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

Abstract

Ultraviolet photoelectron spectroscopy (UPS) is a key technique to determine the work function (Φ) 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 Φ. For such samples, the measured SECO spectra exhibit actually two cut‐offs, one representing the high Φ surface component and the other one corresponding to an area‐averaged Φ value. By combining Kelvin probe force microscopy and electrostatic modeling, it is quantitatively demonstrated that the electrostatic potential of the high Φ areas leads to an additional energy barrier for the electrons emitted from the low Φ areas. Theoretical predictions of the induced energy barrier dependence on the Φ‐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.

Importance of Substrate Work Function Homogeneity for Reliable Ionization Energy Determination by Photoelectron Spectroscopy

“…This feature, however, is rather important, as it is the SECO due to electrons leaving the sample from regions where CBP molecules sit on top of the FSH‐SAM; the fit shown in Figure (c) provides this high Φ to be 4.85 eV, in good agreement also with the KPFM data for the bare FSH‐areas. In more detail, the apparent low kinetic energy onset at 4.37 eV corresponds to an area‐averaged Φ value (), since electrons leaving the CBP on top of the CSH have to overcome an additional energy barrier formed by the adjacent high local work function material . Using the values from KPFM and , where a is the area fraction of CSH, we find a of 4.37 eV, in excellent agreement with the low kinetic energy onset identified in Figure (a).…”

“…Accordingly, it would be desirable to be able to identify in the spectra also the corresponding not yet discussed (and apparent) features, that is, the HOMO level emission from CBP adsorbed on CSH‐areas, and the high Φ SECO from the FSH‐areas. This can indeed be done with a fitting routine described earlier, and reference valence spectra of CBP deposited on a homogeneous substrate (available from our previous work, and shown in color as fit‐components in the figure), as demonstrated in Figure (c) and (d). Closer inspection of the SECO spectrum actually reveals a shoulder‐like structure at higher kinetic energy than the apparent onset.…”

“…The first method to obtain lateral Φ contrast substrates for molecular semiconductor deposition comprised the use of 1‐hexadecanethiol (CSH) and 1H,1H,2H,2H‐perfluorodecanethiol (FSH), which have oppositely oriented dipole moments, in the form of SAMs on gold. The SAM substrates were fabricated by microcontact printing and backfilling of CSH and FSH SAMs on polycrystalline Au, as described in detail in previous work . The second method, featuring wider stripes of Φ contrast, comprised first evaporating a thick film of aluminum on float glass and subsequently evaporating ≈5 nm gold on top through a shadow mask with a finger structure (for substrate characterization see below).…”

“…In a previous study we have demonstrated how, simply as a consequence of electrostatics, heterogeneity of the surface potential of a laterally micro‐structured sample substantially influences Φ determined by UPS . Notably, it was shown that when the surface consists of dissimilar work function materials, two SECOs might be observed, depending on the area ratio of the two materials.…”

Electrode Work Function Reduction by Polyethylenimine Interlayers: Choice of Solvent and Residual Solvent Removal for Superior Functionality

“…The work function of multicomponent surfaces is an average of the local work function values of the individual components. [ 24 ]…”