Orientation-Selective Growth of Single-Atomic-Layer Gold Nanosheets via van der Waals Interlocking and Octanethiolate-Confined Molecular Channels

Single-atomic-layer gold nanosheets have been synthesized on the Au(111) substrate guided by vertical molecular channels confined by an octanethiolate (OT) surfactant, in great contrast to the typical hexagonal or triangular islands observed in homoepitaxy on the (111) plane of fcc metals. This anomalous growth mode shows a strong dependence on the coverage of the surfactant and hence demonstrates the possibility of using the surfactant coverage as a useful control parameter in fine-tuning the shape and aspect ratio for the growth of metal nanorods. The Au sheet grows preferentially in the ⟨112̅⟩ direction, and its width changes in a quantized manner in steps of 6.5a (∼1.85 nm), where a is the nearest-neighbor distance for Au atoms. This unexpected symmetry-breaking growth mode is proposed to arise from the interaction between the edges of the growing Au sheet and the OT overlayer, which behaves as a molecular carpet covering both the Au(111) substrate and the Au sheet. The growth of the Au sheet deforms the molecular carpet. Due to the crystalline structure of the OT layer, the energy involved in distorting the molecular carpet is directionally anisotropic. This hinders the growth of the Au sheet in certain directions leading to the symmetry-breaking growth. Direct comparison with an ethanethiol monolayer confirms the importance of the length of the alkyl chain in controlling the shape of the Au sheet.