Chemically Alternating Langmuir-Blodgett Thin Films as a Model for Molecular Depth Profiling by Mass Spectrometry

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C₆₀) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling.