Spatially and Temporally Resolved Mapping of Contact Electrification on Stand-Alone Ultrathin Glass Materials via Kelvin Probe Force Microscopy

Contact electrification (CE) remains a critical challenge in advanced material technologies where uncontrolled surface charging can compromise manufacturability, reliability, and performance of the materials for practical applications. Ultrathin glass with micrometer-scale thickness is a state-of-the-art specialty oxide material for flexible touchscreens in new-generation electronic devices. Despite extensive studies on CE on thermally grown oxide thin films, the physical and chemical properties of the stand-alone ultrathin oxide materials could be very different and thus lead to distinct CE behaviors. Such behaviors have not been experimentally investigated due to the challenge of their ultrathin form factor as well as the lack of experimental methods that would allow the successful study of CE on stand-alone ultrathin glass materials. Here, we, for the first time, visualize and quantify CE-induced surface charges on ultrathin glasses using sideband-mode Kelvin probe force microscopy (KPFM). To enable the KPFM measurement, we have established experimental strategies, including electrode preparation enabling the measuring circuit, and surface cleaning procedures improving surface activation and hydrophilicity. Nanosized atomic force microscopy (AFM) probes were used to scan and induce triboelectric charges on the stand-alone glass surfaces with a variety of thicknesses (30–100 μm) under ultrapure N₂ conditions. Time-dependent measurements reveal the surface charges on a 30 μm-thick glass sample decay from 4.47 to 0.37 V in 240 min. Moreover, we found that electrostatic charges exhibit a capacitor-like discharging behavior primarily through the bulk material yielding a long relaxation time constant of ∼41 min, which is different from the lateral surface discharging behavior in a thermally grown SiO₂ thin film reported previously. Furthermore, the thickness-dependent surface charging effect was characterized for the ultrathin glass substrates, where the change in contact potential difference between the charged and uncharged region (ΔV_CPD) was found to remain nearly constant across this thickness range from 1.39 ± 0.17 V at 30 μm to 1.34 ± 0.29 V at 100 μm. A self-capacitance analytical model was developed and employed to estimate the corresponding surface charge density (σ), yielding comparable values of 136.26 ± 16.25 μC/m² at 30 μm and 131.44 ± 28.41 μC/m² at 100 μm. Additionally, the external bias applied to the AFM tips can be used to enhance, suppress, or invert the intrinsic CE response of glass materials. This work extends nanoscale CE characterization beyond oxide thin films to stand-alone oxide materials, providing a framework to understand and manipulate electrostatic charging in glass systems for practical applications.