1 research outputs found
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 C<sub>60</sub> 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 C<sub>60</sub> is deposited on the
H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels
of C<sub>60</sub> are pinned to the vacuum level (Fermi level (<i>E</i><sub>F</sub>) + WF). As a result of the energy level alignment,
on the F-SAM, the relative energy from <i>E</i><sub>F</sub> of the highest occupied molecular orbital of C<sub>60</sub> almost
equals that of the lowest unoccupied molecular orbital, implying that
the C<sub>60</sub> film on the F-SAM exhibits both p- and n-type (ambipolar)
charge transport properties, while C<sub>60</sub> is known as a typical
n-type semiconductor. The energetics are preserved even with multilayered
C<sub>60</sub> 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