Evidencing intermolecular effects with core-level photoelectron spectroscopy via the accurate density functional calculation of core-electron binding energies on model systems: gamma-APS as a test

gamma-Aminopropyltrihydroxysilane (gamma-APS) is a molecule which is used as an adhesion promoter in microelectronics, for the coating of oxidized silicon wafers with polyimide films. The Si/gamma-APS interface has been studied recently using X-ray photoelectron spectroscopy, and evidenced a need for reference spectra of both gamma-APS and its thermal byproducts: as gamma-APS oligomerizes readily upon warming, no gas phase, reference, or XPS spectrum of this compound can be obtained experimentally. Thus, spectral features emerging from Si/gamma-APS interactions are difficult to separate from structural fingerprints of gamma-APS alone, This phenomenon further hinders the follow up by XPS of structural modifications the molecule undergoes upon thermal treatments. A recent procedure of computing very accurate core-electron binding energies (CEBEs) via density functional theory (DFT) is used as a guide to propose a pseudo reference spectrum. The computed CEBEs of the various core levels of the isolated. molecule are found in excellent agreement with the experimental XPS spectra recorded upon spin coating the compound on a silicon wafer at room temperature, with an average absolute deviation (aad) for C Is, Id Is, and O Is levels of only 0,13 eV, i.e., of the order of experimental resolution. The same procedure is then conducted on isolated ionic structures presumably formed when thick gamma-APS layers have undergone thermal treatment in a H2O/CO2 atmosphere, A very bad agreement is found between theory and experiment on these isolated ions, with aad's as large as 4.91 V. Upon actually computing the CEBEs on larger molecular models in which (i) ions are paired and then in which (ii) ion pairs are further solvated by one up to four water molecules, the aad reduces to 0,31 eV. We suggest, on the instance of gamma-APS, that (i) the accurate calculation of CEBEs has now come to be a tractable and reliable alternative as a hand for spectrum decomposition when gas-phase reference XPS spectra are not available for calibration and that (ii) the availability of an accurate and tractable theoretical procedure to compute CEBEs, compatible with experimental precision, enables XPS to give some information on intermolecular effects, although this spectroscopy involves core ionizations.