Transport band gap opening at metal-organic interfaces

TL;DR

This study investigates how the transport band gap of copper phthalocyanine (CuPc) at metal interfaces varies with film thickness and substrate type, revealing image charge effects and charge transfer phenomena using spectroscopy techniques.

Contribution

It provides detailed insights into the thickness-dependent band gap opening and interface charge transfer mechanisms at CuPc/metal interfaces, combining photoelectron spectroscopy methods.

Findings

Transport band gap of CuPc opens from 1.4 eV to 2.2 eV with increasing thickness.

Interface dipoles depend on the metal substrate work functions.

Charge transfer dynamics vary with film thickness and substrate, affecting Fermi level positioning.

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 $(1.4 \pm 0.3)$ eV for 1 nm thickness to $(2.2 \pm 0.3)$ 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.