Formation of the Calcium/Poly(3-Hexylthiophene) Interface: Structure and Energetics

Abstract

The adsorption of Ca on poly(3-hexylthiophene) (P3HT) has been studied by adsorption microcalorimetry, atomic beam/surface scattering, X-ray photoelectron spectroscopy (XPS), low-energy He(+) ion scattering spectroscopy (LEIS), and first-principles calculations. The sticking probability of Ca on P3HT is initially 0.35 and increases to almost unity by 5 ML. A very high initial heat of adsorption in the first 0.02 ML (625-500 kJ/mol) is attributed to the reaction of Ca with defect sites or residual contamination. Between 0.1 and 0.5 ML, there is a high and nearly constant heat of adsorption of 405 kJ/mol, which we ascribe to Ca reacting with subsurface sulfur atoms from the thiophene rings of the polymer. This is supported by the absence of LEIS signal for Ca and the shift of the S 2p XPS binding energy by -2.8 eV for reacted S atoms. The heat of adsorption decreases above 0.6 ML coverage, reaching the sublimation enthalpy of Ca, 178 kJ/mol, by 4 ML. This is attributed to the formation of Ca nanoparticles and eventually a continuous solid Ca film, on top of the polymer. LEIS and XPS measurements, which show only a slow increase of the signals related to solid Ca, support this model. Incoming Ca atoms are subject to a kinetic competition between diffusing into the polymer to react with subsurface thiophene units versus forming or adding to three-dimensional Ca clusters on the surface. At approximately 1.6 ML Ca coverage, Ca atoms have similar probabilities for either process, with the former dominating at lower coverage. Ultimately about 1.6 ML of Ca (1.2 x 10(15) atoms/cm(2)) reacts with S atoms, corresponding to a reacted depth of approximately 3 nm, or nearly five monomer-unit layers. Density-functional theory calculations confirm that the heat of reaction and the shift of the S 2p signal are consistent with Ca abstracting S from the thiophene rings to form small CaS clusters.

Calcium Thin Film Growth on Phenyl-C61-Butyric Acid Methyl Ester (PCBM): Interface Structure and Energetics

“…A very similar effect was reported for Ca adsorption on semiconducting polymers. 23,34,45 The O 1s peak also shifts considerably, but toward lower BE instead. This is clearly due to the conversion of the methyl ester groups to Ca carboxylates mentioned above, which has a stronger effect on O 1s BEs than band bending, which would also affect the O 1s BEs for any unreacted components of the PCBM in the same way as it does the C 1s BEs.…”

“…This same result has been observed for Ca adsorption on polymer surfaces. 23,24,27,34,35 We propose that reaction II above, which occurs between Ca and the methyl ester groups of PCBM, is similar to the following reaction: (1)…”

Growth and electronic structure of Sm on thin Al2O3/Ni3Al(111) films

“…The beamline and endstation equipment have been described in detail previously. 53,54 Briefly, this beamline is connected to a bending magnet and equipped with three gratings that cover photon energies from 10 to 250 eV with a resolving power (E/ E) > 1000. The endstation is equipped with an ARUPS 10 electron analyzer.…”

Epitaxial growth of ultrathin ZrO2(111) films on Pt(111)

“…The endstation equipment has been described in detail previously [12,13]. Briefly, the U18 beamline is connected to a bending magnet and equipped with three gratings that cover photon energies from 10 to 250 eV with a resolving power (E/ΔE) > 1000.…”