Multi-Modal Surface and Structural Characterization of Ultrathin Foils for Optimized Particle Transmission in Time-of-Flight Space Plasma Analyzers

Time-of-flight plasma analyzers use ultrathin carbon foils to generate secondary electrons for precise particle time-tagging. Standard amorphous carbon foils (0.5–1.0 µm) cause angular scattering and energy loss, limiting low-energy ion resolution. Monolayer graphene (~0.345 nm) could in principle minimize straggling and maximize transmission, but integration is hindered by contamination, wrinkling, and fragility. We present a comparative structural and chemical analysis of single-layer graphene and amorphous carbon foils using optical microscopy, Keyence VK-X3050 profilometry, and Thermo K-Alpha XPS, linking surface properties to plasma analyzer performance. Graphene foils exhibited >200× fewer holes, yet 5–6× higher roughness (Ra ≈ 37 nm vs. 6–7 nm) with >20% wrinkle coverage >200 nm high, potentially introducing scattering anisotropies. XPS showed PMMA-like contamination (~1.2 ± 0.2 nm, ~34% coverage), sp² suppression, and O/C ratios of 0.17–0.23, with metal residues from transfer processes. These heterogeneities exceed the <5% contamination threshold for angular scattering parity, degrading low-energy ion resolution and TOF timing. Present foils remain limited by surface and topographic inhomogeneity despite graphene’s theoretical plasma diagnostic advantage.

This work is supported by Princeton University’s Office of Undergraduate Research Student Initiated Internship Program (OURSIP) through the Hewlett Foundation and by the US DOE Contract No. DE-AC02-09CH11466.