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 in practical applications. Ultrathin glass with micrometer-scale thickness is a state-of-the-art specialty oxide material for flexible touchscreens in next-generation electronic devices. Here, we visualize and quantify CE-induced surface charges on ultrathin glass using sideband-mode Kelvin probe force microscopy (KPFM). Nanoscale atomic force microscopy (AFM) probes are used to scan and induce triboelectric charges on stand-alone glass surfaces under ultra-pure N$_2$ conditions. Time-dependent measurements reveal that surface charges on a 30~$μ$m-thick glass sample decay from 4.47V to 0.37V over 240minutes. Furthermore, electrostatic charges are found to exhibit capacitor-like discharging behavior primarily through the bulk material, yielding a long relaxation time constant of approximately 41minutes. This behavior differs from the lateral surface discharging observed in thermally grown SiO$2$ thin films reported previously. A self-capacitance analytical model is developed to estimate the corresponding surface charge density ($σ$), yielding comparable values of 136.26~$\pm$16.25$μ$C/m$^2$ at 30~$μ$m and 131.44~$\pm$28.41$μ$C/m$^2$ at 100~$μ$m. Additionally, external bias applied to AFM tips can be used to enhance, suppress, or invert the intrinsic CE response of glass materials.

💡 Research Summary

The authors present a comprehensive study of contact electrification (CE) on stand‑alone ultrathin glass substrates ranging from 30 µm to 100 µm in thickness, using sideband‑mode Kelvin probe force microscopy (KPFM) to directly visualize and quantify the resulting surface charges. Recognizing that prior KPFM investigations have been limited to nanometer‑scale oxide films on conductive silicon, the team developed a set of experimental strategies to overcome the challenges posed by thick insulating glass. First, a 5 nm chromium seed layer followed by a 50 nm gold film was deposited on the backside of each glass piece, creating a well‑defined ground electrode that forms a parallel‑plate capacitor with the AFM tip. This configuration mitigates the severe attenuation of tip‑sample capacitance that occurs in bulk dielectrics and stabilizes the electrical environment for accurate surface potential measurements.

Surface preparation was rigorously controlled: samples underwent a standard solvent wash, a high‑temperature KOH etch (1 M, 80 °C, 20 min) to remove residual organics and activate the surface, and an ultrasonic rinse in ultrapure water. These steps reduced the root‑mean‑square roughness from ~0.5 nm to ~0.3 nm and lowered the water contact angle from 32.5° to <5°, indicating a highly hydrophilic surface that promotes charge trapping. The enhanced cleanliness translated into a larger contact‑potential‑difference (ΔV_CPD) between charged and uncharged regions, increasing from ~1 V (sonicated) to ~2.6 V (KOH‑treated).

All measurements were performed in a nitrogen‑purged glovebox with <0.1 ppm H₂O and O₂ to suppress moisture‑induced charge dissipation. Contact charging was induced by scanning a n‑type Si AFM tip over a 2 × 0.5 µm² area at 50 nN normal load and 0.4 Hz scan rate. Subsequent sideband‑mode KPFM scans over a larger 4 × 1 µm² region captured the spatial distribution of surface potential with high lateral resolution, as the sideband technique isolates short‑range electrostatic forces at the tip apex, minimizing long‑range cantilever contributions.

Time‑dependent studies on a 30 µm sample revealed that after three charging cycles the initial ΔV_CPD of 4.47 V decayed to 0.37 V after 240 minutes. The decay follows an exponential law ΔV(t)=ΔV₀ exp(−t/τ) with a relaxation time constant τ≈41 min, indicating that charge dissipation occurs primarily through bulk‑mediated, capacitor‑like discharge rather than lateral surface diffusion. This behavior contrasts with previously reported lateral spreading in thin SiO₂ films.

Thickness dependence was examined by comparing 30 µm and 100 µm samples. Despite a five‑fold increase in thickness, the average ΔV_CPD remained essentially constant (1.39 ± 0.17 V vs. 1.34 ± 0.29 V). Using a self‑capacitance analytical model, the surface charge density σ was estimated as 136 ± 16 µC m⁻² for 30 µm glass and 131 ± 28 µC m⁻² for 100 µm glass, confirming that the amount of trapped charge is largely independent of thickness within this range.

An additional set of experiments demonstrated that applying an external DC bias to the AFM tip can modulate the CE response: positive bias enhances charge accumulation, negative bias suppresses it, and sufficiently large bias can even invert the sign of the generated surface charge. This controllability suggests a pathway for active management of triboelectric charging in practical devices.

Overall, the paper makes four key contributions: (1) it extends high‑resolution KPFM mapping of CE to micrometer‑scale, stand‑alone glass substrates; (2) it identifies bulk‑dominated, capacitor‑like discharge with a long relaxation time as the dominant relaxation mechanism; (3) it provides quantitative models linking ΔV_CPD, thickness, and surface charge density; and (4) it shows that tip‑bias engineering can actively tune the CE outcome. These insights are directly relevant to the design of flexible displays, touch panels, and energy‑harvesting systems where uncontrolled electrostatic charging of glass can lead to dust attraction, adhesion loss, dielectric breakdown, and device failure. By offering a clear experimental framework and quantitative parameters, the work equips engineers and researchers with the tools needed to predict, monitor, and mitigate triboelectric effects in next‑generation glass‑based technologies.