Improving the Relative Calculations of Volta Potential Differences Acquired from Scanning Kelvin Probe Force Microscopy (SKPFM) By Comparing Inert Standards to First-Principle Calculations
Authors: Efaw Corey , da Silva Thiago , Davis Paul , Li Lan , Hurley Michael 
Published: February 2020

Metal corrosion is an electrochemical process typically controlled by features such as grain boundaries, defects, coating/metal interfaces, composition, and local chemical environments. Advances in characterization tools have progressed the means to resolve these features with hopes of better understanding corrosion initiation and progression. However, many localized techniques, such as SECM, SRET, and electrochemical microcells do not provide sub-micron spatial resolution, as well as require in-situ observation of the kinetic processes to investigate corrosion mechanisms.

Scanning Kelvin probe force microscopy (SKPFM) has presented a non-destructive alternative to better understand corrosion mechanisms on the micro to nano-scale. SKPFM measures surface electronic properties wherein the Volta potential difference (VPD) between Kelvin probe and surface is collected. This Volta potential difference have been directly correlated to corrosion potentials, and thus provides strong predictability of the corrosion mechanisms seen on complex microstructures without degrading the material. When co-localized with elemental characterization provided from SEM/EDS, Volta potential differences can be correlated to elemental composition.

A major drawback with SKPFM is the lack of reproducible results seen for materials’ measured Volta potential differences. There are many reasons for the inconsistency, including variations in experimental conditions (i.e. temperature and relative humidity), operation system and scanning technique, and sample surface conditions (i.e. crystallographic orientation, adsorbed species, roughness, and surface terminating atoms). This work focuses on another plausible factor to this VPD inconsistency by observing the effects of probe choice, as well as probe wear from regular probe usage.

To understand these effects, it is important to distinguish that SKPFM spatially maps the Volta potential difference between the surface and the probe (Δψ), which can be linked to the difference in modified work functions between the surface and probe (Δφ). It can be seen from this relationship that any variation in the Kelvin probe's makeup or its wear characteristics would shift the observed Volta potential difference of the material without the material itself characteristically changing. This is why it is common practice to compare the changes in Volta potential differences seen in a singular image, and why comparing results directly to others' published work is notably difficult to accomplish.

This work attempts to address the inconsistencies seen from published SKPFM results by developing a relative scaling of Kelvin probe work function. This can be accomplished by mapping relatively inert standards just prior to imaging the material of interest and calculating Volta potential differences between the material’s phases and the inert standard. Much like in common electrochemical experimentation, SKPFM will utilize the standard as a “reference electrode”. The relative work functions of these inert standards were approximated with first-principle calculations by density function theory (DFT). From this work, we hope that the discrepancies seen from SKPFM results can start to be resolved through improving the calculations of various material's relative Volta potentials.

Figure 1