Photoemission investigation of interaction between L‐cysteine and silver surface

Abstract

In this study, the formation of Ag–S bond was systematically elucidated by thickness‐dependent ultraviolet photoelectron spectroscopy (UPS) in order to understand the L‐cysteine interaction with silver surface. A clean Ag(111) as the model system for silver surface was used, and L‐cysteine films on silver substrate were formed by vacuum evaporation. The orbital configurations at the interface was estimated including work function, secondary electron cutoff (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection barrier, and ionization energy. A clear spectral feature was appeared in between Fermi edge and HOMO of L‐cysteine, and the feature can be attributed to the formation of Ag–S bonding. In the case of SECO, the maximum shift was 0.46 eV to the higher binding energy side at the nominal thickness of 1 Å. However, from the nominal thickness of 2 Å, SECO started to shift to the lower binding energy side, and at 16 Å, the SECO shifted to a value of around 0.4 eV to the lower binding energy side to almost cancel the initial vacuum level shift. This behavior can be attributed to weakening of the silver‐sulfur bond with increasing of L‐cysteine coverage referring to the literature. The photoelectron yield spectroscopy (PYS) was also performed as an additional spectroscopic work, which exhibited that the work function of silver once decreased and then recovered at low coverage. This behavior can also be assigned to a weakening the interaction of L‐cysteine with silver by increasing of L‐cysteine coverage.