Probing the Roughness of Porphyrin Thin Films with X-ray Photoelectron Spectroscopy.

Thin-film growth of molecular systems is of interest for many applications, such as for instance organic electronics. In this study, we demonstrate how X-ray photoelectron spectroscopy (XPS) can be used to study the growth behavior of such molecular systems. In XPS, coverages are often calculated assuming a uniform thickness across a surface. This results in an error for rough films, and the magnitude of this error depends on the kinetic energy of the photoelectrons analyzed. We have used this kinetic-energy dependency to estimate the roughnesses of thin porphyrin films grown on rutile TiO2 (110). We used two different molecules: cobalt (II) monocarboxyphenyl-10,15,20-triphenylporphyrin (CoMCTPP), with carboxylic-acid anchor groups, and cobalt (II) tetraphenylporphyrin (CoTPP), without anchor groups. We find CoMCTPP to grow as rough films at room temperature across the studied coverage range, whereas for CoTPP the first two layers remain smooth and even; depositing additional CoTPP results in rough films. Although, XPS is not a common technique for measuring roughness, it is fast and provides information of both roughness and thickness in one measurement.

Methanol adsorption and decomposition on Pt/CeO2(111)/Cu(111) thin film model catalyst.

Adsorption and desorption of methanol on Pt particles on a CeO(2)(111)/Cu(111) thin film surface and on an ion-eroded Pt(111) single crystal were investigated by X-ray photoelectron spectroscopy and soft X-ray synchrotron radiation photoelectron spectroscopy (PES). Resonant PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Multilayers of methanol were adsorbed at low temperature and subsequently desorbed by heating to 600 K. Methanol desorption is accompanied by the formation of chemisorbed methoxy -OCH(3). Cerium oxide surface is strongly reduced by methanol, which was detected via the transition Ce(4+) --> Ce(3+) and an increase of the Ce 4f electronic state occupancy. Partial C-O bond scission and formation of atomic carbon was observed on the Pt particles as well as on the rough Pt(111) surface. On Pt/CeO(2)(111), all traces of surface carbon and residual hydrocarbons disappear at 500 K.