Investigation of electron-induced cross-linking of self-assembled monolayers by scanning tunneling microscopy

Ultrathin membranes with subnanometer pores enabling molecular size-selective separation were generated on surfaces via electron-induced cross-linking of self-assembled monolayers (SAMs). The evolution of p-terphenylthiol (TPT) SAMs on Au(111) surfaces into cross-linked monolayers was observed with a scanning tunneling microscope. As the irradiation dose was increased, the cross- linked regions continued to grow and a large number of subnanometer voids appeared. Their equivalent diameter is 0.5 +/- 0.2 nm and the areal density is approximate to 1.7 x 10(17) m(-2). Supported by classical molecular dynamics simulations, we propose that these voids may correspond to free volumes inside a cross-linked monolayer.