Ultraviolet photoelectron spectroscopic study on the interface electronic structure of the L‐cysteine on Pd surface

Abstract

L‐cysteine is one of the most versatile biomolecules with a unique metal‐binding ability. L‐cysteine has an outstanding role in the bioelectronics field as a linker between proteins of biomolecules and metal electrodes of the inorganic metals through multiple functional groups. The interface electronic structures between L‐cysteine with metals deserve further investigation for applications in bioelectronics. However, the interface electronic structures of L‐cysteine and metals have not been well understood. We have previously reported the existence of a new state between the highest occupied molecular orbital (HOMO) of L‐cysteine and the Fermi level of the metals for L‐cysteine/Au(111), L‐cysteine/Ag(111), and L‐cysteine/Cu(111) using photoemission spectroscopy and attributed the formation of the new state to an interaction of the d band with HOMO of L‐cysteine. In this study, the electronic structure at the interfaces of L‐cysteine on a Palladium (Pd) surface is investigated by ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation including work function, secondary electron cutoff (SECO), and HOMO onset; the position of an interface state, charge injection barrier, and ionization energy are estimated. It is observed that thin‐film spectra of L‐cysteine on Pd surfaces in the valance top region are different from the L‐cysteine thick films, and this can be attributed to an interaction between a sulfur‐originated state of L‐cysteine HOMO with Pd d orbitals. Also, a 0.6‐eV SECO shift is estimated due to the charge transferring between L‐cysteine and Pd. The results of SECO further confirm the weakening of the Pd–sulfur bond with increasing L‐cysteine coverage on Pd.