ABSTRACT
Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.
ABSTRACT
Pre-nucleation dynamics, nucleation and templated self-assembly of a conjugated planar aromatic molecule are investigated by photoemission electron microscopy (PEEM). The high resolution of individual molecular layers in PEEM, in combination with a numerical simulation, reveals the dynamic behaviour of molecules during the pre-nucleation deposition period and their temperature dependence. The in situ deposition of p-sexiphenyl (6P) molecules on Cu(110) and Cu(110) 2 × 1-O surfaces in ultrahigh vacuum, when monitored by PEEM in real-time allows (a) layer densities, (b) meta-stable layer filling by 6P molecules, (c) dynamic surface redistributions during layer filling and (d) critical density spontaneous dewetting to be accurately measured. The comparison of 6P deposited on Cu(110) to Cu(110) 2 × 1-O enables temperature dependent 6P nucleation processes on Cu(110) to be elucidated from PEEM. The interplay between energetically stable molecular arrangements and kinetically stabilised arrangements is shown to dominate the pre- and post-nucleation processes. In combination with additional data obtained during post-nucleation deposition times, such as surface diffusion anisotropies and nucleation energies, it is concluded that the pre-requisite for 6P nucleation, in a lying down orientation, is the formation of a double tilted layer with at least one layer being meta-stable.
ABSTRACT
Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large pi-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.
