Transport band gap opening at metal–organic interfaces

Abstract

The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule substrate distance. Due to the image charge potential given by the metallic substrates the transport band gap of CuPc "opens" from (1.4 ± 0.3) eV for 1 nm thickness to (2.2 ± 0.3) eV, and saturates at this value above 10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV for Au and Co substrates, respectively, predominantly depend on the metal substrate work functions. X-ray photoelectron spectroscopy measurements using synchrotron radiation provide detailed information on the interaction between CuPc and the two metal substrates. While charge transfer from the Au or Co substrate to the Cu metal center is present only at sub-monolayer coverages, the authors observe a net charge transfer from the molecule to the Co substrate for films in the nm range. Consequently, the Fermi level is shifted as in the case of a p-type doping of the molecule. This is, however, a competing phenomenon to the energy band shifts due to the image charge potential.

Surface-confined 2D polymerization of a brominated copper-tetraphenylporphyrin on Au(111)

“…It should be noted that DFT generally underestimates energy gaps. On the other hand, the HOMO-LUMO gap is significantly reduced in the proximity of a surface compared to a thick layer 37 or the gas phase as calculated here, which leads to the seemingly good agreement for CuTPP and the H-saturated CuTPP-polymer with the experimental values. After debromination the HOMO-LUMO gap is strongly reduced due to C atoms with unsaturated bonds.…”

“…Because the N orbitals contribute to the highest occupied (HOMO) as well as lowest unoccupied (LUMO) molecular orbital, the excitation energy can be correlated with the energy gap between the HOMO and LUMO. 37 However, the distance of the shake-up satellite to the corresponding core-level peak could be smaller than the actual HOMO-LUMO gap due to reorganization of the charge within the molecule upon photoexcitation and, thus, changed final state screening of the core hole. 38 The separation from the main N 1s peak at 398 eV is 1.2 eV for the completely debrominated molecules.…”

Multiplex movie of concerted rotation of molecules on a 2D material

“…This is supported by comparison with the calculated momentum distributions of the lowest unoccupied molecular orbitals (LUMO and LUMO') of isolated CuPc, which are overlaid on the experimental data as LUMO + LUMO'. Two excitation mechanisms can play a role in the transient LUMO population: a direct excitation from HOMO to the LUMO corresponding to an optical gap of ~1.1 eV, which is slightly smaller than the reported 1.4 eV for thin lms on a metal substrate (35). Due to the offresonant excitation and the momentum distribution of the ( LUMO + LUMO'), we consider the most probable excitation channel to be via hot electron scattering from the Ti 3d band into the LUMO.…”

Hexacene on Cu(110) and Ag(110): Influence of the Substrate on Molecular Orientation and Interfacial Charge Transfer

“…With UPS, we also obtain the work functions of both interfaces in focus. , Molecules close to a metal interface tend to have a reduced HOMO–LUMO gap by the polarization of the metal. − Consequently the LUMO level moves closer to the Fermi level E F .…”