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
S. Kranias et al., 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, JOURNAL OF PHYSICAL CHEMISTRY B, 101(49), 1997, pp. 10254-10261
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.