Depth profiling of Langmuir-Blodgett films with a buckminsterfullerene probe

Bombardment with C₆₀⁺ primary ions of monolayer and multilayer barium arachidate Langmuir-Blodgett (LB) films is investigated. The behavior of cluster versus atomic (Ga⁺) bombardment is monitored by the barium-cationized arachidate ion (mass-to-charge ratio (m/z) 449) and a characteristic fragment ion (m/z 209) using 1-, 7-, and 15-layer model systems. The removal rate of material from the films is shown to be on the order of several hundred molecules per C₆₀ impact, a value 100-fold larger than Ga⁺ impact. The enhancement in secondary ion yield is also shown to be larger for the 15-layer film (400x) than for the monolayer film (100x). Moreover, most of the increase in yield is shown to be associated with ejection of sputtered species rather than an increase in ionization probability. High yields associated with cluster bombardment are also shown to be amenable to depth profiling experiments in which the two ions can be monitored as the film is being removed. In this modality, chemical damage associated with bombardment is removed before it can accumulate on the surface. Due to the similarity of fatty acid LB films to cellular membranes, these results suggest that C ₆₀⁺ primary ion beams may improve the prospects for TOF-SIMS studies of biological systems.