Unoccupied electron states of ultrathin films of thiophene–phenylene cooligomers on the surface of polycrystalline gold

Unoccupied electronic states in the energy range from 5 to 20 eV above the Fermi level have been studied in ultrathin films of dimethyl-substituted thiophene–phenylene cooligomers CH$_3$-phenylene–thiophene–thiophene–phenylene–CH$_3$ (CH$_3$–PTTP–CH$_3$) on polycrystalline gold surfaces of two types: the ex situ Au layer thermally deposited in a special chamber and the in situ Au surface prepared inside an analytical chamber. The film structure is studied by the X-ray diffraction (XRD) method. The formation of a superposition of the amorphous phase and the crystalline phase with period 3.8 nm is discussed. The energy positions of the maxima of the unoccupied electronic states and the character of formation of the boundary potential barrier have been studied by the total current spectroscopy (TCS). The structures of the FSTCS maxima of the 5–7-nm-thick CH$_3$–PTTP–CH$_3$ films are not different when using various types of Au substrates and the ZnO semiconductor surface prepared by atomic layer deposition (ALD). As a CH$_3$–PTTP–CH$_3$ layer is deposited on the ex situ Au and in situ Au surfaces, the electron work function increases insignificantly (by 0.1 eV) as the coating thickness increases to 5–7 nm. At such thicknesses of the CH$_3$–PTTP–CH$_3$ films, the electron work function is 4.7 $\pm$ 0.1 eV in the case of the ex situ Au substrate and 4.9 $\pm$ 0.1 eV in the case of the in situ Au substrate. A possible influence of the processes of physicochemical interaction at the boundary between the film and the substrate on the formation of the boundary potential barrier in the structures under study is discussed.