Topography Influence on Noble Metals’ Work Function Measured In Vacuo by Photoelectron Spectroscopy and Kelvin Probe Force Microscopy

The work function (WF), the minimum energy required to extract an electron from a material's surface into a vacuum, is affected by the parameters of the surface of the material, such as the surface topography. In the literature, it has been shown that changes in roughness affect the WF of metals; however, no clear relation between these two parameters was formulated. The paper presents the dependence of the WF on surface roughness for gold and silver thin films deposited on stainless steel substrates with roughness in the range of nanometers to micrometers. The WF measurements were performed using in-vacuo ultraviolet photoelectron spectroscopy (UPS) and in-vacuo Kelvin probe force microscopy (KPFM), ensuring that the surface remained free from air contamination and oxidation. The topography of the samples was characterized using both 3D laser confocal microscopy and atomic force microscopy. A study of the WF dependence on topography parameters, amplitude and slope, revealed that WF decreases with increasing roughness. Depending on the choice of roughness parameter, a logarithmic or linear relationship was observed. This behavior is attributed to the relationship between amplitude and slope parameters, which was validated experimentally. The decrease of the WF with increasing roughness, according to the literature, is linked to the electronic redistribution of charges and potential step at the surface of a material, which gives rise to the surface dipole, which depends sensitively on the surface's atomic arrangement, namely, a roughness increase. The WF measurements using in-vacuo UPS and KPFM, performed on two samples with different morphologies, showed similar results. Additionally, after exposure to air, WF measurements via KPFM revealed a reduction of 0.45 eV compared to vacuum conditions. Overall, these results evidenced the critical aspect of in-vacuo measurement for the WF.