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.
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