Growth and interfacial studies of conjugated oligomer films on Si and SiO2 substrates

The growth of [2,5-bis(4-styryl)styryl] 1,4-dioctyloxybenzene, (Ooct-OPV5) oligomer films on Si (100)-(2×1) and Si (111)-(7×7) reconstructed surfaces as well as on a ${\mathrm{SiO}}_2$ film over a Si (100) wafer was studied by x-ray photoelectron spectroscopy (XPS). Ooct-OPV5 resembles poly (p-phenylenevinylene) (PPV), a polymer that is widely used in organic light emitting diodes. High purity oligomer films of up to 18 nm thickness were prepared on the clean substrates by stepwise evaporation in ultrahigh vacuum conditions and a layerwise growth of films was observed on all substrates. The electronic structure of the oligomer interface with n-doped Si (111) was investigated by combined x ray and ultraviolet photoelectron spectroscopies (XPS) and (UPS). The $\mathrm{C}\text{\hspace{0.05em}1s}$ XPS peak of the bulk oligomer consisted of three components, all associated with oligomer functional groups at binding energies 285.05, 285.75, and 287.15 eV, respectively. During growth, both $\mathrm{C}\text{\hspace{0.05em}1s}$ and $\mathrm{O}\text{\hspace{0.05em}1s}$ peaks in the film exhibited an upward BE shift of 0.45±0.05 eV, from which the total band bending at the interface was evaluated. The depletion region in the organic film during the interface was found to be ∼90 Å thick. The UP spectra of the oligomeric film exhibited characteristic peaks that resemble those of PPV, and the oligomer work function was found to be 4.00±0.05 eV. The interface between the two materials in contact proved to be nonreactive and no detectable electric dipole was observed.