Pentacene grown on self-assembled monolayer: Adsorption energy, interface dipole, and electronic properties

The energetic, the morphology, the interface dipole formation, and the electronic states of pentacene grown on a self-assembled monolayer (SAM) of benzenethiolate on the Cu(100) surface are investigated by complementary structural and spectroscopic techniques. The growth morphology of the heterostructure has been investigated by atomic force microscopy (AFM), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission spectroscopy, inferring tightly packed arrangement of grains constituted of pentacene molecules with nearly perpendicular orientation, when a single-layer is deposited on top of the benzenethiolate-SAM. The adsorption energy of the pentacene single layer on the benzenethiolate buffer layer (E-a = 1.16 eV 112 kJ/mol) is found much weaker than for pentacene on the copper surface, as estimated by thermal desorption spectroscopy (TDS). The evolution of the spectral density of electronic states in the valence band, obtained by high-resolution ultraviolet photoelectron spectroscopy (HR-UPS), confirms the weak interaction of the pentacene molecules with the benzenethiolate-SAM and the formation of a semiconducting heterostructure, with a hole injection barrier reduced to 0.95 eV with respect to the pentacene/Cu interface.