Molecular Ion Formation by Photoinduced Electron Transfer at the Tetracyanoquinodimethane/Au(111) Interface

Abstract

Optically induced processes in organic materials are essential for light harvesting, switching, and sensor technologies. Here we studied the electronic properties of the tetracyanoquinodimethane(TCNQ)/Au(111) interface by using two-photon photoemission spectroscopy. For this interface we demonstrated the lack of charge-transfer interactions, but we found a significant increase in the sample work function due to UV-light illumination, while the electronic structure of the TCNQ-derived states remain unaffected. Thereby the work function of the interface can be tuned over a wide range via the photon dose. We assigned this to a photoinduced metal-to-molecule electron transfer creating negative ions. The electrons are bound by a small potential barrier. Thus thermal activation reverses the process resulting in the original work function value. The presented photoinduced charge transfer at the TCNQ/Au(111) interface can be used for continuous work function tuning across the substrate's work function, which can be applied in device-adapted hole-injection layers or organic UV-light sensors.

Electronic structure of an iron porphyrin derivative on Au(1 1 1)

“…The photoemission peak close to the high-energy cut-off can be assigned to the first (n = 1) image potential state (IPS) located 3.87±0.07 eV above E F . This corresponds to an energy of 0.57 eV below the vacuum level, and has been found for other adsorbatecovered Au(111) surfaces [34,23,46,47,48,49].…”

“…The photoemission peak close to the high-energy cut-off can be assigned to the first (n = 1) image potential state (IPS) located 3.87±0.07 eV above E F . This corresponds to an energy of 0.57 eV below the vacuum level, and has been found for other adsorbatecovered Au(111) surfaces [34,23,46,47,48,49]. Figure 5 shows the two-color 2PPE spectra of 1 ML FeTPyP/Au(111) recorded with different photon energies and the corresponding analysis of the photon-energy dependent energetic position of the observed photoemission peaks.…”

A Comparative Computational Study of the Adsorption of TCNQ and F4-TCNQ on the Coinage Metal Surfaces

“…The adsorption of TCNQ and F4-TCNQ on different metal surfaces has been the subject of many different experimental and theoretical studies. ,,− From the early results, it was clear that both molecules adsorb flat on most metal surfaces, accepting negative charge from the substrate and causing an increase in the work function of the system. Maybe the only exception is the adsorption of TCNQ on the Au(111) surface.…”

“…In here, a decrease of the work function has been experimentally measured, 44,55 and both theory and experiments seem to indicate that TCNQ is physisorbed on the Au(111) surface and there is no charge transfer between the molecule and the gold substrate. 39,43,52,55 In some cases, molecular distortion and/or surface reconstruction have also been reported. 16,38,40,[46][47][48]51,53 Upon adsorption, the molecule has been claimed to adopt a bent geometry, with the cyano groups closer to the surface than the central carbon ring, while the metal atoms directly underneath the nitrogen atoms are slightly pulled outward the surface.…”

Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation

“…Similar shifts have been observed for many organic molecules on Au(111), where charge transfer to the substrate is negligible and ascribed to a modification of the image charge and work function (Φ). 13 , 53 , 82 − 87 Indeed, we find a reduction of the work function for all three molecules, the largest one for TAPP-H by 0.74 eV.…”