Protein Electronic Energy Transport Levels Derived from High-Sensitivity Near-UV and Constant Final State Yield Photoemission Spectroscopy

Proteins are attractive as functional components in molecular junctions. However, control-ling the electronic charge transport via proteins, held between two electrodes, requires in-formation on their frontier orbital energy level alignment relative to the electrodes’ Fermi level (EF), which normally requires studies of UV Photoemission Spectroscopy (UPS) with HeI excitation. Such excitation is problematic for proteins, which can denature under stand-ard measuring conditions. Here we use high-sensitivity soft UV photoemission spectroscopy (HS-UPS) combined with Constant Final State Yield Spectroscopy (CFS-YS) to get this in-formation for electrode/protein contacts. Monolayers of the redox protein Azurin, (Az) and its Apo-form on Au substrates, have HOMO onset energies, obtained from CFS-YS, differ by ~ 0.2 eV, showing crucial role of the Cu redox centre in the electron transport process. We find that combined HS-UPS / CFS-YS measurements agree with the Photoelectron Yield Spectroscopy (PYS), showing potential of the HS-UPS + CFS-YS as a powerful tool to char-acterize and map the energetics of a protein-electrode interfaces, which will aid optimizing design of devices with targeted electronic properties, as well as for novel applications.