However, it was also interesting to note significant differences in the SAM’s surface morphology evolution during the reaction at 250 K compared to higher temperatures; we hypothesise that this is due to reduced thiol mobility on the Au(111) surface, as well as increased binding coherence between chains in the standing up phase, at lower temperatures. Further investigations on this subject are ongoing.

STM studies of atomic hydrogen’s reaction with alkanethiolate self-assembled monolayers as a function of temperature and chain length

ACS Spring Meeting 2021

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