The electronic structure and polymerization of a self-assembled monolayer

Irradiation-induced modifications of electronic structure in the monomolecular insulator [1,1';4',1''-terphenyl]-4,4''-dimethanethiol (TPDMT) films have been investigated by photoemission and inverse photoemission. A dominant effect is cross-linking of the TPDMT film, which result in a quasipolymeric material with a smaller gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) but improved the dielectric properties. The cross-linking processes are also accompanied by changes in molecular orientation. The photoemission intensities of organic molecular layers generally obey the Debye-Waller temperature dependence but not always. With the example of a monomolecular film formed from [1,1';4',1''-terphenyl]-4,4''-dimethanethiol, we show that pronounced deviations from Debye-Waller temperature behavior are possible and are likely caused by temperature dependent changes in molecular orientation.