Chromium- and Monolayer Graphene-Based Coating for THz-Assisted Atom Probe Tomography of Sol-Gel Silica Specimens

Journal article, 2026

Single-cycle terahertz (THz)-assisted atom probe tomography (APT) has emerged as a promising technique for triggering field evaporation under conditions that may reduce thermally activated contributions compared with conventional laser pulsing in conductive materials. While free carriers in metallic samples facilitate efficient THz near-field coupling at the specimen apex, nonmetallic materials like sol-gel silica lack this advantage, rendering positive THz pulses ineffective. However, negative THz pulses produce well-resolved mass spectra, which are compared with the mass spectra obtained with laser assisted APT( UV 343 nm and NIR 800 nm) to evaluate mass resolving power, signal-to-noise ratio and chemical composition. Moreover, we study the impact of the coating on the THz-APT results. Two metallization strategies are applied: (i) chromium (Cr) sputter coating via PECS and (ii) graphene encapsulation. Cr coating enables successful ion evaporation under positive THz pulses, improving mass resolution and reducing noise, while graphene encapsulation further enhances spectral clarity by sharpening peaks and reducing thermal tails. 3D reconstructions confirm effective deposition of both coatings over the tungsten-silica junction and reveal species segregation under positive THz pulses, highlighting the strong dependence of evaporation behavior on the applied field conditions.