Interpreting Enigmatic Work Function Spectra in UPS

Work function measurements using Ultraviolet Photoemission Spectroscopy (UPS) can produce complex and difficult to interpret spectra. In contrast, the theory is simple and dates to Einstein’s quantum theory of light. By grounding a conducting sample, photoemission from the Fermi level has the kinetic energy of the light source, and the work function corresponds to the minimum observed electron energy. Experimentally, detectors cannot measure very low energy electrons, so the sample is biased negatively to increase the onset energy to an efficient range. Unfortunately, the minimum kinetic energy can be difficult to identify in practice when the spectral onset is not simple, particularly on materials with a small density of states near the E_f. This talk provides examples of work function measurements from photoemission onsets for clean and dirty elemental metals, oxides, and carbon nanostructures. While spectra for clean elemental metals are “textbook”, complex surfaces can exhibit intensity at energies below the true onset. The origins and practical approaches to identify and mitigate deceptive features are discussed.