Modification of Pb quantum well states by the adsorption of organic molecules

Abstract

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.