Enhanced stability of triboelectric charge by hydrophilic air-stable radicals

The stability of triboelectric charge has received much attention due to academic interest and numerous applications. An example of the latter includes increasing the lifetime of air filtration that uses the charges to capture microdust. We studied charge retention on different self-assembled monolayers (SAMs) on Si wafers. The goal is to investigate the role of radicals on charge retention and eventually find a way to increase charge stability. Pt-coated atomic force microscopy probes were brought into contact with different SAMs to tribocharge the SAMs. The charge retention was recorded over time by the non-contact tapping mode of Kelvin Probe Force Microscopy (KPFM). The KPFM images showed that charges on hydrophilic surfaces generally tend to dissipate faster than those on hydrophobic surfaces, which indicates water vapor could enhance the charge dissipation. Interestingly, the charge retention in the presence of stable radicals (TEMPO) dissipated much slower (>12 h), showing out of the general trend. To verify the role of radicals in enhancing charge retention, radical density on the surface was tuned by two separate approaches: the treatments of TEMPO SAMs with tetrabutylammonium fluoride as a SAM etchant and ascorbic acid as a radical scavenger. After these treatments, the charge destabilized by lowering radical density on the surface.