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 u sed as an adhesion promoter in microelectronics, for the coating of ox idized 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 ther mal byproducts: as gamma-APS oligomerizes readily upon warming, no gas phase, reference, or XPS spectrum of this compound can be obtained ex perimentally. Thus, spectral features emerging from Si/gamma-APS inter actions are difficult to separate from structural fingerprints of gamm a-APS alone, This phenomenon further hinders the follow up by XPS of s tructural 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 guid e to propose a pseudo reference spectrum. The computed CEBEs of the va rious core levels of the isolated. molecule are found in excellent agr eement with the experimental XPS spectra recorded upon spin coating th e compound on a silicon wafer at room temperature, with an average abs olute 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 th ick gamma-APS layers have undergone thermal treatment in a H2O/CO2 atm osphere, A very bad agreement is found between theory and experiment o n these isolated ions, with aad's as large as 4.91 V. Upon actually co mputing the CEBEs on larger molecular models in which (i) ions are pai red 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 spec trum decomposition when gas-phase reference XPS spectra are not availa ble for calibration and that (ii) the availability of an accurate and tractable theoretical procedure to compute CEBEs, compatible with expe rimental precision, enables XPS to give some information on intermolec ular effects, although this spectroscopy involves core ionizations.