Substrate-enhanced time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) can provide chemical information about the uppermost monolayers of a surface. In SIMS, keV projectiles bombard a sample, in turn supplying surface composition. An obstacle to using SIMS for routine characterization, however, has been low molecular secondary ion yields. Previous studies have demonstrated that secondary ion yields in organic SIMS can be significantly improved by using polyatomic projectiles. To further improve the performance of TOF-SIMS for molecular characterization, novel substrates were investigated from which to sputter molecular species.;This research investigation focused on developing methods to increase secondary ion yields from ultrathin surfaces in TOF-SIMS. Different substrates and projectiles were examined to determine optimum parameters for improved performance. First, an aminoethanethiol (AET) self-assembled monolayer on a gold substrate has a protonated amine terminal group at neutral pH. This surface was used to adsorb organic and inorganic anions from solution. The anions are bound to the positively-charged surface via electrostatic interactions. The anions reversibly adsorb to the surface, and the AET substrate can be used repeatedly. Secondary ion yields increased up to ∼500 times compared to conventional surfaces such as bare Au and Si surfaces and stainless steel supports used in SIMS. Differences in yield enhancements for intact and fragment ions using atomic and polyatomic projectiles such as (CsI)nCs + (n = 0--2) and (NaF)nNa + (n = 0--2) were also studied.;Second, molecular characterization is dramatically improved when a self-assembled monolayer or bilayer surface is photooxidized prior to inserting the sample into the mass spectrometer. Secondary ion yields are improved when the thiolate group is oxidized to a sulfonate moiety. It is thought that the electron affinity for the sulfonate may be greater and have a weaker bond to the gold substrate compared to the thiolate moiety. Novel bilayer architectures on gold were characterized by TOF-SIMS and by spontaneous desorption mass spectrometry, the first time to our knowledge.