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.

Adsorption-induced modification of the hot electron lifetime in a Pb/Ag111 quantum well system

“…Despite these well-defined momentum space emission pattern, the CE lifetime maps of the molecular and Pb-derived states reveal homogeneous lifetimes for all momenta. This might not be surprising for the molecular states which only exhibit characteristic momentum space signatures for much larger momenta that are inaccessible in our tr-2PMM experiment [38,[53][54][55]. However, we also do not observe any signature of a ringlike feature in the CE lifetime map of the Pb-derived side band as found for the CE lifetime map of the bare Pb/Ag(111) bilayer system for identical excitation conditions (p-polarized excitation).…”

“…In the next step, we focus on the modifications of the ultrafast electron dynamics of the Pb/Ag(111) bilayer system by the adsorption of the aromatic molecule PTCDA. In this multilayer system, the interaction and corresponding charge transfer across the PTCDA/Pb interface quench the QWS in the Pb layer [38]. This can potentially alter the momentum-dependent refilling processes within the Pb/Ag(111) bilayer system.…”

“…To gain further insights into the inter-or intraband scattering processes at interfaces, we turn to the singleparticle electron dynamics of one layer lead (Pb) on an Ag(111) single crystal surface. This model system was selected as an exemplary case from the manifold of ultrathin layers on surfaces that can host quantum confined electrons and quantum well states (QWSs) [9,11,12,15,38]. In the particular case of the Pb/Ag(111) interface, the band structure reveals two distinct bands in the Pb layer: (i) a parabolic, free electron-like QWS with parabolic dispersion in the center of the surface Brillouin zone and p z orbital character and (ii) a Pb side band with almost linear dispersion and p x/y orbital character [38].…”

“…This model system was selected as an exemplary case from the manifold of ultrathin layers on surfaces that can host quantum confined electrons and quantum well states (QWSs) [9,11,12,15,38]. In the particular case of the Pb/Ag(111) interface, the band structure reveals two distinct bands in the Pb layer: (i) a parabolic, free electron-like QWS with parabolic dispersion in the center of the surface Brillouin zone and p z orbital character and (ii) a Pb side band with almost linear dispersion and p x/y orbital character [38]. These two bands are expected to dominate the ultrafast electron dynamics of such ultrathin Pb films.…”

Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy

“…The momentum-resolved photoemission data were obtained by momentum microscopy at the University of Kaiserslautern 37,38 . The core level and normal incidence standing waves experiments were conducted at the hard x-ray Photoemission (HAXPES) and x-ray standing wave (XSW) end station at beamline I09 at the synchrotron light source Diamond in Didcot, UK 39 .…”