ESR13: Structure and Reactivity of Lutetium bis-Phthalocyanine Thin Films / Thesis

Université Pierre et Marie Curie (UPMC), France 

Mattia Farronato

 

Abstract

In this thesis work I studied the structure and reac􀆟vity of Lute􀆟um bis-phthalocyanine (LuPc2) thin
films deposited on metallic surfaces. The knowledge of the structure of epitaxial organic thin films is
extremely important to design the next genera􀆟on of devices based on organic electronics, because
crucial parameters as carrier mobility, and to an extent charge recombina􀆟on, cri􀆟cally depend on the
molecular configura􀆟on and thin film ordering. The second part of the work deals with the reac􀆟vity
of molecular thin films towards atmospheric gases. This is par􀆟cularly important when considering the
life􀆟me of the device, as at the moment this is one of the greatest hindrance to the extensive use of
organic electronic devices in everyday life. Phthalocyanine were chosen due to their already wide use
in devices and their ease of process. In par􀆟cular we chose LuPc2 because, due to the double decker
molecular geometry (see Figure 1) they should present a different reac􀆟vity respect to single decker
phthalocyanine, as the second macrocycle should hinder the reac􀆟vity on the metallic ca􀆟on.
The work was carried out with a surface science approach, the thin films were prepared by thermal
evapora􀆟on under ultra-high vacuum condi􀆟ons and analysed by means of Scanning Tunnelling
Microscopy (STM) and X-ray Diffrac􀆟on (XRD) to achieve a complete characteriza􀆟on of the structure
and the morphology. Than the chemical changes a􀅌er reac􀆟on towards gases were studied by means
of X-ray Photoemission Spectroscopy (XPS) and Near Edge X-ray Absorp􀆟on Fine Structure
Spectroscopy (NEXAFS) to study.
We resolved the structure of a LuPc2 thin film deposited on Au(111), showing that the molecules adopt
a β structure, the interac􀆟on with the substrate via the relaxa􀆟on of the surface reconstruc􀆟on, and
demonstrated the templa􀆟ng effect of the substrate via the epitaxial rela􀆟ons with the overlayer. We
then present the surface morphology at the molecular scale, showing the forma􀆟on of large (tens of
nm) islands composed by a double layer of molecules. From a careful analysis of the STM images we
are able to show the stacking geometry, as well as the rela􀆟ve orienta􀆟on of the different domains
and compare this results with the one obtained by XRD.
We tested the reac􀆟vity of these thin films towards atmospheric gases, in par􀆟cular oxygen and water,
showing a low reac􀆟vity and managing to demonstrate the adsorp􀆟on sites, which indeed are not the
central ca􀆟on, but rather on the macrocycle. We also showed how molecular oxygen is a greater threat
to the molecular stability than water.
Figure 1 Left: β structure of LuPc2; right: proposed absorption geometry for O2 molecule

See a recent presentation.