Molecular-Scale Desorption Processes and the Alternating Missing-Row Phase of Alkanethiol Self-Assembled Monolayers on Au(111)

The structural changes as a function of the immersion time of predeposited hexanethiol self-assembled monolayers (SAMs) chemisorbed on an Au(111) surface in diethyl ether at room temperature have been investigated by scanning tunneling microscopy (STM). High-resolution STM images show molecular-scale morphological changes of the monolayers resulting from the desorption of molecules from the surface and provide a significant implication for the stability of the monolayers in pure organic solvent. Defects in the ordered monolayer were mainly initiated at domain boundaries and around depressions, followed by molecular defects in closely packed domains. After the immersion of predeposited SAMs in pure diethyl ether for 10 days, the structure of the monolayer was completely changed to a disordered phase. Meanwhile, the recrystallization of the resulting disordered phase at 4 °C for 6 months gave rise to a well-ordered 6 × √3 structure having alternating missing rows in the nearest-neighbor direction, reflecting that the disordered phase originates from low surface coverage. This is the first STM result to show that the low-density molecular structure appeared by the recrystallization of the disordered phase.