The adsorption and desorption of n-alkanethiol monolayers on Au(111) have been studied under ultrahigh-vacuum condition by the use of scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and Auger electron spectroscopy (AES). Molecularly resolved STM observations for the alkanethiol monolayers have revealed that at least four different phases evolve during growth, which results in a multistep growth of the monolayer. The desorption species drastically changes at a critical coverage, which is accompanied by a structure change from a low-density flat-lying phase to a denser standing-up phase: While the latter phase bimolecularly desorbs as disulfides, the former phase unimolecularly desorbs as thiolate radicals. The coverage-dependent change of the desorption mode is explained in terms of the difference in the molecule-substrate bonding.

The coverage dependence of desorption behavior of n-alkanethiol monolayers on Au(111) has been studied using thermal desorption spectroscopy (TDS) together with scanning tunneling microscopy (STM) to investigate the molecule−substrate interactions. STM observations indicated four different phases during the growth. We present for the first time evidence for a drastic coverage-dependent change of desorption species, which is parallel with a structure change from a low-density phase to a denser phase:  while the thiolates in the latter phase bimolecularly desorb as disulfides, those in the former phase unimolecularly desorb as thiolate radicals.

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