Studies of self-assembled monolayers on highly oriented pyrolytic graphite using scanning tunneling microscopy and computational simulation

Abstract

We have ingestigated the self-assembled monolayers (SAMs) formed on highly oriented pyrolytic graphite (HOPG) using scanning tunneling microscopy and computational simulation. 1) The SAMs formed by binary fatty acids mixture demonstrated that the phase-separated monolayers were formed instead of the phase-mixed monolayers. A meta-sable state of the monolayers was also observed before it was converted to a thermo stable structure. 2) Only the perylene derivative with smallest attached groups formed stable monolayers on HOPG. The bulky perylene derivatives were hard to be immobilized on the graphite surface due to weaker interaction between the substrate and surface. 3) A chiral monolayer formed from the hexaalkyl hexaphenylbenzene (HHB) on an HOPG substrate. The chirality of the SAMs was due to the different binding sites that the adsorbates could have upon physisorption onto HOPG. Different unit cells were also observed. 4) A general equation to describe the conditions for formation of 2 dimensional crystals on HOPG surfaces was derived. The factors including concentration, surface structure of graphite, molecular size of adsorbates, flatness of adsorbates, symmetry of adsorbates, H-bonding, and temperature were taken into consideration and discussed.