Reduced Pd density of states in Pd/SAM/Au junctions: the role of adsorbed hydrogen atoms

Experiments have shown that a Pd monolayer deposited electrochemically on a Au-supported self-assembled monolayer (SAM) of 4-mercaptopyridine (Mpy) exhibits a strongly reduced Pd local density of states (LDOS) at the Fermi energy (Ef). Understanding the origin of this modified electronic structure is crucial for the use of the sandwich design as a platform for future nanoelectronics. Here we suggest that hydrogen adsorption might be the origin of the modified electronic properties. We performed periodic density functional theory calculation to explore the influence of hydrogen adsorption on the geometric and electronic structure of a Pd/Mpy/Au(111) complex. Dissociative adsorption of H2 on a Pd monolayer on top of a Mpy SAM is a strongly exothermic process leading to atomic hydrogen atoms preferentially located at the hollow sites. Due to the formation of a strong Pd–H bond the Pd–SAM interaction realized via one-fold N–Pd bonds is substantially weakened. Upon hydrogen adsorption, the Pd LDOS becomes significantly modified exhibiting a drastic reduction of the density of states at Ef. The calculated spectra are in a good agreement with the experiment for a hydrogen coverage corresponding to two monolayers which is still thermodynamically allowed.