Excitation and Relaxation Dynamics of Two-Dimensional Photoexcited Electrons on Alkanethiolate Self-Assembled Monolayers

Abstract

The electron dynamics in alkanethiolate selfassembled monolayers (Cn-SAMs; n = 6−18, where n is the number of alkyl carbons) formed on Au(111) surfaces has been investigated by time-and angle-resolved two-photon photoemission spectroscopy. The time evolution of photoexcited electrons flowing down into image potential states (IPSs) formed on standing-up structure of SAMs is resolved twodimensionally; the electron lifetime in the IPS increases with chain length, from sub-ps to 100 ps. The chain length dependence of the IPS lifetime is particularly marked at shorter chain lengths of n = 6−10, whereas it becomes milder at chain lengths above n = 10, whose alkyl layer thickness is ≈10 Å. This thickness dependence can be explained by two competitive channels for the decay of IPS electrons: one is electronic coupling of IPS with unoccupied bulk Au states and an interfacial state localized at the Au−S linkage, and the other is IPS electron decay to the Au substrate through a tunneling barrier of insulating alkyl chains. The former is most influential at shorter chain lengths, while the latter is solely dominant at longer chain lengths. In addition, the photon energy dependence of the IPS intensity revealed that electron injection into the IPS is mediated effectively by an electron excitation into interfacial resonance formed in the alkyl layer.

Photoexcited State Confinement in Two-Dimensional Crystalline Anthracene Monolayer at Room Temperature

“…Figure a shows single-color 2PPE spectra with various photon energies ( h ν) ranging from h ν 1 = 4.47 to 5.17 eV. A peak at 3.84 eV above the Fermi level ( E F ) originates from the first IPS formed outside the surface showing selectivity for p-polarized incident photons (Figure b) and a 2D-free electron-like dispersive nature parallel to the surface (Figure c). , The IPS observation also guarantees the molecular uniformity from the viewpoint of electronic structures because IPS is observable only on an atomically flat surface. , …”

“…It seems reasonable that IPS electrons are accepted by LUMO+ n in anthracene, leading to a LUMO at E F +2.5 eV (Figure b, inset), because anthracene molecules have a positive electron affinity (∼0.5 eV) and the IPS wave function partially penetrates into the anthracene layer . In fact, the LUMO electrons survive with a lifetime of 64 ps (Figure S7), which is comparable to IPS lifetimes for CH 3 -C11-SAM (23.2 ps) and CH 3 -C17-SAM (50.4 ps). , A longer lifetime of the LUMO electrons in Ant-C11-SAM might be caused by electron stabilization by the polarization of anthracene moieties. On the other hand, the lifetime of S-CTE (110 fs) is much shorter than that of IPS, showing that such a high energy exciton quickly relaxes into lower-lying S n , including S 1 .…”

“…A peak at 3.84 eV above the Fermi level (E F ) originates from the first IPS formed outside the surface showing selectivity for p-polarized incident photons (Figure 2b) and a 2D-free electron-like dispersive nature parallel to the surface (Figure 2c). 29,30 The IPS observation also guarantees the molecular uniformity from the viewpoint of electronic structures because IPS is observable only on an atomically flat surface. 31,32 At hν 1 > 4.6 eV, another peak strongly appears at E F +3.17 eV, and it is maximized at hν 1 = 4.90 eV, implying a resonance electron−hole excitation involving occupied and unoccupied states.…”

“…The intensity profiles (Figure b) are fitted by an IPS lifetime of 1100 fs. In the case of alkanethiolate-SAMs, namely, having no anthracene terminals, the IPS lifetime is very long [23.2 ps for dodecanethiolate (CH 3 -C11-SAM)], owing to the insulating properties of alkyl chains. , The shortened IPS lifetime of Ant-C11-SAM shows that the IPS electron relaxes to lower-lying electronic states in the 2D anthracene crystal. It seems reasonable that IPS electrons are accepted by LUMO+ n in anthracene, leading to a LUMO at E F +2.5 eV (Figure b, inset), because anthracene molecules have a positive electron affinity (∼0.5 eV) and the IPS wave function partially penetrates into the anthracene layer .…”

“…45 In fact, the LUMO electrons survive with a lifetime of 64 ps (Figure S7), which is comparable to IPS lifetimes for CH 3 -C11-SAM (23.2 ps) and CH 3 -C17-SAM (50.4 ps). 29,30 A longer lifetime of the LUMO electrons in Ant-C11-SAM might be caused by electron stabilization by the polarization of anthracene moieties. On the other hand, the lifetime of S-CTE (110 fs) is much shorter than that of IPS, showing that such a high energy exciton quickly relaxes into lower-lying S n , including S 1 .…”