Molecular Level Characterization of Thin Paraffinic Films Using NEXAFS Spectroscopy

ABSTRACTBecause near edge X-ray absorption fine structures (NEXAFS) are due to dipole transitions from core levels to unoccupied molecular orbitals, it is an ideal technique for obtaining molecular level information about the orientations of ordered species. Such information is important in assessing the relative importance of various intermolecular forces. For example in long chain molecules with bulky functional groups it is not clear a priori whether the attractive van der Waals forces will overcome the repulsive steric interactions to allow for ordering in thin films. To investigate this we have performed C-K edge NEXAFS spectroscopy studies of thin films of the paraffinic molecules C36H74 and (C18H37)2(NCO)C6H4CO2−+NH2(C18H37)2. The films were grown on a Si(111) surface covered with a native oxide. For the normal alkane, the spectra are dominated by three resolvable (Δhυ≈0.5 eV) features: one (C-H)* peak and two σ*(C-C) peaks. The polarization dependence of these peaks indicates that sub-250 Å C36H74 films have the chains of the molecules aligned along the surface normal. For ≈ 2000 Å films the molecular axes are inclined at angle of 32° with respect to the surface normal, in agreement with the crystalline structure determined by X-ray diffraction. In addition to the major structures of the alkane chains, the C-K NEXAFS of (C18H37)2(NCO)C6H4CO2−+NH2(C18H37)2 has a distinct π* peak due to the phenyl ring. Analysis of these resonances indicates that the C-C chains are also aligned along the surface normal, while the phenyl ring is inclined at an angle of 42° with respect to the normal. In conclusion NEXAFS spectroscopy has been used to show that, for the molecules studied, the attractive van der Waals interactions dominate the repulsive forces thus allowing the molecules to orient with their chains aligned along the surface normal in thin films formed on native oxide covered Si(111).