Structural Transitions of Octanethiol Self-Assembled Monolayers on Gold Nanoplates after Mild Thermal Annealing

Abstract

Single-crystalline gold nanoplates with a thickness of less than 100 nm were synthesized by the reduction of HAuCl4 in water with cetyltrimethylammonium chloride, and their structures were confirmed by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. The formation and surface structures of octanethiol (OT) self-assembled monolayers (SAMs) on gold nanoplates were examined by means of atomic force microscopy and scanning tunnelling microscopy (STM). Molecularly resolved STM observation showed that OT SAMs on gold nanoplates at 25 degrees C for 24 h were composed of two mixed phases containing well-ordered (root 3 x root 3)R30 degrees and c(4 x 2) domains and disordered domain, which are comparable to the formation of fully covered (root 3 x root 3)R30 degrees or c(4 x 2) structures on conventional Au(111) films. After thermal annealing of the precovered OT SAMs on gold nanoplates at 70 degrees C for 30 min, we clearly observed the structural transitions of OT SAMs from the two mixed phases to the loosely packed, uniform 6 x root 3 phase, which is mainly driven by the optimization of van der Waals interactions between alkyl chains via the rearrangement of OT molecules in the two mixed domains to reach thermodynamically stable SAMs. Our results herein provide new insights into the formation and structural behaviors of alkanethiol SAMs on size-confined gold nanostructures.