Revealing Different Bonding Modes of Self-Assembled Octadecylphosphonic Acid Monolayers on Oxides by Time-of-Flight Secondary Ion Mass Spectrometry: Silicon vs Aluminum

Condensed octadecylphosphonic acid (OPA) dimers, i.e., two OPA molecules combined with the loss of a water molecule, were detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on OPA self-assembled monolayers (SAMs) that are only weakly bonded on the native oxide layer of a silicon wafer. In contrast, these condensed dimers were absent on OPA SAMs formed on the oxide layer of an aluminum film, where the OPA molecules are chemically bonded on the substrate through a P-O-Al linkage. These observations lead us to conclude that the OPA molecules in their SAMs have to be free from chemical bonding with the substrate in order for the primary ion beam to generate ion fragments of the condensed dimer. We demonstrate that the detection of condensed OPA dimers serves as an analytical criterion for TOF-SIMS to reveal the bonding mode of OPA molecules in their SAMs on different oxides.