Reliable Work Function Determination of Multicomponent Surfaces and Interfaces: The Role of Electrostatic Potentials in Ultraviolet Photoelectron Spectroscopy

Ultraviolet photoelectron spectroscopy (UPS) is a key technique to determine the work function (phi) of surfaces by measuring the secondary-electron cut-off (SECO). However, the interpretation of SECO spectra as obtained by UPS is not straightforward for multicomponent surfaces, and it is not comprehensively understood to what extent the length scale of inhomogeneity impacts the SECO. Here, this study unravels the physics governing the energy distribution of the SECO by experimentally and theoretically determining the electrostatic landscape above surfaces with defined patterns of phi. For such samples, the measured SECO spectra exhibit actually two cut-offs, one representing the high phi surface component and the other one corresponding to an area-averaged phi value. By combining Kelvin probe force microscopy and electrostatic modeling, it is quantitatively demonstrated that the electrostatic potential of the high phi areas leads to an additional energy barrier for the electrons emitted from the low phi areas. Theoretical predictions of the induced energy barrier dependence on the phi-pattern length scale and sample bias are further experimentally verified. These findings establish a solid base for reliable SECO interpretation of heterogeneous surfaces and improved reliability of interfacial energy-level diagrams from UPS experiments.