Orbital alignment at p-sexiphenyl and coronene/layered materials interfaces measured with photoemission spectroscopy

Abstract

The energy level alignment at the interfaces between para-sexiphenyl/highly oriented pyrolytic graphite (HOPG), coronene/SnS2, and coronene/HOPG were determined using in situ thin film deposition in combination with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements. The organic thin films were grown in multiple steps by vapor deposition, then sequentially characterized in situ after each growth step. The vacuum cleaved single crystals of SnS2 and HOPG substrates provided clean, atomically flat, and chemically inert surfaces, allowing for the investigation of the phenomena of band bending and interface dipoles without the interference of chemical reactions or morphological problems. Due to the distinctly different work functions of the HOPG (Φ=4.65 eV) and SnS2 (Φ=5.45 eV) substrates, the observed shifts in the binding energies of the organic overlayer related XPS core level emission lines could be associated with band bending resulting from Fermi level equilibration between the organic thin films and substrates. Possible occurrence of screening effects due to the different polarization energies of the two substrates can, however, complicate the ability to precisely measure band bending. Low intensity XPS work function measurements enabled the detection of the overlayer-thickness-dependent onset of charging phenomena in the UPS measurements. This allowed the precise determination of the highest occupied molecular orbital alignment of the organic molecules at the investigated interfaces.