Electron-Beam-Induced Damage of Alkanethiolate Self-Assembled Monolayers (SAMs): Dependence on Monolayer Structure and Substrate Conductivity

摘要

We report the first studies of the relative importance of substrate conductivity and monolayer structure on the electron-beam-induced damage of alkanethiolate self-assembled monolayers (SAMs) adsorbed on Au and GaAs(001) using time-of-flight secondary-ion mass spectrometry. The results clearly show that the extent of damage observed is strongly dependent on the electrical conductivity of the substrate; at a given electron dose, the amount of degradation is greatest for SAMs adsorbed on the least conductive substrate, semi-insulating GaAs(001). This is because there is a buildup of static charge at the substrate/SAM interface, whereas for an electrically conductive substrate, electrons can be conducted away from the surface, leading to less electron-beam-induced damage. The monolayer structure also greatly affects the amount of electron beam damage. Disordered SAMs, such as nonanethiol adsorbed on Au, undergo more degradation at a given electron dose than ordered SAMs, such as octadecanethiol (ODT) adsorbed on Au. Comparison of the data for undecanethiol (UDT) on conducting GaAs, a disordered SAM, and ODT on semi-insulating GaAs, an ordered SAM, suggests that the detailed 2D monolayer structure plays a more important role than the electrical conductivity of the substrate in determining the extent of electron-beam-induced damage. In addition, differences in the detailed structure of SAMs on Au and GaAs affect the reaction pathways observed. These findings explain previously reported results that much higher electron beam doses are required to damage SAMs on metals compared with SAMs adsorbed on semiconductors and insulators.