X-ray Photoelectron Spectroscopy with Electrical Modulation Can Be Used to Probe Electrical Properties of Liquids and Their Interfaces at Different Stages.

Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.