The c(4X2) Superlattice of n-Alkanethiol Monolayers Self-Assembled on Au(111)

We report the results of an ultrahigh vacuum scanning tunneling microscope (STM) study of n-alkanethiol [CH sub(3)(CH sub(2)) sub(n-1)SH] (C sub(n)) monolayers self-assembled on Au(111) single-crystal surfaces. The STM images show that the saturation coverage self-assembled monolayer (SAM) unit cell measures 0.86 x 1.0 nm and contains four thiolate molecules. The observed unit cell is consistent with results from recent X-ray, electron, and helium diffraction studies. This unit cell corresponds to a c(4x2) superlattice of a basic ( square root 3x square root 3)R30 dense-packed monolayer. The domain size ranges from 5.0 to 15 nm with domains separated by molecular-width rotational boundaries, antiphase boundaries, and stacking faults. SAMs composed of longer chain thiols (C sub(8) and C sub(10)) were stable in vacuum for over 40 days.