Fig. 3: Charging behaviour correlates with the presence of adventitious carbon.
From: Adventitious carbon breaks symmetry in oxide contact electrification
a, Spectra from ToF-SIMS show a cocktail of adsorbed species, especially carbon-based moieties. After plasma treatment (inset plots) or bake (Extended Data Fig. 2), peaks corresponding to these are greatly reduced. The H2O peak increases, highlighting the quick readsorption and equilibration of water, whereas peaks corresponding to the SiO2 matrix are broadly unchanged. b, Spatially resolved ToF-SIMS shows the spread of carbon species about the surface. These species are removed by plasma or baking but then readsorb over time. c, Although a measured charging rate is constant after our standard cleaning and 72-h storage, it evolves over time directly after plasma/baking. The relaxation half-life is typically around 10 h. d, We measure the time-dependent atomic composition of a surface’s outermost atomic layer using LEIS, exposing it to air between measurements. The signal for C is reduced to virtually zero immediately after plasma, then comes back over the same order of time as the charging relaxation. The H signal follows a similar trend, although with an offset at t = 0 h owing to the fast equilibration of silanols and surface water. The Si and O increase immediately after plasma as the bulk is exposed but then decrease as the carbon layer develops. We observe the same trends if we keep a sample in UHV, although with a much longer timescale (about 36 h, dashed line).
