Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers

We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C₆₀ molecules deposited on alkanethiolate self-assembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C₆₀ is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C₆₀ are pinned to the vacuum level (Fermi level (E_F) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from E_F of the highest occupied molecular orbital of C₆₀ almost equals that of the lowest unoccupied molecular orbital, implying that the C₆₀ film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C₆₀ is known as a typical n-type semiconductor. The energetics are preserved even with multilayered C₆₀ films at least up to ∼5 nm in thickness, showing that the dipole layers induced by SAMs are robust against molecular overlayers. Such a spectroscopic study on the energy levels for organic films will be of importance for further development of organic thin film devices.