Charge Separation at the Molecular Monolayer Surface: Observation and Control of the Dynamics

Abstract

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an n-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (EF), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10-C18). We have also evaluated molecular conductivity at the specific electron energy of EF + 3.7 eV. The tunneling decay parameter, β, was fitted by β90 K = 0.097 Å(-1) and βRT = 0.13 Å(-1). These values were one order smaller than that at around EF by conventional contact probe methods.

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.…”

“…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.…”

“…An image potential state (IPS) of dodecanethiolate-(C12-) SAM survives for as long as 49 ps at 90 K, indicating strong insulating properties of n-alkanethiolate SAM (Cx-, where x is the number of carbon atoms in the alkyl chains) [10]. Since organic molecules are highly ordered structures whose height may be tuned by changing the lengths of their alkyl chains, the electronic properties of functional organic molecules deposited on SAMs are of considerable interest.…”