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.