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

Abstract

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.

Vibrational Spectra of Thiolate-Protected Gold Nanocluster with Infrared Reflection Absorption Spectroscopy: Size- and Temperature-Dependent Ordering Behavior of Organic Monolayer

“…The C12 SAM was prepared by immersing an Au(111) single-crystal substrate into 0.5 mM ethanol solution of dodecanethiol (C 12 H 25 SH), for 20 h at room temperature . The substrate was washed with ethanol to remove excess thiol molecules.…”

“…The C12 SAM was prepared by immersing an Au(111) single-crystal substrate into 0.5 mM ethanol solution of dodecanethiol (C 12 H 25 SH), for 20 h at room temperature. 26 The substrate was washed with ethanol to remove excess thiol molecules. Semifluorinated alkanethiol SAM (F-SAM) was prepared in the same manner but with a 0.68 mM ethanol s o l u t i o n o f 1 H , 1 H , 2 H , 2 H -p e r fl u o r o d e c a n e t h i o l (CF 3 (CF 2 ) 7 (CH 2 ) 2 SH), for 24 h. 26 The SAMs of PhC n H 2n SH (n = 1−3), referred to as CnPh SAM, were produced by immersing an Au(111) substrate into a 1 mM dichloromethane solution at room temperature, for 24 h. 27 The substrate was washed with dichloromethane to remove excess thiol molecules.…”

Characterization of floating-gate memory device with thiolate-protected gold and gold-palladium nanoclusters

“…The results show that it is possible to control the floating-gate behavior with the size-and composition-dependencies of precisely synthesized metal nanoclusters. Since the energy level alignment can be also tuned by changing the surface dipole with chemical modifications of surface overlayers, 51 the combination of nanoclusters and surface overlayers will be a widely applicable method to precisely control the energy levels for the floating-gate behavior.…”