Comparative Analysis of Work Function Measurement Techniques Based on Scanning Probe Microscopy

In this study, we compare three scanning probe techniques- $I\left(z\right)$ spectroscopy, field-emission resonances (FER), and Kelvin probe force spectroscopy (KPFS)-for quantitative local work function measurements. Using Ag(111) as a reference and applying the methods to Ir(111) and Au(111), we find a strong method dependence. $I\left(z\right)$ spectroscopy shows substantial uncertainty due to broad data dispersion and is reliable only on flat surfaces. We show that the triangular-barrier FER extrapolation method lacks robustness, whereas numerical FER analysis based on an image potential-corrected Schrödinger model yields consistent values. KPFS provides the most direct and reliable work function determination once the tip is calibrated. Applying these methods to superconducting Nb, we extract the work functions of (111) and (110) surfaces and its distinct oxygen-reconstructed phases.