Substrate’s surface electrons disrupt molecular self-assembly

Designing nanostructures on substrates requires control of the self-assembly process to create stable structures with desired properties, but achieving ordered assembly can be difficult. Molecular interactions with the substrate can create complicated effects.

Earlier experiments with the organic compound anthraquinone (AQ) on Cu(111) substrates showed the formation of large and highly regular surface pore structures with AQ defining the periphery, an unexpected result traced to a delicate interaction between AQ and copper surface electrons. This raised questions of how slight differences in surface crystallography and electronic structure of the otherwise highly similar Au(111) would affect the development of porous surface layers.

As they report in The Journal of Chemical Physics, DeLoach, Conrad, Einstein, and Dougherty used scanning tunneling microscopy (STM) to observe previously unreported, coverage-dependent structures in self-assembled monolayers of AQ on Au(111) surfaces.

At high coverage on Au(111), AQ assembles in an ordered molecular layer, while at low coverage, chiral hexamers coexist with a 2-D molecular gas. However, the intermediate surface coverage presents the biggest unanswered questions. Here, the Au(111) surface electronic structure does not permit ordered, large pores to form at all, due to very small changes in the energy of its surface electrons. Instead, porous molecular assemblies on Au(111) are strongly disordered, with a distribution of shapes and sizes very different from those on Cu(111).

Quantitative analysis showed that the pore size distribution is well-described by a log-normal distribution function. The researchers propose a model, adapted from economics, in which kinetics drive random vacancy attachment rates that determine the ultimate statistical pore size distribution. They suggest that this model can be extended to similarly disordered assemblies reported in other monolayer systems.

Source: “Coverage dependent molecular assembly of anthraquinone on Au(111),” by Andrew S. DeLoach, Brad R. Conrad, T. L. Einstein, and Daniel B. Dougherty, Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4999623 .