Impact of annealing treatment on adlayer structures of terphenylmethanethiol self-assembled monolayers on Au(111)

The scanning tunneling microscopy (STM) technique was employed to investigate the adlayer structure and morphology of Self-assembled monolayers (SAMs) fabricated from terphenylmethanethiol (TP1) molecules on Au (111) by preparing from an ethanolic solution at room temperature and subsequently annealed at various temperatures, reaching up to 468 K. Upon heat treatment, the TP1 adlayer exhibited a sequence of distinct structural phases characterized by varying molecular arrangements, namely alpha, beta, gamma, delta, epsilon, sigma, and theta, emerging successively for both the as-prepared and annealed samples. The progressive transformation from alpha ->beta -> gamma ->delta ->epsilon ->sigma ->alpha ->theta phases, driven by escalating annealing temperatures, entailed a gradual decrease in molecular packing density until the transition from gamma ->delta, followed by a subsequent increase leading to the formation of the most densely packed molecular network with molecules standing almost upright, referred to as the alpha phase, which is the same phase observed for non-heat-treated films. Interestingly, the desorption process for the annealed SAMs occurred from the densest packing TP1 molecular structure of the alpha phase, contrary to those of biphenyl-based organothiol and n-alkanethiol SAMs where desorption takes place from lying-down striped phases. Almost all TP1 molecules were desorbed at an annealing temperature of 468 K, leading to a significant alteration in the surface structure. During the annealing process, our results showed that the gold surface underwent remarkable changes, including the disappearance of the vacancy islands and the emergence of surface cracks in the form of trenches, along with the formation of rounded monoatomic gold islands with a densely packed alpha phase.