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

Abstract

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 TiO 2 (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.

Surface Reactions and Electronic Structure of Carboxylic Acid Porphyrins Adsorbed on TiO2(110)

“…This observation indicates either a disordered monolayer or a monolayer consisting of tilted molecules at the angle in which there is no dependence with the photon polar incidence angle (54.7°) . Recently, we have used angle-resolved photoelectron spectroscopy to study the growth of Co-MCTPP on TiO 2 (110), and we found that monolayer deposition results in a disordered layer that grows in three dimensions . Thus, the NEXAFS measurements corresponding to the MCTPP monolayer could indicate a disordered monolayer.…”

“…30 Recently, we have used angle-resolved photoelectron spectroscopy to study the growth of Co-MCTPP on TiO 2 (110), and we found that monolayer deposition results in a disordered layer that grows in three dimensions. 31 Thus, the NEXAFS measurements corresponding to the MCTPP monolayer could indicate a disordered monolayer. Hence, under the assumption of no preferred azimuthal orientation, our NEXAFS data suggest that as we go from low coverages to monolayer coverages, the number of molecules with the macrocycle tilted away from the surface increases as there is a transition from flat-lying molecules to a disordered layer.…”

Phenylphosphonic Acid on Rutile TiO2(110): Using Theoretically Predicted O 1s Spectra to Identify the Adsorption Binding Modes

“…This requires us to correct for the moderate changes in attenuation of the Ti 3p signal as the emission angles change from −20 to 60°, while the E -field polar angle relative to the surface normal changes from 10 to 90°. We did this by extrapolating the attenuation of the Ti 3p signal by the phosphonic acid monolayer we observed at 100 eV kinetic energy to a kinetic energy of 260 eV, based on literature data. − This results in a correction factor of 1.6 for the inelastic mean free path (IMFP).…”

Accurate Determination of Adsorption-Energy Differences of Metalloporphyrins on Rutile TiO2(110) 1 × 1

“…Coverages were calculated in two different ways: for the initial depositions, coverages were calculated based on the C 1s-to-Ti 2p ratios. For the deposited amounts (<4 ML), attenuation of the Ti 2p substrate signal is only modest (<40%) and thus any deviation from perfect layer-by-layer growth should only have a small effect on the calculated coverages. , After annealing to 525 K at 20 K/min, the multilayers have desorbed and only the monolayer remains. The heating rate was chosen low enough to achieve equilibrium, such that our two different starting points (A on B and B on A) resulted in almost identical remaining monolayers.…”