Thickness-dependent electronic properties and molecular orientation of diradical metal complex thin films grown on SiO2

We studied the thickness dependence of electronic structure and molecular orientation of neutral, square planar, diamagnetic diradical metal complex, bis(o-diiminobenzosemiquinonate) nickel(II) complexes (Ni(DIBSQ)2), thin films grown on a SiO2 substrate by using ultraviolet photoelectron spectroscopy in combination with metastable atom electron spectroscopy. We observed that the small hole-injection barrier of 0.46 eV for the monolayer film, and the barrier increases continuously with the film thickness and becomes larger than the electron-injection barrier. We further found that molecules are oriented with their long-axes nearly perpendicular to the surface both in the monolayer and multilayer films independent of the film thickness.