Charge transfer
It was shown that charge transfer at many interfaces between organic semiconductors and metal substrates occur. In some cases even a bidirectional charge transfer determines the interface energetics. Combined x-ray phtoemission (XPS) and x-ray excited Auger electron spectroscopy (XAES) measurements allow for example the discussion of such site-specific charge transfer processes.
Also for transition metal phthalocyanine (TMPc) interfaces different interaction channels are known, illustrated by a yellow or red background in Fig. 1. The charge transfer via the transition metal however is an important channel and may affect also the HOMO/LUMO related electronic structure and thus states important for charge carrier transport. Moreover, a charge transfer involving the transion metal can result in a drastic change of electronic and magnetic interface properties. This might be in particular important for possible future applications in spintronic nanodevices such as spin valves or spin memory devices.
Both X-ray absorption (XAS) and XPS) spectra contain valuable information about the electronic configuration of d-levels of the central metal atom of TMPcs. Their shape is determined to a large extent by multiplet effects; the reason is the strong overlap of the core wave function with the valence wave functions. As an example, we show XAS spectra for a monolayer (ML) coverage CoPc on Ag(111) and compare them to spectra representative for the bulk (2.8 nm) below.
Fig. 2 CoPc on Ag(111): Comparison of L3 edge XAS spectra for coverages in the monolayer range to thin films taken at grazing and normal incidence of the incoming synchrotron light. The changes in the spectra point to a charge transfer to the Co ion and a hybridization at the interface.
Related pulications:
Charge transfer between transition metal phthalocyanines and metal substrates: The role of the transition metal, H. Peisert, J. Uihlein, F. Petraki T. Chassé Journal of Electron Spectroscopy and Related Phenomena, (2015) online.
CoPc and CoPcF16 on gold: Site specific charge transfer processes, F. Petraki, H. Peisert, J. Uihlein, U. Aygül, T. Chassé Beilstein Journal of Nanotechnology, 5 (2014) 524–531.