Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical π-type bonds are not strong enough to alter the alloy, while localized σ-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy's surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules.

Cuprous oxide (Cu 2 O) is a promising material for photoelectrochemical energy conversion due to its small direct band gap, high absorbance, and its Earth-abundant constituents. High conversion efficiencies require transport of photoexcited charges to the interface without energy loss. We studied the electron dynamics in Cu 2 O(111) by time-resolved two-photon photoemission for different surface defect densities in order to elucidate the influence on charge carrier transport. On the pristine bulk terminated surface, the principal conduction bands could be resolved, and ultrafast, elastic transport of electrons to the surface was observed. On a reconstructed surface the carrier transport is strongly suppressed and defect states dominate the spectra. Evidence for surface oxygen vacancies acting as efficient carrier traps is provided, what is important for further engineering of Cu 2 O based photoelectrodes. The conversion of solar energy into fuel in photoelectrochemical cells (PEC) represents a sustainable way for energy conversion and storage, a typical route being the production of hydrogen via water splitting 1-3. Upon light absorption in some semiconducting electrode material, photoexcited charges are generated that are separated and transported to the solid-electrolyte interface. Here they drive some catalyst-promoted redox chemistry that stores their energy in chemical bonds 4. This concept promises to be both ecologically and economically attractive if electrodes with high conversion efficiencies can be built from cheap, Earth-abundant materials that are stable in the aggressive chemical environment of a PEC cell 5. Cuprous oxide (Cu 2 O) has attracted much attention recently as electrode material for photo-electrochemical water splitting 6-10. It is a p-type semiconductor with a band gap of 2.1 eV and has a maximum theoretical solarto-hydrogen (STH) conversion efficiency of 18% 8. The bare material is unstable towards reduction to metallic Cu under electrochemical hydrogen evolution conditions, but protective nanolayers of n-type TiO 2 or ZnO can stabilize the electrode. The alignment of the conduction bands and the band bending near the p-n junction make for the charge separation and transport of photoexcited electrons to the oxide-electrolyte interface 6. Ga 2 O 3 nanolayers showed an even better band alignment for charge transport due to the very small conduction band offset. Recently a nanostructured photocathode design with Cu 2 O/Ga 2 O 3 /TiO 2 buried junctions and with NiMo as hydrogen evolution catalyst was demonstrated 9. It was implemented conformally in a coaxial nanowire structure to match the light absorption depth to the much shorter minority carrier diffusion length. It combines efficient light absorption with high positive onset voltage and shows high photocurrent density, paving the way towards efficient water splitting devices based entirely on Earth-abundant materials. Electrode performance is very sensitive to the presence of interfacial or bulk defects, which typically form states wit...

The successful implementation of nanoscale materials in next generation optoelectronic devices crucially depends on our ability to functionalize and design low dimensional materials according to the desired field of application. Recently, organic adsorbates have revealed an enormous potential to alter the occupied surface band structure of tunable materials by the formation of tailored molecule-surface bonds. Here, we extend this concept of adsorption-induced surface band structure engineering to the unoccupied part of the surface band structure. This is achieved by our comprehensive investigation of the unoccupied band structure of a lead (Pb) monolayer film on the Ag(1 1 1) surface prior and after the adsorption of one monolayer of the aromatic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using two-photon momentum microscopy, we show that the unoccupied states of the Pb/Ag(1 1 1) bilayer system are dominated by a parabolic quantum well state (QWS) in the center of the surface Brillouin zone with Pb p orbital character and a side band with almost linear dispersion showing Pb p orbital character. After the adsorption of PTCDA, the Pb side band remains completely unaffected while the signal of the Pb QWS is fully suppressed. This adsorption induced change in the unoccupied Pb band structure coincides with an interfacial charge transfer from the Pb layer into the PTCDA molecule. We propose that this charge transfer and the correspondingly vertical (partially chemical) interaction across the PTCDA/Pb interface suppresses the existence of the QWS in the Pb layer. Our results hence unveil a new possibility to orbital selectively tune and control the entire surface band structure of low dimensional systems by the adsorption of organic molecules.

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