Primary ion fluence dependence in time-of-flight SIMS of self-assembled monolayer of alkyl thiol molecules on Au(1 1 1)—Discussion of static limit

The nondestructive nature of static secondary ion mass spectrometry (SIMS) in the context of studies of self-assembled monolayers (SAMs) of organic molecules has been examined by measuring the primary ion fluence dependence of secondary ion signals with two well-known SAMs, C18H37SH on Au(1 1 1) and C18H37PO3H2 on freshly cleaved mica. This SIMS analysis is challenging because the bonding nature is delicate and the areal molecular density is less than 1015 cm−2. In SIMS, it is prevalently assumed that if the primary ion fluence is confined to not more than 1 × 1012 cm−2, all secondary ion signals should not change by more than 10% and the practically defined static condition is satisfied. Our results from time-of-flight SIMS with the common primary ions of Bi3+, Bi+ and Ar+, indicate that this prevalent static assumption fails for both model SAMs. The SIMS results from the phosphyl case, which have been recently published, consistently display the evidence of bombardment-induced damage. In comparison, the thiol case presented here shows much more complex primary ion fluence dependence of SIMS signals. It is therefore concluded that practical static analysis should use primary ion fluence not more than 1 × 1011 cm−2 or should simply record and report the effects of primary ion fluence.