Water Vapor Adsorption Effect on Silica Surface Electrostatic Patterning

This work verifies a model for the creation and dissipation of reproducible electric potential patterns on silica surfaces, based on water adsorption, ionization, and ion migration under applied electric potential. Samples were thin silica films grown on silicon wafers and partially covered with sets of parallel gold stripe interdigitated electrodes that are normally used for Kelvin force microscope calibration. Noncontact electric potential measurements with a 20 nm spatial resolution were done using the Kelvin method under controlled atmosphere, in an atomic force microscope (AFM) with a Kelvin force attachment (KFM) mounted within an environmental chamber. Patterns were observed in micrographs acquired while one electrode set was biased and the other was grounded and when both were short-circuited and grounded. Electrostatic charging and discharging are much faster at high relative humidity, showing that the charged or discharged silica states are both changed faster under high humidity, while pattern preservation is effective under low humidity. The results are explained considering surface conductance and the partitioning of water cluster ions both in the solid-gas interfaces and the atmosphere, under the biased electrode potential.