Electronic States of Alkanethiolate Self-Assembled Monolayers on Au(111) Studied by Two-Photon Photoemission Spectroscopy

Abstract

Two-photon photoemission (2PPE) spectroscopy has been employed to probe the electronic states of n-alkanethiolate self-assembled monolayers (SAMs) on an Au(111) substrate fabricated in a wet chemical process. Electronic states newly formed in the SAM formation were observed below and above the Fermi level (E F) for various alkyl chain lengths of C12-, C18-, and C22-SAMs. At 3.5–3.7 eV above E F, an unoccupied state originating from a bond between gold and sulfur atoms appears, a state that shows little dispersion with parallel to the surface. The peak position of the unoccupied state depends on the substrate temperature, and it was stabilized with increasing temperature; E F +3.7 eV at 85 K and E F +3.5 eV at 330 K for C18-SAM. The stabilization of the state is attributed to the increase of intermolecular interaction at sulfur atoms with their neighboring S atoms, which is caused by a change in the tilt angle of the alkyl chains in the SAM: on increasing the temperature, the interaction between S atoms in the SAM is promoted by the more upright alkyl chains.

Electronic states of titanyl phthalocyanine films on alkanethiolate self-assembled monolayers probed by two-photon photoemission

“…The spectral features closely match those of bare C6-C22-SAMs measured in the same energy region [28]. Of the two unoccupied states in the energy region, one was derived from chemical bonding between gold and sulfur atoms, denoted as "C", E F +3.6 eV at RT [16], the other derived from an image potential state (IPS, E F +3.7 eV [10]) formed outside the SAM. Both of them were simply filled by electrons from a Au(1 1 1) substrate.…”

“…2, the occupied state derived from the Au(1 1 1) substrate, located 2.05 eV below Fermi level [16], is also observed at around 2.6-2.8 eV of intermediate energy with higher excitation energy of 4.65-4.85 eV. Fig.…”

“…Therefore, it seems reasonable to consider that a bare C8-SAM area remains by forming the TiOPc islands with low-ML thicknesses. The energy positions of the states, previously reported at RT [10,16], are indicated at the bottom of Fig. 2.…”

“…The preparation of C8-SAMs/Au(1 1 1) substrate and 2PPE measurements have been detailed elsewhere [10,[16][17][18], and are merely outlined here. An Au(1 1 1) single crystal was cleaned by repeated cycles of Ar + ion sputtering (0.6 kV, ∼3.0 A, 15 min) and annealing (683 K, 1 h) in an ultrahigh vacuum (UHV) chamber.…”

“…Fig. 3 is a schematic of the energy levels observed at the TiOPc/SAM surface obtained from the UPS and 2PPE spectra [10,16,32]. The HOMO (E F −1.4 eV) and the CTE (E F +2.9 eV) in addition to "C" (E F +3.6 eV) and IPS (E F +3.7 eV) are indicated in the figure.…”